Nakamura Sachiko, Iida Yudai, Murotani Yuta, Matsunaga Ryusuke, Terai Hirotaka, Shimano Ryo
Cryogenic Research Center, the University of Tokyo, Yayoi, Tokyo, 113-0032, Japan.
Department of Physics, the University of Tokyo, Hongo, Tokyo, 113-0033, Japan.
Phys Rev Lett. 2019 Jun 28;122(25):257001. doi: 10.1103/PhysRevLett.122.257001.
The Higgs mode in superconductors, i.e., the collective amplitude mode of the order parameter, does not associate with charge nor spin fluctuations, therefore it does not couple to the electromagnetic field in the linear response regime. Contrary to this common understanding, here, we demonstrate that if the dc supercurrent is introduced into the superconductor, the Higgs mode becomes infrared active and is directly observed in the linear optical conductivity measurement. We observed a sharp resonant peak at ω=2Δ in the optical conductivity spectrum of a thin-film NbN in the presence of dc supercurrent, showing a reasonable agreement with the recent theoretical prediction. The method as proven by this work opens a new pathway to study the Higgs mode in a wide variety of superconductors.
超导体中的希格斯模式，即序参量的集体振幅模式，与电荷和自旋涨落均无关联，因此在线性响应区域内它不与电磁场耦合。与这种普遍认知相反，在此我们证明，如果将直流超电流引入超导体，希格斯模式会变为红外活性的，并能在线性光导率测量中直接观测到。我们在存在直流超电流的情况下，于薄膜NbN的光导率谱中观测到在ω = 2Δ处有一个尖锐的共振峰，这与近期的理论预测相当吻合。这项工作所证实的方法为研究各种超导体中的希格斯模式开辟了一条新途径。
