523 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
Single-electron transistors in electromagnetic environments
The current-voltage (I-V) characteristics of single-electron transistors
(SETs) have been measured in various electromagnetic environments. Some SETs
were biased with one-dimensional arrays of dc superconducting quantum
interference devices (SQUIDs). The purpose was to provide the SETs with a
magnetic-field-tunable environment in the superconducting state, and a
high-impedance environment in the normal state. The comparison of SETs with
SQUID arrays and those without arrays in the normal state confirmed that the
effective charging energy of SETs in the normal state becomes larger in the
high-impedance environment, as expected theoretically. In SETs with SQUID
arrays in the superconducting state, as the zero-bias resistance of the SQUID
arrays was increased to be much larger than the quantum resistance R_K = h/e^2
= 26 kohm, a sharp Coulomb blockade was induced, and the current modulation by
the gate-induced charge was changed from e periodic to 2e periodic at a bias
point 0<|V|<2D_0/e, where D_0 is the superconducting energy gap. The author
discusses the Coulomb blockade and its dependence on the gate-induced charge in
terms of the single Josephson junction with gate-tunable junction capacitance.Comment: 8 pages with 10 embedded figures, RevTeX4, published versio
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
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
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
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
Suppression of \bbox{T_c} in superconducting amorphous wires
The suppression of the mean field temperature of the superconducting
transition, , in homogeneous amorphous wires is studied. We develop a
theory that gives in situations when the dynamically enhanced Coulomb
repulsion competes with the contact attraction. The theory accurately describes
recent experiments on --suppression in superconducting wires, after a
procedure that minimizes the role of nonuniversal mechanisms influencing
is applied.Comment: RevTeX, 4 pages, 3 figure
Superconductor-Insulator Transition in a Disordered Electronic System
We study an electronic model of a 2D superconductor with onsite randomness
using Quantum Monte Carlo simulations. The superfluid density is used to track
the destruction of superconductivity in the ground state with increasing
disorder. The non-superconducting state is identified as an insulator from the
temperature dependence of its d.c. resistivity. The value of
at the superconductor-insulator transition appears to be non-universal.Comment: PostScript, 4 pages, figures include
The Field-Tuned Superconductor-Insulator Transition with and without Current Bias
The magnetic-field-tuned superconductor-insulator transition has been studied
in ultrathin Beryllium films quench-condensed near 20 K. In the zero-current
limit, a finite-size scaling analysis yields the scaling exponent product vz =
1.35 +/- 0.10 and a critical sheet resistance R_{c} of about 1.2R_{Q}, with
R_{Q} = h/4e^{2}. However, in the presence of dc bias currents that are smaller
than the zero-field critical currents, vz becomes 0.75 +/- 0.10. This new set
of exponents suggests that the field-tuned transitions with and without dc bias
currents belong to different universality classes.Comment: RevTex 4 pages, 4 figures, and 1 table minor change
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