8,094 research outputs found
A spin pump turnstile: parametric pumping of a spin-polarized current through a nearly-closed quantum dot
We investigate parametric pumping of a spin-polarized current through a
nearly-closed quantum dot in a perpendicular magnetic field. Pumping is
achieved by tuning the tunnel couplings to the left and right lead - thereby
operating the quantum dot as a turnstile - and changing either the magnetic
field or a gate-voltage. We analyze the quantum dynamics of a pumping cycle and
the limiting time scales for operating the quantum dot turnstile as a pure spin
pump. The proposed device can be used as a fully controllable double-sided and
bipolar spin filter and to inject spins "on demand".Comment: 5 pages, 2 figures, one reference correcte
Structure of human transthyretin complexed with bromophenols: a new mode of binding
The binding of two organohalogen substances, pentabromophenol (PBP) and 2,4,6-tribromophenol (TBP), to human transthyretin (TTR), a thyroid hormone transport protein, has been studied by in vitro competitive binding assays and by X-ray crystallography. Both compounds bind to TTR with high affinity, in competition with the natural ligand thyroxine (
Finite size effects and localization properties of disordered quantum wires with chiral symmetry
Finite size effects in the localization properties of disordered quantum
wires are analyzed through conductance calculations. Disorder is induced by
introducing vacancies at random positions in the wire and thus preserving the
chiral symmetry. For quasi one-dimensional geometries and low concentration of
vacancies, an exponential decay of the mean conductance with the wire length is
obtained even at the center of the energy band. For wide wires, finite size
effects cause the conductance to decay following a non-pure exponential law. We
propose an analytical formula for the mean conductance that reproduces
accurately the numerical data for both geometries. However, when the
concentration of vacancies increases above a critical value, a transition
towards the suppression of the conductance occurs.
This is a signature of the presence of ultra-localized states trapped in
finite regions of the sample.Comment: 5 figures, revtex
Rectification of displacement currents in an adiabatic electron pump
Rectification of ac displacement currents generated by periodic variation of
two independent gate voltages of a quantum dot can lead to a dc voltage linear
in the frequency. The presence of this rectified displacement current could
account for the magnetic field symmetry observed in a recent measurement on an
adiabatic quantum electron pump by Switkes et al. [Science 283, 1905 (1999)].Comment: 2 pages, RevTeX; 1 figur
Crossover from weak localization to weak antilocalization in a disordered microbridge
We calculate the weak localization correction in the double crossover to
broken time-reversal and spin-rotational symmetry for a disordered microbridge
or a short disordered wire using a scattering-matrix approach. Whereas the
correction has universal limiting values in the three basic symmetry classes,
the functional form of the magnetoconductance is affected by eventual
non-homogeneities in the microbridge.Comment: 5 pages, RevTeX; 2 figure
Nonequilibrium theory of Coulomb blockade in open quantum dots
We develop a non-equilibrium theory to describe weak Coulomb blockade effects
in open quantum dots. Working within the bosonized description of electrons in
the point contacts, we expose deficiencies in earlier applications of this
method, and address them using a 1/N expansion in the inverse number of
channels. At leading order this yields the self-consistent potential for the
charging interaction. Coulomb blockade effects arise as quantum corrections to
transport at the next order. Our approach unifies the phase functional and
bosonization approaches to the problem, as well as providing a simple picture
for the conductance corrections in terms of renormalization of the dot's
elastic scattering matrix, which is obtained also by elementary perturbation
theory. For the case of ideal contacts, a symmetry argument immediately allows
us to conclude that interactions give no signature in the averaged conductance.
Non-equilibrium applications to the pumped current in a quantum pump are worked
out in detail.Comment: Published versio
Endstates in multichannel spinless p-wave superconducting wires
Multimode spinless p-wave superconducting wires with a width W much smaller
than the superconducting coherence length \xi are known to have multiple
low-energy subgap states localized near the wire's ends. Here we compare the
typical energies of such endstates for various terminations of the wire: A
superconducting wire coupled to a normal-metal stub, a weakly disordered
superconductor wire and a wire with smooth confinement. Depending on the
termination, we find that the energies of the subgap states can be higher or
lower than for the case of a rectangular wire with hard-wall boundaries.Comment: 10 pages, 7 figure
Andreev reflection from non-centrosymmetric superconductors and Majorana bound state generation in half-metallic ferromagnets
We study Andreev reflection at an interface between a half metal and a
superconductor with spin-orbit interaction. While the absence of minority
carriers in the half metal makes singlet Andreev reflection impossible, the
spin-orbit interaction gives rise to triplet Andreev reflection, i.e., the
reflection of a majority electron into a majority hole or vice versa. As an
application of our calculation, we consider a thin half metal film or wire
laterally attached to a superconducting contact. If the half metal is disorder
free, an excitation gap is opened that is proportional to the spin-orbit
interaction strength in the superconductor. For electrons with energy below
this gap a lateral half-metal--superconductor contact becomes a perfect triplet
Andreev reflector. We show that the system supports localized Majorana end
states in this limit.Comment: 14 pages, 3 figure
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