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

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).

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