Abstract-Ultrafast nonthermal terahertz electrodynamics and possible quantum energy transfer in the Nb 3 Sn superconductor

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

https://journals.aps.org/prb/abstract/10.1103/PhysRevB.99.094504

We report terahertz (THz) electrodynamics of a moderately clean A15 superconductor (SC) following ultrafast excitation to manipulate quasiparticle (QP) transport. In the Martensitic normal state, we observe a photo enhancement in the THz conductivity using optical pulses, while the opposite is observed for the THz pump. This demonstrates wavelength-selective nonthermal control of conductivity distinct from sample heating. The photo enhancement persists up to an additional critical temperature, above the SC one, from a competing electronic order. In the SC state, the fluence dependence of pair-breaking kinetics together with an analytic model provides an implication for a “one photon to one Cooper pair” nonresonant energy transfer during the 35-fs laser pulse; i.e., the fitted photon energy $\hslash {\omega }^{}$absorption to create QPs set by $2{\Delta }_{SC}/\hslash \omega =0.33\%$. This 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.

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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.

Abstract-Terahertz electrodynamics and superconducting energy gap of NbN

Kyung Ik Sim, Young Chan Jo, Taewoo Ha, Jong Hyeon Kim, Jae Hoon Kim, Hirotake Yamamori

https://link.springer.com/article/10.3938%2Fjkps.71.571

We have measured the transmission spectra of the conventional Bardeen-Cooper-Schrieffer (BCS) superconductor niobium nitride (NbN) thin films (Tc = 11 K) using terahertz time-domain spectroscopy (THz-TDS) over the spectral range of 10 - 110 cm −1 and the temperature range of 3.9 - 295 K. We extracted both the real part, σ1, and the imaginary part, σ2, of the optical conductivity, σ̃ = σ1 + iσ2, independently and simultaneously, without a Kramers-Kronig analysis. The superconducting gap Δ(T) was observed in the real part of the conductivity, σ1, below Tc = 11 K with a maximum value of 2Δ(0) = 30 cm −1 and the gap ratio 2Δ(0)/kBTc = 3.92.