Peak effect, vortex-lattice melting line, and order-disorder transition in conventional and high-Tcsuperconductors

Abstract

We investigate the order-disorder transition line from a Bragg glass to an amorphous vortex glass in the H − T phase diagram of three-dimensional type-II superconductors taking into account both pinning-caused and thermal fluctuations of the vortex lattice. Our approach is based on the Lindemann criterion and on results of the collective pinning theory and generalizes previous work of other authors. It is shown that the shapes of the order-disorder transition line and the vortex lattice melting curve are determined only by the Ginzburg number, which characterizes thermal fluctuations, and by a parameter which describes the strength of the quenched disorder in the fluxline lattice. In the framework of this unified approach we obtain the H − T phase diagrams for both conventional and high-Tc superconductors. Several well-known experimental results concerning the fishtail effect and the phase diagram of high-Tc superconductors are naturally explained by assuming that a peak effect in the critical current density versus H signalizes the order-disorder transition line in superconductors with point defects.

First-Order Transition in the Magnetic Vortex Matter in SuperconductingMgB2Tuned by Disorder

“…Simultaneously, the initially narrow peak is enhanced, transforming to the so-called ''fishtail effect'' reminiscent of that in cuprate superconductors [15][16][17]. Both the temperature and defect density dependence of H OD are in very good agreement with the scenario of proliferation of dislocations [4], in conjunction with pinning mediated by fluctuations in the mean free path.…”

“…(1). Moreover, one obtains that gðTÞ/H c2 ðTÞ=H c2 ð0Þ as predicted for pinning induced by fluctuations in the mean free path [4,5]. T c pinning would lead to gðTÞ/½H c2 ðTÞ=H c2 ð0Þ À1=3 , in utter disagreement with the experimental data.…”

“…Namely, the role of disorder can be tuned very easily by changing the external magnetic field H a . Increasing the field leads to an order-disorder (OD) transition revealed by a peak in the critical current density j c , that has been the subject of much theoretical work [1][2][3][4][5]. However, no consensus has been reached thus far as to the mechanism of the transition.…”

“…Larkin and Ovchinnikov [14] noted that this is exacerbated by the gradual softening of the vortex lattice because of the nonlocality of its tilt modulus. Kierfeld et al [3] and Mikitik et al [4] contended that the peak occurs at the field H OD at which the pinning energy gained by disordering the vortex ensemble outweighs the energy of elastic deformation due to the generation of edge and screw dislocations. Finally, in Ref.…”

“…Note that, as pointed out by Pissas et al [12]in pristine samples, the peak becomes unmeasurable above a temperature T Ã that is progressively shifted towards T c for increasing defect density, reaching $0:94T c for irradiation with 5:1 Â 10 19 e À cm À2 . In spite of the widely varying values of h OD ð0Þ H OD ð0Þ=H c2 ð0Þ one can show that the mechanism for the peak effect is the same, and corresponds to the proliferation of a dislocation network in the vortex lattice [3,4]. To set the stage, note that just below H onset , the j c ð5K Þ values ($10 6 Am À2 in pristine samples and 10 7 Am À2 after irradiation with 2:7 Â 10 19 e À =cm 2 ) correspond to translational correlation lengths [14] R c largely exceeding the penetration depth [R c $ 20 m and 10 m respectively, while ab % 500 nm], which means that nonlocality of the elastic constants, and the scenario of Ref.…”