arXiv subjects

Kevin F. Garrity

Publications and source records attributed to Kevin F. Garrity.

30 records · Page 2Linked to original sources

arXiv2016

High-throughput first principles search for new ferroelectrics

We use a combination of symmetry analysis and high-throughput density functional theory calculations to search for new ferroelectric materials. We use two search strategies to identify candidate materials. In the first strategy, we start with non-polar materials and look for unrecognized energy-lowering polar distortions. In the second strategy, we consider polar materials and look for related higher symmetry structures. In both cases, if we find new structures with the correct symmetries that are also close in energy to experimentally known structures, then the material is likely to be switchable in an external electric field, making it a candidate ferroelectric. We find sixteen candidate materials, with variety of properties that are rare in typical ferroelectrics, including large polarization, hyperferroelectricity, antiferroelectricity, and multiferroism.

cond-mat.mtrl-sci↗

arXiv2016

First principles search for $n$-type oxide, nitride, and sulfide thermoelectrics

Oxides have many potentially desirable characteristics for thermoelectric applications, including low cost and stability at high temperatures, but thus far there are few known high $zT$ $n$-type oxide thermoelectrics. In this work, we use high-throughput first principles calculations to screen transition metal oxides, nitrides, and sulfides for candidate materials with high power factors and low thermal conductivity. We find a variety of promising materials, and we investigate these materials in detail in order to understand the mechanisms that cause them to have high power factors. These materials all combine a high density of states near the Fermi level with dispersive bands, reducing the trade-off between the Seebeck coefficient and the electrical conductivity, but they do so for several different reasons. In addition, our calculations indicate that many of our candidate materials have low thermal conductivity.

cond-mat.mtrl-sci↗

arXiv2016

Flux states and topological phases from spontaneous time-reversal symmetry breaking in CrSi(Ge)Te$_3$-based systems

We study adatom-covered single layers of CrSiTe$_3$ and CrGeTe$_3$ using first-principles calculations based on hybrid functionals. We find that the insulating ground state of a monolayer of La (Lu) deposited on single-layer CrSiTe$_3$ (CrGeTe$_3$) carries spontaneously generated current loops around the Cr sites, These "flux states" induce antiferromagnetically ordered orbital moments on the Cr sites and are also associated with nontrivial topological properties. The calculated Chern numbers for these systems are predicted to be $\pm 1$ even in the absence of spin-orbit coupling, with sizable gaps on the order of 100 meV. The flux states and the associated topological phases result from spontaneous time-reversal symmetry breaking due to the presence of nonlocal Coulomb interactions.

cond-mat.mtrl-sci↗

arXiv2016

Intertwined Rashba, Dirac and Weyl Fermions in Hexagonal Hyperferroelectrics

By means of density functional theory based calculations, we study the role of spin-orbit coupling in the new family of ABC hyperferroelectrics [Phys. Rev. Lett. 112, 127601 (2014)]. We unveil an extremely rich physics strongly linked to ferroelectric properties, ranging from the electric control of bulk Rashba effect to the existence of a three dimensional topological insulator phase, with concomitant topological surface states even in the ultrathin film limit. Moreover, we predict that the topological transition, as induced by alloying, is followed by a Weyl semi-metal phase of finite concentration extension, which is robust against disorder, putting forward hyperferroelectrics as promising candidates for spin-orbitronic applications.

cond-mat.mtrl-sci↗

arXiv2014

Antiferroelectricity in thin film ZrO2 from first principles

Density functional calculations are performed to investigate the experimentally-reported field-induced phase transition in thin-film ZrO2 (J. Muller et al., Nano. Lett. 12, 4318). We find a small energy difference of ~ 1 meV/f.u. between the nonpolar tetragonal and polar orthorhombic structures, characteristic of antiferroelectricity. The requisite first-order transition between the two phases, which atypically for antiferroelectrics have a group-subgroup relation, results from coupling to other zone-boundary modes, as we show with a Landau-Devonshire model. Tetragonal ZrO2 is thus established as a previously unrecognized lead-free antiferroelectric with excellent dielectric properties and compatibility with silicon. In addition, we demonstrate that a ferroelectric phase of ZrO2 can be stabilized through epitaxial strain, and suggest an alternative stabilization mechanism through continuous substitution of Zr by Hf.

