Alex Zunger
Semiconductor Quantum Structures and Devices
University of Colorado Boulder
About
Research focus
Semiconductor Quantum Structures and Devices
Impact
1,118 pubs · 103,009 cites · h-index 147
Affiliations
Publications
Original Research
FEB 25ZhangPhysical Review Materials
Upward band gap bowing and negative mixing enthalpy in multi-component cubic halide perovskite alloys
Semiconductor compounds are often alloyed to obtain target physical properties that are absent in the individual components. Conventional tetrahedral semiconductors generally have lower alloy gaps than the composition average gap of the constituents (``downward bowing''). We designed via DFT multi-component halide perovskite alloys that have significant upward bowing. Such alloys have a rather low mixing enthalpy, suggesting stability towards phase separation. The enabling idea is to mix perovskites with B atoms that have low lying s-orbitals in the valence band, with a compound that has IB atoms (e.g., Cd) with s-orbitals in the conduction band. The ensuing s-s repulsion opens the alloy gap with respect to the constituents' gap.
Original Research
JUL 2025XiongPhysical Review X
Prototypes of Nonrelativistic Spin Splitting and Polarization in Symmetry Broken Antiferromagnets
Antiferromagnets that break both space-time reversal and translation-spin-rotation symmetries were recently predicted [L.-D. Yuan, Z. Wang, J.-W. Luo, E. I. Rashba, and A. Zunger, ] to possess splitting between the otherwise spin-degenerate energy bands even without the relativistic spin-orbit coupling (SOC). Here, we point out that such nonrelativistic spin splitting (NRSS)—in particular, “spin splitting type 4” (SST-4) symmetry-broken antiferromagnets—can be divided into subgroups having distinct patterns of spin splitting and spin textures, depending on additional auxiliary symmetries of spin interconversion and polarity. These SST-4 subgroups include the <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"> <a:mi>α</a:mi> </a:math> -type (no spin-interconverting symmetry) having spin splitting at the Brillouin zone center, as well as the <c:math xmlns:c="http://www.w3.org/1998/Math/MathML" display="inline"> <c:mi>β</c:mi> </c:math> subgroup in which a rotation symmetry is applied and determines the alternating spin texture and the <e:math xmlns:e="http://www.w3.org/1998/Math/MathML" display="inline"> <e:mi>γ</e:mi> </e:math> subgroup having exclusively reflection spin-interconverting symmetry. Unlike ferrimagnets, the <g:math xmlns:g="http://www.w3.org/1998/Math/MathML" display="inline"> <g:mi>α</g:mi> </g:math> -type compounds are shown to have tiny net magnetization at finite temperature and thus avoid the adverse effect of the stray field. The <i:math xmlns:i="http://www.w3.org/1998/Math/MathML" display="inline"> <i:mi>α</i:mi> </i:math> and <k:math xmlns:k="http://www.w3.org/1998/Math/MathML" display="inline"> <k:mi>β</k:mi> </k:math> subgroups can be either polar or nonpolar, whereas the <m:math xmlns:m="http://www.w3.org/1998/Math/MathML" display="inline"> <m:mi>γ</m:mi> </m:math> subgroup is polar only, providing a basis for possible switching by external fields. The combination of NRSS-enabling and auxiliary symmetries is used here as a filter for identifying previously synthesized compounds as specific prototypes. Their characteristic splitting and spin polarization are calculated by density functional theory to the benefit of potential future experimental testing. Interesting results are as follows: (i) SOC-independent NRSS can exceed the magnitude of the SOC-induced Rashba and Dresselhaus spin splitting in semiconductors. (ii) Examples of predicted <o:math xmlns:o="http://www.w3.org/1998/Math/MathML" display="inline"> <o:mi>α</o:mi> </o:math> -type insulating compounds include <q:math xmlns:q="http://www.w3.org/1998/Math/MathML" display="inline"> <q:mrow> <q:msub> <q:mi>BiCrO</q:mi> <q:mn>3</q:mn> </q:msub> </q:mrow> </q:math> (nonpolar) and <s:math xmlns:s="http://www.w3.org/1998/Math/MathML" display="inline"> <s:mrow> <s:msub> <s:mi>Mn</s:mi> <s:mn>2</s:mn> </s:msub> <s:msub> <s:mi>ScSbO</s:mi> <s:mn>6</s:mn> </s:msub> </s:mrow> </s:math> (polar), the