Understanding electron and nuclear spin dynamics in Cr$^{5+}$ doped K$_{3}$NbO$_{8}$

Chromium(V) doped in the diamagnetic host potassium niobate, a simple spin $S=\raise.5ex\hbox{$\scriptstyle 1$}\kern-.1em/ \kern-.15em\lower.25ex\hbox{$\scriptstyle 2$} $, $I$ = 0 system, has been proposed as an alternative standard for field calibration and g-standard for high-field EPR [1]. This system constitutes a dilute two-level model relevant for use as a electron spin qubit [2] and as such coherent electron spin manipulation at X-band ($\sim $9.5 GHz) was observed over a wide range temperature. Rabi oscillations are observed for the first time in a spin system based on transition metal oxides up to room temperature. At 4 K, a Rabi frequency $\Omega _{R}$ of 20 MHz together with the phase coherence relaxation (spin-spin relaxation) time, $T_{2}$ of $\sim $10 $\mu $s results in the single qubit figure of merit $Q_{M}$ (=$\Omega _{R}T_{2}$/$\pi )$ as about 500, showing that a diluted ensemble of Cr(V)$ ($S = 1/2) doped K$_{3}$NbO$_{8}$ is a potential candidate for solid-state quantum information processing. Also, the field and temperature dependence of the $T_{1}$ (spin-lattice relaxation) and $T_{2}$ times was investigated [3] for a further understanding of the relaxation mechanisms governing the phase decoherence in this system. These studies show that the coupling of the electron spin with the neighboring $^{39}$K nuclei ($I$ = 3/2) is one of the prominent $T_{2}$ mechanisms. The hyperfine and quadrupole interactions with $^{39}$K nuclei was resolved by using the high-frequency (240 GHz) pulsed electron nuclear double resonance (ENDOR). \\[3pt] [1]. B. Cage, A. Weekley, L. -C. Brunel and N. S. Dalal, \textit{Anal. Chem.} \textbf{71}, 1951 \textbf{(}1999). \\[0pt] [2]. S. Nellutla, K.-Y. Choi, M. Pati, J. van Tol, I. Chiroescu and N. S. Dalal, \textit{Phys. Rev. Lett.} \textbf{99}, 137601 (2007). \\[0pt] [3]. S. Nellutla, G. W. Morley, M. Pati, N. S. Dalal and J. van Tol, \textit{Phys. Rev. B.} \textbf{78}, 054426 (2008).