Phonon spectroscopy of lithium-doped KBr

We have measured the uniaxial stress dependence of phonon absorption lines in KBr doped with ${\mathrm{Li}}^{+}$ over the frequency range 75-225 GHz. Several zero-stress lines are observed which exhibit a large (∼40%) isotope shift when ${}_{}{}^6{}_{}{}^{}\mathrm{Li}$ replaces ${}_{}{}^7{}_{}{}^{}\mathrm{Li}$, clearly indicating a tunneling behavior. The general behavior of this system including the magnitude of the stress coupling is similar to other paraelectric-paraelastic centers found in the alkali halide. However, in detail, it is more complex than expected from the simple off-center tunneling models. Similar results were found earlier in an extensive paraelectric resonance study. These investigations indicate the need for further theoretical work.