31 research outputs found
Magneto-infrared modes in InAs-AlSb-GaSb coupled quantum wells
We have studied a series of InAs/GaSb coupled quantum wells using
magneto-infrared spectroscopy for high magnetic fields up to 33T within
temperatures ranging from 4K to 45K in both Faraday and tilted field
geometries. This type of coupled quantum wells consists of an electron layer in
the InAs quantum well and a hole layer in the GaSb quantum well, forming the
so-called two dimensional electron-hole bilayer system. Unlike the samples
studied in the past, the hybridization of the electron and hole subbands in our
samples is largely reduced by having narrower wells and an AlSb barrier layer
interposed between the InAs and the GaSb quantum wells, rendering them weakly
hybridized. Previous studies have revealed multiple absorption modes near the
electron cyclotron resonance of the InAs layer in moderately and strongly
hybridized samples, while only a single absorption mode was observed in the
weakly hybridized samples. We have observed a pair of absorption modes
occurring only at magnetic fields higher than 14T, which exhibited several
interesting phenomena. Among which we found two unique types of behavior that
distinguishes this work from the ones reported in the literature. This pair of
modes is very robust against rising thermal excitations and increasing magnetic
fields alligned parallel to the heterostructures. While the previous results
were aptly explained by the antilevel crossing gap due to the hybridization of
the electron and hole wavefunctions, i.e. conduction-valence Landau level
mixing, the unique features reported in this paper cannot be explained within
the same concept. The unusual properties found in this study and their
connection to the known models for InAs/GaSb heterostructures will be
disccused; in addition, several alternative ideas will be proposed in this
paper and it appears that a spontaneous phase separation can account for most
of the observed features
Quantum, Multi-Body Effects and Nuclear Reaction Rates in Plasmas
Detailed calculations of the contribution from off-shell effects to the
quasiclassical tunneling of fusing particles are provided. It is shown that
these effects change the Gamow rates of certain nuclear reactions in dense
plasma by several orders of magnitude.Comment: 11 pages; change of content: added clarification of one of the
important steps in the derivatio
Tuning the conductance of molecular junctions: transparent versus tunneling regimes
We present a theoretical study of the transport characteristics of molecular
junctions, where first-row diatomic molecules are attached to (001) gold and
platinum electrodes. We find that the conductance of all of these junctions is
of the order of the conductance quantum unit , spelling out that they
belong to the transparent regime. We further find that the transmission
coefficients show wide plateaus as a function of the energy, instead of the
usual sharp resonances that signal the molecular levels in the tunneling
regime. We use Caroli's model to show that this is a rather generic property of
the transparent regime of a junction, which is driven by a strong effective
coupling between the delocalized molecular levels and the conduction channels
at the electrodes. We analyse the transmission coefficients and chemical
bonding of gold/Benzene and gold/Benzene-dithiolate (BDT) junctions to
understand why the later show large resistances, while the former are highly
conductive.Comment: 9 pages, 7 figure
Unconventional pairing in excitonic condensates under spin-orbit coupling
It is shown that the Rashba and Dresselhaus spin orbit couplings enhance the
conclusive power in the experiments on the excitonic condensed state by at
least three low temperature effects. First, spin orbit coupling facilitates the
photoluminescense measurements via enhancing the bright contribution in the
otherwise dominantly dark exciton condensed state. The second is the presence
of a power law temperature dependence of the thermodynamic observables in low
temperatures and the weakening of the second order transition at the critical
temperature. The third is the appearance of the nondiagonal elements in the
static spin susceptibility.Comment: 4 pages, 4 figures, 1 tabl
Excitonic condensation in a symmetric electron-hole bilayer
Using Diffusion Monte Carlo simulations we have investigated the ground state
of a symmetric electron-hole bilayer and determined its phase diagram at T=0.
We find clear evidence of an excitonic condensate, whose stability however is
affected by in-layer electronic correlation. This stabilizes the electron-hole
plasma at large values of the density or inter-layer distance, and the Wigner
crystal at low density and large distance. We have also estimated pair
correlation functions and low order density matrices, to give a microscopic
characterization of correlations, as well as to try and estimate the condensate
fraction.Comment: 4 pages, 3 figures, 2 table
Finite temperature amplitudes and reaction rates in Thermofield dynamics
We propose a method for calculating the reaction rates and transition
amplitudes of generic process taking place in a many body system in
equilibrium. The relationship of the scattering and decay amplitudes as
calculated in Thermo Field Dynamics the conventional techniques is established.
It is shown that in many cases the calculations are relatively easy in TFD.Comment: 32 pages, RevTex, 2 PS figures, to appear in Phys. Rev.
Transient electric current through an Aharonov-Bohm ring after switching of a Two-Level-System
Response of the electronic current through an Aharonov-Bohm ring after a
two-level-system is switched on is calculated perturbatively by use of
non-equilibrium Green function. In the ballistic case the amplitude of the
Aharonov-Bohm oscillation is shown to decay to a new equilibrium value due to
scattering into other electronic states. Relaxation of Altshuler-Aronov-Spivak
oscillation in diffusive case due to dephasing effect is also calculated. The
time scale of the relaxation is determined by characteristic relaxation times
of the system and the splitting of two-level-system. Oscillation phases are not
affected. Future experimental studies of current response may give us direct
information on characteristic times of mesoscopic systems
The Effect of low Momentum Quantum Fluctuations on a Coherent Field Structure
In the present work the evolution of a coherent field structure of the
Sine-Gordon equation under quantum fluctuations is studied. The basic equations
are derived from the coherent state approximation to the functional
Schr\"odinger equation for the field. These equations are solved asymptotically
and numerically for three physical situations. The first is the study of the
nonlinear mechanism responsible for the quantum stability of the soliton in the
presence of low momentum fluctuations. The second considers the scattering of a
wave by the Soliton. Finally the third problem considered is the collision of
Solitons and the stability of a breather.
It is shown that the complete integrability of the Sine-Gordon equation
precludes fusion and splitting processes in this simplified model.
The approximate results obtained are non-perturbative in nature, and are
valid for the full nonlinear interaction in the limit of low momentum
fluctuations. It is also found that these approximate results are in good
agreement with full numerical solutions of the governing equations. This
suggests that a similar approach could be used for the baby Skyrme model, which
is not completely integrable. In this case the higher space dimensionality and
the internal degrees of freedom which prevent the integrability will be
responsable for fusion and splitting processes. This work provides a starting
point in the numerical solution of the full quantum problem of the interaction
of the field with a fluctuation.Comment: 15 pages, 9 (ps) figures, Revtex file. Some discussion expanded but
conclusions unchanged. Final version to appear in PR