Spontaneous Symmetry Breaking of Acceptors in “Blue” Diamonds

The electronic Raman transition between the lower $1s\left(p_{3/2}\right)$ and the higher $1s\left(p_{1/2}\right)$ state of a hole bound to a boron acceptor in diamond, examined under the high resolution of a Fabry-Pérot interferometer, reveals a doublet separated by $(0.81\pm 0.15)$ ${\mathrm{cm}}^{-1\mathrm{}}$, indicative of a spontaneous symmetry breaking of the fourfold degenerate ground state. The direct transition between the levels into which $1s\left(p_{3/2}\right):{\Gamma }_8$ resolves, with a Raman shift of $(0.80\pm 0.04)$ ${\mathrm{cm}}^{-1\mathrm{}}$, provides a remarkable confirmation of the symmetry lowering shown to arise from a static Jahn-Teller distortion.