Giant g-factors of natural impurities in synthetic quartz

Abstract

We report the observation of g-factors of natural paramagnetic impurities in a pure synthetic quartz crystal at milli-Kelvin temperatures. Measurements are made by performing spectroscopy using multiple high-Q Whispering Gallery Modes sustained in the crystal. Extreme sensitivity of the method at low temperatures allows the determination of natural residual impurities introduced during the crystal growth. We observe g-factors that significantly differ from integer multiples of the electron g-factor in vacuum, and with values of up to 7.6, which reveals much stronger coupling between impurities and the crystal lattice than in previous studies. Both substitutional and interstitial ions are proposed as candidates for the observed interactions.Crystalline quartz is very important material extensively used in different areas of science and technology including optics, acoustics and device physics. In particular, unprecedented acoustic quality factors both at liquid helium and milli-Kelvin temperatures have been recently demonstrated 1-3 . To further progress these areas, ultra-pure materials are required, which depend on efficient refining and identification of residual impurities. These impurities are believed to be responsible for limitations in quality factors 4 and generation of the flicker noise as well as nonlinear effects at low temperatures 5 . In addition to applications of quartz itself, this material serves as a case study for understanding different defects in other silica-based materials.Quartz is one of the most widely used materials due to its exceptional purity. This originates in the stable atomic configuration, which allows only few elements from the periodic table to be present in the quartz crystalline structure as an impurity 6 . Nevertheless, the Electron Paramagnetic Resonance (EPR) studies of this material are relatively easy due to the very narrow linewidths which increase the method sensitivity. Very narrow linewidths are explained by the fact that none of the host constitutive nuclei (most abundent isotopes) have a spin moment 7 . This makes quartz crystal a good candidate as a host material for Cavity Quantum Electrodynamics experiments, which interact spin ensemble impurities to microwave frequency photons 8,9 .The imperfection of quartz crystals are related to substitutional and interstitial impurity ions (Al, H, Cu, Ag, Ge, P, Ti, Fe, etc) as well as vacancy centres (E ) associated with oxygen ions missing in the crystal structure. Trivalent substitutional ions such as Al 3+ , Fe 3+ , Ge 3+ and Ti 3+ are typically accompanied by monovalent impurity ions, such as H + , Li + , Na + , which are interstitially positioned in the crystal as charge compensators. There is a large number of experimental and theoretical studies dedicated to different representatives of these impurities and corresponding bonds 6,7,10 . domain using various methods such as Infrared Spectroscopy, dielectric relaxation spectroscopy and thermoluminesence 11 , etc. In contrast, microwave spectroscopy has been limited mo...

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