Spin currents driven by the Higgs mode in magnetic superconductors

Abstract

Higgs mode in superconducting materials describes slowly-decaying oscillations of the order parameter amplitude. We demonstrate that in magnetic superconductors with built-in spin-splitting field 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 non-adiabatic spin torques generated by these spin currents allow for the magnetic detection of the Higgs mode by measuring the precession of 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 opens the new direction of superconducting optospintronics.

Signatures of the Higgs mode in transport through a normal-metal–superconductor junction

“…The resonant behavior of |I 2 | can serve as a signature of the presence of the Higgs mode in the superconductor. A similar observation has been recently reported for spin currents driven by the Higgs mode [27].…”

“…This is in contrast to Ref. [27], where particle-hole asymmetry is due to the exchange field in the superconductor induced by an external magnetic field. A detailed derivation of I 2 using circuit theory [42][43][44] can be found in the Supplemental Material [41].…”

“…A linear coupling enabled by the presence of a supercurrent was theoretically proposed [29] and experimentally verified [30]. Very recently, it was theoretically predicted that a Higgs mode can be observed through its effect on the time-dependent spin current in a ferromagnet-superconductor junction [27].…”

“…To couple the Higgs mode to the charge current, particle-hole symmetry has to be broken. Inducing a finite spin splitting in the superconductor is one way to achieve this [27]. As a potentially simpler alternative, we propose to apply a DC voltage bias to a normal-metal- superconductor (NS) junction.…”

“…Inserting Eqs. ( 1) and (4) into the Usadel equation (2) leads to the retarded and advanced components of the nonstationary term [27],…”