W. G. Farr | Searcharxiv

arXiv2015

Evidence of Dilute Ferromagnetism in Rare-Earth doped YAG

This work demonstrates strong coupling regime between an Erbium ion spin ensemble and microwave Hybrid Cavity-Whispering Gallery Modes in a Yttrium Aluminium Garnet dielectric crystal. Coupling strengths of $220$~MHz and mode quality factors in excess of $10^6$ are demonstrated. Moreover, the magnetic response of high-Q modes demonstrates behaviour which is unusual for paramagnetic systems. This behaviour includes hysteresis and memory effects. Such qualitative change of the system's magnetic field response is interpreted as a phase transition of Rare Earth ion impurities. This phenomenon is similar to the phenomenon of dilute ferromagnetism in semiconductors. The clear temperature dependence of the phenomenon is demonstrated.

cond-mat.mtrl-sci↗

arXiv2013

Spin bath maser in a cryogenically cooled sapphire whispering gallery mode resonator

We report the observation of a mechanism of maser generation in an ensemble of inter-coupled, inhomogeneously broadened two-level systems, enhanced by high quality factor electromagnetic cavity modes. In this previously unobserved form of population inversion, an inseparable quantum system leads to cavity-enhanced stimulated emission arising from interactions within an ensemble of two-level systems, as opposed to a traditional ensemble of noninteracting identical three level systems. The effect is observed in a cryogenically cooled whispering gallery mode sapphire resonator containing dilute Fe$^{3+}$ impurity ions. These ions exhibit strong spin-lattice interaction, leading to both electron spin resonance broadening and phonon mediated spin-spin coupling. The maser effect is due to a $\left|1/2\right\rangle \rightarrow \left|3/2\right\rangle$ energy transition in electron spin angular momentum observed at zero external magnetic field. Both continuous and oscillating regimes are observed with corresponding thresholds both in detuning frequency and incident power.

cond-mat.mes-hall↗