Quasiparticle spin resonance and coherence in superconducting aluminium.

C H L Quay,C Strunk,M Weideneder,Y Chiffaudel,M Aprili

doi:10.1038/ncomms9660

Abstract

Conventional superconductors were long thought to be spin inert; however, there is now increasing interest in both (the manipulation of) the internal spin structure of the ground-state condensate, as well as recently observed long-lived, spin-polarized excitations (quasiparticles). We demonstrate spin resonance in the quasiparticle population of a mesoscopic superconductor (aluminium) using novel on-chip microwave detection techniques. The spin decoherence time obtained (∼100 ps), and its dependence on the sample thickness are consistent with Elliott–Yafet spin–orbit scattering as the main decoherence mechanism. The striking divergence between the spin coherence time and the previously measured spin imbalance relaxation time (∼10 ns) suggests that the latter is limited instead by inelastic processes. This work stakes out new ground for the nascent field of spin-based electronics with superconductors or superconducting spintronics.

Highlights

Conventional superconductors were long thought to be spin inert; there is increasing interest in both the internal spin structure of the ground-state condensate, as well as recently observed long-lived, spin-polarized excitations
If one thinks of spins as classical magnetic moments, T1 is the time over which they align with an external magnetic field, while T2 is the time over which Larmorlike precessions of the spins around the external field remain phase coherent2. (T1 is sometimes called the longitudinal or spin-lattice relaxation time and T2 the transverse relaxation time.) T1BT2 for conduction electrons in most normal metals[3,5,6,7]
The striking effect can be understood by considering that spin imbalance in superconductors can be thought of, in a simple picture, as having contributions from both a spin-dependent shift in the quasiparticle

Summary

Introduction

Conventional superconductors were long thought to be spin inert; there is increasing interest in both (the manipulation of) the internal spin structure of the ground-state condensate, as well as recently observed long-lived, spin-polarized excitations (quasiparticles). We demonstrate spin resonance in the quasiparticle population of a mesoscopic superconductor (aluminium) using novel on-chip microwave detection techniques. This work stakes out new ground for the nascent field of spin-based electronics with superconductors or superconducting spintronics. In a typical electron spin resonance (ESR) experiment, electrons are immersed in an external homogenous static magnetic field, H. Microwave radiation creates a perturbative transverse magnetic field (perpendicular to the static field) of frequency fRF. The power P(H, fRF) absorbed by the spins from the microwave field is determined, usually by measuring the fraction of the incident microwaves that is not absorbed, that is, either transmitted or reflected. When H is tuned to its resonance value, Hres 1⁄4 2pfRF=g—with g the gyromagnetic ratio—the electron spins precess around H and P(H, fRF) is maximal. T2 1⁄4 2/(gDH), where DH is the full width at half maximum of the power resonance as a function of H

Methods

Results

Conclusion