Showing papers by "David Vanderbilt published in 2014"

Weyl semimetals from noncentrosymmetric topological insulators

Abstract: We study the problem of phase transitions from three-dimensional topological to normal insulators without inversion symmetry. In contrast with the conclusions of some previous work, we show that a Weyl semimetal always exists as an intermediate phase regardless of any constriant from lattice symmetries, although the interval of the critical region is sensitive to the choice of path in the parameter space and can be very narrow. We demonstrate this behavior by carrying out first-principles calculations on the noncentrosymmetric topological insulators LaBiTe3 and LuBiTe3 and the trivial insulator BiTeI. We find that a robust Weyl-semimetal phase exists in the solid solutions LaBi1−xSbxTe3 and LuBi1−xSbxTe3 for x ≈ 38.5%‐41.9% and x ≈40.5%‐45.1%, respectively. A low-energy effective model is also constructed to describe the critical behavior in these two materials. In BiTeI, a Weyl semimetal also appears with applied pressure, but only within a very small pressure range, which may explain why it has not been experimentally observed.

Wannier center sheets in topological insulators

Abstract: We argue that various kinds of topological insulators (TIs) can be insightfully characterized by an inspection of the charge centers of the hybrid Wannier functions, defined as the orbitals obtained by carrying out a Wannier transform on the Bloch functions in one dimension while leaving them Bloch-like in the other two. From this procedure, one can obtain the Wannier charge centers (WCCs) and plot them in the two-dimensional projected Brillouin zone. We show that these WCC sheets contain the same kind of topological information as is carried in the surface energy bands, with the crucial advantage that the topological properties of the bulk can be deduced from bulk calculations alone. The distinct topological behaviors of these WCC sheets in trivial, Chern, weak, strong, and crystalline TIs are first illustrated by calculating them for simple tight-binding models. We then present the results of first-principles calculations of the WCC sheets in the trivial insulator ${\mathrm{Sb}}_{2}$Se${}_{3}$, the weak TI KHgSb, and the strong TI ${\mathrm{Bi}}_{2}$Se${}_{3}$, confirming the ability of this approach to distinguish between different topological behaviors in an advantageous way.

Non-hysteretic colossal magnetoelectricity in a collinear antiferromagnet

Abstract: The manipulation of magnetic ordering with applied electric fields is of pressing interest for new magnetoelectric devices and information storage applications. Recently, such magnetoelectric control was realized in multiferroics. However, their magnetoelectric switching is often accompanied by significant hysteresis, resulting from a large barrier, separating different ferroic states. Hysteresis prevents robust switching, unless the applied field overcomes a certain value (coercive field). Here we address the role of a switching barrier on magnetoelectric control, and identify a material, collinear antiferromagnetic and pyroelectric Ni3TeO6, in which magnetoelectric switching occurs without hysteresis. The barrier between two magnetic states in the vicinity of a spin–flop transition is almost flat, and thus small changes in external electric/magnetic fields allow to switch the ferroic state through an intermediate state in a continuous manner, resulting in a colossal magnetoelectric response. This colossal magnetoelectric effect resembles the large piezoelectric effect at the morphotropic phase boundary in ferroelectrics. Usually magnetoelectric switching is accompanied by hysteresis, which is a consequence of the large barrier between different magnetoelectric states. Here, the authors show that in the antiferromagnet Ni3TeO6magnetoelectric switching of magnetization as well as polarization occur without hysteresis.

Dimerization-Induced Cross-Layer Quasi-Two-Dimensionality in Metallic IrTe 2

Abstract: The crystal structure of layered metal ${\mathrm{IrTe}}_{2}$ is determined using single-crystal x-ray diffraction. At $T=220\text{ }\text{ }\mathrm{K}$, it exhibits Ir and Te dimers forming a valence-bond crystal. Electronic structure calculations reveal an intriguing quasi-two-dimensional electronic state, with planes of reduced density of states cutting diagonally through the Ir and Te layers. These planes are formed by the dimers exhibiting a signature of covalent bonding character development. Evidence for significant charge disproportionation among the dimerized and nondimerized Ir (charge order) is presented. We argue that the structural transition is driven by the Ir dimerization and bonding, while electronic correlations (dynamical mean field theory corrections to density functional theory) and spin orbit coupling play a secondary role.

