arXiv subjects

Warren E. Pickett

Publications and source records attributed to Warren E. Pickett.

At least 37 records · Page 2Linked to original sources

arXiv2019

Spin-Orbit Coupling Induced Degeneracy in the Anisotropic Unconventional Superconductor UTe$_2$

The orthorhombic uranium dichalcogenide UTe$_2$ displays superconductivity below 1.7 K, with the anomalous feature of retaining 50$\%$ of normal state (ungapped) carriers, according to heat capacity data from two groups. Incoherent transport that crosses over from above 50 K toward a low temperature, Kondo lattice Fermi liquid regime indicates strong magnetic fluctuations and the need to include correlation effects in theoretical modeling. We report density functional theory plus Hubbard U (DFT+U) results for UTe$_2$ to provide a platform for modeling its unusual behavior, focusing on ferromagnetic (FM, time reversal breaking) long range correlations along the ${\hat a}$ axis as established by magnetization measurements and confirmed by our calculations. States near the Fermi level are dominated by the $j=\frac{5}{2}$ configuration, with the $j_z=\pm\frac{1}{2}$ sectors being effectively degenerate and half-filled. Unlike the small-gap insulating nonmagnetic electronic spectrum, the FM Fermi surfaces are large (strongly metallic) and display low dimensional features, reminiscent of the FM superconductor UGe$_2$.

cond-mat.supr-con↗

arXiv2019

Strong Particle-Hole Symmetry Breaking in a 200 kelvin Superconductor

The superconducting state of metals has long provided a classic example of particle-hole symmetry (PHS) at low energy. Fermionic self-energy results based on first principles theory for the electron-phonon coupling in H$_3$S presented here illustrate strong PHS-breaking dynamics arising from the underlying sharp structure in the fermionic density of states. Thus H$_3$S is not only the superconductor with the highest critical temperature $T_c$ (through 2018), but its low energy, low temperature properties deviate strongly from textbook behavior. The minor momentum and band dependence of the fermionic self-energy allows evaluation of the momentum-resolved and zone-averaged spectral densities and interacting thermal distribution function, all of which clearly illustrate strong particle-hole asymmetry.

cond-mat.supr-con↗

arXiv2019

How Compressed Hydrides Produce Room Temperature Superconductivity

The 2014-2015 prediction, discovery, and confirmation of record high temperature superconductivity above 200K in H$_3$S, followed by the 2018 extension to superconductivity in the 250-280K range in lanthanum hydride, marks a new era in the longstanding quest for room temperature superconductivity: quest achieved, at the cost of supplying 1.5-2 megabars of pressure. Predictions of numerous high temperature superconducting metal hydrides $XH_n$ ($X$=metal) have appeared, but are providing limited understanding of what drives the high transition temperature T$_c$, or what limits T$_c$. We apply an opportunistic atomic decomposition of the coupling function to show, first, that the $X$ atom provides coupling strength as commonly calculated, but is it irrelevant for superconductivity; in fact, it is important for analysis that its contribution is neglected. Five $X$H$_n$ compounds, predicted to have T$_c$ in the 150-300K range, are analyzed consistently for their relevant properties, revealing some aspects that confront conventional wisdom. A phonon frequency -- critical temperature ($ω_2$-T$_c$) phase diagram is obtained that reveals a common phase instability limiting T$_c$ at the {\it low pressure} range of each compound. The hydrogen scattering strength is identified and found to differ strongly over the hydrides. A quantity directly proportional to T$_c$ in these hydrides is identified.

cond-mat.supr-con↗

arXiv2018

Atomic-layer-resolved composition and electronic structure of the cuprate Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$ from soft x-ray standing-wave photoemission

A major remaining challenge in the superconducting cuprates is the unambiguous differentiation of the composition and electronic structure of the CuO$_2$ layers and those of the intermediate layers. The large c axis for these materials permits employing soft x-ray (930.3 eV) standing wave (SW) excitation in photoemission that yields atomic layer-by-atomic layer depth resolution of these properties. Applying SW photoemission to Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$ yields the depth distribution of atomic composition and the layer-resolved densities of states. We detect significant Ca presence in the SrO layers and oxygen bonding to three different cations. The layer-resolved valence electronic structure is found to be strongly influenced by the supermodulation structure--as determined by comparison to DFT calculations, by Ca-Sr intermixing, and by the Cu 3d-3d Coulomb interaction, further clarifying the complex interactions in this prototypical cuprate. Measurements of this type for other quasi-two-dimensional materials with large-c represent a promising future direction.

