# publications

## 2024

- Electron Glass Phase with Resilient Zhang-Rice Singlets in LiCu3O3A. Consiglio, G. Gatti, E. Martino, and 21 more authors
*Physical Review Letters*Mar 2024LiCu3O3 is an antiferromagnetic mixed valence cuprate where trilayers of edge-sharing Cu(II)O (3d9) are sandwiched in between planes of Cu(I) (3d10) ions, with Li stochastically substituting Cu(II). Angle-resolved photoemission spectroscopy (ARPES) and density functional theory reveal two insulating electronic subsystems that are segregated in spite of sharing common oxygen atoms: a Cu dz2/O pz derived valence band (VB) dispersing on the Cu(I) plane, and a Cu 3dx2−y2/O 2px,y derived Zhang-Rice singlet (ZRS) band dispersing on the Cu(II)O planes. First-principle analysis shows the Li substitution to stabilize the insulating ground state, but only if antiferromagnetic correlations are present. Li further induces substitutional disorder and a 2D electron glass behavior in charge transport, reflected in a large 530 meV Coulomb gap and a linear suppression of VB spectral weight at EF that is observed by ARPES. Surprisingly, the disorder leaves the Cu(II)-derived ZRS largely unaffected. This indicates a local segregation of Li and Cu atoms onto the two separate corner-sharing Cu(II)O2 sub-lattices of the edge-sharing Cu(II)O planes, and highlights the ubiquitous resilience of the entangled two hole ZRS entity against impurity scattering.

## 2023

- Dynamics and resilience of the unconventional charge density wave in ScV6Sn6 bilayer kagome metalManuel Tuniz, Armando Consiglio, Denny Puntel, and 22 more authors
*Communications Materials*Dec 2023Long-range electronic ordering descending from a metallic parent state constitutes a rich playground to study the interplay of structural and electronic degrees of freedom. In this framework, kagome metals are in the most interesting regime where both phonon and electronically mediated couplings are significant. Several of these systems undergo a charge density wave transition. However, to date, the origin and the main driving force behind this charge order is elusive. Here, we use the kagome metal ScV6Sn6 as a platform to investigate this problem, since it features both a kagome-derived nested Fermi surface and van-Hove singularities near the Fermi level, and a charge-ordered phase that strongly affects its physical properties. By combining time-resolved reflectivity, first principles calculations and photo-emission experiments, we identify the structural degrees of freedom to play a fundamental role in the stabilization of charge order, indicating that ScV6Sn6 features an instance of charge order predominantly originating from phonons.

- Mott insulators with boundary zerosN. Wagner, L. Crippa, A. Amaricci, and 9 more authors
*Nature Communications*Nov 2023The topological classification of electronic band structures is based on symmetry properties of Bloch eigenstates of single-particle Hamiltonians. In parallel, topological field theory has opened the doors to the formulation and characterization of non-trivial phases of matter driven by strong electron-electron interaction. Even though important examples of topological Mott insulators have been constructed, the relevance of the underlying non-interacting band topology to the physics of the Mott phase has remained unexplored. Here, we show that the momentum structure of the Green’s function zeros defining the “Luttinger surface" provides a topological characterization of the Mott phase related, in the simplest description, to the one of the single-particle electronic dispersion. Considerations on the zeros lead to the prediction of new phenomena: a topological Mott insulator with an inverted gap for the bulk zeros must possess gapless zeros at the boundary, which behave as a form of “topological antimatter” annihilating conventional edge states. Placing band and Mott topological insulators in contact produces distinctive observable signatures at the interface, revealing the otherwise spectroscopically elusive Green’s function zeros.

- Observation of Termination-Dependent Topological Connectivity in a Magnetic Weyl Kagome LatticeFederico Mazzola, Stefan Enzner, Philipp Eck, and 20 more authors
*Nano Letters*Sep 2023Engineering surfaces and interfaces of materials promises great potential in the field of heterostructures and quantum matter designers, with the opportunity to drive new many-body phases that are absent in the bulk compounds. Here, we focus on the magnetic Weyl kagome system Co3Sn2S2 and show how for the terminations of different samples the Weyl points connect differently, still preserving the bulk-boundary correspondence. Scanning tunneling microscopy has suggested such a scenario indirectly, and here, we probe the Fermiology of Co3Sn2S2 directly, by linking it to its real space surface distribution. By combining micro-ARPES and first-principles calculations, we measure the energy-momentum spectra and the Fermi surfaces of Co3Sn2S2 for different surface terminations and show the existence of topological features depending on the top-layer electronic environment. Our work helps to define a route for controlling bulk-derived topological properties by means of surface electrostatic potentials, offering a methodology for using Weyl kagome metals in responsive magnetic spintronics.

- Strongly anisotropic spin and orbital Rashba effect at a tellurium – noble metal interfaceB. Geldiyev, M. Ünzelmann, P. Eck, and 19 more authors
*Physical Review B*Sep 2023We study the interplay of lattice, spin, and orbital degrees of freedom in a two-dimensional model system: a flat square lattice of Te atoms on a Au(100) surface. The atomic structure of the Te monolayer is determined by scanning tunneling microscopy and quantitative low-energy electron diffraction. Using spin- and angle-resolved photoelectron spectroscopy and density functional theory, we observe a Te-Au interface state with highly anisotropic Rashba-type spin-orbit splitting at the ¯¯¯X point of the Brillouin zone. Based on a profound symmetry and tight-binding analysis, we show how in-plane square lattice symmetry and broken inversion symmetry at the Te-Au interface together enforce a remarkably anisotropic orbital Rashba effect which strongly modulates the spin splitting.

- Flat band separation and robust spin Berry curvature in bilayer kagome metals
*Domenico Di Sante*, Chiara Bigi, Philipp Eck, and 16 more authors*Nature Physics*Aug 2023Kagome materials have emerged as a setting for emergent electronic phenomena that encompass different aspects of symmetry and topology. It is debated whether the XV6Sn6 kagome family (where X is a rare-earth element), a recently discovered family of bilayer kagome metals, hosts a topologically non-trivial ground state resulting from the opening of spin–orbit coupling gaps. These states would carry a finite spin Berry curvature, and topological surface states. Here we investigate the spin and electronic structure of the XV6Sn6 kagome family. We obtain evidence for a finite spin Berry curvature contribution at the centre of the Brillouin zone, where the nearly flat band detaches from the dispersing Dirac band because of spin–orbit coupling. In addition, the spin Berry curvature is further investigated in the charge density wave regime of ScV6Sn6 and it is found to be robust against the onset of the temperature-driven ordered phase. Utilizing the sensitivity of angle-resolved photoemission spectroscopy to the spin and orbital angular momentum, our work unveils the spin Berry curvature of topological kagome metals and helps to define its spectroscopic fingerprint.

- Topological band inversion in HgTe(001): Surface and bulk signatures from photoemissionRaphael C. Vidal, Giovanni Marini, Lukas Lunczer, and 15 more authors
*Physical Review B*Mar 2023HgTe is a versatile topological material and has enabled the realization of a variety of topological states, including two- and three-dimensional (3D) topological insulators and topological semimetals. Nevertheless, a quantitative understanding of its electronic structure remains challenging, in particular, due to coupling of the Te 5p-derived valence electrons to Hg 5d core states at shallow binding energy. We present a joint experimental and theoretical study of the electronic structure in strained HgTe(001) films in the 3D topological-insulator regime, based on angle-resolved photoelectron spectroscopy and density functional theory. The results establish detailed agreement in terms of: (i) electronic band dispersions and orbital symmetries, (ii) surface and bulk contributions to the electronic structure, and (iii) the importance of Hg 5d states in the valence-band formation. Supported by theory, our experiments directly image the paradigmatic band inversion in HgTe, underlying its nontrivial band topology.

- Recipe for higher order topology on the triangular latticePhilipp Eck, Yuan Fang,
*Domenico Di Sante*, and 2 more authors*Physical Review B*Mar 2023We present a recipe for an electronic two-dimensional (2D) higher order topological insulator (HOTI) on a triangular lattice that can be realized in a large family of materials. The essential ingredient is mirror symmetry breaking, which allows for a finite quadrupole moment and trivial Z2 index. The competition between spin-orbit coupling and the symmetry-breaking terms gives rise to four topologically distinct phases; the HOTI phase appears when symmetry breaking dominates, including in the absence of spin-orbit coupling. We identify triangular monolayer adsorbate systems on the (111) surface of zincblende/diamond type substrates as ideal material platforms and predict the HOTI phase for X=(Al,B,Ga) on SiC.

- Role of electronic correlations in the kagome-lattice superconductor LaRh3B2Savita Chaudhary, Shama, Jaskaran Singh, and 4 more authors
*Physical Review B*Feb 2023LaRh3B2 crystallizes in a layered structure where Rh atoms form a perfect kagome lattice. The material shows superconductivity at Tc≈2.6 K and no signature for density wave instabilities. We report our measurements of electronic transport, magnetization, and heat capacity in the normal and superconducting state, and we derive normal and superconducting parameters. From first-principles calculations of the electronic band structure, we identify all features of kagome bands predominantly formed by the Rh d orbitals: a flat band, Dirac cones, and van Hove singularities. The calculation of the phonon dispersions and electron-phonon coupling suggests a strong similarity between LaRh3B2 and AV3Sb5 (A=K,Cs,Rb). For LaRh3B2, it matches quantitatively with the observed Tc, supporting a conventional phonon-mediated pairing mechanism. By comparison to the AV3Sb5 family, we conjecture that there is a reduced importance of electron correlations in LaRh3B2.

