arXiv subjects

W. E. Pickett

Publications and source records attributed to W. E. Pickett.

At least 55 records · Page 3Linked to original sources

arXiv2010

The Role of Crystal Symmetry in the Magnetic Instabilities of $β$-YbAlB$_4$ and $α$-YbAlB$_4$

Density functional theory methods are applied to investigate the properties of the new superconductor $β$-YbAlB$_4$ and its polymorph $α$-YbAlB$_4$. We utilize the generalized gradient approximation + Hubbard U (GGA+U) approach with spin-orbit(SO) coupling to approximate the effects of the strong correlations due to the open $4f$ shell of Yb. We examine closely the differences in crystal bonding and symmetry of $β$-YbAlB$_4$ and $α$-YbAlB$_4$. The in-plane bonding structure amongst the dominant itinerant electrons in the boron sheets is shown to differ significantly. Our calculations indicate that, in both polymorphs, the localized 4$f$ electrons hybridize strongly with the conduction sea when compared to the related materials YbRh$_{2}$Si$_{2}$ and YbB$_{2}$. Comparing $β$-YbAlB$_4$ to the electronic structure of related crystal structures indicates a key role of the 7-member boron coordination of the Yb ion in $β$-YbAlB$_4$ in producing its enhanced Kondo scale and superconductivity. The Kondo scale is shown to depend strongly on the angle between the B neighbors and the Yb ion, relative to the $x-y$ plane, which relates some of the physical behavior to structural characteristics.

cond-mat.str-el↗

arXiv2010

Dirac Point Degenerate with Massive Bands at a Topological Quantum Critical Point

The quasi-linear bands in the topologically trivial skutterudite insulator CoSb$_3$ are studied under adiabatic, symmetry-conserving displacement of the Sb sublattice. In this cubic, time-reversal and inversion symmetric system, a transition from trivial insulator to topological point Fermi surface system occurs through a critical point in which massless (Dirac) bands are {\it degenerate} with massive bands. Spin-orbit coupling does not alter the character of the transition. The mineral skutterudite (CoSb$_3$) is very near the critical point in its natural state.

cond-mat.str-el↗

arXiv2010

Crystal Symmetry and Magnetic Order in Iron Pnictides: a Tight Binding Wannier Function Analysis

To perform a local orbital analysis of electronic and magnetic interactions, we construct the Wannier functions (WFs) of the Fe $3d$ orbitals in the parent compound of the recently discovered iron pnictide superconductors, LaFeAsO, and a comparison material LaFePO. Comparing the WFs for the stripe antiferromagnetic order with those for no magnetic order, the difference is a significant spreading (``{\it de}localization'') of specifically the $d_{xy}$ and $d_{xz}$ (but not $d_{yz}$) WFs, where parallel Fe spins lie along the $x$ direction. The WF basis gives a tight-binding representation of the first principles, density functional based Fe-derived bands. Comparing hopping parameters, it is found that changes due to stripe antiferromagnetism, even if it is weak, enables more isotropic hopping involving spin-majority electrons in the Fe $3d_{xz}$ (but not the $3d_{yz}$) orbital. This change, counterintuitively, actually reinforces electronic anisotropy. Further insight is gained by comparing the WFs of LaFeAsO and LaFePO, identifying how the difference in WFs is related to the difference in hopping integrals and showing how the pnictide atom is influential in forming the stripe antiferromagnetism. Kinetic energy considerations suggest that orbital fluctuation, in addition to spin fluctuation, may contribute to the decrease in observed ordered moment compared to the calculated values.

cond-mat.supr-con↗

arXiv2010

Parallel electron-hole bilayer conductivity from electronic interface reconstruction

The perovskite SrTiO$_3$-LaAlO$_3$ structure has advanced to a model system to investigate the rich electronic phenomena arising at polar interfaces. Using first principles calculations and transport measurements we demonstrate that an additional SrTiO$_3$ capping layer prevents structural and chemical reconstruction at the LaAlO$_3$ surface and triggers the electronic reconstruction at a significantly lower LaAlO$_3$ film thickness than for the uncapped systems. Combined theoretical and experimental evidence (from magnetotransport and ultraviolet photoelectron spectroscopy) suggests two spatially separated sheets with electron and hole carriers, that are as close as 1 nm.

