We report a direct measurement of the low-energy optical conductivity of large-area single crystal YbMgGaO4, a newly found spin-liquid candidate material, by means of time domain terahertz spectroscopy. No magnetic resonance absorption was observed in polarization dependent measurement down to 1.5K indicating absence of magnetic order. From magneto-terahertz measurement up to 7T within Faraday geometry, we observed an absorption feature which is linearly proportional to the applied magnetic field, yielding evidence for a Zeeman splitting of magnetic moment, from which we extract the value of in-plane g-factors g‖~4.2. Furthermore, we measure reflectivity and transmittance spectra between 10000cm−1 and 20000cm−1 using FTIR, our measurement reveal that the fine structure spectrum of YbMgGaO4 is similar to a single ion Yb3+ absorption features arising from ground state 2F7/2to 2F5/2 excitations in trigonal CEF. From optical measurement, we suggest that YbMgGaO4 is a nonmagnetic insulator and long range order do not develop down to 1.5K.
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