Seminars Archive

Eliashberg-type superconductivity enhancement effects in BCS superconductors driven by narrow-band THz pulses

J. Demsar
Institute of Physics, Johannes Gutenberg University Mainz, Germany

Abstract
Back in the 1960s numerous groups demonstrated amplification of critical current, superconducting gap, and the critical temperature in clean-limit BCS superconductors illuminated by microwaves or ultrasound [1,2]. These counterintuitive results were explained by Eliashberg [3], demonstrating the enhancement of the superconducting order parameter in a BCS superconductor might be a result of radiation-induced non-thermal electronic distribution [3]. Under continuous irradiation, the superconductivity enhancement effects were generally weak (of the order of 1%) and limited to excitations at sub-gap frequencies in clean-limit superconductors [1,2]. Here, I present studies of gap dynamics in BCS superconductors NbN and MgB2, driven by tuned, intense, narrowband THz pulses. Systematic studies of pair-breaking efficiency as a function of temperature, excitation frequency and excitation density [4,5] provide evidence for pronounced (of the order of 1) Eliashberg-type enhancement effects in both, clean-limit (MgB2) and dirty limit (NbN) superconductors, and for both sub-gap and above-gap pumping, implying a highly nonthermal quasiparticle distribution proceeding on the 100 ps timescale.

[1] T.M. Klapwijk, J.N. van den Bergh, J.E. Mooij, J. Low Temp. Phys. 26, 385 (1977).
[2] J. Demsar, J. Low Temp. Phys. 201, 676-709 (2020) and the references therein.
[3] B.I. Ivlev, S.G. Lisitsyn, G.M. Eliashberg, J. Low Temp. Phys. 10, 449 (1973).
[4] M. Beck, et al., Phys. Rev. Lett. 110, 267003 (2013).
[5] S. Sobolev, et al., Phys. Rev. Lett. 131, 186903 (2023).