Showing papers by "David Vanderbilt published in 2015"
TL;DR: In this article, it was shown that a semi-Dirac structure with spin-orbit coupling can be formed by merging of three conventional Dirac points, and that the nontrivial topology with a nonzero Chern number emerges naturally from this kind of semi-drac structure, establishing a general scenario that provides a different route to the formation of Chern insulating states in practical materials systems.
Abstract: By combining first-principles calculations with Wannier-based tight-binding modeling, we demonstrate that a TiO2/VO2 heterostructure that was previously proposed as a prototypical semi-Dirac system becomes a Chern insulator (quantum anomalous Hall insulator) in the presence of spin-orbit coupling. We show that this occurs only when the semi-Dirac structure is of a special type that can be formed by the merging of three conventional Dirac points. Our results reveal how the nontrivial topology with a nonzero Chern number emerges naturally from this kind of semi-Dirac structure, establishing a general scenario that provides a different route to the formation of Chern-insulating states in practical materials systems.
72 citations
TL;DR: In this article, the presence of chiral degeneracies in realistic band structures of ferromagnetic materials was investigated using bcc Fe as a playground, and the authors found ubiquitous Weyl points and demonstrated that Chern numbers can be transferred between Fermi sheets.
Abstract: Using bcc Fe as a playground, the authors investigate the presence of chiral degeneracies in realistic band structures of ferromagnetic materials. They find ubiquitous Weyl points and demonstrate that Chern numbers can be transferred between Fermi sheets by varying the Fermi level or an external parameter such as the magnetization direction.
64 citations
TL;DR: The discovery of a metamagnetic phase transition in a polar antiferromagnet Ni_{3}TeO_{6} that occurs at 52 T is reported, providing new insights on design principles for materials with strong magnetoelectric coupling.
Abstract: We report the discovery of a metamagnetic phase transition in a polar antiferromagnet Ni_{3}TeO_{6} that occurs at 52 T. The new phase transition accompanies a colossal magnetoelectric effect, with a magnetic-field-induced polarization change of 0.3 μC/cm^{2}, a value that is 4 times larger than for the spin-flop transition at 9 T in the same material, and also comparable to the largest magnetically induced polarization changes observed to date. Via density-functional calculations we construct a full microscopic model that describes the data. We model the spin structures in all fields and clarify the physics behind the 52 T transition. The high-field transition involves a competition between multiple different exchange interactions which drives the polarization change through the exchange-striction mechanism. The resultant spin structure is rather counterintuitive and complex, thus providing new insights on design principles for materials with strong magnetoelectric coupling.
53 citations
TL;DR: First-principles calculations predict a ferrimagnetic (udu) ground state, optimal switching path along the c-axis, and transition to a lower energy udu-udd magnetic double cell.
Abstract: Mn(2+)2 Fe(2+)W(6+)O6 , a new polar magnetic phase, adopts the corundum-derived Ni3TeO6 -type structure with large spontaneous polarization (PS) of 67.8 μC cm(-2), complex antiferromagnetic order below ≈75 K, and field-induced first-order transition to a ferrimagnetic phase below ≈30 K. First-principles calculations predict a ferrimagnetic (udu) ground state, optimal switching path along the c-axis, and transition to a lower energy udu-udd magnetic double cell.
49 citations
TL;DR: Yokosuk et al. as discussed by the authors proposed a new method for complex phase materials, which is based on the concept of complex phase phase materials (CPSM), which can be applied to complex phases of complex materials.
Abstract: Michael O. Yokosuk,1 Sergey Artyukhin,2 Amal al-Wahish,1 Xueyun Wang,2 Junjie Yang,3 Zhiqiang Li,4 Sang-Wook Cheong,2,3 David Vanderbilt,2 and Janice L. Musfeldt1,5 1Department of Chemistry, University of Tennessee, Knoxville, Tennessee 37996, USA 2Department of Physics and Astronomy and the Rutgers Center for Emergent Materials, Rutgers, The State University of New Jersey, Piscataway, New Jersey 08854, USA 3Laboratory for Pohang Emergent Materials and Max Plank POSTECH Center for Complex Phase Materials, Pohang University of Science and Technology, Pohang 790-784, Korea 4National High Magnetic Field Laboratory, Tallahassee, Florida 32310, USA 5Department of Physics and Astronomy, University of Tennessee, Knoxville, Tennessee 37996, USA (Received 26 July 2015; published 8 October 2015)
30 citations
TL;DR: In this article, the surface polarization is formulated in terms of a Berry phase, with the hybrid Wannier representation providing a natural basis for study of the effect of surface polarization.
Abstract: The term ``surface polarization'' is introduced to describe the in-plane polarization existing at the surface of an insulating crystal when the in-plane surface inversion symmetry is broken. Here, the surface polarization is formulated in terms of a Berry phase, with the hybrid Wannier representation providing a natural basis for study of this effect. Tight-binding models are used to demonstrate how the surface polarization reveals itself via the accumulation of charges at the corners and edges for a two dimensional rectangular lattice and for GaAs respectively.
26 citations
TL;DR: In this paper, the magnetic charges and spin-lattice and spinelectronic ME constants in hexagonal manganites and ferrites were analyzed. But the conditions under which exchange striction leads to an enhanced Z m values and anomalously large in-plane spin-Lattice ME eects were not discussed.
