328 research outputs found
Electron transport and energy relaxation in dilute magnetic alloys
We consider the effect of the RKKY interaction between magnetic impurities on
the electron relaxation rates in a normal metal. The interplay between the RKKY
interaction and the Kondo effect may result in a non-monotonic temperature
dependence of the electron momentum relaxation rate, which determines the Drude
conductivity. The electron phase relaxation rate, which determines the
magnitude of the weak localization correction to the resistivity, is also a
non-monotonic function of temperature. For this function, we find the
dependence of the position of its maximum on the concentration of magnetic
impurities. We also relate the electron energy relaxation rate to the
excitation spectrum of the system of magnetic impurities. The energy relaxation
determines the distribution function for the out-of-equilibrium electrons.
Measurement of the electron distribution function thus may provide information
about the excitations in the spin glass phase.Comment: 15 pages, 5 figure
Quantum Disorder and Quantum Chaos in Andreev Billiards
We investigate the crossover from the semiclassical to the quantum
description of electron energy states in a chaotic metal grain connected to a
superconductor. We consider the influence of scattering off point impurities
(quantum disorder) and of quantum diffraction (quantum chaos) on the electron
density of states. We show that both the quantum disorder and the quantum chaos
open a gap near the Fermi energy. The size of the gap is determined by the mean
free time in disordered systems and by the Ehrenfest time in clean chaotic
systems. Particularly, if both times become infinitely large, the density of
states is gapless, and if either of these times becomes shorter than the
electron escape time, the density of states is described by random matrix
theory. Using the Usadel equation, we also study the density of states in a
grain connected to a superconductor by a diffusive contact.Comment: 20 pages, 10 figure
Quantum correction to the Kubo formula in closed mesoscopic systems
We study the energy dissipation rate in a mesoscopic system described by the
parametrically-driven random-matrix Hamiltonian H[\phi(t)] for the case of
linear bias \phi=vt. Evolution of the field \phi(t) causes interlevel
transitions leading to energy pumping, and also smears the discrete spectrum of
the Hamiltonian. For sufficiently fast perturbation this smearing exceeds the
mean level spacing and the dissipation rate is given by the Kubo formula. We
calculate the quantum correction to the Kubo result that reveals the original
discreteness of the energy spectrum. The first correction to the system
viscosity scales proportional to v^{-2/3} in the orthogonal case and vanishes
in the unitary case.Comment: 4 pages, 3 eps figures, REVTeX
Fine structure of Vavilov-Cherenkov radiation near the Cherenkov threshold
We analyze the Vavilov-Cherenkov radiation (VCR) in a dispersive
nontransparent dielectric air-like medium both below and above the Cherenkov
threshold, in the framework of classical electrodynamics. It is shown that the
transition to the subthreshold energies leads to the destruction of
electromagnetic shock waves and to the sharp reduction of the frequency domain
where VCR is emitted. The fine wake-like structure of the Vavilov-Cherenkov
radiation survives and manifests the existence of the subthreshold radiation in
the domain of anomalous dispersion. These domains can approximately be defined
by the two phenomenological parameters of the medium, namely, the effective
frequency of oscillators and the damping describing an interaction with the
other degrees of freedom.Comment: 9 pages, 6 figure
Quantum interference and the formation of the proximity effect in chaotic normal-metal/superconducting structures
We discuss a number of basic physical mechanisms relevant to the formation of
the proximity effect in superconductor/normal metal (SN) systems. Specifically,
we review why the proximity effect sharply discriminates between systems with
integrable and chaotic dynamics, respectively, and how this feature can be
incorporated into theories of SN systems. Turning to less well investigated
terrain, we discuss the impact of quantum diffractive scattering on the
structure of the density of states in the normal region. We consider ballistic
systems weakly disordered by pointlike impurities as a test case and
demonstrate that diffractive processes akin to normal metal weak localization
lead to the formation of a hard spectral gap -- a hallmark of SN systems with
chaotic dynamics. Turning to the more difficult case of clean systems with
chaotic boundary scattering, we argue that semiclassical approaches, based on
classifications in terms of classical trajectories, cannot explain the gap
phenomenon. Employing an alternative formalism based on elements of
quasiclassics and the ballistic -model, we demonstrate that the inverse
of the so-called Ehrenfest time is the relevant energy scale in this context.
