270 research outputs found

### Density of states of the interacting two-dimensional electron gas

We study the influence of electron-electron interactions on the density of
states (DOS) of clean 2D electron gas. We confirm the linear cusp in the DOS
around the Fermi level, which was obtained previously. The cusp crosses over to
a pure logarithmic dependence further away from the Fermi surface.Comment: RevTeX, 3 pages, no figure

### Magnetoresistance and dephasing in a two-dimensional electron gas at intermediate conductances

We study, both theoretically and experimentally, the negative
magnetoresistance (MR) of a two-dimensional (2D) electron gas in a weak
transverse magnetic field $B$. The analysis is carried out in a wide range of
zero-$B$ conductances $g$ (measured in units of $e^2/h$), including the range
of intermediate conductances, $g\sim 1$. Interpretation of the experimental
results obtained for a 2D electron gas in GaAs/In$_x$Ga$_{1-x}$As/GaAs single
quantum well structures is based on the theory which takes into account terms
of higher orders in $1/g$, stemming from both the interference contribution and
the mutual effect of weak localization (WL) and Coulomb interaction. We
demonstrate that at intermediate conductances the negative MR is described by
the standard WL "digamma-functions" expression, but with a reduced prefactor
$\alpha$. We also show that at not very high $g$ the second-loop corrections
dominate over the contribution of the interaction in the Cooper channel, and
therefore appear to be the main source of the lowering of the prefactor,
$\alpha\simeq 1-2/\pi g$. We further analyze the regime of a "weak insulator",
when the zero-$B$ conductance is low $g(B=0)<1$ due to the localization at low
$T$, whereas the Drude conductance is high, $g_0>>1.$ In this regime, while the
MR still can be fitted by the digamma-functions formula, the experimentally
obtained value of the dephasing rate has nothing to do with the true one. The
corresponding fitting parameter in the low-$T$ limit is determined by the
localization length and may therefore saturate at $T\to 0$, even though the
true dephasing rate vanishes.Comment: 36 pages, 16 figure

### Weak localization effect on thermomagnetic phenomena

The quantum transport equation (QTE) is extended to study weak localization
(WL) effects on galvanomagnetic and thermomagnetic phenomena. QTE has many
advantages over the linear response method (LRM): (i) particle-hole asymmetry
which is necessary for the Hall effect is taken into account by the
nonequilibrium distribution function, while LRM requires expansion near the
Fermi surface, (ii) when calculating response to the temperature gradient, the
problem of WL correction to the heat current operator is avoided, (iii)
magnetic field is directly introduced to QTE, while the LRM deals with the
vector potential and and special attention should be paid to maintain gauge
invariance, e.g. when calculating the Nernst effect the heat current operator
should be modified to include the external magnetic field. We reproduce in a
very compact form known results for the conductivity, the Hall and the
thermoelectric effects and then we study our main problem, WL correction to the
Nernst coefficient (transverse thermopower).Comment: 20 pages 2 figure

### Mesoscopic Aharonov-Bohm oscillations in metallic rings

We study the amplitude of mesoscopic Aharonov-Bohm oscillations in
quasi-one-dimensional (Q1D) diffusive rings. We consider first the
low-temperature limit of a fully coherent sample. The variance of oscillation
harmonics is calculated as a function of the length of the leads attaching the
ring to reservoirs. We further analyze the regime of relatively high
temperatures, when the dephasing due to electron-electron interaction
suppresses substantially the oscillations. We show that the dephasing length
L_phi^AB governing the damping factor exp(-2pi R /L_phi^AB) of the oscillations
is parametrically different from the common dephasing length for the Q1D
geometry. This is due to the fact that the dephasing is governed by energy
transfers determined by the ring circumference 2pi R, making L_phi^AB
R-dependent.Comment: 16 pages, 4 figures, to appear in proceedings of NATO/Euresco
Conference "Fundamental Problems of Mesoscopic Physics: Interactions and
Decoherence", Granada (Spain), September 200

### Electron neutrino mass scale in spectrum of Dirac equation with the 5-form flux term on the AdS(5)xS(5) background

Dimensional reduction from 10 to 5 dimensions of the IIB supergravity Dirac
equation written down on the AdS(5)xS(5) (+ self-dual 5-form) background
provides the unambiguous values of bulk masses of Fermions in the effective 5D
Randall Sundrum theory. The use of "untwisted" and "twisted" (hep-th/0012378)
boundary conditions at the UV and IR ends of the warped space-time results in
two towers of spectrum of Dirac equation: the ordinary one which is linear in
spectral number and the "twisted" one exponentially decreasing with growth of
spectral number. Taking into account of the Fermion-5-form interaction
(hep-th/9811106) gives the electron neutrino mass scale in the "twisted"
spectrum of Dirac equation. Profiles in extra space of the eigenfunctions of
left and right "neutrinos" drastically differ which may result in the extremely
small coupling of light right neutrino with ordinary matter thus joining it to
plethora of candidates for Dark Matter.Comment: 11 page

