Abstract-Terahertz Second-Harmonic Generation from Lightwave Acceleration of Symmetry-Breaking Nonlinear Supercurrents

C. Vaswani, M. Mootz, C. Sundahl, D. H. Mudiyanselage, J. H. Kang, X. Yang, D. Cheng, C. Huang, R. H. J. Kim, Z. Liu, L. Luo, I. E. Perakis, C. B. Eom, and J. Wang

https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.124.207003

We report terahertz (THz) light-induced second harmonic generation, in superconductors with inversion symmetry that forbid even-order nonlinearities. The THz second harmonic emission vanishes above the superconductor critical temperature and arises from precession of twisted Anderson pseudospins at a multicycle, THz driving frequency that is not allowed by equilibrium symmetry. We explain the microscopic physics by a dynamical symmetry breaking principle at sub-THz-cycle by using quantum kinetic modeling of the interplay between strong THz-lightwave nonlinearity and pulse propagation. The resulting nonzero integrated pulse area inside the superconductor leads to light-induced nonlinear supercurrents due to subcycle Cooper pair acceleration, in contrast to dc-biased superconductors, which can be controlled by the band structure and THz driving field below the superconducting gap.

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

X. Yang, C. Vaswani, C. Sundahl, M. Mootz, L. Luo, J. H. Kang, I. E. Perakis, C. B. Eom,  J. Wang, 

Fig. 1: Pseudo-spin coherent oscillations forbidden by equilibrium symmetry and strong HH generation nonlinearities.

https://www.nature.com/articles/s41566-019-0470-y

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.

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

X.Yang, C. Vaswani, C. Sundahl, M. Mootz, P. Gagel, L. Luo, J. H. Kang, P. P. Orth, I. E. Perakis, C. B. Eom, J. Wang,

https://www.nature.com/articles/s41563-018-0096-3

Sudden’ quantum quench and prethermalization have become a cross-cutting theme for discovering emergent states of matter. Yet this remains challenging in electron matter, especially superconductors. The grand question of what is hidden underneath superconductivity (SC) 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.