Magnetocaloric studies of the peak effect in Nb

Abstract

We report a magnetocaloric study of the peak effect and Bragg glass transition in a Nb single crystal. The thermomagnetic effects due to vortex flow into and out of the sample are measured.The magnetocaloric signature of the peak effect anomaly is identified. It is found that the peak effect disappears in magnetocaloric measurements at fields significantly higher than those reported in previous ac-susceptometry measurements. Investigation of the superconducting to normal transition reveals that the disappearance of the bulk peak effect is related to inhomogeneity broadening of the superconducting transition. The emerging picture also explains the concurrent disappearance of the peak effect and surface superconductivity, which was reported previously in the sample under investigation. Based on our findings we discuss the possibilities of multicriticality associated with the disappearance of the peak effect.

Emergence of Quasi-Long-Range Order below the Bragg Glass Transition

“…It has nearly cylindrical shape with the [111] crystallographic direction parallel to the cylinder axis. It has zero field T c 9:16 K, extrapolated H c2 0 5600 Oe, Ginzburg-Landau parameter 1 0 3:4, and residual resistivity ratio 300 = 10 12 [6,9]. These values are consistent with approximately 1 at.…”

“…If the statistical weight of the displacement field is Gaussian, the correlation function in the vicinity of reciprocal lattice point Q 0 follows C Q 0 z / e ÿ1=4Q 2 0 'z , and as a result the structure function is a Lorentzian: Sq z / 1=q 2 z 1 4 Q 2 0 ' 2 , which yields L z 2 0 = 2 '. For our sample we have previously determined all relevant superconducting properties, as well as the transition width T c [9]. We thus estimate L z 200 m.…”

“…However, QLRO fails to be fully established on field cooling at lowest temperature. Only with subsequent warming did true QLRO (to resolution limit) appear in the vortex lattice, presumably due to annealing of the trapped defects.The single crystal Nb sample in this work was previously studied using SANS, ac-susceptometry, calorimetry, and magnetocalorimetry [6,9]. It has nearly cylindrical shape with the [111] crystallographic direction parallel to the cylinder axis.…”

“…The single crystal Nb sample in this work was previously studied using SANS, ac-susceptometry, calorimetry, and magnetocalorimetry [6,9]. It has nearly cylindrical shape with the [111] crystallographic direction parallel to the cylinder axis.…”

A procedure to extract the magnetocaloric parameters of the single phases from experimental data of a multiphase system

“…15 Therefore, relevant information about the transition can be extracted from the exponent n (Ref. 7) and, in fact, MCE can be used as a tool to explore the fundamental physics in very different systems such as superconducting materials 18 or graphene. 19 However, several factors can yield an incorrect determination of n. Some of them can arise from experimental aspects, such as experimental indetermination of T C , 20 or the use of a sample with a non negligible demagnetizing factor, N D .…”