We use complementary optical spectroscopy methods to directly measure the
lowest crystal-field energies of the rare-earth quantum magnet
LiY1−xHoxF4, including their hyperfine splittings, with more
than 10 times higher resolution than previous work. We are able to observe
energy level splittings due to the 6Li and 7Li
isotopes, as well as non-equidistantly spaced hyperfine transitions originating
from dipolar and quadrupolar hyperfine interactions. We provide refined crystal
field parameters and extract the dipolar and quadrupolar hyperfine constants
AJ=0.02703±0.00003cm−1 and B=0.04±0.01cm−1, respectively. Thereupon we determine all crystal-field
energy levels and magnetic moments of the 5I8 ground state manifold,
including the (non-linear) hyperfine corrections. The latter match the
measurement-based estimates. The scale of the non-linear hyperfine corrections
sets an upper bound for the inhomogeneous line widths that would still allow
for unique addressing of a selected hyperfine transition. e.g. for quantum
information applications. Additionally, we establish the far-infrared,
low-temperature refractive index of LiY1−xHoxF4.Comment: 9 pages, 6 Figures, 3 Table