Spin current cross-correlations as a probe of magnon coherence

Abstract

Motivated by the important role of the normalized second order coherence function, often called $g^{(2)}$, in the field of quantum optics, we propose a method to determine magnon coherence in solid-state devices. Namely, we show that the cross-correlations of pure spin-currents injected by a ferromagnet into two metal leads, normalized by their dc value, replicate the behavior of $g^{(2)}$ when magnons are driven far from equilibrium. We consider two scenarios: driving by ferromagnetic resonance, which leads to the coherent occupation of a single mode, and driving by heating of the magnons, which leads to an excess of incoherent magnons. We find an enhanced normalized cross-correlation in the latter case, thereby demonstrating bunching of nonequilibrium thermal magnons due to their bosonic statistics. Our results contribute to the burgeoning field of quantum magnonics, which seeks to explore and exploit the quantum nature of magnons.

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• Ultrastrong magnon-photon coupling and entanglement in superconductor/ferromagnet nanostructures
• Quantum many-body states and Green functions of nonequilibrium electron-magnon systems: Localized spin operators vs. their mapping to Holstein-Primakoff bosons
• Spin-Energy Correlation in Degenerate Weakly-Interacting Fermi Gases
• Fingerprints of Universal Spin-Stiffness Jump in Two-Dimensional Ferromagnets
• Key Observable for Linear Thermalization
• Driven-dissipative Quantum Dynamics in Cavity Magnon-Polariton System