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.

中文翻译：

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时间分辨太赫兹光谱研究的掺杂不足的YBa2Cu3O6 + x中的光诱导非平衡响应

近来，强激光脉冲已成为一种在复杂量子材料中不同阶次之间进行调谐的工具。在不同的光诱导现象中，瞬态超导性远高于铜酸盐中的平衡转变温度，这是特别有吸引力的。这些实验的关键是特定声子模式的共振泵浦，据信这可以通过抑制竞争顺序或稍微改变结构来诱导超导相干。在这里，我们对掺杂不足引起的光致非平衡响应进行了全面的研究。${\mathrm{Ba}}_2{铜}_3{\mathrm{Ø}}_{6+X}$。我们发现，在光激发下，超导状态的约瑟夫森等离子体边缘最初被准粒子激发伴随着去除，随后在短时间内以低于静态等离子体边缘的频率重新出现。在正常状态下，反射谱中的泵浦脉冲确实会引起增强或较弱的边缘状形状，并伴有电导率的实部和虚部同时上升。我们比较了近红外和中红外激发之间的泵激效应，并排除了声子泵激作为上述光诱导效应的一种情况。我们进一步阐述，正常状态下的瞬态响应不太可能由光诱导的超导性解释。