arXiv subjects

Warren E. Pickett

Publications and source records attributed to Warren E. Pickett.

70 records · Page 4Linked to original sources

arXiv2008

Anisotropy and Magnetism in the LSDA+U Method

Consequences of anisotropy (variation of orbital occupation) and magnetism, and their coupling, are analyzed for LSDA+U functionals, both the commonly used ones as well as less commonly applied functionals. After reviewing and extending some earlier observations for an isotropic interaction, the anisotropies are examined more fully and related to use with the local density (LDA) or local spin density (LSDA) approximations. The total energies of all possible integer configurations of an open $f$ shell are presented for three functionals, where some differences are found to be dramatic. Differences in how the commonly used "around mean field" (AMF) and "fully localized limit" (FLL) functionals perform are traced to such differences. The LSDA+U interaction term, applied self-consistently, usually enhances spin magnetic moments and orbital polarization, and the double-counting terms of both functionals provide an opposing, moderating tendency ("suppressing the magnetic moment"). The AMF double counting term gives magnetic states a significantly larger energy penalty than does the FLL counterpart.

cond-mat.str-el↗

arXiv2008

Disorder-Induced Stabilization of the Pseudogap in Strongly Correlated Systems

The interplay of strong interaction and strong disorder, as contained in the Anderson-Hubbard model, is addressed using two non-perturbative numerical methods: the Lanczos algorithm in the grand canonical ensemble at zero temperature and Quantum Monte Carlo. We find distinctive evidence for a zero-energy anomaly which is robust upon variation of doping, disorder and interaction strength. Its similarities to, and differences from, pseudogap formation in other contexts, including perturbative treatments of interactions and disorder, classical theories of localized charges, and in the clean Hubbard model, are discussed.

cond-mat.str-el↗

arXiv2007

Magnetic Moment Collapse-Driven Mott Transition in MnO

The metal-insulator transition in correlated electron systems, where electron states transform from itinerant to localized, has been one of the central themes of condensed matter physics for more than half a century. The persistence of this question has been a consequence both of the intricacy of the fundamental issues and the growing recognition of the complexities that arise in real materials, even when strong repulsive interactions play the primary role. The initial concept of Mott was based on the relative importance of kinetic hopping (measured by the bandwidth) and on-site repulsion of electrons. Real materials, however, have many additional degrees of freedom that, as is recently attracting note, give rise to a rich variety of scenarios for a ``Mott transition.'' Here we report results for the classic correlated insulator MnO which reproduce a simultaneous moment collapse, volume collapse, and metallization transition near the observed pressure, and identify the mechanism as collapse of the magnetic moment due to increase of crystal field splitting, rather than to variation in the bandwidth.

cond-mat.str-el↗

arXiv2006

Correlation-Driven Charge Order at a Mott Insulator - Band Insulator Digital Interface

To study digital Mott insulator LaTiO3 and band insulator SrTiO3 interfaces, we apply correlated band theory (LDA+U) to (n,m) multilayers, 1<n,m<9. If the on-site repulsion on Ti is large enough to model the magnetic insulating behavior of cubic bulk LaTiO3, the charge imbalance at the interface is found in all cases to be accommodated by disproportionation (Ti4+ + Ti3+), charge ordering, and Ti3+ d_xy-orbital ordering, with antiferromagnetic exchange coupling between the spins in the interface layer. Lattice relaxation affects the conduction behavior by shifting (slightly but importantly) the lower Hubbard band, but the disproportionation and orbital ordering are robust against relaxation.

cond-mat.mtrl-sci↗

arXiv2006

First Principles Study of the Electronic and Vibrational Properties of LiNbO2

In the layered transition metal oxide LiNbO$_2$ the Nb$^{3+}$ ($4d^2$) ion is trigonal-prismatically coordinated with O ions, with the resulting crystal field leading to a single band system for low energy properties. A tight-binding representation shows that intraplanar second neighbor hopping $t_2 = 100$ meV dominates the first neighbor interaction ($t_1 = 64$ meV). The first and third neighbor couplings are strongly modified by oxygen displacements of the symmetric Raman-active vibrational mode, and electron-phonon coupling to this motion may provide the coupling mechanism for superconductivity in Li-deficient samples (where $T_c = 5$ K). We calculate all zone-center phonon modes, identify infrared (IR) and Raman active modes, and report LO-TO splitting of the IR modes. The Born effective charges for the metal ions are found to have considerable anisotropy reflecting the degree to which the ions participate in interlayer coupling and covalent bonding. Insight into the microscopic origin of the valence band density, composed of Nb $d_{z^2}$ states with some mixing of O $2p$ states, is obtained from examining Wannier functions for these bands.

