4,273 research outputs found
Influence of carrier lifetime on quantum criticality and superconducting Tc of (TMTSF)_2ClO_4
This work presents and analyzes electrical resistivity data on the organic
superconductor (TMTSF)ClO and their anion substituted alloys
(TMTSF)(ClO)(ReO) along the least conducting
axis. Nonmagnetic disorder introduced by finite size domains of anion ordering
on non Fermi liquid character of resistivity is investigated near the
conditions of quantum criticality. The evolution of the -linear resistivity
term with anion disorder shows a limited decrease in contrast with the complete
suppression of the critical temperature as expected for unconventional
superconductivity beyond a threshold value of . The resulting breakdown of
scaling between both quantities is compared to the theoretical predictions of a
linearized Boltzmann equation combined to the scaling theory of umklapp
scattering in the presence of disorder induced pair-breaking for the carriers.Comment: 13 pages, 8 figure
Different Behavior of Magnetic Impurities in Crystalline and Ammorphous States of Superconductors
It has been observed that the effect of magnetic impurities in a
superconductor is drastically different depending on whether the host
superconductor is in a crystalline or an amorphous state. Based on the recent
theory of Kim and Overhauser (KO), it is shown that as the system is getting
disordered, the initial slope of the depression is decreasing by a
factor , when the mean free path becomes smaller
than the BCS coherence length , which is in agreement with
experimental findings. In addition, for a superconductor in a crystalline state
in the presence of magnetic impurities the superconducting transition
temperature drops sharply from about 50% of (for a pure
system) to zero near the critical impurity concentration. This {\sl pure limit
behavior} was indeed found by Roden and Zimmermeyer in crystalline Cd.
Recently, Porto and Parpia have also found the same {\sl pure limit behavior}
in superfluid He-3 in aerogel, which may be understood within the framework of
the KO theory.Comment: 7 figures, 20 pages, latex, to appear in Superconductor Science and
Technolog
Transport properties and point contact spectra of Ni_xNb_{1-x} metallic glasses
Bulk resistivity and point contact spectra of Ni_xNb_{1-x} metallic glasses
have been investigated as functions of temperature (0.3-300K) and magnetic
field (0-12T). Metallic glasses in this family undergo a superconducting phase
transition determined by the Nb concentration. When superconductivity was
suppressed by a strong magnetic field, both the bulk sample R(T) and the point
contact differential resistance curves of Ni_xNb_{1-x} showed logarithmic
behavior at low energies, which is explained by a strong electron - "two level
system" coupling. We studied the temperature, magnetic field and contact
resistance dependence of Ni_{44}Nb_{56} point-contact spectra in the
superconducting state and found telegraph-like fluctuations superimposed on
superconducting characteristics. These R(V) characteristics are extremely
sensitive detectors for slow relaxing "two level system" motion.Comment: 4 pages, 5 figure
Superconducting and Magnetic Properties of Nb/Pd_1-xFe_x/Nb Triple Layers
The superconducting and magnetic properties of Nb/Pd_1-xFe_x/Nb triple layers
with constant Nb layer thickness d_Nb=200 A and different interlayer
thicknesses are investigated. The thickness dependence of the magnetization and
of the superconducting transition temperature shows that for small iron
concentration x the Pd_1-xFe_x layer is likely to be in the paramagnetic state
for very thin films whereas ferromagnetic order is established for x>=0.13. The
parallel critical field B_c2II(T) exhibits a crossover from two-dimensional
(2D) behavior where the Nb films are coupled across the interlayer, towards a
2D behavior of coupled Nb films with increasing d_PdFe or x. This 2D-2D
crossover allows a determination of the penetration depth xi_F of Cooper pairs
into the Pd_1-xFe_x layer as a function of x. For samples with a ferromagnetic
interlayer xi_ is found to be independent of x.Comment: 9 pages, 8 figure
Reentrant superconductivity in superconductor/ferromagnetic-alloy bilayers
We studied the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) like state
establishing due to the proximity effect in superconducting Nb/Cu41Ni59
bilayers. Using a special wedge-type deposition technique, series of 20-35
samples could be fabricated by magnetron sputtering during one run. The layer
thickness of only a few nanometers, the composition of the alloy, and the
quality of interfaces were controlled by Rutherford backscattering
spectrometry, high resolution transmission electron microscopy, and Auger
spectroscopy. The magnetic properties of the ferromagnetic alloy layer were
characterized with superconducting quantum interference device (SQUID)
magnetometry. These studies yield precise information about the thickness, and
demonstrate the homogeneity of the alloy composition and magnetic properties
along the sample series. The dependencies of the critical temperature on the Nb
and Cu41Ni59 layer thickness, Tc(dS) and Tc(dF), were investigated for constant
thickness dF of the magnetic alloy layer and dS of the superconducting layer,
respectively. All types of non-monotonic behaviors of Tc versus dF predicted by
the theory could be realized experimentally: from reentrant superconducting
behavior with a broad extinction region to a slight suppression of
superconductivity with a shallow minimum. Even a double extinction of
superconductivity was observed, giving evidence for the multiple reentrant
behavior predicted by theory. All critical temperature curves were fitted with
suitable sets of parameters. Then, Tc(dF) diagrams of a hypothetical F/S/F
spin-switch core structure were calculated using these parameters. Finally,
superconducting spin-switch fabrication issues are discussed in detail in view
of the achieved results.Comment: 34 pages, 9 figure
Minigap, Parity Effect and Persistent Currents in SNS Nanorings
We have evaluated a proximity-induced minigap in the density of states (DOS)
of SNS junctions and SNS nanorings at an arbitrary concentration of
non-magnetic impurities. We have demonstrated that an isotropic energy minigap
in the electron spectrum opens up already at arbitrarily weak disorder, while
angle resolved DOS at higher energies can remain strongly anisotropic. The
minigap value can be tuned by passing a supercurrent through an
SNS junction or by applying a magnetic flux to an SNS ring. A
non-monotonous dependence of on has been found at weak
disorder. We have also studied persistent currents in isolated SNS nanorings.
For odd number of electrons in the ring we have found a non-trivial
current-phase (current-flux) relation which -- at relatively high disorder --
may lead to a -junction state and spontaneous currents in the ground state
of the system.Comment: 7 pages, 8 figure
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