- Electronic correlations and universal long-range scaling in kagome metals
*Domenico Di Sante*, Bongjae Kim, Werner Hanke, and 4 more authors*Physical Review Research*Jan 2023We investigate the real-space profile of effective Coulomb interactions in correlated kagome materials. By particularizing to KV3Sb5, Co3Sn2S2, FeSn, and Ni3In, we analyze representative cases that exhibit a large span of correlation-mediated phenomena, and contrast them to prototypical perovskite transition metal oxides. From our constrained random phase approximation studies we find that the on-site interaction strength in kagome metals not only depends on the screening processes at high energy, but also on the low-energy hybridization profile of the electronic density of states. Our results indicate that rescaled by the on-site interaction amplitude, all kagome metals exhibit a universal long-range Coulomb behavior.

## 2022

- Real-space obstruction in quantum spin Hall insulatorsPhilipp Eck, Carmine Ortix, Armando Consiglio, and 6 more authors
*Physical Review B*Nov 2022The recently introduced classification of two-dimensional insulators in terms of topological crystalline invariants has been applied so far to “obstructed” atomic insulators characterized by a mismatch between the centers of the electronic Wannier functions and the ionic positions. We extend this notion to quantum spin Hall insulators in which the ground state cannot be described in terms of time-reversal symmetric localized Wannier functions. A system equivalent to graphene in all its relevant electronic and topological properties except for a real-space obstruction is identified and studied via symmetry analysis as well as with density functional theory. The low-energy model comprises a local spin-orbit coupling and a nonlocal symmetry breaking potential, which turn out to be the essential ingredients for an obstructed quantum spin Hall insulator. An experimental fingerprint of the obstruction is then measured in a large-gap triangular quantum spin Hall material.

- Observation of room temperature excitons in an atomically thin topological insulatorMarcin Syperek, Raul Stuehler, Armando Consiglio, and 10 more authors
*Nature Communications*Oct 2022Optical spectroscopy of ultimately thin materials has significantly enhanced our understanding of collective excitations in low-dimensional semiconductors. This is particularly reflected by the rich physics of excitons in atomically thin crystals which uniquely arises from the interplay of strong Coulomb correlation, spin-orbit coupling (SOC), and lattice geometry. Here we extend the field by reporting the observation of room temperature excitons in a material of non-trivial global topology. We study the fundamental optical excitation spectrum of a single layer of bismuth atoms epitaxially grown on a SiC substrate (hereafter bismuthene or Bi/SiC) which has been established as a large-gap, two-dimensional (2D) quantum spin Hall (QSH) insulator. Strongly developed optical resonances are observed to emerge around the direct gap at the K and K’ points of the Brillouin zone, indicating the formation of bound excitons with considerable oscillator strength. These experimental findings are corroborated, concerning both the character of the excitonic resonances as well as their energy scale, by ab-initio GW and Bethe-Salpeter equation calculations, confirming strong Coulomb interaction effects in these optical excitations. Our observations provide evidence of excitons in a 2D QSH insulator at room temperature, with excitonic and topological physics deriving from the very same electronic structure.

- Deep Learning the Functional Renormalization Group
*Domenico Di Sante*, Matija Medvidović, Alessandro Toschi, and 4 more authors*Physical Review Letters*Sep 2022We perform a data-driven dimensionality reduction of the scale-dependent four-point vertex function characterizing the functional renormalization group (FRG) flow for the widely studied two-dimensional t−t′ Hubbard model on the square lattice. We demonstrate that a deep learning architecture based on a neural ordinary differential equation solver in a low-dimensional latent space efficiently learns the FRG dynamics that delineates the various magnetic and d-wave superconducting regimes of the Hubbard model. We further present a dynamic mode decomposition analysis that confirms that a small number of modes are indeed sufficient to capture the FRG dynamics. Our Letter demonstrates the possibility of using artificial intelligence to extract compact representations of the four-point vertex functions for correlated electrons, a goal of utmost importance for the success of cutting-edge quantum field theoretical methods for tackling the many-electron problem.

- Twofold van Hove singularity and origin of charge order in topological kagome superconductor CsV3Sb5Mingu Kang, Shiang Fang, Jeong-Kyu Kim, and 14 more authors
*Nature Physics*Mar 2022The layered vanadium antimonides AV3Sb5 (A = K, Rb, Cs) are a recently discovered family of topological kagome metals that exhibit a range of strongly correlated electronic phases including charge order and superconductivity. However, it is not yet understood how the distinctive electronic structure of the kagome lattice is linked to the observed many-body phenomena. Here we combine angle-resolved photoemission spectroscopy and density functional theory to reveal multiple kagome-derived van Hove singularities (vHS) coexisting near the Fermi level of CsV3Sb5 and analyse their contribution to electronic symmetry breaking. The vHS are characterized by two distinct sublattice flavours (p-type and m-type), which originate, respectively, from their pure and mixed sublattice characters. These twofold vHS flavours of the kagome lattice critically determine the pairing symmetry and unconventional ground states emerging in the AV3Sb5 series. We establish that, among the multiple vHS in CsV3Sb5, the m-type vHS of the dxz/dyz kagome band and the p-type vHS of the dxy/dx2–y2 kagome band are located very close to the Fermi level, setting the stage for electronic symmetry breaking. The former band is characterized by pronounced Fermi surface nesting, while the latter exhibits a higher-order vHS. Our work reveals the essential role of kagome-derived vHS for the collective phenomena realized in the AV3Sb5 family.

- Triplet Superconductivity from Nonlocal Coulomb Repulsion in an Atomic Sn Layer Deposited onto a Si(111) SubstrateSebastian Wolf,
*Domenico Di Sante*, Tilman Schwemmer, and 2 more authors*Physical Review Letters*Apr 2022Atomic layers deposited on semiconductor substrates introduce a platform for the realization of the extended electronic Hubbard model, where the consideration of electronic repulsion beyond the on-site term is paramount. Recently, the onset of superconductivity at 4.7 K has been reported in the hole-doped triangular lattice of tin atoms on a silicon substrate. Through renormalization group methods designed for weak and intermediate coupling, we investigate the nature of the superconducting instability in hole-doped Sn=Sið111Þ. We find that the extended Hubbard nature of interactions is crucial to yield triplet pairing, which is f-wave (p-wave) for moderate (higher) hole doping. In light of persisting challenges to tailor triplet pairing in an electronic material, our finding promises to pave unprecedented ways for engineering unconventional triplet superconductivity.

- Evidence of a 2D Electron Gas in a Single‐Unit‐Cell of Anatase TiO2 (001)Alessandro Troglia, Chiara Bigi, Ivana Vobornik, and 10 more authors
*Advanced Science*Apr 2022The formation and the evolution of electronic metallic states localized at the surface, commonly termed 2D electron gas (2DEG), represents a peculiar phenomenon occurring at the surface and interface of many transition metal oxides (TMO). Among TMO, titanium dioxide (TiO2), particularly in its anatase polymorph, stands as a prototypical system for the development of novel applications related to renewable energy, devices and sensors, where understanding the carrier dynamics is of utmost importance. In this study, angle-resolved photo-electron spectroscopy (ARPES) and X-ray absorption spectroscopy (XAS) are used, supported by density functional theory (DFT), to follow the formation and the evolution of the 2DEG in TiO2 thin films. Unlike other TMO systems, it is revealed that, once the anatase fingerprint is present, the 2DEG in TiO2 is robust and stable down to a single-unit-cell, and that the electron filling of the 2DEG increases with thickness and eventually saturates. These results prove that no critical thickness triggers the occurrence of the 2DEG in anatase TiO2 and give insight in formation mechanism of electronic states at the surface of TMO.

- Van Hove tuning of AV3Sb5 kagome metals under pressure and strainArmando Consiglio, Tilman Schwemmer, Xianxin Wu, and 5 more authors
*Physical Review B*Apr 2022From first-principles calculations, we investigate the structural and electronic properties of the kagome metals AV3Sb5 (A=Cs, K, Rb) under isotropic and anisotropic pressure. Charge-ordering patterns are found to be unanimously suppressed, while there is a significant rearrangement of p-type and m-type Van Hove point energies with respect to the Fermi level. Already for moderate tensile strain along the V plane and compressive strain normal to the V layer, we find that a Van Hove point can be shifted to the Fermi energy. Such a mechanism provides an invaluable tuning knob to alter the correlation profile in the kagome metal, and suggests itself for further experimental investigation. It might allow us to reconcile possible multidome superconductivity in kagome metals not only from phonons but also from the viewpoint of unconventional pairing.

## 2021

- Nature of Unconventional Pairing in the Kagome Superconductors AV3Sb5 (A = K;Rb;Cs)Xianxin Wu, Tilman Schwemmer, Tobias Müller, and 10 more authors
*Physical Review Letters*Oct 2021The recent discovery of AV3Sb5 (A=K,Rb,Cs) has uncovered an intriguing arena for exotic Fermi surface instabilities in a kagome metal. Among them, superconductivity is found in the vicinity of multiple van Hove singularities, exhibiting indications of unconventional pairing. We show that the sublattice interference mechanism is central to understanding the formation of superconductivity in a kagome metal. Starting from an appropriately chosen minimal tight-binding model with multiple van Hove singularities close to the Fermi level for AV3Sb5, we provide a random phase approximation analysis of superconducting instabilities. Nonlocal Coulomb repulsion, the sublattice profile of the van Hove bands, and the interaction strength turn out to be the crucial parameters to determine the preferred pairing symmetry. Implications for potentially topological surface states are discussed, along with a proposal for additional measurements to pin down the nature of superconductivity in AV3Sb5.

