Isotropic negative area compressibility, which is very rare, is observed in KBBF and the related mechanism is investigated by combined high-pressure X-ray diffraction (XRD) experiments and first-principles calculations. The strong mechanical anisotropy leads to a large Poisson's ratio and high figure of merit for the acoustic-optics effect, giving KBBF potential applications as smart strain converters and deep-ultraviolet (DUV) acoustic-optic devices.

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First-principles calculations in combination with group theory analyses were employed to study the spin-polarized electronic structures of C B V N centers consisting of a nitrogen vacancy and a substitutional carbon atom in hexagonal boron nitride (h-BN) monolayer with different charge states.It is clarified that the paramagnetic neutral C B V N center is stable in the n-type h-BN monolayer. The neutral defect center possesses a triplet (S=1) ground state and with two spin-conserved optical vertical transition. Its spin coherence time is estimated to be 3.9 ms at T = 0 K by a simple scheme combining the mean-field theory and the first-principles calculations. The results indicate that the neutral C B V N center is very suitable for achieving spin qubit.