8,418 research outputs found
Electron spin relaxation in semiconductors and semiconductor structures
We suggest an approach to the problem of free electron spin evolution in a
semiconductor with arbitrary anisotropy or quantum structure in a magnetic
field. The developed approach utilizes quantum kinetic equations for average
spin components. These equations represent the relaxation in terms of
correlation functions for fluctuating effective fields responsible for spin
relaxation. In a particular case when autocorrelation functions are dominant,
the kinetic equations reduce to the Bloch equations. The developed formalism is
applied to the problem of electron spin relaxation due to exchange scattering
in a semimagnetic quantum well (QW) as well as to the spin relaxation in a QW
due to Dyakonov-Perel mechanism.Comment: 9 pages, 1 postscript figur
On the macroscopic quantization in mesoscopic rings and single-electron devices
In this letter the phenomenon of macroscopic quantization is investigated
using the particle on the ring interacting with the dissipative environment as
an example. It is shown that the phenomenon of macroscopic quantization has the
clear physical origin in that case. It follows from the angular momentum
conservation combined with momentum quantization for bare particle on the ring
. The existence an observable which can take only integer values in the zero
temperature limit is rigorously proved. With the aid of the mapping between
particle on the ring and Ambegaokar-Eckern-Schon model, which can be used to
describe single-electron devices, it is demonstrated that this observable is
analogous to the "effective charge" introduced by Burmistrov and Pruisken for
the single-electron box problem. Different consequences of the revealed physics
are discussed, as well as a generalization of the obtained results to the case
of more complicated systems.Comment: 4.5 page
Persistent currents in nanorings and quantum decoherence by Coulomb interaction
Employing instanton technique we evaluate equilibrium persistent current (PC)
produced by a quantum particle moving in a periodic potential on a ring and
interacting with a dissipative environment formed by diffusive electron gas.
The model allows for detailed non-perturbative analysis of interaction effects
and -- depending on the system parameters -- yields a rich structure of
different regimes. We demonstrate that at low temperatures PC is exponentially
suppressed at sufficiently large ring perimeters where
the dephasing length is set by interactions and does not depend
on temperature. This behavior represents a clear example of quantum decoherence
by electron-electron interactions at .Comment: 13 pages, 4 figure
Persistent currents in quantum phase slip rings
We investigate the effect of interacting quantum phase slips on persistent
current and its fluctuations in ultrathin superconducting nanowires and
nanorings pierced by the external magnetic flux. We derive the effective action
for these systems and map the original problem onto an effective sine-Gordon
theory on torus. We evaluate both the flux dependent persistent current and the
critical radius of the ring beyond which this current gets exponentially
suppressed by quantum fluctuations. We also analyze fluctuations of persistent
current caused by quantum phase slips. At low temperatures the supercurrent
noise spectrum has the form of coherent peaks which can be tuned by the
magnetic flux. Experimental observation of these peaks can directly demonstrate
the existence of plasma modes in superconducting nanorings.Comment: 11 pages, 5 figure
Fluctuations of persistent current
We theoretically analyze equilibrium fluctuations of persistent current (PC)
in nanorings. We demonstrate that these fluctuations persist down to zero
temperature provided the current operator does not commute with the total
Hamiltonian of the system. For a model of a quantum particle on a ring we
explicitly evaluate PC noise power which has the form of sharp peaks at
frequencies set by the corresponding interlevel distances. In rings with many
conducting channels a much smoother and broader PC noise spectrum is expected.
