Showing papers by "Alex Zunger published in 2020"

Polymorphous nature of cubic halide perovskites

Abstract: Many common crystal structures can be described by a single (or very few) repeated structural motif (``monomorphous structures'') such as octahedron in cubic halide perovskites. Interestingly, recent accumulated evidence suggests that electronic structure calculations based on such macroscopically averaged monomorphous cubic ($Pm\text{\ensuremath{-}}3m$) halide perovskites obtained from x-ray diffraction, show intriguing deviations from experiment. These include systematically too small band gaps, dielectric constants dominated by the electronic, negative mixing enthalpy of alloys, and significant deviations from the measured pair distribution function. We show here that a minimization of the systems $T=0$ internal energy via density functional theory reveals a distribution of different low-symmetry local motifs, including tilting, rotations, and B-atom displacements (``polymorphous networks''). This is found only if one allows for larger-than-minimal cell size that does not geometrically exclude low symmetry motifs. As the (super) cell size increases, the energy is lowered relative to the monomorphous cell, and stabilizes after \ensuremath{\sim}32 formula units (\ensuremath{\ge}160 atoms) are included. Being a result of nonthermal energy minimization in the internal energy without entropy, this correlated set of displacements must represent the intrinsic geometry preferred by the underlying chemical bonding (lone pair bonding), and as such has a different origin than the normal, dynamic thermal disorder modeled by molecular dynamics. Indeed, the polymorphous network, not the monomorphous ansatz, is the kernel structure from which high temperature thermal agitation develops. The emerging physical picture is that the polymorphous network has an average structure with high symmetry, yet the local structural motifs have low symmetries. We find that, compared with monomorphous counterparts, the polymorphous networks have significantly lower predicted total energies, larger band gaps, and ionic dominated dielectric constants, and agree much more closely with the observed pair distribution functions. An analogous polymorphous situation is found in the paraelectric phase of a few cubic oxide perovskites where local polarization takes the role of local displacements in halide perovskites, and in the paramagnetic phases of a few $3d$ oxides where the local spin configuration takes that role.

Giant momentum-dependent spin splitting in centrosymmetric low-Z antiferromagnets

Abstract: The traditional bulk Rashba effect predicts spin-orbit coupling induced wave-vector-dependent spin splitting between energy bands in nonmagnetic solids that lack spatial inversion symmetry. The authors show here how the violation of magnetic symmetry leads instead to a few prototypes of momentum-dependent spin splitting of energy bands that are induced by antiferromagnetism. This discovery broadens the spintronics playing field to include centrosymmetric compounds, even made of light elements with low spin-orbit coupling. Atomistic density functional calculations show that giant splitting above 200 meV is possible between top two valence bands.

Understanding doping of quantum materials

Abstract: Doping mobile carriers into ordinary semiconductors such as Si, GaAs, and ZnO was the enabling step in the electronic and optoelectronic revolutions. The recent emergence of a class of "Quantum Materials", where uniquely quantum interactions between the components produce specific behaviors such as topological insulation, unusual magnetism, superconductivity, spin-orbit-induced and magnetically-induced spin splitting, polaron formation, and transparency of electrical conductors, pointed attention to a range of doping-related phenomena associated with chemical classes that differ from the traditional semiconductors. These include wide-gap oxides, compounds containing open-shell d electrons, and compounds made of heavy elements yet having significant band gaps. The atomistic electronic structure theory of doping that has been developed over the past two decades in the sub-field of semiconductor physics has recently been extended and applied to quantum materials. The present review focuses on explaining the main concepts needed for a basic understanding of the doping phenomenology and indeed peculiarities in quantum materials from the perspective of condensed matter theory, with the hope of forging bridges to the chemists that have enabled the synthesis of some of the most interesting compounds in this field.

Symmetry-breaking polymorphous descriptions for correlated materials without interelectronic U

Abstract: Correlated materials with open-shell $d$ and $f$ ions having degenerate band-edge states show a rich variety of interesting properties ranging from metal-insulator transition to unconventional superconductivity. The textbook view for the electronic structure of these materials is that mean-field approaches are inappropriate, as the interelectronic interaction U is required to open a band gap between the occupied and unoccupied degenerate states while retaining symmetry. We show that the latter scenario often defining what Mott insulators are is in fact not needed for the $3d$ binary oxides MnO, FeO, CoO, and NiO. The mean-field-like band theory can indeed lift such degeneracies in the binaries when nontrivial unit-cell representations (polymorphous networks) are allowed to break symmetries, in conjunction with a recently developed nonempirical exchange and correlation density functional without an on-site interelectronic interaction U. We explain how density-functional theory in the polymorphous representation achieves band-gap opening in correlated materials through a separate mechanism from the Mott-Hubbard approach. We show the method predicts magnetic moments and gaps for the four binary monoxides in both the antiferromagnetic and paramagnetic phases, offering an effective alternative to symmetry-conserving approaches for studying a range of functionalities in open $d$- and $f$-shell complex materials.