- Momentum for Catalysis: How Surface Reactions Shape the RuO2 Flat Surface StateVedran Jovic, Armando Consiglio, Kevin E. Smith, and 5 more authors
*ACS Catalysis*Jan 2021The active (110) surface of the benchmark oxygen evolution catalyst RuO2 spans a ﬂat-band surface state (FBSS) between the surface projections of its Dirac nodal lines (DNLs) that deﬁne the electronic properties of this functional semimetal. Monitoring well-known surface adsorption processes of H2, O2, NO, and CO by in operando angle-resolved photoemission spectroscopy, we selectively modify the oxidation state of individual Ru surface sites and identify the electronic nature of the FBSS: stabilized by bridging oxygen Obr pz, the FBSS disperses along ⟨001⟩ oriented chains of bridging Rubr 4dz2 orbitals, collapses upon Obr removal, yet remains surprisingly unaﬀected by the oxidation state of the undercoordinated 1f-cus-Ru species. This directly reﬂects in the ability of RuO2(110) to oxidize CO and H2 along with its inability to oxidize NO, demonstrating the FBSS’s active role in catalytic charge transfer processes at the oxygen bridge sites. Our synergetic approach provides momentum-resolved insights to the interplay of a catalyst’s delocalized electronic band structure and the localized orbitals of its surface reactantsa route toward a microscopic understanding of heterogeneous catalysis.

- Momentum-space signatures of Berry flux monopoles in the Weyl semimetal TaAsM. Ünzelmann, H. Bentmann, T. Figgemeier, and 14 more authors
*Nature Communications*Dec 2021Abstract Since the early days of Dirac flux quantization, magnetic monopoles have been sought after as a potential corollary of quantized electric charge. As opposed to magnetic monopoles embedded into the theory of electromagnetism, Weyl semimetals (WSM) exhibit Berry flux monopoles in reciprocal parameter space. As a function of crystal momentum, such monopoles locate at the crossing point of spin-polarized bands forming the Weyl cone. Here, we report momentum-resolved spectroscopic signatures of Berry flux monopoles in TaAs as a paradigmatic WSM. We carried out angle-resolved photoelectron spectroscopy at bulk-sensitive soft X-ray energies (SX-ARPES) combined with photoelectron spin detection and circular dichroism. The experiments reveal large spin- and orbital-angular-momentum (SAM and OAM) polarizations of the Weyl-fermion states, resulting from the broken crystalline inversion symmetry in TaAs. Supported by first-principles calculations, our measurements image signatures of a topologically non-trivial winding of the OAM at the Weyl nodes and unveil a chirality-dependent SAM of the Weyl bands. Our results provide directly bulk-sensitive spectroscopic support for the non-trivial band topology in the WSM TaAs, promising to have profound implications for the study of quantum-geometric effects in solids.

- Nanoscale synthesis of ionic analogues of bilayer silicene with high carrier mobilityDmitry V. Averyanov, Peitao Liu, Ivan S. Sokolov, and 6 more authors
*Journal of Materials Chemistry C*Dec 2021High carrier mobility of both electrons and holes is found in nanofilms of layered SrAl 2 Si 2 integrated with silicon. The salient feature of its atomic structure is anionic bilayers [Al 2 Si 2 ] 2− , isostructural and isoelectronic to bilayer silicene. , Design of materials with special properties benefits from establishing deep structural and electronic analogies between emerging and existing materials. The Zintl anion [Al 2 Si 2 ] 2− is both isostructural and isoelectronic to bilayer silicene; it thus makes a promising building block to assemble electronic materials. Here, we show that nanoscale films of SrAl 2 Si 2 , a semimetal formed by alternating [Al 2 Si 2 ] and Sr layers, exhibit a high carrier mobility, exceeding 10 000 cm 2 V −1 s −1 . The dominant role of the anionic bilayers in the electronic structure and transport properties is established by band structure calculations. To synthesize monocrystalline epitaxial films of SrAl 2 Si 2 with atomically sharp interfaces, a general two-step route involving a sacrificial 2D template is devised. A distinct advantage of the films is their natural integration with silicon technology. The results establish a platform for engineering layered ionic nanomaterials.

- Design and realization of topological Dirac fermions on a triangular latticeMaximilian Bauernfeind, Jonas Erhardt, Philipp Eck, and 8 more authors
*Nature Communications*Sep 2021Large-gap quantum spin Hall insulators are promising materials for room-temperature applications based on Dirac fermions. Key to engineer the topologically non-trivial band ordering and sizable band gaps is strong spin-orbit interaction. Following Kane and Mele’s original suggestion, one approach is to synthesize monolayers of heavy atoms with honeycomb coordination accommodated on templates with hexagonal symmetry. Yet, in the majority of cases, this recipe leads to triangular lattices, typically hosting metals or trivial insulators. Here, we conceive and realize “indenene”, a triangular monolayer of indium on SiC exhibiting non-trivial valley physics driven by local spin-orbit coupling, which prevails over inversion-symmetry breaking terms. By means of tunneling microscopy of the 2D bulk we identify the quantum spin Hall phase of this triangular lattice and unveil how a hidden honeycomb connectivity emerges from interference patterns in Bloch px ± ipy-derived wave functions.

- From high Tc to low Tc : Multiorbital effects in transition metal oxidesMichael Klett, Tilman Schwemmer, Sebastian Wolf, and 8 more authors
*Physical Review B*Sep 2021Despite the structural resemblance of certain cuprate and nickelate parent compounds there is a striking spread of Tc among such transition metal oxide superconductors. We adopt a minimal two-orbital eg model which covers cuprates and nickelate heterostructures in different parametric limits, and analyze its superconducting instabilities. The joint consideration of interactions, doping, fermiology, and in particular the eg orbital splitting allows us to explain the strongly differing pairing propensities in cuprate and nickelate superconductors.

## 2020

- Turbulent hydrodynamics in strongly correlated Kagome metals
*Domenico Di Sante*, Johanna Erdmenger, Martin Greiter, and 6 more authors*Nature Communications*Aug 2020A current challenge in condensed matter physics is the realization of strongly correlated, viscous electron fluids. These fluids can be described by holography, that is, by mapping them onto a weakly curved gravitational theory via gauge/gravity duality. The canonical system considered for realizations has been graphene. In this work, we show that Kagome systems with electron fillings adjusted to the Dirac nodes provide a much more compelling platform for realizations of viscous electron fluids, including non-linear effects such as turbulence. In particular, we find that in Scandium Herbertsmithite, the fine-structure constant, which measures the effective Coulomb interaction, is enhanced by a factor of about 3.2 as compared to graphene. We employ holography to estimate the ratio of the shear viscosity over the entropy density in Sc-Herbertsmithite, and find it about three times smaller than in graphene. These findings put the turbulent flow regime described by holography within the reach of experiments. Viscous electron fluids are predicted in strongly correlated systems but remain challenging to realize. Here, the authors predict enhanced effective Coulomb interaction and reduced ratio of the shear viscosity over entropy density in a Kagome metal, inferring turbulent flow of viscous electron fluids.

- Electronic structure of epitaxial perovskite films in the two-dimensional limit: Role of the surface terminationP. Schuetz, M. Kamp,
*D. Di Sante*, and 5 more authors*Applied Physics Letters*May 2020An often-overlooked property of transition metal oxide thin films is their microscopic surface structure and its effect on the electronic properties in the ultrathin limit. Contrary to the expected conservation of the perovskite stacking order in the (001) direction, heteroepitaxially grown SrIrO3 films on TiO2-terminated SrTiO3 are found to exhibit a terminating SrO surface layer. The proposed mechanism for the self-organized conversion involves the adsorption of excess oxygen ions at the apical sites of the IrO2-terminated surface and the subsequent decomposition of the IrO6 octahedra into gaseous molecular IrO3 and the remaining SrO-terminated surface. Whereas the ab initio calculated electronic structure of SrO-terminated SrIrO3 in the monolayer limit exhibits a striking similarity to bulk Sr2IrO4, the broken octahedral symmetry at the IrO2-terminated surface would mix the otherwise crystal field split e(g) and t(2g) states, resulting in distinctly different low-energy electronic states. Published under license by AIP Publishing.

- Orbital-Driven Rashba Effect in a Binary Honeycomb Monolayer AgTeMaximilian Unzelmann, Hendrik Bentmann, Philipp Eck, and 12 more authors
*Physical Review Letters*Apr 2020The Rashba effect is fundamental to the physics of two-dimensional electron systems and underlies a variety of spintronic phenomena. It has been proposed that the formation of Rashba-type spin splittings originates microscopically from the existence of orbital angular momentum (OAM) in the Bloch wave functions. Here, we present detailed experimental evidence for this OAM-based origin of the Rashba effect by angle-resolved photoemission (ARPES) and two-photon photoemission experiments for a monolayer AgTe on Ag(111). Using quantitative low-energy electron diffraction analysis, we determine the structural parameters and the stacking of the honeycomb overlayer with picometer precision. Based on an orbitalsymmetry analysis in ARPES and supported by first-principles calculations, we unequivocally relate the presence and absence of Rashba-type spin splittings in different bands of AgTe to the existence of OAM.