We discuss some fundamental difficulties related to the formulation of low
energy theories of mesoscopic chaotic systems in general and how they prevent
us from analysing the gap structure in a rigorous manner. Given these
difficulties, we argue that the proximity effect represents a basic and
challenging test phenomenon for theories of quantum chaotic systems.Comment: 21 pages (two-column), 6 figures; references adde
Conductance of Mesoscopic Systems with Magnetic Impurities
We investigate the combined effects of magnetic impurities and applied
magnetic field on the interference contribution to the conductance of
disordered metals. We show that in a metal with weak spin-orbit interaction,
the polarization of impurity spins reduces the rate of electron phase
relaxation, thus enhancing the weak localization correction to conductivity.
Magnetic field also suppresses thermal fluctuations of magnetic impurities,
leading to a recovery of the conductance fluctuations. This recovery occurs
regardless the strength of the spin-orbit interaction. We calculate the
magnitudes of the weak localization correction and of the mesoscopic
conductance fluctuations at an arbitrary level of the spin polarization induced
by a magnetic field. Our analytical results for the ``'' Aharonov-Bohm
conductance oscillations in metal rings can be used to extract spin and
gyromagnetic factor of magnetic impurities from existing experimental data.Comment: 18 pages, 8 figure
Gap Fluctuations in Inhomogeneous Superconductors
Spatial fluctuations of the effective pairing interaction between electrons
in a superconductor induce variations of the order parameter which in turn lead
to significant changes in the density of states. In addition to an overall
reduction of the quasi-particle energy gap, theory suggests that mesoscopic
fluctuations of the impurity potential induce localised tail states below the
mean-field gap edge. Using a field theoretic approach, we elucidate the nature
of the states in the `sub-gap' region. Specifically, we show that these states
are associated with replica symmetry broken instanton solutions of the
mean-field equations.Comment: 11 pages, 3 figures included. To be published in PRB (Sept. 2001
Measurement of the Ds lifetime
We report precise measurement of the Ds meson lifetime. The data were taken
by the SELEX experiment (E781) spectrometer using 600 GeV/c Sigma-, pi- and p
beams. The measurement has been done using 918 reconstructed Ds. The lifetime
of the Ds is measured to be 472.5 +- 17.2 +- 6.6 fs, using K*(892)0K+- and phi
pi+- decay modes. The lifetime ratio of Ds to D0 is 1.145+-0.049.Comment: 5 pages, 2 figures submitted to Phys. Lett.
Confirmation of the Double Charm Baryon Xi_cc+ via its Decay to p D+ K-
We observes a signal for the double charm baryon Xi_cc+ in the charged decay
mode Xi_cc+ -> p D+ K- to complement the previously reported decay Xi_cc+ ->
Lambda_c K- pi+ in data from SELEX, the charm hadro-production experiment
(E781) at Fermilab. In this new decay mode we observe an excess of 5.62 events
over an expected background estimated by event mixing to be 1.38+/-0.13 events.
The Poisson probability that a background fluctuation can produce the apparent
signal is less than 6.4E-4. The observed mass of this state is
(3518+/-3)MeV/c^2, consistent with the published result. Averaging the two
results gives a mass of (3518.7+/-1.7)MeV/c^2. The observation of this new weak
decay mode confirms the previous SELEX suggestion that this state is a double
charm baryon. The relative branching ratio Gamma(Xi_cc+ -> pD+K-)/Gamma(Xi_cc+
-> Lambda_c K- pi+) = 0.36+/-0.21.Comment: 11 pages, 6 included eps figures. v2 includes improved statistical
method to determine significance of observation. Submitted to PL
- …