cond-mat.mtrl-sci↗

arXiv2010

Sr$_2$VO$_3$FeAs: A Nanolayered Bimetallic Iron Pnictide Superconductor

One of the unifying concepts in the iron-pnictide superconductors, both for the mechanism of magnetic ordering and of unconventional order parameter character, has been the electron and hole Fermi surfaces that are approximately nested. Using the density functional methods that have predicted Fermi surfaces correctly in SrFe$_2$P$_2$, we find that the recently reported superconducting Sr$_2$VO$_3$FeAs, with $T_c$=37 K and no apparent competition between magnetism and superconductivity, possesses different Fermi surface geometry and character than previous classes of iron pnictides. The intervening layer (a V bilayer) gives rise to bands that cross the Fermi level. Coupling to the FeAs layer is small except for interaction along the zone boundary, however that coupling degrades the Fermi surface nesting. Sr$_2$VO$_3$FeAs, with its alternating layers of open shell atoms, deserves further close study that should help to understand the origin of the properties of iron pnictide compounds.

cond-mat.supr-con↗

arXiv2010

NaAlSi: a self-doped semimetallic superconductor with free electrons and covalent holes

The layered ternary sp conductor NaAlSi, possessing the iron-pnictide "111" crystal structure, superconducts at 7 K. Using density functional methods, we show that this compound is an intrinsic (self-doped) low-carrier-density semimetal with a number of unusual features. Covalent Al-Si valence bands provide the holes, and free-electron-like Al 3s bands, which propagate in the channel between the neighboring Si layers, dip just below the Fermi level to create the electron carriers. The Fermi level (and therefore the superconducting carriers) lies in a narrow and sharp peak within a pseudogap in the density of states. The small peak arises from valence bands which are nearly of pure Si, quasi-two-dimensional, flat, and coupled to Al conduction bands. Isostructural NaAlGe, which is not superconducting above 1.6 K, has almost exactly the same band structure except for one missing piece of small Fermi surface. Certain deformation potentials induced by Si and Na displacements along the c-axis are calculated and discussed. It seems likely that the mechanism of pairing is related to that of several other lightly doped two-dimensional nonmagnetic semiconductors (TiNCl, ZrNCl, HfNCl), which is not well understood but apparently not of phonon origin.

cond-mat.supr-con↗

arXiv2009

Orbital order, stacking defects and spin-fluctuations in the $p$-electron molecular solid RbO$_2$

We examine magnon and orbiton behavior in localized O$_2$ anti-bonding molecular $π^*$ orbitals using an effective Kugel-Khomskii Hamiltonian derived from a two band Hubbard model with hopping parameters taken from {\em ab initio} density functional calculations. The considerable difference between intraband and interband hoppings leads to a strong coupling between the spin wave dispersion and the orbital ground state, providing a straightforward way of experimentally determining the orbital ground state from the measured magnon dispersion. The near degeneracy of different orbital ordered states leads to stacking defects which further modulate spin-fluctuation spectra. Proliferation of orbital domains disrupts long-range magnetic order, thus causing a significant reduction in the observed Néel temperature.

cond-mat.str-el↗

arXiv2009

Antiphase magnetic boundaries in iron-based superconductors: A first-principle density-functional theory study

Superconductivity arises in the layered iron-pnictide compounds when magnetic long range order disappears. We use first principles density functional methods to study magnetic arrangements that may compete with long range order near the phase boundary. Specifically, we study the energetics and charge density distribution (through calculation of the electric field gradients) for ordered supercells with varying densities of antiphase magnetic boundaries. We quantify the amount by which Fe atoms with low spin moments at the antiphase boundaries have higher energies than Fe atoms with high spin moments away from the antiphase boundaries. These disruptions in magnetic order should be useful in accounting for experimental data such as electric field gradients and hyperfine fields on both Fe and As atoms.

cond-mat.supr-con↗

arXiv2009

Competing Phases, Strong Electron-Phonon Interaction and Superconductivity in Elemental Calcium under High Pressure

The observed "simple cubic" (sc) phase of elemental Ca at room temperature in the 32-109 GPa range is, from linear response calculations, dynamically unstable. By comparing first principle calculations of the enthalpy for five sc-related (non-close-packed) structures, we find that all five structures compete energetically at room temperature in the 40-90 GPa range, and three do so in the 100-130 GPa range. Some competing structures below 90 GPa are dynamically stable, i.e., no imaginary frequency, suggesting that these sc-derived short-range-order local structures exist locally and can account for the observed (average) "sc" diffraction pattern. In the dynamically stable phases below 90 GPa, some low frequency phonon modes are present, contributing to strong electron-phonon (EP) coupling as well as arising from the strong coupling. Linear response calculations for two of the structures over 120 GPa lead to critical temperatures in the 20-25 K range as is observed, and do so without unusually soft modes.

