274 research outputs found
Scaling and Decoherence in the Out-of-Equilibrium Kondo Model
We study the Kondo effect in quantum dots in an out-of-equilibrium state due
to an applied dc-voltage bias. Using the method of infinitesimal unitary
transformations (flow equations), we develop a perturbative scaling picture
that naturally contains both equilibrium coherent and non-equilibrium
decoherence effects. This framework allows one to study the competition between
Kondo effect and current-induced decoherence, and it establishes a large regime
dominated by single-channel Kondo physics for asymmetrically coupled quantum
dots.Comment: 4 pages, 3 figures; v2: minor changes (typos corrected, esp. in Eqs.
(3), (4), references updated, improved layout for figures
Quantum first order phase transitions
The scaling theory of critical phenomena has been successfully extended for
classical first order transitions even though the correlation length does not
diverge in these transitions. In this paper we apply the scaling ideas to
quantum first order transitions. The usefulness of this approach is illustrated
treating the problems of a superconductor coupled to a gauge field and of a
biquadratic Heisenberg chain, at zero temperature. In both cases there is a
latent energy associated with their discontinuous quantum transitions. We
discuss the effects of disorder and give a general criterion for it's relevance
in these transitions.Comment: 6 pages, 2 figures, misprints corrected and a reference added.
Version published in PHYSICA
The phase diagram of magnetic ladders constructed from a composite-spin model
White's density matrix renormalization group ({DMRG}) method has been applied
to an composite-spin model, which can also be considered as a
two-leg ladder model. By appropriate choices of the coupling constants this
model allows not only to study how the gap is opened around the gapless
integrable models, but also to interpolate continuously between models with
different spin lengths. We have found indications for the existence of several
different massive phases.Comment: 30 pages, 8 Postscript figure
Phase Transitions Between Topologically Distinct Gapped Phases in Isotropic Spin Ladders
We consider various two-leg ladder models exhibiting gapped phases. All of
these phases have short-ranged valence bond ground states, and they all exhibit
string order. However, we show that short-ranged valence bond ground states
divide into two topologically distinct classes, and as a consequence, there
exist two topologically distinct types of string order. Therefore, not all
gapped phases belong to the same universality class. We show that phase
transitions occur when we interpolate between models belonging to different
topological classes, and we study the nature of these transitions.Comment: 11 pages, 16 postscript figure
Stability of the Haldane phase in anisotropic magnetic ladders
We have considered the properties of anisotropic two-leg ladder models with
S=1/2 or S=1 spins on the rungs, using White's density matrix renormalization
group method. We have generalized the method by taking into account the
symmetries of the model in order to reduce the dimensions of the matrix to be
diagonalized, thereby making possible to consider more states. The boundaries
in the parameter space of the extended region, where the Haldane phase exists,
are estimated.Comment: 19 pages, 5 figure
Nonequilibrium Current in the One Dimensional Hubbard Model at Half-Filling
Nonlinear transport in the one dimensional Hubbard model at half-filling
under a finite bias voltage is investigated by the adaptive time-dependent
density matrix renormalization group method. For repulsive on-site interaction,
dielectric breakdown of the Mott insulating ground state to a current-carrying
nonequilibrium steady state is clearly observed when the voltage exceeds the
charge gap. It is found that by increasing the voltage further the
current-voltage characteristics are scaled only by the charge gap and the
scaling curve exhibits almost linear dependence on the voltage whose slope is
suppressed by the electron correlation. In the case of attractive interaction
the linear conductance is the perfect one which agrees with the
prediction by the Luttinger liquid theory.Comment: 4 pages, 7 figure
Spectral sum rules for the Tomonaga-Luttinger model
In connection with recent publications we discuss spectral sum rules for the
Tomonaga-Luttinger model without using the explicit result for the one-electron
Green's function. They are usefull in the interpretation of recent high
resolution photoemission spectra of quasi-one-dimensional conductors. It is
shown that the limit of infinite frequency and band cut\-off do not commute.
Our result for arbitrary shape of the interaction potential generalizes an
earlier discussion by Suzumura. A general analytical expression for the
spectral function for wave vectors far from the Fermi wave vector is
presented. Numerical spectra are shown to illustrate the sum rules.Comment: 9 pages, REVTEX 3.0, 2 figures added as postscript file
Boundary effects on one-particle spectra of Luttinger liquids
We calculate one-particle spectra for a variety of models of Luttinger
liquids with open boundary conditions. For the repulsive Hubbard model the
spectral weight close to the boundary is enhanced in a large energy range
around the chemical potential. A power law suppression, previously predicted by
bosonization, only occurs after a crossover at energies very close to the
chemical potential. Our comparison with exact spectra shows that the effects of
boundaries can partly be understood within the Hartree-Fock approximation.Comment: 4 pages including 4 figures, revised version, to be published in
Phys. Rev. B, January 200
Flow equation solution for the weak to strong-coupling crossover in the sine-Gordon model
A continuous sequence of infinitesimal unitary transformations, combined with
an operator product expansion for vertex operators, is used to diagonalize the
quantum sine-Gordon model for 2 pi < beta^2 < infinity. The leading order of
this approximation already gives very accurate results for the single-particle
gap in the strong-coupling phase. This approach can be understood as an
extension of perturbative scaling theory since it links weak to strong-coupling
behavior in a systematic expansion. The approach should also be useful for
other strong-coupling problems that can be formulated in terms of vertex
operators.Comment: 4 pages, 1 figure, minor changes (typo in Eq. (3) corrected,
references added), published versio
Search for the Nondimerized Quantum Nematic Phase in the Spin-1 Chain
Chubukov's proposal concerning the possibility of a nondimerized quantum
nematic phase in the ground-state phase diagram of the bilinear-biquadratic
spin-1 chain is studied numerically. Our results do not support the existence
of this phase, but they rather indicate a direct transition from the
ferromagnetic into the dimerized phase.Comment: REVTEX, 14 pages +8 PostScript figure
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