Low-field optically detected magnetic resonance of a coupled triplet-doublet defect pair in diamond

Microwave-induced changes in the optical emission of the N-V center in diamond have previously been attributed to magnetic resonance of the defect in its ${}_{}{}^3{}_{}{}^{}$A ground state [E. van Oort et al., J. Phys. C 21, 4385 (1988)]. In this paper, the focus is on the origin of the hyperfine splittings that are observed in the optically detected magnetic-resonance (ODMR) spectra at zero and low magnetic fields. Detailed analysis of the anisotropic behavior of the line splittings in a magnetic field shows that the hyperfine structure arises from triplet-doublet magnetic dipolar interactions of N-V centers and ${\mathit{P}}_1$ centers (with S=1/2). The ODMR spectrum of the N-V center appears to display the nitrogen hyperfine interaction characteristic of the ${\mathit{P}}_1$ center.