Light-induced new collective modes in the superconductor ${\mathrm{La}}_{1.905}{\mathrm{Ba}}_{0.095}{\mathrm{CuO}}_4$

We report near and midinfrared pump, $c$-axis terahertz probe measurements on a superconducting single crystal ${\mathrm{La}}_{1.905}{\mathrm{Ba}}_{0.095}{\mathrm{CuO}}_4$ with $T_c=32$ K. The measurements reveal that the pump-induced change occurs predominantly at the Josephson plasma edge position below $T_c$. Upon excitation by strong near-infrared pulses, the superconducting state is severely disturbed and incoherent quasiparticle excitations develop in the frequency regime above the static plasma edge. However, within a very short time delay $(\sim 1.5$ ps) we observe the reappearance of a very sharp Josephson plasma edge at a frequency lower than the static Josephson plasma edge and the emergence of a new light-induced edge at a higher energy. The results imply that the light can induce new Josephson plasmon modes with different coupling strengths. A similar but weaker effect is observed for the midinfrared pump. No pump-induced effect is detected above $T_c$.

Figure 1

Crystal structure and equilibrium optical spectra. (a) Both the pump and THz probe electric fields are polarized to the $c$ axis of the sample. (b) The FIR reflectance spectra measured by FTIR at two selective temperatures 35 and 6 K above and below $T_c$, respectively. The reflectance extension to lower energy (dot curves) is achieved by the time-domain THz measurement. (c) and (d) Static THz spectra in the time domain $E\left(t\right)$ and their Fourier-transformed spectra in the frequency domain $E\left(\omega \right)$ at 35 and 6 K. The dashed line indicates the plasma edge dip position.

Figure 2

Pump-induced changes by $1.28\text{−}\mu \text{m}$ excitations at 6 K. (a) The relative electric field change at the THz peak position as a function of time delay after excitation. (b) Inset: The pump-induced relative change $\mathrm{\Delta }E\left(t\right)/E_{\text{peak}}$ in the time domain at $\tau =0$ ps. Main panel: The Fourier-transformed spectrum of $\mathrm{\Delta }E\left(t\right)$. (c) Frequency domain THz spectrum before and after excitation at $\tau =0$ ps. (d) The reflectivity spectra before and after excitation at $\tau =0$ ps. The inset shows the ratio of the reflectivity change relative to the static values. The penetration depth mismatch is not considered here.

Figure 3

Calculated optical constants from a multilayer model at different time delays after 1.28-$\mu \mathrm{m}$ excitations. (a)–(d) show the reflectance spectral changes at several representative time delays. (e)–(h) show the corresponding energy loss function spectra. (i)–(l) show the real part of conductivity spectra after excitations. (m)–(o) are intensity plots of the spectral evolution of $R\left(\omega \right),\text{Im}[-1/\varepsilon (\omega \left)\right]$, and ${\sigma }_1\left(\omega \right)$ as a function of time delay.

Figure 4

Pump-induced changes by 15-$\mu \mathrm{m}$ excitations at 6 K. (a) The relative electric field change at the THz peak position as a function of time delay after excitation. (b) The pump-induced relative change of $\mathrm{\Delta }E\left(t\right)/E_{\text{peak}}$ in the time domain (inset) and the Fourier-transformed $\mathrm{\Delta }E(\omega ,\tau )$ in the frequency domain (main panel) at $\tau =0$ ps. (c) and (d) Calculated reflectivity and real part of conductivity from a multilayer model at two time delays $\tau =0$ and 30 ps and their comparisons with static spectra.