480 research outputs found
Quantum Effects in Small-Capacitance Single Josephson Junctions
We have measured the current-voltage (I-V) characteristics of
small-capacitance single Josephson junctions at low temperatures (T=0.02-0.6
K), where the strength of the coupling between the single junction and the
electromagnetic environment was controlled with one-dimensional arrays of dc
SQUIDs. The single-junction I-V curve is sensitive to the impedance of the
environment, which can be tuned IN SITU. We have observed Coulomb blockade of
Cooper-pair tunneling and even a region of negative differential resistance,
when the zero-bias resistance R_0' of the SQUID arrays is much higher than the
quantum resistance R_K = h/e^2 = 26 kohm. The negative differential resistance
is evidence of coherent single-Cooper-pair tunneling within the theory of
current-biased single Josephson junctions. Based on the theory, we have
calculated the I-V curves numerically in order to compare with the experimental
ones at R_0' >> R_K. The numerical calculation agrees with the experiments
qualitatively. We also discuss the R_0' dependence of the
single-Josephson-junction I-V curve in terms of the superconductor-insulator
transition driven by changing the coupling to the environment.Comment: 11 pages with 14 embedded figures, RevTeX4, final versio
Spin flip scattering at Al surfaces
Non-local measurements are performed on a multi terminal device to
determine the spin diffusion length and in combination with resistivity
measurements also the spin relaxation time in Al films. By varying the
thickness of Al we determine the contribution to spin relaxation from surface
scattering. From the temperature dependence of the spin diffusion length it is
established that the spin relaxation is impurity dominated at low temperature.
A comparison of the spin and momentum relaxation lengths for different
thicknesses reveals that the spin flip scattering at the surfaces is weak
compared to that within the bulk of the Al films.Comment: 11 pages, 5 figure
Enhanced spin accumulation in a superconductor
A lateral array of ferromagnetic tunnel junctions is used to inject and
detect non-equilibrium quasi-particle spin distribution in a superconducting
strip made of Al. The strip width and thickness is kept below the quasi
particle spin diffusion length in Al. Non-local measurements in multiple
parallel and antiparallel magnetic states of the detectors are used to in-situ
determine the quasi-particle spin diffusion length. A very large increase in
the spin accumulation in the superconducting state compared to that in the
normal state is observed and is attributed to a diminishing of the
quasi-particle population by opening of the gap below the transition
temperature.Comment: 6 pages, 4 figures; accepted for publication in Journal of Applied
Physic
Nonlinearities and Parametric Amplification in Superconducting Coplanar Waveguide Resonators
Experimental investigations of the nonlinear properties of superconducting
niobium coplanar waveguide resonators are reported. The nonlinearity due to a
current dependent kinetic inductance of the center conductor is strong enough
to realize bifurcation of the nonlinear oscillator. When driven with two
frequencies near the threshold for bifurcation, parametric amplification with a
gain of +22.4 dB is observed.Comment: 4 pages, 5 figures, 1 table. In version 2: Added appendix with model
description and fits to measurements. Minor corrections and rephrasin
Spin injection and relaxation in a mesoscopic superconductor
We study spin accumulation and spin relaxation in a superconducting nanowire.
Spins are injected and detected by using a set of magnetic tunnel contact
electrodes, closely spaced along the nanowire. We observe a giant enhancement
of the spin accumulation of up to five orders of magnitude on transition into
the superconducting state, consistent with the expected changes in the density
of states. The spin relaxation length decreases by an order of magnitude from
its value in the normal state. These measurements combined with our theoretical
model, allow us to distinguish the individual spin flip mechanisms present in
the transport channel. Our conclusion is that magnetic impurities rather than
spin-orbit coupling dominate spin-flip scattering in the superconducting state.Comment: 5 pages, 5 figure
Escape from a zero current state in a one dimensional array of Josephson junctions
A long one dimensional array of small Josephson junctions exhibits Coulomb
blockade of Cooper pair tunneling. This zero current state exists up to a
switching voltage, Vsw, where there is a sudden onset of current. In this paper
we present histograms showing how Vsw changes with temperature for a long array
and calculations of the corresponding escape rates. Our analysis of the problem
is based on the existence of a voltage dependent energy barrier and we do not
make any assumptions about its shape. The data divides up into two temperature
regimes, the higher of which can be explained with Kramers thermal escape
model. At low temperatures the escape becomes independent of temperature.Comment: 4 pages 5 figure
Observation of shot-noise-induced asymmetry in the Coulomb blockaded Josephson junction
We have investigated the influence of shot noise on the IV-curves of a single
mesoscopic Josephson junction. We observe a linear enhancement of zero-bias
conductance of the Josephson junction with increasing shot noise power.
Moreover, the IV-curves become increasingly asymmetric. Our analysis on the
asymmetry shows that the Coulomb blockade of Cooper pairs is strongly
influenced by the non-Gaussian character of the shot noise.Comment: 4 pages, 5 figures, RevTE
On modeling and measuring viscoelasticity with dynamic Atomic Force Microscopy
The interaction between a rapidly oscillating atomic force microscope tip and
a soft material surface is described using both elastic and viscous forces with
a moving surface model. We derive the simplest form of this model, motivating
it as a way to capture the impact dynamics of the tip and sample with an
interaction consisting of two components: interfacial or surface force, and
bulk or volumetric force. Analytic solutions to the piece-wise linear model
identify characteristic time constants, providing a physical explanation of the
hysteresis observed in the measured dynamic force quadrature curves. Numerical
simulation is used to fit the model to experimental data and excellent
agreement is found with a variety of different samples. The model parameters
form a dimensionless impact-rheology factor, giving a quantitative physical
number to characterize a viscoelastic surface that does not depend on the tip
shape or cantilever frequency.Comment: 13 pages, 7 figure
Phase-Charge Duality of a Josephson junction in a fluctuating electromagnetic environment
We have measured the current-voltage characteristics of a single Josephson
junction placed in a high impedance environment. The transfer of Cooper pairs
through the junction is governed by overdamped quasicharge dynamics, leading to
Coulomb blockade and Bloch oscillations. Exact duality exists to the standard
overdamped phase dynamics of a Josephson junction, resulting in a dual shape of
the current-voltage characteristic, with current and voltage changing roles. We
demonstrate this duality with experiments which allow for a quantitative
comparison with a theory that includes the effect of fluctuations due to finite
temperature of the electromagnetic environment
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