295 research outputs found
Hydrodynamic Equation for the Breakdown of the Quantum Hall Effect in a Uniform Current
The hydrodynamic equation for the spatial and temporal evolution of the
electron temperature T_e in the breakdown of the quantum Hall effect at
even-integer filling factors in a uniform current density j is derived from the
Boltzmann-type equation, which takes into account electron-electron and
electron-phonon scatterings. The derived equation has a drift term, which is
proportional to j and to the first spatial derivative of T_e. Applied to the
spatial evolution of T_e in a sample with an abrupt change of the width along
the current direction, the equation gives a distinct dependence on the current
direction as well as a critical relaxation, in agreement with the recent
experiments.Comment: 4 pages, 1 Postscript figure, corrected equations, to be published in
J. Phys. Soc. Jpn. 70 (2001) No.
Electronic Processes at the Breakdown of the Quantum Hall Effect
Microscopic processes giving the energy gain and loss of a two-dimensional
electron system in long-range potential fluctuations are studied theoretically
at the breakdown of the quantum Hall effect in the case of even-integer filling
factors. The Coulomb scattering within a broadened Landau level is proposed to
give the gain, while the phonon scattering to give the loss. The energy balance
equation shows that the electron temperature T_e and the diagonal conductivity
sigma_{xx} exhibit a bistability above the lower critical electric field
E_{c1}. Calculated values of E_{c1} as well as T_e and sigma_{xx} at E_{c1} are
in agreement with the observed values in their orders of magnitude.Comment: 4 pages, 2 Postscript figures, submitted to the Journal of the
Physical Society of Japa
Field-induced breakdown of the quantum Hall effect
A numerical analysis is made of the breakdown of the quantum Hall effect
caused by the Hall electric field in competition with disorder. It turns out
that in the regime of dense impurities, in particular, the number of localized
states decreases exponentially with the Hall field, with its dependence on the
magnetic and electric field summarized in a simple scaling law. The physical
picture underlying the scaling law is clarified. This intra-subband process,
the competition of the Hall field with disorder, leads to critical breakdown
fields of magnitude of a few hundred V/cm, consistent with observations, and
accounts for their magnetic-field dependence \propto B^{3/2} observed
experimentally. Some testable consequences of the scaling law are discussed.Comment: 7 pages, Revtex, 3 figures, to appear in Phys. Rev.
Superconductivity in novel BiS2-based layered superconductor LaO1-xFxBiS2
Layered superconductors have provided some interesting fields in condensed
matter physics owing to the low dimensionality of their electronic states. For
example, the high-Tc (high transition temperature) cuprates and the Fe-based
superconductors possess a layered crystal structure composed of a stacking of
spacer (blocking) layers and conduction (superconducting) layers, CuO2 planes
or Fe-Anion layers. The spacer layers provide carriers to the conduction layers
and induce exotic superconductivity. Recently, we have reported
superconductivity in the novel BiS2-based layered compound Bi4O4S3. It was
found that superconductivity of Bi4O4S3 originates from the BiS2 layers. The
crystal structure is composed of a stacking of BiS2 superconducting layers and
the spacer layers, which resembles those of high-Tc cuprate and the Fe-based
superconductors. Here we report a discovery of a new type of BiS2-based layered
superconductor LaO1-xFxBiS2, with a Tc as high as 10.6 K.Comment: 23 pages, 5 figures, 1 table (table caption has been revised), to
appear in J. Phys. Soc. Jp
Thermal and Tunneling Pair Creation of Quasiparticles in Quantum Hall Systems
We make a semiclassical analysis of thermal pair creations of quasiparticles
at various filling factors in quantum Hall systems. It is argued that the gap
energy is reduced considerably by the Coulomb potential made by impurities. It
is also shown that a tunneling process becomes important at low temperature and
at strong magnetic field. We fit typical experimental data excellently based on
our semiclassical results of the gap energy.Comment: 6 pages, 6 PS figures, to be published in Phys.Rev.
Hydrodynamic Equations in Quantum Hall Systems at Large Currents
Hydrodynamic equations (HDEQs) are derived which describe spatio-temporal
evolutions of the electron temperature and the chemical potential of
two-dimensional systems in strong magnetic fields in states with large diagonal
resistivity appearing at the breakdown of the quantum Hall effect. The
derivation is based on microscopic electronic processes consisting of drift
motions in a slowly-fluctuating potential and scattering processes due to
electron-electron and electron-phonon interactions. In contrast with the usual
HDEQs, one of the derived HDEQs has a term with an energy flux perpendicular to
the electric field due to the drift motions in the magnetic field. As an
illustration, the current distribution is calculated using the derived HDEQs.Comment: 10 pages, 2 Postscript figures, to be published in J. Phys. Soc. Jpn.
71 (2002) No.
Integer Quantum Hall Effect with Realistic Boundary Condition : Exact Quantization and Breakdown
A theory of integer quantum Hall effect(QHE) in realistic systems based on
von Neumann lattice is presented. We show that the momentum representation is
quite useful and that the quantum Hall regime(QHR), which is defined by the
propagator in the momentum representation, is realized. In QHR, the Hall
conductance is given by a topological invariant of the momentum space and is
quantized exactly. The edge states do not modify the value and topological
property of in QHR. We next compute distribution of current based
on effective action and find a finite amount of current in the bulk and the
edge, generally. Due to the Hall electric field in the bulk, breakdown of the
QHE occurs. The critical electric field of the breakdown is proportional to
and the proportional constant has no dependence on Landau levels in
our theory, in agreement with the recent experiments.Comment: 48 pages, figures not included, some additions and revision
Spin Degree of Freedom in a Two-Dimensional Electron Liquid
We have investigated correlation between spin polarization and
magnetotransport in a high mobility silicon inversion layer which shows the
metal-insulator transition. Increase in the resistivity in a parallel magnetic
field reaches saturation at the critical field for the full polarization
evaluated from an analysis of low-field Shubnikov-de Haas oscillations. By
rotating the sample at various total strength of the magnetic field, we found
that the normal component of the magnetic field at minima in the diagonal
resistivity increases linearly with the concentration of ``spin-up'' electrons.Comment: 4 pages, RevTeX, 6 eps-figures, to appear in PR
Strong, Ultra-narrow Peaks of Longitudinal and Hall Resistances in the Regime of Breakdown of the Quantum Hall Effect
With unusually slow and high-resolution sweeps of magnetic field, strong,
ultra-narrow (width down to ) resistance peaks are observed in
the regime of breakdown of the quantum Hall effect. The peaks are dependent on
the directions and even the history of magnetic field sweeps, indicating the
involvement of a very slow physical process. Such a process and the sharp peaks
are, however, not predicted by existing theories. We also find a clear
connection between the resistance peaks and nuclear spin polarization.Comment: 5 pages with 3 figures. To appear in PR
Thermohydrodynamics in Quantum Hall Systems
A theory of thermohydrodynamics in two-dimensional electron systems in
quantizing magnetic fields is developed including a nonlinear transport regime.
Spatio-temporal variations of the electron temperature and the chemical
potential in the local equilibrium are described by the equations of
conservation with the number and thermal-energy flux densities. A model of
these flux densities due to hopping and drift processes is introduced for a
random potential varying slowly compared to both the magnetic length and the
phase coherence length. The flux measured in the standard transport experiment
is derived and is used to define a transport component of the flux density. The
equations of conservation can be written in terms of the transport component
only. As an illustration, the theory is applied to the Ettingshausen effect, in
which a one-dimensional spatial variation of the electron temperature is
produced perpendicular to the current.Comment: 10 pages, 1 figur
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