Anisotropic response of the moving vortex lattice in superconducting Mo(1−x)Gex amorphous films

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Abstract

We have performed magnetic susceptibility measurements in Mo1−xGex amorphous thin films biased with an electrical current using anisotropic coils. We tested the symmetry of the vortex response changing the relative orientation between the bias current and the susceptibility coils. We found a region in the DC current–temperature phase diagram where the dynamical vortex structures behave anisotropically. In this region the shielding capability of the superconducting currents measured by the susceptibility coils is less effective along the direction of vortex motion compared to the transverse direction. This anisotropic response is found in the same region where the peak effect in the critical current is developed. On rising temperature the isotropic behavior is recovered.

Highlights

► We implement a new variation of the typical mutual inductance technique. ► The technique permits measurements of susceptibility and transport at the same time. ► We report the IdcT dynamical phase diagram of superconducting MoGe films. ► Measurements at low T indicates the presence of a transverse critical current. ► We found two different anisotropic behaviors where is present the peak effect.

Introduction

The study of the dynamic behavior of elastic media in the presence of different static potentials has been center of attention in the recent years. Superconducting vortices are almost ideal systems for the study of the phenomenology of these systems because relevant properties such as density, interactions and external force can be easily controlled.

Much work has been done on the problem of moving vortex systems. Depending on the velocity and type of static potential it is expected to arise different dynamical phases. In the elastic regime, Giamarchi and Le Doussal [1] have predicted the existence of a moving-Bragg-glass phase for high driving currents and a weak disordered static potential. This is a phase free of dislocations, with a power law decay of the positional correlations which are anisotropic with respect to flow direction. On another hand, Balents et al. [2] have argued for the existence of a smectic phase. In this phase, vortices move along well defined static channels, holding correlation perpendicular to the channels, but uncorrelated along them. On increasing disorder plastic motion is expected close to the depinning force showing a mixture of moving and fixed vortices [3]. In the later case the vortex motion is also through meandered static flow patterns. A common expected feature of all these dynamical states is the anisotropic response of the moving structure to perturbations parallel or perpendicular to the vortex motion.

A number of experimental works have been done investigating these phenomena. Transport measurements report changes in the dynamical properties of the vortex matter close to the peak effect [4], [5], [6], [7] that were attributed to a crossover from plastic to ordered motion. More recently Kokubo and coworkers [8] have proven this dynamical ordering from mode locking experiments. Snapshots of moving vortex structures have been obtained in decoration experiments [9], [10] that have shown evidence for these smectic and moving glass phases of vortices. However one of the most striking properties of these quasi-ordered driven phases is the existence of barriers to a small force transverse to the direction of motion. These barriers would originate an effective transverse critical current [11] or a change in the Hall noise spectrum [12]. Although the numerical results present a clear evidence of these effects the experimental confirmation is evasive. Recently Lefebvre et al. [13] have shown transport measurements in superconducting metallic amorphous tapes that reveal the existence of a transverse critical current although these results show big quantitative discrepancies with numerical simulations.

Mutual inductance techniques have proven to be a valuable tool for measuring superconducting characteristics of two dimensional systems [14], [15]. In previous works it was shown that a variation of this technique based in special shaped coils is capable of detecting the anisotropic character of dynamical phases [16], [17]. In this work we extend these susceptibility measurements to Mo1−xGex amorphous films that reveal the anisotropic characteristic of dynamical vortex phases. This technique allows the measurement of susceptibility and transport at the same time obtaining susceptibility information with applied current to the sample.

Section snippets

Experimental details

Mutual inductance measurements were performed using planar serpentine coils of geometry similar to those described in a previous paper [17]. It was showed that the implementation of rectangular coils with high aspect ratio allow the direct measurement of the anisotropy. Shielding currents are induced in the sample by the primary coils and therefore they are predominantly parallel to the long side of the coil, as it is shown in Fig. 1d. As a consequence the voltage generated in the reception

Results and discussion

The principal result of this paper is represented in Fig. 2 where the susceptibility of the sample is plotted as a function of temperature, on measuring it with an electrical current through the sample for the two current orientations and an applied magnetic field perpendicular to the film. In the following we define χ as the susceptibility data taken where the bias current is parallel to the long side of the coils (Iy, y direction), and χ where the bias current on the sample is perpendicular

Conclusions

Our results clearly indicate that the response of the moving vortex lattice in α-MoGe have an anisotropic characteristic in the region close to the depinning transition. Our results show the existence of two different regimes of anisotropic behavior. At lower temperatures there is a finite response in the y direction and the response in the perpendicular direction is current independent. This is a region where there is a transverse critical current. Rising temperature and/or the magnitude of

Acknowledgments

We are very grateful to M. Hesselberth and P.H. Kes for helping us with the deposition of the MoGe films on the planar coils. This project was financially supported by CNEA and CONICET. M.I.D., D.E.S and H.P. researchers of CONICET.

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Present address: Departamento de Física, Facultad de Ciencias Exactas y Naturales, Universidad Nacional de Buenos Aires, Ciudad Autónoma de Buenos Aires, Argentina.

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