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

Abstract

We report on critical currents and voltage noise measurements in Niobium strips in the superconducting state, in the presence of a bulk vortex lattice (B < BC2) and in the surface superconducting state (Bc2 < B < BC3). For homogeneous surfaces, the correlation length of the current fluctuations can be associated with the electromagnetic skin depth of vortex superficial instabilities. The modification of the surface state by means of low energy irradiation induces a strong modification of the critical current and of the noise. The appearance of a corner frequency in the spectral domain can be linked with the low wave-vectors of the artificial corrugation. Since this latter occurs only for B < BC2, we propose that the long-range interactions allow the correlation length to extend up to values imposed by the surface topography.PACS numbers: 71.27.+a,72.70.+m,72.20.My Noise measurements are powerful tools to go inside the origin of the vortex pinning, and of the dynamical interactions between the vortex lattice and the sample disorder. This noise, generated during the lattice flow, is called the flux-flow noise [1]. It is generally characterized by the shape of its spectral density, and by its power. As it is often proposed in noise analysis, it is convenient to define a correlation length, within which the fluctuations are correlated. This defines the fluctuator of the system. 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. The coupling to the bulk was shown to be due to the conservation of the total current. 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. A following change in the noise spectral density, the noise power, or eventually in the underlying statistics, would give some clues to understand the spectrum and the fluctuator origins. Wherever it takes place, it has been made clear for years that the existence of flux-flow noise is intimately linked to the nature of the relevant disorder, i.e. to the vortex pinning. In the conclusion of his review article, Clem states that such a flux-flow noise theory " should be intimately related to an appropriate theory of critical current density " [1]...

New Insight Into the Fluctuations of the Moving Vortex Lattice: Non-Gaussian Noise and Lévy Flights

“…[16,17]). In contrast, the observed properties support a description of the noise in terms of surface critical current fluctuations, which has been verified in bulk samples of low-T c superconductors [4,18]. We will now focus on the low current regime.…”

“…To reveal the existence of long time correlations between the fluctuations, the use of the second spectrum is an efficient and complementary tool. The second spectra are observed to be frequency-independent Details on the measuring condition of the bulk sample can be found in [18]. Inset: log-log plot of the histogram (d).…”

“…T = 4.2K. Solid curves denote the Gaussian fits.Details on the measuring condition of the bulk sample can be found in[18]. Inset: log-log plot of the histogram (d).…”

Flux-lines lattice order and critical current studied by time-of-flight small-angle neutron scattering

“…Fitting the experimental J c (exp) of our Niobium sample with the surface pinning expression J c (surf ) = 2/t × ε × sinθ gives a very good agreement with a critical angle θ = 4 deg (Fig.8). This is a large, but still reasonable value for a bulk sample with unpolished surfaces [33]. We do not exclude that edge currents play also a role for the critical current.…”

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

“…All the samples under study are cut from the same ingot of high purity polycrystalline bulk niobium previously used for transport and noise measurements [15] or small angle neutron scattering [16]. In order to start with samples of similar surface states and with a moderate roughness, a buffer chemical polishing (BCP) was first performed on all the samples.…”