7,419 research outputs found
Conductivity in a disordered one-dimensional system of interacting fermions
Dynamical conductivity in a disordered one-dimensional model of interacting
fermions is studied numerically at high temperatures and in the
weak-interaction regime in order to find a signature of many-body localization
and vanishing d.c. transport coefficients. On the contrary, we find in the
regime of moderately strong local disorder that the d.c. conductivity sigma0
scales linearly with the interaction strength while being exponentially
dependent on the disorder. According to the behavior of the charge stiffness
evaluated at the fixed number of particles, the absence of the many-body
localization seems related to an increase of the effective localization length
with the interaction.Comment: 4 pages, 5 figures, submitted to PR
Wave functions in the neighborhood of a toroidal surface; hard vs. soft constraint
The curvature potential arising from confining a particle initially in
three-dimensional space onto a curved surface is normally derived in the hard
constraint limit, with the degree of freedom normal to the
surface. In this work the hard constraint is relaxed, and eigenvalues and wave
functions are numerically determined for a particle confined to a thin layer in
the neighborhood of a toroidal surface. The hard constraint and finite layer
(or soft constraint) quantities are comparable, but both differ markedly from
those of the corresponding two dimensional system, indicating that the
curvature potential continues to influence the dynamics when the particle is
confined to a finite layer. This effect is potentially of consequence to the
modelling of curved nanostructures.Comment: 4 pages, no fig
On the origin of unusual transport properties observed in densely packed polycrystalline CaAl_{2}
A possible origin of unusual temperature behavior of transport coefficients
observed in densely packed polycrystalline CaAl_{2} compound [M. Ausloos et
al., J. Appl. Phys. 96, 7338 (2004)] is discussed, including a power-like
dependence of resistivity with and N-like form of the
thermopower. All these features are found to be in good agreement with the
Shklovskii-Efros localization scenario assuming polaron-mediated hopping
processes controlled by the Debye energy
Photon deflection by a Coulomb field in noncommutative QED
In noncommutative QED photons present self-interactions in the form of triple
and quartic interactions. The triple interaction implies that, even though the
photon is electrically neutral, it will deflect when in the presence of an
electromagnetic field. If detected, such deflection would be an undoubted
signal of noncommutative space-time. In this work we derive the general
expression for the deflection of a photon by any electromagnetic field. As an
application we consider the case of the deflection of a photon by an external
static Coulomb field.Comment: 07 pages, some typos corrected, accepted for publication in JP
Low temperature terahertz spectroscopy of n-InSb through a magnetic field driven metal-insulator transition
We use fiber-coupled photoconductive emitters and detectors to perform
terahertz (THz) spectroscopy of lightly-doped n-InSb directly in the cryogenic
(1.5 K) bore of a high-field superconducting magnet. We measure transmission
spectra from 0.1-1.1 THz as the sample is driven through a metal-insulator
transition (MIT) by applied magnetic field. In the low-field metallic state,
the data directly reveal the plasma edge and magneto-plasmon modes. With
increasing field, a surprisingly broad band (0.3-0.8 THz) of low transmission
appears at the onset of the MIT. This band subsequently collapses and evolves
into the sharp 1s -> 2p- transition of electrons `frozen' onto isolated donors
in the insulating state.Comment: 4 pages, 3 figure
Fermi-Hubbard physics with atoms in an optical lattice
The Fermi-Hubbard model is a key concept in condensed matter physics and
provides crucial insights into electronic and magnetic properties of materials.
Yet, the intricate nature of Fermi systems poses a barrier to answer important
questions concerning d-wave superconductivity and quantum magnetism. Recently,
it has become possible to experimentally realize the Fermi-Hubbard model using
a fermionic quantum gas loaded into an optical lattice. In this atomic approach
to the Fermi-Hubbard model the Hamiltonian is a direct result of the optical
lattice potential created by interfering laser fields and short-ranged
ultracold collisions. It provides a route to simulate the physics of the
Hamiltonian and to address open questions and novel challenges of the
underlying many-body system. This review gives an overview of the current
efforts in understanding and realizing experiments with fermionic atoms in
optical lattices and discusses key experiments in the metallic,
band-insulating, superfluid and Mott-insulating regimes.Comment: Posted with permission from the Annual Review of of Condensed Matter
Physics Volume 1 \c{opyright} 2010 by Annual Reviews,
http://www.annualreviews.or
Mott physics and first-order transition between two metals in the normal state phase diagram of the two-dimensional Hubbard model
For doped two-dimensional Mott insulators in their normal state, the
challenge is to understand the evolution from a conventional metal at high
doping to a strongly correlated metal near the Mott insulator at zero doping.