Hyperferroelectrics: Proper Ferroelectrics with Persistent Polarization

Abstract: All known proper ferroelectrics are unable to polarize normal to a surface or interface if the resulting depolarization field is unscreened, but there is no fundamental principle that enforces this behavior. In this work, we introduce hyperferroelectrics, a new class of proper ferroelectrics which polarize even when the depolarization field is unscreened, this condition being equivalent to instability of a longitudinal optic mode in addition to the transverse-optic-mode instability characteristic of proper ferroelectrics. We use first-principles calculations to show that several recently discovered hexagonal ferroelectric semiconductors have this property, and we examine its consequences both in the bulk and in a superlattice geometry.

Wannier Center Sheets in Topological Insulators

Abstract: We argue that various kinds of topological insulators (TIs) can be insightfully characterized by an inspection of the charge centers of the hybrid Wannier functions, defined as the orbitals obtained by carrying out a Wannier transform on the Bloch functions in one dimension while leaving them Bloch-like in the other two. From this procedure, one can obtain the Wannier charge centers (WCCs) and plot them in the two-dimensional projected Brillouin zone. We show that these WCC sheets contain the same kind of topological information as is carried in the surface energy bands, with the crucial advantage that the topological properties of the bulk can be deduced from bulk calculations alone. The distinct topological behaviors of these WCC sheets in trivial, Chern, weak, strong, and crystalline TIs are first illustrated by calculating them for simple tight-binding models. We then present the results of first-principles calculations of the WCC sheets in the trivial insulator ${\mathrm{Sb}}_{2}$Se${}_{3}$, the weak TI KHgSb, and the strong TI ${\mathrm{Bi}}_{2}$Se${}_{3}$, confirming the ability of this approach to distinguish between different topological behaviors in an advantageous way.

Effective J = 1/2 insulating state in Ruddlesden-Popper iridates: An LDA+DMFT study

Abstract: Using ab initio methods for correlated electrons in solids, we investigate the metal-insulator transition across the Ruddlesden-Popper (RP) series of iridates and explore the robustness of the ${J}_{\mathrm{eff}}=1/2$ state against band effects due to itineracy, tetragonal distortion, octahedral rotation, and Coulomb interaction. We predict the effects of epitaxial strain on the optical conductivity, magnetic moments, and ${J}_{\mathrm{eff}}=1/2$ ground-state wave functions in the RP series. To describe the solution of the many-body problem in an intuitive picture, we introduce a concept of energy-dependent atomic states, which strongly resemble the atomic ${J}_{\mathrm{eff}}=1/2$ states but with coefficients that are energy or time dependent. We demonstrate that the deviation from the ideal ${J}_{\mathrm{eff}}=1/2$ state is negligible at short time scales for both single- and double-layer iridates, while it becomes quite significant for ${\mathrm{Sr}}_{3}{\mathrm{Ir}}_{2}{\mathrm{O}}_{7}$ at long times and low energy. Interestingly, ${\mathrm{Sr}}_{2}{\mathrm{IrO}}_{4}$ is positioned very close to the $SU(2)$ limit, with only $\ensuremath{\sim}3%$ deviation from the ideal ${J}_{\mathrm{eff}}=1/2$ situation.

