Anisotropic Josephson coupling of d-vectors arising from interplay with frustrated spin textures

Abstract: We propose a Cooper pair analogue of Dzyaloshinskii-Moriya's anisotropic coupling for $d$-vectors in an inhomogeneous unconventional superconductor, which can arise when itinerant electrons couple to a local exchange field of frustrated spin moments. Using perturbative Green's function methods, we examine an $s$-$d$ model on a geometrically frustrated lattice with noncollinear spin texture that breaks time-reversal and parity symmetries, leading to spin-triplet pairing correlations. The effective Josephson tunneling in the presence of such an exchange field leads to an anisotropic Josephson coupling between $d$-vectors, which prefers a spatially inhomogeneous pairing order. Furthermore, we discuss anomalous vortices arising from the spatially inhomogeneous $d$-vector in the absence of a magnetic field for nonunitary triplet pairing order. We also propose a Josephson diode effect in which the diode efficiency is proportional to the spin chirality of the local exchange field. Our results can help understand the experiments in superconducting proximitized Mn$_3$Ge and in 4Hb-TaS$_2$.