Photoinduced Nonequilibrium Response in Underdoped ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{6+x}$ Probed by Time-Resolved Terahertz Spectroscopy

Intense laser pulses have recently emerged as a tool to tune between different orders in complex quantum materials. Among different light-induced phenomena, transient superconductivity far above the equilibrium transition temperature in cuprates is particularly attractive. Key to those experiments was the resonant pumping of specific phonon modes, which was believed to induce superconducting phase coherence by suppressing the competing orders or modifying the structure slightly. Here, we present a comprehensive study of photoinduced nonequilibrium response in underdoped ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{6+x}$. We find that upon photoexcitations, the Josephson plasma edge in the superconducting state is initially removed accompanied by quasiparticle excitations, and subsequently reappears at a frequency lower than the static plasma edge within a short time. In the normal state, an enhancement or weaker edgelike shape is indeed induced by pump pulses in the reflectance spectrum accompanied by simultaneous rises in both real and imaginary parts of conductivity. We compare the pump-induced effects between near- and midinfrared excitations and exclude phonon pumping as a scenario for the photoinduced effects above. We further elaborate that the transient responses in the normal state are unlikely to be explained by photoinduced superconductivity.

Popular Summary

The pursuit of higher superconducting transition temperatures is an evergreen frontier in condensed-matter physics. Light-induced superconductivity above room temperature has been in the spotlight since it was first reported; however, there are two critical issues remaining. One is whether the observed optical signatures can be unambiguously identified as evidence for superconductivity. The second is whether phonon resonant excitation is necessary for those signatures. Aiming at addressing those two issues, we present a comprehensive study of the transient optical responses in underdoped ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{6+x}$ after systematic measurements and analyses.

We measure the light-induced nonequilibrium optical constants along the $c$ axis (perpendicular to the copper-oxygen planes) of underdoped ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{6+x}$ bulk samples in the terahertz regime. All of our results can be explained by suppression of superconductivity and the generation of quasiparticles without invoking light-induced superconductivity. Furthermore, by comparing the results when we tune the excitation energy near and far away from a specific phonon mode claimed as the key to inducing superconductivity, we observe no substantial difference, which indicates that we can exclude phonon resonant excitation as a scenario for the light-induced phenomenon.

Our work shows that simultaneous rises in both real and imaginary parts of conductivity may not be seen as an indication of transient superconductivity. In addition, the phonon resonant excitation scenario can be excluded in ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{6+x}$. These results shed light on the experimental and theoretical field of the light-induced phenomenon, especially light-induced superconducting behavior.

Figure 1

Sample characterization and broadband reflectivity spectra along the $c$ axis in the equilibrium state. (a) Temperature-dependent magnetization measurements. (b) Out-of-plane reflectance spectra of YBCO are governed by phonons in the far-infrared region. In superconducting states, sharp Josephson plasmon edges show up in the terahertz regime. Dashed lines are the low-frequency extrapolations used for Kramers-Kronig transformation. The gray shaded areas show the energy scale and spectral range of pump pluses. (c),(d) The colored thick lines show the real and imaginary parts of conductivity in $x=0.55$ at 60 and 300 K. The black and gray thin lines show the fitting results of the Drude-Lorentz model. (e),(f) Reflected terahertz electric field in time and frequency domain measured by a time-domain terahertz spectroscopy system, whose measurement range is plotted as a shaded area in (b)–(d).

Figure 2

Pump-induced changes after excitation of $1.28\mu \mathrm{m}$ at 5 K. (a) The decay procedure of $|\mathrm{\Delta }{E}_{\max }|/E_{\mathrm{peak}}$ after being excited by a fluence of $1\mathrm{mJ}/{\mathrm{cm}}^2$ (peak electric field of $\sim 3.9\mathrm{MV}/\mathrm{cm}$). $E_{\mathrm{peak}}$ is the maximum value of $E_0(t)$ at a static position. (b) The pump-induced relative change $\mathrm{\Delta }E(t,\tau )/E_{\mathrm{peak}}$ in the time domain at 3 and 40 ps. (c) Fourier transformed spectrum of $\mathrm{\Delta }E(t,\tau )$. Gray lines represent the Fourier transformation of the static reflected electric field divided by a coefficient. (d) Transient reflectivity $R(\omega ,\tau )$. (e) Real part of conductivity ${\sigma }_1(\omega ,\tau )$. (f) Imaginary part of conductivity ${\sigma }_2(\omega ,\tau )$. Some portions are plotted in color-fading dots; for the light-induced measurement signal there has almost vanishing spectral weight, which makes those calculated data points unreliable. Optical constants in the static state are plotted in gray lines.

Figure 3

Pump-induced changes after excitation at $1.28\mu \mathrm{m}$ by a fluence of $2\mathrm{mJ}/{\mathrm{cm}}^2$ (peak electric field of $\sim 5.5\mathrm{MV}$/cm) in the normal state. (a) The decay procedure of $|\mathrm{\Delta }{E}_{\max }|/E_{\mathrm{peak}}$ at 60 K is plotted in a solid line. Three representative time delays are labeled in colored triangles. The decay procedure profile at 5 K is plotted in a dashed line for comparison. (b) The pump-induced relative change $\mathrm{\Delta }E(t,\tau )/E_{\mathrm{peak}}$ in the time domain at three different time delays. (c) Fourier transformed spectrum of $\mathrm{\Delta }E(t,\tau )$. Gray lines indicate the Fourier transformation of the static reflected electric field divided by a coefficient. (d),(f) Transient reflectivity $R(\omega ,\tau )$, real part of conductivity ${\sigma }_1(\omega ,\tau )$, imaginary part of conductivity ${\sigma }_2(\omega ,\tau )$, respectively. The colored dots are experimental data and the black lines are fitting curves. Optical constants in the static state are plotted in gray lines.

Figure 4

Comparison between the pump-induced effects after the excitation of $15\mu \mathrm{m}$ (solid lines) and $1.28\mu \mathrm{m}$ (dashed lines) pulses at $1\mathrm{mJ}/{\mathrm{cm}}^2$. (a) The decay procedure of $|\mathrm{\Delta }{E}_{\max }|/E_{\mathrm{peak}}$ at 5 and 60 K. (b) The pump-induced change of the terahertz electric field at 5 K in the time domain. (c)–(e) Transient optical constants, $R(\omega ,\tau )$, ${\sigma }_1(\omega ,\tau )$, ${\sigma }_2(\omega ,\tau )$ at 5 K. (f) The pump-induced change of the terahertz electric field at 60 K in the time domain. (g)–(i) The transient optical constants at 60 K in the normal state. Gray lines are the optical constants in the equilibrium state.