3,006 research outputs found
Metastable bound state of a pair of two-dimensional spatially separated electrons in anti-parallel magnetic fields
We propose a new mechanism for binding of two equally charged carriers in a
double-layer system subjected by a magnetic field of a special form. A field
configuration for which the magnetic fields in adjacent layers are equal in
magnitude and opposite in direction is considered. In such a field an
additional integral of motion - the momentum of the pair P arises. For the case
when in one layer the carrier is in the zero (n=0) Landau level while in the
other layer - in the first (n=1) Landau level the dependence of the energy of
the pair on its momentum E(P} is found. This dependence turns out to be
nonmonotonic one : a local maximum and a local minimum appears, indicating the
emergence of a metastable bound state of two carrier with the same sign of
electrical charge.Comment: 7 page
Involvement of Mhc Loci in immune responses that are not Ir-gene-controlled
Twenty-nine randomly chosen, soluble antigens, many of them highly complex, were used to immunize mice of two strains, C3H and B10.RIII. Lymphnode cells from the immunized mice were restimulated in vitro with the priming antigens and the proliferative response of the cells was determined. Both strains were responders to 28 of 29 antigens. Eight antigens were then used to immunize 11 congenic strains carrying different H-2 haplotypes, and the T-cell proliferative responses of these strains were determined. Again, all the strains responded to seven of the eight antigens. These experiments were then repeated, but this time -antibodies specific for the A (AA) or E (EE) molecules were added to the culture to block the in vitro responsiveness. In all but one of the responses, inhibition with both A-specific and E-specific antibodies was observed. The response to one antigen (Blastoinyces) was exceptional in that some strains were nonresponders to this antigen. Furthermore, the response in the responder strains was blocked with A-specific, but not with E-specific, antibodies. The study demonstrates that responses to antigens not controlled by Irr genes nevertheless require participation of class II Mhc molecules. In contrast to Ir gene-controlled responses involving either the A- or the E-molecule controlling loci (but never both), the responses not Ir-controlled involve participation of both A- and E-controlling loci. The lack of Ir-gene control is probably the result of complexity of the responses to multiple determinants. There is thus no principal difference between responses controlled and those not controlled by Ir genes: both types involve the recognition of the antigen, in the context of Mhc molecules
Charged vortices in superfluid systems with pairing of spatially separated carriers
It is shown that in a magnetic field the vortices in superfluid electron-hole
systems carry a real electrical charge. The charge value depends on the
relation between the magnetic length and the Bohr radiuses of electrons and
holes. In double layer systems at equal electron and hole filling factors in
the case of the electron and hole Bohr radiuses much larger than the magnetic
length the vortex charge is equal to the universal value (electron charge times
the filling factor).Comment: 4 page
Correlation-function spectroscopy of inelastic lifetime in heavily doped GaAs heterostructures
Measurements of resonant tunneling through a localized impurity state are
used to probe fluctuations in the local density of states of heavily doped
GaAs. The measured differential conductance is analyzed in terms of correlation
functions with respect to voltage. A qualitative picture based on the scaling
theory of Thouless is developed to relate the observed fluctuations to the
statistics of single particle wavefunctions. In a quantitative theory
correlation functions are calculated. By comparing the experimental and
theoretical correlation functions the effective dimensionality of the emitter
is analyzed and the dependence of the inelastic lifetime on energy is
extracted.Comment: 41 pages, 14 figure
Drag in paired electron-hole layers
We investigate transresistance effects in electron-hole double layer systems
with an excitonic condensate. Our theory is based on the use of a minimum
dissipation premise to fix the current carried by the condensate. We find that
the drag resistance jumps discontinuously at the condensation temperature and
diverges as the temperature approaches zero.Comment: 12 pages, 1 Figure, .eps file attache
Charged Many-Electron -- Single Hole Complexes in a Double Quantum Well near a Metal Plate
It has been shown that the presence of a metal plate near a double quantum
well with spatially separated electron and hole layers may lead to a drastic
reconstruction of the system state with the formation of stable charged
complexes of several electrons bound to a spatially separated hole. Complexes
of both the Fermi and the Bose statistics may coexist in the ground state and
their relative densities may be changed with the change of the electron and
hole densities. The stability of the charged complexes may be increased by an
external magnetic field perpendicular to the layers plane.Comment: to appear in Phys.Rev.Lett. 77, No.7 (1996). 4 pages, RevTeX, 1
figur
Magneto-Conductance Anisotropy and Interference Effects in Variable Range Hopping
We investigate the magneto-conductance (MC) anisotropy in the variable range
hopping regime, caused by quantum interference effects in three dimensions.
