arXiv subjects

Warren E. Pickett

Publications and source records attributed to Warren E. Pickett.

At least 55 records · Page 3Linked to original sources

arXiv2014

Dielectric Response of Electron-doped Ionic Superconductor Li$_x$ZrNCl

When electron doped, the layered transition metal nitrides ${\cal T}$NCl (${\cal T}$ = group IVB transition metal ion) become impressive superconductors with critical temperature T$_c$= 15-26K. Here we take the most studied member, ZrNCl, as a representative and calculate the dielectric response $ε(ω)$ versus frequency and concentration of doped electronic carriers. The static dielectric constant $ε_{\infty}$=5 is reproduced extremely well. We establish that the differences between rigid band modeling and virtual crystal treatment are small, and compare also with actual lithium doping using supercells. We obtain the variations upon changing the doping level of the reflectivity and energy loss function as well, many of which are found not to be correlated with the observed (non)variation of T$_c(x)$. The main plasmon peaks appear where the electron gas model suggests, in the range 1.2-2.0 eV for $x$ varying from 0.16 to 0.50.

cond-mat.supr-con↗

arXiv2014

Confinement-driven transitions between topological and Mott phases in (LaNiO3)$_N$/(LaAlO3)$_M$(111) superlattices

A set of broken symmetry two-dimensional ground states are predicted in (111)-oriented (LaNiO$_3$)$_N$/(LaAlO$_3$)$_M$ ($N$/$M$) superlattices, based on density functional theory (DFT) calculations including a Hubbard $U$ term. An unanticipated Jahn-Teller distortion with $d_{z^2}$ orbital polarization and a FM Mott insulating (and multiferroic) phase emerges in the double perovskite (1/1), that shows strong susceptibility to strain-controlled orbital engineering. The LaNiO$_3$ bilayer with graphene topology has a switchable multiferroic (ferromagnetic (FM) and ferroelectric) insulating ground state with inequivalent Ni sites. Beyond $N=3$ the confined LaNiO$_3$ slab undergoes a metal-to-insulator transition through a half-semimetallic phase with conduction originating from the interfaces. Antiferromagnetic arrangements allow combining motifs of the bilayer and single trigonal layer band structures in designed artificial mixed phases.

cond-mat.str-el↗

arXiv2013

Massive symmetry breaking in LaAlO$_3$/SrTiO$_3$(111) quantum wells: a three-orbital, strongly correlated generalization of graphene

Density functional theory calculations with an on-site Coulomb repulsion term (GGA+U method) reveal competing ground states in (111) oriented (LaAlO$_3$)$_M$/(SrTiO$_3$)$_N$ superlattices with n-type interfaces, ranging from spin, orbital polarized, Dirac point Fermi surface to charge ordered flat band phases. These are steered by the interplay of (i) Hubbard U, (ii) SrTiO$_3$ quantum well thickness and (iii) crystal field spitting tied to in-plane strain. In the honeycomb lattice bilayer case N=2 under tensile strain inversion symmetry breaking drives the system from a ferromagnetic Dirac point (massless Weyl semimetal) to a charge ordered multiferroic (ferromagnetic and ferroelectric) flat band massive (insulating) phase. With increasing SrTiO$_3$ quantum well thickness an insulator-to-metal transition occurs.

cond-mat.str-el↗

arXiv2012

Formal Valence, $d$ Occupation, and Charge-Order Transitions

While the formal valence and charge state concepts have been tremendously important in materials physics and chemistry, their very loose connection to actual charge leads to uncertainties in modeling behavior and interpreting data. We point out, taking several transition metal oxides (La$_2$VCuO$_6$, YNiO$_3$, CaFeO$_3$, AgNiO$_2$, V$_4$O$_7$) as examples, that while dividing the crystal charge into atomic contributions is an ill-posed activity, the 3d occupation of a cation (and more particularly, differences) is readily available in first principles calculations. We discuss these examples, which include distinct charge states and charge-order (or disproportionation) systems, where different "charge states" of cations have identical 3d orbital occupation. Implications for theoretical modeling of such charge states and charge-ordering mechanisms are discussed.

