21 research outputs found
Electronic Structure, Electron-Phonon Coupling, and Multiband Effects in MgB2
We review the current situation in the theory of superconducting and
transport properties of MgB2. First principle calculations of of the electronic
structure and electron-phonon coupling are discussed and compared with the
experiment. We also present a brief description of the multiband effects in
superconductivity and transport, and how these manifest themselves in MgB2. We
also mention some yet open questions.Comment: Physica C, in pres
First- principle calculations of magnetic interactions in correlated systems
We present a novel approach to calculate the effective exchange interaction
parameters based on the realistic electronic structure of correlated magnetic
crystals in local approach with the frequency dependent self energy. The analog
of ``local force theorem'' in the density functional theory is proven for
highly correlated systems. The expressions for effective exchange parameters,
Dzialoshinskii- Moriya interaction, and magnetic anisotropy are derived. The
first-principle calculations of magnetic excitation spectrum for ferromagnetic
iron, with the local correlation effects from the numerically exact QMC-scheme
is presented.Comment: 17 pages, 3 Postscript figure
Exchange Effects in the Invar Hardening: as a test case
An increase of the critical resolved shear stress of Invar alloys (Invar
hardening) with a lowering temperature is explained. The effect is caused by a
growth of the exchange interaction between dangling -electron states of
dislocation cores and paramagnetic obstacles (e.g., Ni atoms in FeNi alloys)
which occurs below the Curie temperature. The spins of the two electrons align
along the magnetization due to the exchange interaction with the surrounding
atoms of the ferromagnetic. The exchange interaction between the dislocations
and obstacles is enhanced in Invars due to a strong growth of the magnetic
moments of atoms under the action of elastic strains near the dislocation
cores. Parameters characterizing the exchange interaction are determined for
the case of the FeNi Invar. The influence of the internal
magnetic field on the dislocation detachment from the obstacles is taken into
account. The obtained temperature dependence of the critical resolved shear
stress in the FeNi Invar agrees well with the available
experimental data. Experiments facilitating a further check of the theoretical
model are suggested.Comment: 8 pages, 2 figure
Sensitivity of the Mott Transition to Non-cubic Splitting of the Orbital Degeneracy: Application to NH3 K3C60
Within dynamical mean-field theory, we study the metal-insulator transition
of a twofold orbitally degenerate Hubbard model as a function of a splitting
\Delta of the degeneracy. The phase diagram in the U-\Delta plane exhibits
two-band and one-band metals, as well as the Mott insulator. The correlated
two-band metal is easily driven to the insulator state by a strikingly weak
splitting \Delta << W of the order of the Kondo-peak width zW, where z << 1 is
the metal quasiparticle weight. The possible relevance of this result to the
insulator-metal transition in the orthorhombic expanded fulleride NH3 K3C60 is
discussed.Comment: revtex, 15 pages including 6 ps figures. Submitted to Phys. Rev.
Lattice QCD Simulations in External Background Fields
We discuss recent results and future prospects regarding the investigation,
by lattice simulations, of the non-perturbative properties of QCD and of its
phase diagram in presence of magnetic or chromomagnetic background fields.
After a brief introduction to the formulation of lattice QCD in presence of
external fields, we focus on studies regarding the effects of external fields
on chiral symmetry breaking, on its restoration at finite temperature and on
deconfinement. We conclude with a few comments regarding the effects of
electromagnetic background fields on gluodynamics.Comment: 31 pages, 10 figures, minor changes and references added. To appear
in Lect. Notes Phys. "Strongly interacting matter in magnetic fields"
(Springer), edited by D. Kharzeev, K. Landsteiner, A. Schmitt, H.-U. Ye
Flux Phase as a Dynamic Jahn-Teller Phase: Berryonic Matter in the Cuprates?
There is considerable evidence for some form of charge ordering on the
hole-doped stripes in the cuprates, mainly associated with the low-temperature
tetragonal phase, but with some evidence for either charge density waves or a
flux phase, which is a form of dynamic charge-density wave. These three states
form a pseudospin triplet, demonstrating a close connection with the E X e
dynamic Jahn-Teller effect, suggesting that the cuprates constitute a form of
Berryonic matter. This in turn suggests a new model for the dynamic Jahn-Teller
effect as a form of flux phase. A simple model of the Cu-O bond stretching
phonons allows an estimate of electron-phonon coupling for these modes,
explaining why the half breathing mode softens so much more than the full
oxygen breathing mode. The anomalous properties of provide a coupling
(correlated hopping) which acts to stabilize density wave phases.Comment: Major Revisions: includes comparisons with specific cuprate phonon
modes, 16 eps figures, revte