182 research outputs found
Keijsers, Shklyarevskii and van Kempen Reply
Answer to the Comment on ``Point-Contact Study of Fast and Slow Two-Level
Fluctuators in Metallic Glasses'' by Jan von Delft et al.Comment: 3 pages, no figures, accepted Phys. Rev. Letter
Loss of quantum coherence due to non-stationary glass fluctuations
Low-temperature dynamics of insulating glasses is dominated by a macroscopic
concentration of tunneling two-level systems (TTLS). The distribution of the
switching/relaxation rates of TTLS is exponentially broad, which results in
non-equilibrium state of the glass at arbitrarily long time-scales. Due to the
electric dipolar nature, the switching TTLS generate fluctuating
electromagnetic fields. We study the effect of the non-thermal slow fluctuators
on the dephasing of a solid state qubit. We find that at low enough
temperatures, non-stationary contribution can dominate the stationary (thermal)
one, and discuss how this effect can be minimized.Comment: 4 page
Zero-bias anomalies of point contact resistance due to adiabatic electron renormalization of dynamical defects
We study effect of the adiabatic electron renormalization on the parameters
of the dynamical defects in the ballistic metallic point contact. The upper
energy states of the ``dressed'' defect are shown to give a smaller
contribution to a resistance of the contact than the lower energy ones. This
holds both for the "classical" renormalization related to defect coupling with
average local electron density and for the "mesoscopic" renormalization caused
by the mesoscopic fluctuations of electronic density the dynamical defects are
coupled with. In the case of mesoscopic renormalization one may treat the
dynamical defect as coupled with Friedel oscillations originated by the other
defects, both static and mobile. Such coupling lifts the energy degeneracy of
the states of the dynamical defects giving different mesoscopic contribution to
resistance, and provides a new model for the fluctuator as for the object
originated by the electronic mesoscopic disorder rather than by the structural
one. The correlation between the defect energy and the defect contribution to
the resistance leads to zero-temperature and zero-bias anomalies of the point
contact resistance.
A comparison of these anomalies with those predicted by the Two Channel Kondo
Model (TCKM) is made. It is shown, that although the proposed model is based on
a completely different from TCKM physical background, it leads to a zero-bias
anomalies of the point contact resistance, which are qualitatively similar to
TCKM predictions.Comment: 6 pages, to be published in Phys. Rev.
Magnetoresistance of p-GaAs/AlGaAs structures in the vicinity of metal-insulator transition: Effect of superconducting leads
Experimental and theoretical studies on transport in semiconductor samples
with superconducting electrodes are reported. We focus on the samples close to
metal-insulator transition. In metallic samples, a peak of negative
magnetoresistance at fields lower than critical magnetic field of the leads was
observed. This peak is attributed to restoration of a single-particle tunneling
emerging with suppression of superconductivity. The experimental results allow
us to estimate tunneling transparency of the boundary between superconductor
and metal. In contrast, for the insulating samples no such a peak was observed.
We explain this behavior as related to properties of transport through the
contact between superconductor and hopping conductor. This effect can be used
to discriminate between weak localization and strong localization regimes.Comment: 10 pages, 3 fi
Point contact spectroscopy of hopping transport: effects of a magnetic field
The conductance of a point contact between two hopping insulators is expected
to be dominated by the individual localized states in its vicinity. Here we
study the additional effects due to an external magnetic field. Combined with
the measured conductance, the measured magnetoresistance provides detailed
information on these states (e.g. their localization length, the energy
difference and the hopping distance between them). We also calculate the
statistics of this magnetoresistance, which can be collected by changing the
gate voltage in a single device. Since the conductance is dominated by the
quantum interference of particular mesoscopic structures near the point
contact, it is predicted to exhibit Aharonov-Bohm oscillations, which yield
information on the geometry of these structures. These oscillations also depend
on local spin accumulation and correlations, which can be modified by the
external field. Finally, we also estimate the mesoscopic Hall voltage due to
these structures.Comment: 7 pages, 5 figur
Non-Gaussian dephasing in flux qubits due to 1/f-noise
Recent experiments by F. Yoshihara et al. [Phys. Rev. Lett. 97, 167001
(2006)] and by K. Kakuyanagi et al. (cond-mat/0609564) provided information on
decoherence of the echo signal in Josephson-junction flux qubits at various
bias conditions. These results were interpreted assuming a Gaussian model for
the decoherence due to 1/f noise. Here we revisit this problem on the basis of
the exactly solvable spin-fluctuator model reproducing detailed properties of
the 1/f noise interacting with a qubit. We consider the time dependence of the
echo signal and conclude that the results based on the Gaussian assumption need
essential reconsideration.Comment: Improved fitting parameters, new figur
Spontaneous current generation in gated nanostructures
We have observed an unusual dc current spontaneously generated in the
conducting channel of a short-gated GaAs transistor. The magnitude and
direction of this current critically depend upon the voltage applied to the
gate. We propose that it is initiated by the injection of hot electrons from
the gate that relax via phonon emission. The phonons then excite secondary
electrons from asymmetrically distributed impurities in the channel, which
leads to the observed current
Decoherence in qubits due to low-frequency noise
The efficiency of the future devices for quantum information processing is
limited mostly by the finite decoherence rates of the qubits. Recently a
substantial progress was achieved in enhancing the time, which a solid-state
qubit demonstrates a coherent dynamics. This progress is based mostly on a
successful isolation of the qubits from external decoherence sources. Under
these conditions the material-inherent sources of noise start to play a crucial
role. In most cases the noise that quantum device demonstrate has 1/f spectrum.
This suggests that the environment that destroys the phase coherence of the
qubit can be thought of as a system of two-state fluctuators, which experience
random hops between their states. In this short review we discuss the current
state of the theory of the decoherence due to the qubit interaction with the
fluctuators. We describe the effect of such an environment on different
protocols of the qubit manipulations - free induction and echo signal. It turns
out that in many important cases the noise produced by the fluctuators is
non-Gaussian. Consequently the results of the interaction of the qubit with the
fluctuators are not determined by the pair correlation function only.
We describe the effect of the fluctuators using so-called spin-fluctuator
model. Being quite realistic this model allows one to evaluate the qubit
dynamics in the presence of one fluctuator exactly. This solution is found, and
its features, including non-Gaussian effects are analyzed in details. We extend
this consideration for the systems of large number of fluctuators, which
interact with the qubit and lead to the 1/f noise. We discuss existing
experiments on the Josephson qubit manipulation and try to identify
non-Gaussian behavior.Comment: 25 pages, 7 figure
Nonlinear absorption of surface acoustic waves by composite fermions
Absorption of surface acoustic waves by a two-dimensional electron gas in a
perpendicular magnetic field is considered. The structure of such system at the
filling factor close to 1/2 can be understood as a gas of {\em composite
fermions}. It is shown that the absorption at can be strongly
nonlinear, while small deviation form 1/2 will restore the linear absorption.
Study of nonlinear absorption allows one to determine the force acting upon the
composite fermions from the acoustic wave at turning points of their
trajectories.Comment: 7 pages, 1 figure, submitted to Europhysics letter
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