40 research outputs found
The nonlinear effects in 2DEG conductivity investigation by an acoustic method
The parameters of two-dimensional electron gas (2DEG) in a GaAs/AlGaAs
heterostructure were determined by an acoustical (contactless) method in the
delocalized electrons region (2.5T). Nonlinear effects in Surface
Acoustic Wave (SAW) absorption by 2DEG are determined by the electron heating
in the electric field of SAW, which may be described in terms of electron
temperature . The energy relaxation time is determined
by the scattering at piezoelectric potential of acoustic phonons with strong
screening. At different SAW frequencies the heating depends on the relationship
between and 1 and is determined either by the
instantaneously changing wave field (), or by the
average wave power ().Comment: RevTeX, 5 pages, 3 PS-figures, submitted to Physica Status
Sol.(Technical corrections in PS-figs
Electron localization in sound absorption oscillations in the quantum Hall effect regime
The absorption coefficient for surface acoustic waves in a piezoelectric
insulator in contact with a GaAs/AlGaAs heterostructure (with two-dimensional
electron mobility at T=4.2K) via a small
gap has been investigated experimentally as a function of the frequency of the
wave, the width of the vacuum gap, the magnetic field, and the temperature. The
magnetic field and frequency dependencies of the high-frequency conductivity
(in the region 30-210 MHz) are calculated and analyzed. The experimental
results can be explained if it assumed that there exists a fluctuation
potential in which current carrier localization occurs. The absorption of the
surface acoustic waves in an interaction with two-dimensional electrons
localized in the energy "tails" of Landau levels is discussed.Comment: RevTeX 6 pages+6 EPS pic
Experimental study of weak antilocalization effect in a high mobility InGaAs/InP quantum well
The magnetoresistance associated with quantum interference corrections in a
high mobility, gated InGaAs/InP quantum well structure is studied as a function
of temperature, gate voltage, and angle of the tilted magnetic field.
Particular attention is paid to the experimental extraction of phase-breaking
and spin-orbit scattering times when weak anti- localization effects are
prominent. Compared with metals and low mobility semiconductors the
characteristic magnetic field in high mobility
samples is very small and the experimental dependencies of the interference
effects extend to fields several hundreds of times larger. Fitting experimental
results under these conditions therefore requires theories valid for arbitrary
magnetic field. It was found, however, that such a theory was unable to fit the
experimental data without introducing an extra, empirical, scale factor of
about 2. Measurements in tilted magnetic fields and as a function of
temperature established that both the weak localization and the weak
anti-localization effects have the same, orbital origin. Fits to the data
confirmed that the width of the low field feature, whether a weak localization
or a weak anti-localization peak, is determined by the phase-breaking time and
also established that the universal (negative) magnetoresistance observed in
the high field limit is associated with a temperature independent spin-orbit
scattering time.Comment: 13 pages including 10 figure
Magnetic-field-dependent zero-bias diffusive anomaly in Pb oxide-n-InAs structures: Coexistence of two- and three-dimensional states
The results of experimental and theoretical studies of zero-bias anomaly
(ZBA) in the Pb-oxide-n-InAs tunnel structures in magnetic field up to 6T are
presented. A specific feature of the structures is a coexistence of the 2D and
3D states at the Fermi energy near the semiconductor surface. The dependence of
the measured ZBA amplitude on the strength and orientation of the applied
magnetic field is in agreement with the proposed theoretical model. According
to this model, electrons tunnel into 2D states, and move diffusively in the 2D
layer, whereas the main contribution to the screening comes from 3D electrons.Comment: 8 double-column pages, REVTeX, 9 eps figures embedded with epsf,
published versio
Interaction effects and phase relaxation in disordered systems
This paper is intended to demonstrate that there is no need to revise the
existing theory of the transport properties of disordered conductors in the
so-called weak localization regime. In particular, we demonstrate explicitly
that recent attempts to justify theoretically that the dephasing rate
(extracted from the magnetoresistance) remains finite at zero temperature are
based on the profoundly incorrect calculation. This demonstration is based on a
straightforward evaluation of the effect of the electron-electron interaction
on the weak localization correction to the conductivity of disordered metals.
Using well-controlled perturbation theory with the inverse conductance as
the small parameter, we show that this effect consists of two contributions.
First contribution comes from the processes with energy transfer smaller than
the temperature. This contribution is responsible for setting the energy scale
for the magnetoresistance. The second contribution originates from the virtual
processes with energy transfer larger than the temperature. It is shown that
the latter processes have nothing to do with the dephasing, but rather manifest
the second order (in ) correction to the conductance. This correction is
calculated for the first time. The paper also contains a brief review of the
existing experiments on the dephasing of electrons in disordered conductors and
an extended qualitative discussion of the quantum corrections to the
conductivity and to the density of electronic states in the weak localization
regime.Comment: 34 pages, 13 .eps figure
MODERN ANTIPLATELET THERAPY OF ISCHEMIC HEART DISEASE PATIENTS WITH HIGH RISK OF THROMBOTIC EVENTS: DATA FROM EVIDENCE-BASED MEDICINE AND REAL PRACTICE
Aim. To study a rate of clopidogrel use in patients having indications for clopidogrel therapy according with up to date guidelines. Material and methods. Database of patients experienced acute myocardial infarction (MI) and database of patients experienced angiography because of stable ischemic heart disease (IHD) were used to evaluate clopidogrel therapy implementation. Patients with acute MI (n=84) and patients with IHD after angioplasty (n=239) were interviewed. Results. Only 14 of 84 (16.6%) patients after acute MI and 112 of 239 (47%) patients after angioplasty were taken clopidogrel within 12 months. Conclusion. The majority of patients after MI and coronary angioplasty do not receive clopidogrel despite the need of dual antiplatelet agents therapy