反常磁体中的自旋极化超电流与约瑟夫森二极管效应
Spin-polarized supercurrents and Josephson diode effect in altermagnets
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中文总结 AI 辅助
本研究通过理论分析，探究了d波反常磁体结的约瑟夫森效应，在强自旋极化 regime 中发现其可产生效率超30%的电荷二极管效应与100%的自旋二极管效应。
中文摘要 AI 辅助
我们对置于两个BCS超导体（SC）之间的d波反常磁体（AM）结中的约瑟夫森效应开展了系统的理论研究。总体而言，SC/AM界面具有自旋活性，采用自旋依赖的δ势建模，允许局域交换场矢量取任意方向。该模型基于全量子（Gor'kov）和准经典（Eilenberger）格林函数技术构建，应用于两种不同情形：（i）弱自旋极化AM（交换场远小于费米能）和（ii）强自旋极化AM（交换场与费米能相当）。我们将模型应用于SC/AM/SC结构，分析约瑟夫森电流相位关系（CPR）。在弱自旋极化 regime 中，CPR呈现常规约瑟夫森效应；无论反常磁体取向如何，该结会发生0-π转变，且系统表现出与铁磁结或反铁磁结类似的特性。为探究自旋极化电流和非互易输运（本工作的核心结果），我们重点研究强自旋极化 regime，在此 regime 中区分两种情形：结内共面交换场分布呈现常规约瑟夫森效应，但CPR中存在纯净且稳定的长程二次谐波；相反，非共面交换场分布会在结内产生所谓的量子几何相位，导致约瑟夫森CPR中无相反转中心，进而出现约瑟夫森二极管效应，其电荷二极管效率显著大于30%，自旋二极管效率达100%。
英文摘要
We present a systematic theoretical study of the Josephson effect in junctions consisting of a d-wave altermagnet (AM) placed between two BCS superconductors (SC). In general, the SC/AM interfaces are spin-active and modeled by spin-dependent $δ$ potentials, allowing for an arbitrary direction of the local exchange field vector. The model is formulated within the fully quantum (Gor'kov) and quasiclassical (Eilenberger) Green's function technique, applied to two distinct cases of (i) a weakly spin-polarized AM (exchange field much smaller compared to the Fermi energy) and (ii) a strongly spin-polarized AM (exchange field comparable to the Fermi energy). We apply our model to the SC/AM/SC geometry, accounting for the Josephson current-phase relation (CPR). In the weakly spin-polarized regime, the CPR displays the normal Josephson effect. Irrespective of the orientation of the altermagnet, the junction undergoes the $0-π$ transition. Depending on the orientation, the system displays the features similar to those of a ferromagnetic or an antiferromagnetic junction. To investigate the spin-polarized currents and nonreciprocal transport as the central results of the present work, we put the main focus on the strongly spin-polarized regime. Within this regime, we distinguish two cases. A coplanar exchange field profile across the junctions displays the normal Josephson effect; however, with a pure and stable long-range second harmonic in the CPR. In contrast, a noncoplanar exchange field profile gives rise to the so-called quantum geometric phases across the junction, leading to the absence of the phase-inversion center in the Josephson CPR. As a result, a Josephson diode effect emerges with a significant charge diode efficiency larger than 30% and a perfect spin diode efficiency of 100%.
