Abstract
Development of ultrafast lasers and nonlinear optical techniques over the last two decades provides tools to access real-time dynamics of low energy excitations in superconductors. For example, time-resolved THz spectroscopy and time- and angular-resolved photoemission spectroscopy provide access to the real-time dynamics of the superconducting gap amplitude. Such studies enable determination of microscopic parameters like quasi-particle recombination rates, pair-breaking rates and electron–boson coupling constants. Recently, intense THz pulses have been used to probe the nonlinear dynamics, including observation of collective modes. Moreover, using low-frequency electromagnetic pulses, there are several reports of amplification of superconductivity in both conventional and unconventional superconductors. Starting with a brief historical overview of the pioneering work, where non-equilibrium phenomena in superconductors were investigated using quasi-continuous excitation, we review some of the insights that are provided by using real-time approaches. We focus on conventional BCS superconductors, whose ground state is reasonably well understood, and address similarities and open questions related to the corresponding studies in high-{T}_{c} superconductors.
Highlights
The field of non-equilibrium superconductivity [1, 2] started soon after the first observation of the superconducting gap using optical spectroscopy [3] and the development of the BCS theory
In the last two decades or so, the time-resolved techniques, with their ability to probe the evolution of optical constants, electronic band structure, magnetization and crystal structure with femtosecond time resolution contributed to further our understanding of out-of-equilibrium superconductors
As addressed above, when at kBT
Summary
The field of non-equilibrium superconductivity [1, 2] started soon after the first observation of the superconducting gap using optical spectroscopy [3] and the development of the BCS theory. Given that small energy gap Δ on the order of a meV (depending on the material), superconductors were first considered as potential detectors for far-infrared radiation [4]. While the first real-time experiments with picosecond time resolution were performed on conventional superconductor Pb [9], the field started to blossom following the early studies on cuprate superconductors [10–18]. The first systematic real-time studies of dynamics in conventional superconductors were performed substantially later [19–22] Such studies still represent a small fraction of the body of work performed on superconductors, still largely dominated by studies of cuprate and pnictide high-temperature superconductors. For a review, focusing on ultrafast spectroscopy in correlated high-temperature superconductors, we refer the interested reader to a recent review [23]
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