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...
Quasi‐elastic light scattering spectroscopy intensity–intensity autocorrelation functions [S(k,t)] and static light scattering intensities of 1 MDa hydroxypropylcellulose in aqueous solutions were measured. With increasing polymer concentration, over a narrow concentration range, S(k,t) gained a slow relaxation. The transition concentration for the appearance of the slow mode (ct) was also the transition concentration for the solution‐like/melt‐like rheological transition (c+) at which the solution shear viscosity [ηp(c)] passed over from a stretched exponential to a power‐law concentration dependence. To a good approximation, we found ct[η] ≈ c+[η] ≈ 4, [η] being the intrinsic viscosity. The appearance of the slow mode did not change the light scattering intensity (I): from a concentration lower than ct to a concentration greater than ct, I/c fell uniformly with increasing concentration. The slow mode thus did not arise from the formation of compact aggregates of polymer chains. If the polymer slow mode arose from long‐lived structures that were not concentration fluctuations, the structures involved much of the dissolved polymer. At 25 °C, the mean relaxation rate of the slow mode approximately matched the relaxation rate for the diffusion of 0.2‐μm‐diameter optical probes observed with the same scattering vector. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 323–333, 2005
The vortex matter in bulk type II superconductors serves as a prototype system for studying the random pinning problem in condensed matter physics. Since the vortex lattice is embedded in an atomic lattice, small‐angle neutron scattering (SANS) is the only technique that allows for direct structural studies. In traditional SANS methods, the scattering intensity is a measure of the structure factor averaged over the entire sample. Recent studies in vortex physics have shown that it is highly desirable to develop a SANS technique that is capable of resolving the spatial inhomogeneities in the bulk vortex state. This article reports a novel slicing neutron diffraction technique using atypical collimation and an areal detector, which allows for observing the three‐dimensional disorder of the vortex matter inside an as‐grown Nb single crystal.
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