False metals, real insulators, and degenerate gapped metals

Abstract: This paper deals with a significant family of compounds predicted by simplistic electronic structure theory to be metals but are, in fact, insulators. This false metallic state has been traditionally attributed in the literature to reflect the absence of proper treatment of electron-electron correlation ("Mott insulators") whereas, in fact, even mean-field like density functional theory describes the insulating phase correctly if the restrictions posed on the simplistic theory are avoided. Such unwarranted restrictions included different forms of disallowing symmetry breaking described in this article. As science and technology of conductors have transitioned from studying simple elemental metals such as Al or Cu to compound conductors such as binary or ternary oxides and pnictides, a special class of degenerate but gapped metals has been noticed. Their presumed electronic configurations show the Fermi level inside the conduction band or valence band, yet there is an "internal band gap" between the principal band edges. The significance of this electronic configuration is that it might be unstable towards the formation of states inside the internal band gap when the formation of such states costs less energy than the energy gained by transferring carriers from the conduction band to these lower energy acceptor states, changing the original (false) metal to an insulator.

False metals, real insulators, and degenerate gapped metals

Abstract: This paper deals with a significant family of compounds predicted by simplistic electronic structure theory to be metals but are, in fact, insulators. This false metallic state has been traditionally attributed in the literature to reflect the absence of proper treatment of electron-electron correlation (“Mott insulators”) whereas, in fact, even mean-field like density functional theory describes the insulating phase correctly if the restrictions posed on the simplistic theory are avoided. Such unwarranted restrictions included different forms of disallowing symmetry breaking described in this article. As the science and technology of conductors have transitioned from studying simple elemental metals such as Al or Cu to compound conductors such as binary or ternary oxides and pnictides, a special class of degenerate but gapped metals has been noticed. Their presumed electronic configurations show the Fermi level inside the conduction band or valence band, yet there is an “internal band gap” between the principal band edges. The significance of this electronic configuration is that it might be unstable toward the formation of states inside the internal band gap when the formation of such states costs less energy than the energy gained by transferring carriers from the conduction band to these lower energy acceptor states, changing the original (false) metal to an insulator. The analogous process also exists for degenerate but gapped metals with the Fermi level inside the valence band, where the energy gain is defined by transfer of electrons from the donor level to the unoccupied part of the valence band. We focus here on the fact that numerous electronic structure methodologies have overlooked some physical factors that could stabilize the insulating alternative, predicting instead false metals that do not really exist (note that this is in general not a physical phase transition, but a correction of a previous error in theory that led to a false prediction of a metal). Such errors include: (i) ignoring spin symmetry breaking, such as disallowing magnetic spin ordering in CuBi2O4 or disallowing the formation of polymorphous spin networks in paramagnetic LaTiO3 and YTiO3; (ii) ignoring structural symmetry breaking, e.g., not enabling energy-lowering bond disproportionation (Li-doped TiO2, SrBiO3, and rare-earth nickelates), or not exploring pseudo-Jahn–Teller-like distortions in LaMnO3, or disallowing spontaneous formation of ordered vacancy compounds in Ba4As3 and Ag3Al22O34; and (iii) ignoring spin–orbit coupling forcing false metallic states in CaIrO3 and Sr2IrO4. The distinction between false metals vs real insulators is important because (a) predicting theoretically that a given compound is metal even though it is found to be an insulator often creates the temptation to invoke high order novel physical effects (such as correlation in d-electron Mott insulators) to explain what was in effect caused by a more mundane artifact in a lower-level mean-field band theory, (b) recent prediction of exotic physical effects such as topological semimetals were unfortunately based on the above compounds that were misconstrued by theory to be metal, but are now recognized to be stable insulators not hosting exotic effects, and (c) practical technological applications based on stable degenerate but gapped metals such as transparent conductors or electrides for catalysis must rely on the systematically correct and reliable theoretical classification of metals vs insulators.

