87 research outputs found
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
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.
Spin degree of freedom in two dimensional exciton condensates
We present a theoretical analysis of a spin-dependent multicomponent
condensate in two dimensions. The case of a condensate of resonantly
photoexcited excitons having two different spin orientations is studied in
detail. The energy and the chemical potentials of this system depend strongly
on the spin polarization . When electrons and holes are located in two
different planes, the condensate can be either totally spin polarized or spin
unpolarized, a property that is measurable. The phase diagram in terms of the
total density and electron-hole separation is discussed.Comment: 4 pages, 3 figures, Accepted for publication in Physical Review
Letter
Full Counting Statistics of Multiple Andreev Reflections in incoherent diffusive superconducting junctions
We present a theory for the full distribution of current fluctuations in
incoherent diffusive superconducting junctions, subjected to a voltage bias.
This theory of full counting statistics of incoherent multiple Andreev
reflections is valid for arbitrary applied voltage. We present a detailed
discussion of the properties of the first four cumulants as well as the low and
high voltage regimes of the full counting statistics. The work is an extension
of the results of Pilgram and the author, Phys. Rev. Lett. 94, 086806 (2005).Comment: Included in special issue Spin Physics of Superconducting
heterostructures of Applied Physics A: Materials Science & Processin
Suppression of non-Poissonian shot noise by Coulomb correlations in ballistic conductors
We investigate the current injection into a ballistic conductor under the
space-charge limited regime, when the distribution function of injected
carriers is an arbitrary function of energy F_c(epsilon). The analysis of the
coupled kinetic and Poisson equations shows that the injected current
fluctuations may be essentially suppressed by Coulomb correlations, and the
suppression level is determined by the shape of F_c(epsilon). This is in
contrast to the time-averaged quantities: the mean current and the spatial
profiles are shown to be insensitive to F_c(epsilon) in the leading-order terms
at high biases. The asymptotic high-bias behavior for the energy resolved
shot-noise suppression has been found for an arbitrary (non-Poissonian)
injection, which may suggest a new field of investigation on the optimization
of the injected energy profile to achieve the desired noise-suppression level.Comment: extended version 4 -> 8 pages, examples and figure adde
Suppression of non-Poissonian shot noise by Coulomb correlations in ballistic conductors
We investigate the current injection into a ballistic conductor under the
space-charge limited regime, when the distribution function of injected
carriers is an arbitrary function of energy F_c(epsilon). The analysis of the
coupled kinetic and Poisson equations shows that the injected current
fluctuations may be essentially suppressed by Coulomb correlations, and the
suppression level is determined by the shape of F_c(epsilon). This is in
contrast to the time-averaged quantities: the mean current and the spatial
profiles are shown to be insensitive to F_c(epsilon) in the leading-order terms
at high biases. The asymptotic high-bias behavior for the energy resolved
shot-noise suppression has been found for an arbitrary (non-Poissonian)
injection, which may suggest a new field of investigation on the optimization
of the injected energy profile to achieve the desired noise-suppression level.Comment: extended version 4 -> 8 pages, examples and figure adde
Multiple Andreev Reflection and Giant Excess Noise in Diffusive Superconductor/Normal-Metal/Superconductor Junctions
We have studied superconductor/normal metal/superconductor (SNS) junctions
consisting of short Au or Cu wires between Nb or Al banks. The Nb based
junctions display inherent electron heating effects induced by the high thermal
resistance of the NS boundaries. The Al based junctions show in addition
subharmonic gap structures in the differential conductance dI/dV and a
pronounced peak in the excess noise at very low voltages V. We suggest that the
noise peak is caused by fluctuations of the supercurrent at the onset of
Josephson coupling between the superconducting banks. At intermediate
temperatures where the supercurrent is suppressed a noise contribution ~1/V
remains, which may be interpreted as shot noise originating from large multiple
charges.Comment: 7 pages, 7 figures, extended versio
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