Transport and thermodynamic properties of iron-based superconductors

Iron-based superconductors (FeSCs) are a novel family of high-temperature superconductors. The rich phase diagrams exhibited by these compounds under various doping regimes, their multi-band electronic structure, the high superconducting critical temperature with exotic realization of order parameters, all contribute to this system being of considerable theoretical interest.;In this Thesis, we report on our past work aimed at addressing possible signatures of the exotic superconducting (SC) order parameters (OPs), the coexistence with spin-density wave (SDW) phase, and fluctuations effects, as reflected in various transport or thermodynamic properties of these materials.;We present a theoretical description of the differential conductance of point contacts between a normal metal and a multi-band superconductor with s+/--wave symmetry. We demonstrate that the interband impurity scattering broadens the coherent peak near the superconducting gap and significantly reduces its height even at relatively low scattering rates for an extended s+/--wave gap. Our mean-field treatment of the zero-temperature London penetration depth of a clean multi-band superconductor in the case when both SC and SDW orders coexist shows that the supefluid density closely follows the evolution of the superconducting order parameter as doping is increased, saturating to a BCS value in the pure superconducting state. Furthermore, a strong anisotropic in-pane penetration depth is shown to be induced by the SDW order.;Beyond mean-field level, however, the monotonic behavior of both penetration depth, and specific heat jump is modified into a sharp peak near the tetracritical point - a point of intersection of four phase transition lines. We show that in the case of specific heat jump, this effect originates from thermal fluctuations of the SDW OP near the tetracritical point. Thermal fluctuations result in a power-law dependence of the specific heat jump fluctuation correction that is stronger than the contribution of mass renormalization due to quantum fluctuations of SDW in the vicinity of the putative critical point beneath the superconducting dome.