latter having spin splitting of 158 meV and 160 meV in the valence and conduction bands, respectively. (iii) The <u:math xmlns:u="http://www.w3.org/1998/Math/MathML" display="inline"> <u:mi>β</u:mi> </u:math> -type ( <w:math xmlns:w="http://www.w3.org/1998/Math/MathML" display="inline"> <w:mrow> <w:msub> <w:mrow> <w:mtext>Cu</w:mtext> </w:mrow> <w:mrow> <w:mn>2</w:mn> </w:mrow> </w:msub> <w:msub> <w:mrow> <w:mtext>Y</w:mtext> </w:mrow> <w:mrow> <w:mn>2</w:mn> </w:mrow> </w:msub> <w:msub> <w:mrow> <w:mtext>O</w:mtext> </w:mrow> <w:mrow> <w:mn>5</w:mn> </w:mrow> </w:msub> </w:mrow> </w:math> and <y:math xmlns:y="http://www.w3.org/1998/Math/MathML" display="inline"> <y:mrow> <y:msub> <y:mrow> <y:mtext>FeF</y:mtext> </y:mrow> <y:mrow> <y:mn>2</y:mn> </y:mrow> </y:msub> </y:mrow> </y:math> ) and <ab:math xmlns:ab="http://www.w3.org/1998/Math/MathML" display="inline"> <ab:mi>γ</ab:mi> </ab:math> -type compounds ( <cb:math xmlns:cb="http://www.w3.org/1998/Math/MathML" display="inline"> <cb:mrow> <cb:mrow> <cb:msub> <cb:mrow> <cb:mtext>Mn</cb:mtext> </cb:mrow> <cb:mrow> <cb:mn>4</cb:mn> </cb:mrow> </cb:msub> <cb:msub> <cb:mrow> <cb:mtext>Nb</cb:mtext> </cb:mrow> <cb:mrow> <cb:mn>2</cb:mn> </cb:mrow> </cb:msub> <cb:msub> <cb:mrow> <cb:mtext>O</cb:mtext> </cb:mrow> <cb:mrow> <cb:mn>9</cb:mn> </cb:mrow> </cb:msub> </cb:mrow> </cb:mrow> </cb:math> and <eb:math xmlns:eb="http://www.w3.org/1998/Math/MathML" display="inline"> <eb:mrow> <eb:msub> <eb:mrow> <eb:mtext>FeScO</eb:mtext> </eb:mrow> <eb:mrow> <eb:mn>3</eb:mn> </eb:mrow> </eb:msub> </eb:mrow> </eb:math> ) are distinguished both by their auxiliary symmetries and polarity. The spin textures of <gb:math xmlns:gb="http://www.w3.org/1998/Math/MathML" display="inline"> <gb:mi>γ</gb:mi> </gb:math> -type compounds are mirror reflected with spin degeneracy of the wave vectors on that mirror. These observations will likely broaden the experimental playing field of NRSS physics significantly.
Original Research
JAN 2025XiongPhysical review. B./Physical review. B
Symmetry breaking forms split-off flat bands in quantum oxides controlling metal versus insulator phases
The crystal structure used as an input to electronic structure calculation of conventionally bonded solids consists primarily of the standard crystallographic degrees of freedom. Quantum solids sometimes have additional microscopic degrees of freedom nested within the crystallographic structure. These might include local motifs including positional (Peierls dimers, deformed octahedra, and Jahn-Teller distortions), magnetic (local moment configurations), and dipolar (local ferroelectric configurations). Such motifs can be observed experimentally via local probes that avoid averaging and theoretically as distinct total energy-lowering features relative to more simplified average crystallographic structures. Here, we examine the ability of electronic structure methods independent of strong correlation physics to explain the broad phenomenology of metal-insulator selectivity in quantum oxides by energy-lowering symmetry breaking. We do this by avoiding the restriction of considering only (i) electron-electron interactions in a fixed unresponsive lattice---as done in Mott strong correlation explanations---allowing, however, (ii) local positional and magnetic motifs that coexist within a crystallographic landscape. We find, in a broad range of quantum oxides with different symmetry-breaking modes, the formation of split-off flat bands that can also control metallic vs insulating characteristics. The split-off band effect is common to magnetic and nonmagnetic materials, including both binary and ternary oxides. One finds that, when such local symmetry-breaking motifs are considered in mean-field-like (e.g., density functional theory) electronic structure calculations of quantum oxides, they explain many of the observed trends in metal vs insulator phases discussed otherwise in prior literature via strong correlation in symmetry-unbroken view.
