Study of the energy gap structure in iron-based superconductors using London penetration depth and controlled artificial disorder

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xviii CHAPTER 1. GENERAL INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . 1 1.1 Basic Overview of Iron-based Superconductors . . . . . . . . . . . . . . . . . . . . . 1 1.1.1 Families of iron-based superconductors . . . . . . . . . . . . . . . . . . . . . . 2 1.1.2 Tuning parameters and phase diagrams . . . . . . . . . . . . . . . . . . . . . 2 1.2 Iron-based Superconductors: Energy Gap Structure and Symmetry . . . . . . . . . . 7 1.2.1 Iron-based superconductor a multiband system . . . . . . . . . . . . . . . . 9 1.2.2 Derivation of s-wave and d -wave gaps in a multiband superconductor . . . . 13 1.2.3 Brief overview of experimental methods to probe the energy gap structure and symmetry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 1.3 Effects of Disorder on Superconducting Critical Temperature and Low Temperature London Penetration Depth for Different Gap Structures. . . . . . . . . . . . . . . . . 23 1.3.1 Basic overview of different types of disorder . . . . . . . . . . . . . . . . . . . 23 1.3.2 Effects of non-magnetic, point-like disorder on superconducting critical temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 1.3.3 Effects of non-magnetic disorder on the energy gap structure . . . . . . . . . 31 1.4 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32 CHAPTER 2. EXPERIMENTAL DETERMINATION OF THE SUPERCONDUCTING GAP STRUCTURE: LONDON PENETRATION DEPTH AND DISORDER . . . . . . . 38 2.1 London Penetration Depth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38 2.2 Measurement of London Penetration Depth: Tunnel-diode Resonator Method . . . . 42 2.2.1 Basic properties of a tunnel-diode circuit . . . . . . . . . . . . . . . . . . . . 42 2.2.2 Calibration of penetration depth from TDR frequency measurement . . . . . 46 2.2.3 Screening of samples using the TDR dipper probe . . . . . . . . . . . . . . . 50 2.3 Electron Irradiation to Introduce Point-like Disorder . . . . . . . . . . . . . . . . . . 51 2.4 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54