arXiv subjects

David Vanderbilt

Publications and source records attributed to David Vanderbilt.

At least 73 records · Page 4Linked to original sources

arXiv2018

Covalency-driven collapse of strong spin-orbit coupling in face-sharing iridium octahedra

We report $\textit{ab-initio}$ density functional theory calculation and Raman scattering results to explore the electronic structure of Ba$_5$CuIr$_3$O$_{12}$ single crystals. This insulating iridate, consisting of face-sharing IrO$_6$ octahedra forming quasi-one-dimensional chains, cannot be described by the local $j_{\rm eff}$=1/2 moment picture commonly adopted for discussing electronic and magnetic properties of iridate compounds with IrO$_6$ octahedra. The shorter Ir-Ir distance in the face-sharing geometry, compared to corner- or edge-sharing structures, leads to strong covalency between neighboring Ir. Then this strong covalency results in the formation of molecular orbitals (MO) at each Ir trimers as the low-energy electronic degree of freedom. The theoretically predicted three-peak structure in the joint density of states, a distinct indication of deviation from the $j_{\rm eff}$=1/2 picture, is verified by observing the three-peak structure in the electronic excitation spectrum by Raman scattering.

cond-mat.str-el↗

arXiv2018

Trimer Bonding States on the Surface of Transition-metal Dichalcogenide TaTe2

We report a comprehensive study on the surface structural and electronic properties of TaTe2 at room temperature. The surface structure was investigated using both low energy electron diffraction intensity versus voltage and density functional theory calculations. The relaxed structures obtained from the two methods are in good agreement, which is very similar to the bulk, maintaining double zigzag trimer chains. The calculated density of states indicates that such structure originates from the trimer bonding states of the Ta dxz and dxy orbitals. This work will further provide new insights towards the understanding of the charge density wave phase transition in TaTe2 at low temperature.

cond-mat.mtrl-sci↗

arXiv2018

Nature of the Magnetic Interactions in Sr$_3$NiIrO$_6$

Iridates abound with interesting magnetic behaviours because of their strong spin-orbit coupling. Sr$_3$NiIrO$_6$ brings together the spin-orbital entanglement of the Ir$^{4+}$ ion with a 3$d$ Ni cation and a one-dimensional crystal structure. It has a ferrimagnetic ground state with a 55 T coercive field. We perform a theoretical study of the magnetic interactions in this compound, and elucidate the role of anisotropic symmetric exchange as the source of its strong magnetic anisotropy. Our first-principles calculations reproduce the magnon spectra of this compound and predict a signature in the cross sections that can differentiate the anisotropic exchange from single-ion anisotopy.

cond-mat.mtrl-sci↗

arXiv2018

Current-density implementation for calculating flexoelectric coefficients

The flexoelectric effect refers to polarization induced in an insulator when a strain gradient is applied. We have developed a first-principles methodology based on density-functional perturbation theory to calculate the elements of the bulk, clamped-ion flexoelectric tensor. In order to determine the transverse and shear components directly from a unit cell calculation, we calculate the current density induced by the adiabatic atomic displacements of a long-wavelength acoustic phonon. Previous implementations based on the charge-density response required supercells to capture these components. Our density-functional-theory implementation requires the development of an expression for the current density that is valid for the case of nonlocal pseudopotentials, and long-wavelength phonon perturbations. We benchmark our methodology on simple systems of isolated noble gas atoms, and apply it to calculate the clamped-ion flexoelectric constants for a variety of technologically important cubic oxides. We also discuss some technical issues that are associated with the definition of current density in a nonlocal pseudopotential context, and their relevance to the calculation of macroscopic response properties of crystals.

cond-mat.mtrl-sci↗

arXiv2018

Geometric and nongeometric contributions to the surface anomalous Hall conductivity

A static electric field generates circulating currents at the surfaces of a magnetoelectric insulator. The anomalous Hall part of the surface conductivity tensor describing such bound currents can change by multiples of $e^2/h$ depending on the insulating surface preparation, and a bulk calculation does not fix its quantized part. To resolve this ambiguity, we develop a formalism for calculating the full surface anomalous Hall conductivity in a slab geometry. We identify a Berry-curvature term, closely related to the expression for the bulk anomalous Hall conductivity, whose value can change by quantized amounts by adjusting the surface Hamiltonian. In addition, the surface anomalous Hall conductivity contains a nongeometric part that does not depend on the surface preparation.