Understanding electronic peculiarities in tetragonal FeSe as local structural symmetry breaking

Abstract: Traditional band theory of perfect crystalline solids often uses as input the structure deduced from diffraction experiments; when modeled by the minimal unit cell this often produces a spatially averaged model. The present study illustrates that this is not always a safe practice unless one examines if the intrinsic bonding mechanism is capable of benefiting from the formation of a distribution of lower symmetry local environments that differ from the macroscopically averaged structure. This can happen either due to positional or to magnetic symmetry breaking. By removing the constraint of a small crystallographic cell, the energy minimization in the density functional theory finds atomic and spin symmetry breaking, not evident in conventional diffraction experiments but being found by local probes such as atomic pair distribution function analysis. Here we report that large atomic and electronic anomalies in bulk tetragonal FeSe emerge from the existence of distributions of local positional and magnetic moment motifs. The found symmetry-broken motifs obtained by minimization of the internal energy represent what chemical bonding in the tetragonal phase prefers as intrinsic energy lowering (stabilizing) static distortions. This explains observations of band renormalization, predicts orbital order and enhanced nematicity, and provides unprecedented close agreement with spectral function measured by photoemission and local atomic environment revealed by the pair distribution function. While the symmetry-restricted strong correlation approach has been argued previously to be the exclusive theory needed for describing the main peculiarities of FeSe, we show here that the symmetry-broken mean-field approach addresses numerous aspects of the problem, provides intuitive insight into the electronic structure, and opens the door for large-scale mean-field calculations for similar $d$-electron quantum materials.

The Rashba Scale: Emergence of Band Anti-crossing as a Design Principle for Materials with Large Rashba Coefficient

Abstract: Summary The spin-orbit-induced spin splitting of energy bands in low-symmetry compounds (the Rashba effect) has a long-standing relevance to spintronic applications and the fundamental understanding of symmetry breaking in solids, yet the knowledge of what controls its magnitude in different materials is difficult to anticipate. Indeed, rare discoveries of compounds with large Rashba coefficients are invariably greeted as pleasant surprises. We advance the understanding of the “Rashba Scale” using the “inverse design” approach by formulating theoretically the relevant design principle and then identifying compounds satisfying it. We show that the presence of energy band anti-crossing provides a causal design principle of compounds with large Rashba coefficients, leading to the identification via first-principles calculations of 34 rationally designed strong Rashba compounds. Since topological insulators must have band anti-crossing, this establishes an interesting cross-functionality of “topological Rashba insulators” that may provide a platform for the simultaneous control of spin splitting and spin polarization.

Inverse design of compounds that have simultaneously ferroelectric and Rashba cofunctionality

Abstract: Cofunctionality---the coexistence of different (possibly even contraindicated) properties in the same compound---is an exciting prospect that never fails to deliver interesting surprises, as in transparent while electrically conducting compounds, topological insulators having Rashba spin split bands, or electrically conducting while thermally insulating (thermoelectric) compounds. Here we study a cofunctionality of ferroelectricity and Rashba effect that combines the directional helical spin-polarization characteristic of the energy bands of bulk Rashba compounds with the existence of two opposite electric polarization states, characteristic of atomic displacements in displacive ferroelectrics. Flipping of the ferroelectric polarization (e.g., via an applied electric field) would result in the reversal of the Rashba spin polarization. However, thus far, only very few (essentially one) compounds have been found to be ferroelectric Rashba semiconductors (FERSCs), e.g., GeTe ($R3m$). In this paper, we propose a general strategy for the identification of compounds that possess cofunctionalities and apply it to perform an inverse design, finding compounds that simultaneously have ferroelectricity and Rashba spin splitting. The inverse design combining functionality ${f}_{1}$ with ${f}_{2}$ involves definition and utilization of causal factors that enable the said functionalities, and involves three steps: (1) screening materials that satisfy the enabling DPs common to the two functionalities ${f}_{1}$ and ${f}_{2}$; (2) filtering the materials according to DPs that are unique to each of the individual functionalities ${f}_{1}$ and ${f}_{2}$; and (3) by using the ensuing two compound lists of compounds $C({f}_{1})$ and $C({f}_{2})$ identifying the intersection between them, i.e., compounds $C({f}_{1},{f}_{2})$ possessing simultaneous ${f}_{1}$ and ${f}_{2}$. Based on this process, we design 52 atomic combinations not suspected to be FERSCs that were previously synthesized. Of these 52 compounds, we find 24 FERSCs that are thermodynamically stable (i.e., reside on the convex hull) and, at the same time, feature larger spin splitting (large than 25 meV). These include ${\mathrm{BrF}}_{5}(Cmc{2}_{1})$, $\mathrm{TlI}{\mathrm{O}}_{3}(R3m)$, $\mathrm{Zn}{\mathrm{I}}_{2}{\mathrm{O}}_{6}(P{2}_{1})$, $\mathrm{LaTa}{\mathrm{O}}_{4}(Cmc{2}_{1})$, ${\mathrm{Tl}}_{3}{\mathrm{S}}_{3}\mathrm{Sb}(R3m)$, ${\mathrm{Sn}}_{2}{\mathrm{P}}_{2}{\mathrm{Se}}_{6}$ (Pc), and ${\mathrm{Bi}}_{2}\mathrm{Si}{\mathrm{O}}_{5}(Cmc{2}_{1})$ that have Rashba spin splitting of 31, 57, 111, 40, 90, 67, and 76 meV, respectively. Density functional theory calculations illustrate the reversal of the spin texture when the electric polarization is flipped. This paper validates a general approach for the search of other cofunctionalities based on causal enabling design principles rather than uncovering machine correlations.

