Dissipative and fluctuating effects in Ba(Fe1-x Rhx)2As2 iron-based superconductors

Abstract

This thesis addresses dissipative and fluctuating mechanisms, taking place both in the superconducting, and in the normal phase of the Rh-doped family of 122 iron-based superconductors. These novel materials have been holding the stage of superconductivity, and renewing the interest in high temperature superconductors, for the last six years. Iron-pnictides are extremely type-II superconductors, with a Ginzurg-Landau parameter k-100, and extremely high upper critical fields, that can reach up to 100 T! Moreover, the lower critical field Hc1 is of the order of few mT, so there is a very broad range of magnetic fields in which the behavior of the vortex matter can vary significantly. Nuclear Magnetic Resonance (NMR) is the more widely employed experimental tool in this thesis, given its microscopic nature, and the sensitivity to kHz-MHz fluctuations, that well match the vortices dynamic timescale. However, not many NMR studies concerning the dynamic in the iron-pnictides of the vortex lattice have been carried out, so far. One possible reason may be that the vortex dynamic is strongly related to technological applications, whereas NMR usually probes the equilibrium state. Even though from a technological point of view this regime is not very appealing, it can anyhow help to grasp some information about the nature of the solid vortex phase. The thesis is organized as follows: Part I includes the first two chapters. The first chapter presents an overview of structural, electronic and magnetic properties of the materials. A whole section reports the main NMR results obtained on 122 iron-pnictides, up to now. The second chapter presents the basic aspects of NMR, which are needed to understand the experimental results. Part II includes chapters 3-7. The third chapter discusses the onset of unconventional fluctuative phenomena of the order parameter, taking place above Tc. Such issues were studied by SQUID magnetometry. Chapter 4 debates an NMR study of the anomalous enhancement of the spin-spin decay rate, in the normal state. The phenomenology of these systems becomes even richer, below the transition temperature, where the vortex lattice dynamic, in equilibrium condition (Chapter 5), and out-of-equilibrium (Chapter 6) can be investigated by NMR. Finally, chapter 7 deals with novel methodologies to study the vortex lattice melting transition. Part III is devoted to the appendices, which describe both the experimental equipment, and some theoretical background.