Ultrasonic attenuation, AC magnetic susceptibility and small-angle neutron scattering studies of vortex states in single-crystal niobium and vanadium-titanium alloys

We present recent experimental results on vortex structure and dynamics in low-Tc type-II superconductors (single-crystal niobium and V-21at.%Ti binary alloy), using ultrasonic attenuation, ac magnetic susceptibility, and small-angle neutron scattering.;Vortex lines in type-II superconductors form an ideal lattice in the absence of thermal fluctuations and defects in the atomic lattice. However, in real systems the magnetic field vs. temperature phase diagrams of type-II superconductors are determined by all of the above factors. In many weakly-pinned vortex lattices, the transition from an elastic regime with dominant intervortex interactions to a plastic regime with dominant pinning interactions, manifests itself in an anomalous increase in the critical current density in a narrow region of field and temperature right below the upper critical field, also known as the "peak effect". It was recently demonstrated via neutron scattering that the peak effect in niobium is in the neighborhood of a first-order disordering transition of the vortex lattice. It was suggested that a distinct phase exists between the peak effect line and the bulk superconducting transition.;It is quite interesting that the peak effect also exists in certain strongly-disordered systems, such as binary alloy superconductors. The mechanism behind its occurrence is not well-understood.;We verify the extrapolation scheme used in identifying the bulk superconducting transition in niobium via ultrasonic attenuation techniques with concurrent emphac-susceptibility. Measurements of the attenuation with shear waves suggest the presence of oxygen and hydrogen impurities in the same niobium sample.;The magnetic field vs. temperature phase diagram of the strongly-disordered alloy V-21at.%Ti is mapped via mutual-inductance ac-susceptibility and the presence of a peak effect verified. Neutron scattering measurements show that an ordered state exists deep in the mixed state of the superconductor. Energy Dispersive X-ray Spectroscopy shows the presence of two atomic phases, one of which (referred to as "phase-B") spans a network around regions of the sample's nominal composition (referred to as "phase-A"). It is proposed that this network could be responsible for the ordered vortex state as well as for the peak effect in V-21at.%Ti. This also opens the possibility that the peak effect in the strongly-disordered binary alloys may be physically very different from weakly-disordered vortex lattices.