1,641 research outputs found
Mechanism of charge transfer/disproportionation in LnCu3Fe4O12 (Ln: Lanthanides)
The Fe-Cu intersite charge transfer and Fe charge disproportionation are
interesting phenomena observed in some LnCu3Fe4O12 (Ln: Lanthanides) compounds
containing light and heavy Ln atoms, respectively. We show that a change in the
spin state is responsible for the intersite charge transfer in the light Ln
compounds. At the high spin state, such systems prefer an unusual Cu-d^8
configuration, whereas at the low spin state they retreat to the normal Cu-d^9
configuration through a charge transfer from Fe to Cu-3d_{xy} orbital. We find
that the strength of the crystal field splitting and the relative energy
ordering between Cu-3d_{xy} and Fe-3d states are the key parameters,
determining the intersite charge transfer (charge disproportionation) in light
(heavy) Ln compounds. It is further proposed that the size of Ln affects the
onsite interaction strength of Cu-3d states, leading to a strong modification
of the Cu-L_3 edge spectrum, as observed by the X-ray absorption spectroscopy.Comment: 6 pages, 5 figures, 1 table. To appear in PR
Theoretical Analysis of Electronic and Magnetic Properties of NaVO: Crucial Role of the Orbital Degrees of Freedom
Using realistic low-energy model with parameters derived from the
first-principles electronic structure calculation, we address the origin of the
quasi-one-dimensional behavior in orthorhombic NaVO, consisting of the
double chains of edge-sharing VO octahedra. We argue that the geometrical
aspect alone does not explain the experimentally observed anisotropy of
electronic and magnetic properties of NaVO. Instead, we attribute the
unique behavior of NaVO to one particular type of the orbital ordering,
which respects the orthorhombic symmetry. This orbital ordering acts to
divide all states into two types: the `localized' ones, which are
antisymmetric with respect to the mirror reflection , and
the symmetric `delocalized' ones. Thus, NaVO can be classified as the
double exchange system. The directional orientation of symmetric orbitals,
which form the metallic band, appears to be sufficient to explain both
quasi-one-dimensional character of interatomic magnetic interactions and the
anisotropy of electrical resistivity.Comment: 16 pages, 4 figure
Origin of giant bulk Rashba splitting: Application to BiTeI
We theoretically propose the necessary conditions for realization of giant
Rashba splitting in bulk systems. In addition to (i) the large atomic
spin-orbit interaction in an inversion-asymmetric system, the following two
conditions are further required; (ii) a narrow band gap, and (iii) the presence
of top valence and bottom conduction bands of symmetrically the same character.
As a representative example, using the first principles calculations, the
recently discovered giant bulk Rashba splitting system BiTeI is shown to fully
fulfill all these three conditions. Of particular importance, by predicting the
correct crystal structure of BiTeI, different from what has been believed thus
far, the third criterion is demonstrated to be met by a negative crystal field
splitting of the top valence bands.Comment: 3 figure
Flat-Band Ferromagnetism in Organic Polymers Designed by a Computer Simulation
By coupling a first-principles, spin-density functional calculation with an
exact diagonalization study of the Hubbard model, we have searched over various
functional groups for the best case for the flat-band ferromagnetism proposed
by R. Arita et al. [Phys. Rev. Lett. {\bf 88}, 127202 (2002)] in organic
polymers of five-membered rings. The original proposal (poly-aminotriazole) has
turned out to be the best case among the materials examined, where the reason
why this is so is identified here. We have also found that the ferromagnetism,
originally proposed for the half-filled flat band, is stable even when the band
filling is varied away from the half-filling. All these make the ferromagnetism
proposed here more experimentally inviting.Comment: 11 pages, 13figure
Mechanisms of enhanced orbital dia- and paramagnetism: Application to the Rashba semiconductor BiTeI
We study the magnetic susceptibility of a layered semiconductor BiTeI with
giant Rashba spin splitting both theoretically and experimentally to explore
its orbital magnetism. Apart from the core contributions, a large
temperature-dependent diamagnetic susceptibility is observed when the Fermi
energy E_F is near the crossing point of the conduction bands, while the
susceptibility turns to be paramagnetic when E_F is away from it. These
features are consistent with first-principles calculations, which also predict
an enhanced orbital magnetic susceptibility with both positive and negative
signs as a function of E_F due to band (anti)crossings. Based on these
observations, we propose two mechanisms for an enhanced paramagnetic orbital
susceptibility.Comment: 4 figures; added reference
Breakdown of an Electric-Field Driven System: a Mapping to a Quantum Walk
Quantum transport properties of electron systems driven by strong electric
fields are studied by mapping the Landau-Zener transition dynamics to a quantum
walk on a semi-infinite one-dimensional lattice with a reflecting boundary,
where the sites correspond to energy levels and the boundary the ground state.