Original Research
JAN 2025SabinoMaterials Horizons
Alloying multiple halide perovskites on the same sublattice in search of stability and target band gaps
that of long-range ordered double perovskites of the same compositions, seeking examples of ordered phases emerging from disordered alloys. Our findings provide fundamental insights into the electronic and structural behavior of HP alloys, offering valuable design principles for the development of stable and efficient materials for next-generation photovoltaic and optoelectronic devices.
Original Research
SEP 2024Zhangnpj Computational Materials
Hidden magnetism and split off flat bands in the insulator metal transition in VO2
Transition metal d -electron oxides with an odd number of electrons per unit cell are expected to form metals with partially occupied energy bands, but exhibit in fact a range of behaviors, being either insulators, or metals, or having insulator-metal transitions. Traditional explanations involved predominantly electron-electron interactions in fixed structural symmetry. The present work focuses instead on the role of symmetry breaking local structural motifs. Viewing the previously observed V-V dimerization in VO 2 as a continuous knob, reveals in density functional calculations the splitting of an isolated flat band from the broad conduction band. This leads past a critical percent dimerization to the formation of the insulating phase while lowering the total energy. In VO 2 this transition is found to have a rather low energy barrier approaching the thermal energy at room temperature, suggesting energy-efficient switching in neuromorphic computing. Interestingly, sufficient V-V dimerization suppresses magnetism, leading to the nonmagnetic insulating state, whereas magnetism appears when dimerization is reduced, forming a metallic state. This study opens the way to design novel functional quantum materials with symmetry breaking-induced flat bands.
Original Research
JUL 2024Xiong
Energy-lowering symmetry breaking creates a flat-band insulator in paramagnetic Nb3Cl8
Ordinary band structure calculations of quantum materials often incorrectly predicted metallic, instead of insulating electronic structure, motivating Mott-Hubbard strong electron correlation as a gapping mechanism. More recently, allowing the formation of local structural symmetry breaking motifs in otherwise ordinary band theory was shown to lower the total energy while predicting insulating gaps when they are observed. An important counter example was recently pointed out whereby the flat band formed in Nb3Cl8 by symmetry breaking is by itself partially occupied, thus failing to account for the observed insulating state. It is shown here that a generalized symmetry breaking involving the cooperative structural and magnetic effects produces in mean-field-like density functional theory an energy lowering insulating phase.
Original Research
JUL 2024SabinoPhysical review. B./Physical review. B
Impact of symmetry breaking and spin-orbit coupling on the band gap of halide perovskites
Halide perovskite (HP) materials have recently emerged as a class of semiconductors with immense promise for various optoelectronic applications, ranging from solar cells to light-emitting diodes. One of the unique attributes of HPs is their tunable band gaps with different factors governing their value. The first factor is related to relativistic corrections [``mass-Darwin,'' connected to the ns² lone pairs, and spin-orbit coupling (SOC)] that induce an orbital shift or degeneracy splitting, resulting in a band-gap reduction. The second factor involves the structural configuration: in HPs the local symmetry of each Wyckoff position tends to be broken, inducing an opening of the band gap. Based on high-throughput density functional theory calculations, this paper systematically studies a possible self-cancelation on the band-gap correction for HPs when the polymorphous configuration---structural effects---and the SOC---electronic effects---are included. Our results indicate that the nature of interplay between SOC and symmetry breaking (SB) is that they are independent decoupling effects to describe the band-gap magnitude in halide perovskites. As a result of that, we observe a transitivity of the band-gap description; i.e., if we know the band gap of halide perovskites without SB and SOC, we can independently add the effects of band-gap reduction due to SOC and band-gap opening due to SB, regardless of the order in which these effects are considered.