### Theory of Interaction Effects in N-S Junctions out of Equilibrium

We consider a normal metal - superconductor (N-S) junction in the regime,
when electrons in the normal metal are driven out of equilibrium. We show that
the non-equilibrium fluctuations of the electron density in the N-layer cause
the fluctuations of the phase of the order parameter in the S-layer. As a
result, the density of states in the superconductor deviates from the BCS form,
most notably the density of states in the gap becomes finite. This effect can
be viewed as a result of the time reversal symmetry breaking due to the
non-equilibrium, and can be described in terms of a low energy collective mode
of the junction, which couples normal currents in N-layer and supercurrents.
This mode is analogous to the Schmid-Sch\"{o}n mode. To interpret their
measurements of the tunneling current, Pothier {\em et. al} [Phys. Rev. Lett.
{\bf 79}, 3490 (1997)] had to assume that the energy relaxation rate in the
normal metal is surprisingly high. The broadening of the BCS singularity of the
density of states in the S-layer manifest itself similarly to the broadening of
the distribution function. Mechanism suggested here can be a possible
explanation of this experimental puzzle. We also propose an independent
experiment to test our explanation.Comment: 16 pages, 2 .eps figure

### Phase Relaxation of Electrons in Disordered Conductors

Conduction electrons in disordered metals and heavily doped semiconductors at
low temperatures preserve their phase coherence for a long time: phase
relaxation time $\tau_\phi$ can be orders of magnitude longer than the momentum
relaxation time. The large difference in these time scales gives rise to well
known effects of weak localization, such as anomalous magnetoresistance. Among
other interesting characteristics, study of these effects provide quantitative
information on the dephasing rate $1/\tau_\phi$. This parameter is of
fundamental interest: the relation between $\hbar/\tau_\phi$ and the
temperature $T$ (a typical energy scale of an electron) determines how well a
single electron state is defined. We will discuss the basic physical meaning of
$1/\tau_\phi$ in different situations and its difference from the energy
relaxation rate. At low temperatures, the phase relaxation rate is governed by
collisions between electrons. We will review existing theories of dephasing by
these collisions or (which is the same) by electric noise inside the sample. We
also discuss recent experiments on the magnetoresistance of 1D systems: some of
them show saturation of $1/\tau_\phi$ at low temperatures, the other do not. To
resolve this contradiction we discuss dephasing by an external microwave field
and by nonequilibrium electric noise.Comment: Order of figures and references corrected; one reference added; 15
pages, 2 figures, lecture given on 10th International Winterschool on New
Developments in Solid State Physics, Mauterndorf, Salzburg, Austria; 23-27
Feb. 199

### Tunneling ``zero-bias'' anomaly in the quasi-ballistic regime

For the first time, we study the tunneling density of states (DOS) of the
interacting electron gas beyond the diffusive limit. A strong correction to the
DOS persists even at electron energies exceeding the inverse transport
relaxation time, which could not be expected from the well-known
Altshuler-Aronov-Lee (AAL) theory. This correction originates from the
interference between the electron waves scattered by an impurity and by the
Friedel oscillation this impurity creates. Account for such processes also
revises the AAL formula for the DOS in the diffusive limit.Comment: 4 pages, 2 .eps figures, submitted to Phys. Rev. Let

### Interaction-induced dephasing of Aharonov-Bohm oscillations

We study the effect of the electron-electron interaction on the amplitude of
mesoscopic Aharonov-Bohm oscillations in quasi-one-dimensional (Q1D) diffusive
rings. We show that the dephasing length L_phi^AB governing the damping factor
exp(-2piR / L_phi^AB) of the oscillations is parametrically different from the
common dephasing length for the Q1D geometry. This is due to the fact that the
dephasing is governed by energy transfers determined by the ring circumference
2piR, making L_phi^AB R-dependent.Comment: 4 pages, 2 figures. Minor changes, final version published in PR

### Conductance fluctuations in a quantum dot under almost periodic ac pumping

It is shown that the variance of the linear dc conductance fluctuations in an
open quantum dot under a high-frequency ac pumping depends significantly on the
spectral content of the ac field. For a sufficiently strong ac field
$\gamma\tau_{\phi}<< 1$, where $1/\tau_{\phi}$ is the dephasing rate induced by
ac noise and $\gamma$ is the electron escape rate, the dc conductance
fluctuations are much stronger for the harmonic pumping than in the case of the
noise ac field of the same intensity. The reduction factor $r$ in a static
magnetic field takes the universal value of 2 only for the white--noise
pumping. For the strictly harmonic pumping $A(t)=A_{0}\cos\omega t$ of
sufficiently large intensity the variance is almost insensitive to the static
magnetic field $r-1= 2\sqrt{\tau_{\phi}\gamma} << 1$. For the quasi-periodic ac
field of the form $A(t)=A_{0} [\cos(\omega_{1} t)+\cos(\omega_{2} t)]$ with
$\omega_{1,2} >> \gamma$ and $\gamma\tau_{\phi} << 1$ we predict the novel
effect of enchancement of conductance fluctuations at commensurate frequencies
$\omega_{2}/\omega_{1}=P/Q$.Comment: 4 pages RevTex, 4 eps figures; the final version to appear in
Phys.Rev.

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