- Robust dx2-y2-wave superconductivity of infinite-layer nickelatesXianxin Wu,
*Domenico Di Sante*, Tilman Schwemmer, and 4 more authors*Physical Review B*Feb 2020Motivated by the recent observation of superconductivity in strontium-doped NdNiO2, we study the superconducting instabilities in this system from various vantage points. Starting with first-principles calculations, we construct two distinct tight-binding models, a simpler single-orbital as well as a three-orbital model, both of which capture the key low-energy degrees of freedom to varying degrees of accuracy. We study superconductivity in both models using the random phase approximation. We then analyze the problem at stronger coupling, and study the dominant pairing instability in the associated t-J model limit. In all instances, the dominant pairing tendency is in the d(x2-y2) channel, analogous to the cuprate superconductors.

- Kagome metal-organic frameworks as a platform for strongly correlated electronsMarius Fuchs, Peitao Liu, Tilman Schwemmer, and 4 more authors
*Journal of Physics: Materials*Feb 2020By using first-principles calculations we put forward the Cu-dicyanoanthracene lattice as a platform to investigate strong electronic correlations in the family of Kagome metal-organic frameworks. We show that the low-energy model is composed by molecular orbitals which arrange themselves in a typical Kagome lattice at n = 2/3 filling, where the Fermi level lies at the Dirac point. The Coulomb interaction matrix expressed in this molecular orbitals basis, as obtained by large-scale constrained random-phase approximation calculations, is characterized by local U and non-local parameters exceeding more than ten times the Kagome bandwidth. For such Kagome systems, our findings suggest the possible emergence of peculiar electron–electron collective phenomena, such as an exotic valence bond solid order characterized by modulated bond strengths.

- Observation of tunable single-atom Yu-Shiba-Rusinov statesArtem Odobesko,
*Domenico Di Sante*, Alexander Kowalski, and 5 more authors*Physical Review B*Nov 2020The coupling of a spin to an underlying substrate is the basis for a plethora of phenomena. In the case of a metallic substrate, Kondo screening of the adatom magnetic moment can occur. As the substrate turns superconducting, an intriguing situation emerges where pair breaking due to the adatom spins leads to Yu-Shiba-Rusinov bound states, but also intertwines with Kondo phenomena. Through scanning tunneling spectroscopy, we analyze the interdependence of Kondo screening and superconductivity. Our data obtained on single Fe adatoms on Nb(110) show that the coupling and the resulting YSR states are strongly adsorption site-dependent and reveal a quantum phase transition at a Kondo temperature comparable to the superconducting gap. The experimental signatures are rationalized by combined density-functional theory and continuous-time quantum Monte Carlo calculations to rigorously treat magnetic and hybridization effects on equal footing.

## 2019

- Interplay of Dirac Nodes and Volkov-Pankratov Surface States in Compressively Strained HgTeDavid M. Mahler, Julian-Benedikt Mayer, Philipp Leubner, and 8 more authors
*Physical Review X*Aug 2019Preceded by the discovery of topological insulators, Dirac and Weyl semimetals have become a pivotal direction of research in contemporary condensed matter physics. While easily accessible from a theoretical viewpoint, these topological semimetals pose a serious challenge in terms of experimental synthesis and analysis to allow for their unambiguous identification. In this work, we report on detailed transport experiments on compressively strained HgTe. Because of the superior sample quality in comparison to other topological semimetallic materials, this enables us to resolve the interplay of topological surface states and semimetallic bulk states to an unprecedented degree of precision and complexity. As our gate design allows us to precisely tune the Fermi level at the Weyl and Dirac points, we identify a magnetotransport regime dominated by Weyl/Dirac bulk state conduction for small carrier densities and by topological surface state conduction for larger carrier densities. As such, similar to topological insulators, HgTe provides the archetypical reference for the experimental investigation of topological semimetals.

- Unconventional superconductivity in a doped quantum spin Hall insulatorXianxin Wu, Mario Fink, Werner Hanke, and 2 more authors
*Physical Review B*Jul 2019A monolayer of jacutingaite (Pt2HgSe3) has recently been identified as a novel quantum spin Hall insulator. By first-principles calculations, we study its Fermiology in the doped regime and unveil a type-I and type-II van Hove singularity for hole and electron doping, respectively. We find that the common link between the propensity for a topological band gap at pristine filling and unconventional superconductivity at finite doping is rooted in the longer-ranged hybridization integrals on the honeycomb lattice. In a combined effort of random phase approximation and functional renormalization group, we find chiral d-wave order for the type-I and odd-parity f -wave order for the type-II regime. When longer-ranged Coulomb interaction is included, a propensity of the type-II regime towards a topological p(x)+ip(y)-wave order emerges.

- Triplet superconductivity in the Dirac semimetal germanene on a substrate
*Domenico Di Sante*, Xianxin Wu, Mario Fink, and 2 more authors*Physical Review B*May 2019The success of graphene and its emerging Dirac physics has stimulated the quest for versatile and tunable electronic properties in atomically thin systems, leading to the discovery of various chemical classes of two-dimensional (2D) compounds. In particular, honeycomb lattices of group-IV elements, such as silicene and germanene, have been found experimentally. Whether it is a necessity of synthesis or a desired feature for application purposes, most 2D materials demand a supporting substrate. In this Rapid Communication, by combining ab initio simulations with multiorbital functional renormalization group analysis of Fermi surface instabilities, we highlight the constructive impact of substrates to enable the realization of exotic electronic quantum states of matter, where the buckling emerges as the decisive material parameter adjustable by the commensuration. At the example of germanene deposited on MoS2, an experimentally characterized superstructure, we find that the coupling between the monolayer and the substrate, together with the buckled hexagonal geometry, conspire to provide a highly suited scenario for unconventional triplet superconductivity.

- Orbital Fingerprint of Topological Fermi Arcs in the Weyl Semimetal TaPChul-Hee Min, Hendrik Bentmann, Jennifer N. Neu, and 15 more authors
*Physical Review Letters*Mar 2019The monopnictides TaAs and TaP are well-established Weyl semimetals. Yet, a precise assignment of Fermi arcs, accommodating the predicted chiral charge of the bulk Weyl points, has been difficult in these systems, and the topological character of different surface features in the Fermi surface is not fully understood. Here, employing a joint analysis from linear dichroism in angle-resolved photoemission and fast-principles calculations, we unveil the orbital texture on the full Fermi surface of TaP(001). We observe pronounced switches in the orbital texture at the projected Weyl nodes, and show how they facilitate a topological classification of the surface band structure. Our findings establish a critical role of the orbital degrees of freedom in mediating the surface-bulk connectivity in Weyl semimetals.

- Custodial glide symmetry of quantum spin Hall edge modes in monolayer WTe2Seulgi Ok, Lukas Muechler,
*Domenico Di Sante*, and 3 more authors*Physical Review B*Mar 2019A monolayer of WTe2 has been shown to display quantum spin Hall (QSH) edge modes persisting up to 100 K in transport experiments. Based on density-functional theory calculations and symmetry-based model building including the role of correlations and substrate support, we develop an effective electronic model for WTe2 that fundamentally differs from other prototypical QSH settings: we find a remarkably strong transverse localization of QSH edge modes in WTe2 related to the glide symmetry due to which the topological gap opens away from high-symmetry points in momentum space. While the indirect bulk gap is much smaller, a large direct gap of up to 1 eV in the Brillouin zone region of the dispersing edge modes determines their properties.

- Fe/GeTe(111) heterostructures as an avenue towards spintronics based on ferroelectric Rashba semiconductorsJagoda Slawinska,
*Domenico Di Sante*, Sara Varotto, and 3 more authors*Physical Review B*Feb 2019By performing density functional theory and Green’s functions calculations, complemented by x-ray photoemission spectroscopy, we investigate the electronic structure of Fe/GeTe(111), a prototypical ferromagnetic/Rashba-ferroelectric interface. We reveal that such a system exhibits several intriguing properties resulting from the complex interplay of exchange interaction, electric polarization, and spin-orbit coupling. Despite a rather strong interfacial hybridization between Fe and GeTe bands, resulting in a complete suppression of the surface states of the latter, the bulk Rashba bands are hardly altered by the ferromagnetic overlayer. This could have a deep impact on spin-dependent phenomena observed at this interface, such as spin-to-charge interconversion, which are likely to involve bulk rather than surface Rashba states.

- Towards topological quasifreestanding stanene via substrate engineering
*Domenico Di Sante*, Philipp Eck, Maximilian Bauernfeind, and 5 more authors*Physical Review B*Jan 2019In search for a new generation of spintronics hardware, material candidates for room temperature quantum spin Hall effect (QSHE) have become a contemporary focus of investigation. Inspired by the original proposal for QSHE in graphene, several heterostructures have been synthesized, aiming at a hexagonal monolayer of heavier group IV elements promoting the QSHE bulk gap via increased spin-orbit coupling. So far, the monolayer/substrate coupling, which can manifest itself in strain, deformation, and hybridization, has proven to be detrimental to the aspired QSHE conditions for the monolayer. For stanene, the Sn analog of graphene, we investigate how an interposing buffer layer mediates between monolayer and substrate in order to optimize the QSHE setting. From a detailed density functional theory study, we highlight the principal mechanisms induced by such a buffer layer to accomplish quasifreestanding stanene in its QSHE phase. We complement our theoretical predictions by presenting attempts to grow a buffer layer on SiC(0001) on which stanene can be deposited.

