739 research outputs found
Dissipation in Quasi One-Dimensional Superconducting Single-Crystal Sn Nanowires
Electrical transport measurements were made on single-crystal Sn nanowires to
understand the intrinsic dissipation mechanisms of a one-dimensional
superconductor. While the resistance of wires of diameter larger than 70 nm
drops precipitately to zero at Tc near 3.7 K, a residual resistive tail
extending down to low temperature is found for wires with diameters of 20 and
40 nm. As a function of temperature, the logarithm of the residual resistance
appears as two linear sections, one within a few tenths of a degree below Tc
and the other extending down to at least 0.47 K, the minimum temperature of the
measurements. The residual resistance is found to be ohmic at all temperatures
below Tc of Sn. These findings are suggestive of a thermally activated phase
slip process near Tc and quantum fluctuation-induced phase slip process in the
low temperature regime. When the excitation current exceeds a critical value,
the voltage-current (V-I) curves show a series of discrete steps in approaching
the normal state. These steps cannot be fully understood with the classical
Skocpol-Beasley-Tinkham phase slip center model (PSC), but can be qualitatively
accounted for partly by the PSC model modified by Michotte et al.Comment: 7 pages, 5 figures. To be appeared on Physical Review B 71, 200
Fluctuation Conductivity in Unconventional Superconductors near Critical Disorder
The fluctuation conductivity in bulk superconductors with
non s-wave pairing and with nonmagnetic disorder of strength is studied at
low and within the Gaussian approximation. It is shown by assuming a quasi
two-dimensional (2D) electronic state that, only if the gap function
d_\mu({\p}) is, as in a 2D p-wave pairing state, linear in the in-plane
(relative) momentum {\p}_\perp, the in-plane fluctuation conductivity on the
line is weakly divergent in low limit. The present result may be
useful in clarifying the true gap function of spin-triplet
through resistivity measurements.Comment: 8 pages, 1 figure, to be published in J. Phys. Soc. Jpn. 70, No.10
(2001
Current-voltage characteristics of quasi-one-dimensional superconductors: An S-curve in the constant voltage regime
Applying a constant voltage to superconducting nanowires we find that its
IV-characteristic exhibits an unusual S-behavior. This behavior is the direct
consequence of the dynamics of the superconducting condensate and of the
existence of two different critical currents: j_{c2} at which the pure
superconducting state becomes unstable and j_{c1}<j_{c2} at which the phase
slip state is realized in the system.Comment: 4 pages, 5 figures, replaced with minor change
High-field muSR studies of superconducting and magnetic correlations in cuprates above Tc
The advent of high transverse-field muon spin rotation (TF-muSR) has led to
recent muSR investigations of the magnetic-field response of cuprates above the
superconducting transition temperature T_c. Here the results of such
experiments on hole-doped cuprates are reviewed. Although these investigations
are currently ongoing, it is clear that the effects of high field on the
internal magnetic field distribution of these materials is dependent upon a
competition between superconductivity and magnetism. In La_{2-x}Sr_xCuO_4 the
response to the external field above Tc is dominated by heterogeneous spin
magnetism. However, the magnetism that dominates the observed inhomogeneous
line broadening below x ~ 0.19 is overwhelmed by the emergence of a completely
different kind of magnetism in the heavily overdoped regime. The origin of the
magnetism above x ~ 0.19 is currently unknown, but its presence hints at a
competition between superconductivity and magnetism that is reminiscent of the
underdoped regime. In contrast, the width of the internal field distribution of
underdoped YBa_2Cu_3O_y above Tc is observed to track Tc and the density of
superconducting carriers. This observation suggests that the magnetic response
above Tc is not dominated by electronic moments, but rather inhomogeneous
fluctuating superconductivity.Comment: 28 pages, 11 figures, 104 reference
Transition from synchronous to asynchronous superfluid phase slippage in an aperture array
We have investigated the dynamics of superfluid phase slippage in an array of
apertures. The magnitude of the dissipative phase slips shows that they occur
simultaneously in all the apertures when the temperature is around 10 mK below
the superfluid transition, and subsequently lose their simultaneity as the
temperature is lowered. We find that when periodic synchronous phase slippage
occurs, the synchronicity exists from the very first phase slip, and therefore
is not due to mode locking of interacting oscillators. When the system is
allowed to relax freely from a given initial energy, the total number of phase
slips that occur and the energy left in the system after the last phase slip
depends reproducibly on the initial energy. We find the energy remaining after
the final phase slip is a periodic function of the initial system energy. This
dependence directly reveals the discrete and dissipative nature of the phase
slips and is a powerful diagnostic for investigation of synchronicity in the
array. When the array slips synchronously, this periodic energy function is a
sharp sawtooth. As the temperature is lowered and the degree of synchronicity
drops, the peak of this sawtooth becomes rounded, suggesting a broadening of
the time interval over which the array slips. The underlying mechanism for the
higher temperature synchronous behavior and the following loss of synchronicity
at lower temperatures is not yet understood. We discuss the implications of our
measurements and pose several questions that need to be resolved by a theory
explaining the synchronous behavior in this quantum system. An understanding of