To this end, we solve the cellular dynamical mean-field equations for the
two-dimensional Hubbard model using a plaquette as the reference quantum
impurity model and continuous-time quantum Monte Carlo method as impurity
solver. The normal-state phase diagram as a function of interaction strength
, temperature , and filling shows that, upon increasing towards
the Mott insulator, there is a surface of first-order transition between two
metals at nonzero doping. That surface ends at a finite temperature critical
line originating at the half-filled Mott critical point. Associated with this
transition, there is a maximum in scattering rate as well as thermodynamic
signatures. These findings suggest a new scenario for the normal-state phase
diagram of the high temperature superconductors. The criticality surmised in
these systems can originate not from a T=0 quantum critical point, nor from the
proximity of a long-range ordered phase, but from a low temperature transition
between two types of metals at finite doping. The influence of Mott physics
therefore extends well beyond half-filling.Comment: 27 pages, 16 figures, LaTeX, published versio
Nearly frozen Coulomb Liquids
We show that very long range repulsive interactions of a generalized
Coulomb-like form , with (-dimensionality),
typically introduce very strong frustration, resulting in extreme fragility of
the charge-ordered state. An \textquotedbl{}almost frozen\textquotedbl{} liquid
then survives in a broad dynamical range above the (very low) melting
temperature which is proportional to . This
\textquotedbl{}pseudogap\textquotedbl{} phase is characterized by unusual
insulating-like, but very weakly temperature dependent transport, similar to
experimental findings in certain low carrier density systems.Comment: 5 pages,4 figure
Valence-bond theory of highly disordered quantum antiferromagnets
We present a large-N variational approach to describe the magnetism of
insulating doped semiconductors based on a disorder-generalization of the
resonating-valence-bond theory for quantum antiferromagnets. This method
captures all the qualitative and even quantitative predictions of the
strong-disorder renormalization group approach over the entire experimentally
relevant temperature range. Finally, by mapping the problem on a hard-sphere
fluid, we could provide an essentially exact analytic solution without any
adjustable parameters.Comment: 5 pages, 3 eps figure
How are turbulent sensible heat fluxes and snow melt rates affected by a changing snow cover fraction?
The complex interaction between the atmospheric boundary layer and the heterogeneous land surface is typically not resolved in numerical models approximating the turbulent heat exchange processes. In this study, we consider the effect of the land surface heterogeneity on the spatial variability of near-surface air temperature fields and on snow melt processes. For this purpose we calculated turbulent sensible heat fluxes and daily snow depth depletion rates with the physics-based surface process model Alpine3D. To account for the effect of a heterogeneous land surface (such as patchy snow covers) on the local energy balance over snow, Alpine3D is driven by twodimensional atmospheric fields of air temperature and wind velocity, generated with the non-hydrostatic atmospheric model Advanced Regional Prediction System. The atmospheric model is initialized with a set of snow distributions [snow cover fraction (SCF) and number of snow patches] and atmospheric conditions (wind velocities) for an idealized flat test site. Numerical results show that the feedback of the heterogeneity of the land surface (snow, no snow) on the near-surface variability of the atmospheric fields result in a significant increase in the mean air temperature ΔTa = 1.8 K (3.7 and 4.9 K) as the SCF is decreased from a continuous snow cover to 55% (25 and 5%). Mean air temperatures over snow heavily increase with increasing initial wind velocities and weakly increase with an increasing number of snow patches. Surface turbulent sensible heat fluxes and daily snow depth depletion rates are strongly correlated to mean air temperatures, leading to 22–40% larger daily snow depth depletion rates for patchy snow covers. Numerical results from the idealized test site are compared with a test site in complex terrain. As slope-induced atmospheric processes (such as the development of katabatic flows) modify turbulent sensible heat fluxes, the variation of the surface energy balance is larger in complex terrain than for an idealized flat test site
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