868 research outputs found
Fluctuating "order parameter" for a quantum chaotic system with partially broken time-reversal symmetry
The functional defined as the squared modulus of the spatial average of the
wave function squared, plays the role of an ``order parameter'' for the
transition between Hamiltonian ensembles with orthogonal and unitary symmetry.
Upon breaking time-reversal symmetry, the order parameter crosses over from one
to zero. We compute its distribution in the crossover regime and find that it
has large fluctuations around the ensemble average. These fluctuations imply
long-range spatial correlations in the eigenfunction and non-Gaussian
perturbations of eigenvalues, in precise agreement with results by Fal'ko and
Efetov and by Taniguchi, Hashimoto, Simons, and Altshuler. As a third
implication of the order-parameter fluctuations we find correlations in the
response of an eigenvalue to independent perturbations of the system.Comment: 4 pages, REVTeX-3.0, 1 figure. Reference added to Y. V. Fyodorov and
A. D. Mirlin, Phys. Rev. B 51, 13403 (1995
Thermopower of Single-Channel Disordered and Chaotic Conductors
We show (analytically and by numerical simulation) that the zero-temperature
limit of the distribution of the thermopower S of a one-dimensional disordered
wire in the localized regime is a Lorentzian, with a disorder-independent width
of 4 pi^3 k_B^2 T/3e\Delta (where T is the temperature and \Delta the mean
level spacing). Upon raising the temperature the distribution crosses over to
an exponential form exp(-2|S|eT/\Delta). We also consider the case of a chaotic
quantum dot with two single-channel ballistic point contacts. The distribution
of S then has a cusp at S=0 and a tail |S|^{-1-\beta} log|S| for large S (with
\beta=1,2 depending on the presence or absence of time-reversal symmetry).Comment: To be published in Superlattices and Microstructures, special issue
on the occasion of Rolf Landauer's 70th birthda
Distribution of parametric conductance derivatives of a quantum dot
The conductance G of a quantum dot with single-mode ballistic point contacts
depends sensitively on external parameters X, such as gate voltage and magnetic
field. We calculate the joint distribution of G and dG/dX by relating it to the
distribution of the Wigner-Smith time-delay matrix of a chaotic system. The
distribution of dG/dX has a singularity at zero and algebraic tails. While G
and dG/dX are correlated, the ratio of dG/dX and is independent
of G. Coulomb interactions change the distribution of dG/dX, by inducing a
transition from the grand-canonical to the canonical ensemble. All these
predictions can be tested in semiconductor microstructures or microwave
cavities.Comment: 4 pages, RevTeX, 3 figure
The Thermopower of Quantum Chaos
The thermovoltage of a chaotic quantum dot is measured using a current
heating technique. The fluctuations in the thermopower as a function of
magnetic field and dot shape display a non-Gaussian distribution, in agreement
with simulations using Random Matrix Theory. We observe no contributions from
weak localization or short trajectories in the thermopower.Comment: 4 pages, 3 figures, corrected: accidently omitted author in the
Authors list, here (not in the article
Conditions for Adiabatic Spin Transport in Disordered Systems
We address the controversy concerning the necessary conditions for the
observation of Berry phases in disordered mesoscopic conductors. For this
purpose we calculate the spin-dependent conductance of disordered
two-dimensional structures in the presence of inhomogeneous magnetic fields.
Our numerical results show that for both, the overall conductance and quantum
corrections, the relevant parameter defining adiabatic spin transport scales
with the square root of the number of scattering events, in generalization of
Stern's original proposal [Phys. Rev. Lett. 68, 1022 (1992)]. This could hinder
a clear-cut experimental observation of Berry phase effects in diffusive
metallic rings.Comment: 5 pages, 4 figures. To appear in Phys. Rev. B (Rapid Communications
Berry phase and adiabaticity of a spin diffusing in a non-uniform magnetic field
An electron spin moving adiabatically in a strong, spatially non-uniform
magnetic field accumulates a geometric phase or Berry phase, which might be
observable as a conductance oscillation in a mesoscopic ring. Two contradicting
theories exist for how strong the magnetic field should be to ensure
adiabaticity if the motion is diffusive. To resolve this controversy, we study
the effect of a non-uniform magnetic field on the spin polarization and on the
weak-localization effect. The diffusion equation for the Cooperon is solved
exactly. Adiabaticity requires that the spin-precession time is short compared
to the elastic scattering time - it is not sufficient that it is short compared
to the diffusion time around the ring. This strong condition severely
complicates the experimental observation.Comment: 16 pages REVTEX, including 3 figure
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