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The model magnet LiY1-xHoxF4 has been shown to exhibit a variety of quantum many-body phenomena, such as quantum phase transitions, quantum annealing, long lived coherent oscillations and long-range entanglement, making LiY1-xHoxF4 a promising candidate for the implementation of solid state qubits. The magnetic moment of the Holmium atoms stems from the well screened f-shell electrons and the dynamics is largely dominated by dipolar interaction which can be tuned by doping concentration x. The energy levels of the rare-earth magnetic ion develop as follows: The degeneracy of the free-atom electron states arranged by the native strong spin-orbit interaction is lifted by the tetragonal crystal lattice symmetry (point group S4) and subsequently further split by the hyperfine interaction with the nuclear spin I=7/2.
Earlier work optically probed the transition from the eightfold hyperfine-split ground state to the second excited state in a Fourier transform infrared (FTIR) spectrometer with a lab infrared source and 1.2 m optical path difference (OPD), hence with limited signal to noise ratio and resolution. We present data using high brilliance synchrotron radiation light in the far infrared regime from the Swiss Light Source (SLS) at Paul Scherrer Institut in Switzerland taken with a high resolution FTIR spectrometer featuring 11 m OPD allowing us to probe the ground state to second excited state transition hyperfine lines with unprecedented precision of 0.00077 cm-1 which corresponds to 23 MHz. This precision allows us to extract the full width half maximum (FWHM) of the hyperfine linewidths as function of temperature and three different concentrations (x=0.3%, 0.25%, 0.1%). We observe Arrhenius behavior of the linewidths as a function of temperature and decreasing linewidths for decreasing concentrations. For the lowest doping x = 0.1% and T=6 K we find an average FWHM of 0.006 cm-1, which corresponds to 180 MHz and a lower bound lifetime of 0.46 ns.
As a next step, we push towards a more detailed examination of the absorption line shapes and intensities, and measurements of lower Holmium doping concentrations as well as other compounds with sharp absorption lines in the infrared regime.
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