When no spin-orbit scattering is included, there is an increase in the
localization length (as in two dimensions), producing a large positive MC. By
contrast, with spin-orbit scattering present, there is no change in the
localization length, and only a small increase in the overall tunneling
amplitude. The numerical data for small magnetic fields , and hopping
lengths , can be collapsed by using scaling variables , and
in the perpendicular and parallel field orientations
respectively. This is in agreement with the flux through a `cigar'--shaped
region with a diffusive transverse dimension proportional to . If a
single hop dominates the conductivity of the sample, this leads to a
characteristic orientational `finger print' for the MC anisotropy. However, we
estimate that many hops contribute to conductivity of typical samples, and thus
averaging over critical hop orientations renders the bulk sample isotropic, as
seen experimentally. Anisotropy appears for thin films, when the length of the
hop is comparable to the thickness. The hops are then restricted to align with
the sample plane, leading to different MC behaviors parallel and perpendicular
to it, even after averaging over many hops. We predict the variations of such
anisotropy with both the hop size and the magnetic field strength. An
orientational bias produced by strong electric fields will also lead to MC
anisotropy.Comment: 24 pages, RevTex, 9 postscript figures uuencoded Submitted to PR
Charged hydrogenic problem in a magnetic field: Non-commutative translations, unitary transformations, and coherent states
An operator formalism is developed for a description of charged electron-hole
complexes in magnetic fields. A novel unitary transformation of the Hamiltonian
that allows one to partially separate the center-of-mass and internal motions
is proposed. We study the operator algebra that leads to the appearance of new
effective particles, electrons and holes with modified interparticle
interactions, and their coherent states in magnetic fields. The developed
formalism is used for studying a two-dimensional negatively charged
magnetoexciton . It is shown that Fano-resonances are present in the
spectra of internal transitions, indicating the existence of
three-particle quasi-bound states embedded in the continuum of higher Landau
levels.Comment: 9 pages + 2 figures, accepted in PRB, a couple of typos correcte
Effect of dephasing on mesoscopic conductance fluctuations in quantum dots with single channel leads
We consider the distribution of conductance fluctuations in disordered
quantum dots with single channel leads. Using a perturbative diagrammatic
approach, valid for continuous level spectra, we describe dephasing due to
processes within the dot by considering two different contributions to the
level broadening, thus satisfying particle number conservation. Instead of a
completely non-Gaussian distribution, which occurs for zero dephasing, we find
for strong dephasing that the distribution is mainly Gaussian with
non-universal variance and non-Gaussian tails.Comment: 11 pages in REVTeX two-column format; 6 eps figures included;
submitted to Phys. Rev.
Conductance Fluctuations in a Disordered Double-Barrier Junction
We consider the effect of disorder on coherent tunneling through two barriers
in series, in the regime of overlapping transmission resonances. We present
analytical calculations (using random-matrix theory) and numerical simulations
(on a lattice) to show that strong mode-mixing in the inter-barrier region
induces mesoscopic fluctuations in the conductance of universal magnitude
for a symmetric junction. For an asymmetric junction, the
root-mean-square fluctuations depend on the ratio of the two tunnel
resistances according to ,
where in the presence (absence) of time-reversal symmetry.Comment: 12 pages, REVTeX-3.0, 2 figures, submitted to Physical Review
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