Instabilities in the Vortex Matter and the Peak Effect Phenomenon

Abstract

In single crystals of 2H-NbSe2, we identify for the first time a crossover from a weak collective to a strong pinning regime in the vortex state which is not associated with the peak effect phenomenon. Instead, we find the crossover is associated with an anomalous history dependent magnetization response. In the dc magnetic field (Bdc)-temperature (T) vortex matter phase diagram we demarcate this pinning crossover boundary. We also delineate another boundary which separates the strong pinning region from a thermal fluctuation dominated regime, and find that a peak effect appears on this boundary.

Peak Effect in the Critical Current of Type II Superconductors with Strong Magnetic Vortex Pinning

“…The OD transition was suggested to be a thermodynamic phase transition induced by thermal fluctuations or pinning centers [2], which has been confirmed experimentally by the direct structural observation of the vortex lattice, such as small angle neutron scattering (SANS) [9], and muon spin relaxation [17]. Also, the investigation of the reversibility of the OD transition provided strong support for its thermodynamical nature [18].However, the exact nature of the PE phenomenon remains controversial [6,7,10,[19][20][21]. Recently, across the PE regime no noticeable change in the order of the vortex lattice was observed by SANS [19], and by Bitter decoration [20].…”

“…Recently, across the PE regime no noticeable change in the order of the vortex lattice was observed by SANS [19], and by Bitter decoration [20]. It was also reported that the PE is situated on a boundary separating the strong pinning regime from the thermal fluctuations dominated regime, while the weak-strong pinning crossover is located far from the PE regime [7]. On the other hand, for some superconductors with strong pinning centers, it has been found that the occurrence of the PE depends strongly on the strength and amount of the pinning centers [3,6,22,23].…”

“…However, the exact nature of the PE phenomenon remains controversial [6,7,10,[19][20][21]. Recently, across the PE regime no noticeable change in the order of the vortex lattice was observed by SANS [19], and by Bitter decoration [20].…”

“…Finally, in order to clarify the role of thermal fluctuations [7], we studied the effects of thermal fluctuations on the PE. Shown in Fig.…”

“…Fy, 74.25.Qt One pronounced phenomenon in type-II superconductors is the so called peak effect (PE), which is the appearance of a peak in the critical current density J c before decreasing to 0 with increasing temperature or field [1]. The PE has widely been observed in a variety of low and high temperature superconductors by different experimental techniques [2], such as transport [3][4][5], magnetization [6,7], and ac-susceptibility [8][9][10]. It was proposed long ago that the PE originated from softening of the elastic moduli of vortex lattice [1], which caused by the competitions between elastic energy E el , pinning energy E pin of vortex lattice and the energy of thermal fluctuations E th [11].…”