Structural evidence for an edge-contaminated vortex phase in a Nb crystal using neutron diffraction

Abstract

We report the first direct observation of a disordered vortex matter phase existing near the edge of a bulk type-II superconductor Nb using a novel position-sensitive neutron diffraction technique. This "edge-contaminated" vortex state was implicated in previous studies using transport techniques and was postulated to have played a significant role in the behavior of vortex dynamics in a wide range of type-II superconductors. It is found that upon thermal annealing, the vortex matter in the bulk undergoes reordering, suggesting that the edge-contaminated bulk vortex state is metastable. The edge vortex state remains disordered after repeated thermal annealing, indicating spatial coexistence of a vortex glass with a Bragg glass. This observation resolves many outstanding issues concerning the peak effect in type-II superconductors. PACS numbers: 74.25.UvVortex matter (VM) in type-II superconductors continues to be a subject of fascination. A longstanding issue is the nature of the ground state of the vortex lines in the presence of atomic impurities and other forms of quenched disorder acting as random pinning centers. Early calculations[1, 2] and scaling arguments [3] suggested the absence of long-range order even for weak random pinning. Thus an ordered VM phase was not expected in any real type-II superconductors contrary to the neutron diffraction experiments in which sharp Bragg peaks were observed in the VM phase of Nb [4,5]. A possible reconcilliation has since been proposed in the Bragg glass model [6,7] which predicts that the vortex lines form a topologically ordered lattice with quasi-longrange-order (QLRO) characterized by a power-law structure factor [8,9]. However, the latest theoretical treatments seem to again keep the issue in the open [10][11][12] and further experimental progress is urgently needed.The existence of QLRO implies that there must be a true order-disorder phase transition in the VM. An outstanding question is how this putative phase transition is related to the ubiquitous peak effect [13][14][15][16][17][18][19][20][21][22][23][24][25][26][27], an intriguing phenomenon that when disorder is reduced in sample preparation, a type-II superconductor often exhibits a sharp minimum in the resistance versus temperature (T ), or a peak in the critical current versus T curves[13]. The peak effect has attracted attention [28,29] since it was first observed in Nb [13,14], but remains poorly understood. There is clear evidence for a first-order melting transition at the peak effect from magnetization [19,20], neutron diffraction [21], and heat capacity[26] measurements. It is also known that the peak effect can disappear at high or low magnetic fields [25,30,31], and some samples display neither a peak effect nor any sign of a phase transition [32,33]. This "lack-of-universality" appears to be related to another puzzle in the VM physics: in some samples, the zero-fieldcooled (ZFC) VM state is ordered, but the field-cooled (FC) state is disordered, indicative of supercooling at a first-order tra...

Magnetic small-angle neutron scattering

“…In addition to a transport current or an AC field, an ordering of the VL can in some cases be achieved by a simple thermal cycling (Daniilidis et al, 2007;Marziali Bermudez et al, 2017). Conversely, the application of a field oscillation will introduce an edge "contamination" of the VL at surfaces where vortices enter and leave the sample (Hanson et al, 2011). While FIG.…”

Evidence for Cooper pair diffraction on the vortex lattice of superconducting niobium

“…[17]). A weak pinning 17 excludes also an influence of the peak effect [32][33][34] on the measured field distributions. The BCS-Gor'kov model breaks down for T ≤ 0.5 K as shown by the dasheddotted lines in Fig.…”

Dynamic Reorganization of Vortex Matter into Partially Disordered Lattices

“…Eventually, vortex entanglement and the proliferation of VL dislocations increase the effective pinning, producing a sudden rise of the critical current J c known as the Peak Effect (PE) [7,8], which is the fingerprint of the ODT in vortex matter. While both the highpinning disordered phase and the low-pinning ordered phase, above and below the ODT, are widely accepted in the literature, intermediate responses have been ascribed to surface contamination and partial reordering by the probing transport current [9,10,13,14]. However, in the narrow transitional region adjacent to the PE, for which a multidomain phase has been theoretically proposed [15], non-invasive techniques have shown that pinning can be partially decreased or even increased by applying dc currents [16,17] To address this question, we performed an experiment in a clean NbSe 2 single crystal, combining small angle neutron scattering (SANS) with in-situ linear ac susceptibility measurements.…”