2,834 research outputs found
Microscopic conditions favoring itinerant ferromagnetism: Hund's rule coupling and orbital degeneracy
The importance of Hund's rule coupling for the stabilization of itinerant
ferromagnetism is investigated within a two-band Hubbard model. The magnetic
phase diagram is calculated by finite-temperature quantum Monte Carlo
simulations within the dynamical mean-field theory. Ferromagnetism is found in
a broad range of electron fillings whereas antiferromagnetism exists only near
half filling. The possibility of orbital ordering at quarter filling is also
analyzed.Comment: 5 pages, 6 figures, RevTeX, final version contains an additional
phase diagram for smaller Hund's rule coupling. to appear in Eur. Phys. J. B
(1998
T-matrix formulation of real-space dynamical mean-field theory and the Friedel sum rule for correlated lattice fermions
We formulate real-space dynamical mean-field theory within scattering theory.
Thereby the Friedel sum rule is derived for interacting lattice fermions at
zero temperature.Comment: 7 pages, no figures, extended and corrected versio
Multitude of phases in correlated lattice fermion systems with spin-dependent disorder
The magnetic phases induced by the interplay between disorder acting only on
particles with a given spin projection ("spin-dependent disorder") and a local
repulsive interaction is explored. To this end the magnetic ground state phase
diagram of the Hubbard model at half-filling is computed within dynamical
mean-field theory combined with the geometric average over disorder, which is
able to describe Anderson localization. Five distinct phases are identified: a
ferromagnetically polarized metal, two types of insulators, and two types of
spin-selective localized phases. The latter four phases possess different
long-range order of the spins. The predicted phase diagram may be tested
experimentally using cold fermions in optical lattices subject to
spin-dependent random potentials.Comment: 8 pages, 9 figures, revised versio
Correlated-Electron Theory of Strongly Anisotropic Metamagnets
We present the first correlated-electron theory of metamagnetism in strongly
anisotropic antiferromagnets. Quantum-Monte-Carlo techniques are used to
calculate the field vs. temperature phase diagram of the infinite-dimensional
Hubbard model with easy axis. A metamagnetic transition scenario with 1.~order
and 2.~order phase transitions is found. The apparent similarities to the phase
diagram of FeBr and to mean-field results for the Ising model with
competing interactions are discussed.Comment: 4 pages, RevTeX + one uuencoded ps-file including 3 figure
Spin-selective localization of correlated lattice fermions
The interplay between local, repulsive interactions and disorder acting only
on one spin orientation of lattice fermions ("spin-dependent disorder") is
investigated. The nonmagnetic disorder vs. interaction phase diagram is
computed using Dynamical Mean-Field Theory in combination with the geometric
average over disorder. The latter determines the typical local density of
states and is therefore sensitive to Anderson localization. The effect of
spin-dependent disorder is found to be very different from that of conventional
disorder. In particular, it destabilizes the metallic solution and leads to a
novel spin-selective, localized phase at weak interactions and strong disorder
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Strong-coupling solution of the bosonic dynamical mean-field theory
We derive an approximate analytical solution of the self-consistency
equations of the bosonic dynamical mean-field theory (B-DMFT) in the
strong-coupling limit. The approach is based on a linked-cluster expansion in
the hybridization function of normal bosons around the atomic limit. The
solution is used to compute the phase diagram of the bosonic Hubbard model for
different lattices. We compare our results with numerical solutions of the
B-DMFT equations and numerically exact methods, respectively. The very good
agreement with those numerical results demonstrates that our approach captures
the essential physics of correlated bosons both in the Mott insulator and in
the superfluid phase. Close to the transition into the superfluid phase the
momentum distribution function at zero momentum is found to be strongly
enhanced already in the normal phase. The linked-cluster expansion also allows
us to compute dynamical properties such as the spectral function of bosons. The
evolution of the spectral function across the transition from the normal to the
superfluid phase is seen to be characteristically different for the interaction
driven and density driven transition, respectively.Comment: 8 pages, 6 figure
A mean-field theory of Anderson localization
Anderson model of noninteracting disordered electrons is studied in high
spatial dimensions. We find that off-diagonal one- and two-particle propagators
behave as gaussian random variables w.r.t. momentum summations. With this
simplification and with the electron-hole symmetry we reduce the parquet
equations for two-particle irreducible vertices to a single algebraic equation
for a local vertex. We find a disorder-driven bifurcation point in this
equation signalling vanishing of diffusion and onset of Anderson localization.
There is no bifurcation in where all states are localized. A natural
order parameter for Anderson localization pops up in the construction.Comment: REVTeX4, 4 pages, 2 EPS figure
Dissipative Currents in Superfluid 3He Weak Links
We calculate the current-pressure relation for pinholes connecting two
volumes of bulk superfluid 3He-B. The theory of multiple Andreev reflections,
adapted from superconducting weak links, leads to a nonlinear dependence of the
dc current on pressure bias. In arrays of pinholes one has to take into account
oscillations of the texture at the Josephson frequency. The associated
radiation of spin waves from the junction leads to an additional dissipative
current at small biases, in quantitative agreement with measurements.Comment: 4 pages, 3 figures; updated to the published versio
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