Organization of flux line system and voltage oscillations in superconducting MgB2

Abstract

The influence of bidirectional square wave (BSW) current on the evolution of the V-t curves was studied in a polycrystalline sample of MgB 2 . Regular sinusoidal-like voltage oscillations were observed at well-defined values of the amplitude (I BSW ) and period (P I ) of the BSW current, temperature, and external magnetic field. The voltage oscillations were interpreted mainly in terms of short-and long-range spatial correlations in a flux line system, relaxation effects, and the dynamic competition between pinning and depinning. The oscillating mode was also discussed in terms of sliding charge density waves associated with density fluctuations. Alternatively, the coupling between conventional thermally activated flux creep theory and annealing of the disordered states was considered as another possible mechanism in describing the voltage oscillations. Due to the complex interplay of pinning and driving forces, the oscillating mode is correlated to the drifting of flux lines, which are in motion, and it is assumed that the majority of flux lines try to traverse the sample, but do not leave it entirely. The symmetry in voltage oscillations was attributed to the equality between flux exit and entry along the sample edges. In this process, the majority of flux lines leave the sample and the direction of motion of flux lines reverses. During the oscillatory motion, we suggest that the interaction between flux lines and pinning centers may be elastic coupling in nature, because of rigidity of the flux line lattice in MgB 2 . Non-oscillatory behavior of V-t curves measured for BSW drive with low P I was attributed pre-dominantly to the effect of pinning on the distribution of the flux lines. The vortex capacitor model could be applied to explain the voltage decays in these V-t curves. However, the voltage decays associated with switching of I BSW to dc current (I dc ) were correlated to current-induced quenched disorder and to the glassy state relaxation. Further, angular dependence of voltage oscillations reveals that the anisotropic effects are nearly negligible on the evolution of V-t curves. It was shown that the voltage oscillations can be described well by an empirical expression V(t) $ sin(wt þ /), where w is the frequency and / is the phase angle. Finally, at well-defined values of I BSW and P I of BSW current, H, and T, it was argued that the MgB 2 sample behaves like a double-integrator.