cond-mat.str-el↗

arXiv2012

Phenomenology of a semi-Dirac semi-Weyl semi-metal

We extend the study of fermionic particle-hole symmetric semi-Dirac (alternatively, semi-Weyo) dispersion of quasiparticles, $\varepsilon_K = \pm \sqrt{(k_x^2/2m)^2 + (vk_y)^2)} = \pm \varepsilon_0 \sqrt{K_x^4 + K_y^2}$ in dimensionless units, discovered computationally in oxide heterostructures by Pardo and collaborators. This unique system a highly anisotropic sister phase of both (symmetric) graphene and what has become known as a Weyl semimetal, with $ ^{1/2} \approx v$ independent of energy, and $ ^{1/2} \propto m^{-1/2}\sqrt{\varepsilon}$ being very strongly dependent on energy ($\varepsilon$) and depending only on the effective mass $m$. Each of these systems is distinguished by bands touching (alternatively, crossing) at a point Fermi surface, with one consequence being that for this semi-Dirac system the ratio $|χ_{orb}/χ_{sp}|$ of orbital to spin susceptibilities diverges at low doping. We extend the study of the low-energy behavior of the semi-Dirac system, finding the plasmon frequency to be highly anisotropic while the Hall coefficient scales with carrier density in the usual manner. The Faraday rotation behavior is also reported. For Klein tunneling for normal incidence on an arbitrarily oriented barrier, the kinetic energy mixes both linear (massless) and quadratic (massive) contributions depending on orientation. Analogous to graphene, perfect transmission occurs under resonant conditions, except for the specific orientation that eliminates massless dispersion. Comparisons of the semi-Dirac system are made throughout with both other types of point Fermi surface systems.

cond-mat.mes-hall↗

arXiv2011

Pressure-induced metal-insulator and spin-state transition in low-valence layered nickelates

Ab initio calculations predict a metal-insulator transition at zero temperature to occur in La$_4$Ni$_3$O$_8$ at moderate pressures as a result of a pressure-induced spin-state transition. The spin-state transition that is seen at 105 K at ambient pressure from a low-temperature high-spin state to a high-temperature low-spin state has been observed to be shifted to lower temperatures as pressure is applied. From our calculations we find that a smaller unit cell volume favors the metallic low-spin state, which becomes more stable at 5 GPa. Similar physics should take place in the related compound La$_3$Ni$_2$O$_6$, but on a different energy scale, which may account for why the transition has not been observed in this material.

cond-mat.str-el↗

arXiv2011

Tunable 2-dimensional/3-dimensional electron gases by submonolayer La doping of SrTiO_{3}

First-principles calculation was used to study the structural and electronic features of the low dimensional oxide structure, SrTiO_{3}/Sr_{1-x}La_{x}TiO_{3} (x=0.25) superlattices, constructed by submonolayer low dimensional La doping into SrTiO_{3}. We demonstrate a dimensionality crossover from three-dimensional to two-dimensional (3D \to 2D) electronic behavior in the system. Two types of carriers, one confined to 2D and the other extended, exhibit distinct tunable (3D \to 2D) transport characteristics that will enable the study of many properties (e.g., superconductivity) through this change in dimensionality.

cond-mat.str-el↗

arXiv2011

Metal-insulator transition in layered nickelates La$_3$Ni$_2$O$_{7-δ}$ ($δ$=0.0, 0.5, 1)

Three low-valence layered nickelates with general formula La$_3$Ni$_2$O$_{7-δ}$ ($δ$=0.0, 0.5, 1) are studied by ab initio techniques. Both the insulating and metallic limits are analyzed, together with the compound at the Mott transition ($δ$= 0.5; Ni$^{2+}$), that shows insulating behavior, with all Ni atoms in a S=1 high-spin state. The compound in the $δ$= 1 limit (La$_3$Ni$_2$O$_6$), with mean formal valence Ni$^{1.5+}$ and hence nominally metallic, nevertheless shows a correlated {\it molecular} insulating state, produced by the quantum confinement of the NiO$_2$ bilayers and the presence of mainly d$_{z^2}$ bands (bonding-antibonding split) around the gap. The metallic compound shows a larger bandwidth of the e$_g$ states that can sustain the experimentally observed paramagnetic metallic properties. The evolution of the in-plane antiferromagnetic coupling with the oxygen content is discussed, and also the similarities of this series of compounds with the layered superconducting cuprates.

cond-mat.str-el↗

arXiv2011

Evaluation of Compensated Magnetism in La$_2$VCuO$_6$: Exploration of Charge States

We present an ab initio study of double perovskite La$_2$VCuO$_6$, which was one of the earliest compounds suggested as a potential compensated half-metal. Two charge and spin configurations close in energy have been identified. (i) The originally envisioned spin-compensated V$^{4+}$:d$^1$/Cu$^{2+}$:d$^1$ configuration is comprised of antialigned S=1/2 cations. This state is a spin-compensated half-metal for moderate values of U (the on-site Coulomb repulsion strength on the metal ions) and is insulating for larger values of U. (ii) An unanticipated non-magnetic solution V$^{5+}$:d$^0$/Cu$^{+}$:d$^{10}$ consists of an empty- and a full-shell ion, both spherically symmetric, that leads to a band insulator. This ionic band insulator is calculated to be the ground state at small and moderate values of U. The different distortions of the perovskite structure that occur for each state are central in determining the energy differences. Treating the Cu$^{2+}$ Jahn-Teller distortion self-consistently is particularly important for the magnetic solution.