cond-mat.supr-con↗

arXiv2018

Superconducting Phases in Lithium Decorated Graphene LiC 6

A study of possible superconducting phases of graphene has been constructed in detail. A realistic tight binding model, fit to ab initio calculations, accounts for the Li-decoration of graphene with broken lattice symmetry, and includes $s$ and $d$ symmetry Bloch character that influences the gap symmetries that can arise. The resulting seven hybridized Li-C orbitals that support nine possible bond pairing amplitudes. The gap equation is solved for all possible gap symmetries. One band is weakly dispersive near the Fermi energy along $Γ\rightarrow M$ where its Bloch wave function has linear combination of $d_{x^2-y^2}$ and $d_{xy}$ character, and is responsible for $ d_{x^{2}-y^{2}}$ and $d_{xy}$ pairing with lowest pairing energy in our model. These symmetries almost preserve properties from a two band model of pristine graphene. Another part of this band, along $K\rightarrow Γ$, is nearly degenerate with upper $s$ band that favors extended $s$ wave pairing which is not found in two band model. Upon electron doping to a critical chemical potential $μ_1=0.22 eV$ the pairing potential decreases, then increases until a second critical value $μ_2$=1.3 eV at which a phase transition to a new phase which is not appear in two band model. This phase in the pristine graphene converts to usual extended s-wave pairing.

cond-mat.supr-con↗

arXiv2018

Pressure-induced Frustration of Magnetic Coupling in Elemental Europium

Applying linear response and the magnetic force theorem in correlated density functional theory, the inter-sublattice exchange constants of antiferromagnetic Eu are calculated and found to vanish near the pressure of P$_c$=82 GPa, just where magnetic order is observed experimentally to be lost. The Eu $4f^7$ moment remains unchanged at high pressure, again in agreement with spectroscopic measurements, leaving the picture of perfect frustration of interatomic Ruderman-Kittel-Kasuya-Yoshida couplings in a broad metallic background, leaving a state of electrons strongly exchange coupled to arbitrarily oriented, possibly quasistatic local moments. This strongly frustrated state gives way to superconductivity at T$_c$=1.7K, observed experimentally. These phenomena, and free energy considerations related to correlations, suggest an unusual phase of matter that is discussed within the scenarios of the Doniach Kondo lattice phase diagram, the metallic spin glass class, and itinerant spin liquid or spin gas systems.

cond-mat.str-el↗

arXiv2017

Room Temperature Superconductivity Revolution: Foreshadowed by Victorians, Enabled by Millenials

Room temperature superconductivity has been the most prominent, highly ambitious, but still imaginable, acme of materials physics for half a century. The struggle toward this revolution was foreshadowed by a Victorian novelist and championed,unsuccessfully, by dogged physicists in the 1960s to 1980s who had a workable theory but uncompliant materials. Discovery of superconductivity of H$_3$S at 200 K in the 160-200 GPa pressure range has renewed anticipation of yet higher values of the critical temperature T$_c$. With the several reports of metalization of hydrogen, and theoretical extensions enabled by modern algorithms and unprecedented computational hardware and spurred forward by the Materials Genome Initiative, it is possible that the room temperature precipice has thereby already been breached in a silent revolution. This concise note draws analogies of this development with an earlier revolution.

cond-mat.supr-con↗

arXiv2017

Survey of the class of isovalent antiperovskite alkaline earth-pnictide compounds

