609 research outputs found
Strain-driven light polarization switching in deep ultraviolet nitride emitters
Residual strain plays a critical role in determining the crystalline quality
of nitride epitaxial layers and in modifying their band structure; this often
leads to several interesting physical phenomena. It is found, for example, that
compressive strain in AlxGa1-xN layers grown on AlyGa1-yN (x<y) templates
results in an anti-crossing of the valence bands at considerably much higher Al
composition than expected. This happens even in the presence of large and
negative crystal field splitting energy for AlxGa1-xN layers. A judicious
magnitude of the compressive strain can support vertical light emission (out of
the c-plane) from AlxGa1-xN quantum wells up to x\approx 0.80, which is
desirable for the development of deep ultraviolet light-emitting diodes
designed to operate below 250nm with transverse electric polarization
characteristics
Magnetotransport of coupled electron-holes
The carriers in InAs-GaSb double quantum wells are hybrid ``electron-holes''.
We study the magnetotransport properties of such particles using a
two-component Keldysh technique, which results in a semi-analytic expression
for the small-field current. We show that zero temperature current can be large
even when the Fermi energy lies within the hybridization gap, a result which
cannot be understood within a semiclassical (Boltzmann) approach. Magnetic
field dependence of the conductance is also affected significantly by the
hybridization of electrons and holes.Comment: 4 pages, 2 figure
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
Shot noise suppression in multimode ballistic Fermi conductors
We have derived a general formula describing current noise in multimode
ballistic channels connecting source and drain electrodes with Fermi electron
gas. In particular (at ), the expression describes the
nonequilibrium ''shot'' noise, which may be suppressed by both Fermi
correlations and space charge screening. The general formula has been applied
to an approximate model of a 2D nanoscale, ballistic MOSFET. At large negative
gate voltages, when the density of electrons in the channel is small, shot
noise spectral density approaches the Schottky value , where
is the average current. However, at positive gate voltages, when the
maximum potential energy in the channel is below the Fermi level of the
electron source, the noise can be at least an order of magnitude smaller than
the Schottky value, mostly due to Fermi effects.Comment: 4 page
Nonlinear voltage dependence of the shot noise in mesoscopic degenerate conductors with strong electron-electron scattering
It is shown that measurements of zero-frequency shot-noise can provide
information on electron-electron interaction, because the strong interaction
results in the nonlinear voltage dependence of the shot noise in metallic
wires. This is due to the fact that the Wiedemann-Franz law is no longer valid
in the case of considerable electron-electron interaction. The deviations from
this law increase the noise power and make it dependent strongly on the ratio
of electron-electron and electron-impurity scattering rates.Comment: 4 pages, 2 figures, revised version according to referee's comment
Current noise in long diffusive SNS junctions in the incoherent MAR regime
Spectral density of current fluctuations at zero frequency is calculated for
a long diffusive SNS junction with low-resistive interfaces. At low
temperature, T << Delta, the subgap shot noise approaches linear voltage
dependence, S=(2/ 3R)(eV + 2Delta), which is the sum of the shot noise of the
normal conductor and voltage independent excess noise. This result can also be
interpreted as the 1/3-suppressed Poisson noise for the effective charge q =
e(1+2Delta/eV) transferred by incoherent multiple Andreev reflections (MAR). At
higher temperatures, anomalies of the current noise develop at the gap
subharmonics, eV = 2Delta/n. The crossover to the hot electron regime from the
MAR regime is analyzed in the limit of small applied voltages.Comment: improved version, to be published in Phys. Rev.
Frequency-Dependent Shot Noise as a Probe of Electron-Electron Interaction in Mesoscopic Diffusive Contacts
The frequency-dependent shot noise in long and narrow mesoscopic diffusive
contacts is numerically calculated. The case of arbitrarily strong
electron-electron scattering and zero temperature of electrodes is considered.
For all voltages, the noise increases with frequency and tends to finite
values. These limiting values are larger than the Poissonian noise and increase
nearly as voltage to power 4/3. This allows one to experimentally determine the
parameters of electron-electron interaction.Comment: 3 pages, RevTeX, 3 eps figure
Microscopic analysis of shot-noise suppression in nondegenerate diffusive conductors
We present a theoretical investigation of shot-noise suppression due to
long-range Coulomb interaction in nondegenerate diffusive conductors.
Calculations make use of an ensemble Monte Carlo simulator self-consistently
coupled with a one-dimensional Poisson solver. We analyze the noise in a
lightly doped active region surrounded by two contacts acting as thermal
reservoirs. By taking the doping of the injecting contacts and the applied
voltage as variable parameters, the influence of elastic and inelastic
scattering in the active region is investigated. The transition from ballistic
to diffusive transport regimes under different contact injecting statistics is
analyzed and discussed. Provided significant space-charge effects take place
inside the active region, long-range Coulomb interaction is found to play an
essential role in suppressing the shot noise at . In the elastic
diffusive regime, momentum space dimensionality is found to modify the
suppression factor , which within numerical uncertainty takes values
respectively of about 1/3, 1/2 and 0.7 in the 3D, 2D and 1D cases. In the
inelastic diffusive regime, shot noise is suppressed to the thermal value.Comment: 11 pages, 13 figure
Effect of screening on shot noise in diffusive mesoscopic conductors
Shot noise in diffusive mesoscopic conductors, at finite observation
frequencies (comparable to the reciprocal Thouless time
), is analyzed with an account of screening. At low frequencies,
the well-known result is recovered. This result is valid at
arbitrary for wide conductors longer than the screening length.
However, at least for two very different systems, namely, wide and short
conductors, and thin conductors over a close ground plane, noise approaches a
different fundamental level, , at .Comment: 5 pages, 3 figures. Published version. Also available in the
journal's format at
http://hana.physics.sunysb.edu/~yehuda/cv/papers/shotnoise.pd
Engineering Superfluidity in Electron-Hole Double Layers
We show that band-structure effects are likely to prevent superfluidity in
semiconductor electron-hole double-layer systems. We suggest the possibility
that superfluidity could be realized by the application of uniaxial pressure
perpendicular to the electron and hole layers.Comment: 4 pages, includes 3 figure
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