Broadband electron paramagnetic resonance spectroscopy of Er 167 : LiYF 4 7 at millikelvin temperatures

Rare-earth spin ensembles are a promising platform for microwave quantum memory applications due to their hyperfine transitions, which can exhibit exceptionally long coherence times when using an operation point with zero first-order Zeeman (ZEFOZ) shift. In this work, we use broadband electron paramagnetic resonance (EPR) spectroscopy on <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"> <a:mmultiscripts> <a:mi>Er</a:mi> <a:mprescripts/> <a:none/> <a:mn>167</a:mn> </a:mmultiscripts> </a:math> : <b:math xmlns:b="http://www.w3.org/1998/Math/MathML"> <b:mmultiscripts> <b:mi>LiYF</b:mi> <b:mn>4</b:mn> <b:none/> <b:mprescripts/> <b:none/> <b:mn>7</b:mn> </b:mmultiscripts> </b:math> single crystals at subkelvin temperatures. By fitting the spin Hamiltonian to the zero-field spectrum, we obtain refined parameters of the magnetic field-independent interactions, such as the hyperfine and quadrupole interaction. We also study the influence of the quadrupole interaction on the hyperfine splitting in the zero and low magnetic field range by analyzing EPR spectra between <c:math xmlns:c="http://www.w3.org/1998/Math/MathML"> <c:mrow> <c:mn>0</c:mn> <c:mspace width="0.16em"/> <c:mi mathvariant="normal">m</c:mi> <c:mi mathvariant="normal">T</c:mi> </c:mrow> </c:math> and <g:math xmlns:g="http://www.w3.org/1998/Math/MathML"> <g:mrow> <g:mn>50</g:mn> <g:mspace width="0.16em"/> <g:mi mathvariant="normal">m</g:mi> <g:mi mathvariant="normal">T</g:mi> </g:mrow> </g:math> . Our findings highlight the broadband EPR spectroscopy approach as a powerful tool for the precise determination of the spin Hamiltonian parameters and for the characterization of hyperfine transitions in terms of their selection rules and linewidth.