Abstract
${\text{Cr}}^{5+}$-doped ${\text{K}}_{3}{\text{NbO}}_{8}$, considered to be useful as an electron spin qubit, has been investigated by pulsed $X$ band $(\ensuremath{\sim}9.7\text{ }\text{GHz})$ and 240 GHz electron paramagnetic resonance and electron nuclear double resonance (ENDOR). Comparison of the low temperature electronic spin-lattice relaxation rate $1/{T}_{1}$ at 9.7 and 240 GHz shows that it is 250 times faster at 240 GHz than at $X$ band. On the other hand, spin-spin relaxation rate $1/{T}_{2}$ appears largely frequency independent and is very likely related to the superhyperfine (SHF) coupling of the ${\text{Cr}}^{5+}$ electron with the surrounding potassium and niobium nuclei. This coupling was investigated by hyperfine sublevel correlation spectroscopy at 9.7 GHz and pulsed Mims ENDOR at 240 GHz. The use of high frequency and field enabled us to unambiguously measure the hyperfine and quadrupole couplings of the $^{39}\text{K}$ in spite of its small magnetic moment. We find that the largest $^{39}\text{K}$ SHF coupling is positive, with 0.522 and 0.20 MHz as its isotropic and dipolar parts, respectively. $^{93}\text{N}\text{b}$ ENDOR was dominantly due to its quadrupolar interaction with a coupling of about 0.8 MHz and a SHF coupling of about 0.08 MHz. The significance of these data to spin qubit studies is pointed out.
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