A specific feature of PC noise is that its spectrum can be tuned by an external
magnetic flux indicating the presence of quantum coherence in the system.Comment: 12 pages, 3 figur
Bunch-by-bunch detection of coherent transverse modes from digitized single-bpm signals in the Tevatron
A system was developed for bunch-by-bunch detection of transverse proton and
antiproton coherent oscillations based on the signal from a single
beam-position monitor (BPM) located in a region of the ring with large
amplitude functions. The signal is digitized over a large number of turns and
Fourier-analyzed offline with a dedicated algorithm. To enhance the signal, the
beam is excited with band-limited noise for about one second, and this was
shown not to significantly affect the circulating beams even at high
luminosity. The system is used to measure betatron tunes of individual bunches
and to study beam- beam effects. In particular, it is one of the main
diagnostic tools in an ongoing study of nonlinear beam-beam compensation
studies with Gaussian electron lenses. We present the design and operation of
this tool, together with results obtained with proton and antiproton bunches.Comment: 4 pp. 14th Beam Instrumentation Workshop (BIW10) 2-6 May 2010: Santa
Fe, New Mexic
Quantum Phase Slip Noise
Quantum phase slips (QPS) generate voltage fluctuations in superconducting
nanowires. Employing Keldysh technique and making use of the phase-charge
duality arguments we develop a theory of QPS-induced voltage noise in such
nanowires. We demonstrate that quantum tunneling of the magnetic flux quanta
across the wire yields quantum shot noise which obeys Poisson statistics and is
characterized by a power law dependence of its spectrum on the
external bias. In long wires decreases with increasing frequency
and vanishes beyond a threshold value of at .
Quantum coherent nature of QPS noise yields non-monotonous dependence of
on at small .Comment: 4.5 pages + 2-page supplemental material, 3 figure
Shot noise in ultrathin superconducting wires
Quantum phase slips (QPS) may produce non-equilibrium voltage fluctuations in
current-biased superconducting nanowires. Making use of the Keldysh technique
and employing the phase-charge duality arguments we investigate such
fluctuations within the four-point measurement scheme and demonstrate that shot
noise of the voltage detected in such nanowires may essentially depend on the
particular measurement setup. In long wires the shot noise power decreases with
increasing frequency and vanishes beyond a threshold value of
at Comment: 5 pages, 2 figure
Anisotropy of heavy hole spin splitting and interference effects of optical polarization in semiconductor quantum wells subjected to an in-plane magnetic field
Strong effects of optical polarization anisotropy observed previously in the
quantum wells subjected to the in-plane magnetic field arrive at complete
description within microscopic approach. Theory we develop involves two sources
of optical polarization. First source is due to correlations between electron
and heavy hole (HH) phases of -functions arising due to electron Zeeman
spin splitting and joint manifestation of low-symmetry and Zeeman interactions
of HH in an in-plane magnetic field. In this case, four possible
phase-controlled electron-HH transitions constitute the polarization effect,
which can reach its maximal amount (1) at low temperatures when only one
transition survives. Other polarization source stems from the admixture of
excited light-holes (LH) states to HH by low-symmetry interactions. The
contribution of this mechanism to total polarization is relatively small but it
can be independent of temperature and magnetic field. Analysis of different
mechanisms of HH splitting exhibits their strong polarization anisotropy. Joint
action of these mechanisms can result in new peculiarities, which should be
taken into account for explanation of different experimental situations.Comment: 8 pages, 5 postscript figure
Mechanisms of Carrier-Induced Ferromagnetism in Diluted Magnetic Semiconductors
Two different approaches to the problem of carrier-induced ferromagnetism in
the system of the disordered magnetic ions, one bases on self-consistent
procedure for the exchange mean fields, other one bases on the RKKY
interaction, used in present literature as the alternative approximations is
analyzed. Our calculations in the framework of exactly solvable model show that
two different contributions to the magnetic characteristics of the system
represent these approaches. One stems from the diagonal part of carrier-ion
exchange interaction that corresponds to mean field approximation. Other one
stems from the off-diagonal part that describes the interaction between ion
spins via free carriers. These two contributions can be responsible for the
different magnetic properties, so aforementioned approaches are complementary,
not alternative. A general approach is proposed and compared with different
approximations to the problem under consideration.Comment: 5 pages, RevTe
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