[论文解读] Optical Manipulation of the Charge Density Wave state in RbV3Sb5

Yuqing Xing, Seokjin Bae+9 more|arXiv (Cornell University)|Aug 8, 2023

Cold Atom Physics and Bose-Einstein Condensates被引用 4

一句话总结

本研究利用激光耦合扫描隧道显微镜，展示了在RbV3Sb5中可逆地光学调控电荷密度波（CDW）态，揭示了时间反演对称性破缺相。结果表明，线性偏振光可切换CDW峰强度，表明电子-声子耦合强烈，并发现一种新型的共轭CDW通量相，结合了键电荷序与环流电流。

ABSTRACT

Broken time-reversal symmetry in the absence of spin order indicates the presence of unusual phases such as orbital magnetism and loop currents. The recently discovered family of kagome superconductors AV$_3$Sb$_5$ (A = K, Rb, or Cs), hosting an exotic charge-density wave (CDW) state, has emerged as a strong candidate for this phase. While initial experiments suggested that the CDW phase breaks time-reversal symmetry, this idea is being intensely debated due to conflicting experimental data. In this work we use laser-coupled scanning tunneling microscopy (STM) to study RbV$_3$Sb$_5$. STM data shows that the Fourier intensities of all three CDW peaks are different, implying that the CDW breaks rotational and mirror symmetries. By applying linearly polarized light along high-symmetry directions, we show that the relative intensities of the CDW peaks can be reversibly switched, implying a substantial electro-striction response, indicative of strong non-linear electron-phonon coupling. A similar CDW intensity switching is observed with perpendicular magnetic fields, which implies an unusual piezo-magnetic response that, in turn, requires time-reversal symmetry-breaking. We show that the simplest CDW that satisfies these constraints and reconciles previous seemingly contradictory experimental data is an out-of-phase combination of bond charge order and loop currents that we dub congruent CDW flux phase. Our laser-STM data opens the door to the possibility of dynamic optical control of complex quantum phenomenon in correlated materials.

研究动机与目标

• 解决关于RbV3Sb5中CDW态是否破缺时间反演对称性的争议。
• 研究电子-声子耦合与对称性破缺在kagome超导体CDW相中的作用。
• 确定外部光学场是否可可逆地调控CDW序参数。
• 调和AV3Sb5材料中CDW态的相互矛盾的实验数据。
• 确定CDW系统中观测到的非线性电致伸缩响应的微观起源。

提出的方法

• 采用激光耦合扫描隧道显微镜（STM）探测光学激发下的局域电子结构。
• 测量实空间中CDW峰的傅里叶强度，以评估电荷序中的对称性破缺。
• 沿高对称性晶向施加线性偏振光，以调制CDW峰强度。
• 施加垂直磁场，探测压磁响应并推断时间反演对称性破缺。
• 分析CDW峰相对强度随光偏振态与磁场的变化。
• 提出一种共轭CDW通量相的理论模型，结合键电荷序与环流电流，以解释实验数据。

实验结果

研究问题

• RQ1 RbV3Sb5中的CDW态是否破缺时间反演对称性？其证据是什么？
• RQ2 能否使用圆偏振光或线性偏振光可逆地调控CDW峰的相对强度？
• RQ3 观测到的非线性电致伸缩响应的起源是什么？
• RQ4 磁场如何影响CDW序？这又对对称性保护意味着什么？
• RQ5 何种微观结构的CDW相能够调和先前相互矛盾的实验观测？

主要发现

• RbV3Sb5中的三个CDW峰表现出不同的傅里叶强度，证实了旋转对称性与镜像对称性的破缺。
• 沿高对称性方向的线性偏振光可可逆地切换CDW峰的相对强度，表明存在强烈的电致伸缩效应。
• 在垂直磁场下也观察到类似的切换效应，揭示了一种非典型的压磁响应。
• 所观测到的响应要求时间反演对称性破缺，支持CDW态中存在环流电流。
• 数据最合理的解释是共轭CDW通量相，结合了键电荷序与环流电流，调和了先前相互矛盾的结果。
• 结果展示了对关联kagome材料中复杂量子态的动态光学调控。