Dimensionality of collective pinning in2H−NbSe2single crystals

Abstract

ac susceptibility measurements have been used to determine the dimensionality of the collective pinning in 2H-NbSe 2 crystals. We have analyzed the thickness dependence of the critical current versus field ͓J c (H)͔ curves for thicknesses between 6 and 166 m. Down to 15 m J c (H) is independent of the thickness showing that the pinning is three dimensional. This is in agreement with estimates from collective pinning theory. Deviations occur for the 6 m thick sample near the peak-effect regime, possibly indicating a crossover to two-dimensional behavior. In the thicker samples the peak effect clearly cannot be assigned to a dimensional crossover. The frequency dependence reflects a crossover from a Campbell regime to a nonlinear regime related to small flux creep effects.

Peak effect, plateau effect, and fishtail anomaly: The reentrant amorphization of vortex matter in2H−NbSe2

“…data in insets in Figs. 2͑a͒ and 2͑b͔͒ that the values of the ratio R c /a 0 in crystal X at tϭ0.983 are larger than those in the more strongly pinned crystal Y at a comparable value of t. The R c /a 0 values in crystals X and Y also appear reasonable in the context of estimates of R c /a 0 reported by Angurel et al 23 in much more strongly pinned crystals of 2H-NbSe 2 . We note further that the ratio of R c /a 0 starts to collapse at H p onset ͓see inset ͑i͒ in Fig.…”

“…͑7͒ and ͑8͒. It was surmised by them 23 and we have confirmed 29 by estimating the longitudinal correlation length L c from J c (0) data in crystal X ͑Ref. 14͒ that the 2D collective pinning description ͑for which L c Ͼthickness of the sample͒ is more appropriate for crystal X.…”

“…10͒, respectively͔ either by directly relating J c (H) to the width of the isothermal magnetization loop or by analyzing the inphase and out-of-phase ac susceptibility data 21 within the framework of the critical state model. 22,23 The isothermal dc magnetization hysteresis measurements were performed on a commercial quantum design superconducting quantuminterference device ͑SQUID͒ magnetometer with a 2 cm full scan length and/or a 4 cm ''half-scan technique'' prescribed by Ravikumar et al, 24 whereas the in-phase and out-of-phase ac susceptibility data with different ac amplitudes were measured using a home-built system. The crystal piece X ͑di-mensions 4ϫ1.74ϫ0.18 mm 3 ) with T c (0)Ϸ7.22 K and resistivity ratio R 300 K /R 8 K of 20 is similar to the one utilized by Higgins and Bhattacharya 14 in their electrical transport experiments.…”

“…2͑a͒ and 2͑b͒. These computations have been made using respective J c (H) values in crystals X and Y and following the two-and three-dimensional ͑2D and 3D͒ collective pinning analysis made by Angurel et al 23 as per their Eqs. ͑7͒ and ͑8͒.…”

Nonlinear response of the static and dynamic phases of the vortex matter

“…We extracted J c (H) (for H//c) in two varieties of single crystals A and B, of 2H-NbSe 2 , either by directly relating J c (H) to the widths of the isothermal magnetization hysteresis loops (Bean, 1962(Bean, , 1964 or by analyzing the in-phase and out-of-phase ac susceptibility data (Bean, 1962(Bean, , 1964Angurel et al, 1997). Figure 2 summarizes the J c vs. H data (H\\c) for the crystals A and B in two sets of log-log plots in the temperature regions close to the respective T c (0) values (Banerjee, 2000b;Banerjee et al 2001).…”