## 2018

- The origin of Mooij correlations in disordered metalsSergio Ciuchi,
*Domenico Di Sante*, Vladimir Dobrosavljevic, and 1 more author*Npj Quantum Materials*Sep 2018Sufficiently disordered metals display systematic deviations from the behavior predicted by semi-classical Boltzmann transport theory. Here the scattering events from impurities or thermal excitations can no longer be considered as additive-independent processes, as asserted by Matthiessen’s rule following from this picture. In the intermediate region between the regime of good conduction and that of insulation, one typically finds a change of sign of the temperature coefficient of resistivity, even at elevated temperature spanning ambient conditions, a phenomenology that was first identified by Mooij in 1973. Traditional weak coupling approaches to identify relevant corrections to the Boltzmann picture focused on long-distance interference effects such as "weak localization", which are especially important in low dimensions (1D and 2D) and close to the zero-temperature limit. Here we formulate a strong-coupling approach to tackle the interplay of strong disorder and lattice deformations (phonons) in bulk three-dimensional metals at high temperatures. We identify a polaronic mechanism of strong disorder renormalization, which describes how a lattice locally responds to the relevant impurity potential. This mechanism, which quantitatively captures the Mooij regime, is physically distinct and unrelated to Anderson localization, but realizes early seminal ideas of Anderson himself, concerning the interplay of disorder and lattice deformations.

- Role of spin-orbit coupling in the electronic structure of IrO2Pranab Kumar Das, Jagoda Slawinska, Ivana Vobornik, and 15 more authors
*Physical Review Materials*Jun 2018The delicate interplay of electronic charge, spin, and orbital degrees of freedom is in the heart of many novel phenomena across the transition metal oxide family. Here, by combining high-resolution angle-resolved photoemission spectroscopy and first principles calculations (with and without spin-orbit coupling), the electronic structure of the rutile binary iridate, IrO2, is investigated. The detailed study of electronic bands measured on a high-quality single crystalline sample and use of a wide range of photon energy provide a huge improvement over the previous studies. The excellent agreement between theory and experimental results shows that the single-particle DFT description of IrO2 band structure is adequate, without the need of invoking any treatment of correlation effects. Although many observed features point to a 3D nature of the electronic structure, clear surface effects are revealed. The discussion of the orbital character of the relevant bands crossing the Fermi level sheds light on spin-orbit-coupling-driven phenomena in this material, unveiling a spin-orbit-induced avoided crossing, a property likely to play a key role in its large spin Hall effect.

- Ferroelectric Control of the Spin Texture in GeTeChristian Rinaldi, Sara Varotto, Marco Asa, and 10 more authors
*Nano Letters*May 2018The electric and nonvolatile control of the spin texture in semiconductors would represent a fundamental step toward novel electronic devices combining memory and computing functionalities. Recently, GeTe has been theoretically proposed as the father compound of a new class of materials, namely ferroelectric Rashba semiconductors. They display bulk bands with giant Rashba-like splitting due to the inversion symmetry breaking arising from the ferroelectric polarization, thus allowing for the ferroelectric control of the spin. Here, we provide the experimental demonstration of the correlation between ferroelectricity and spin texture. A surface-engineering strategy is used to set two opposite predefined uniform ferroelectric polarizations, inward and outward, as monitored by piezoresponse force microscopy. Spin and angular resolved photoemission experiments show that these GeTe(111) surfaces display opposite sense of circulation of spin in bulk Rashba bands. Furthermore, we demonstrate the crafting of nonvolatile ferroelectric patterns in GeTe films at the nanoscale by using the conductive tip of an atomic force microscope. Based on the intimate link between ferroelectric polarization and spin in GeTe, ferroelectric patterning paves the way to the investigation of devices with engineered spin configurations.

- Origin of the pressure-dependent Tc valley in superconducting simple cubic phosphorusXianxin Wu, Harald O. Jeschke,
*Domenico Di Sante*, and 3 more authors*Physical Review Materials*Mar 2018Motivated by recent experiments, we investigate the pressure-dependent electronic structure and electron-phonon (e-ph) coupling for simple cubic phosphorus by performing first-principles calculations within the full potential linearized augmented plane-wave method. As a function of increasing pressure, our calculations show a valley feature in T-c, followed by an eventual decrease for higher pressures. We demonstrate that this T-c valley at low pressures is due to two nearby Lifshitz transitions, as we analyze the band-resolved contributions to the e-ph coupling. Below the first Lifshitz transition, the phonon hardening and shrinking of the gamma Fermi surface with s-orbital character results in a decreased Tc with increasing pressure. After the second Lifshitz transition, the appearance of delta Fermi surfaces with 3d-orbital character generate strong e-ph interband couplings in alpha delta and beta delta channels, and hence lead to an increase of T-c. For higher pressures, the phonon hardening finally dominates, and T-c decreases again. Our study reveals that the intriguing T-c valley discovered in experiment can be attributed to Lifshitz transitions, while the plateau of T-c detected at intermediate pressures appears to be beyond the scope of our analysis. This strongly suggests that aside from e-ph coupling, electronic correlations along with plasmonic contributions may be relevant for simple cubic phosphorus. Our findings hint at the notion that increasing pressure can shift the low-energy orbital weight towards d character, and as such even trigger an enhanced importance of orbital-selective electronic correlations despite an increase of the overall bandwidth.

- Tunable metal-insulator transition, Rashba effect and Weyl Fermions in a relativistic charge-ordered ferroelectric oxideJiangang He,
*Domenico Di Sante*, Ronghan Li, and 3 more authors*Nature Communications*Feb 2018Controllable metal-insulator transitions (MIT), Rashba-Dresselhaus (RD) spin splitting, and Weyl semimetals are promising schemes for realizing processing devices. Complex oxides are a desirable materials platform for such devices, as they host delicate and tunable charge, spin, orbital, and lattice degrees of freedoms. Here, using first-principles calculations and symmetry analysis, we identify an electric-field tunable MIT, RD effect, and Weyl semimetal in a known, charge-ordered, and polar relativistic oxide Ag2BiO3 at room temperature. Remarkably, a centrosymmetric BiO6 octahedral-breathing distortion induces a sizable spontaneous ferroelectric polarization through Bi3+/Bi5+ charge disproportionation, which stabilizes simultaneously the insulating phase. The continuous attenuation of the Bi3+/Bi5+ disproportionation obtained by applying an external electric field reduces the band gap and RD spin splitting and drives the phase transition from a ferroelectric RD insulator to a paraelectric Dirac semimetal, through a topological Weyl semimetal intermediate state. These findings suggest that Ag2BiO3 is a promising material for spin-orbitonic applications.

## 2017

- Dimensionality-Driven Metal-Insulator Transition in Spin-Orbit-Coupled SrIrO3P. Schuetz,
*D. Di Sante*, L. Dudy, and 10 more authors*Physical Review Letters*Dec 2017Upon reduction of the film thickness we observe a metal-insulator transition in epitaxially stabilized, spin-orbit-coupled SrIrO3 ultrathin films. By comparison of the experimental electronic dispersions with density functional theory at various levels of complexity we identify the leading microscopic mechanisms, i.e., a dimensionality-induced readjustment of octahedral rotations, magnetism, and electronic correlations. The astonishing resemblance of the band structure in the two-dimensional limit to that of bulk Sr2IrO4 opens new avenues to unconventional superconductivity by "clean" electron doping through electric field gating.

- Dipole Order in Halide Perovskites: Polarization and Rashba Band SplittingsShunbo Hu, Heng Gao, Yuting Qi, and 8 more authors
*Journal of Physical Chemistry C*Oct 2017ABX(3) (A = organic cation; B = Sn, Pb; and X = halogen) organohalide perovskites have recently attracted much attention for their photovoltaic applications. Such hybrid compounds are derived from the replacement of the inorganic monovalent metal element by an organic cation, for example, methylammonium ion (MA = CH3NH3) and formamidinium ion (FA = +HC(NH2)(2)). In particular, since the organic cations are polar, it is interesting to investigate their possible long-range ordering and the corresponding Rashba spin-split bands. In this work, by using density functional theory calculations, we estimate the ferroelectric polarization corresponding to a complete ordering of dipole moments for the optimized structures of 12 perovskite halides, with A = MA, FA; B = Pb, Sn; X = Cl, Br, I. The adiabatic path and functional mode analysis have been discussed for all cases. The calculated values of the polarization may be as high as a conventional inorganic ferroelectric compound, such as BaTiO3. The concomitant inversion symmetry breaking, coupled to the sizable spin-orbit coupling of Pb and Sn, results in a fairly large Rashba spin-splitting effect for both valence and conduction bands. We highlight a rather anisotropic dispersion of spin-orbit split bands which gives rise to different Rashba parameters in different directions perpendicular to the polar axis in k-space. Furthermore, we found a weak and positive correlation between the magnitude of polarization and relevant spin-split band parameters. Since the mechanism for enhanced carrier lifetime in 3D Rashba materials is connected to the reduced recombination rate due to the spin-forbidden transition, our study could aid in the understanding of the fundamental physics of organometal halide perovskites and the optimization and design of materials for better performance.

- Giant Rashba Splitting in Pb1-xSnxTe (111) Topological Crystalline Insulator Films Controlled by Bi Doping in the BulkValentine V. Volobuev, Partha S. Mandal, Marta Galicka, and 11 more authors
*Advanced Materials*Jan 2017The topological properties of lead-tin chalcogenide topological crystalline insulators can be widely tuned by temperature and composition. It is shown that bulk Bi doping of epitaxial Pb1-xSnxTe (111) films induces a giant Rashba splitting at the surface that can be tuned by the doping level. Tight binding calculations identify their origin as Fermi level pinning by trap states at the surface.

