135 research outputs found
Microwave response of an NS ring coupled to a superconducting resonator
A long phase coherent normal (N) wire between superconductors (S) is
characterized by a dense phase dependent Andreev spectrum . We probe this
spectrum in a high frequency phase biased configuration, by coupling an NS ring
to a multimode superconducting resonator. We detect a dc flux and frequency
dependent response whose dissipative and non dissipative components are related
by a simple Debye relaxation law with a characteristic time of the order of the
diffusion time through the N part of the ring. The flux dependence exhibits
periodic oscillations with a large harmonics content at temperatures
where the Josephson current is purely sinusoidal. This is explained considering
that the populations of the Andreev levels are frozen on the time-scale of the
experiments.Comment: 5 pages,4 figure
Proximity DC squids in the long junction limit
We report the design and measurement of
Superconducting/normal/superconducting (SNS) proximity DC squids in the long
junction limit, i.e. superconducting loops interrupted by two normal metal
wires roughly a micrometer long. Thanks to the clean interface between the
metals, at low temperature a large supercurrent flows through the device. The
dc squid-like geometry leads to an almost complete periodic modulation of the
critical current through the device by a magnetic flux, with a flux periodicity
of a flux quantum h/2e through the SNS loop. In addition, we examine the entire
field dependence, notably the low and high field dependence of the maximum
switching current. In contrast with the well-known Fraunhoffer-type
oscillations typical of short wide junctions, we find a monotonous gaussian
extinction of the critical current at high field. As shown in [15], this
monotonous dependence is typical of long and narrow diffusive junctions. We
also find in some cases a puzzling reentrance at low field. In contrast, the
temperature dependence of the critical current is well described by the
proximity effect theory, as found by Dubos {\it et al.} [16] on SNS wires in
the long junction limit. The switching current distributions and hysteretic IV
curves also suggest interesting dynamics of long SNS junctions with an
important role played by the diffusion time across the junction.Comment: 12 pages, 16 figure
Analytical calculation of the excess current in the OTBK theory
We present an analytical derivation of the excess current in Josephson
junctions within the Octavio-Tinkham-Blonder-Klapwijk theory for both symmetric
and asymmetric barrier strengths. We confirm the result found numerically by
Flensberg et al. for equal barriers [Phys. Rev. B 38, 8707 (1988)], including
the prediction of negative excess current for low transparencies, and we
generalize it for differing barriers. Our analytical formulae provide for
convenient fitting of experimental data, also in the less studied, but
practically relevant case of the barrier asymmetry.Comment: 13 pages, 3 figures, submitted to Superconductor Science and
Technolog
Bubble dynamics in DNA
The formation of local denaturation zones (bubbles) in double-stranded DNA is
an important example for conformational changes of biological macromolecules.
We study the dynamics of bubble formation in terms of a Fokker-Planck equation
for the probability density to find a bubble of size n base pairs at time t, on
the basis of the free energy in the Poland-Scheraga model. Characteristic
bubble closing and opening times can be determined from the corresponding first
passage time problem, and are sensitive to the specific parameters entering the
model. A multistate unzipping model with constant rates recently applied to DNA
breathing dynamics [G. Altan-Bonnet et al, Phys. Rev. Lett. 90, 138101 (2003)]
emerges as a limiting case.Comment: 9 pages, 2 figure
Spin accumulation induced resistance in mesoscopic ferromagnet/ superconductor junctions
We present a description of spin-polarized transport in mesoscopic
ferromagnet-superconductor (F/S) systems, where the transport is diffusive, and
the interfaces are transparent. It is shown that the spin reversal associated
with Andreev reflection generates an excess spin density close to the F/S
interface, which leads to a spin contact resistance. Expressions for the
contact resistance are given for two terminal and four terminal geometries. In
the latter the sign depends on the relative magnetization of the ferromagnetic
electrodes.Comment: RevTeX 10 pages, 4 figures, submitted to Phys.Rev. Let
Effects of Spin-Orbit Interactions on Tunneling via Discrete Energy Levels in Metal Nanoparticles
The presence of spin-orbit scattering within an aluminum nanoparticle affects
measurements of the discrete energy levels within the particle by (1) reducing
the effective g-factor below the free-electron value of 2, (2) causing avoided
crossings as a function of magnetic field between predominantly-spin-up and
predominantly-spin-down levels, and (3) introducing magnetic-field-dependent
changes in the amount of current transported by the tunneling resonances. All
three effects can be understood in a unified fashion by considering a simple
Hamiltonian. Spin-orbit scattering from 4% gold impurities in superconducting
aluminum nanoparticles produces no dramatic effect on the superconducting gap
at zero magnetic field, but we argue that it does modify the nature of the
superconducting transition in a magnetic field.Comment: 10 pages, 5 figures. Submitted to Phys. Rev.
Harnessing nuclear spin polarization fluctuations in a semiconductor nanowire
Soon after the first measurements of nuclear magnetic resonance (NMR) in a
condensed matter system, Bloch predicted the presence of statistical
fluctuations proportional to in the polarization of an ensemble of
spins. First observed by Sleator et al., so-called "spin noise" has
recently emerged as a critical ingredient in nanometer-scale magnetic resonance
imaging (nanoMRI). This prominence is a direct result of MRI resolution
improving to better than 100 nm^3, a size-scale in which statistical spin
fluctuations begin to dominate the polarization dynamics. We demonstrate a
technique that creates spin order in nanometer-scale ensembles of nuclear spins
by harnessing these fluctuations to produce polarizations both larger and
narrower than the natural thermal distribution. We focus on ensembles
containing ~10^6 phosphorus and hydrogen spins associated with single InP and
GaP nanowires (NWs) and their hydrogen-containing adsorbate layers. We monitor,
control, and capture fluctuations in the ensemble's spin polarization in
real-time and store them for extended periods. This selective capture of large
polarization fluctuations may provide a route for enhancing the weak magnetic
signals produced by nanometer-scale volumes of nuclear spins. The scheme may
also prove useful for initializing the nuclear hyperfine field of electron spin
qubits in the solid-state.Comment: 18 pages, 5 figure
Tunneling Via Individual Electronic States in Ferromagnetic Nanoparticles
We measure electron tunneling via discrete energy levels in ferromagnetic
cobalt particles less than 4 nm in diameter, using non-magnetic electrodes. Due
to magnetic anisotropy, the energy of each tunneling resonance shifts as an
applied magnetic field rotates the particle's magnetic moment. We see both
spin-increasing and decreasing tunneling transitions, but we do not observe the
spin degeneracy at small magnetic fields seen previously in non-magnetic
materials. The tunneling spectrum is denser than predicted for independent
electrons, possibly due to spin-wave excitations.Comment: 4 pages, 4 figures. Improved by comments from referees, to appear in
Phys. Rev. Let
Proximity Induced Josephson-Quasiparticle Process in a Single Electron Transistor
We have performed the first experiments in a superconductor - normal metal -
superconductor single electron transistor in which there is an extra
superconducting strip partially overlapping the normal metal island in good
metal-to-metal contact. Superconducting proximity effect gives rise to current
peaks at voltages below the quasiparticle threshold. We interpret these peaks
in terms of the Josephson-quasiparticle process and discuss their connection
with the proximity induced energy gap in the normal metal island.Comment: 4 pages + 4 figure
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