75 research outputs found
Quantum Phase Slips in one-dimensional Josephson Junction Chains
We have studied quantum phase-slip (QPS) phenomena in long one-dimensional
Josephson junction series arrays with tunable Josephson coupling. These chains
were fabricated with as many as 2888 junctions, where one sample had a tunable
weak link in the middle. Measurements were made of the zero-bias resistance,
, as well as current-voltage characteristics (IVC). The finite is
explained by QPS and shows an exponential dependence on with a
distinct change in the exponent at . When the IVC
clearly shows a remnant of the Coulomb blockade, which evolves to a
zero-current state with a sharp critical voltage as is tuned to a smaller
value. The zero-current state below the critical voltage is due to coherent QPS
and we show that these are enhanced at the central weak link. Above the
critical voltage a negative differential resistance is observed which nearly
restores the zero-current state
Imaging high-speed friction at the nanometer scale
Friction is a complicated phenomenon involving nonlinear dynamics at
different length and time scales[1, 2]. The microscopic origin of friction is
poorly understood, due in part to a lack of methods for measuring the force on
a nanometer-scale asperity sliding at velocity of the order of cm/s.[3, 4]
Despite enormous advance in experimental techniques[5], this combination of
small length scale and high velocity remained illusive. Here we present a
technique for rapidly measuring the frictional forces on a single asperity (an
AFM tip) over a velocity range from zero to several cm/s. At each image pixel
we obtain the velocity dependence of both conservative and dissipative forces,
revealing the transition from stick-slip to a smooth sliding friction[1, 6]. We
explain measurements on graphite using a modified Prandtl-Tomlinson model that
takes into account the damped elastic deformation of the asperity. With its
greatly improved force sensitivity and very small sliding amplitude, our method
enables rapid and detailed surface mapping of the full velocity-dependence of
frictional forces with less than 10~nm spatial resolution.Comment: 7 pages, 4 figure
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
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