The few reported members of the antiperovskite structure class $Ae_3Pn_APn_B$ of alkaline earth ($Ae$ = Ca,Sr,Ba) pnictides ($Pn$ = N,P,As,Sb,Bi) compounds are all based on the B-site anion $Pn_B$=N. All fit can be categorized as narrow gap semiconductors, making them of interest for several reasons. Because chemical reasoning suggests that more members of this class may be stable, we provide here a density functional theory (DFT) based survey of this entire class of $3\times5\times5$ compounds. We determine first the relative energetic stability of the distribution of pairs of $Pn$ ions in the A and B sites of the structure, finding that the $B$ site always favors the small pnictogen anion. The trends of the calculated energy gaps with $Ae$ cation and $Pn$ anions are determined, and we study effects of spin-orbit coupling as well as two types of gap corrections to the conventional DFT electronic spectrum. Because there have been suggestions that this class harbors topological insulating phases, we have given this possibility attention and found that energy gap corrections indicate the cubic structures will provide at most a few topological insulators. Structural instability is addressed by calculating phonon dispersion curves for a few compounds, with one outcome being that distorted structures should be investigated further for thermoelectric and topological character. Examples of the interplay between spin-orbit coupling and strain on the topological nature are provided. A case study of $\mathrm{Ca_3BiP}$ including the effect of strain illustrates how a topological semimetal can be transformed into topological insulator and Dirac semimetal.

cond-mat.mtrl-sci↗

arXiv2017

All 3d electron-hole bilayers in CrN/MgO(111) multilayers

CrN/MgO(111) multilayers modeled via \textit{ab initio} calculations give rise to nanoscale, scalable, spatially separated two-dimensional electron and hole gases (2DEG+2DHG), each confined to its own CrN interface. Due to the Cr $3d^3$ configuration, both electron and hole gases are based on correlated transition metal layers involving bands of $3d$ character. Transport calculations predict each subsystem will have a large thermopower, on the order of 250 $μ$$V /K$ at room temperature. These heterostructures combine a large thermoelectric efficiency with scalable nanoscale conducting sheets; for example, operating at a temperature difference of 50K, 40 bilayers could produce a 1 V voltage with a film thickness of 100 nm.

cond-mat.mes-hall↗

arXiv2016

Nitride Multilayers as a Platform for Parallel Two-Dimensional Electron-Hole Gases: MgO/ScN(111)

At interfaces between insulating oxides LaAlO$_3$ and SrTiO$_3$, a two dimensional electron gas (2DEG) has been observed and well studied, while the predicted hole gas (2DHG) has not been realized due to the strong tendency of holes in oxygen $2p$ orbitals to localize. Here we propose, via ab initio calculations, an unexplored class of materials for the realization of parallel two dimensional (2D), two carrier (electron+hole) gases: nitride-oxide heterostructures, with (111)-oriented ScN and MgO as the specific example. Beyond a critical thickness of five ScN layers, this interface hosts spatially separated conducting Sc-$3d$ electrons and N-$2p$ holes, each confined to $\sim$two atomic layers -- the transition metal nitride provides both gases. A guiding concept is that the N$^{3-}$ anion should promote robust two carrier 2D hole conduction compared to that of O$^{2-}$; metal mononitrides are mostly metallic and even superconducting while most metal monoxides are insulating. A second positive factor is that the density of transition metal ions, hence of a resulting 2DG, is about three times larger for a rocksalt (111) interface than for a perovskite(001) interface. Our results, including calculation of Hall coefficient and thermopower for each conducting layer separately, provide guidance for new exploration, both experimental and theoretical, on nitride-based conducting gases that should promote study of long sought exotic states viz. new excitonic phases and distinct, nanoscale parallel superconducting nanolayers.

cond-mat.str-el↗

arXiv2015

Disturbing the dimers: electron- and hole-doping in the intermetallic insulator FeGa$_3$

Insulating FeGa$_3$ poses peculiar puzzles beyond the occurrence of an electronic gap in an intermetallic compound. This Fe-based material has a very distinctive structural characteristic with the Fe atoms occurring in dimers. The insulating gap can be described comparably well in either the weakly correlated limit or the strongly correlated limit within density functional theory viewpoints, where the latter corresponds to singlet formation on the Fe$_2$ dimers. Though most of the calculated occupied Wannier functions are an admixture of Fe $3d$ and Ga $4s$ or $4p$ states, there is a single bonding-type Wannier function per spin centered on each Fe$_2$ dimer. Density functional theory methods have been applied to follow the evolution of the magnetic properties and electronic spectrum with doping, where unusual behavior is observed experimentally. Both electron and hole doping are considered, by Ge and Zn on the Ga site, and by Co and Mn on the Fe site, the latter introducing direct disturbance of the Fe$_2$ dimer. Results from weakly and strongly correlated pictures are compared. Regardless of the method, magnetism including itinerant phases appears readily with doping. The correlated picture suggests that in the low doping limit Mn (for Fe) produces an in-gap hole state, while Co (for Fe) introduces a localized electronic gap state.

