Observation of sharp resistance anomalies near T_c in granular superconductors

Abstract

Abstract. Granular films prepared from well-defined In clusters embedded in Kr ( = 10-40 at ?la Kr) show sharp resistance anomalies very close to the superconducting transition temperature. The observed resistance peak(s), which go(es) above the normal state resistance R , and which can be as high as = 1.5 R,, can be explained by a "mesoscopic" effect: Superconducting percolation aggregates of size tP, larger than the superconducting coherence length ts, but smaller than the quasiparticle relaxation length AQ,, have a superconducting gap, but are not yet able to carry a supercurrent.Keywords: Granular superconductors; Nonequilibrium superconductivity; Metal clusters.The superconducting properties of granular metals built from metal clusters embedded in an insulating matrix have been the subjet of both experimental and theoretical work for many years [I]. The discovery of the high-T, oxides with their original granular structure renewed and intensified the interest in this field. One of the suprising features of granular metals is the observation of a metal cluster size dependence of the superconducting transition temperature T,, which is usually higher than that of the bulk material and which increases with decreasing metal cluster size L [2, 31. A consequence of this is that granular metals with a metal cluster size distribution P(L) will have a distribution P( T,) in the superconducting transition temperature, if the clusters are not too strongly coupled as it is the case for metal volume fractions uM < 1. Such a distribution P (T,) will not only result in a broadened transition, but it may also lead to non-equilibrium superconducting phenomena connected with the occurrence of normal conductor (N)-superconductor (S) interfaces. Just below the highest superconducting transition temperature (T,)max, usually that of the smallest clusters, these interfaces occur on a length scale which is given by the cluster size L. This length is much smaller than other characteristic length scales, i. e. the superconducting coherence length ts, or the charge imbalance relaxation length A,, connected with the quasiparticle diffusion from the N-into the S-state. The study of non-equilibrium phenomena on such a "mesoscopic" length scale is of special interest.Non-equilibrium phenomena at N-S interfaces usually show up in the resistance curve R (7') just below T, [4]. For this reason we have studied the superconducting transition in granular In films by resistance measurements very close to T,. These granular films were built from In clusters with different cluster size distributions P ( L ) . In clusters have a L-dependent T, with T, 2 : 4.1 K for L L 5 nm and T, = 3.7 K for L 2 6 nm (51. The resistance curves R ( T ) of such films measured by standard four terminal dc-method

On the Interplay Between T c -Inhomogeneities at Long Length Scales and Thermal Fluctuations Around the Average Superconducting Transition in Cuprates

“…In particular, for H parallel to the c direction the ∆ρ (H) T saturation value is reached, at each temperature, for smaller field amplitudes than for H parallel to the ab planes. This behaviour of ∆ρ (T,H) is quite similar to that observed by other groups in other HTSC and LTSC compounds having ρ (T,H) peaks near the superconducting transition and attributed by these authors to intrinsic effects[29,[31][32][33][34][35] (compare, in particular, Figs. 7(a) and (b) with Fig.…”

“…It is now well established that the presence of T c -inhomogeneities at long length scales but non-uniformly distributed may generate various types of striking anomalies in the transport properties around the superconducting transition in HTSC, including the so-called anomalous peaks of the in-plane resistivity [25] the thermoelectric power [26] and the magnetoconductivity. [27,28] In addition to their intrinsic interest, these results may also provide an alternative explanation, in terms of temperature independent current redistributions associated with non uniformly distributed T c -inhomogeneities, of the anomalous resistivity and magnetoresistivity peaks observed above the average superconducting transition by different groups in other LTSC [29][30][31] and HTSC [32][33][34][35] compounds and that are being attributed to different, and in some cases not well settled, intrinsic effects. [29][30][31][32][33][34][35] In the case of the thermopower, it is also possible to explain the anomalous S(T) peaks observed in YBCO samples in terms of oxygen content inhomogeneities.…”

Resistance anomaly and excess voltage in inhomogeneous superconducting aluminum thin films

“…[1][2][3][4][5][6][7][8][9][10][11][12][13] These systems range from one dimensional ͑1D͒ to bulk samples, conventional to compound superconductors, and single component to multicomponent systems. Due to the diversity in sample configurations and material properties, it is unclear how ͑and if͒ the various observations are connected to each other.…”

Observation of excess resistance anomaly at resistive transitions in Ag/Au nanostructures

“…While there is no known analogue of the ERA in nonsuperconducting metallic nanostructure, the diversity of superconducting devices and materials that exhibit such a phenomenon has led to considerable debate in regard to its microscopic origin. An increase in resistivity in the vicinity of T C can arise from quasi-particle tunneling between the superconducting-normal (S-N) puddles in case of a granular superconductor [31] or vortex dynamics in the fluctuation regime [22,32]. Large excess resistance in granular doped nano-diamond films has been attributed to superconducting transition through an intermediate Bosonic insulating phase [23].…”