The Leggett mode associated with out-of-phase oscillations of superconducting phase in multi-band superconductors usually is heavy due to the interband coupling, which makes its excitation and detection difficult. We report on the existence of a massless Leggett mode in three-band superconductors with time-reversal-symmetry-breaking (TRSB). The mass of the Leggett mode is small close to the TRSB transition and vanishes at the transition point, and thus locates within the smallest superconducting energy gap, which makes it stable and detectable. The mass of the Leggett mode can be measured by Raman spectroscopy. The thermodynamic consequences of this massless mode and possible realization in iron-based superconductors are also discussed.
Time-Reversal Symmetry Breaking Superconducting State and Collective Modes in Kagome Superconductors
Capital Normal University（首都师范大学）；Kavli Institute for Theoretical Sciences, University of Chinese Academy of Sciences（中国科学院大学理论科学研究所）；Beijing National Laboratory for Condensed Matter Physics and Institute of Physics, Chinese Academy of Sciences（中国科学院物理研究所北京凝聚态物理国家研究中心）；School of Physical Sciences, University of Chinese Academy of Sciences（中国科学院大学物理科学学院）；Institute of Theoretical Physics, Chinese Academy of Sciences（中国科学院理论物理研究所）
Short-time dynamics in $s+is$-wave superconductor with incipient bands
基于最近在K掺杂BaFe $_2$ As $_2$超导合金中观察到的时间反转对称破缺态，我们采用一个双空穴双电子袋的有效四带模型，用$s+is$波序参量理论研究了超导态的集体模和短时动力学.作为空穴掺杂的结果，超导$s+is$状态出现于初始电子带，并且作为$s^{\pm}$（高空穴数）和$s^{++}$（低空穴数）之间的中间状态出现。振幅和相位模式耦合，产生各种集体模式。在$s^{\pm}$态中，我们发现了频率类似于没有Leggett模的两带模型的Higgs模，而在$s+is$和$s^{++}$态中，我们发现了一个新的耦合集体软模.最后，我们比较我们的结果与$s+id$解决方案，并发现类似的行为的集体模式。
