18,418 research outputs found
Interlayer tunneling in counterflow experiments on the excitonic condensate in quantum Hall bilayers
The effect of tunneling on the transport properties of} quantum Hall double
layers in the regime of the excitonic condensate at total filling factor one is
studied in counterflow experiments. If the tunnel current is smaller than a
critical , tunneling is large and is effectively shorting the two layers.
For tunneling becomes negligible. Surprisingly, the transition
between the two tunneling regimes has only a minor impact on the features of
the filling-factor one state as observed in magneto-transport, but at currents
exceeding the resistance along the layers increases rapidly
Surface segregation and the Al problem in GaAs quantum wells
Low-defect two-dimensional electron systems (2DESs) are essential for studies
of fragile many-body interactions that only emerge in nearly-ideal systems. As
a result, numerous efforts have been made to improve the quality of
modulation-doped AlGaAs/GaAs quantum wells (QWs), with an emphasis
on purifying the source material of the QW itself or achieving better vacuum in
the deposition chamber. However, this approach overlooks another crucial
component that comprises such QWs, the AlGaAs barrier. Here we show
that having a clean Al source and hence a clean barrier is instrumental to
obtain a high-quality GaAs 2DES in a QW. We observe that the mobility of the
2DES in GaAs QWs declines as the thickness or Al content of the
AlGaAs barrier beneath the QW is increased, which we attribute to
the surface segregation of Oxygen atoms that originate from the Al source. This
conjecture is supported by the improved mobility in the GaAs QWs as the Al cell
is cleaned out by baking
Electromechanical tuning of vertically-coupled photonic crystal nanobeams
We present the design, the fabrication and the characterization of a tunable
one-dimensional (1D) photonic crystal cavity (PCC) etched on two
vertically-coupled GaAs nanobeams. A novel fabrication method which prevents
their adhesion under capillary forces is introduced. We discuss a design to
increase the flexibility of the structure and we demonstrate a large reversible
and controllable electromechanical wavelength tuning (> 15 nm) of the cavity
modes.Comment: 11 pages, 4 figure
Rotating Black Hole Entropy from Two Different Viewpoints
Using the brick-wall method, we study the entropy of Kerr-Newman black hole
from two different viewpoints, a rest observer at infinity and zero angular
momentum observer near horizon. We investigate this with scalar field in the
canonical quantization approach. An observer at infinity can take one of the
two possible frequency ranges; one is with positive frequencies only and the
other is with the whole range including negative frequencies. On the other
hand, a zero angular momentum observer near horizon can take positive
frequencies only. For the observer at infinity the superradiant modes appear in
either choice of the frequency ranges and the two results coincide. For the
zero angular momentum observer superradiant modes do not appear due to absence
of ergoregion. The resulting entropies from the two viewpoints turn out to be
the same.Comment: LaTeX 18 pages, 2 figures, Minor modifications in section 3(ZAMO
Relativistic Generalization of the Gamow Factor for Fermion Pair Production or Annihilation
In the production or annihilation of a pair of fermions, the initial-state or
final-state interactions often lead to significant effects on the reaction
cross sections. For Coulomb-type interactions, the Gamow factor has been
traditionally used to take into account these effects. However the Gamow factor
needs to be modified when the magnitude of the coupling constant or the
relative velocity of two particles increases. We obtain the relativistic
generalization of the Gamow factor in terms of the overlap of the Feynman
amplitude with the relativistic wave function of two fermions with an
attractive Coulomb-type interaction. An explicit form of the corrective factor
is presented for the spin-singlet S-wave state. While the corrective factor
approaches the Gamow factor in the non-relativistic limit, we found that the
Gamow factor significantly over-estimates the effects when the coupling
constant or the velocity is large.Comment: 16 pages, 4 figures in LaTe
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