125 research outputs found
Magnetoresistance of a quantum dot with spin-active interfaces
We study the zero-bias magnetoresistance MR of an interacting quantum dot
connected to two ferromagnetic leads and capacitively coupled to a gate voltage
source Vg. We investigate the effects of the spin-activity of the contacts
between the dot and the leads by introducing an effective exchange field in an
Anderson model. This spin-activity makes easier negative MR effects, and can
even lead to a giant MR effect with a sign tunable with Vg. Assuming a twofold
orbital degeneracy, our approach allows to interpret in an interacting picture
the MR(Vg) measured by S. Sahoo et al. [Nature Phys. 2, 99 (2005)] in single
wall carbon nanotubes with ferromagnetic contacts. If this experiment is
repeated on a larger Vg-range, we expect that the MR(Vg) oscillations are not
regular like in the presently available data, due to Coulomb interactions.Comment: 9 pages, 6 figures, to appear in Phys. Rev.
Supercurrent in Nb/InAs-Nanowire/Nb Josephson junctions
We report on the fabrication and measurements of planar mesoscopic Josephson
junctions formed by InAs nanowires coupled to superconducting Nb terminals. The
use of Si-doped InAs-nanowires with different bulk carrier concentrations
allowed to tune the properties of the junctions. We have studied the junction
characteristics as a function of temperature, gate voltage, and magnetic field.
In junctions with high doping concentrations in the nanowire Josephson
supercurrent values up to 100\,nA are found. Owing to the use of Nb as
superconductor the Josephson coupling persists at temperatures up to 4K. In all
junctions the critical current monotonously decreased with the magnetic field,
which can be explained by a recently developed theoretical model for the
proximity effect in ultra-small Josephson junctions. For the low-doped
Josephson junctions a control of the critical current by varying the gate
voltage has been demonstrated. We have studied conductance fluctuations in
nanowires coupled to superconducting and normal metal terminals. The
conductance fluctuation amplitude is found to be about 6 times larger in
superconducting contacted nanowires. The enhancement of the conductance
fluctuations is attributed to phase-coherent Andreev reflection as well as to
the large number of phase-coherent channels due to the large superconducting
gap of the Nb electrodes.Comment: 5 Figure, submitted to Journal of Applied Physic
Shot noise of large charge quanta in superconductor/semiconductor/superconductor junctions
We have found experimentally that the noise of ballistic electron transport
in a superconductor/semiconductor/superconductor junction is enhanced relative
to the value given by the general relation, S_V=2eIR^2coth(eV/2kT), for two
voltage regions in which this expression reduces to its thermal and shot noise
limits. The noise enhancement is explained by the presence of large charge
quanta, with effective charge q*=(1+2Delta/eV)e, that generate a noise spectrum
S_V=2q*IR^2, as predicted in Phys. Rev. Lett. 76, 3814 (1996). These charge
quanta result from multiple Andreev reflections at each junction interface,
which are also responsible for the subharmonic gap structure observed in the
voltage dependence of the junction's conductance.Comment: 5 pages, 5 figures, submitted to Physical Review B as a Rapid
Communication. v2 author name in reference corrected. v3 added references. v4
clarifications in the text and reference added thanks to C. Urbin
Carrier relaxation in GaAs v-groove quantum wires and the effects of localization
Carrier relaxation processes have been investigated in GaAs/AlGaAs v-groove
quantum wires (QWRs) with a large subband separation (46 meV). Signatures of
inhibited carrier relaxation mechanisms are seen in temperature-dependent
photoluminescence (PL) and photoluminescence-excitation (PLE) measurements; we
observe strong emission from the first excited state of the QWR below ~50 K.
