1,002 research outputs found
Propagation of a hole on a Neel background
We analyze the motion of a single hole on a N\'eel background, neglecting
spin fluctuations. Brinkman and Rice studied this problem on a cubic lattice,
introducing the retraceable-path approximation for the hole Green's function,
exact in a one-dimensional lattice. Metzner et al. showed that the
approximationalso becomes exact in the infinite-dimensional limit. We introduce
a new approach to this problem by resumming the Nagaoka expansion of the
propagator in terms of non-retraceable skeleton-paths dressed by
retraceable-path insertions. This resummation opens the way to an almost
quantitative solution of the problemin all dimensions and, in particular sheds
new light on the question of the position of the band-edges. We studied the
motion of the hole on a double chain and a square lattice, for which deviations
from the retraceable-path approximation are expected to be most pronounced. The
density of states is mostly adequately accounted for by the
retra\-ce\-able-path approximation. Our band-edge determination points towards
an absence of band tails extending to the Nagaoka energy in the spectrums of
the double chain and the square lattice. We also evaluated the spectral density
and the self-energy, exhibiting k-dependence due to finite dimensionality. We
find good agreement with recent numerical results obtained by Sorella et al.
with the Lanczos spectra decoding method. The method we employ enables us to
identify the hole paths which are responsible for the various features present
in the density of states and the spectral density.Comment: 26 pages,Revte
Excitonic order at strong-coupling: pseudo-spins, doping, and ferromagnetism
A tight binding model is introduced to describe the strong interaction limit
of excitonic ordering. At stoichiometry, the model reduces in the strong
coupling limit to a pseudo-spin model with approximate U(4) symmetry. Excitonic
order appears in the pseudo-spin model as in-plane pseudo-magnetism. The U(4)
symmetry unifies all possible singlet and triplet order parameters describing
such states. Super-exchange, Hunds-rule coupling, and other perturbations act
as anisotropies splitting the U(4) manifold, ultimately stabilizing a
paramagnetic triplet state. The tendency to ferromagnetism with doping
(observed experimentally in the hexaborides) is explained as a spin-flop
transition to a different orientation of the U(4) order parameter. The physical
mechanism favoring such a reorientation is the enhanced coherence (and hence
lower kinetic energy) of the doped electrons in a ferromagnetic background
relative to the paramagnet. A discussion of the physical meaning of various
excitonic states and their experimental consequences is also provided.Comment: 16 pages, 5 figure
On the derivation of the t-J model: electron spectrum and exchange interactions in narrow energy bands
A derivation of the t-J model of a highly-correlated solid is given starting
from the general many-electron Hamiltonian with account of the
non-orthogonality of atomic wave functions. Asymmetry of the Hubbard subbands
(i.e. of ``electron'' and ``hole''cases) for a nearly half-filled bare band is
demonstrated. The non-orthogonality corrections are shown to lead to occurrence
of indirect antiferromagnetic exchange interaction even in the limit of the
infinite on-site Coulomb repulsion. Consequences of this treatment for the
magnetism formation in narrow energy bands are discussed. Peculiarities of the
case of ``frustrated'' lattices, which contain triangles of nearest neighbors,
are considered.Comment: 4 pages, RevTe
Gauge Invariance and Hall Terms in the Quasiclassical Equations of Superconductivity
This paper presents a careful derivation of the quasiclassical equations of
superconductivity so that a manifest gauge invariance is retained with respect
to the space-time arguments of the quasiclassical Green's function .
The terms responsible for the Hall effect naturally appear from the derivation.
The equations are applicable to clean as well as dirty superconductors for an
arbitrary external frequency much smaller than the Fermi energy. Thus, they
will form a basis toward a complete microscopic understanding of the Hall
effect in type-II superconductors.Comment: 9 pages, 1 figur
Disorder-quenched Kondo effect in mesosocopic electronic systems
Nonmagnetic disorder is shown to quench the screening of magnetic moments in
metals, the Kondo effect. The probability that a magnetic moment remains free
down to zero temperature is found to increase with disorder strength.
Experimental consequences for disordered metals are studied. In particular, it
is shown that the presence of magnetic impurities with a small Kondo
temperature enhances the electron's dephasing rate at low temperatures in
comparison to the clean metal case. It is furthermore proven that the width of
the distribution of Kondo temperatures remains finite in the thermodynamic
(infinite volume) limit due to wave function correlations within an energy
interval of order , where is the elastic scattering time. When
time-reversal symmetry is broken either by applying a magnetic field or by
increasing the concentration of magnetic impurities, the distribution of Kondo
temperatures becomes narrower.Comment: 17 pages, 7 figures, new results on Kondo effect in quasi-1D wires
added, 6 Refs. adde
Typical support and Sanov large deviations of correlated states
Discrete stationary classical processes as well as quantum lattice states are
asymptotically confined to their respective typical support, the exponential
growth rate of which is given by the (maximal ergodic) entropy. In the iid case
the distinguishability of typical supports can be asymptotically specified by
means of the relative entropy, according to Sanov's theorem. We give an
extension to the correlated case, referring to the newly introduced class of
HP-states.Comment: 29 pages, no figures, references adde
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