Optical excitation in the cuprates has been shown to induce transient superconducting correlations above the thermodynamic transition temperature $T_C$, as evidenced by the terahertz-frequency optical properties in the nonequilibrium state. In ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{6+x}$, this phenomenon has so far been associated with the nonlinear excitation of certain lattice modes and the creation of new crystal structures. In other compounds, like ${\mathrm{La}}_{2-x}{\mathrm{Ba}}_x{\mathrm{CuO}}_4$, similar effects were reported also for excitation at near-infrared frequencies, and were interpreted as a signature of the melting of competing orders. However, to date, it has not been possible to systematically tune the pump frequency widely in any one compound, to comprehensively compare the frequency-dependent photosusceptibility for this phenomenon. Here, we make use of a newly developed nonlinear optical device, which generates widely tunable high-intensity femtosecond pulses, to excite ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{6.5}$ throughout the entire optical spectrum (3–750 THz). In the far-infrared region (3–24 THz), signatures of nonequilibrium superconductivity are induced only for excitation of the 16.4- and 19.2-THz vibrational modes that drive $c$-axis apical oxygen atomic positions. For higher driving frequencies (25–750 THz), a second resonance is observed around the charge transfer band edge at approximately 350 THz. These findings highlight the importance of coupling to the electronic structure of the ${\mathrm{CuO}}_2$ planes, mediated either by a phonon or by charge transfer.

中文翻译：

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YBa2Cu3O6.5中光诱导初期超导的泵浦频率共振

铜酸盐中的光激发已显示出在热力学转变温度以上会诱发瞬态超导相关性 ${Ť}_C$由非平衡状态下的太赫兹频率光学特性证明。在${\mathrm{Ba}}_2{铜}_3{\mathrm{Ø}}_{6+X}$到目前为止，这种现象与某些晶格模式的非线性激发以及新晶体结构的产生有关。在其他化合物中，例如${啦}_{2-X}{巴}_X{\mathrm{氧化铜}}_4$，据报道在近红外频率激发也有类似的作用，并被解释为竞争顺序融化的标志。但是，迄今为止，不可能系统地在任何一种化合物中广泛地调节泵浦频率，以全面比较这种现象随频率变化的光敏性。在这里，我们利用了新开发的非线性光学器件，该器件会产生可广泛调节的高强度飞秒脉冲，从而激发${\mathrm{Ba}}_2{铜}_3{\mathrm{Ø}}_{6.5}$整个光谱范围（3-750 THz）。在远红外区域（3–24 THz），仅在激发驱动力的16.4和19.2 THz振动模式时才诱发非平衡超导信号$C$轴顶端氧原子位置。对于更高的驱动频率（25–750 THz），在电荷转移带边缘大约350 THz处观察到第二个共振。这些发现凸显了耦合到电子结构的重要性。${\mathrm{氧化铜}}_2$ 由声子或电荷转移介导的平面。