论文标题
通过Terahertz第三次谐波生成，揭示了超导和伪超导体中的集体模式的强耦合
Revealing strong coupling of collective modes between superconductivity and pseudogap in cuprate superconductor by terahertz third harmonic generation
论文作者
论文摘要
固体不同自由度之间相互作用的研究对于了解材料的功能至关重要。这种相互作用的一个显着例子是超导性（SC），电荷密度波（CDW），伪PGAP状态（PG）和其他外来阶段之间的相互交织的耦合或竞争。非线性Terahertz（THZ）第三次谐波生成（THG）光谱的最新出现为探索超导级参数的集体（HIGGS）模式提供了有力的工具，以及与交织/竞争阶段的相互作用。在这项研究中，我们报告了YBA $ _2 $ cu $ _3 $ o $ $ $ _ {6+x} $（ybco）薄膜的YBA $ _2 $ _2 $ _2 $ _2 $ _2 $ _cu $ _2 $ _2 $ _3 $ _3 $ _3 $ _3 $（ybco）薄膜的非线性光谱。我们确定一个特征温度$ t_ {thg} $，在此下方出现了三阶suscepetility $χ^{（3）} $。值得注意的是，$ t_ {thg} $与pseudogap的跨界温度$ t^*$相吻合。进入超导状态后，THG急剧增加，但在$ t_c $附近表现出异常的倾角特征，在最佳掺杂样品中更清楚地看到。令人惊讶的是，我们直接在测量的THG信号的实时波形中观察到一个跳动结构。时域波形的傅立叶变换给出了下方和上方的两个独立模式。该观察结果强烈表明，额外的模式，大概是希格斯模式，以$ t_c $和夫妻的形式出现到已经在$ t^*$下开发的模式出现。强耦合效果为超导性和伪群之间的相互作用提供了新的见解。结果明确地表明，伪阶段不是超导性的先驱，而是一个独特的顺序。
The study of interaction between different degrees of freedom in solids is of fundamental importance to understand the functionalities of materials. One striking example of such interaction is the intertwined coupling or competition between superconductivity (SC), charge density wave (CDW), pseudogap state (PG), and other exotic phases in cuprate superconductors. Recent emergence of nonlinear Terahertz (THz) third harmonic generation (THG) spectroscopy provides a powerful tool for exploring the collective (Higgs) modes of superconductivity order parameters, and its interaction with intertwined/competing phases. In this study, we report on nonlinear THz THG spectroscopy of the YBa$_2$Cu$_3$O$_{6+x}$ (YBCO) thin films with different doping. We identify a characteristic temperature $T_{THG}$, below which third order suscepetility $χ^{(3)}$ emerges. Notably, the $T_{THG}$ is coincident with the crossover temperature $T^*$ of pseudogap in a wide range doping of phase diagram. Upon entering the superconducting state, THG increases sharply but exhibits an abnormal dip feature near $T_c$ which is more clearly seen in optimally doped sample. Strikingly, we observe a beating structure directly in the measured real time waveform of THG signal. Fourier transformation of the time domain waveform gives two separate modes below and above original THG frequency. The observation strongly indicates that an additional mode, presumably Higgs mode, appears at $T_c$ and couples to the mode already developed below $T^*$. The strong coupling effect offers new insight into the interplay between superconductivity and pseudogap. The result unambiguously suggests that the pseudogap phase is not a precursor of superconductivity but represents a distinct order.