Original Research
OCT 2023SabinoAdvanced Energy Materials
Light‐Induced Frenkel Defect Pair Formation Can Lead to Phase‐Segregation of Otherwise Miscible Halide Perovskite Alloys
Alloys of ABX 3 halide perovskites (HP) exhibit unique phase behavior compared to traditional III‐V and II‐VI semiconductor alloys used in solar cells. While the latter typically have good mutual miscibility when their mixed components are size matched, and phase‐segregate when size mismatched, HP alloys show good miscibility in the dark but can phase‐segregate under light. Quantum mechanical calculations described herein reveal light‐induced defect formation and migration hold the key. Specifically, the interaction between a halogen vacancy V X with halogen interstitial X i forming together a Frenkel‐pair defect emerges as the enabler for phase‐segregation in HP alloys. At a threshold bromine composition in the Br‐I alloys, the photogenerated holes in the valence band localize, creating thereby a doubly‐charged iodine Frenkel‐pair (V I + I i ) 2+ . Faster migration of iodine over bromine interstitial into the vacant iodine V I site leads to the formation of iodine‐rich and iodine‐depleted regions, establishing phase‐segregation. Removal of the mobile defects–the agent of segregation–by dark thermal annealing, supplies the opposing force, leading to reversal of phase‐segregation. This atomistic understanding can enable some control of the phase‐segregation by selecting substituting elements on the B site–such as replacing some Pb by Sn–that are unable to form stable Frenkel defects.
Original Research
AUG 2023YuanNature Communications
Uncovering spin-orbit coupling-independent hidden spin polarization of energy bands in antiferromagnets
Many textbook physical effects in crystals are enabled by some specific symmetries. In contrast to such 'apparent effects', 'hidden effect X' refers to the general condition where the nominal global system symmetry would disallow the effect X, whereas the symmetry of local sectors within the crystal would enable effect X. Known examples include the hidden Rashba and/or hidden Dresselhaus spin polarization that require spin-orbit coupling, but unlike their apparent counterparts are demonstrated to exist in non-magnetic systems even in inversion-symmetric crystals. Here, we discuss hidden spin polarization effect in collinear antiferromagnets without the requirement for spin-orbit coupling (SOC). Symmetry analysis suggests that antiferromagnets hosting such effect can be classified into six types depending on the global vs local symmetry. We identify which of the possible collinear antiferromagnetic compounds will harbor such hidden polarization and validate these symmetry enabling predictions with first-principles density functional calculations for several representative compounds. This will boost the theoretical and experimental efforts in finding new spin-polarized materials.
Original Research
APR 2023YuanAdvanced Materials
Degeneracy Removal of Spin Bands in Collinear Antiferromagnets with Non‐Interconvertible Spin‐Structure Motif Pair
Energy bands in antiferromagnets are supposed to be spin degenerate in the absence of spin-orbit coupling (SOC). Recent studies have identified formal symmetry conditions for antiferromagnetic crystals in which this degeneracy can be lifted, spin splitting,even in the vanishing SOC (i.e., non-relativistic) limit. Materials having such symmetries could enable spin-split antiferromagnetic spintronics without the burden of using heavy-atom compounds. However, the symmetry conditions that involve spin and magnetic symmetry are not always effective as practical material selection filters. Furthermore, these symmetry conditions do not readily disclose trends in the magnitude and momentum dependence of the spin-splitting energy. Here, it is shown that the formal symmetry conditions enabling spin-split antiferromagnets can be interpreted in terms of local motif pairs, such as octahedra or tetrahedra, each carrying opposite magnetic moments. Collinear antiferromagnets with such a spin-structure motif pair, whose components interconvert by neither translation nor spatial inversion, will show spin splitting. Such a real-space motif-based approach enables an easy way to identify and design materials (illustrated in real example materials) having spin splitting without the need for SOC, and offers insights into the momentum dependence and magnitude of the spin splitting.
