25,630 research outputs found
Method and apparatus for making a heat insulating and ablative structure Patent
Method and apparatus for fabrication of heat insulating and ablative reentry structur
Actively Tuned and Spatially Trapped Polaritons
We report active tuning of the polariton resonance of quantum well excitons
in a semiconductor microcavity using applied stress. Starting with the quantum
well exciton energy higher than the cavity photon mode, we use stress to reduce
the exciton energy and bring it into resonance with the photon mode. At the
point of zero detuning, line narrowing and strong increase of the
photoluminescence are seen. By the same means, we create an in-plane harmonic
potential for the polaritons, which allows trapping, potentially making
Bose-Einstein condensation of polaritons analogous to trapped atoms possible.
We demonstrate drift of the polaritons into this trap.Comment: 10 pages, 5 figure
Degenerate states of narrow semiconductor rings in the presence of spin orbit coupling: Role of time-reversal and large gauge transformations
The electron Hamiltonian of narrow semiconductor rings with the Rashba and
Dresselhaus spin orbit terms is invariant under time-reversal operation
followed by a large gauge transformation. We find that all the eigenstates are
doubly degenerate when integer or half-integer quantum fluxes thread the
quantum ring. The wavefunctions of a degenerate pair are related to each other
by the symmetry operation. These results are valid even in the presence of a
disorder potential. When the Zeeman term is present only some of these
degenerate levels anticross
Scaling limit of a non-relativistic model
I calculate the structure function for scattering from the two-body bound
state in its lowest level in a non-relativistic model of confined scalar
``quarks'' of masses and . The scaling limit in exists and is non-vanishing only for the values
and which correspond to the fractions of
the momentum of the two-body system carried by each of the ``quarks.'' In the
scaling limit, the interference from scattering off of the two ``quarks''
vanishes. Thus the scaling limit of this model agrees with the parton picture.Comment: 10 pages, 3 figures not included, in LaTex, UMD 92-22
Imaging Transport Resonances in the Quantum Hall Effect
We use a scanning capacitance probe to image transport in the quantum Hall
system. Applying a DC bias voltage to the tip induces a ring-shaped
incompressible strip (IS) in the 2D electron system (2DES) that moves with the
tip. At certain tip positions, short-range disorder in the 2DES creates a
quantum dot island in the IS. These islands enable resonant tunneling across
the IS, enhancing its conductance by more than four orders of magnitude. The
images provide a quantitative measure of disorder and suggest resonant
tunneling as the primary mechanism for transport across ISs.Comment: 4 pages, 4 figures, submitted to PRL. For movies and additional
infomation, see http://electron.mit.edu/scanning/; Added scale bars to
images, revised discussion of figure 3, other minor change
Observation of a cyclotron harmonic spike in microwave-induced resistances in ultraclean GaAs/AlGaAs quantum wells
We report the observation of a colossal, narrow resistance peak that arises
in ultraclean (mobility 3X10^7cm^2/Vs) GaAs/AlGaAs quantum wells (QWs) under
millimeterwave irradiation and a weak magnetic field. Such a spike is
superposed on the 2nd harmonic microwave-induced resistance oscillations (MIRO)
but having an amplitude > 300% of the MIRO, and a typical FWHM ~50 mK,
comparable with the Landau level width. Systematic studies show a correlation
between the spike and a pronounced negative magnetoresistance in these QWs,
suggesting a mechanism based on the interplay of strong scatterers and smooth
disorder. Alternatively, the spike may be interpreted as a manifestation of
quantum interference between the quadrupole resonance and the higher-order
cyclotron transition in well-separated Landau levels.Comment: 4pages, 4figure
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