cond-mat.str-el↗

arXiv2011

Electronic Correlation and Transport Properties of Nuclear Fuel Materials

Actinide elements, such as uranium and plutonium, and their compounds are best known as nuclear materials. When engineering optimal fuel materials for nuclear power, important thermophysical properties to be considered are melting point and thermal conductivity. Understanding the physics underlying transport phenomena due to electrons and lattice vibrations in actinide systems is a crucial step toward the design of better fuels. Using first principle LDA+DMFT method, we conduct a systematic study on the correlated electronic structures and transport properties of select actinide carbides, nitrides, and oxides, many of which are nuclear fuel materials. We find that different mechanisms, electrons--electron and electron--phonon interactions, are responsible for the transport in the uranium nitride and carbide, the best two fuel materials due to their excellent thermophysical properties. Our findings allow us to make predictions on how to improve their thermal conductivities.

cond-mat.str-el↗

arXiv2010

Spin and Charge Fluctuations in the $α$-structure Layered Nitride Superconductors

To explore conditions underlying the superconductivity in electron-doped TiNCl where T$_c$ = 16 K, we calculate the electronic structure, Wannier functions and spin and charge susceptibilities using first-principles density functional theory. TiNCl is the first high-temperature superconductor discovered in the $α$-structure of the layered transition-metal nitride family MNCl (M=Ti, Zr, Hf). We construct a tight-binding model based on Wannier functions derived from the band structure, and consider explicit electronic interactions in a multi-band Hubbard Hamiltonian, where the interactions are treated with the random phase approximation (RPA) to calculate spin and charge susceptibility. The results show that, consistent with TiNCl being a nonmagnetic material, spin fluctuations do not dominate over charge fluctuations and both may have comparable impact on the properties of the doped system.

cond-mat.supr-con↗

arXiv2010

Quantum Confinement Induced Molecular Mott Insulating State in La$_4$Ni$_3$O$_8$

The recently synthesized layered nickelate La$_4$Ni$_3$O$_8$, with its cuprate-like NiO$_2$ layers, seemingly requires a Ni1 ($d^8$)+2Ni2 ($d^9$) charge order, together with strong correlation effects, to account for its insulating behavior. Using density functional methods including strong intra-atomic repulsion (Hubbard U), we obtain an insulating state via a new mechanism: {\it without charge order}, Mott insulating behavior arises based on quantum coupled, spin-aligned Ni2-Ni1-Ni2 $d_{z^2}$ states across the trilayer (rather than based on atomic states), with antiferromagnetic ordering within layers. The weak and frustrated magnetic coupling between cells may account for the small spin entropy that is removed at the Néel transition at 105 K and the lack of any diffraction peak at the Néel point.

cond-mat.str-el↗

arXiv2010

First-Principles Study of Electronic and Vibrational Properties of BaHfN$_2$

The transition metal nitride BaHfN$_2$, which consists of weakly bonded neutral slabs of closed shell ions, has structural and chemical similarities to other layered nitrides which have impressive superconducting T$_c$ when electron doped: A$_x$HfNCl, A$_x$ZrNCl, A$_x$TiNCl, with $T_c= 25.5$, $15.2$ and $16.5$ K, respectively for appropriate donor (A) concentrations $x$. These similarities suggest the possibility of BaHfN$_2$ being another relatively high T$_c$ nitride upon doping, with effects of structure and the role of specific transition metal ions yet to be understood. We report first-principles electronic structure calculations for stoichiometric BaHfN$_2$ using density functional theory with plane-wave basis sets and separable dual-space Gaussian pseudopotentials. An indirect band gap of 0.8 eV was obtained and the lowest conduction band is primarily of Hf 5$d_{xy}$ character, similar to $β$-ZrNCl and $α$-TiNCl. The two N sites, one in the Hf layer and another one in the Ba layer, were found to have very anisotropic Born effective charges (BEC):deviations from the formal charge (-3) are opposite for the two sites, and opposite for the two orientations (in-plane, out of plane). LO-TO splittings and comparison of BECs and dielectric constant tensors to those of related compounds are discussed, and the effect of electron doping on the zone-center phonons is reported.

cond-mat.mtrl-sci↗

arXiv2010

Electron Confinement, Orbital Ordering, and Orbital Moments in $d^0$-$d^1$ Oxide Heterostructures