- Realizing double Dirac particles in the presence of electronic interactions
*Domenico Di Sante*, Andreas Hausoel, Paolo Barone, and 3 more authors*Physical Review B*Sep 2017Double Dirac fermions have recently been identified as possible quasiparticles hosted by three-dimensional crystals with particular nonsymmorphic point-group symmetries. Applying a combined approach of ab initio methods and dynamical mean-field theory, we investigate how interactions and double Dirac band topology conspire to form the electronic quantum state of Bi2CuO4. We derive a downfolded eight-band model of the pristine material at low energies around the Fermi level. By tuning the model parameters from the free band structure to the realistic strongly correlated regime, we find a persistence of the double Dirac dispersion until its constituting time-reversal symmetry is broken due to the onset of magnetic ordering at the Mott transition. Our calculations suggest that the double Dirac fermions in Bi2CuO4 can be restored by experimentally accessible hydrostatic pressures. In light of the growing attention to the topological quantum chemistry approach, our results on Bi2CuO4 show how many-body effects must be included beyond the static mean-field level for reliable predictions on new materials.

- Disorder-Driven Metal-Insulator Transitions in Deformable Lattices
*Domenico Di Sante*, Simone Fratini, Vladimir Dobrosavljević, and 1 more author*Physical Review Letters*Jan 2017We show that, in the presence of a deformable lattice potential, the nature of the disorder-driven metal-insulator transition is fundamentally changed with respect to the noninteracting (Anderson) scenario. For strong disorder, even a modest electron-phonon interaction is found to dramatically renormalize the random potential, opening a mobility gap at the Fermi energy. This process, which reflects disorder-enhanced polaron formation, is here given a microscopic basis by treating the lattice deformations and Anderson localization effects on the same footing. We identify an intermediate “bad insulator” transport regime which displays resistivity values exceeding the Mott-Ioffe-Regel limit and with a negative temperature coefficient, as often observed in strongly disordered metals. Our calculations reveal that this behavior originates from significant temperature-induced rearrangements of electronic states due to enhanced interaction effects close to the disorder-driven metal-insulator transition.

- Three-Dimensional Electronic Structure of the Type-II Weyl Semimetal WTe2
*Domenico Di Sante*, Pranab Kumar Das, C. Bigi, and 15 more authors*Physical Review Letters*Jul 2017By combining bulk sensitive soft-x-ray angular-resolved photoemission spectroscopy and first-principles calculations we explored the bulk electron states of WTe2, a candidate type-II Weyl semimetal featuring a large nonsaturating magnetoresistance. Despite the layered geometry suggesting a two-dimensional electronic structure, we directly observe a three-dimensional electronic dispersion. We report a band dispersion in the reciprocal direction perpendicular to the layers, implying that electrons can also travel coherently when crossing from one layer to the other. The measured Fermi surface is characterized by two well-separated electron and hole pockets at either side of the Γ point, differently from previous more surface sensitive angle-resolved photoemission spectroscopy experiments that additionally found a pronounced quasiparticle weight at the zone center. Moreover, we observe a significant sensitivity of the bulk electronic structure of WTe2 around the Fermi level to electronic correlations and renormalizations due to self-energy effects, previously neglected in first-principles descriptions.

## 2016

- Robust spin-polarized midgap states at step edges of topological crystalline insulatorsPaolo Sessi,
*Domenico Di Sante*, Andrzej Szczerbakow, and 11 more authors*Science*Dec 2016Topological crystalline insulators are materials in which the crystalline symmetry leads to topologically protected surface states with a chiral spin texture, rendering them potential candidates for spintronics applications. Using scanning tunneling spectroscopy, we uncover the existence of one-dimensional (1D) midgap states at odd-atomic surface step edges of the three-dimensional topological crystalline insulator (Pb,Sn)Se. A minimal toy model and realistic tight-binding calculations identify them as spin-polarized flat bands connecting two Dirac points. This nontrivial origin provides the 1D midgap states with inherent stability and protects them from backscattering. We experimentally show that this stability results in a striking robustness to defects, strong magnetic fields, and elevated temperature.

- Lone-Pair-Electron-Driven Ionic Displacements in a Ferroelectric Metal-Organic HybridWen-Ping Zhao, Chao Shi, Alessandro Stroppa, and 3 more authors
*Inorganic Chemistry*Oct 2016A displacive-type mechanism, which accounts for the occurrence of ferroelectricity in most inorganic ferroelectrics, is rarely found in molecule-based ferroelectrics. Its role is often covered by the predominant order-disorder one. Herein, we report a lone-pair-electron-driven displacive-type ferroelectric organic-inorganic hybrid compound, [H(2)dmdap][SbCl5] (\textlessbold\textgreater1\textless/bold\textgreater; dmdap = N,N-dimethyl-1,3-diaminopropane). The structure of \textlessbold\textgreater1\textless/bold\textgreater features a typical zigzag chain of [SbCl5](infinity) containing cis-connected anionic octahedra. The compound undergoes a second-order paraelectric-ferroelectric phase transition at 143 K (P2(1)/c \textless-\textgreater Pc) with a saturation polarization of 1.36 mu C.cm(-2) and a coercive field of 3.5 kV.cm(-1) at 119 K. Theoretical study discloses the ferroelectricity mainly originating from the relative displacements of the Sb and Cl ions in the crystal lattice, which are driven by the 5s(2) lone-pair electrons of the Sb-III center. Furthermore, on the basis of analysis, possible routes are suggested to enhance ferroelectric polarization in this class of compounds.

- Analogies between Jahn-Teller and Rashba spin physicsAlessandro Stroppa, Paolo Barone,
*Domenico Di Sante*, and 3 more authors*International Journal of Quantum Chemistry*Oct 2016In developing physical theories, analogical reasoning has been found to be very powerful, as attested by a number of important historical examples. An analogy between two apparently different phenomena, once established, allows one to transfer information and bring new concepts from one phenomenon to the other. Here, we discuss an important analogy between two widely different physical problems, namely, the Jahn-Teller distortion in molecular physics and the Rashba spin splitting in condensed matter physics. By exploring their conceptual and mathematical features and by searching for the counterparts between them, we examine the orbital texture in Jahn-Teller systems, as the counterpart of the spin texture of the Rashba physics, and put forward a possible way of experimentally detecting the orbital texture. Finally, we discuss the analogy by comparing the coexistence of linear Rashba+Dresselhaus effects and Jahn-Teller problems for specific symmetries, which allow for nontrivial spin and orbital textures, respectively.

- Layer-dependent quantum cooperation of electron and hole states in the anomalous semimetal WTe2Pranab Kumar Das,
*D. Di Sante*, I. Vobornik, and 13 more authors*Nature Communications*Feb 2016The behaviour of electrons and holes in a crystal lattice is a fundamental quantum phenomenon, accounting for a rich variety of material properties. Boosted by the remarkable electronic and physical properties of two-dimensional materials such as graphene and topological insulators, transition metal dichalcogenides have recently received renewed attention. In this context, the anomalous bulk properties of semimetallic WTe2 have attracted considerable interest. Here we report angle-and spin-resolved photoemission spectroscopy of WTe2 single crystals, through which we disentangle the role of W and Te atoms in the formation of the band structure and identify the interplay of charge, spin and orbital degrees of freedom. Supported by first-principles calculations and high-resolution surface topography, we reveal the existence of a layer-dependent behaviour. The balance of electron and hole states is found only when considering at least three Te-W-Te layers, showing that the behaviour of WTe2 is not strictly two dimensional.

- Giant Rashba-Type Spin Splitting in Ferroelectric GeTe(111)Marcus Liebmann, Christian Rinaldi,
*Domenico Di Sante*, and 18 more authors*Advanced Materials*Jan 2016Photoelectron spectroscopy in combination with piezoforce microscopy reveals that the helicity of Rashba bands is coupled to the nonvolatile ferroelectric polarization of GeTe(111). A novel surface Rashba band is found and fingerprints of a bulk Rashba band are identified by comparison with density functional theory calculations.

- Experimental and theoretical studies of structural phase transition in a novel polar perovskite-like [C2H5NH3][Na0.5Fe0.5(HCOO)(3)] formateMaciej Ptak, Miroslaw Maczka, Anna Gagor, and 5 more authors
*Dalton Transactions*Jan 2016We report the synthesis, single crystal X-ray diffraction, and thermal, dielectric, Raman and infrared studies of a novel heterometallic formate [C2H5NH3][Na0.5Fe0.5(HCOO)(3)] ( EtANaFe). The thermal studies show that EtANaFe undergoes a second-order phase transition at about 360 K. X-ray diffraction data revealed that the high-temperature structure is monoclinic, space group P2(1)/n, with dynamically disordered ethyl-ammonium (EtA(+)) cations. EtANaFe possesses a polar low-temperature structure with the space group Pn and, in principle, is ferroelectric below 360 K. Dielectric data show that the reciprocal of the real part of dielectric permittivity above and below the phase transition temperature follows the Curie-Weiss, as expected for a ferroelectric phase transition. Based on theoretical calculations, we estimated the polarization as (0.2, 0, 0.8) mu C cm(-2), i.e., lying within the ac plane. The obtained data also indicate that the driving force of the phase transition is ordering of EtA(+) cations. However, this ordering is accompanied by significant distortion of the metal formate framework.

