Double electron-electron resonance measurements of diamond to determine {\it T}$_2$ dependence on concentration of paramagnetic impurities*

A nitrogen-vacancy (NV) center in diamond is a promising candidate for investigation of fundamental sciences and applications to a nanoscale magnetic field sensing device because of unique properties of a NV center in diamond including capability to detect optically detected magnetic resonance (ODMR) signals from a single NV and initialize its spin state. Fundamental studies and applications of NV centers relay on coherent control of the NV centers that is limited by decoherence time ({\it T}$_2$) and, as often observed, {\it T}$_2$ is limited by paramagnetic impurity contents in diamond crystals. In this work, we will investigate {\it T}$_2$ dependence on concentration of nitrogen impurities in type-Ib and type-IIa diamond crystals. For precise determination of the nitrogen concentration, we employ a home-built high-frequency electron spin resonance spectrometer which enables broadband double electron-electron resonance spectroscopy with high spectral resolution.~[1,2] \\$[1]$ F.~H.~Cho, V.~Stepanov and S.~Takahashi, Rev. Sci. Instrum. \textbf{85} , 075110 (2014). \\$[2]$ F.~H.~Cho, V.~Stepanov, C.~Abeywardana and S.~Takahashi, Methods Enzymol. 563 , \textbf{95} (2015).