cond-mat.supr-con↗

arXiv2009

Orbital-ordering driven structural distortion in metallic SrCrO3

In contrast to the previous reports that the divalent perovskite SrCrO$_3$ was believed to be cubic structure and nonmagnetic metal, recent measurements suggest coexistence of majority tetragonally distorted weak antiferromagnetic phase and minority nonmagnetic cubic phase. Within the local (spin) density approximation (L(S)DA) our calculations confirm that a slightly tetragonally distorted phase indeed is energetically favored. Using the correlated band theory method (LDA+ Hubbard U) as seems to be justified by the unusual behavior observed in SrCrO$_3$, above the critical value $U_c$=4 eV only the distorted phase undergoes an orbital-ordering transition, resulting in $t_{2g}^2 --> d_{xy}^1$($d_{xz}d_{yz}$)$^1$ corresponding to the filling of the $d_{xy}$ orbital but leaving the other two degenerate. The Fermi surfaces of the cubic phase are simple with nesting features, although the nesting wavevectors do not correlate with known data. This is not uncommon in perovskites; the strongly directional d-d bonding often leads to boxlike Fermi surfaces, and either the nesting is not strong enough, or the matrix elements are not large enough, to promote instabilities. Fixed spin moment calculations indicate the cubic structure is just beyond a ferromagnetic Stoner instability (IN(0)~1.1) in L(S)DA, and that the energy is unusually weakly dependent on the moment out to 1.5$μ_B$/Cr (varying only by 11 meV/Cr), reflecting low energy long-wavelength magnetic fluctuations. We observe that this system shows strong magneto-phonon coupling (change in Cr local moment is ~7.3 $μ_B$/Å) for breathing phonon modes.

cond-mat.str-el↗

arXiv2009

Tight-binding modeling and low-energy behavior of the semi-Dirac point

We develop a tight-binding model description of semi-Dirac electronic spectra, with highly anisotropic dispersion around point Fermi surfaces, recently discovered in electronic structure calculations of VO$_2$/TiO$_2$ nano-heterostructures. We contrast their spectral properties with the well known Dirac points on the honeycomb lattice relevant to graphene layers and the spectra of bands touching each other in zero-gap semiconductors. We also consider the lowest order dispersion around one of the semi-Dirac points and calculate the resulting electronic energy levels in an external magnetic field. We find that these systems support apparently similar electronic structures but diverse low-energy physics.

cond-mat.str-el↗

arXiv2009

Charge Fluctuations and the Valence Transition in Yb under Pressure

We present a dynamical mean field theory study of the valence transition ($f^{14} \to f^{13}$) in elemental, metallic Yb under pressure. Our calculations reproduce the observed valence transition as reflected in the volume dependence of the $4f$ occupation. The transition is accelerated by heating, and suggests quasiparticle or Kondo-like structure in the spectra of the trivalent end state, consistent with the early lanthanides. Results for the local charge fluctuations and susceptibility, however, show novel signatures uniquely associated with the valence transition itself, indicating that Yb is a fluctuating valence material in contrast to the intermediate valence behavior seen in the early trivalent lanthanides Ce, Pr, and Nd.

cond-mat.str-el↗

arXiv2008

Avoiding the polarization catastrophe in LaAlO3 overlayers on SrTiO3(001) through a polar distortion

A pronounced uniform polar distortion extending over several unit cells enables thin LaAlO3 overlayers on SrTiO3(001) to counteract the charge dipole and thereby neutralize the "polarization catastrophe" that is suggested by simple ion-counting. This unanticipated mechanism, obtained from density functional theory calculations, allows several unit cells of the LaAlO3 overlayer to remain insulating (hence, fully ionic). The band gap of the system, defined by occupied O $2p$ states at the surface and unoccupied Ti 3d states at the interface in some cases $\sim$20 Ådistant, decreases with increasing thickness of the LaAlO3-film before an insulator-to-metal transition and a crossover to an electronic reconstruction occurs at around five monolayers of LaAlO3.