- Intertwined Rashba, Dirac, and Weyl Fermions in Hexagonal Hyperferroelectrics
*Domenico Di Sante*, Paolo Barone, Alessandro Stroppa, and 3 more authors*Physical Review Letters*Aug 2016By means of density functional theory based calculations, we study the role of spin-orbit coupling in the new family of ABC hyperferroelectrics [Garrity, Rabe, and Vanderbilt Phys. Rev. Lett. 112, 127601 (2014)]. We unveil an extremely rich physics strongly linked to ferroelectric properties, ranging from the electric control of bulk Rashba effect to the existence of a three-dimensional topological insulator phase, with concomitant topological surface states even in the ultrathin film limit. Moreover, we predict that the topological transition, as induced by alloying, is followed by a Weyl semimetal phase of finite concentration extension, which is robust against disorder, putting forward hyperferroelectrics as promising candidates for spin-orbitronic applications.

- Possibility of combining ferroelectricity and Rashba-like spin splitting in monolayers of the 1T-type transition-metal dichalcogenides MX2 (M = Mo, W; X = S, Se, Te)Emilie Bruyer,
*Domenico Di Sante*, Paolo Barone, and 3 more authors*Physical Review B*Nov 2016First-principles calculations were carried out to explore the possible coupling between spin-polarized electronic states and ferroelectric polarization in monolayers of transition-metal dichalcogenides MX2 (M=Mo,W;X=S,Se,Te) with distorted octahedrally coordinated 1T structures. For d2 metal ions, two competing metal clustering effects can take place, where metal ions are arranged in trimers or zigzag chains. Among these, the former structural distortion comes along with an improper ferroelectric phase which persists in the monolayer limit. Switchable Rashba-like spin-polarization features are predicted in the trimerized polytype, which can be permanently tuned by acting on its ferroelectric properties. The polar trimerized structure is found to be stable for 1T−MoS2 only, while the nonpolar polytype with zigzag metal clustering is predicted to stabilize for other transition-metal dichalcogenides with d2 metal ions.

## 2015

- Strain Tuning of Ferroelectric Polarization in Hybrid Organic Inorganic Perovskite CompoundsSaurabh Ghosh,
*Domenico Di Sante*, and Alessandro Stroppa*Journal of Physical Chemistry Letters*Nov 2015Metal organic frameworks (MOFs) are hybrid crystalline compounds comprised of an extended ordered network made up of organic molecules, organic linkers and metal cations. In particular, MOFs with the same topology as inorganic perovskites have been shown to possess interesting properties, e.g., coexistence of ferroelectric and magnetic ordering. Using first-principles density functional theory, we have investigated the effect of strain on the compounds C(NH2)(3)Cr(HCOO)(3) and (CH3CH2NH3)Mn(HCOO)(3). Here, we show that compressive strain can substantially increase the ferroelectric polarization by more than 300%, and we discuss the mechanism involved in the strain enhancement of polarization. Our study highlights the complex interplay between strain and organic cations’ dipoles and put forward the possibility of tuning of ferroelectric polarization through appropriate thin film growing.

- Gate-tunable quantum oscillations in ambipolar Cd3As2 thin filmsYanwen Liu, Cheng Zhang, Xiang Yuan, and 9 more authors
*Npg Asia Materials*Oct 2015Electrostatic doping in materials can lead to various exciting electronic properties, such as metal-insulator transition and superconductivity, by altering the Fermi level position or introducing exotic phases. Cd3As2, a three-dimensional (3D) analog of graphene with extraordinary carrier mobility, was predicted to be a 3D Dirac semimetal, a feature confirmed by recent experiments. However, most research so far has been focused on metallic bulk materials that are known to possess ultra-high mobility and giant magneto-resistance but limited carrier transport tunability. Here we report on the first observation of a gate-induced transition from band conduction to hopping conduction in single-crystalline Cd3As2 thin films via electrostatic doping by solid electrolyte gating. The extreme charge doping enables the unexpected observation of p-type conductivity in a similar to 50-nm-thick Cd3As2 thin film grown by molecular beam epitaxy. More importantly, the gate-tunable Shubnikov-de Haas oscillations and the temperature-dependent resistance reveal a unique band structure and bandgap opening when the dimensionality of Cd3As2 is reduced. This is also confirmed by our first-principle calculations. The present results offer new insights toward nanoelectronic and optoelectronic applications of Dirac semimetals in general and provide new routes in the search for the intriguing quantum spin Hall effect in low-dimension Dirac semimetals, an effect that is theoretically predicted but not yet experimentally realized.

- Robustness of Rashba and Dirac Fermions against Strong Disorder
*Domenico Di Sante*, Paolo Barone, Evgeny Plekhanov, and 2 more authors*Scientific Reports*Jun 2015By addressing the interplay between substitutional disorder and spin-orbit-coupling in chalcogenide alloys, we predict a strong robustness of spectral features at the Fermi energy. Indeed, supplementing our state of the art first-principles calculations with modeling analysis, we show that the disorder self-energy is vanishingly small close to the band gap, thus i) allowing for bulk Rashba-like spin splitting to be observed in ferroelectric alloys by means of Angle Resolved PhotoEmission Spectroscopy, and ii) protecting the band-character inversion related to the topological transition in recently discovered Topological Crystalline Insulators. Such a protection against strong disorder, which we demonstrate to be general for three dimensional Dirac systems, has potential and valuable implications for novel technologies, as spintronics and/or spinorbitronics.

- Formation and Observation of a Quasi-Two-Dimensional dxy Electron Liquid in Epitaxially Stabilized Sr2−xLaxTiO4 Thin FilmsY. F. Nie,
*D. Di Sante*, S. Chatterjee, and 5 more authors*Physical Review Letters*Aug 2015We report the formation and observation of an electron liquid in Sr2−xLaxTiO4, the quasi-two-dimensional counterpart of SrTiO3, through reactive molecular-beam epitaxy and in situ angle-resolved photoemission spectroscopy. The lowest lying states are found to be comprised of Ti 3dxy orbitals, analogous to the LaAlO3/SrTiO3 interface and exhibit unusually broad features characterized by quantized energy levels and a reduced Luttinger volume. Using model calculations, we explain these characteristics through an interplay of disorder and electron-phonon coupling acting cooperatively at similar energy scales, which provides a possible mechanism for explaining the low free carrier concentrations observed at various oxide heterostructures such as the LaAlO3/SrTiO3 interface.

- Emergence of ferroelectricity and spin-valley properties in two-dimensional honeycomb binary compounds
*Domenico Di Sante*, Alessandro Stroppa, Paolo Barone, and 2 more authors*Physical Review B*Apr 2015By means of density functional theory calculations, we predict that several two-dimensional AB binary monolayers, where A and B atoms belong to group IV or III-V, are ferroelectric. Dipoles arise from the buckled structure, where the A and B ions are located on the sites of a bipartite corrugated honeycomb lattice with trigonal symmetry. We discuss the emerging valley-dependent properties and the coupling of spin and valley physics, which arise from the loss of inversion symmetry, and explore the interplay between ferroelectricity and Rashba spin-splitting phenomena. We show that valley-related properties originate mainly from the binary nature of AB monolayers, while the Rashba spin-texture developing around valleys is fully controllable and switchable by reversing the ferroelectric polarization.

## 2014

- Toward Truly Single Crystalline GeTe Films: The Relevance of the Substrate SurfaceRuining Wang, Jos E. Boschker, Emilie Bruyer, and 6 more authors
*Journal of Physical Chemistry C*Dec 2014The growth of GeTe thin films on a Si(111)-(root 3 x root 3)R30 degrees-Sb surface is reported. At growth onset, the rapid formation of fully relaxed crystalline GeTe(0001)-(1 x 1) is observed. During growth, a GeTe(0001)-(root 3 x root 3)R30 degrees surface reconstruction is also detected. Indeed, density functional theory (DFT) simulations indicate that the reconstructed GeTe(0001)-(root 3 x root 3)R30 degrees structure is energetically competing with the GeTe(0001)-(1 x 1) reconstruction. The out-of-plane a-GeTe\textless0001\textgreater\textbar\textbarSi\textless111\textgreater and in-plane alpha-GeTe\textless-1010\textgreater\textbar\textbarSi\textless-211\textgreater epitaxial relationships are confirmed by X-ray diffraction (XRD). Suppression of rotational twist and reduction of twinned domains are achieved. The formation of rotational domains in GeTe grown on Si(111)-(7 x 7) is explained by domain matched coincidence lattice formation with the Si(111)-(1 x 1) surface. Atomic force microscopy (AFM) images show the coalescence of well-oriented islands with subnanometer roughness on their top part. van der Pauw measurements are performed to verify the electric properties of the films. The quality of epitaxial GeTe thin film is discussed and related to the crystalline structure of GeTe and its rhombohedrally distorted resonant bonds.

- Topological Tuning in Three-Dimensional Dirac SemimetalsAwadhesh Narayan,
*Domenico Di Sante*, Silvia Picozzi, and 1 more author*Physical Review Letters*Dec 2014We study with first-principles methods the interplay between bulk and surface Dirac fermions in three dimensional Dirac semimetals. By combining density functional theory with the coherent potential approximation, we reveal a topological phase transition in Na3Bi1-xSbx and Cd-3[As1-xPx](2) alloys, where the material goes from a Dirac semimetal to a trivial insulator upon changing Sb or P concentrations. Tuning the composition allows us to engineer the position of the bulk Dirac points in reciprocal space. Interestingly, the phase transition coincides with the reversal of the band ordering between the conduction and valence bands.

