Piezospectroscopy of the triple acceptor neutral copper in germanium

Experimental and theoretical results are presented for the behaviour of the optical absorption lines of neutral copper, a triple acceptor, in germanium under applied uniaxial compression. The ground state for the three equivalent holes is found to be the four-fold degenerate Λ₈ state of $\text{T}{}_{\mathrm{d}}$. The interpretation of the results requires that, for F6〈111〉, there be no splitting of either the ground state or the excited state of the D-line. However, this interpretation produces a violation of the predicted selection rules for the G components with F6〈100〉.