  The methods for controlling spin states of negatively charged
nitrogen-vacancy (NV) centers using microwave (MW) or radiofrequency (RF)
excitation fields for electron spin and nuclear spin transitions are effective
in strong magnetic fields where a level anti-crossing (LAC) occurs. A LAC can
also occur at zero field in the presence of transverse strain or electric
fields in the diamond crystal, leading to mixing of the spin states. In this
paper, we investigate zero-field LAC of NV centers using dual-frequency
excitation spectroscopy. Under RF modulation of the spin states, we observe
sideband transitions and Autler-Townes splitting in the optically detected
magnetic resonance (ODMR) spectra. Numerical simulations show that the
splitting originates from Landau-Zener transition between electron spin
|$\pm$1> states, which potentially provides a new way of manipulating NV center
spin states in zero or weak magnetic field.