cond-mat.mtrl-sci↗

arXiv2008

Magnetism Driven by Anion Vacancies in Superconducting $α$--FeSe$_{1-x}$

To study the microscopic electronic and magnetic interactions in the substoichiometric iron chalcogenide FeSe$_{1-x}$ which is observed to superconduct at x~1/8 up to $T_c$=27 K, we use first principles methods to study the Se vacancy in this nearly magnetic FeSe system. The vacancy forms a ferrimagnetic cluster of eight Fe atoms, which for the ordered x=1/8 alloy leads to half metallic conduction. Similar magnetic clusters are obtained for FeTe$_{1-x}$ and for BaFe$_2$As$_2$ with an As vacancy, although neither of these are half metallic. Based on fixed spin density results, we suggest the low energy excitations in FeSe$_{1-x}$ are antiparamagnon-like with short correlation length.

cond-mat.supr-con↗

arXiv2008

The Delicate Electronic and Magnetic Structure of the LaOFePn System (Pn = pnictogen)

The occurrence of high temperature superconductivity, and the competition with magnetism, in stoichiometric and doped LaOFeAs and isostructural iron-oxypnictides is raising many fundamental questions about the electronic structure and magnetic interactions in this class of materials. There are now sufficient experimental data that it may be possible to identify the important issues whose resolution will lead to the understanding of this system. In this paper we address a number of the important issues. One important characteristic is the Fe-As distance (or more abstractly the pnictogen (Pn) height $z$(Pn)); we present results for the effect of $z$(Pn) on the electronic structure, energetics, and Fe magnetic moment. We also study LaOFeAs under pressure, and investigate the effects of both electron and hole doping within the virtual crystal approximation. The electric field gradients for all atoms in the LaOFeAs compound are presented (undoped and doped) and compared with available data. The observed $(π,π,π)$ magnetic order is studied and compared with the computationally simpler $(π,π,0)$ order which is probably a very good model in most respects. We investigate the crucial role of the pnictogen atom in this class, and predict the structures and properties of the N and Sb counterparts that have not yet been reported experimentally. At a certain volume a gap opens at the Fermi level in LaOFeN, separating bonding from antibonding bands and suggesting directions for a better simple understanding of the seemingly intricate electronic structure of this system. Finally, we address briefly on the possible effects of post-lanthanum rare earths, which have been observed to enhance the superconducting critical temperature substantially.

cond-mat.mtrl-sci↗

arXiv2008

Magnetism from $2p$ States in Alkaline Earth Monoxides: Trends with Varying N Impurity Concentration

$2p$-based magnetic moments and magnetic coupling are studied with density functional based methods for substitutional N in the alkaline earth monoxide series MgO, CaO, SrO, BaO. The hole is rather strongly localized near the N$^{2-}$ ion, being somewhat more so when strong on-site interactions are included in the calculations. Strong magnetic coupling is obtained in the itinerant electron limit but decreases strongly in the localized limit in which the Coulomb repulsion within the N $2p$ shell (U) is much greater than the N $2p$ impurity bandwidth (W). We find that this limit is appropriate for realistic N concentrations. Ordering on a simple cubic sublattice may maximize the magnetic coupling due to its high directionality.

cond-mat.str-el↗

arXiv2008

Electron-Hole Symmetry and Magnetic Coupling in Antiferromagnetic LaOFeAs

When either electron or hole doped at concentrations $x\sim 0.1$, the LaOFeAs family displays remarkably high temperature superconductivity with T$_c$ up to 55 K. In the most energetically stable $\vec Q_M = (π,π)$ antiferromagnetic (AFM) phase comprised of tetragonal-symmetry breaking alternating chains of aligned spins, there is a deep pseudogap in the Fe 3d states centered at the Fermi energy, and very strong magnetophonon coupling is uncovered. Doping (of either sign) beyond $x \sim 0.1$ results in Fe 3d heavy mass carriers ($m^*\sim 4-8$) with a large Fermi surface. Calculated Fe-Fe transverse exchange couplings $J_{ij}(R)$ reveal that exchange coupling is strongly dependent on the AFM symmetry and Fe-As distance.

cond-mat.supr-con↗

arXiv2008

Pressure-driven magnetic moment collapse in the ground state of MnO

The zero temperature Mott transition region in antiferromagnetic, spin S=5/2 MnO is probed using the correlated band theory LSDA+U method. The first transition encountered is an insulator-insulator volume collapse within the rocksalt structure that is characterized by an unexpected Hund's rule violating `spin-flip' moment collapse. This spin-flip to S=1/2 takes fullest advantage of the anisotropy of the Coulomb repulsion, allowing gain in the kinetic energy (which increases with decreasing volume) while retaining a sizable amount of the magnetic exchange energy. While transition pressures vary with the interaction strength, the spin-flip state is robust over a range of interaction strengths and for both B1 and B8 structures.

cond-mat.str-el↗