Chern insulator at a magnetic rocksalt interface

Abstract: Considerable efforts have recently been devoted to the experimental realization of a two-dimensional Chern insulator, i.e., a system displaying a quantum anomalous Hall effect. However, existing approaches such as those based on magnetic doping of topological-insulator thin films have resulted in small band gaps, restricting the effect to low temperatures. We use first-principles calculations to demonstrate that an interface between thin films of the topologically trivial ferromagnetic insulators EuO and GdN can result in a band inversion and a nonzero Chern number. Both materials are stoichiometric and the interface is nonpolar and lattice-matched, which should allow this interface to be achievable experimentally. We show that the band structure can be tuned by layer thickness or epitaxial strain, and can result in Chern insulators with gaps of over 0.1 eV.

Comment on "weyl fermions and the anomalous Hall effect in metallic ferromagnets"

Abstract: We point out that, contrary to an assertion by Chen et al. [Phys. Rev. B 88, 125110 (2013)], the nonquantized part of the intrinsic anomalous Hall conductivity can indeed be expressed as a Fermi-surface property even when Weyl points are present in the band structure.

Quantum anomalous Hall phase in (001) double-perovskite monolayers via intersite spin-orbit coupling

Abstract: Using tight-binding models and first-principles calculations, we demonstrate the possibility to achieve a quantum anomalous Hall (QAH) phase on a two-dimensional square lattice, which can be realized in monolayers of double perovskites. We show that effective intersite spin-orbit coupling between ${e}_{g}$ orbitals can be induced perturbatively, giving rise to a QAH state. Moreover, the effective spin-orbit coupling can be enhanced by octahedral rotations. Based on first-principles calculations, we propose that this type of QAH state could be realized in ${\mathrm{La}}_{2}{\mathrm{MnIrO}}_{6}$ monolayers, with the size of the gap as large as 26 meV in the ideal case. We observe that the electronic structure is sensitive to structural distortions, and that an enhanced Hubbard $U$ tends to stabilize the nontrivial gap.

Spin-orbit spillage as a measure of band inversion in insulators

Abstract: We propose a straightforward and effective approach for quantifying the band inversion induced by spin-orbit coupling in band insulators. In this approach we define a quantity as a function of wave vector in the Brillouin zone (BZ) measuring the mismatch, or ``spillage,'' between the occupied states of a system with and without SOC. Plots of the spillage throughout the BZ provide a ready indication of the number and location of band inversions driven by SOC. To illustrate the method, we apply this approach to the two-band Dirac model, the 2D Kane-Mele model, a 2D Bi bilayer with applied Zeeman field, and to first-principles calculations of some 3D materials including both trivial and ${\mathbb{Z}}_{2}$ topological insulators. We argue that the distribution of spillage in the BZ is closely related to the topological indices in these systems. Our approach provides a fresh perspective for understanding topological character in band theory, and should be helpful in searching for new materials with nontrivial band topology.

Dynamical magnetic charges and linear magnetoelectricity

Abstract: A theoretical study of the magnetic charge in two different magnetoelectric materials gives an explanation of the origin of the magnetoelectric effect in each material, and offers insight on the mechanisms that could enhance it.

Non-hysteretic colossal magnetoelectricity in a collinear antiferromagnet

Abstract: The manipulation of magnetic ordering with applied electric fields is of pressing interest for new magnetoelectric devices and information storage applications. Recently, such magnetoelectric control was realized in multiferroics. However, their magnetoelectric switching is often accompanied by significant hysteresis, resulting from a large barrier, separating different ferroic states. Hysteresis prevents robust switching, unless the applied field overcomes a certain value (coercive field). Here we address the role of a switching barrier on magnetoelectric control, and identify a material, collinear antiferromagnetic and pyroelectric Ni3TeO 6, in which magnetoelectric switching occurs without hysteresis. The barrier between two magnetic states in the vicinity of a spin–flop transition is almost flat, and thus small changes in external electric/magnetic fields allow to switch the ferroic state through an intermediate state in a continuous manner, resulting in a colossal magnetoelectric response. This colossal magnetoelectric effect resembles the large piezoelectric effect at the morphotropic phase boundary in ferroelectrics.