Quantum interference induces a distribution localized around the ground state,
and when the electric field is strengthened, a delocalization transition occurs
describing breakdown of the original electron system.Comment: 4 pages, 3 figures, Journal-ref adde
Symmetry Breaking and Bifurcations in the Periodic Orbit Theory: II -- Spheroidal Cavity --
We derive a semiclassical trace formula for the level density of the
three-dimensional spheroidal cavity. To overcome the divergences and
discontinuities occurring at bifurcation points and in the spherical limit, the
trace integrals over the action-angle variables are performed using an improved
stationary phase method. The resulting semiclassical level density oscillations
and shell energies are in good agreement with quantum-mechanical results. We
find that the births of three-dimensional orbits through the bifurcations of
planar orbits in the equatorial plane lead to considerable enhancement of shell
effect for superdeformed shapes.Comment: 49 pages, 18 figures, using PTPTeX.cls(included), submitted to Prog.
Theor. Phy
Superconductivity induced by inter-band nesting in the three-dimensional honeycomb lattice
In order to study whether the inter-band nesting can favor superconductivity
arising from electron-electron repulsion in a three-dimensional system, we have
looked at the repulsive Hubbard model on a stack of honeycomb (i.e.,
non-Bravais) lattices with the FLEX method, partly motivated by the
superconductivity observed in MgB2. By systematically changing the shape of
Fermi surface with varied band filling n and the third-direction hopping, we
have found that the pair scattering across the two-bands is indeed found to
give rise to gap functions that change sign across the bands and behave as an
s- or d-wave within each band. This implies (a) the electron repulsion can
assist gapful pairing when a phonon-mechanism pairing exists, and (b) the
electron repulsion alone, when strong enough, can give rise to a d-wave-like
pairing, which should be, for a group-theoretic reason, a time-reversal broken
d+id with point nodes in the gap
High-temperature Superconductivity in Layered Nitrides \beta-LiMNCl (M = Ti, Zr, Hf): Insights from Density-functional Theory for Superconductors
We present an ab initio analysis with density functional theory for
superconductors (SCDFT) to understand the superconducting mechanism of doped
layered nitrides \beta-LiMNCl (M=Ti, Zr, and Hf). The current version of
SCDFT is based on the Migdal-Eliashberg theory and has been shown to reproduce
accurately experimental superconducting-transition temperatures Tc of a wide
range of phonon-mediated superconductors. In the present case, however, our
calculated Tc4.3 K (M=Zr) and 10.5 K (M=Hf) are found to be less
than a half of the experimental Tc. In addition, Tc obtained in the present
calculation increases with the doping concentration x, opposite to that
observed in the experiment. Our results indicate that we need to consider some
elements missing in the present SCDFT based on the Migdal-Eliashberg theory.Comment: 18 pages, 13 figures, submitted to Physical Review
Spin-triplet superconductivity in repulsive Hubbard models with disconnected Fermi surfaces: a case study on triangular and honeycomb lattices
We propose that spin-fluctuation-mediated spin-triplet superconductivity may
be realized in repulsive Hubbard models with disconnected Fermi surfaces. The
idea is confirmed for Hubbard models on triangular (dilute band filling) and
honeycomb (near half-filling) lattices using fluctuation exchange
approximation, where triplet pairing order parameter with f-wave symmetry is
obtained. Possible relevance to real superconductors is suggested.Comment: 5 pages, 6 figures, RevTeX, uses epsf.sty and multicol.st
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