Original Research
FEB 2023GuanPhysical review. B./Physical review. B
Hidden Zeeman-type spin polarization in bulk crystals
Exploring hidden effects that have been overlooked given the nominal global crystal symmetry but are indeed visible in solid-state materials has been a fascinating subject of research recently. Here, we introduce a hidden Zeeman-type spin polarization (HZSP) in nonmagnetic bulk crystals with sublattice structures. In the momentum space of these crystals, the doubly degenerate bands formed in a certain plane can exhibit a uniform spin configuration with opposite spin orientations perpendicular to this plane, whereas such degenerate states are spatially separated in a pair of real-space sectors. Interestingly, we find that HZSP can manifest itself in both centrosymmetric and noncentrosymmetric materials. We further demonstrate the important role of nonsymmorphic twofold screw-rotational symmetry played in the formation of HZSP. Moreover, two representative material examples, i.e., centrosymmetric WSe₂ and noncentrosymmetric BaBi₄O₇, are identified to show HZSP via first-principles calculations. Our finding thus not only opens different perspectives for hidden spin polarization research but also significantly broadens the range of materials towards spintronics applications.
Original Research
DEC 2022YuanUniversity of Colorado Boulder
Uncovering hidden spin polarization of energy bands in antiferromagnets
Many textbook physical effects in crystals are enabled by some specific symmetries. In contrast to such ‘apparent effects’, ‘hidden effect X’ refers to the general condition where the nominal global system symmetry would disallow the effect X, whereas the symmetry of local sectors within the crystal would enable effect X. Known examples include the hidden Rashba and/or hidden Dresselhaus spin polarization that require spin orbit coupling, but (unlike the apparent Rashba and Dresselhaus counterparts) can exist even in inversion-symmetric non-magnetic crystals. Here we point out that the spin splitting effect that does not require spin-orbit coupling (SOC) can have a hidden spin polarization counterpart in antiferromagnets. We show that such hidden, SOC-independent effects reflect intrinsic properties of the perfect crystal rather than an effect due to imperfections, opening the possibility for experimental realization, and offering a potential way to switch antiferromagnetic ordering.
Original Research
OCT 2022GambinoPhysical review. B./Physical review. B
Density functional description of spin, lattice, and spin-lattice dynamics in antiferromagnetic and paramagnetic phases at finite temperatures
Describing the (a) electronic and magnetic properties (EMPs) of compounds generally requires the specification of (b) the type of spin configurations one is considering [e.g., antiferromagnetic (AFM) or paramagnetic (PM) phases, with or without spin short-range order (SRO)] and lattice structure (e.g., atomic displacements, possible symmetry breaking) of such phases at a given temperature. Indeed, studying the interplay between the spin configuration and lattice structure (SCLS) and the ensuing EMPs has been an outstanding challenge in the theory of matter. The traditional approach of electronic phases of matter has generally focused on the interelectronic interactions, regarding the lattice structure as a spectator degree of freedom (DOF), often fixed from an external source (experiment or model assumptions). However, one expects that the EMPs of a compound can generally respond self-consistently to changes in SCLS (including symmetry), and at the same time, the SCLS can change in response to different electron and spin distributions visited during the calculation of the EMPs. This ping-pong-like interplay where structure affects electronic properties and the latter affect structure is indeed a cornerstone of much of the intricacy of understanding quantum materials. However, there is a limited understanding of the theory required to determine the SCLS at finite temperature in a way that can affect the EMPs and vice versa. We use here a practical, density functional theory (DFT)-based approach that provides the SCLS as a function of temperature, involving the description of spin, lattice, and spin-lattice dynamics of AFM and PM phases, thus providing the required ping-pong partners to the description of the EMPs of different phases. We distinguish three levels of dynamics: (I) dynamics of the spin DOFs treated via noncollinear Heisenberg Monte Carlo solved with exchange energies obtained from first-principles DFT cluster expansion, (II) dynamics of the lattice DOFs treated by ab initio molecular dynamics (AIMD) employing a fixed, representative spin configuration from Level I at the simulated temperature, and (III) coupling of spin and lattice dynamics via Landau-Lifshitz-Gilbert spin dynamics combined with AIMD. Such SCLSs at each of the three levels are used as inputs to DFT supercell calculations, providing the EMPs at each temperature. The results of this sequence include electronic band structures, bandgaps, density of states, as well as the statistical distribution of local moments and the SRO parameters, each as a function of temperature. Herein, we define a path to include temperature in magnetic insulators at different levels of spin dynamics by intercommunication between electronic structure theory and statistical mechanics. Using NiO as a test case, we address the separability of the DOFs in magnetic insulators for a minimal description of EMPs, demonstrating that inclusion of spin dynamics and, to some level, lattice dynamics is enough to explain the EMPs.