This is attributed to reduced inter-subband relaxation via phonon scattering
between localized states. Theoretical calculations and experimental results
indicate that the pinch-off regions, which provide additional two-dimensional
confinement for the QWR structure, have a blocking effect on relaxation
mechanisms for certain structures within the v-groove. Time-resolved PL
measurements show that efficient carrier relaxation from excited QWR states
into the ground state, occurs only at temperatures > 30 K. Values for the low
temperature radiative lifetimes of the ground- and first excited-state excitons
have been obtained (340 ps and 160 ps respectively), and their corresponding
localization lengths along the wire estimated.Comment: 9 pages, 8 figures, submitted to Phys. Rev. B Attempted to correct
corrupt figure
Andreev reflection and enhanced subgap conductance in NbN/Au/InGaAs-InP junctions
We report on the fabrication of highly transparent superconductor/normal
metal/two-dimensional electron gas junctions formed by a superconducting NbN
electrode, a thin (10nm) Au interlayer, and a two-dimensional electron gas in a
InGaAs/InP heterostructure. High junction transparency has been achieved by
exploiting of a newly developed process of Au/NbN evaporation and rapid
annealing at 400C. This allowed us to observe for the first time a decrease in
the differential resistance with pronounced double-dip structure within the
superconducting energy gap in superconductor-2DEG proximity systems. The effect
of a magnetic field perpendicular to the plane of the 2DEG on the differential
resistance of the interface was studied. It has been found that the reduced
subgap resistance remains in high magnetic fields. Zero-field data are analyzed
within the previously established quasiclassical model for the proximity
effect.Comment: 15 pages, 5 figure
The Fano-Rashba effect
We analyze the linear conductance of a semiconductor quantum wire containing
a region where a local Rashba spin-orbit interaction is present. We show that
Fano lineshapes appear in the conductance due to the formation of quasi bound
states which interfere with the direct transmission along the wire, a mechanism
that we term the Fano-Rashba effect. We obtain the numerical solution of the
full Schr\"odinger equation using the quantum-transmitting-boundary method. The
theoretical analysis is performed using the coupled-channel model, finding an
analytical solution by ansatz. The complete numerical solution of the
coupled-channel equations is also discussed, showing the validity of the ansatz
approach.Comment: 5 pages, proceedings of ICN+T 2006 (Basel, Switzerland, 30/7-4/9),
accepted, to appear in J. Phys.: Conf. Se
Spin-orbit coupling and phase-coherence in InAs nanowires
We investigated the magnetotransport of InAs nanowires grown by selective
area metal-organic vapor phase epitaxy. In the temperature range between 0.5
and 30 K reproducible fluctuations in the conductance upon variation of the
magnetic field or the back-gate voltage are observed, which are attributed to
electron interference effects in small disordered conductors. From the
correlation field of the magnetoconductance fluctuations the phase-coherence
length l_phi is determined. At the lowest temperatures l_phi is found to be at
least 300 nm, while for temperatures exceeding 2 K a monotonous decrease of
l_phi with temperature is observed. A direct observation of the weak
antilocalization effect indicating the presence of spin-orbit coupling is
masked by the strong magnetoconductance fluctuations. However, by averaging the
magnetoconductance over a range of gate voltages a clear peak in the
magnetoconductance due to the weak antilocalization effect was resolved. By
comparison of the experimental data to simulations based on a recursive
two-dimensional Green's function approach a spin-orbit scattering length of
approximately 70 nm was extracted, indicating the presence of strong spin-orbit
coupling.Comment: 8 pages, 7 figure
Longitudinal photocurrent spectroscopy of a single GaAs/AlGaAs v-groove quantum wire
Modulation-doped GaAs v-groove quantum wires (QWRs) have been fabricated with
novel electrical contacts made to two-dimensional electron-gas (2DEG)
reservoirs. Here, we present longitudinal photocurrent (photoconductivity/PC)
spectroscopy measurements of a single QWR. We clearly observe conductance in
the ground-state one-dimensional subbands; in addition, a highly
temperature-dependent response is seen from other structures within the
v-groove. The latter phenomenon is attributed to the effects of structural
topography and localization on carrier relaxation. The results of
power-dependent PC measurements suggest that the QWR behaves as a series of
weakly interacting localized states, at low temperatures
Особенности химизма блеклых руд участка ЭМИ Светлинского эпитермального рудного поля (Хабаровский край)
We experimentally studied the Josephson supercurrent in Nb/InN-nanowire/Nb junctions. Large
critical currents up to 5.7 μA have been achieved, which proves the good coupling of the nanowire
to the superconductor. The effect of a magnetic field perpendicular to the plane of the Josephson
junction on the critical current has been studied. The observed monotonous decrease in the critical
current with magnetic field is explained by the magnetic pair-breaking effect in planar Josephson
junctions of ultra-narrow width [J. C. Cuevas and F. S. Bergeret, Phys. Rev. Lett. 99, 217002 (2007)]
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