Ultra-High Cooperativity Interactions between Magnons and Resonant Photons in a YIG sphere

TL;DR

This study demonstrates ultra-high cooperativity interactions between magnons and resonant photons in a YIG sphere at millikelvin temperatures, achieving record coupling strength and revealing complex mode interactions and symmetry breaking effects.

Contribution

The paper reports the first observation of ultrastrong magnon-photon coupling in a YIG sphere with a cooperativity exceeding 10^7, along with detailed mode characterization and symmetry-breaking phenomena.

Findings

Achieved a coupling strength of 7.11 GHz with a cooperativity of approximately 1.5×10^7.

Observed complex modifications of photon mode frequencies indicating beyond simple models.

Detected time reversal symmetry breaking due to gyrotropic permeability of YIG.

Abstract

Resonant photon modes of a 5mm diameter YIG sphere loaded in a cylindrical cavity in the 10-30GHz frequency range are characterised as a function of applied DC magnetic field at millikelvin temperatures. The photon modes are confined mainly to the sphere, and exhibited large mode filling factors in comparison to previous experiments, allowing ultrastrong coupling with the magnon spin wave resonances. The largest observed coupling between photons and magnons is $2g/2\pi=7.11$ GHz for a 15.5 GHz mode, corresponding to a cooperativity of $C=1.51\pm0.47\times10^7$. Complex modifications beyond a simple multi-oscillator model, of the photon mode frequencies were observed between 0 and 0.1 Tesla. Between 0.4 to 1 Tesla, degenerate resonant photon modes were observed to interact with magnon spin wave resonances with different couplings strengths, indicating time reversal symmetry breaking due to the gyrotropic permeability of YIG. Bare dielectric resonator mode frequencies were determined by detuning magnon modes to significantly higher frequencies with strong magnetic fields. By comparing measured mode frequencies at 7 Tesla with Finite Element modelling, a bare dielectric permittivity of $15.96\pm0.02$ of the YIG crystal has been determined at about $20$ mK.