cond-mat.str-el↗

arXiv2015

Design of Mott and topological phases on buckled 3d-oxide honeycomb lattices

Perovskite bilayers with (111)-orientation combine a honeycomb lattice as a key feature with the strongly correlated, multiorbital nature of electrons in transition metal oxides. In a systematic DFT+$U$ study of (111)-oriented (La$X$O$_3$)$_2$/(LaAlO$_3$)$_4$ superlattices, we establish trends in the evolution of ground states versus band filling in (111)-oriented (La$X$O$_3$)$_2$/(LaAlO$_3$)$_4$ superlattices, with $X$ spanning the entire $3d$ transition metal series. The competition between local quasi-cubic and global triangular symmetry triggers unanticipated broken symmetry phases, with mechanisms ranging from Jahn-Teller distortions, to charge-, spin-, and orbital-ordering. LaMnO$_3$, where spin-orbit coupling opens a sizable gap in the Dirac-point Fermi surface, emerges as a topological Chern insulator.

cond-mat.mtrl-sci↗

arXiv2015

Interplay between Spin-Orbit Coupling and Strong Correlation Effects: Comparison of Three Osmate Double Perovskites: Ba$_{2}A$OsO$_{6}$ ($A=$Na, Ca, Y)

High formal valence Os-based double perovskites are a focus of current interest because they display strong interplay of large spin orbit coupling and strong electronic correlation. Here we present the electronic and magnetic characteristics of a sequence of three cubic Os based double perovskites Ba$_{2}$$A$OsO$_{6}$, ($A=$Na, Ca, Y), with formal valences (charge states) of Os$^{+7}~(d^{1})$, Os$^{+6}(d^{2})$, Os$^{+5}(d^{3})$. For these first principles based calculations we apply an ``exact exchange for correlated electrons'' functional, with exact exchange applied in a hybrid fashion solely to the Os($5d$) states. While Ba$_{2}$NaOsO$_{6}$ is a ferromagnetic Dirac-Mott insulator studied previously, the other two show antiferromagnetic ordering while all retain the cubic (undistorted) structure. For comparison purposes we have investigated only the ferromagnetic ordered phase. A metal-insulator transition is predicted to occur upon rotating the direction of magnetization in all three materials, reflecting the central role of spin orbit coupling in these small gap osmates. Surprises arising from comparing formal charge states with the radial charge densities are discussed. Chemical shielding factors and orbital susceptibilities are provided for comparison with future nuclear magnetic resonance data.

cond-mat.str-el↗

arXiv2015

van Hove singularities and spectral smearing in high temperature superconducting H3S

The superconducting phase of hydrogen sulfide at Tc=200 K observed by Drozdov and collaborators at pressures around 200 GPa is simple bcc Im-3m H3S, predicted beforehand by Duan {\it et al.}, has experimental confirmation. The various "extremes" that are involved -- pressure, implying extreme reduction of volume, extremely high H phonon energy scale around 1500K, extremely high temperature for a superconductor -- necessitates a close look at new issues raised by these characteristics in relation to high Tc. We use first principles methods to analyze the H3S electronic structure, particularly the van Hove singularities (vHs) and the effect of sulfur. Focusing on the two closely spaced vHs near the Fermi level that give rise to the impressively sharp peak in the density of states, the implications of strong coupling Migdal-Eliashberg theory are assessed. The electron spectral density smearing due to virtual phonon emission and absorption needs to be included explicitly to obtain accurate theoretical predictions and current understanding. Means for increasing Tc in H3S-like materials are addressed.

cond-mat.supr-con↗

arXiv2015

Organometallic-like localization of 4d-derived spins in an inorganic conducting niobium suboxide

