Low-Temperature EPR Measurements onin situVacuum-Cleaved Silicon

The $g=2.0055$ EPR signal from a silicon crystal well cleaved in situ in ultrahigh vacuum at low temperatures is sensitive to gases and can be as low as (3 ±{} 2) ×{} 10¹² spins cm^-2, which is more than an order of magnitude lower than reported previously for a single crystal or crushed powers. The signal is now believed to arise mainly not from electrons at ordinary surfaces but ones trapped in atomic mismatch regions in the bases of microcracks.