In cuprate high-$T_c$ superconductors, resonant excitation of certain lattice vibrations has been shown to induce transient terahertz reflectivity features suggestive of nonequilibrium superconductivity above the critical temperature $T_c$. A microscopic mechanism for these observations is still lacking. Here, time-resolved measurements of scattering-angle- and polarization-dependent second-harmonic generation in driven ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{6+x}$ reveal a three-order-of-magnitude amplification of a 2.5-THz electronic mode, which is unique because of its symmetry, momentum, and temperature dependence. A theory for amplification of finite-momentum Josephson plasma polaritons, which are assumed to be well formed below $T_c$ but incoherent throughout the pseudogap phase, explains all these observations. A theoretical solution for the Fresnel-Floquet reflection that starts from the coherently oscillating Josephson plasma polaritons provides a possible mechanism for the nonequilibrium superconductorlike terahertz reflectivity reported earlier. Beyond the immediate case of cuprates, this work underscores the role of nonlinear mode mixing to amplify fluctuating modes above the transition temperature in a wide range of materials.

中文翻译：

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DrivenYBa2Cu3O6+x 中超导波动的放大

在铜酸盐高${吨}_C$超导体，某些晶格振动的共振激发已被证明会引起瞬态太赫兹反射率特征，表明高于临界温度的非平衡超导性${吨}_C$. 这些观察的微观机制仍然缺乏。在这里，驱动中散射角和偏振相关的二次谐波产生的时间分辨测量${\mathrm{YBa}}_2{铜}_3{\mathrm{○}}_{6+X}$揭示了 2.5-THz 电子模式的三数量级放大，这是独一无二的，因为它具有对称性、动量和温度依赖性。一种放大有限动量约瑟夫逊等离子体极化子的理论，假设如下所示${吨}_C$但在整个赝间隙阶段不连贯，解释了所有这些观察结果。从相干振荡的约瑟夫逊等离子体极化子开始的菲涅耳-弗洛凯反射的理论解决方案为之前报道的非平衡超导体类太赫兹反射率提供了可能的机制。除了铜酸盐的直接案例之外，这项工作还强调了非线性模式混合的作用，以放大各种材料中高于转变温度的波动模式。