125 research outputs found
Correlations in a two-dimensional Bose gas with long range interactions
We study the correlations of two-dimensional dipolar excitons in coupled
quantum wells with a dipole -- dipole repulsive interaction. We show that at
low concentrations, the Bose degeneracy of the excitons is accompanied by
strong multi-particle correlations and the system behaves as a Bose liquid. At
high concentration the particles interaction suppresses quantum coherence and
the system behaves similar to a classical liquid down to a temperature lower
than typical for a Bose gas. We evaluate the interaction energy per particle
and the resulting blue shift of the exciton luminescence that is a direct tool
to measure the correlations. This theory can apply to other systems of bosons
with extended interaction.Comment: 11 pages including 2 figure
Effective mass and band nonparabolicity in remote doped Si/Si0.8Ge0.2 quantum wells
The effective masses in remote doped Si/Si0.8Ge0.2/Si quantum wells having sheet densities, Ns in the range 2 × 1011–1.1 × 1012 cm – 2 have been determined from the temperature dependencies of the Shubnikov–de Haas oscillations. The values obtained increase with magnetic field and Ns. This behavior is taken as evidence for the nonparabolicity of the valence band and accounts for the discrepancies in previously reported masses. Self-consistent band structure calculations for a triangular confinement of the carriers have also been carried out and provide confirmation of the increase in mass with Ns. Theory and experiment give extrapolated Gamma point effective masses of 0.21 and 0.20 of the free-electron mass, respectively
Nonlinear acoustic and microwave absorption in glasses
A theory of weakly-nonlinear low-temperature relaxational absorption of
acoustic and electromagnetic waves in dielectric and metallic glasses is
developed. Basing upon the model of two-level tunneling systems we show that
the nonlinear contribution to the absorption can be anomalously large. This is
the case at low enough frequencies, where freqeuency times the minimal
relaxation time for the two-level system are much less than one. In dielectric
glasses, the lowest-order nonlinear contribution is proportional to the wave's
intensity. It is negative and exhibits anomalous frequency and temperature
dependencies. In metallic glasses, the nonlinear contribution is also negative,
and it is proportional to the square root of the wave's intensity and to the
frequency. Numerical estimates show that the predicted nonlinear contribution
can be measured experimentally
Cyclotron resonance in a two-dimensional electron gas with long-range randomness
We show that the the cyclotron resonance in a two-dimensional electron gas
has non-trivial properties if the correlation length of the disorder is larger
than the de Broglie wavelength: (a) the lineshape assumes three different forms
in strong, intermediate, and weak magnetic fields (b) at the transition from
the intermediate to the weak fields the linewidth suddenly collapses due to an
explosive growth in the fraction of electrons with a diffusive-type dynamics.Comment: A few typos correcte
Exciton correlations in coupled quantum wells and their luminescence blue shift
In this paper we present a study of an exciton system where electrons and
holes are confined in double quantum well structures. The dominating
interaction between excitons in such systems is a dipole - dipole repulsion. We
show that the tail of this interaction leads to a strong correlation between
excitons and substantially affects the behavior of the system. Making use of
qualitative arguments and estimates we develop a picture of the exciton -
exciton correlations in the whole region of temperature and concentration where
excitons exist. It appears that at low concentration degeneracy of the excitons
is accompanied with strong multi-particle correlation so that the system cannot
be considered as a gas. At high concentration the repulsion suppresses the
quantum degeneracy down to temperatures that could be much lower than in a Bose
gas with contact interaction. We calculate the blue shift of the exciton
luminescence line which is a sensitive tool to observe the exciton - exciton
correlations.Comment: 27 pages in PDF and DVI format, 8 figure
The Quantum Hall Effect in Drag: Inter-layer Friction in Strong Magnetic Fields
We study the Coulomb drag between two spatially separated electron systems in
a strong magnetic field, one of which exhibits the quantum Hall effect. At a
fixed temperature, the drag mimics the behavior of in the quantum
Hall system, in that it is sharply peaked near the transitions between
neighboring plateaux. We assess the impact of critical fluctuations near the
transitions, and find that the low temperature behavior of the drag measures an
exponent that characterizes anomalous low frequency dissipation; the
latter is believed to be present following the work of Chalker.Comment: 13 pages, Revtex 2.0, 1 figure upon request, P-93-11-09
Coulomb Drag Between Parallel Ballistic Quantum Wires
The Coulomb drag between parallel, {\it ballistic} quantum wires is studied
theoretically in the presence of a perpendicular magnetic field B. The
transresistance R_D shows peaks as a function of the Fermi level and splitting
energy between the 1D subbands of the wires. The sharpest peaks appear when the
Fermi level crosses the subband extrema so that the Fermi momenta are small.
Two other kinds of peaks appear when either {\it intra}- or {\it inter}-subband
transitions of electrons have maximum probability; the {\it intra}-subband
transitions correspond to a small splitting energy. R_D depends on the field B
in a nonmonotonic fashion: it decreases with B, as a result of the suppression
of backscattering, and increases sharply when the Fermi level approaches the
subband bottoms and the suppression is outbalanced by the increase of the
Coulomb matrix elements and of the density of states.Comment: Text 14 pages in Latex/Revtex format, 4 Postscript figures. Phys.
Rev. B,in pres
Excitonic instability and electric-field-induced phase transition towards a two dimensional exciton condensate
We present an InAs-GaSb-based system in which the electric-field tunability
of its 2D energy gap implies a transition towards a thermodynamically stable
excitonic condensed phase. Detailed calculations show a 3 meV BCS-like gap
appearing in a second-order phase transition with electric field. We find this
transition to be very sharp, solely due to exchange interaction, and so, the
exciton binding energy is greatly renormalized even at small condensate
densities. This density gradually increases with external field, thus enabling
the direct probe of the Bose-Einstein to BCS crossover.Comment: LaTex, 11 pages, 3 ps figures, To appear in PR
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