Isotope-Induced Symmetry Change in Dynamic Semiconductor Defects

Two hydrogen nuclei and a hole bind to the triple-acceptor copper in germanium. The result is a single acceptor whose electronic states are modified by coupling to the zero-point motion of the nuclei. That motion displays a qualitative change from rotation to libration induced by an increase in hydrogen isotopic mass. The acceptor $A\left(\mathrm{Cu}{\mathrm{H}}_2\right)$ has full tetrahedral symmetry and a complex ground-state manifold. All heavier isotope combinations display only a single ground-state component of symmetry lower than tetrahedral.