Supercurrent decay in ballistic magnetic Josephson junctions

Hervé Ness,Andrey E Antipov,Mark Van Schilfgaarde,Roman M Lutchyn,Ivan A Sadovskyy

doi:10.1038/s41524-021-00694-3

Abstract

We investigate transport properties of ballistic magnetic Josephson junctions and establish that suppression of supercurrent is an intrinsic property of the junctions, even in absence of disorder. By studying the role of ferromagnet thickness, magnetization, and crystal orientation we show how the supercurrent decays exponentially with thickness and identify two mechanisms responsible for the effect: (i) large exchange splitting may gap out minority or majority carriers leading to the suppression of Andreev reflection in the junction, (ii) loss of synchronization between different modes due to the significant dispersion of the quasiparticle velocity with the transverse momentum. Our results for Nb/Ni/Nb junctions are in good agreement with recent experimental studies. Our approach combines density functional theory and the Bogoliubov-de Gennes model and opens a path for material composition optimization in magnetic Josephson junctions and superconducting magnetic spin valves.

Highlights

Coherent quantum tunneling of Cooper pairs through a thin barrier is one of the first examples of macroscopic quantum coherent phenomena
Our main qualitative conclusions are supported by microscopic calculations for Nb/ Ni/Nb magnetic Josephson junction (MJJ)
We found that the Nb/Ni/Nb junction is a suitable system for comparison with the simulations because of the long mean free path in Ni relative to the junction thickness and the quasiballistic nature of quasiparticle propagation in the ferromagnet

Summary

Introduction

Coherent quantum tunneling of Cooper pairs through a thin barrier is one of the first examples of macroscopic quantum coherent phenomena. Most of the previous studies focused on conventional Josephson junctions (JJs) consisting of two s-wave superconductors (S) that are connected by an insulating (I) or a normal (N) region[3,4]. The flow of supercurrent through a JJ depends on the superconducting phase difference φ between two superconductors and, in general, is characterized by the current-phase relationship J(φ) (CPR). In conventional JJs CPR should be periodic with 2π, I(φ) = I(φ + 2π) which follows from the Bardeen–Cooper–Schrieffer theory[5]. This result is a manifestation of a 2e charge of Cooper pairs and is used in metrology to measure electron charge. CPR can be expanded in Fourier harmonics, IðφÞ 1⁄4 nIn sinðnφÞ

Methods

Results

Conclusion