扭曲d波超导双层中的集体相位模式
Collective phase modes in twisted $d$-wave superconducting bilayers
AI总结：
本文理论分析扭曲d波超导双层中的低能相位涨落，发现高阶约瑟夫森耦合修正相位动力学，导致相对相位刚度在正常态有限，并在d+id'相中抑制相对相位刚度而增强整体相位刚度，相对相位涨落具有小但有限的激发隙，接近手性量子临界点时强烈抑制。
AI中文摘要：
扭曲铜酸盐双层已被预测可承载高温手性$d+id'$超导性，其源于高阶约瑟夫森耦合过程。在这种二维超导系统中，超导序参量相位的长波涨落构成无能隙集体模式，因此在零温下仍然显著。这里，我们对扭曲$d$波超导双层中的低能相位涨落进行理论分析，系统地保留所有阶的约瑟夫森耦合。我们证明，高阶约瑟夫森耦合过程导致相位动力学的非平凡修正。特别地，即使在正常态，两层之间的相对相位刚度也是有限的，这是层间隧穿明确诱导的层内U(1)对称性破缺的直接结果。相位涨落抑制$d+id'$相中的相对相位刚度，但增强整体相位刚度。此外，相对相位涨落获得一个小但有限的激发隙，该激发隙不再等于约瑟夫森等离子体频率，并且在接近手性量子临界点时被强烈抑制，表明超导态内约瑟夫森相位锁定的失效。
英文摘要：
Twisted cuprate bilayers have been predicted to host high-temperature chiral $d+id'$ superconductivity, originating from higher-order Josephson coupling processes. In such two-dimensional superconducting systems, long-wavelength fluctuations in the phase of the superconducting order parameter constitute gapless collective modes and therefore remain significant even at zero temperature. Here, we perform a theoretical analysis of the low-energy phase fluctuations in twisted $d$-wave superconducting bilayers within a self-consistent harmonic approximation, systematically retaining Josephson coupling to all orders. We demonstrate that higher-order Josephson coupling processes lead to nontrivial modifications of the phase dynamics. The momentum-resolved summand of the relative-phase stiffness is nonzero even in the normal state because interlayer tunneling explicitly breaks the intralayer U(1) symmetry, but its momentum integral vanishes for the continuum dispersion. The relative-phase stiffness is smaller in the $d+id'$ phase than in the $d$-wave phase, while the overall-phase stiffness has the opposite behavior. Furthermore, phase fluctuations strongly soften the Josephson plasma frequency near a twist angle of $45^\circ$ and also substantially reduce the Josephson critical current.