cond-mat.mes-hall↗

arXiv2018

Nearly triple nodal point topological phase in half-metallic GdN

Recent developments in topological semimetals open a way to realize relativistic dispersions in condensed matter systems. One recently studied type of topological feature is the "triple nodal point" where three bands become degenerate. In contrast to Weyl and Dirac nodes, triple nodal points, which are protected by a rotational symmetry, have nodal lines attached, so that a characterization in terms of a chirality is not possible. Previous studies of triple nodal points considered nonmagnetic systems, although an artificial Zeeman splitting was used to probe the topological nature. Here instead we treat a ferromagnetic material, half-metallic GdN, where the splitting of the triple nodal points comes from the spin-orbit coupling. The size of the splitting ranges from 15 to 150 meV depending on the magnetization orientation, enabling a transition between a Weyl-point phase and a "nearly triple nodal point" phase that exhibits very similar surface spectra and transport properties compared to a true triple-node system. The rich topological surface states, manipulable via the orientation of the magnetization, make half-metallic GdN a promising platform for future investigations and applications.

cond-mat.mtrl-sci↗

arXiv2018

Quantum theory of mechanical deformations

We construct a general metric-tensor framework for treating inhomogenous adiabatic deformations applied to crystalline insulators, by deriving an effective time-dependent Schrödinger equation in the undistorted frame. The response can be decomposed into "static" and "dynamic" terms that correspond, respectively, to the amplitude and the velocity of the distortion. We then focus on the dynamic contributon, which takes the form of a gauge field entering the effective Hamiltonian, in the linear-response limit. We uncover an intimate relation between the dynamic response to the rotational component of the inhomogeneous deformation and the diamagnetic response to a corresponding inhomogeneous magnetic field. We apply this formalism to the theory of flexoelectric response, where we resolve a previous puzzle by showing that the currents generated by the dynamic term, while real, generate no bound charges even at surfaces, and so may be dropped from a practical theory of flexoelectricity.

cond-mat.mtrl-sci↗

arXiv2018

Nexus networks in carbon honeycombs

Nexus metals represent a new type of topological material in which nodal lines merge at nexus points. Here, we propose novel networks in nexus systems through intertwining between nexus fermions and additional nodal lines. These nexus networks can be realized in several recently synthesized carbon honeycomb materials. In these carbon honeycombs, we demonstrate a phase transition between a nexus network and a system with triply-degenerate points and additional nodal lines. The Landau level spectra show unusual magnetic transport properties in the nexus networks. Our results pave the way toward realizations of new topological materials with novel transport properties beyond standard Weyl/Dirac semimetals.

cond-mat.mtrl-sci↗

arXiv2018

Microscopic Theory of Spin Toroidization in Periodic Crystals

Using the semiclassical theory of electron dynamics, we derive a gauge-invariant expression for the spin toroidization in a periodical crystal. We show that the spin toroidization is comprised of two contributions: one is due to the configuration of a classical spin array, while the other comes from the coordinate shift of the electron as spin carrier in response to the inhomogeneous magnetic field. We then establish a direct and elengant relation between our spin toroidization and the antisymmetric magnetoelectric polarizability in insulators. Finally, we demonstrate our spin toroidization in a tight-binding model and show that it is a genuine bulk quantity.

cond-mat.mes-hall↗

arXiv2018

Symmorphic intersecting nodal rings in semiconducting layers

The unique properties of topological semimetals have strongly driven efforts to seek for new topological phases and related materials. Here, we identify a critical condition for the existence of linked nodal rings (LNRs) in symmorphic crystals, and propose that three types of LNRs, named as alpha-, beta- and gamma-type, can be obtained by stacking semiconducting layers. Several honeycomb structures are suggested to be topological LNR semimetals, including layered and "hidden" layered structures. Transitions between the three types of LNRs can be driven by external strains. Interesting surface states other than drumhead states are found in these topological materials.