Hole antidoping of oxides

Abstract: In standard doping, adding charge carrier to a compound results in a shift of the Fermi level towards the conduction band for electron doping and towards the valence band for hole doping. We discuss the curious case of antidoping, where the direction of band movements in response to doping is reversed. Specifically, $p$-type antidoping moves the previously occupied bands to the principal conduction band resulting in an increase of band gap energy and reduction of electronic conductivity. We find that this is a generic behavior for a class of materials: early transition and rare-earth metal (e.g., Ti, Ce) oxides where the sum of composition-weighed formal oxidation states is positive; such compounds tend to form the well-known electron-trapped intermediate bands localized on the reduced cation orbitals. What is less known is that doping by a hole annihilates a single trapped electron on a cation. The latter thus becomes electronically inequivalent with respect to the normal cation in the undoped lattice, thus representing a symmetry-breaking effect. We give specific theoretical predictions for target compounds where hole antidoping might be observed experimentally: Magn\'eli-like phases (i.e., $\mathrm{Ce}{\mathrm{O}}_{2\text{\ensuremath{-}}x}$ and $\mathrm{Ti}{\mathrm{O}}_{2\text{\ensuremath{-}}x}$) and ternary compounds (i.e., $\mathrm{B}{\mathrm{a}}_{2}\mathrm{T}{\mathrm{i}}_{6}{\mathrm{O}}_{13}$ and $\mathrm{B}{\mathrm{a}}_{4}\mathrm{T}{\mathrm{i}}_{12}{\mathrm{O}}_{27}$), and note that this unique behavior opens the possibility of unconventional control of materials conductivity by doping.

Ferri-chiral compounds with potentially switchable Dresselhaus spin splitting

Abstract: Spin splitting of energy bands can be induced by relativistic spin-orbit interactions in materials without inversion symmetry Whereas polar space-group symmetries permit Rashba (R-1) spin splitting with helical spin textures in momentum space, which could be reversed upon switching a ferroelectric polarization via applied electric fields, the ordinary Dresselhaus effect $(\mathrm{D}\text{\ensuremath{-}}{1}_{\mathrm{A}})$ is active in materials exhibiting nonpolar noncentrosymmetric crystal classes with atoms occupying exclusively nonpolar lattice sites Consequently, the spin-momentum locking induced by $\mathrm{D}\text{\ensuremath{-}}{1}_{\mathrm{A}}$ is not electric field switchable Here we find a type of ferri-chiral materials with an alternative type of Dresselhaus symmetry, referred to as $\mathrm{D}\text{\ensuremath{-}}{1}_{\mathrm{B}}$, exhibiting crystal class constraints similar to $\mathrm{D}\text{\ensuremath{-}}{1}_{\mathrm{A}}$ (all dipoles add up to zero), but unlike $\mathrm{D}\text{\ensuremath{-}}{1}_{\mathrm{A}}$, at least one polar site is occupied The spin splitting is associated with the crystalline chirality, which in principle could be reversed upon a change in chirality Focusing on alkali metal chalcogenides, we identify ${\mathrm{NaCu}}_{5}{\mathrm{S}}_{3}$ in the nonenantiomorphic ferri-chiral structure, which exhibits ${\mathrm{CuS}}_{3}$ chiral units differing in the magnitude of their Cu displacements We then synthesize ${\mathrm{NaCu}}_{5}{\mathrm{S}}_{3}$ (space group $P{6}_{3}22$) and confirm its ferri-chiral structure with powder x-ray diffraction Our electronic structure calculations demonstrate it exhibits $\mathrm{D}\text{\ensuremath{-}}{1}_{\mathrm{B}}$ spin splitting as well as a ferri-chiral phase transition, revealing spin splitting interdependent on chirality Our electronic structure calculations show that a few percent biaxial tensile strain can reduce (or nearly quench) the switching barrier separating the monodomain ferri-chiral $P{6}_{3}22$ states We compute the circular dichroism absorption spectrum of the two ferri-chiral orientations and discuss what type of external stimuli might switch the chirality so as to reverse the (nonhelical) Dresselhaus $\mathrm{D}\text{\ensuremath{-}}{1}_{\mathrm{B}}$ spin texture Our study suggests the design of ferri-chiral crystals as potential spintronic and optoelectronic materials

