8,239 research outputs found
Finite temperature properties of the triangular lattice t-J model, applications to NaCoO
We present a finite temperature () study of the t-J model on the
two-dimensional triangular lattice for the negative hopping , as relevant
for the electron-doped NaCoO (NCO). To understand several aspects of
this system, we study the -dependent chemical potential, specific heat,
magnetic susceptibility, and the dynamic Hall-coefficient across the entire
doping range. We show systematically, how this simplest model for strongly
correlated electrons describes a crossover as function of doping () from a
Pauli-like weakly spin-correlated metal close to the band-limit (density )
to the Curie-Weiss metallic phase () with pronounced
anti-ferromagnetic (AFM) correlations at low temperatures and Curie-Weiss type
behavior in the high-temperature regime. Upon further reduction of the doping,
a new energy scale, dominated by spin-interactions () emerges (apparent both
in specific heat and susceptibility) and we identify an effective interaction
, valid across the entire doping range. This is distinct from
Anderson's formula, as we choose here , hence the opposite sign of the
usual Nagaoka-ferromagnetic situation. This expression includes the subtle
effect of weak kinetic AFM - as encountered in the infinitely correlated
situation (). By explicit computation of the Kubo-formulae, we
address the question of practical relevance of the high-frequency expression
for the Hall coefficient . We hope to clarify some open questions
concerning the applicability of the t-J model to real experimental situations
through this study
Critical Dynamics of a Two-dimensional Superfluid near a Non-Thermal Fixed Point
Critical dynamics of an ultracold Bose gas far from equilibrium is studied in
two spatial dimensions. Superfluid turbulence is created by quenching the
equilibrium state close to zero temperature. Instead of immediately
re-thermalizing, the system approaches a meta-stable transient state,
characterized as a non-thermal fixed point. A focus is set on the vortex
density and vortex-antivortex correlations which characterize the evolution
towards the non-thermal fixed point and the departure to final
(quasi-)condensation. Two distinct power-law regimes in the vortex-density
decay are found and discussed in terms of a vortex binding-unbinding transition
and a kinetic description of vortex scattering. A possible relation to decaying
turbulence in classical fluids is pointed out. By comparing the results to
equilibrium studies of a two-dimensional Bose gas, an intuitive understanding
of the location of the non-thermal fixed point in a reduced phase space is
developed.Comment: 11 pages, 13 figures; PRA versio
Uniform shrinking and expansion under isotropic Brownian flows
We study some finite time transport properties of isotropic Brownian flows.
Under a certain nondegeneracy condition on the potential spectral measure, we
prove that uniform shrinking or expansion of balls under the flow over some
bounded time interval can happen with positive probability. We also provide a
control theorem for isotropic Brownian flows with drift. Finally, we apply the
above results to show that under the nondegeneracy condition the length of a
rectifiable curve evolving in an isotropic Brownian flow with strictly negative
top Lyapunov exponent converges to zero as with positive
probability
Nonequilibrium Kondo Effect in a Quantum Dot Coupled to Ferromagnetic Leads
We study the Kondo effect in the electron transport through a quantum dot
coupled to ferromagnetic leads, using a real-time diagrammatic technique which
provides a systematic description of the nonequilibrium dynamics of a system
with strong local electron correlations. We evaluate the theory in an extension
of the `resonant tunneling approximation', introduced earlier, by introducing
the self-energy of the off-diagonal component of the reduced propagator in spin
space. In this way we develop a charge and spin conserving approximation that
accounts not only for Kondo correlations but also for the spin splitting and
spin accumulation out of equilibrium. We show that the Kondo resonances, split
by the applied bias voltage, may be spin polarized. A left-right asymmetry in
the coupling strength and/or spin polarization of the electrodes significantly
affects both the spin accumulation and the weight of the split Kondo resonances
out of equilibrium. The effects are observable in the nonlinear differential
conductance. We also discuss the influence of decoherence on the Kondo
resonance in the frame of the real-time formulation.Comment: 13 pages, 13 figure
Thermal spin transport and spin-orbit interaction in ferromagnetic/non-magnetic metals
In this article we extend the currently established diffusion theory of
spin-dependent electrical conduction by including spin-dependent
thermoelectricity and thermal transport. Using this theory, we propose new
experiments aimed at demonstrating novel effects such as the spin-Peltier
effect, the reciprocal of the recently demonstrated thermally driven spin
injection, as well as the magnetic heat valve. We use finite-element methods to
model specific devices in literature to demonstrate our theory. Spin-orbit
effects such as anomalous-Hall, -Nernst, anisotropic magnetoresistance and
spin-Hall are also included in this model
On the chiral anomaly in non-Riemannian spacetimes
The translational Chern-Simons type three-form coframe torsion on a
Riemann-Cartan spacetime is related (by differentiation) to the Nieh-Yan
four-form. Following Chandia and Zanelli, two spaces with non-trivial
translational Chern-Simons forms are discussed. We then demonstrate, firstly
within the classical Einstein-Cartan-Dirac theory and secondly in the quantum
heat kernel approach to the Dirac operator, how the Nieh-Yan form surfaces in
both contexts, in contrast to what has been assumed previously.Comment: 18 pages, RevTe
Intrinsic Geometric Structure of Quantum Gravity
We couple c=-2 matter to 2-dimensional gravity within the framework of dynamical triangulations. We use a very fast algorithm, special to the c=-2 case, in order to test scaling of correlation functions defined in terms of geodesic distance and we determine the fractal dimension d_H with high accuracy. We find d_H=3.58(4), consistent with a prediction coming from the study of diffusion in the context of Liouville theory, and that the quantum space-time possesses the same fractal properties at all distance scales similarly to the case of pure gravity
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