非均匀超导体中的超声衰减
Ultrasonic attenuation in inhomogeneous superconductors
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中文总结 AI 辅助
该研究基于实空间Bogoliubov–de Gennes公式，揭示相干因子可调控束缚态对超声衰减的贡献，发现超声能区分谱特征相似但对称性与自旋结构不同的束缚态。
中文摘要 AI 辅助
能隙内束缚态可在态密度中产生显著特征，但未必会在超声衰减中产生对应信号。我们证明，相干因子可作为束缚态对超声衰减贡献的滤波器。在相位偏置的超导体-正常金属-超导体结中，当声子频率与能隙内能级的相位依赖间距匹配时，安德烈夫束缚态会在衰减中产生共振。在d_{x²-y²}超导体的(110)晶界处情况不同：尽管非磁性杂质诱导的束缚态具有大的谱权重，但相关相干因子几乎抵消，导致其在衰减中仍保持弱效应。当晶界附近出现局域磁性时，这种抵消被消除，束缚态发生自旋劈裂，且会出现清晰的衰减峰，我们认为该峰由低温下准粒子对发射声子的过程主导。我们采用实空间Bogoliubov–de Gennes公式研究超声衰减，并通过均匀s波和d波超导体的已知低温行为对该方法进行基准验证。结果表明，超声可区分具有相似谱特征但不同对称性和自旋结构的束缚态。
英文摘要
Subgap bound states can produce pronounced features in the density of states without necessarily giving rise to a corresponding signal in ultrasonic attenuation. We show that the coherence factors act as a filter for the contribution of bound states to ultrasonic attenuation. In a phase-biased superconductor--normal--superconductor junction, the Andreev bound states produce a resonance in the attenuation when the phonon frequency matches the phase-dependent separation between the subgap levels. The situation is different at a $(110)$ grain boundary in a $d_{x^2-y^2}$ superconductor. There, nonmagnetic impurity-induced bound states remain weak in the attenuation despite their large spectral weight, because the relevant coherence factors nearly cancel. When local magnetism develops near the boundary, this cancellation is removed, the bound states become spin split, and a clear attenuation peak appears, which we argue is governed by a process involving emission of phonons by pair of quasiparticles at low temperatures. We obtain these results using a real-space Bogoliubov--de Gennes formulation of ultrasonic attenuation and benchmark the method against the known low-temperature behavior of homogeneous $s$- and $d$-wave superconductors. The results show that ultrasound can distinguish bound states with similar spectral signatures but different symmetry and spin structure.