Prediction of low-Z collinear and noncollinear antiferromagnetic compounds having momentum-dependent spin splitting even without spin-orbit coupling

Abstract: Recent study (Yuan et al, Phys Rev B 102, 014422 (2020)) revealed a SOC-independent spin splitting and spin polarization effect induced by antiferromagnetic ordering which do not necessarily require breaking of inversion symmetry or the presence of SOC, hence can exist even in centrosymmetric, low-Z light element compounds, considerably broadening the material base for spin polarization In the present work we develop the magnetic symmetry conditions enabling such effect, dividing the 1651 magnetic space groups into 7 different spin splitting prototypes (SST-1 to SST-7) We use the 'Inverse Design' approach of first formulating the target property (here, spin splitting in low-Z compounds not restricted to low symmetry structures), then derive the enabling physical design principles to search realizable compounds that satisfy these a priori design principles This process uncovers 422 magnetic space groups (160 centrosymmetric and 262 non-centrosymmetric) that could hold AFM-induced, SOC-independent spin splitting and spin polarization We then search for stable compounds following such enabling symmetries We investigate the electronic and spin structures of some selected prototype compounds by density functional theory (DFT) and find spin textures that are different than the traditional Rashba-Dresselhaus patterns We provide the DFT results for all antiferromagnetic spin splitting prototypes (SST-1 to SST-4) and concentrate on revealing of the AFM-induced spin splitting prototype (SST-4) The symmetry design principles along with their transformation into an Inverse Design material search approach and DFT verification could open the way to their experimental examinationM) The symmetry design principles along with their transformation into an Inverse Design material search approach and DFT verification could open the way to their experimental examination

The Rashba Scale: Emergence of Band Anti-Crossing as a Design Principle for Materials with Large Rashba coefficient

Abstract: The spin-orbit -induced spin splitting of energy bands in low symmetry compounds (the Rashba Effect) has a long-standing relevance to spintronic applications and to the fundamental understanding of symmetry breaking in solids, yet the knowledge of what controls its magnitude in different materials is difficult to anticipate. Indeed, rare discoveries of compounds with large Rashba coefficients are invariably greeted as pleasant surprises. We advance the understanding of the "Rashba Scale" using the "inverse design" approach by formulating theoretically the relevant design principle and then identifying compounds that satisfy it. We show that the presence of energy band anti-crossing provides a causal design principle of compounds with large Rashba coefficients, leading to the identification via first-principles calculations of 34 rationally designed strong-Rashba compounds. Since topological insulators must have band anti crossing, this leads us to establish an interesting cross functionality of "Topological Rashba Insulators" (TRI) that may provide a platform for the simultaneous control of spin splitting and spin-polarization.

Emergence of an upper bound to the electric field controlled Rashba spin splitting in InAs nanowires

Abstract: The experimental assessment of the strength ($\alpha_R$) of the Rashba spin-orbit coupling is rather indirect and involves the measurement of the spin relaxation length from magnetotransport, together with a model of weak antilocalization. The analysis of the spin relaxation length in nanowires, however, clouds the experimental assessment of the $\alpha_R$ and leads to the prevailing belief that it can be tuned freely with electric field--a central tenant of spintronics. Here, we report direct theory of $\alpha_R$ leading to atomistic calculations of the spin band structure of InAs nanowires upon application of electric field-- a direct method that does not require a theory of spin relaxation. Surprisingly, we find an {\it upper bound} to the electric field tunable Rashba spin splitting and the ensuing $\alpha_R$; for InAs nanowires, $\alpha_R$ is pinned at about 170 meVA irrespective of the applied field strength. We find that this pinning is due to the quantum confined stark effect, that reduces continuously the nanowire band gap with applied electric field, leading eventually to band gap closure and a considerable increase in the density of free carriers. This results in turn in a strong screening that prevents the applied electric field inside the nanowire from increasing further beyond around 200 kV/cm for InAs nanowires. Therefore, further increase in the gate voltage will not increase $\alpha_R$. This finding clarifies the physical trends to be expected in nanowire Rashba SOC and the roles played by the nano size and electric field.