Structure of the flux lines lattice inNbSe2: Equilibrium state and influence of the magnetic history

Abstract

We have performed small-angle neutron scattering (SANS) of the flux line lattice (FLL) in a F e doped N bSe2 sample which presents a large peak effect in the critical current. The scattered intensity and the width of the Bragg peaks of the equilibrium FLL indicate an ordered structure in the peak effect zone. The history dependence in the FLL structure has been studied using field cooled and zero field cooled procedures, and each state shows the same intensity of Bragg scattering and good orientational order. These results strongly suggest that the peak effect is unrelated to a bulk disordering transition, and confirm the role of a heterogeneous distribution of screening current.The peak effect in the critical current of a type II superconductor is a long standing problem in the field of superconductivity. Since its observation [1, 2], no clear consensus has emerged as to its origin. Most models are based on the Pippard idea [3] who attributed the peak effect to the loss of rigidity of the flux lines lattice (FLL), which falls close to B c2 more rapidly than the pinning strength due to inhomogeneities. Larkin and Ovchinikov (LO) have treated the complex problem of the elastic FLL in the presence of random pinning centers [4]. The main issues are the statistical calculations of a volume V c over which the lattice remains correlated and the corresponding pinning force per unit volume F p ∝ V −1/2 c . In addition to the intense theoretical interest, there is the possible link between the FLL bulk disorder and the critical current. Other interpretations of the critical current deal with dominant pinning of flux lines by the surface [5,6], without significant role of FLL bulk order. Recently, the explanation of the peak effect as a genuine phase transition has been addressed both theoretically and experimentally, and the peak effect is now considered as a classical example of an order-disorder transition in an elastic system [7]. A superconductor with a peak effect is remarkable by its anomalous transport properties. They have been tackled by quite thorough series of measurements in N bSe 2 , with a large focus on the metastable properties [8]. Two coexisting macroscopic FLL states with different pinning strengths were observed [9] and the kinetics between these two states can explain most of the anomalous transport properties. Paltiel et al have also shown that the critical current in the peak effect zone is heterogeneous and more important close to the edge of the samples [10]. However, the technique of Hall probe does not allow concluding on the structure of the FLL. The genuine nature of FLL states with the two different critical currents remains unknown, even if one is usually assumed to be much more disordered following LO approach [10]. However, this latter assumption needs to be experimentally controlled using a structural probe: indeed, a correlation between the critical current and the FLL bulk order can not be a priori assumed, as shown by different Small Angle Neutron Scattering (SANS) experiments [11,12,1...

Decomposing the Bragg glass and the peak effect in a Type-II superconductor

“…Unfortunately the experimental picture is neither so simple nor general. In Fe-doped NbSe 2 , a well-ordered vortex lattice can be observed by SANS at fields much higher than H pp , leading to the conclusion that the peak effect is unrelated to a bulk order–disorder transition 60 . This conclusion is shared by an earlier decoration study on clean and Fe-doped NbSe 2 61 .…”

Magnetic small-angle neutron scattering

“…associating the PE with a bulk order-disorder transition is widely accepted, this is not universal. This is due to some observations of an ordered VL above the PE (Pautrat et al, 2009(Pautrat et al, , 2007 and studies which indicate that this is caused by surface rather than bulk pinning (Pautrat et al, 2012). Here, one should note that few SANS measurements are performed under the same exact conditions (transport currents, AC fields) as those used to determine the PE.…”

Filamentary flow of vortices in aBi2Sr2Ca1Cu2O8<

“…More recently, a neutron-scattering measurements of FFL in BSCCO and NbSe 2 have shown that the Bragg peaks were observed beyond the field generally associated with destruction of the FFL, showing the persistence of FFL. 4,7 Another remarkable behavior in Fig. 1 is the multiple steps in the I-V curves measured upon increasing or decreasing bias current near the PE.…”

“…1-3 Peak effect ͑PE͒, i.e., a sharp increase in the critical current 0 H-I c curve, is an important observation for tracking these topological phase transitions in both lowand high-T c superconductors. [4][5][6][7] In the low-T c superconductors, the PE appears near 0 H c2 , when a transition from ordered to a strongly disordered pinning state occurs in the vortex lattices. 8,9 In high-T c superconductors, a pronounced PE or fishtail in which J c shows a sharp increase, was also observed and attributed to a transition from order to disorder ͑as in Refs.…”

“…Measurements of I-V characteristics vs temperature ͑T͒ and magnetic field ͑B͒ have only been used to investigate the nature of pinning and structural transitions in vortex state in the low-T c materials. 4,5,9,19 However, in high-T c superconductors, those measurements were technically impossible to perform due to the problem of contacts heating and the limitation of the applied current. In order to avoid that, one must apply only a very small current.…”