cond-mat.supr-con↗

arXiv2006

Design for a Room Temperature Superconductor

The vision of ``room temperature superconductivity'' has appeared intermittently but prominently in the literature since 1964, when W. A. Little and V. L. Ginzburg began working on the `problem of high temperature superconductivity' around the same time. Since that time the prospects for room temperature superconductivity have varied from gloom (around 1980) to glee (the years immediately after the discovery of HTS), to wait-and-see (the current feeling). Recent discoveries have clarified old issues, making it possible to construct the blueprint for a viable room temperature superconductor.

cond-mat.supr-con↗

arXiv2006

The Next Breakthrough in Phonon-Mediated Superconductivity

If history teaches us anything, it is that the next breakthrough in superconductivity will not be the result of surveying the history of past breakthroughs, as they have almost always been a matter of serendipity resulting from undirected exploration into new materials. Still, there is reason to reflect on recent advances, work toward higher T_c of even an incremental nature, and recognize that it is important to explore avenues currently believed to be unpromising even as we attempt to be rational. In this paper we look at two remarkable new unusually high temperature superconductors (UHTS), MgB2 with Tc=40 K and (in less detail) high pressure Li with Tc=20 K, with the aim of reducing their unexpected achievements to a simple and clear understanding. We also consider briefly other UHTS systems that provide still unresolved puzzles; these materials include mostly layered structures, and several with strongly bonded C-C or B-C substructures. What may be possible in phonon-coupled superconductivity is reconsidered in the light of the discussion.

cond-mat.supr-con↗

arXiv2005

Half metallic digital ferromagnetic heterostructure composed of a $δ$-doped layer of Mn in Si

We propose and investigate the properties of a digital ferromagnetic heterostructure (DFH) consisting of a $δ$-doped layer of Mn in Si, using \textit{ab initio} electronic-structure methods. We find that (i) ferromagnetic order of the Mn layer is energetically favorable relative to antiferromagnetic, and (ii) the heterostructure is a two-dimensional half metallic system. The metallic behavior is contributed by three majority-spin bands originating from hybridized Mn-$d$ and nearest-neighbor Si-$p$ states, and the corresponding carriers are responsible for the ferromagnetic order in the Mn layer. The minority-spin channel has a calculated semiconducting gap of 0.25 eV. Analysis of the total and partial densities of states, band structure, Fermi surfaces and associated charge density reveals the marked two-dimensional nature of the half metallicity. The band lineup is found to be favorable for retaining the half metal character to near the Curie temperature ($T_{C}$). Being Si based and possibly having a high $T_{C}$ as suggested by an experiment on dilutely doped Mn in Si, the heterostructure may be of special interest for integration into mature Si technologies for spintronic applications.

cond-mat.mtrl-sci↗

arXiv2000

Magneto-electronic Properties of a Ferrimagnetic Semiconductor: The Hybrid Cupromanganite CaCu3Mn4O12

The mixed manganite-cuprate CaCu3Mn4O12 is found, using density functional methods, to be a narrow gap (90 meV calculated) ferrimagnetic semiconductor. Cu (formally S=1/2) antialigns with Mn (formally S=3/2), and the net spin moment is 9 μ_B consistent with the formal spins. Holes have Cu d_{xy}-O p_σ (i.e. antibonding dpσ) character with spins aligned antiparallel to the net magnetization; electrons have the opposite spin and have mixed Cu d_{xy} - Mn e_g character. Thermally excited electrons and holes will each be fully spin polarized, but in opposite directions. The properties of this material are strongly tied to the distorted quadruple perovskite structure, which is closely related to the skutterudite structure. The observed resistivity, magnetoresistance, and magnetization are discussed in terms of our results.

cond-mat.str-el↗

arXiv2000

Double exchange-driven spin pairing at the (001) surface of manganites

The (001) surface of La_{1-x}Ca_xMnO_3 system in various magnetic orderings is studied by first principle calculations. A general occurrence is that z^2 dangling bond charge -- which is ``invisible'' in the formal valence picture -- is promoted to the bulk gap/Fermi level region. This drives a double-exchange-like process that serves to align the surface Mn spin with its subsurface neighbor, regardless of the bulk magnetic order. For heavy doping, the locally ``ferromagnetic'' coupling is very strong and the moment enhanced by as much as 30% over the bulk value.