- Tunable ferroelectric polarization and its interplay with spin-orbit coupling in tin iodide perovskitesAlessandro Stroppa,
*Domenico Di Sante*, Paolo Barone, and 5 more authors*Nature Communications*Dec 2014Ferroelectricity is a potentially crucial issue in halide perovskites, breakthrough materials in photovoltaic research. Using density functional theory simulations and symmetry analysis, we show that the lead-free perovskite iodide (FA) SnI3, containing the planar formamidinium cation FA, (NH2CHNH2)(+), is ferroelectric. In fact, the perpendicular arrangement of FA planes, leading to a ’weak’ polarization, is energetically more stable than parallel arrangements of FA planes, being either antiferroelectric or ’strong’ ferroelectric. Moreover, we show that the ’weak’ and ’strong’ ferroelectric states with the polar axis along different crystallographic directions are energetically competing. Therefore, at least at low temperatures, an electric field could stabilize different states with the polarization rotated by pi/4, resulting in a highly tunable ferroelectricity appealing for multistate logic. Intriguingly, the relatively strong spin-orbit coupling in noncentrosymmetric (FA)SnI3 gives rise to a co-existence of Rashba and Dresselhaus effects and to a spin texture that can be induced, tuned and switched by an electric field controlling the ferroelectric state.

- Engineering relativistic effects in ferroelectric SnTeE. Plekhanov, P. Barone,
*D. Di Sante*, and 1 more author*Physical Review B*Oct 2014Spin-orbit coupling is increasingly seen as a rich source of novel phenomena, as shown by the recent excitement around topological insulators and Rashba effects. We here show that the addition of ferroelectric degrees of freedom to a semiconductor featuring topologically nontrivial properties, such as SnTe, merges the intriguing field of spin-orbit-driven physics with nonvolatile functionalities appealing for spintronics. By using a variety of modeling techniques, we show that a strikingly rich sequence of phases can be induced in SnTe, when going from a room-temperature cubic phase to a low-temperature ferroelectric structure, ranging from a topological crystalline insulator to a time-reversal-invariant Z2 topological insulator to a “ferroelectric Rashba semiconductor,” exhibiting a huge electrically controllable Rashba effect in the bulk band structure.

- Strong interplay between electron-phonon interaction and disorder in low-doped systems
*Domenico Di Sante*, and Sergio Ciuchi*Physical Review B*Aug 2014The effects of doping on the spectral properties of low-doped systems are investigated by means of the coherent potential approximation to describe the distributed disorder induced by the impurities and the phonon-phonon noncrossing approximation to characterize a wide class of electron-phonon interactions that dominate the low-energy spectral features. When disorder and electron-phonon interaction work on comparable energy scales, a strong interplay between them arises, the effect of disorder can no longer be described as a mere broadening of the spectral features, and the phonon signatures are still visible despite the presence of strong disorder. As a consequence, the disorder-induced metal-insulator transition is strongly affected by a weak or moderate electron-phonon coupling, which is found to stabilize the insulating phase.

- Improper origin of polar displacements at CaTiO3 and CaMnO3 twin wallsPaolo Barone,
*Domenico Di Sante*, and Silvia Picozzi*Physical Review B*Apr 2014Recent interest in novel functionalities arising at domain walls of ferroic materials naturally calls for a microscopic understanding. To this end, first-principles calculations have been performed in order to provide solid evidence of polar distortions in the twin walls of nonpolar CaTiO3 and magnetic CaMnO3. We show that such polar displacements arise from rotation and/or tilting octahedral distortions—cooperatively acting at the twin wall in both considered systems—rather than from a proper secondary ferroelectric instability, as often believed. Our results are in excellent agreement with experimental observations of domain walls in CaTiO3. In addition, we show that magnetic properties at the twin wall in CaMnO3 are also modified, thus suggesting an unexplored route to achieve and detect multiferroic ordering in a single-phase material.

## 2013

- Tuning the Ferroelectric Polarization in a Multiferroic Metal-Organic Framework
*Domenico Di Sante*, Alessandro Stroppa, Prashant Jain, and 1 more author*Journal of the American Chemical Society*Dec 2013We perform density functional theory calculations on a recently synthesized metal-organic framework (MOP) with a perovskite-like topology ABX(3), i.e., [CH3CH2NH3]Mn(HCOO)(3), and predict a multiferroic behavior, i.e., a coexistence of ferroelectricity and ferromagnetism. A peculiar canted ordering of the organic A-cation dipole moments gives rise to a ferroelectric polarization of similar to 2 mu C/cm(2). Starting from these findings, we show that by choosing different organic A cations, it is possible to tune the ferroelectric polarization and increase it up. to 6 mu C/cm(2). The possibility of changing the magnitude and/or the canting of the organic molecular dipole opens new routes toward engineering ferroelectric polarization in the new class of multiferroic metal-organic frameworks.

- Beyond standard local density approximation in the study of magnetoelectric effects in Fe/BaTiO3 and Co/BaTiO3 interfaces
*Domenico Di Sante*, Kunihiko Yamauchi, and Silvia Picozzi*Journal of Physics-Condensed Matter*Feb 2013First-principles density functional theory (DFT) simulations for Fe/BaTiO3 and Co/BaTiO3 junctions have been performed with different treatments of the exchange-correlation potential, ranging from standard semilocal density approximations to a Hubbard-like approach and to hybrid functionals. With the aim of elucidating the role of correlations in the microscopic interplay between ferroelectricity and magnetism in the interfacial region, we find that, compared to standard DFT approximations, Hubbard-like approaches and hybrid functionals do not qualitatively modify the physical origin behind magnetoelectric effects driven by interfacial orbital hybridization. Rather, more accurate treatments of correlations for both Fe/BaTiO3 and Co/BaTiO3 interfaces predict a stronger change of the interface magnetization upon switching the direction of polarization in the ferroelectric layer.

- Electric Control of the Giant Rashba Effect in Bulk GeTe
*Domenico Di Sante*, Paolo Barone, Riccardo Bertacco, and 1 more author*Advanced Materials*Jan 2013Relativistic effects, including the Rashba effect, are increasingly seen as key ingredients in spintronics. A link between Rashba physics and the field of ferroelectrics is established by predicting giant Rashba spin-splitting in bulk GeTe (see the Figure showing the band-structure as well as in-plane and out-of-plane spin polarization for a constant energy cut).

- Pressure-induced topological phase transitions in rocksalt chalcogenidesPaolo Barone, Tomáš Rauch,
*Domenico Di Sante*, and 3 more authors*Physical Review B*Jul 2013By means of a comprehensive theoretical investigation, we show that external pressure can induce topological phase transitions in IV–VI semiconducting chalcogenides with a rocksalt structure. These materials satisfy mirror symmetries that are needed to sustain topologically protected surface states, at variance with time-reversal symmetry that is responsible for gapless edge states in Z2 topological insulators. The band inversions at high-symmetry points in the Brillouin zone that are related by mirror symmetry are brought about by an “asymmetric” hybridization between cation and anion sp orbitals. By working out the microscopic conditions to be fulfilled in order to maximize this hybridization, we identify materials in the rocksalt chalcogenide class that are prone to undergo a topological phase transition induced by pressure and/or alloying. Our model analysis is fully confirmed by complementary advanced first-principles calculations and ab initio-based tight-binding simulations.

- Strain engineering of topological properties in lead-salt semiconductorsPaolo Barone, Domenico Di Sante, and Silvia Picozzi
*physica status solidi (RRL) – Rapid Research Letters*Jul 2013Rock-salt chalcogenide SnTe represents the simplest realization of a topological insulator where a crystal symmetry allows for the appearance of surface metallic states. Here, we theoretically predict that strain, as realized in thin films grown on (001) substrates, may induce a transition to a topological crystalline insulating phase in related lead-salt chalcogenides. Furthermore, relevant topological properties of the surface states, such as the location of the Dirac cones on the surface Brillouin zone or the decay length of edge states, appear to be tunable with strain, with potential implications for technological devices benefiting from those additional degrees of freedom. (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

## 2012

- Structural, electronic and ferroelectric properties of croconic acid crystal: a DFT study
*Domenico Di Sante*, Alessandro Stroppa, and Silvia Picozzi*Physical Chemistry Chemical Physics*Jul 2012The recent discovery of high polarization at room temperature in croconic acid crystals as large as 21 mu C cm(-2) [Horiuchi et al., Nature, 2010, 463, 789] has lead to renewed interest in organic ferroelectrics, a promising class of materials for future electronic devices. We present here an extended ab initio study of this molecular crystal, using different approximations for the exchange-correlation functionals, ranging from local and semi-local types to more sophisticated hybrid functionals and van der Waals corrected functionals. Furthermore, by using distortion mode analysis, we focus on the different contributions to the polarization and on their microscopic origins.

## 2011

- Polar distortions in hydrogen-bonded organic ferroelectricsAlessandro Stroppa,
*Domenico Di Sante*, Sachio Horiuchi, and 3 more authors*Physical Review B*Jul 2011Although ferroelectric compounds containing hydrogen bonds were among the first to be discovered, organic ferroelectrics are relatively rare. The discovery of high polarization at room temperature in croconic acid [Horiuchi et al., Nature (London) 463, 789 (2010)] has led to a renewed interest in organic ferroelectrics. We present an ab initio study of two ferroelectric organic molecular crystals, 1-cyclobutene-1,2-dicarboxylic acid (CBDC) and 2-phenylmalondialdehyde (PhMDA). By using a distortion-mode analysis we shed light on the microscopic mechanisms contributing to the polarization, which we find to be as large as 14.3 and 7.0 μC/cm2 for CBDC and PhMDA, respectively. These results suggest that it may be fruitful to search among known but poorly characterized organic compounds for organic ferroelectrics with enhanced polar properties suitable for device applications.