cond-mat.mtrl-sci↗

arXiv2017

Phonon-assisted spin splitting in centrosymmetric crystals

For static crystals it is well known that electronic states are doubly degenerate in their spin degree of freedom in the presence of time reversal and inversion symmetries. This degeneracy can only be lifted by either (i) breaking time reversal symmetry, for example in a ferromagnet, or (ii) breaking inversion symmetry and having spin orbit coupling, for example in the Rashba effect. We propose that spin degeneracy can be lifted in time reversal and inversion symmetric crystals with a combination of lattice vibrations and spin-orbit coupling. We demonstrate this effect in the cubic perovskite CsPbCl$_3$ by performing first principles calculations of the finite temperature band structure, which, in accordance with our prediction, undergoes spin splitting. We also suggest optical and photoemission experiments to examine our predictions. This new understanding dramatically expands the range of materials that can exhibit spin splitting, with potential applications in a variety of technologies such as spintronics and photovoltaics.

cond-mat.mtrl-sci↗

arXiv2017

Composite Weyl nodes stabilized by screw symmetry with and without time reversal

We classify the band degeneracies in 3D crystals with screw symmetry $n_m$ and broken $\mathcal P*\mathcal T$ symmetry, where $\mathcal P$ stands for spatial inversion and $\mathcal T$ for time reversal. The generic degeneracies along symmetry lines are Weyl nodes: Chiral contact points between pairs of bands. They can be single nodes with a chiral charge of magnitude $|χ|=1$ or composite nodes with $|χ|=2$ or $3$, and the possible $χ$ values only depend on the order $n$ of the axis, not on the pitch $m/n$ of the screw. Double Weyl nodes require $n=4$ or 6, and triple nodes require $n=6$. In all cases the bands split linearly along the axis, and for composite nodes the splitting is quadratic on the orthogonal plane. This is true for triple as well as double nodes, due to the presence in the effective two-band Hamiltonian of a nonchiral quadratic term that masks the chiral cubic dispersion. If $\mathcal T$ symmetry is present and $\mathcal P$ is broken there may exist on some symmetry lines Weyl nodes pinned to $\mathcal T$-invariant momenta, which in some cases are unavoidable. In the absence of other symmetries their classification depends on $n$, $m$, and the type of $\mathcal T$ symmetry. With spinless $\mathcal T$ such $\mathcal T$-invariant Weyl nodes are always double nodes, while with spinful $\mathcal T$ they can be single or triple nodes. $\mathcal T$-invariant triples nodes can occur not only on 6-fold axes but also on 3-fold ones, and their in-plane band splitting is cubic, not quadratic as in the case of generic triple nodes. These rules are illustrated by means of first-principles calculations for hcp cobalt, a $\mathcal T$-broken, $\mathcal P$-invariant crystal with $6_3$ symmetry, and for trigonal tellurium and hexagonal NbSi$_2$, which are $\mathcal T$-invariant, $\mathcal P$-broken crystals with 3-fold and 6-fold screw symmetry respectively.

cond-mat.mtrl-sci↗

arXiv2017

Temperature dependence of the bulk Rashba splitting in the bismuth tellurohalides

We study the temperature dependence of the Rashba-split bands in the bismuth tellurohalides BiTe$X$ $(X=$ I, Br, Cl) from first principles. We find that increasing temperature reduces the Rashba splitting, with the largest effect observed in BiTeI with a reduction of the Rashba parameter of $40$% when temperature increases from $0$ K to $300$ K. These results highlight the inadequacy of previous interpretations of the observed Rashba splitting in terms of static-lattice calculations alone. Notably, we find the opposite trend, a strengthening of the Rashba splitting with rising temperature, in the pressure-stabilized topological-insulator phase of BiTeI. We propose that the opposite trends with temperature on either side of the topological phase transition could be an experimental signature for identifying it. The predicted temperature dependence is consistent with optical conductivity measurements, and should also be observable using photoemission spectroscopy, which could provide further insights into the nature of spin splitting and topology in the bismuth tellurohalides.

cond-mat.mtrl-sci↗

arXiv2017

Antiferroelectric topological insulators in orthorhombic $A$MgBi compounds ($A=$ Li, Na, K)

