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Terahertz-light quantum tuning of a metastable emergent phase hidden by superconductivity

Abstract

‘Sudden’ quantum quench and prethermalization have become a cross-cutting theme for discovering emergent states of matter1,2,3,4. Yet this remains challenging in electron matter5,6,7,8,9, especially superconductors10,11,12,13,14. The grand question of what is hidden underneath superconductivity (SC)15 appears universal, but poorly understood. Here we reveal a long-lived gapless quantum phase of prethermalized quasiparticles (QPs) after a single-cycle terahertz (THz) quench of a Nb3Sn SC gap. Its conductivity spectra is characterized by a sharp coherent peak and a vanishing scattering rate that decreases almost linearly towards zero frequency, which is most pronounced around the full depletion of the condensate and absent for a high-frequency pump. Above a critical pump threshold, such a QP phase with coherent transport and memory persists as an unusual prethermalization plateau, without relaxation to normal and SC thermal states for an order of magnitude longer than the QP recombination and thermalization times. Switching to this metastable ‘quantum QP fluid’ signals non-thermal quench of coupled SC and charge-density-wave (CDW)-like orders and hints quantum control beneath the SC.

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Fig. 1: Single-cycle THz quantum quench and phase transition in a Nb3Sn A15 superconductor.
Fig. 2: The distinct spectral features of the gapless quantum state differ from both normal metallic states and thermal behaviours.
Fig. 3: The persisting prethermalized plateau state with non-thermal characteristics and a long-lived memory.
Fig. 4: Predictions of the theoretical model for the hidden gapless quantum state with extraordinary conductivity.

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Acknowledgements

Work at Iowa State University was supported by the Army Research Office under award W911NF-15-1-0135 (THz quantum quench spectroscopy). Work at the University of Wisconsin was supported by funding from the DOE Office of Basic Energy Sciences under award number DE-FG02-06ER46327 (structural and electrical characterizations) and DOE Grant no. DE-SC100387-020 (sample growth). Work at the University of Alabama at Birmingham was supported by start-up funds. The THz instrument was supported in part by the M. W. Keck Foundation (J.W.).

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X.Y. and C.V. performed the THz pump–probe spectroscopy measurements and collected the data. C.S., J.H.K. and C.B.E. grew the samples and performed structural and electrical characterizations. M.M. and I.E.P. developed the theory for the hidden phase calculations. J.W., X.Y., C.V. and L.L. analysed the results with the help of P.P.O. and P.G. The paper was written by J.W. and I.E.P., with discussions from all the authors. J.W. conceived and supervised the project.

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Correspondence to J. Wang.

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Supplementary Information

Supplementary Information, 21 pages, Supplementary Figures 1–3, Supplementary References 1–35

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Yang, X., Vaswani, C., Sundahl, C. et al. Terahertz-light quantum tuning of a metastable emergent phase hidden by superconductivity. Nature Mater 17, 586–591 (2018). https://doi.org/10.1038/s41563-018-0096-3

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