8,362 research outputs found
Electronic Structure of New LiFeAs High-Tc Superconductor
We present results of it ab initio LDA calculations of electronic structure
of "next generation" layered ironpnictide High-Tc superconductor LiFeAs
(Tc=18K). Obtained electronic structure of LiFeAs is very similar to recently
studied ReOFeAs (Re=La,Ce,Pr,Nd,Sm) and AFe2As2 (A=Ba,Sr) compounds. Namely
close to the Fermi level its electronic properties are also determined ma inly
by Fe 3d-orbitals of FeAs4 two-dimensional layers. Band dispersions of LiFeAs
are very similar to the LaOFeAs and BaFe2As2 systems as well as the shape of
the Fe-3d density o f states and Fermi surface.Comment: 4 pages, 5 figures; Electronic structure improved with respect to new
experimental crystal structure dat
Approximation of corner polyhedra with families of intersection cuts
We study the problem of approximating the corner polyhedron using
intersection cuts derived from families of lattice-free sets in .
In particular, we look at the problem of characterizing families that
approximate the corner polyhedron up to a constant factor, which depends only
on and not the data or dimension of the corner polyhedron. The literature
already contains several results in this direction. In this paper, we use the
maximum number of facets of lattice-free sets in a family as a measure of its
complexity and precisely characterize the level of complexity of a family
required for constant factor approximations. As one of the main results, we
show that, for each natural number , a corner polyhedron with basic
integer variables and an arbitrary number of continuous non-basic variables is
approximated up to a constant factor by intersection cuts from lattice-free
sets with at most facets if and that no such approximation is
possible if . When the approximation factor is allowed to
depend on the denominator of the fractional vertex of the linear relaxation of
the corner polyhedron, we show that the threshold is versus .
The tools introduced for proving such results are of independent interest for
studying intersection cuts
First-principles dynamical CPA to finite-temperature magnetism of transition metals
We present here the first-principles dynamical CPA (coherent potential
approximation) combined with the tight-binding LMTO LDA+U method towards
quantitative calculations of the electronic structure and magnetism at finite
temperatures in transition metals and compounds. The theory takes into account
the single-site dynamical charge and spin fluctuations using the functional
integral technique as well as an effective medium. Numerical results for Fe,
Co, and Ni show that the theory explains quantitatively the high-temperature
properties such as the effective Bohr magneton numbers and the excitation
spectra in the paramagnetic state, and describes the Curie temperatures
semiquantitatively.Comment: ICM'09 Proceeding
Quantized Rotation of Atoms From Photons with Orbital Angular Momentum
We demonstrate the coherent transfer of the orbital angular momentum of a
photon to an atom in quantized units of hbar, using a 2-photon stimulated Raman
process with Laguerre-Gaussian beams to generate an atomic vortex state in a
Bose-Einstein condensate of sodium atoms. We show that the process is coherent
by creating superpositions of different vortex states, where the relative phase
between the states is determined by the relative phases of the optical fields.
Furthermore, we create vortices of charge 2 by transferring to each atom the
orbital angular momentum of two photons.Comment: New version, 4 pages and 3 figures, accepted for publication in
Physical Review Letter
Electronic structure and effects of dynamical electron correlation in ferromagnetic bcc-Fe, fcc-Ni and antiferromagnetic NiO
LDA+DMFT method in the framework of the iterative perturbation theory (IPT)
with full LDA Hamiltonian without mapping onto the effective Wannier orbitals.
We then apply this LDA+DMFT method to ferromagnetic bcc-Fe and fcc-Ni as a test
of transition metal, and to antiferromagnetic NiO as an example of transition
metal oxide. In Fe and Ni, the width of occupied 3d bands is narrower than
those in LDA and Ni 6eV satellite appears. In NiO, the resultant electronic
structure is of charge-transfer insulator type and the band gap is 4.3eV. These
results are in good agreement with the experimental XPS. The configuration
mixing and dynamical correlation effects play a crucial role in these results
Spin-dependent Transparency of Ferromagnet/Superconductor Interfaces
Because the physical interpretation of the spin-polarization of a ferromagnet
determined by point-contact Andreev reflection (PCAR) is non-trivial, we have
carried out parameter-free calculations of PCAR spectra based upon a
scattering-theory formulation of Andreev reflection generalized to
spin-polarized systems and a tight-binding linear muffin tin orbital method for
calculating the corresponding scattering matrices. PCAR is found to measure the
spin-dependent interface transparency rather than the bulk polarization of the
ferromagnet which is strongly overestimated by free electron model fitting.Comment: 4 pages, 1figure. submitte
Electronic Structure, Local Moments and Transport in Fe_2VAl
Local spin density approximation calculations are used to elucidate
electronic and magnetic properties of Heusler structure Fe_2VAl. The compound
is found to be a low carrier density semimetal. The Fermi surface has small
hole pockets derived from a triply degenerate Fe derived state at Gamma
compensated by an V derived electron pocket at the X point. The ideal compound
is found to be stable against ferromagnetism. Fe impurities on V sites,
however, behave as local moments. Because of the separation of the hole and
electron pockets the RKKY interaction between such local moments should be
rapidly oscillating on the scale of its decay, leading to the likelihood of
spin-glass behavior for moderate concentrations of Fe on V sites. These
features are discussed in relation to experimental observations of an unusual
insulating state in this compound.Comment: 16 pages, RevTeX, 5 figure
- …