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초록

The negatively charged boron vacancy (VB−) in h-BN is a spin-1 defect functioning as an optically addressable spin qubit in 2D materials. A precise understanding of its spin decoherence is essential to advance it into a robust qubit platform. First-principles quantum many-body simulations are employed to investigate VB− decoherence in dense nuclear spin baths of h-BN under magnetic fields from 0.01 to 3 T, considering isotopic variants h-10B14N, h-11B14N, h-10B15N, and h-11B15N. A transition boundary (TB) is observed where the dominant decoherence mechanism changes: below the TB, sub-microsecond decoherence is governed by independent nuclear spin dynamics, whereas above it, pairwise flip-flops dominate, extending T2 to tens of microseconds. Analytical predictions place the TB at 0.502 T for h-10B14N and 0.205 T for h-11B14N. The larger TB in h-10BN results from the larger nuclear spin of 10B (I = 3), which produces stronger nuclear modulation over a wider field range. The analytical approach also explains the magnetic-field-insensitive fast modulation observed below the TB. These findings clarify the role of dense nuclear spin baths with large nuclear spins (I ≥ 1) in VB− decoherence and provide design principles for isotopically engineered h-BN spin qubits.

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