Terahertz and infrared spectroscopy of novel superconductors

The present thesis is devoted to the investigation of novel superconductors by phase-sensitive terahertz transmission and infrared to ultraviolet spectroscopy. In particular, a nominally undoped Pr2CuO4 superconducting thin film, an FeTe0.5Se0.5 thin film, and a LiFeAs single crystal have been investigated. The emphasis is on the low-frequency part of the optical spectrum (i.e., the terahertz and infrared spectrum), as the goal of the study was to shed light on the size and symmetry of the superconducting gaps and also to determine the temperature dependences as well as the absolute values of the penetration depth, which are key input parameters for models applicable for new superconductors. In addition, niobium has been investigated as a reference, so as to see what is expected from conventional superconductors and to clarify the electrodynamics of niobium. A superconducting Nb thin film with Tc of 8.04 K has been investigated by backward wave oscillator-based (BWO-based) and time-domain terahertz (TDT) spectrometers in the frequency range between 4 and 100 cm−1 for temperatures ranging from 2 to 10 K. From these measurements an energy gap of 22.50 cm−1 = 2.79 meV = 4.02kBTc have been determined. The optical conductivity below Tc could nicely be described by calculations according to the Eliashberg theory, with the electron-phonon interaction evaluated from tunneling measurements. Absolute values of the penetration depth have been calculated from phase-sensitive terahertz measurements. The zero-temperature limit of at T = 0 is found to be 115 ± 5 nm. From this value, a London penetration depth of 43 ± 2 nm has been obtained. The overall temperature dependence of the penetration depth follows a behavior typical for conventional s-wave superconductors. A superconducting Pr2CuO4 film with T0 structure and Tc of 27 K has been investigated by use of optical methods in a wide frequency (5 – 55000 cm−1) and temperature (2 – 300 K) range. A Drude-like peak centered at zero