Experimental signature of transient symmetry breaking in a cavity superconductor

Abstract

Transient states of matter far from equilibrium may exhibit physical properties beyond those allowed by the equilibrium-state crystalline symmetries. We explore ultrafast and direct electronic excitations of transient states in a cavity superconductor by using time-resolved terahertz-pump terahertz-probe spectroscopy. Our results show that the strong terahertz field can transiently modify the symmetries of the electronic subsystems via the injection of a transient supercurrent, leading to high-order nonlinear dynamical responses that are not compatible with the equilibrium-state symmetries, which evidences for transient symmetry breaking on the picosecond time scale. Our study also finds that the strong coupling of the superconductor to the designed microcavities enables the sensitive detection of the nonlinear responses associated to the transient symmetry breaking.

Figures (4)

• Figure 1: (a) 0.4 THz pump-induced changes $\delta E_{\text{probe}}$ of the transmitted probe electric field as a function of pump-probe delay $t_{\text{pp}}$ at the time $t_{\text{gate}}=3$ and 4 ps corresponding to the terahertz probe pulse as marked by the triangles in (d). (b) The frequency-domain spectra corresponding to the traces in (a) exhibit maxima at the fundamental, second, and third harmonics of the pump pulse, as marked by $1f$, $2f$, and $3f$, respectively. (c) Time-domain profile of the 0.4 THz pump pulse. (d) Time-domain traces of the transmitted probe pulses measured at 4 K for pump-probe time-delays $t_{\text{pp}}=0$ and 3 ps. (e) Schematic illustration of the pump-probe experiment on a cavity superconductor sample. (f) Design and dimensions of one cavity unit based on gold microstructure and NbN superconductor thin film.
• Figure 2: Contour plot of $f=0.4$ THz pump pulse induced temporal changes $\delta E_{\text{probe}}$ of the transmitted probe electric field as a function of pump-probe delay $t_{\text{pp}}$ measured at (a) 4 K, (b) 10 K, and (c) 14 K. (d) Temperature dependence of the pump-field induced transient changes $\delta E_{\text{probe}}$ at $t_\text{gate}=3$ ps [Fig. \ref{['Fig_0.4THz4K']}(d)] and (e) the corresponding Fourier power spectra measured at various temperatures above and below $T_c$. (f) Temperature dependence of the pump-field induced transient changes $\delta E_{\text{probe}}$ at $t_\text{gate}=4$ ps [Fig. \ref{['Fig_0.4THz4K']}(d)] and (g) the corresponding Fourier power spectra measured at various temperatures above and below $T_c = 13.5$ K.
• Figure 3: (a) Contour plot of $f=0.5$ THz pump-induced temporal changes $\delta E_{\text{probe}}$ of the transmitted probe electric field as a function of pump-probe delay $t_{\text{pp}}$ measured at 4 K. (b) The corresponding nonlinear temporal modulation at $t_\text{gate}=4$ ps and (c) its Fourier power spectrum. (d) Contour plot of $f=0.5$ THz pump-induced spectral changes of the transmission $\delta T_{\text{probe}}$ as a function of pump-probe delay $t_{\text{pp}}$ measured at 4 K. (e) The corresponding nonlinear spectral modulation at $0.5$ THz and (f) its Fourier power spectrum.
• Figure 4: Fourier power spectra of pump-induced modulation corresponding to the probe frequencies of (a) 0.29, (b) 0.40, (c) 0.50, and (d) 0.59 THz, respectively, for the pump frequency of $f=0.4$ THz, measured at various temperatures, indicating resonantly enhanced spectral detection around the probe frequency of 0.5 THz.