Based on the refined crystal structure comprised of columns of 3*4 planar blocks of NbO6 octahedra and first principles electronic structure methods, we find that orthorhombic (o)-Nb$_{12}$O$_{29}$ introduces a new class of transition metal oxide. The electronic system consists of a large Nb dimer-based localized orbital comparable in size to those in organometallic compounds, yet is tightly bound and weakly interacting with itinerant electronic bands. These local moments, a rare occurrence for Nb, form one-dimensional spin chains that criss-cross perpendicularly oriented conducting nanowires. The local moment bandwidth is comparable to what is seen in rare earth compounds with extremely localized orbitals. The microscopic origin is traced to the local structure of the NbO6 octahedra and associated orbital+spin ordering. The resulting 1D$_s$*1D$_c$ anisotropic two dimensional Heisenberg-Kondo lattice model (s=spin, c=charge) provides a strongly anisotropic spin-fermion lattice system for further study.

cond-mat.str-el↗

arXiv2014

Spin-orbit coupling, strong correlation, and insulator-metal transitions: the J$_{\rm eff}$ =3\2 ferromagnetic Mott insulator Ba$_{2}$NaOsO$_{6}$

The double perovskite Ba$_{2}$NaOsO$_{6}$ (BNOO), an exotic example of a very high oxidation state (heptavalent) osmium $d^1$ compound and also uncommon by being a ferromagnetic Mott insulator without Jahn-Teller (JT) distortion, is modeled using the density functional theory (DFT) hybrid functional based exact exchange for correlated electrons (oeeHyb) method and including spin-orbit coupling (SOC). The experimentally observed narrow gap ferromagnetic insulating ground state is obtained, with easy axis along [110] in accord with experiment, providing support that this approach provides a realistic method for studying this system. The predicted spin density for [110] spin orientation is nearly cubic (unlike for other directions), providing an explanation for the absence of JT distortion. An orbital moment of -0.4$μ_B$ strongly compensates the +0.5$μ_B$ spin moment on Os, leaving a strongly compensated moment more in line with experiment. Remarkably, the net moment lies primarily on the oxygen ions. An insulator-metal transition by rotating the magnetization direction with an external field under moderate pressure is predicted as one consequence of strong SOC, and metallization under moderate pressure is predicted. Comparison is made with the isostructural, isovalent insulator Ba$_2$LiOsO$_6$ which however orders antiferromagnetically.

cond-mat.str-el↗

arXiv2014

Magnetic correlations and pairing in the 1/5-depleted square lattice Hubbard model

We study the single-orbital Hubbard model on the 1/5-depleted square lattice geometry, which arises in such diverse systems as the spin-gap magnetic insulator CaV4O9 and ordered-vacancy iron selenides, presenting new issues regarding the origin of both magnetic ordering and superconductivity in these materials. We find a rich phase diagram that includes a plaquette singlet phase, a dimer singlet phase, a Neel and a block-spin antiferromagnetic phase, and stripe phases. Quantum Monte Carlo simulations show that the dominant pairing correlations at half filling change character from d-wave in the plaquette phase to extended s-wave upon transition to the Neel phase. These findings have intriguing connections to iron-based superconductors, and suggest that some physics of multiorbital systems can be captured by a single-orbital model at different dopings.

cond-mat.str-el↗

arXiv2014

Analysis of charge states in the mixed valent ionic insulator AgO

The doubly ionized $d^9$ copper ion provides, originally in La$_2$CuO$_4$ and later in many more compounds, the platform for high temperature superconductivity when it is forced toward higher levels of oxidation. The nearest chemical equivalent is Ag$^{2+}$, which is almost entirely avoided in nature. AgO is an illustrative example, being an unusual nonmagnetic insulating compound with an open $4d$ shell on one site. This compound has been interpreted in terms of one Ag$^{3+}$ ion at the fourfold site and one Ag$^{+}$ ion that is twofold coordinated. We analyze more aspects of this compound, finding that indeed the Ag$^{3+}$ ion supports only four occupied $4d$-based Wannier functions per spin, while Ag$^+$ supports five, yet their physical charges are nearly equal. The oxygen $2p$ Wannier functions display two distinct types of behavior, one type of which includes conspicuous Ag $4d$ tails. Calculation of the Born effective charge tensor shows that the mean effective charges of the Ag ions differ by about a factor of two, roughly consistent with the assigned formal charges. We analyze the $4d$ charge density and discuss it in terms of recent insights into charge states of insulating (and usually magnetic) transition metal oxides. What might be expected in electron- and hole-doped AgO is discussed briefly.

cond-mat.mtrl-sci↗