the array phase slip process is essential to the optimization of superfluid
`dc-SQUID' gyroscopes and interferometers.Comment: 10 pages, 4 figure
Coulomb drag at \nu = 1/2: Composite fermion pairing fluctuations
We consider the Coulomb drag between two two-dimensional electron layers at
filling factor \nu = 1/2 each, using a strong coupling approach within the
composite fermion picture. Due to an attractive interlayer interaction,
composite fermions are expected to form a paired state below a critical
temperature T_c. We find that above T_c pairing fluctuations make the
longitudinal transresistivity \rho_D increase with decreasing temperature. The
pairing mechanism we study is very sensitive to density variations in the two
layers, and to an applied current. We discuss possible relation to an
experiment by Lilly et al. [Phys. Rev. Lett. 80, 1714 (1998)].Comment: REVTeX, 4 pages, 1 figur
Single domain transport measurements of C60 films
Thin films of potassium doped C60, an organic semiconductor, have been grown
on silicon. The films were grown in ultra-high vacuum by thermal evaporation of
C60 onto oxide-terminated silicon as well as reconstructed Si(111). The
substrate termination had a drastic influence on the C60 growth mode which is
directly reflected in the electrical properties of the films. Measured on the
single domain length scale, these films revealed resistivities comparable to
bulk single crystals. In situ electrical transport properties were correlated
to the morphology of the film determined by scanning tunneling microscopy. The
observed excess conductivity above the superconducting transition can be
attributed to two-dimensional fluctuations.Comment: 4 pages, 4 figure
Universal conductance fluctuations in three dimensional metallic single crystals of Si
In this paper we report the measurement of conductance fluctuations in single
crystals of Si made metallic by heavy doping (n \approx 2-2.5n_c, n_c being
critical composition at Metal-Insulator transition). Since all dimensions (L)
of the samples are much larger than the electron phase coherent length L_\phi
(L/L_\phi \sim 10^3), our system is truly three dimensional. Temperature and
magnetic field dependence of noise strongly indicate the universal conductance
fluctuations (UCF) as predominant source of the observed magnitude of noise.
Conductance fluctuations within a single phase coherent region of L_\phi^3 was
found to be saturated at \approx (e^2/h)^2. An accurate
knowledge of the level of disorder, enables us to calculate the change in
conductance \delta G_1 due to movement of a single scatterer as \delta G_1 \sim
e^2/h, which is \sim 2 orders of magnitude higher than its theoretically
expected value in 3D systems.Comment: Text revised version. 4 eps figs unchange
Current-induced highly dissipative domains in high Tc thin films
We have investigated the resistive response of high Tc thin films submitted
to a high density of current. For this purpose, current pulses were applied
into bridges made of Nd(1.15)Ba(1.85)Cu3O7 and Bi2Sr2CaCu2O8. By recording the
time dependent voltage, we observe that at a certain critical current j*, a
highly dissipative domain develops somewhere along the bridge. The successive
formation of these domains produces stepped I-V characteristics. We present
evidences that these domains are not regions with a temperature above Tc, as
for hot spots. In fact this phenomenon appears to be analog to the nucleation
of phase-slip centers observed in conventional superconductors near Tc, but
here in contrast they appear in a wide temperature range. Under some
conditions, these domains will propagate and destroy the superconductivity
within the whole sample. We have measured the temperature dependence of j* and
found a similar behavior in the two investigated compounds. This temperature
dependence is just the one expected for the depairing current, but the
amplitude is about 100 times smaller.Comment: 9 pages, 9 figures, Revtex, to appear in Phys. Rev.
A superconducting-nanowire 3-terminal electronic device
In existing superconducting electronic systems, Josephson junctions play a
central role in processing and transmitting small-amplitude electrical signals.
However, Josephson-junction-based devices have a number of limitations
including: (1) sensitivity to magnetic fields, (2) limited gain, (3) inability
to drive large impedances, and (4) difficulty in controlling the junction
critical current (which depends sensitively on sub-Angstrom-scale thickness
variation of the tunneling barrier). Here we present a nanowire-based
superconducting electronic device, which we call the nanocryotron (nTron), that
does not rely on Josephson junctions and can be patterned from a single thin
film of superconducting material with conventional electron-beam lithography.
The nTron is a 3-terminal, T-shaped planar device with a gain of ~20 that is
capable of driving impedances of more than 100 k{\Omega}, and operates in
typical ambient magnetic fields at temperatures of 4.2K. The device uses a
localized, Joule-heated hotspot formed in the gate to modulate current flow in
a perpendicular superconducting channel. We have characterized the nTron,
matched it to a theoretical framework, and applied it both as a digital logic
element in a half-adder circuit, and as a digital amplifier for superconducting
nanowire single-photon detectors pulses. The nTron has immediate applications
in classical and quantum communications, photon sensing and astronomy, and its
performance characteristics make it compatible with existing superconducting
technologies. Furthermore, because the hotspot effect occurs in all known
superconductors, we expect the design to be extensible to other materials,
providing a path to digital logic, switching, and amplification in
high-temperature superconductors
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