标题： YBa$_2$Cu$_3$O$_{6.6}$中来自临界超导涨落的增强相干太赫兹发射 显示英文标题

标题： Enhanced coherent terahertz emission from critical superconducting fluctuations in YBa$_2$Cu$_3$O$_{6.6}$

摘要： 相干太赫兹（THz）发射正成为探测量子材料中对称性破缺的强大新工具。 此方法依赖于二阶光学非线性，且与二次谐波产生光谱学互补。 在这里，我们报道了欠掺杂YBa$_2$Cu$_3$O$_{6+x}$中约瑟夫森等离子体的相干太赫兹发射，并发现发射场的振幅在接近超导转变温度 $T_C$时急剧增加。 我们从理论上展示了发射如何通过临界超导涨落而增强，这是临界乳光的一种非线性类比。 这一观察结果预计将在许多热力学和量子相变的研究中具有重要意义。 显示英文摘要

摘要： Coherent terahertz (THz) emission is emerging as a powerful new tool to probe symmetry breakings in quantum materials. This method relies on second order optical nonlinearities and is complementary to second harmonic generation spectroscopy. Here, we report coherent THz emission from Josephson plasmons in underdoped YBa$_2$Cu$_3$O$_{6+x}$, and find that the amplitude of the emitted field increases dramatically close to the superconducting transition temperature, $T_C$. We show theoretically how emission is enhanced by critical superconducting fluctuations, a nonlinear analogue of critical opalescence. This observation is expected to be of general importance for the study of many thermal and quantum phase transitions.