Nature of the Order-disorder Transition of the Vortex Matter in Weakly Pinned Superconductors

Peak effect in weakly pinned superconductors represents order-disorder transition of the vortex matter. It is accompanied by metastability. Experimental manifestation of the metastability is highlighted and is explained by proposing a phenomenological model, in which each metastable vortex state corresponds to a distinct critical current density J c . Existence of a unique stable state corresponding to a J c = J c st is postulated and established experimentally. It is shown that the stable state can be accessed from any metastable state by subjecting the superconductor to an oscillatory field of a small amplitude. Assuming that the stable state is the thermodynamic equilibrium state, we infer the equilibrium magnetization across the peak effect regime. We present an evidence that the order-disorder transition corresponds to a first order transition. Superconducting materials lose their resistance below a critical temperature T c . Besides the zero resistance, they also exhibit Meissner effect (total flux expulsion). Based on their magnetic behavior, superconductors are classified as type I and type II. Application of a magnetic field larger than a critical value H c restores the normal resistance of Type I superconductors even below T c . Most type I superconductors are elemental materials having a critical field not more than few hundred Oersteds. On the other hand, type II superconductors exhibit Meissner effect only below a lower critical field H c1 . Between H c1 and an upper critical field H c2 , magnetic flux penetrates the superconductor in the form of quantized vortices while the resistance remains zero. This state is called the Mixed State or Vortex State. Most of the type II superconductors are alloys and compounds having critical fields in the range of few thousand to several millions of Oersteds. Thus they are extensively used in generating high magnetic fields in particle accelerators, Nuclear Magnetic Resonance Imagers (MRIs), Tokamaks for nuclear fusion experiments and so on. The most popular superconducting materials in use for these applications Dr Gurazada Ravi Kumar is the recipient of the Homi Bhabha Science and Technology Award for the year 2004.