Broadband Relaxation Dynamics of Boron-Vacancy Centers in Hexagonal Boron Nitride

The negatively charged boron vacancy center (VB⁻) in hexagonal boron nitride (hBN) has attracted attention for its potential applications in quantum sensing. While GHz-scale sensing at low magnetic fields has been demonstrated with these defects, their behavior at high fields remains largely unexplored. We investigate the spin relaxation dynamics of VB⁻ centers over temperatures of $15-250$ K and magnetic fields of up to $7$ T, corresponding to a ground-state splitting of ~ 200 GHz. Our results uncover distinct relaxation regimes, transitioning from spin-spin-interaction-driven and disorder-induced stretched exponential dynamics at low temperatures and fields to relaxation dominated by single-phonon processes at elevated magnetic fields. We extract temperature- and magnetic-field-dependent scaling behaviors of the relaxation rate to provide a quantitative picture of the interactions between VB⁻ centers and their environment. Our results pave the way towards high-field, sub-terahertz quantum sensors based on two-dimensional spin-defect platforms.