259 research outputs found
Observation of narrow-band noise accompanying the breakdown of insulating states in high Landau levels
Recent magnetotransport experiments on high mobility two-dimensional electron
systems have revealed many-body electron states unique to high Landau levels.
Among these are re-entrant integer quantum Hall states which undergo sharp
transitions to conduction above some threshold field. Here we report that these
transitions are often accompanied by narrow- and broad-band noise with
frequencies which are strongly dependent on the magnitude of the applied dc
current.Comment: 4 pages, 3 figure
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.
Quantum Response at Finite Fields and Breakdown of Chern Numbers
We show that the response to an electric field, in models of the Integral
Quantum Hall effect, is analytic in the field and has isolated essential
singularity at zero field. We also study the breakdown of Chern numbers
associated with the response of Floquet states. We argue, and give evidence,
that the breakdown of Chern numbers in Floquet states is a discontinuous
transition at zero field. This follows from an observation, of independent
interest, that Chern numbers for finite dimensional Floquet operators are
generically zero. These results rule out the possibility that the breakdown of
the Hall conductance is a phase transition at finite fields for a large class
of models.Comment: 16 pages, 8 eps figures, LaTeX2e with IOP style. Many changes,
including new materia
Improved Theory of the Muonium Hyperfine Structure
Terms contributing to the hyperfine structure of the muonium ground state at
the level of few tenths of kHz have been evaluated. The
radiative correction has been calculated numerically to the precision of 0.02
kHz. Leading terms of order and some relativistic corrections have been evaluated analytically.
The theoretical uncertainty is now reduced to 0.17 kHz. At present, however, it
is not possible to test QED to this precision because of the 1.34 kHz
uncertainty due to the muon mass.Comment: 11 pages + 2 figures (included), RevTeX 3.0, CLNS 94/127
Influence of Landau-level mixing on Wigner crystallization in graphene
Graphene, with its massless linearly-dispersing carriers, in the quantum Hall
regime provides an instructive comparison with conventional two-dimensional
(2D) systems in which carriers have a nonzero band mass and quadratic
dispersion. We investigate the influence of Landau level mixing in graphene on
Wigner crystal states in the Landau level obtained using single
Landau level approximation. We show that the Landau level mixing does not
qualitatively change the phase diagram as a function of partial filling factor
in the level. We find that the inter-Landau level mixing,
quantified by relative occupations of the two Landau levels,
, oscillates around 2% and, in general, remains small () irrespective of the Landau level index . Our results show that the
single Landau level approximation is applicable in high Landau levels, even
though the energy gap between the adjacent Landau levels vanishes.Comment: 6 pages, 3 figure
Creation of super-high-flux photo-neutrons and gamma-rays > 8 MeV using a petawatt laser to irradiate high-Z solid targets
We report the creation of super-high-flux gamma-rays with energy >8 MeV and
photo-neutrons via the (g,n) reaction near giant dipole resonance energies (8 -
20 MeV), using the ~130 J Texas Petawatt laser to irradiate high-Z (Au, Pt, Re,
W) targets of mm - cm thickness, at laser intensities up to ~5x1021W/cm2. We
detected up to ~ several x 1012 gamma-rays > 8 MeV (~3% of incident laser
energy) and ~ 1010 photo-neutrons per shot. Due to the short pulse and narrow
gamma-ray cone (~17o half-width) around laser forward, the peak emergent
gamma-ray flux >8 MeV reached ~1027 gammas/cm2/sec, and the peak emergent
neutron flux reached ~1020 neutrons/cm2/sec. Such intense gamma-ray and neutron
fluxes are among the highest achieved for short-pulse laser experiments. They
will facilitate the study of nuclear reactions requiring super-high-flux of
gamma-rays or neutrons, such as the creation of r-process elements. These
results may also have far-reaching applications for nuclear energy, such as the
transmutation of nuclear waste.Comment: 16 pages, 9 figure
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.
Dynamic Nuclear Polarization in a Quantum Hall Corbino Disk
Electrical polarization of nuclear spins is studied in a Corbino disk under a
breakdown regime of the quantum Hall effect (QHE). Since the edge channels are
completely absent in the Corbino disk, we conclude that the electric current
flowing in the bulk channel of a quantum Hall conductor is relevant to dynamic
nuclear polarization (DNP). A pump and probe measurement demonstrates that DNP
emerges near the critical voltage of the QHE breakdown. The agreement of the
onset voltage of DNP with that of the QHE breakdown indicates that the
underlying origin of DNP is closely related to that of the QHE breakdown.Comment: 3 pages, 4 figure
Theory of Current-Induced Breakdown of the Quantum Hall Effect
By studying the quantum Hall effect of stationary states with high values of
injected current using a von Neumann lattice representation, we found that
broadening of extended state bands due to a Hall electric field occurs and
causes the breakdown of the quantum Hall effect. The Hall conductance agrees
with a topological invariant that is quantized exactly below a critical field
and is not quantized above a critical field. The critical field is proportional
to and is enhanced substantially if the extended states occupy a
small fraction of the system.Comment: 5 pages, RevTeX, final version to appear in PR
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.
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