Non-equilibrium superconductivity in THz-driven two-band superconductor MgB2

Abstract

The phenomenon of superconductivity has attracted the attention of both the fundamental science community and industry and technology for more than a century. Effects related to the macroscopic manifestation of quantum mechanical order give rise to a better understanding of the processes occurring in the micro-world and lead to new opportunities for applied science such as, for example, creation of the Superconducting QUantum Interference Device [1] – SQUID – allowing for measuring very low magnetic fields, based on Josephson effect. Superconducting electromagnets create extremely high magnetic field and are used in magnetic resonance imaging (MRI) and nuclear magnetic resonance (NMR) machines, fusion reactors such as tokamaks etc. The experimental studies of superconductivity in the non-equilibrium started soon after the observation of the superconducting energy gap in optical spectroscopy [2] and the development of the microscopic theory of superconductivity by Bardeen, Cooper and Schrieffer [3]. The first experiments were performed in the quasi-continuous excitation regime, without direct access to the dynamics of superconductors [4]. Later on, with the advent of femtosecond laser technology, time-resolved experiments were utilized to investigate the evolution of the optical properties of superconductors on an ultrafast timescale [5]. This thesis presents the results of the studies on the two-band superconductor MgB2 utilizing time-resolved spectroscopy in the Terahertz (THz) range of electromagnetic radiation. We conducted systematic experiments to obtain the dynamics of the superconducting order in the two-band superconductor MgB2 as a function of temperature, excitation energy density with the resolution in time- and frequency-domain using different excitation and probing methods. Our results show that the initial Cooper-pair breaking dynamics driven by the resonant THz excitation exhibits unexpected slowing down on the tens-of-picoseconds timescale implying strongly non-thermal distribution of the photoexcited quasiparticles. The initial suppression of superconductivity is followed by the recovery of the superconducting state, and the recovery dynamics exhibit temperature and absorbed energy density dependence of the recovery time, both for the case of the resonant excitation with intense THz pulses and the off-resonant excitation with near infrared (NIR) laser pulses. The most non-trivial observation is the temperature dependence of the relative photoinduced gap suppression, which experiences a dip around 0.6·Tc instead of a monotonous in- crease following the quasi-thermal suppression of the gap. This observation suggests the existence of a process competing with the photoinduced gap suppression, leading to a dynamic "cooling" of a superconductor. Such a process can be a consequence of a non-thermal QP distribution considered by Eliashberg in the 1970’s [6]. Compared to the enhancement of superconductivity with microwave radiation, where an increase in Tc of ≈ 1 % was demonstrated, we observe a decrease in the pair-breaking efficiency by a factor of ≈ 2. The spectrally-resolved conductivity dynamics shows that the technique is sensitive to the spectral features associated with the superconducting gap and can provide rich information about the non-equilibrium optical properties of the superconductor and help to better under- stand the complex ground state of the multi-band superconductivity. Detailed interpretation of the obtained results, however, requires further theoretical analysis of both equilibrium and non-equilibrium electrodynamics of multi-band superconductors.