Studies of the vortex phase diagram of the type II superconductors YBa2Cu3O7 and 2H-NbSe2

Abstract

The work presented in this thesis is devoted to clarifying and understanding the phase diagram and the vortex-melting line of two type II superconductors: the fully oxidised YBazCugO? and 2H-NbSe2. With regard to the high temperature superconductor YBaaCuaOy, magnetisation techniques were used in order to investigate the genesis of the peak-effect from the melting transition on three pure single crystals. A phase diagram was mapped, different vortex phases were demarcated and two multicritical points on the melting line Bm(T) were identified. In particular, the unequivocal existence of the lower multicritical point was confirmed. This point of convergence of several boundary lines is located at low field («5.5T) and defines (together with the upper multicritical point) a region in which an ordered elastic solid melts in to a liquid via a first order transition. The intrinsic nature of this point was demonstrated and a possible explanation of its origin was given. The behaviour of the boundary lines, the shape of the phase diagram and the positions of both multicritical points reveal a pronounced and surprising symmetry. The other system investigated in this thesis is the low temperature superconductor 2H- NbSez. Using a 3He insert, DC transport measurements were performed down to 0.3 Kelvin. The resistance of the sample was measured as a fiinction of the magnetic field (for fixed temperatures) and of the temperature (for fixed fields). From these measurements boundary lines and details of the melting transition were extracted. Combining this data with magnetic measurements performed on a similar sample, a complete description of the phase diagram was obtained. The irreversibility line extracted by the latter measurements almost coincides with the melting region obtained with transport techniques. Surprisingly, the melting line does not approach the upper critical field Bc2(T) when reaching very low temperatures (0.3 Kelvin) as the theory predicts. Finally, one of the most interesting feature displayed by 2H-NbSe2 is a fine structure revealed in the melting-process. In particular, a very narrow jump in the melting region is observed and a possible explanation is proposed.

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