cond-mat.mtrl-sci↗

arXiv1999

Magnetic reconstruction at (001) CaMnO$_3$ surface

The Mn-terminated (001) surface of the stable anti-ferromagnetic insulating phase of cubic perovskite CaMnO$_3$ is found to undergo a magnetic reconstruction consisting on a spin-flip process at surface: each Mn spin at the surface flips to pair with that of Mn in the subsurface layer. In spite of very little Mn-O charge transfer at surface, the surface behavior is driven by the $e_g$ states due to $d_{xy}$ $\to$ $d_{z^2}$ charge redistribution. These results, based on local spin density theory, give a double exchange like coupling that is driven by $e_g$ character, not additional charge, and may have relevance to CMR materials.

cond-mat.mtrl-sci↗

arXiv1999

Half-Metallic Ferrimagnetism in Mn_2VAl

We show that Mn_2VAl is a compound for which the generalized gradient approximation (GGA) to the exchange-correlation functional in density functional theory makes a qualitative change in predicted behavior compared to the usual local density approximation (LDA). Application of GGA leads to prediction of Mn_2VAl being a half-metallic ferrimagnet, with the minority channel being the conducting one. The electronic and magnetic structure is analyzed and contrasted with the isostructural enhanced semimetal Fe_2VAl.

cond-mat.mtrl-sci↗

arXiv1998

Prediction of Ferromagnetic Ground State of NaCl-type FeN

Ab-initio results for structural and electronic properties of NaCl-type FeN are presented in a framework of plane-wave and ultrasoft pseudopotentials. Competition among different magnetic ordering is examined. We find the ferromagnetic phase stable overall. Stabilization over the unpolarized phase is obtained by splitting one flat t_2g-type band crossing the Fermi energy. A comparison with CrN is considered. We find large differences in the properties of the two systems that can be addressed to the smaller ionicity and magnetization of FeN.

cond-mat.mtrl-sci↗

arXiv1998

The Josephson Effect in Single Spin Superconductors

The Josephson Effect provides a primary signature of single spin superconductivity (SSS), the as yet unobserved superconducting state which was proposed recently as a low temperature phase of half-metallic antiferromagnets. These materials are insulating in the spin-down channel but are metallic in the spin-up channel. The SSS state is characterized by a unique form of p-wave pairing within a single spin channel. We develop the theory of a rich variety of Josephson effects that arise due to the form of the SSS order parameter. Tunneling is allowed at a SSS-SSS' junction but of course depends on the relative orientation of their order parameters. No current flows between an SSS and an s-wave BCS system due to their orthogonal symmetries, which potentially can be used to distinguish SSS from other superconducting states. Single spin superconductors also offer a means to probe other materials, where tunneling is a litmus test for any form of ``triplet'' order parameter.

cond-mat.supr-con↗

arXiv1997

Spin Density Functional Based Search for Half-Metallic Antiferromagnets

We present results based on local spin density calculations of a computational search for half-metallic (HM) antiferromagnetic (AFM) materials within the class of double perovskite structure oxides LaM'M''O3 that incorporate open shell 3d (or 4d) transition metal ions M', M''. The pairs M'M'' = MnCo, CrFe, CrRu, CrNi, MnV, and VCu are studied. La2VMnO6 is the most promising candidate, with the HM AFM phase more stable by 0.17 eV/cell than the ferromagnetic phase. La2VCuO6 is another promising possibility, but due to S=1/2 ions quantum fluctuations my play an important role in determining the ground state magnetic and electronic structure. This study indicates that HM AFM materials should not be prohibitively difficult to find.

cond-mat.mtrl-sci↗

arXiv1997

Single Spin Superconductivity: Formulation and Ginzburg-Landau Theory

We describe a novel superconducting phase that arises due to a pairing instability of the half-metallic antiferromagnetic (HM AFM) normal state. This single spin superconducting (SSS) phase contains broken time reversal symmetry in addition to broken gauge symmetry, the former due to the underlying magnetic order in the normal state. A classification of normal state symmetries leads to the conclusion that the HM AFM normal phase whose point group contains the inversion operator contains the least symmetry possible which still allows for a zero momentum pairing instability. The Ginzburg-Landau free energy for the superconducting order parameter is constructed consistent with the symmetry of the normal phase, electromagnetic gauge invariance and the crystallographic point group symmetry including inversion. For cubic, hexagonal and tetragonal point groups, the possible symmetries of the superconducting phase are classified, and the free energy is used to construct a generalized phase diagram. We identify the leading candidate out of the possible SSS phases for each point group. The symmetry of the superconducting phase is used to determine the cases where the gap function has generic zeros (point or line nodes) on the Fermi surface. Such nodes always occur, hence thermodynamic properties will have power-law behavior at low temperature.

cond-mat.supr-con↗