2,980 research outputs found
Chemical Differences between K and Na in Alkali Cobaltates
KCoO shares many similarities with NaCoO, as well as some
important differences (no hydration-induced superconductivity has been
reported). At =20 K, KCoO becomes an insulator with a tiny
optical gap as happens in NaCoO at 52 K. This similarity, with a
known common structure, enables direct comparisons to be made. Using the
K-zigzag structure recently reported and the local density approximation, we
compare and contrast these cobaltates at x=0.5. Although the electronic
structures are quite similar as expected, substantial differences are observed
near the Fermi level. These differences are found to be attributable mostly to
the chemical, rather than structural difference: although Na is normally
considered to be fully ion, K has somewhat more highly ionic character than
does Na in these cobaltates.Comment: 5 paper
Reformulation of the LDA+U method for a local orbital basis
We present a new approach to the evaluation of the on-site repulsion energy U
for use in the LDA+U method of Anisimov and collaborators. Our objectives are
to make the method more firmly based, to concentrate primarily on ground state
properties rather than spectra, and to test the method in cases where only
modest changes in orbital occupations are expected, as well as for highly
correlated materials. Because of these objectives, we employ a differential
definition of U. We also define a matrix U, which we find is very dependent on
the environment of the atom in question. The formulation is applied to evaluate
U for transition metal monoxides from VO to NiO using a local orbital basis
set. The resulting values of U are typically only 40-65% as large as values
currently in use. We evaluate the U matrix for the e_g and t_{2g} subshells in
paramagnetic FeO, and illustrate the very different charge response of the e_g
and t_{2g} states. The sensitivity of the method to the choice of the d
orbitals, and to the basis set in general, is discussed.Comment: 6 figure
Half metallic digital ferromagnetic heterostructure composed of a -doped layer of Mn in Si
We propose and investigate the properties of a digital ferromagnetic
heterostructure (DFH) consisting of a -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- and nearest-neighbor Si- 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 (). Being Si based
and possibly having a high 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.Comment: 4 pages, 4 figures, Revised version, to appear in Phys. Rev. Let
Orbital-quenching-induced magnetism in Ba_2NaOsO_6
The double perovskite \bnoo with heptavalent Os () is observed to remain
in the ideal cubic structure ({\it i.e.} without orbital ordering) despite
single occupation of the orbitals, even in the ferromagnetically
ordered phase below 6.8 K. Analysis based on the {\it ab initio} dispersion
expressed in terms of an Os -based Wannier function picture, spin-orbit
coupling, Hund's coupling, and strong Coulomb repulsion shows that the magnetic
OsO cluster is near a moment-less condition due to spin and orbital
compensation. Quenching (hybridization) then drives the emergence of the small
moment. This compensation, unprecedented in transition metals, arises in a
unified picture that accounts for the observed Mott insulating behavior.Comment: in press at Europhysics Letter
Dirac Point Degenerate with Massive Bands at a Topological Quantum Critical Point
The quasi-linear bands in the topologically trivial skutterudite insulator
CoSb 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) is very near
the critical point in its natural state.Comment: 5 pages, 3 figure
Linear response separation of a solid into atomic constituents: Li, Al, and their evolution under pressure
We present the first realization of the generalized pseudoatom concept
introduced by Ball, and adopt the name enatom to minimize confusion. This
enatom, which consists of a unique decomposition of the total charge density
(or potential) of any solid into a sum of overlapping atomiclike contributions
that move rigidly with the nuclei to first order, is calculated using
(numerical) linear response methods, and is analyzed for both fcc Li and Al at
pressures of 0, 35, and 50 GPa. These two simple fcc metals (Li is fcc and a
good superconductor in the 20-40 GPa range) show different physical behaviors
under pressure, which reflects the increasing covalency in Li and the lack of
it in Al. The nonrigid (deformation) parts of the enatom charge and potential
have opposite signs in Li and Al; they become larger under pressure only in Li.
