1,227 research outputs found
A Model for the Voltage Steps in the Breakdown of the Integer Quantum Hall Effect
In samples used to maintain the US resistance standard the breakdown of the
dissipationless integer quantum Hall effect occurs as a series of dissipative
voltage steps. A mechanism for this type of breakdown is proposed, based on the
generation of magneto-excitons when the quantum Hall fluid flows past an
ionised impurity above a critical velocity. The calculated generation rate
gives a voltage step height in good agreement with measurements on both
electron and hole gases. We also compare this model to a hydrodynamic
description of breakdown.Comment: 4 pages including 3 figure
Cytogenetic studies of early myeloid progenitor compartments in Ph<SUP>1</SUP>- positive chronic myeloid leukemia. II. Long-term culture reveals the persistence of Ph<SUP>1</SUP>-negative progenitors in treated as well as newly diagnosed patients
We recently showed that long-term marrow cultures can be used to demonstrate the presence of Philadelphia (Ph1) negative progenitors in patients with newly diagnosed Ph1-positive chronic myeloid leukemia (CML). We now report results for 6 chronic phase patients studied 5-83 mo postdiagnosis and an additional 3 newly diagnosed patients. Marrow metaphases were exclusively Ph1-positive. Clonogenic assays revealed a minor population of Ph1-negative progenitors in 3 cases (1 treated, 2 untreated). Long-term marrow culture adherent layers contained Ph1- negative progenitors in 6 cases (3 treated, 3 untreated). Whenever this occurred, the Ph1-negative population had become the only one detectable within 3-4 wk, and this was always associated with a rapid decline of the Ph1-positive population. For 2 of the 3 cases where Ph1- negative progenitors were not detected, there was a similar rapid decline in the Ph1-positive population in culture. In the other case, Ph1-positive progenitors were maintained at levels typically seen in normal long-term marrow cultures. These results suggest that chromosomally normal stem cells may persist for a considerable period in the marrow of some, but perhaps not all, patients with CML, even in the face of maintenance chemotherapy. In addition, they provide new evidence of heterogeneity in this disease, as shown by the variable ability of Ph1-positive progenitor populations to be maintained in vitro
Validation of magnetophonon spectroscopy as a tool for analyzing hot-electron effects in devices
It is shown that very high precision hot-electron magnetophonon experiments made on n+n−n+-GaAs sandwich device structures which are customized for magnetoresistance measurements can be very accurately modeled by a new Monte Carlo technique. The latter takes account of the Landau quantization and device architecture as well as material parameters. It is proposed that this combination of experiment and modeling yields a quantitative tool for the direct analysis of spatially localized very nonequilibrium electron distributions in small devices and low dimensional structures
Dynamic instabilities in resonant tunneling induced by a magnetic field
We show that the addition of a magnetic field parallel to the current induces
self sustained intrinsic current oscillations in an asymmetric double barrier
structure. The oscillations are attributed to the nonlinear dynamic coupling of
the current to the charge trapped in the well, and the effect of the external
field over the local density of states across the system. Our results show that
the system bifurcates as the field is increased, and may transit to chaos at
large enough fields.Comment: 4 pages, 3 figures, accepted in Phys. Rev. Letter
Electric field inversion asymmetry: Rashba and Stark effects for holes in resonant tunneling devices
We report experimental evidence of excitonic spin-splitting, in addition to
the conventional Zeeman effect, produced by a combination of the Rashba
spin-orbit interaction, Stark shift and charge screening. The
electric-field-induced modulation of the spin-splitting are studied during the
charging and discharging processes of p-type GaAs/AlAs double barrier resonant
tunneling diodes (RTD) under applied bias and magnetic field. The abrupt
changes in the photoluminescence, with the applied bias, provide information of
the charge accumulation effects on the device.Comment: 4 pages, 2 figure
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
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