EPR spectroscopy of Er:CaWO₄ at millikelvin temperatures

Rare-earth-ions are interesting physical systems because they have long lived states and record coherence times, due the screening of the 4f valence shell by the 5s and 5p filled shells. Rare-earths with an odd number of electrons are also paramagnetic, with an electron-spin transition at GHz frequencies in magnetic fields less than 1 Tesla. For such transitions, coherence times around 50-100 us have been measured at temperatures as low as 1.4K [1]. Here we present results from our recent Electron-Paramagnetic-Resonance (EPR) studies of 0.005% Er:CaWO₄. These measurements were recorded in a novel regime for this material: sub-Kelvin temperature down to 10mK, using a superconducting micro-resonator fabricated directly on the material surface and using a superconducting parametric amplifier for the microwave signals [2], [3]. We observe the longest recorded Hahn-echo decay for an electronic spin transition in an Erbium doped material, up to 1 ms.
[1] S. Bertaina et al., Nature nanotechnology, vol. 2, no 1, p. 39, (2007)
[2] A. Bienfait et al., Nature nanotechnology, vol. 11, no 3, p. 253, (2016)
[3] C. Macklin et al., science, vol. 350, issue 6258, p307, (2015)