1,403 research outputs found
Electronic Structure and Magnetic Exchange Coupling in Ferromagnetic Full Heusler Alloys
Density-functional studies of the electronic structures and exchange
interaction parameters have been performed for a series of ferromagnetic full
Heusler alloys of general formula CoMnZ (Z = Ga, Si, Ge, Sn), RhMnZ (Z
= Ge, Sn, Pb), NiMnSn, CuMnSn and PdMnSn, and the connection
between the electronic spectra and the magnetic interactions have been studied.
Different mechanisms contributing to the exchange coupling are revealed. The
band dependence of the exchange parameters, their dependence on volume and
valence electron concentration have been thoroughly analyzed within the Green
function technique.Comment: 9 figures, 6 table
The spin angular gradient approximation in the density functional theory
A spin angular gradient approximation for the exchange correlation magnetic
field in the density functional formalism is proposed. The usage of such
corrections leads to a consistent spin dynamical approach beyond the local
approximation. The proposed technique does not contain any approximations for
the form of potential and can be used in modern full potential band structure
methods. The obtained results indicate that the direct 'potential' exchange in
3d magnets is rather small compared to the indirect 'kinetic' exchange, thus
justifies the dynamical aspect of the local density approximation in 3d metals
Mott Transition in Degenerate Hubbard Models: Application to Doped Fullerenes
The Mott-Hubbard transition is studied for a Hubbard model with orbital
degeneracy N, using a diffusion Monte-Carlo method. Based on general arguments,
we conjecture that the Mott-Hubbard transition takes place for U/W \propto
\sqrt{N}, where U is the Coulomb interaction and W is the band width. This is
supported by exact diagonalization and Monte-Carlo calculations. Realistic
parameters for the doped fullerenes lead to the conclusion that stoichiometric
A_3 C_60 (A=K, Rb) are near the Mott-Hubbard transition, in a correlated
metallic state.Comment: 4 pages, revtex, 1 eps figure included, to be published in Phys.Rev.B
Rapid Com
Metal-insulator transitions: Influence of lattice structure, Jahn-Teller effect, and Hund's rule coupling
We study the influence of the lattice structure, the Jahn-Teller effect and
the Hund's rule coupling on a metal-insulator transition in AnC60 (A= K, Rb).
The difference in lattice structure favors A3C60 (fcc) being a metal and A4C60
(bct) being an insulator, and the coupling to Hg Jahn-Teller phonons favors
A4C60 being nonmagnetic. The coupling to Hg (Ag) phonons decreases (increases)
the value Uc of the Coulomb integral at which the metal-insulator transition
occurs. There is an important partial cancellation between the Jahn-Teller
effect and the Hund's rule coupling.Comment: 4 pages, RevTeX, 3 eps figure, additional material available at
http://www.mpi-stuttgart.mpg.de/docs/ANDERSEN/fullerene
Screening, Coulomb pseudopotential, and superconductivity in alkali-doped Fullerenes
We study the static screening in a Hubbard-like model using quantum Monte
Carlo. We find that the random phase approximation is surprisingly accurate
almost up to the Mott transition. We argue that in alkali-doped Fullerenes the
Coulomb pseudopotential is not very much reduced by retardation
effects. Therefore efficient screening is important in reducing
sufficiently to allow for an electron-phonon driven superconductivity. In this
way the Fullerides differ from the conventional picture, where retardation
effects play a major role in reducing the electron-electron repulsion.Comment: 4 pages RevTeX with 2 eps figures, additional material available at
http://www.mpi-stuttgart.mpg.de/docs/ANDERSEN/fullerene
Nonperturbative approach to the Hubbard model in C60 cluster
We propose a computational scheme for the Hubbard model in the C60 cluster in
which the interaction with the Fermi sea of charges added to the neutral
molecule is switched on sequentially. This is applied to the calculation of the
balance of charging energies, within a low-energy truncation of the space of
states which produces moderate errors for an intermediate range of the
interaction strength.Comment: 5 pages, Revtex, 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.
Theory of Superconducting of doped fullerenes
We develop the nonadiabatic polaron theory of superconductivity of
taking into account the polaron band narrowing and realistic
electron-phonon and Coulomb interactions. We argue that the crossover from the
BCS weak-coupling superconductivity to the strong-coupling polaronic and
bipolaronic superconductivity occurs at the BCS coupling constant independent of the adiabatic ratio, and there is nothing ``beyond'' Migdal's
theorem except small polarons for any realistic electron-phonon interaction. By
the use of the polaronic-type function and the ``exact'' diagonalization in the
truncated Hilbert space of vibrons (``phonons'') we calculate the ground state
energy and the electron spectral density of the molecule. This
allows us to describe the photoemission spectrum of in a wide
energy region and determine the electron-phonon interaction. The strongest
coupling is found with the high-frequency pinch mode and with the
Frenkel exciton. We clarify the crucial role of high-frequency bosonic
excitations in doped fullerenes which reduce the bare bandwidth and the Coulomb
repulsion allowing the intermediate and low-frequency phonons to couple two
small polarons in a Cooper pair. The Eliashberg-type equations are solved for
low-frequency phonons. The value of the superconducting , its pressure
dependence and the isotope effect are found to be in a remarkable agreement
with the available experimental data.Comment: 20 pages, Latex, 4 figures available upon reques
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