Original Research
DEC 2021AktasPhysical Review Research
Piezoelectricity in nominally centrosymmetric phases
Compound phases often display properties that are symmetry forbidden relative to their nominal, average crystallographic symmetry, even if extrinsic reasons (defects, strain, or imperfections) are not apparent. Specifically, breaking the macroscopic inversion symmetry of a centrosymmetric phase can dominate or significantly change its observed properties while the detailed mechanisms and magnitudes of the deviations of symmetry breaking are often obscure. Here, we choose piezoelectricity as a tool to investigate macroscopic inversion-symmetry breaking in nominally centrosymmetric materials as a prominent example and measure resonant piezoelectric spectroscopy (RPS) and Resonant Ultrasound Spectroscopy (RUS) in 15 compounds, 18 samples, and 21 different phases, including unpoled ferroelectrics, paraelectrics, relaxors, ferroelastics, incipient ferroelectrics, and isotropic materials with low defect concentrations, i.e., NaCl, fused silica, and CaF₂. We exclude the flexoelectric effect as a source of the observed piezoelectricity yet observe piezoelectricity in all nominally cubic phases of these samples. By scaling the RPS intensities with those of RUS, we calibrate the effective piezoelectric coefficients using single-crystal quartz as standard. Using this scaling we determine the effective piezoelectric modulus in nominally nonpiezoelectric phases, finding that the ``symmetry-forbidden'' piezoelectric effect ranges from ~1 to 10^-5 pm/V (~0.5% to ~2×10^-5% of the piezoelectric coefficient of poled ferroelectric lead zirconate titanate). The values for the unpoled ferroelectric phase are only slightly higher than those in the paraelectric phase. The extremely low coefficients are well below the detection limit of conventional piezoelectric measurements and demonstrate RPS as a convenient and ultrahighly sensitive method to measure piezoelectricity. We suggest that symmetry-breaking piezoelectricity in nominally centrosymmetric materials and disordered, unpoled ferroelectrics is a common phenomenon.
Original Research
DEC 2021SabinoMaterials Horizons
Intrinsic doping limitations in inorganic lead halide perovskites
(p)F towards the valence band, for p-type doping) to be sufficiently large. We find that, even though in HP's based on Br and Cl there are numerous shallow level dopants that satisfy DP-(i), in contrast DP-(ii) is satisfied only for holes and DP-(iii) fails for both holes and electrons, being the ultimate bottleneck for the n-type doping in Iodide HP's. This suggests an intrinsic mechanism for doping limitations in this class of semiconductors in terms of recognized physical mechanisms.