These results establish a method of construction of the enatom, whose potential
can be used to obtain a real-space understanding of the vibrational properties
and electron-phonon interaction in solids.Comment: 13 pages, 9 figures, 1 table, V2: fixed problem with Fig. 7, V3:
minor correction
Quantum limit of the triplet proximity effect in half-metal - superconductor junctions
We apply the scattering matrix approach to the triplet proximity effect in
superconductor-half metal structures. We find that for junctions that do not
mix different orbital modes, the zero bias Andreev conductance vanishes, while
the zero bias Josephson current is nonzero. We illustrate this finding on a
ballistic half-metal--superconductor (HS) and superconductor -- half-metal --
superconductor (SHS) junction with translation invariance along the interfaces,
and on HS and SHS systems where transport through the half-metallic region
takes place through a single conducting channel. Our calculations for these
physically single mode setups -- single mode point contacts and chaotic quantum
dots with single mode contacts -- illustrate the main strength of the
scattering matrix approach: it allows for studying systems in the quantum
mechanical limit, which is inaccessible for quasiclassical Green's function
methods, the main theoretical tool in previous works on the triplet proximity
effect.Comment: 12 pages, 10 figures; v2: references added, typos correcte
Linear bands, zero-momentum Weyl semimetal, and topological transition in skutterudite-structure pnictides
It was reported earlier [Phys. Rev. Lett. 106, 056401 (2011)] that the
skutterudite structure compound CoSb displays a unique band structure with
a topological transition versus a symmetry-preserving sublattice (Sb)
displacement very near the structural ground state. The transition is through a
massless Dirac-Weyl semimetal, point Fermi surface phase which is unique in
that (1) it appears in a three dimensional crystal, (2) the band critical point
occurs at =0, and (3) linear bands are degenerate with conventional
(massive) bands at the critical point (before inclusion of spin-orbit
coupling). Further interest arises because the critical point separates a
conventional (trivial) phase from a topological phase. In the native cubic
structure this is a zero-gap topological semimetal; we show how spin-orbit
coupling and uniaxial strain converts the system to a topological insulator
(TI). We also analyze the origin of the linear band in this class of materials,
which is the characteristic that makes them potentially useful in
thermoelectric applications or possibly as transparent conductors. We
characterize the formal charge as Co , consistent with the gap, with
its site symmetry, and with its lack of moment. The Sb states are
characterized as (separately, ) -bonded ring states
occupied and the corresponding antibonding states empty. The remaining
(locally) orbitals form molecular orbitals with definite parity centered
on the empty site in the skutterudite structure. Eight such orbitals must
be occupied; the one giving the linear band is an odd orbital singlet
at the zone center. We observe that the provocative linearity of the band
within the gap is a consequence of the aforementioned near-degeneracy, which is
also responsible for the small band gap.Comment: 10 pages, 7 figure
Effect of Local Electron-Electron Correlation in Hydrogen-like Impurities in Ge
We have studied the electronic and local magnetic structure of the hydrogen
interstitial impurity at the tetrahedral site in diamond-structure Ge, using an
empirical tight binding + dynamical mean field theory approach because within
the local density approximation (LDA) Ge has no gap. We first establish that
within LDA the 1s spectral density bifurcates due to entanglement with the four
neighboring sp3 antibonding orbitals, providing an unanticipated richness of
behavior in determining under what conditions a local moment hyperdeep donor or
Anderson impurity will result, or on the other hand a gap state might appear.
Using a supercell approach, we show that the spectrum, the occupation, and the
local moment of the impurity state displays a strong dependence on the strength
of the local on-site Coulomb interaction U, the H-Ge hopping amplitude, the
depth of the bare 1s energy level epsilon_H, and we address to some extent the
impurity concentration dependence. In the isolated impurity, strong interaction
regime a local moment emerges over most of the parameter ranges indicating
magnetic activity, and spectral density structure very near (or in) the gap
suggests possible electrical activity in this regime.Comment: 9 pages, 5 figure
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