Lightwave-driven gapless superconductivity and forbidden quantum beats by terahertz symmetry breaking

Abstract

Light-induced supercurrents chart a path forward for the electromagnetic design of emergent materials phases and collective modes for quantum engineering applications. However, controlled spatial–temporal modulation of the complex order parameter characterizing such non-equilibrium macroscopic quantum states remains elusive. Such ultrafast phase-amplitude modulation can manifest via high harmonic modes beyond those allowed by equilibrium symmetries. Here, we drive moving condensate states via subcycle dynamical symmetry breaking achieved with nonlinear oscillating terahertz photocurrents. These non-equilibrium macroscopic quantum states with broken inversion symmetry are controlled via Cooper pair acceleration by asymmetric and multi-cycle terahertz photoexcitations. The observed supercurrent-carrying states evolve during a lightwave cycle and exhibit three distinguishing features: Anderson pseudo-spin precessions forbidden by equilibrium symmetry, strong high harmonic coherent oscillations assisted by pairing and long-lived gapless superfluidity with minimal condensate quench. Lightwave tuning of persistent photocurrents can be extended for quantum control of unconventional superconductors and topological matter, with implications on quantum gate and sensing functionalities.

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Fig. 1: Pseudo-spin coherent oscillations forbidden by equilibrium symmetry and strong HH generation nonlinearities.
Fig. 2: Lightwave-driven collective modes with different THz nonlinear and asymmetric driving.
Fig. 3: Light-driven gapless superconductivity by non-thermal control of the supercurrent-carrying quantum states.
Fig. 4: Gauge-invariant quantum kinetic calculation of the density matrix for the periodically driven, supercurrent-carrying macroscopic state.

Data availability

The data that support the plots within this paper and other findings of this study are available from the corresponding author upon reasonable request.

Change history

  • 12 July 2019

    An amendment to this paper has been published and can be accessed via a link at the top of the paper.

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Acknowledgements

Work at Iowa State University was supported by the Army Research Office under award no. W911NF-15-1-0135 (THz quantum spectroscopy). Work at the University of Wisconsin was supported by funding from the Department of Energy Office of Basic Energy Sciences under award no. DE-FG02-06ER46327 (structural and electrical characterizations) and Department of Energy grant no. DE-SC100387-020 (thin-film synthesis). The theory work at the University of Alabama, Birmingham was supported by the US Department of Energy, Office of Science, Basic Energy Sciences under award no. DE-SC0019137. The THz spectroscopy instrument was supported in part by the W.M. Keck Foundation.

Author information

X.Y., C.V. and J.W. performed the experimental measurements, collected the data and analysed the results. C.S., J.H.K. and C.B.E. designed, fabricated and characterized the thin film heterostructures. M.M. and I.E.P. developed the theory and performed the quantum kinetic calculations. J.W. supervised the study, and wrote the paper together with I.E.P. and with help from all authors.

Correspondence to J. Wang.

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