cond-mat.mtrl-sci↗

arXiv2014

Chern insulator at a magnetic rocksalt interface

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 non-zero Chern number. Both materials are stoichiometric and the interface is non-polar 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.

cond-mat.mtrl-sci↗

arXiv2013

Wannier Center Sheets in Topological Insulators

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 Sb$_2$Se$_3$, the weak TI KHgSb, and the strong TI Bi$_2$Se$_3$, confirming the ability of this approach to distinguish between different topological behaviors in an advantageous way.

cond-mat.mes-hall↗

arXiv2013

Hyperferroelectrics: proper ferroelectrics with persistent polarization

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.

cond-mat.mtrl-sci↗

arXiv2013

Pseudopotentials for high-throughput DFT calculations

The increasing use of high-throughput density-functional theory (DFT) calculations in the computational design and optimization of materials requires the availability of a comprehensive set of soft and transferable pseudopotentials. Here we present design criteria and testing results for a new open-source "GBRV" ultrasoft pseudopotential library that has been optimized for use in high-throughput DFT calculations. We benchmark the GBRV potentials, as well as two other pseudopotential sets available in the literature, to all-electron calculations in order to validate their accuracy. The results allow us to draw conclusions about the accuracy of modern pseudopotentials in a variety of chemical environments.

cond-mat.mtrl-sci↗

arXiv2012

Chern insulators from heavy atoms on magnetic substrates

We propose searching for Chern insulators by depositing atomic layers of elements with large spin-orbit coupling (e.g., Bi) on the surface of a magnetic insulator. We argue that such systems will typically have isolated surface bands with non-zero Chern numbers. If these overlap in energy, a metallic surface with large anomalous Hall conductivity (AHC) will result; if not, a Chern-insulator state will typically occur. Thus, our search strategy reduces to looking for examples having the Fermi level in a global gap extending across the entire Brillouin zone. We verify this search strategy and identify several candidate systems by using first-principles calculations to compute the Chern number and AHC of a large number of such systems on MnTe, MnSe, and EuS surfaces. Our search reveals several promising Chern insulators with gaps of up to 140\,meV.

cond-mat.mtrl-sci↗

arXiv2012

Orthorhombic $ABC$ semiconductors as antiferroelectrics

We use a first-principles rational-design approach to identify a previously-unrecognized class of antiferroelectric materials in the $Pnma$ MgSrSi structure type. The MgSrSi structure type can be described in terms of antipolar distortions of the nonpolar $P6_{3}/mmc$ ZrBeSi structure type, and we find many members of this structure type are close in energy to the related polar $P6_{3}mc$ LiGaGe structure type, which includes many members we predict to be ferroelectric. We highlight known $ABC$ combinations in which this energy difference is comparable to the antiferroelectric-ferroelectric switching barrier of PbZrO$_{3}$. We calculate structural parameters and relative energies for all three structure types, both for reported and as-yet hypothetical representatives of this class. Our results provide guidance for the experimental realization and further investigation of high-performance materials suitable for practical applications.

cond-mat.mtrl-sci↗

arXiv2012

Hexagonal $ABC$ as semiconducting ferroelectrics

We use a first-principles rational-design approach to identify a previously-unrecognized class of ferroelectric materials in the $P63mc$ LiGaGe structure type. We calculate structural parameters, polarization and ferroelectric well depths both for reported and as-yet hypothetical representatives of this class. Our results provide guidance for the experimental realization and further investigation of high-performance materials suitable for practical applications.

cond-mat.mtrl-sci↗