Abstract
The Higgs mode in superconducting materials describes slowly decaying oscillations of the order parameter amplitude. We demonstrate that in superconductors with a built-in spin-splitting field the Higgs mode is strongly coupled to the spin degrees of freedom, allowing for the generation of time-dependent spin currents. Converting such spin currents to electric signals by spin-filtering elements provides a tool for the second-harmonic generation and the electrical detection of the Higgs mode generated by the external irradiation. The nonadiabatic spin torques generated by these spin currents allow for the magnetic detection of the Higgs mode by measuring the precession of the magnetic moment in the adjacent ferromagnet. We discuss also the reciprocal effect, which is the generation of the Higgs mode by the magnetic precession. Coupling the collective modes in superconductors to light and magnetic dynamics provides an opportunity for the study of superconducting optospintronics.Received 1 July 2019Revised 23 August 2020Accepted 25 August 2020DOI:https://doi.org/10.1103/PhysRevResearch.2.033416Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Research AreasFFLOOptical conductivitySpin currentSuperconducting phase transitionPhysical SystemsFerromagnetic superconductorsHiggs bosonsMultilayer thin filmsTunnel junctionsTechniquesMethods in superconductivityCondensed Matter & Materials Physics
Highlights
Oscillations of the order parameter amplitude in condensed-matter systems are often called Higgs modes (HMs) [1,2,3,4,5], in analogy with the Higgs boson in particle physics [6]
We have demonstrated that spin and charge currents can be effectively generated by the collective amplitude modes of the superconducting order parameter
Owing to the fact that the HM can be generated by external irradiation [27,92], our result paves the way for a conceptually different direction of superconducting optospintronics: the study of spin currents and spin torques generated by light interacting with superconducting materials
Summary
Oscillations of the order parameter amplitude in condensed-matter systems are often called Higgs modes (HMs) [1,2,3,4,5], in analogy with the Higgs boson in particle physics [6] These collective excitations are generic for ordered states such as antiferromagnets, charge density waves [7], superfluids [8,9,10], cold atomic gases [11,12] and superconductors [2,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27]. In usual superconductors the HMs are charge neutral and decoupled from charge current In such systems the observation of HMs has been facilitated by the development of low-temperature terahertz spectroscopy [18,19,24,29,30,31,32]. Higgs modes have been observed indirectly as the AC linear conductance peak in currentcarrying films of NbN [24] and Al [33]
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