We introduce antiferroelectric topological insulators as a new class of functional materials in which an electric field can be used to control topological order and induce topological phase transitions. Using first principles methods, we predict that several alkali-MgBi orthorhombic members of an $ABC$ family of compounds are antiferroelectric topological insulators. We also show that epitaxial strain and hydrostatic pressure can be used to tune the topological order and the band gap of these $ABC$ compounds. Antiferroelectric topological insulators could enable precise control of topology using electric fields, enhancing the applicability of topological materials in electronics and spintronics.

cond-mat.mtrl-sci↗

arXiv2016

Surface theorem for the Chern-Simons axion coupling

The Chern-Simons axion coupling of a bulk insulator is only defined modulo a quantum of e^2/h. The quantized part of the coupling is uniquely defined for a bounded insulating sample, but it depends on the specific surface termination. Working in a slab geometry and representing the valence bands in terms of hybrid Wannier functions, we show how to determine that quantized part from the excess Chern number of the hybrid Wannier sheets located near the surface of the slab. The procedure is illustrated for a tight-binding model consisting of coupled quantum anomalous Hall layers. By slowly modulating the model parameters, it is possible to transfer one unit of Chern number from the bottom to the top surface over the course of a cyclic evolution of the bulk Hamiltonian. When the evolution of the surface Hamiltonian is also cyclic, the Chern pumping is obstructed by chiral touchings between valence and conduction surface bands.

cond-mat.mtrl-sci↗

arXiv2016

Z2Pack: Numerical Implementation of Hybrid Wannier Centers for Identifying Topological Materials

The intense theoretical and experimental interest in topological insulators and semimetals has established band structure topology as a fundamental material property. Consequently, identifying band topologies has become an important, but often challenging problem, with no exhaustive solution at the present time. In this work we compile a series of techniques, some previously known, that allow for a solution to this problem for a large set of the possible band topologies. The method is based on tracking hybrid Wannier charge centers computed for relevant Bloch states, and it works at all levels of materials modeling: continuous k.p models, tight-binding models and ab initio calculations. We apply the method to compute and identify Chern, Z2 and crystalline topological insulators, as well as topological semimetal phases, using real material examples. Moreover, we provide a numerical implementation of this technique (the Z2Pack software package) that is ideally suited for high-throughput screening of materials databases for compounds with non-trivial topologies. We expect that our work will allow researchers to: (a) identify topological materials optimal for experimental probes, (b) classify existing compounds and (c) reveal materials that host novel, not yet described, topological states.

cond-mat.mtrl-sci↗

arXiv2016

Domain walls and ferroelectric reversal in corundum derivatives

Domain walls are the topological defects that mediate polarization reversal in ferroelectrics, and they may exhibit quite different geometric and electronic structures compared to the bulk. Therefore, a detailed atomic-scale understanding of the static and dynamic properties of domain walls is of pressing interest. In this work, we use first-principles methods to study the structures of $180^{\circ}$ domain walls, both in their relaxed state and along the ferroelectric reversal pathway, in ferroelectrics belonging to the family of corundum derivatives. Our calculations predict their orientation, formation energy, and migration energy, and also identify important couplings between polarization, magnetization, and chirality at the domain walls. Finally, we point out a strong empirical correlation between the height of the domain-wall mediated polarization reversal barrier and the local bonding environment of the mobile $A$ cations as measured by bond valence sums. Our results thus provide both theoretical and empirical guidance to future searches for ferroelectric candidates in materials of the corundum derivative family.

cond-mat.mtrl-sci↗

arXiv2016

Temperature effects in the band structure of topological insulators

We study the effects of temperature on the band structure of the Bi$_2$Se$_3$ family of topological insulators using first-principles methods. Increasing temperature drives these materials towards the normal state, with similar contributions from thermal expansion and from electron-phonon coupling. The band gap changes with temperature reach $0.3$ eV at $600$ K, of similar size to the changes caused by electron correlation. Our results suggest that temperature-induced topological phase transitions should be observable near critical points of other external parameters.

cond-mat.mtrl-sci↗