Abstract-Competing Orders and Ultrafast Energy Transfer at the Quantum Limit in a Nb3Sn Superconductor Probed by Terahertz Electrodynamics

X. Yang, X. Zhao, C. Vaswani, C. Sundahl, Y. Yao, M. Mootz, P. P. Orth, J. H. Kang, I. E. Perakis, C-Z Wang, K-M Ho, C. B. Eom, J. Wang

https://arxiv.org/abs/1808.06731

We report the low-energy electrodynamics of a moderately clean A15 superconductor (SC) following ultrafast excitation to understand and manipulate terahertz (THz) quasi--particle (QP) transport by tuning pump photoexcitation of from competing orders. Using 35-fs optical pulses, we observe a non-thermal enhancement in the low frequency conductivity, opposite to that observed for THz pump, which persists up to an additional critical temperature, above the SC one, from an electronic order in the Martensitic normal state. In the SC state, the fluence dependence of pair breaking kinetics together with an analytic model provides evidence for a `one photon-to-one pair' non-resonant energy transfer during the laser pulse. Such initial transfer of photon energy ℏω to QPs at the {\em quantum} limit, set by 2ΔSC/ℏω=0.33%, is more than one order of magnitude smaller than in previously studied BCS SCs, which we attribute to strong electron--phonon coupling and possible influence of phonon condensation in A15 SCs.