Spontaneous Phonon Emission from Spin Defects in Hexagonal Boron Nitride

The spin-relaxation of negatively charged boron-vacancy centers (V_B^-) in hexagonal boron nitride (hBN) at high magnetic fields is dominated by a resonant single-phonon process. In the low-temperature limit, when the spin-splitting exceeds the thermal energy (k_B​T<ℏω), this process occurs through spontaneous phonon emission. While this regime has previously been accessible only at millikelvin temperatures, we realize it at 10 K by driving spin splitting up to 0.2 THz under 7 T magnetic field. As the temperature is reduced below 10 K (for 7 T field), the longitudinal spin relaxation rate (1/T₁) approaches a constant floor, consistent with the onset of spontaneous phonon emission. Our results establish (V_B^-) centers as a promising platform for studying spin–phonon interactions in the quantum regime.