Vortex Properties from Resistive Transport Measurements on Extreme Type-II Superconductors

The nature of vortices in extreme type-II superconductors has been an ever growing field of research all since the discovery of the first highmaterials in 1986. This thesis focuses on the study of vortex phase transitions and vortex-liquid properties through resistive transport measurements on single crystals of mainly YBa Cu O AE (YBCO). Some important results are the following: (i) Location of the vortex glass transition. A study of the second-order vortex glass transition in the magnetic field–temperature ( – ) phase diagram as a function of anisotropy resulted in a simple description of the glass line, over an extended region. Here is the critical temperature, is a parameter, and . No evidence was found for any 3D–2D transition in the studied range of field and anisotropy ( T, ). (ii) Description of the vortex-liquid resistivity in glassy systems. By introducing a modified vortex-glass correlation length , depending on a mean local pinning energy and the thermal energy , an explicit description of the vortex-liquid resistivity close to the second-order phase transition was obtained both for varying and . (iii) Driven vortex dynamics in clean systems. The vortex-solid dynamics below the firstorder melting transition was found to be divided into a flux-creep region with high critical current density at low temperatures, and an almost Ohmic flux-flow regime with low . (iv) Vortex-liquid correlation and multiterminal measurements. Using various multiterminal setups, the vortex liquid in YBCO just above was shown to be rather strongly correlated along the vortex line also in untwinned crystals. The longitudinal and transversal resistivities and their mutual relation were investigated. (v) In-plane anisotropy of YBCO. A new multiterminal configuration allowed measurements of the in-plane anisotropy as a function of temperature and magnetic field direction. The measurements revealed a possible presence of nearly isotropic fluctuations, corresponding to a fluctuation anisotropy  . This value is clearly lower than that obtained from the vortex phase diagram or from resistive measurements, . The thesis starts with a short overview, followed by an introduction to vortices in highand subsequent discussions of various vortex properties. The published papers are shortly summarized and are finally appended.