Non-equilibrium Phenomena in Superconductors Probed by Femtosecond Time-Domain Spectroscopy

Abstract

Development of ultrafast lasers and non-linear 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 non-linear 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. arXiv:2003.11503v1 [cond-mat.supr-con]

Colloquium: Nonthermal pathways to ultrafast control in quantum materials

“…Among others, coherent control of the crystal lattice by driving to anharmonic regimes has been suggested as a mechanism to induce ultrafast phase transitions, such as a lattice-controlled metal-insulator transition (Rini et al, 2007), and lead to states of matter without equilibrium counterparts, including possible light-induced superconductivity (Fausti et al, 2011;Hu et al, 2014;Kaiser et al, 2014b;Mankowsky et al, 2014). In turn, these groundbreaking experiments have stimulated considerable theoretical activity on nonequilibrium superconductivity (Babadi et al, 2017;Coulthard et al, 2017;Dasari and Eckstein, 2018;Denny et al, 2015;Kennes et al, 2017;Kim et al, 2016;Knap et al, 2016;Komnik and Thorwart, 2016;Mazza and Georges, 2017;Murakami et al, 2017;Nava et al, 2018;Okamoto et al, 2016;Raines et al, 2015;Sentef, 2017;Sentef et al, 2016Sentef et al, , 2017Wang et al, 2018a), as well as a lively debate on how we should understand and interpret optical superconducting-like signatures on transient timescales (Demsar, 2020;Zhang et al, 2020). Furthermore, coherent optical driving of the lattice has been suggested as a path towards switching applications (see Sec.…”

Robust hybridization gap in a Kondo Insulator YbB${}_{12}$ probed by femtosecond optical spectroscopy

“…We demonstrate that the (non-linear) dynamics over large range of temperatures and over three orders of magnitude in excitation density can be quantitatively described by the relaxation bottleneck model [11,12]. Quantitative analysis provides access to the microscopic electron-hole recombination rate, R, which is found to be about 3 orders of magnitude lower than in superconductors with comparable gap energy [21][22][23]. We ascribe this observation to an enhanced density of states, which is a result of the charge transfer from localized 4f states that is accompanying hybridization.…”

“…Here, studies on a series of heavy fermions [10][11][12][13][14][15][16][17] as well as on the Kondo insulator SmB 6 [11,12,18] were performed, demonstrating the relaxation of photoexcited carriers to be governed by the presence of the hybridization gap near the Fermi level [12]. The dramatic slowing down of carrier relaxation at low temperatures [10][11][12][13][14][15]17], in some cases by orders of magnitude [12], was qualitatively accounted for by the phenomenological model [12], originally developed to describe the relaxation dynamics in fully-gaped superconductors [19][20][21][22][23]. The model, given by the set of coupled non-linear rate equations, describes the recombination of photoexcited electron-hole pairs across the indirect hybridization gap via emission of large-momentum, high-frequency phonons.…”

Parametric resonance of Josephson plasma waves: A theory for optically amplified interlayer superconductivity in YBa2Cu3O6+x

“…The appearance of this additional blueshifted lower plasmon edge has been interpreted as indication of the photoinduced enhancement of the interlayer Josephson coupling. Earlier theoretical work aimed at understanding photoinduced superconductivity [10][11][12][13][14][15][16][17][18][19][20][21][22][23][24] has not been able to explain the dramatic enhancement of JP edges following excitation of the phonons. Another striking phenomenon that has been recently observed in YBa 2 Cu 3 O 6+x is the coherent excitation of Josephson plasma wave oscillations by the photoexcited phonons [1].…”