Superconductivity in iron-based superconductors

Iron based superconductors(FeSC) are a new class of high temperature superconductors with very intriguing properties. These materials cannot be explained using the 'conventional' logic of the 'conventional' superconductors, and is also different from the Cuprates-the other popular class of high temperature superconductors. A complete description of the superconducting state in these materials requires a thorough understanding of its superconducting order parameter and the mechanism that leads to superconductivity-both of which are unsettled issues.;In this thesis, we attempt to tackle some aspects of these issues. We first discuss, keeping the wisdom of Fermi-liquid theory in mind, the criteria for the superconducting instability in FeSC which is a lattice based system. Superconductivity in lattice based systems is different from well known BCS superconductivity. We make the point that the presence of electron and hole like carriers are crucial for the manifestations of such properties in the FeSCs.;We then present a prescription to analyze the symmetries and structure of the superconducting order parameter (the gap) in generic lattice based systems where only the interaction amongst fermions close to the Fermi surface is important. We demonstrate the effectiveness of this prescription by applying it to the case of FeSCs where we study the evolution of the gap with injecting of carriers (of both hole and electron like). This prescription avoids use of heavy numerical studies and still gives results in excellent agreement with numerical and experimental studies.;Elaborating more on the intriguing nature of FeSCs, we also point to the possibility of a new time reversal symmetry breaking s+is state that is unique to systems like these (due to presence of multiple Fermi pockets of the carriers) and discuss its experimental consequences.