661 research outputs found
Electron localization effects on the low-temperature high-field magnetoresistivity of three-dimensional amorphous superconductors
he electrical resistivity Ï of three-dimensional amorphous superconducting films a-Mo3Si and a-Nb3Ge is measured in magnetic fields ÎŒ0H up to 30 T. At low temperatures and at magnetic fields above the upper critical field Hc2, Ï is temperature independent and decreases as a function of magnetic field. This field dependence is consistent with localization theory in the high-field limit [ÎŒ0Hâ«Ä§/(4eLÏ2), where LÏ is the phase-coherence length]. Above the superconducting transition temperature Tc, the temperature dependence of the conductivity is consistent with inelastic scattering processes which are destructive to the phase coherence for electron localization, thereby allowing estimates for LÏ(T). The Hall effect data on a-Mo3Si, in conjunction with the resistivity data, allow the determination of the carrier concentration and mean free path. The upper critical field is comparable to (in a-Mo3Si) and significantly larger than (in a-Nb3Ge) the Clogston-Chandrasekhar paramagnetic limit. This phenomenon is discussed in the context of electron localization
Evidence for Thermally Activated Spontaneous Fluxoid Formation in Superconducting Thin-Film Rings
We have observed spontaneous fluxoid generation in thin-film rings of the
amorphous superconductor MoSi, cooled through the normal-superconducting
transition, as a function of quench rate and externally applied magnetic field,
using a variable sample temperature scanning SQUID microscope. Our results can
be explained using a model of freezout of thermally activated fluxoids,
mediated by the transport of bulk vortices across the ring walls. This
mechanism is complementary to a mechanism proposed by Kibble and Zurek, which
only relies on causality to produce a freezout of order parameter fluctuations.Comment: 4 pages, 3 figure
Metastable amorphous ferromagnetic phases in palladium-base alloys
The existence of an amorphous phase obtained by rapid quenching from the liquid state, has been recently reported in palladium alloys containing from 16 to 22 at % silicon.(1) Since the ferromagnetic elements iron, cobalt, and nickel, form complete series of solid solutions with palladium, it was anticipated that the amorphous structure could be retained if these elements were substituted for palladium up to a certain concentration
Superconductivity in metal-semiconductor eutectic alloys
In an eutectic alloy obtained by melting the constituents, alternating domains of metal and semiconductor can exist in a well-defined microscopic array. The possibility of superconductivity arising from an interface interaction between the metal and semiconductor has been investigated. Superconductivity has been observed as a bulk property of some eutectic alloys (e.g., Al-Si and Al-Ge). Transition temperatures well in excess of those for the pure metal were found. The effect of rapid cooling the alloys from the liquid state on the microstructure and on the superconducting properties has been studied. It was found that a decrease in the characteristic domain sizes of the metal and semiconductor was accompanied by an increase of the superconducting transition temperature of the alloy. The results suggest that the enhancement of Tc depends on the Fermi energy of the metal. In metal-metal eutectic systems with comparable microstructure, no increase of Tc was observed. Several explanations of the experimental findings are considered. The exciton mechanism discussed by Ginzburg and by Allender, Bray, and Bardeen is considered as a possible means to account for the enhancements of the metal Tc's
Robust dx2-y2 pairing symmetry in high-temperature superconductors
Although initially quite controversial, it has been widely accepted that the
Cooper pairs in optimally doped cuprate superconductors have predominantly
dx2-y2 wavefunction symmetry. The controversy has now shifted to whether the
high-Tc pairing symmetry changes away from optimal doping. Here we present
phase-sensitive tricrystal experiments on three cuprate systems:
Y0.7Ca0.3Ba2Cu3O7-x (Ca-doped Y-123), La2-xSrxCuO4 (La-214) and Bi2Sr2CaCu2O8+x
(Bi-2212),with doping levels covering the underdoped, optimal and overdoped
regions. Our work implies that time-reversal invariant, predominantly dx2-y2
pairing symmetry is robust over a large variation in doping, and underscores
the important role of on-site Coulomb repulsion in the making of
high-temperature superconductivity.Comment: 4 pages, 4 figure
c-axis Josephson Tunnelling in Twinned and Untwinned YBCO-Pb Junctions
Within a microscopic two band model of planes and chains with a pairing
potential in the planes and off diagonal pairing between planes and chains we
find that the chains make the largest contribution to the Josephson tunnelling
current and that through them the d-wave part of the gap contributes to the
current. This is contrary to the usual assumption that for a d-wave tetragonal
superconductor the c-axis Josephson current for incoherent tunnelling into an
s-wave superconductor is zero while that of a d-wave orthorhombic
superconductor with a small s-wave component to its gap it is small but
non-zero. Nevertheless it has been argued that the effect of twins in YBCO
would lead to cancellation between pairs of twins and so the observation of a
current in c-axis YBCO-Pb experiments is evidence against a d-wave type order
parameter. We argue that both theory and experiment give evidence that the two
twin orientations are not necessarily equally abundant and that the ratio of
tunnelling currents in twinned and untwinned materials should be related to the
relative abundance of the two twin orientations.Comment: 6 pages, RevTeX 3.0, 15 PostScript figur
Observation of Andreev bound states in bicrystal grain-boundary Josephson junctions of the electron doped superconductor LaCeCuO
We observe a zero-bias conductance peak (ZBCP) in the ab-plane quasiparticle
tunneling spectra of thin film grain-boundary Josephson junctions made of the
electron doped cuprate superconductor LaCeCuO. An applied magnetic field
reduces the spectral weight around zero energy and shifts it non-linearly to
higher energies consistent with a Doppler shift of the Andreev bound states
(ABS) energy. For all magnetic fields the ZBCP appears simultaneously with the
onset of superconductivity. These observations strongly suggest that the ZBCP
results from the formation of ABS at the junction interfaces, and,
consequently, that there is a sign change in the symmetry of the
superconducting order parameter of this compound consistent with a d-wave
symmetry.Comment: 9 pages, 7 figures; December 2004, accepted for publication in Phys.
Rev.
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