699 research outputs found
Novel multiple-band superconductor SrPt2As2
We present LDA calculated electronic structure of recently discovered
superconductor SrPt2As2 with Tc=5.2K. Despite its chemical composition and
crystal structure are somehow similar to FeAs-based high-temperature
superconductors, the electronic structure of SrPt2As2 is very much different.
Crystal structure is orthorhombic (or tetragonal if idealized) and has layered
nature with alternating PtAs4 and AsPt4 tetrahedra slabs sandwiched with Sr
ions. The Fermi level is crossed by Pt-5d states with rather strong admixture
of As-4p states. Fermi surface of SrPt2As2 is essentially three dimensional,
with complicated sheets corresponding to multiple bands. We compare SrPt2As2
with 1111 and 122 representatives of FeAs-class of superconductors, as well as
with isovalent (Ba,Sr)Ni2As2 superconductors. Brief discussion of
superconductivity in SrPt2As2 is also presented.Comment: 5 pages, 4 figure
Electronic Structure of New Multiple Band Pt-Pnictide Superconductors APt3P
We report LDA calculated band structure, densities of states and Fermi
surfaces for recently discovered Pt-pnictide superconductors APt3P
(A=Ca,Sr,La), confirming their multiple band nature. Electronic structure is
essentially three dimensional, in contrast to Fe pnictides and chalcogenides.
LDA calculated Sommerfeld coefficient agrees rather well with experimental
data, leaving little space for very strong coupling superconductivity,
suggested by experimental data on specific heat of SrPt3P. Elementary estimates
show, that the values of critical temperature can be explained by rather weak
or moderately strong coupling, while the decrease of superconducting transition
temperature Tc from Sr to La compound can be explained by corresponding
decrease of total density of states at the Fermi level N(E_F). The shape of the
density of states near the Fermi level suggests that in SrPt3P electron doping
(such as replacement Sr by La) decreases N(E_F) and Tc, while hole doping (e.g.
partial replacement of Sr with K, Rb or Cs, if possible) would increase N(E_F)
and possibly Tc.Comment: 5 pages, 5 figure
Realistic modeling of strongly correlated electron systems: An introduction to the LDA+DMFT approach
The LDA+DMFT approach merges conventional band structure theory in the local
density approximation (LDA) with a state-of-the-art many-body technique, the
dynamical mean-field theory (DMFT). This new computational scheme has recently
become a powerful tool for ab initio investigations of real materials with
strong electronic correlations. In this paper an introduction to the basic
ideas and the set-up of the LDA+DMFT approach is given. Results for the
photoemission spectra of the transition metal oxide La_{1-x}Sr_xTiO_3, obtained
by solving the DMFT-equations by quantum Monte-Carlo (QMC) simulations, are
presented and are found to be in very good agreement with experiment. The
numerically exact DMFT(QMC) solution is compared with results obtained by two
approximative solutions, i.e., the iterative perturbation theory and the
non-crossing approximation.Comment: 15 pages, 3 figures, SCES-Y2K Conference Proceeding
Optical orientation and alignment of excitons in direct and indirect band gap (In,Al)As/AlAs quantum dots with type-I band alignment
The spin structure and spin dynamics of excitons in an ensemble of
(In,Al)As/AlAs quantum dots (QDs) with type-I band alignment, containing both
direct and indirect band gap dots, are studied. Time-resolved and spectral
selective techniques are used to distinguish between the direct and indirect
QDs. The exciton fine structure is studied by means of optical alignment and
optical orientation techniques in magnetic fields applied in the Faraday or
Voigt geometries. A drastic difference in emission polarization is found for
the excitons in the direct QDs involving a -valley electron and the
excitons in the indirect QDs contributed by an -valley electron. We show
that in the direct QDs the exciton spin dynamics is controlled by the
anisotropic exchange splitting, while in the indirect QDs it is determined by
the hyperfine interaction with nuclear field fluctuations. The anisotropic
exchange splitting is determined for the direct QD excitons and compared with
model calculations
On Microscopic Origin of Integrability in Seiberg-Witten Theory
We discuss microscopic origin of integrability in Seiberg-Witten theory,
following mostly the results of hep-th/0612019, as well as present their
certain extension and consider several explicit examples. In particular, we
discuss in more detail the theory with the only switched on higher perturbation
in the ultraviolet, where extra explicit formulas are obtained using
bosonization and elliptic uniformization of the spectral curve.Comment: 24 pages, 1 figure, LaTeX, based on the talks at 'Geometry and
Integrability in Mathematical Physics', Moscow, May 2006; 'Quarks-2006',
Repino, May 2006; Twente conference on Lie groups, December 2006 and
'Classical and Quantum Integrable Models', Dubna, January 200
Optical orientation and alignment of excitons in direct and indirect band gap (In,Al)As/AlAs quantum dots with type-I band alignment
The spin structure and spin dynamics of excitons in an ensemble of
(In,Al)As/AlAs quantum dots (QDs) with type-I band alignment, containing both
direct and indirect band gap dots, are studied. Time-resolved and spectral
selective techniques are used to distinguish between the direct and indirect
QDs. The exciton fine structure is studied by means of optical alignment and
optical orientation techniques in magnetic fields applied in the Faraday or
Voigt geometries. A drastic difference in emission polarization is found for
the excitons in the direct QDs involving a -valley electron and the
excitons in the indirect QDs contributed by an -valley electron. We show
that in the direct QDs the exciton spin dynamics is controlled by the
anisotropic exchange splitting, while in the indirect QDs it is determined by
the hyperfine interaction with nuclear field fluctuations. The anisotropic
exchange splitting is determined for the direct QD excitons and compared with
model calculations
Electronic structure, magnetic and optical properties of intermetallic compounds R2Fe17 (R=Pr,Gd)
In this paper we report comprehensive experimental and theoretical
investigation of magnetic and electronic properties of the intermetallic
compounds Pr2Fe17 and Gd2Fe17. For the first time electronic structure of these
two systems was probed by optical measurements in the spectral range of 0.22-15
micrometers. On top of that charge carriers parameters (plasma frequency and
relaxation frequency) and optical conductivity s(w) were determined.
Self-consistent spin-resolved bandstructure calculations within the
conventional LSDA+U method were performed. Theoretical interpetation of the
experimental s(w) dispersions indicates transitions between 3d and 4p states of
Fe ions to be the biggest ones. Qualitatively the line shape of the theoretical
optical conductivity coincides well with our experimental data. Calculated by
LSDA+U method magnetic moments per formula unit are found to be in good
agreement with observed experimental values of saturation magnetization.Comment: 16 pages, 5 figures, 1 tabl
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