Original Research
SEP 2021AcostaPhysical review. B./Physical review. B
Different shapes of spin textures as a journey through the Brillouin zone
Crystallographic point group symmetry (CPGS) such as polar and nonpolar crystal classes have long been known to classify compounds that have spin-orbit-induced spin splitting. While taking a journey through the Brillouin zone (BZ) from one k-point to another for a fixed CPGS, it is expected that the wave vector point group symmetry (WPGS) can change, and consequently, a qualitative change in the texture of the spin polarization can occur [the expectation value of spin operator PS_nk₀ in Bloch state $u(n,k)$ and the wave vector k₀]. However, the nature of the spin texture (ST) change is generally unsuspected. In this paper, we determine a full classification of the linear-in-k ST patterns based on the polarity and chirality reflected in the WPGS at k₀. The spin-polarization vector PS_nk₀ controlling the ST is bound to be parallel to the rotation axis and perpendicular to the mirror planes, and hence, symmetry operation types in WPGSs impose symmetry restriction to the ST. For instance, the ST is always parallel to the wave vector k in nonpolar chiral WPGSs since they contain only rotational symmetries. Some consequences of the ST classification based on the symmetry operations in the WPGS include the observation of ST patterns that are unexpected according to the symmetry of the crystal. For example, it is usually established that spin-momentum locking effect (spin vector always perpendicular to the wave vector) requires the crystal inversion symmetry breaking by an asymmetric electric potential. However, we find that polar WPGS can have this effect even in compounds without electric dipoles or external electric fields. We use the determined relation between WPGS and ST as a design principle to select compounds with multiple STs near band edges at different k valleys. Based on high-throughput calculations for 1481 compounds, we find 37 previously fabricated materials with different STs near band edges. The ST classification as well as the predicted compounds with multiple STs can be a platform for potential application for spin-valleytronics and the control of the ST by accessing different valleys.
Original Research
JUN 2021YuanPhysical review. B./Physical review. B
Strong influence of nonmagnetic ligands on the momentum-dependent spin splitting in antiferromagnets
Recent studies show the nonrelativistic antiferromagnetic ordering could generate momentum-dependent spin splitting analogous to the Rashba effect but free from the requirement of relativistic spin-orbit coupling. Whereas the classification of such compounds can be illustrated by different spin-splitting prototypes (SSTs) from symmetry analysis and density-functional-theory calculations, the huge variation in chemical bonding and structures of these diverse compounds possibly clouds the issue of how much of the variation in spin splitting can be traced back to the symmetry-defined characteristics, rather to the underlining chemical and structural diversity. The alternative model Hamiltonian approaches do not confront the issues of chemical and structural complexity but often consider only the magnetic sublattice, dealing with the all-important effects of the nonmagnetic ligands via renormalizing the interactions between the magnetic sites. To this end, we constructed a DFT model Hamiltonian that allows us to study SSTs at constant chemistry while retaining the realistic atomic-scale structure including ligands. This is accomplished by using a single, universal magnetic skeletal lattice (Ni²⁺ ions in rocksalt NiO) and designing small displacements of the nonmagnetic (oxygen) sublattice which produce, by design, the different SST magnetic symmetries. We show that (i) even similar crystal structures having very similar band structures can lead to contrasting behavior of spin splitting vs momentum, and (ii) even subtle deformations of the nonmagnetic ligand sublattice could cause a giant spin splitting in AFM-induced SST. This is a paradigm shift relative to the convention of modeling magnets without considering the nonmagnetic ligand that mediates indirect magnetic interaction (e.g., superexchange).
Review
JAN 2021ZungerChemical Reviews
Understanding Doping of Quantum Materials
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 subfield 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.
Original Research
DEC 2020HuangPhysical review. B./Physical review. B
Ferri-chiral compounds with potentially switchable Dresselhaus spin splitting
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 (D-1A) is active in materials exhibiting nonpolar noncentrosymmetric crystal classes with atoms occupying exclusively nonpolar lattice sites. Consequently, the spin-momentum locking induced by D-1A is not electric field switchable. Here we find a type of ferri-chiral materials with an alternative type of Dresselhaus symmetry, referred to as D-1B, exhibiting crystal class constraints similar to D-1A (all dipoles add up to zero), but unlike D-1A, 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 NaCu₅S₃ in the nonenantiomorphic ferri-chiral structure, which exhibits CuS₃ chiral units differing in the magnitude of their Cu displacements. We then synthesize NaCu₅S₃ (space group P6₃22) and confirm its ferri-chiral structure with powder x-ray diffraction. Our electronic structure calculations demonstrate it exhibits D-1B 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 P6₃22 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 D-1B spin texture. Our study suggests the design of ferri-chiral crystals as potential spintronic and optoelectronic materials.
Original Research
DEC 2020WangPhysical review. B./Physical review. B
Understanding electronic peculiarities in tetragonal FeSe as local structural symmetry breaking
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.