302 research outputs found

### Spin Coulomb drag in the two-dimensional electron liquid

We calculate the spin-drag transresistivity $\rho_{\uparrow \downarrow}(T)$
in a two-dimensional electron gas at temperature $T$ in the random phase
approximation. In the low-temperature regime we show that, at variance with the
three-dimensional low-temperature result [$\rho_{\uparrow\downarrow}(T) \sim
T^2$], the spin transresistivity of a two-dimensional {\it spin unpolarized}
electron gas has the form $\rho_{\uparrow\downarrow}(T) \sim T^2 \ln T$. In the
spin-polarized case the familiar form $\rho_{\uparrow\downarrow}(T) =A T^2$ is
recovered, but the constant of proportionality $A$ diverges logarithmically as
the spin-polarization tends to zero. In the high-temperature regime we obtain
$\rho_{\uparrow \downarrow}(T) = -(\hbar / e^2) (\pi^2 Ry^* /k_B T)$ (where
$Ry^*$ is the effective Rydberg energy) {\it independent} of the density.
Again, this differs from the three-dimensional result, which has a logarithmic
dependence on the density. Two important differences between the spin-drag
transresistivity and the ordinary Coulomb drag transresistivity are pointed
out: (i) The $\ln T$ singularity at low temperature is smaller, in the Coulomb
drag case, by a factor $e^{-4 k_Fd}$ where $k_F$ is the Fermi wave vector and
$d$ is the separation between the layers. (ii) The collective mode contribution
to the spin-drag transresistivity is negligible at all temperatures. Moreover
the spin drag effect is, for comparable parameters, larger than the ordinary
Coulomb drag effect.Comment: 6 figures; various changes; version accepted for publicatio

### Nondissipative Drag Conductance as a Topological Quantum Number

We show in this paper that the boundary condition averaged nondissipative
drag conductance of two coupled mesoscopic rings with no tunneling, evaluated
in a particular many-particle eigenstate, is a topological invariant
characterized by a Chern integer. Physical implications of this observation are
discussed.Comment: 4 pages, no figure. Title modified and significant revision made to
the text. Final version appeared in PR

### Four-Wave mixing in degenerate Fermi gases: Beyond the undepleted pump approximation

We analyze the full nonlinear dynamics of the four-wave mixing between an
incident beam of fermions and a fermionic density grating. We find that when
the number of atoms in the beam is comparable to the number of atoms forming
the grating, the dephasing of that grating, which normally leads to a decay of
its amplitude, is suppressed. Instead, the density grating and the beam density
exhibit large nonlinear coupled amplitude oscillations. In this case four-wave
mixing can persist for much longer times compared to the case of negligible
back-action. We also evaluate the efficiency of the four-wave mixing and show
that it can be enhanced by producing an initial density grating with an
amplitude that is less than the maximum value. These results indicate that
efficient four-wave mixing in fermionic alkali gases should be experimentally
observable.Comment: 9 pages, 8 figure

### Temperature Dependence of Hall Response in Doped Antiferromagnets

Using finite-temperature Lanczos method the frequency-dependent Hall response
is calculated numerically for the t-J model on the square lattice and on
ladders. At low doping, both the high-frequency RH* and the d.c. Hall
coefficient RH0 follow qualitatively similar behavior at higher temperatures:
being hole-like for T > Ts~1.5J and weakly electron-like for T < Ts. Consistent
with experiments on cuprates, RH0 changes, in contrast to RH*, again to the
hole-like sign below the pseudogap temperature T*, revealing a strong
temperature variation for T->0.Comment: LaTeX, 4 pages, 4 figures, submitted to PR

### Dynamics of Fermionic Four-Wave Mixing

We study the dynamics of a beam of fermions diffracted off a density grating
formed by fermionic atoms in the limit of a large grating. An exact description
of the system in terms of particle-hole operators is developed. We use a
combination of analytical and numerical methods to quantitatively explore the
Raman-Nath and the Bragg regimes of diffraction. We discuss the limits in
diffraction efficiency resulting from the dephasing of the grating due the
distribution of energy states occupied by the fermions. We propose several
methods to overcome these limits, including the novel technique of ``atom
echoes''.Comment: 8 pages, 7 figure

### Quasiparticles and c-axis coherent hopping in high T_c superconductors

We study the problem of the low-energy quasiparticle spectrum of the extended
t-J model and analyze the coherent hopping between weakly coupled planes
described by this model. Starting with a two-band model describing the Cu-O
planes and the unoccupied bands associated to the metallic atoms located in
between the planes, we obtain effective hopping matrix elements describing the
c-axis charge transfer. A computational study of these processes shows an
anomalously large charge anisotropy for doping concentrations around and below
the optimal doping.Comment: 4 pages, 3 figure

### Mesoscopic fluctuations of Coulomb drag between quasi-ballistic 1D-wires

Quasiballistic 1D quantum wires are known to have a conductance of the order
of 2e^2/h, with small sample-to-sample fluctuations. We present a study of the
transconductance G_12 of two Coulomb-coupled quasiballistic wires, i.e., we
consider the Coulomb drag geometry. We show that the fluctuations in G_12
differ dramatically from those of the diagonal conductance G_ii: the
fluctuations are large, and can even exceed the mean value, thus implying a
possible reversal of the induced drag current. We report extensive numerical
simulations elucidating the fluctuations, both for correlated and uncorrelated
disorder. We also present analytic arguments, which fully account for the
trends observed numerically.Comment: 10 pages including 7 figures. Minor changes according to referee
report. Accepted for PR

### Interference of a Tonks-Girardeau Gas on a Ring

We study the quantum dynamics of a one-dimensional gas of impenetrable bosons
on a ring, and investigate the interference that results when an initially
trapped gas localized on one side of the ring is released, split via an
optical-dipole grating, and recombined on the other side of the ring. Large
visibility interference fringes arise when the wavevector of the optical dipole
grating is larger than the effective Fermi wavevector of the initial gas.Comment: 7 pages, 3 figure

### Spin effects in the magneto-drag between double quantum wells

We report on the selectivity to spin in a drag measurement. This selectivity
to spin causes deep minima in the magneto-drag at odd fillingfactors for
matched electron densities at magnetic fields and temperatures at which the
bare spin energy is only one tenth of the temperature. For mismatched densities
the selectivity causes a novel 1/B-periodic oscillation, such that negative
minima in the drag are observed whenever the majority spins at the Fermi
energies of the two-dimensional electron gasses (2DEGs) are anti-parallel, and
positive maxima whenever the majority spins at the Fermi energies are parallel.Comment: 4 pages, 3 figure

### Magnetotunneling spectroscopy of mesoscopic correlations in two-dimensional electron systems

An approach to experimentally exploring electronic correlation functions in
mesoscopic regimes is proposed. The idea is to monitor the mesoscopic
fluctuations of a tunneling current flowing between the two layers of a
semiconductor double-quantum-well structure. From the dependence of these
fluctuations on external parameters, such as in-plane or perpendicular magnetic
fields, external bias voltages, etc., the temporal and spatial dependence of
various prominent correlation functions of mesoscopic physics can be
determined. Due to the absence of spatially localized external probes, the
method provides a way to explore the interplay of interaction and localization
effects in two-dimensional systems within a relatively unperturbed environment.
We describe the theoretical background of the approach and quantitatively
discuss the behavior of the current fluctuations in diffusive and ergodic
regimes. The influence of both various interaction mechanisms and localization
effects on the current is discussed. Finally a proposal is made on how, at
least in principle, the method may be used to experimentally determine the
relevant critical exponents of localization-delocalization transitions.Comment: 15 pages, 3 figures include

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