Voltage noise and surface current fluctuations in the superconducting surface sheath

Abstract

We report the first measurements of the voltage noise in the surface superconductivity state of a type-II superconductor. We present strong evidences that surface vortices generates surface current fluctuations whose magnitude can be modified by the pinning ability of the surface. Simple twostage mechanism governed by current conservation appears to describe the data. We conclude that large voltage fluctuations induced by surface vortices exist while the bulk is metallic. Furthermore, this experiment shows that sole surface current fluctuations can account for the noise observed even in the presence of vortices in the bulk.PACS numbers: 74.40.+k, 74.25.Op, 72.70.+m, 74.70.Ad Separating bulk and surface effects is an ubiquitous problem in condensed matter physics. For example, Flicker noise in semi-conductors [1] and 1/f like-noise in metals [2,3] can occur from either bulk or surface localized sources. Similarly, one can cite the long standing debate about surface versus bulk effects in driven nonlinear systems like charge density waves [4] or vortex lattice in superconductors [5,6]. Such debate extends to the origin of the fluctuations responsible for their electronic noise. In any case, the key question is how to determine the relevant sources of disorder. From a technological point of view, noise is a limiting factor for most of applications, and it is necessary to know its origin before expecting its minimization. It is also of theoretical interest to know if, when analyzing disordered systems, boundaries effects are of first importance or if they can be neglected compared to the pure bulk treatment of the problem. A similar question has lead to the proposition that bulk impurities play no major role in the broad band noise generation of charge density waves [7]. As a model system, the case of superconducting vortex lattices can be particularly instructing.The dynamics of a bulk vortex lattice has been heavily studied in the literature. It is both understood and experimentally shown that the vortices, when submitted to an overcritical current I > I c and under steady state conditions, move in the bulk of the sample with a well defined average periodicity [8]. This motion is associated with a resistance R f f ≤ R n and an electrostatic field E = −V L ∧ ω (V L is the line velocity and ω the vortex field) [9]. Experimentally, it was also shown that E can be separated into its mean E and its fluctuating part δE * [10]. The latter is the electric field noise. Combined with the V-I relation V = R f f (I − I c ), one can easily realize that this noise can be expressed as fluctuations in the bulk current (I − I c ), or as fluctuations in the resistance R f f . Most of the theories invoke bulk pinning centers through their interaction with the moving vortices, leading to vortex density fluctuations and to their associated δR * f f fluctuations [10]. Alternatively, noise may arise from over-critical current fluctuations δ(I − I c ) * , while R f f is constant [12]. For the simple reason of current conservation, δ(I...

Experimental study of the correlation length of critical-current fluctuations in the presence of surface disorder: Probing vortex long-range interactions

“…This confirms previous auto and cross correlation experiments of both flux and voltage noises [4], and explains the insensivity of the low frequency noise to bulk perturbations [5], in Pb-In alloys. In these experiments, two parameters, the normalized spectrum of the fluctuator and its correlation length, are experimentally justified but are not explained [3,4]. If the fluctuator is of superficial origin, it should be possible to induce notable changes in the noise characteristics after some surface treatments.…”

“…To the extent that the fluctuators are independent and that the system is large enough, the correlation length can be calculated from the noise power via an usual statistical averaging (the central limit theorem) [2]. Recently, it was shown that no difference can be observed between the flux-flow noise in the mixed state and in the surface superconducting state of a Niobium slab [3]. In other words, the same fluctuator is present with or without a bulk vortex lattice, showing clearly its superficial origin.…”

Change of surface critical current in the surface superconductivity and mixed states of superconducting niobium

“…In the present geometry, B is perpendicular to the large facets of the sample, whereas SSC is theoretically expected for B parallel to the surfaces. This feature has been already reported in Nb [14], NbSe 2 [23], MgB 2 [24] but the explanation is not straightforward.…”

“…However, under a low applied current, the electric resistance reaches its normal state value R n at much higher field, and hysteric magnetization is also observed up to B c3 ≈1.7 B c2 due to the existence of a finite critical current in the surface superconducting (SSC) state [17][18][19]. From V(I) curves (not shown here), we find that the differential resistance dV/d(I-I c )≈ R n as soon as B>B c2 as expected for a metallic bulk [14,20]. Then, from these different measurements, it is possible to identify both critical fields B c2 and B c3 at different temperatures.…”

“…in the surface superconducting (SSC) state, deserves also a special attention [11]. SSC can be also the place of very peculiar and interesting properties as reported in [12][13][14].…”