Abstract: noncollinear spins induces much larger magnetic charges than those that depend on spin-orbit coupling. The hexagonal manganites RMnO3 and ferrites RFeO3 (R = Sc, Y, In, Ho-Lu) exhibit strong couplings between electric, magnetic and structural degrees of freedom, with the transition-metal ions in the basal plane antiferromagnetically coupled through super-exchange so as to form a 120 noncollinear spin arrangement. Here we present a theoretical study of the magnetic charges, and of the spin-lattice and spin-electronic ME constants, in these hexagonal manganites and ferrites, clarifying the conditions under which exchange striction leads to an enhancedZ m values and anomalously large in-plane spin-lattice ME eects.
25 citations
TL;DR: In this article, the authors derived a new formula for the Chern-Simons axion coupling, expressing it as an integral involving Berry curvatures and potentials defined on the Wannier charge center sheets.
Abstract: We study the adiabatic pumping of the Chern-Simons axion (CSA) coupling along a parametric loop characterized by a nonzero second Chern number C^{(2)} from the viewpoint of the hybrid Wannier representation, in which the Wannier charge centers are visualized as sheets defined over a projected 2D Brillouin zone. We derive a new formula for the CSA coupling, expressing it as an integral involving Berry curvatures and potentials defined on the Wannier charge center sheets. We show that a loop characterized by a nonzero C^{(2)} requires a series of sheet-touching events at which 2π quanta of Berry curvature are passed from sheet to sheet, in such a way that e^{2}/h units of CSA coupling are pumped by a lattice vector by the end of the cycle. We illustrate these behaviors via explicit calculations on a model tight-binding Hamiltonian and discuss their implications.
22 citations
Journal Article•
TL;DR: A new formula is derived for the CSA coupling, expressing it as an integral involving Berry curvatures and potentials defined on the Wannier charge center sheets, in such a way that e^{2}/h units of C SA coupling are pumped by a lattice vector by the end of the cycle.
Abstract: We study the adiabatic pumping of the Chern-Simons axion (CSA) coupling along a parametric loop characterized by a nonzero second Chern number C^{(2)} from the viewpoint of the hybrid Wannier representation, in which the Wannier charge centers are visualized as sheets defined over a projected 2D Brillouin zone. We derive a new formula for the CSA coupling, expressing it as an integral involving Berry curvatures and potentials defined on the Wannier charge center sheets. We show that a loop characterized by a nonzero C^{(2)} requires a series of sheet-touching events at which 2π quanta of Berry curvature are passed from sheet to sheet, in such a way that e^{2}/h units of CSA coupling are pumped by a lattice vector by the end of the cycle. We illustrate these behaviors via explicit calculations on a model tight-binding Hamiltonian and discuss their implications.
14 citations
TL;DR: In this article, the authors propose a method for calculating Chern-Simons orbital magnetoelectric coupling, conventionally parametrized in terms of a phase angle, which can be expressed as integrals over 3D, 2D, and 1D manifolds.
Abstract: We propose a method for calculating Chern-Simons orbital magnetoelectric coupling, conventionally parametrized in terms of a phase angle $\ensuremath{\theta}$. According to previous theories, $\ensuremath{\theta}$ can be expressed as a three-dimensional (3D) Brillouin-zone (BZ) integral of the Chern-Simons 3-form defined in terms of the occupied Bloch functions. Such an expression is valid only if a smooth and periodic gauge has been chosen in the entire Brillouin zone, and even then, convergence with respect to the $\mathbf{k}$-space mesh density can be difficult to obtain. In order to solve this problem, we propose to relax the periodicity condition in one direction (say, the ${k}_{z}$ direction) so that a gauge discontinuity is introduced on a two-dimensional (2D) $\mathbf{k}$ plane normal to ${k}_{z}$. The total $\ensuremath{\theta}$ response then has contributions from both the integral of the Chern-Simons 3-form over the 3D bulk BZ and the gauge discontinuity expressed as a 2D integral over the $\mathbf{k}$ plane. Sometimes, the boundary plane may be further divided into subregions by 1D ``vortex loops'' which make a third kind of contribution to the total $\ensuremath{\theta}$, expressed as a combination of Berry phases around the vortex loops. The total $\ensuremath{\theta}$ thus consists of three terms which can be expressed as integrals over 3D, 2D, and 1D manifolds. When time-reversal symmetry is present and the gauge in the bulk BZ is chosen to respect this symmetry, both the 3D and 2D integrals vanish; the entire contribution then comes from the vortex-loop integral, which is either 0 or $\ensuremath{\pi}$ corresponding to the ${\mathbb{Z}}_{2}$ classification of 3D time-reversal-invariant insulators. We demonstrate our method by applying it to the Fu-Kane-Mele model with an applied staggered Zeeman field.
4 citations
Posted Content•
TL;DR: In this paper, the authors provided an overview of the current first-principles perspective on flexoelectric effects in crystalline solids and provided a theoretical analysis of surface effects.
Abstract: In this Chapter we provide an overview of the current first-principles perspective on flexoelectric effects in crystalline solids. We base our theoretical formalism on the long-wave expansion of the electrical response of a crystal to an acoustic phonon perturbation. In particular, we recover the known expression for the piezoelectric tensor from the response at first order in wavevector ${\bf q}$, and then obtain the flexoelectric tensor by extending the formalism to second order in $\bf q$. We put special emphasis on the issue of surface effects, which we first analyze heuristically, and then treat more carefully by presenting a general theory of the microscopic response to an arbitrary inhomogeneous strain. We demonstrate our approach by presenting a full calculation of the flexoelectric response of a SrTiO$_3$ film, where we point out an unusually strong dependence of the bending-induced open-circuit voltage on the choice of surface termination. Finally, we briefly discuss some remaining open issues concerning the methodology and some promising areas for future research.