Effect of pinning and driving force on the metastability effects in weakly pinned superconductors and the determination of spinodal line pertaining to order—disorder transition

Abstract

We explore the effect of varying drive on metastability features exhibited by the vortex matter in single crystals of 2H-NbSe 2 and CeRu 2 with varying degree of random pinning. The metastable nature of vortex matter is reflected in the path dependence of the critical current density, which in turn is probed in a contact-less way via ac-susceptibility measurements. The sinusoidal ac magnetic field applied during ac susceptibility measurements appears to generate a driving force on the vortex matter. In a nascent pinned single crystal of 2H-NbSe 2 , where the peak effect (PE) pertaining to the order-disorder phenomenon is a sharp first order like transition, the supercooling feature below the peak temperature is easily wiped out by the reorganization caused by the ac driving force. In this paper, we elucidate the interplay between the drive and the pinning which can conspire to make the path dependent ac-susceptibility response of different metastable vortex states appear identical. An optimal balance between the pinning and driving force is needed to view the metastability effects in typically weakly pinned specimen of low temperature superconductors. As one uses samples with larger pinning in order to differentiate the response of different metastable vortex states, one encounters a new phenomena, viz., the second magnetization peak (SMP) anomaly prior to the PE. Supercooling/superheating can occur across both the PE and the SMP anomaly and both of these are known to display non-linear characteristic as well. Interplay between the path dependence in the critical current density and the non-linearity in the electromagnetic response determine the metastability effects seen in first and the third harmonic response of the ac susceptibility across the temperature regions of the SMP and the PE. The limiting temperature above which metastability effects cease can be conveniently located in the third harmonic data, and the observed behavior can be rationalized within the Beans Critical State model. A vortex phase diagram showing the different vortex phases for a typically weakly pinned specimen has been constructed via the ac susceptibility data in a crystal of 2H-NbSe 2 which shows the SMP and the PE anomalies. The phase space of coexisting weaker and stronger pinned regions has been identified. It can be bifurcated into two parts, where the order and disorder dominate, respectively. The former part continuously connects to the reentrant disordered vortex phase pertaining to the small bundle pinning regime, where the vortices are far apart, interaction effects are weak and the polycrystalline form of flux line lattice prevails.

On the nature of superconductivity in the anisotropic dichalcogenide NbSe2{CoCp2}x

“…Due to the complex nature of vortex phases, a vast number of detailed studies have been performed in the case of high T c as well as low T c superconductors, see, e.g. [10,33,48,49]. To analyze the complicated vortex phase structure in NbSe 2 {CoCp 2 } 0.26 , further investigations are warranted which also consider the subtle pinning behavior of this type-II superconductor in greater detail, as well as the presence of large van der Waals gaps and intercalated guest molecules which might act as pinning centers.…”

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

“…While theoretically some difference persist as regards the origin of the PE phenomenon, the experimental investigations (Banerjee et al, 1998(Banerjee et al, , 1999a(Banerjee et al, , 1999b(Banerjee et al, , 2000a(Banerjee et al, , 2000b(Banerjee et al, , 2001Bhattacharya & Higgins, 1993;Gammel et al, 1998;Higgins and Bhattacharya, 1996;Marchevsky et al, 2001;Thakur et. al, 2005Thakur et. al, , 2006Troyanovski et al, 1999Troyanovski et al, , 2002 are almost concurrent towards in establishing PE as an order to disorder transformation in the vortex lattice.…”

“…On the basis of correlation between pinning strength and the metastability effects in the elastic region of vortex phase diagram, the crystals can be sequentially enumerated in terms of the progressively enhanced pinning. For instance, in 2H-NbSe 2 crystals, ranging from nomenclature A to C, the J c values vary from 10 A/cm 2 to 1000 A/cm 2 (Banerjee et al, 1998(Banerjee et al, , 1999a(Banerjee et al, , 1999b(Banerjee et al, , 2000a(Banerjee et al, , 2000b(Banerjee et al, , 2001Thakur et al 2005Thakur et al , 2006.…”

“…[Mohan et al 2007;Mohan 2009b] All the above discussions pertain to susceptibility measurements performed in the zero field cooled (ZFC) mode. Detailed studies of the dependence of the thermomagnetic history dependent magnetization response on the pinning (Banerjee et al 1999b, Thakur et al, 2006, had shown an enhancement in the history dependent magnetization response and enhanced metastablility developing in the vortex state as the pinning increases across the PE. While the ZFC and field cooling (FC), '(T) response can be identical in samples with weak pinning, the will show that ''(T) is a more sensitive measure of small difference in the thermomagnetic history dependent response.…”