The (SrTiO$_3$)$_m$/(SrVO$_3$)$_n$ $d^0-d^1$ multilayer system is studied with first principles methods through the observed insulator-to-metal transition with increasing thickness of the SrVO$_3$ layer. When correlation effects with reasonable magnitude are included, crystal field splittings from the structural relaxations together with spin-orbit coupling (SOC) determines the behavior of the electronic and magnetic structures. These confined slabs of SrVO$_3$ prefer $Q_{orb}$=($π,π$) orbital ordering of $\ell_z = 0$ and $\ell_z = -1$ ($j_z=-1/2$) orbitals within the plane, accompanied by $Q_{spin}$=(0,0) spin order (ferromagnetic alignment). The result is a SOC-driven ferromagnetic Mott insulator. The orbital moment of 0.75 $μ_B$ strongly compensates the spin moment on the $\ell_z = -1$ sublattice. The insulator-metal transition for $n = 1 \to 5$ (occurring between $n$=4 and $n$=5) is reproduced. Unlike in the isoelectronic $d^0-d^1$ TiO$_2$/VO$_2$ (rutile structure) system and in spite of some similarities in orbital ordering, no semi-Dirac point [{\it Phys. Rev. Lett.} {\bf 102}, 166803 (2009)] is encountered, but the insulator-to-metal transition occurs through a different type of unusual phase. For n=5 this system is very near (or at) a unique semimetallic state in which the Fermi energy is topologically determined and the Fermi surface consists of identical electron and hole Fermi circles centered at $k$=0. The dispersion consists of what can be regarded as a continuum of radially-directed Dirac points, forming a "Dirac circle".

cond-mat.str-el↗

arXiv2009

Metal-insulator transition through a semi-Dirac point in oxide nanostructures: VO$_2$ (001) layers confined within TiO$_2$

Multilayer (TiO$_2$)$_m$/(VO$_2$)$_n$ nanostructures ($d^1$ - $d^0$ interfaces with no polar discontinuity) show a metal-insulator transition with respect to the VO$_2$ layer thickness in first principles calculations. For $n$ $\geq$ 5 layers, the system becomes metallic, while being insulating for $n$ = 1 and 2. The metal-insulator transition occurs through a semi-Dirac point phase for $n$ = 3 and 4, in which the Fermi surface is point-like and the electrons behave as massless along the zone diagonal in k-space and as massive fermions along the perpendicular direction. We provide an analysis of the evolution of the electronic structure through this unprecedented insulator-to-metal transition, and identify it as resulting from quantum confinement producing a non-intuitive orbital ordering on the V $d^1$ ions, rather than being a specific oxide interface effect. Spin-orbit coupling does not destroy the semi-Dirac point for the calculated ground state, where the spins are aligned along the rutile c-axis, but it does open a substantial gap if the spins lie in the basal plane.

cond-mat.str-el↗

arXiv2009

Compensated magnetism by design in double perovskite oxides

Taking into account Goodenough's superexchange rules, including both full structural relaxation and spin-orbit coupling, and checking strong correlation effects, we look for compensated half metals within the class of oxide double perovskites materials. Identifying likely half metallic (or half semimetallic) antiferromagnets, the full complications including orbital magnetism are included in order to arrive at realistic predictions of designed magnetic compounds with (near) vanishing net moment. After sorting through several candidates that have not been considered previously, two materials, K$_2$MnRhO$_6$ and La$_2$CrWO$_6$, remain as viable candidates. An important factor is obtaining compounds either with very small induced orbital moment (helped by closed subshells) or with an orbital moment that compensates the spin-orbit driven degradation of half metallic character. While thermodynamic stability of these materials cannot be ensured, the development of layer-by-layer oxide deposition techniques does not require that materials be thermodynamically stable to be synthesized.

cond-mat.str-el↗

arXiv2009

Half-metallic semiDirac point generated by quantum confinement in TiO$_2$/VO$_2$ nanostructures

Multilayer VO$_2$/TiO$_2$ nanostructures ($d^1$ - $d^0$ interfaces with no polar discontinuity) are studied with first principles density functional methods including structural relaxation. Quantum confinement of the {\it half metallic} VO$_2$ slab within insulating TiO$_2$ produces an unexpected and unprecendented two-dimensional new state, with a {\it semiDirac} point Fermi surface: spinless charge carriers are effective-mass like along one principal axis, but are massless along the other. Effects of interface imperfection are addressed.

cond-mat.str-el↗

arXiv2009

Anisotropy, Itineracy, and Magnetic Frustration in High-Tc Iron Pnictides

Using first-principle density functional theory calculations combined with insight from a tight-binding representation, dynamical mean field theory, and linear response theory, we have extensively investigated the electronic structures and magnetic interactions of nine ferropnictides representing three different structural classes. The calculated magnetic interactions are found to be short-range, and the nearest ($J_{1a}$) and next-nearest ($J_{2}$) exchange constants follow the universal trend of J_{1a}/2J_{2}\sim 1, despite their itinerant origin and extreme sensitivity to the z-position of As. These results bear on the discussion of itineracy versus magnetic frustration as the key factor in stabilizing the superconducting ground state. The calculated spin wave dispersions show strong magnetic anisotropy in the Fe plane, in contrast to cuprates.

cond-mat.supr-con↗