Spin-polarized Andreev molecules and anomalous nonlocal Josephson effects in altermagnetic junctions

TL;DR

This paper explores how altermagnetism in coupled Josephson junctions induces spin-polarized Andreev molecules, leading to novel nonlocal Josephson effects including $0- ext{pi}$ and $ ext{phi}_0$ transitions and a tunable Josephson diode effect.

Contribution

It demonstrates for the first time that d-wave altermagnetism causes spin-polarized Andreev molecules and results in anomalous nonlocal Josephson effects in coupled junctions.

Findings

Spin-polarized Andreev molecules form due to hybridization of bound states.

Nonlocal Josephson effects include $0- ext{pi}$ and $ ext{phi}_0$ transitions.

Nonreciprocal critical currents enable a tunable Josephson diode effect.

Abstract

Altermagnetism has emerged as a promising ingredient for realizing nontrivial Josephson phases, but so far explored in single Josephson junctions. In this work, we consider the coherent coupling of two Josephson junctions with spin-singlet $s$-wave superconductivity and demonstrate that $d$-wave altermagnetism gives rise to spin-polarized Andreev molecules due to the hybridization of Andreev bound states of each junction when the coupling is weak. Interestingly, these spin-polarized Andreev molecules induce an anomalous nonlocal Josephson effect, where the current flow across one Josephson junction due to phase changes across the other junction develops $0-\pi$ and $\phi_{0}$ transitions originating from altermagnetism. Furthermore, the nonlocal Josephson current carried by spin-polarized Andreev molecules exhibits nonreciprocal critical currents, enabling a nonlocal Josephson diode effect whose polarity is tunable by the altermagnetic strength and right phase. Our findings put forward altermagnetism as a promising arena for designing nonlocal spin Josephson phenomena.