219 research outputs found
Metal-insulator transition in a 2D electron gas: Equivalence of two approaches for determining the critical point
The critical electron density for the metal-insulator transition in a
two-dimensional electron gas can be determined by two distinct methods: (i) a
sign change of the temperature derivative of the resistance, and (ii) vanishing
activation energy and vanishing nonlinearity of current-voltage characteristics
as extrapolated from the insulating side. We find that in zero magnetic field
(but not in the presence of a parallel magnetic field), both methods give
equivalent results, adding support to the existence of a true zero-field
metal-insulator transition.Comment: As publishe
Spin gap in the 2D electron system of GaAs/AlGaAs single heterojunctions in weak magnetic fields
We study the interaction-enhanced spin gaps in the two-dimensional electron
gas confined in GaAs/AlGaAs single heterojunctions subjected to weak magnetic
fields. The values are obtained from the chemical potential jumps measured by
magnetocapacitance. The gap increase with parallel magnetic field indicates
that the lowest-lying charged excitations are accompanied with a single spin
flip at the odd-integer filling factor nu=1 and nu=3, in disagreement with the
concept of skyrmions.Comment: as publishe
Quantum Hall effect at low magnetic fields
The temperature and scale dependence of resistivities in the standard scaling
theory of the integer quantum Hall effect is discussed. It is shown that recent
experiments, claiming to observe a discrepancy with the global phase diagram of
the quantum Hall effect, are in fact in agreement with the standard theory. The
apparent low-field transition observed in the experiments is identified as a
crossover due to weak localization and a strong reduction of the conductivity
when Landau quantization becomes dominant.Comment: 4 pages, 2 figures, minor corrections, to appear in PR
Composite fermions close to the one-half filling of the lowest Landau level revisited
By strictly adhering to the microscopic theory of composite fermions for the
Landau-level filling fractions nu_e = p/(2 p + 1), we reproduce, with
remarkable accuracy, the surface-acoustic-wave (SAW)-based experimental results
by Willett and co-workers concerning two-dimensional electron systems with nu_e
close to 1/2. Our results imply that the electron band mass m_b, as distinct
from the composite fermion mass m_*, must undergo a substantial increase under
the conditions corresponding to nu_e approximately equal to 1/2. In view of the
relatively low aerial electronic densities n_e to which the underlying SAW
experiments correspond, our finding conforms with the experimental results by
Shashkin et al. [Phys. Rev. B 66, 073303 (2002)], concerning two-dimensional
electrons in silicon, that signal sharp increase in m_b for n_e decreasing
below approximately 2 x 10^{11} cm^{-2}. We further establish that a finite
mean-free path l_0 is essential for the observed linearity of the longitudinal
conductivity sigma_{xx}(q) as deduced from the SAW velocity shifts.Comment: 5 pages, 2 postscript figure
Indication of the ferromagnetic instability in a dilute two-dimensional electron system
The magnetic field B_c, in which the electrons become fully spin-polarized,
is found to be proportional to the deviation of the electron density from the
zero-field metal-insulator transition in a two-dimensional electron system in
silicon. The tendency of B_c to vanish at a finite electron density suggests a
ferromagnetic instability in this strongly correlated electron system.Comment: 4 pages, postscript figures included. Revised versio
Observation of the parallel-magnetic-field-induced superconductor-insulator transition in thin amorphous InO films
We study the response of a thin superconducting amorphous InO film with
variable oxygen content to a parallel magnetic field. A field-induced
superconductor-insulator transition (SIT) is observed that is very similar to
the one in normal magnetic fields. As the boson-vortex duality, which is the
key-stone of the theory of the field-induced SIT, is obviously absent in the
parallel configuration, we have to draw conclusion about the theory
insufficiency.Comment: 3 pages, 4 figure
Spin magnetization of strongly correlated electron gas confined in a two-dimensional finite lattice
The influence of disorder and interaction on the ground state polarization of
the two-dimensional (2D) correlated electron gas is studied by numerical
investigations of unrestricted Hartree-Fock equations. The ferromagnetic ground
state is found to be plausible when the electron number is lowered and the
interaction and disorder parameters are suitably chosen. For a finite system at
constant electronic density the disorder induced spin polarization is cut off
when the electron orbitals become strongly localized to the individual network
sites. The fluctuations of the interaction matrix elements are calculated and
brought out as favoring the ferromagnetic instability in the extended and weak
localization regime. The localization effect of the Hubbard interaction term is
discussed.Comment: 7 pages, 9 figure
Scaling analysis of the magnetic-field-tuned quantum transition in superconducting amorphous In-O films
We have studied the magnetic-field-tuned superconductor-insulator quantum
transition (SIT) in amorphous In-O films with different oxygen content and,
hence, different electron density. While for states of the film near the
zero-field SIT the two-dimensional scaling behaviour is confirmed, for deeper
states in the superconducting phase the SIT scenario changes: in addition to
the scaling function that describes the conductivity of fluctuation-induced
Cooper pairs, there emerges a temperature-dependent contribution to the film
resistance. This contribution can originate from the conductivity of normal
electrons.Comment: 4 pages, 5 figures; accepted for publication in JETP Lett. 71 (4),
200
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