Spectroscopy with Phonons on ${\mathrm{Al}}_2$${\mathrm{O}}_3$: ${\mathrm{V}}^{3+}$ Using the Phonon Bremsstrahlung of a Superconducting Tunnel Junction

The sharp edge of the phonon "bremsstrahlung" spectrum of a superconducting tunnel junction was isolated by modulation. This yielded a monochromatic, voltage-tunable phonon source which was applied for phonon spectroscopy on ${\mathrm{V}}^{3+}$ ions in ${\mathrm{Al}}_2$ ${\mathrm{O}}_3$. At a frequency corresponding to the ground-state splitting of ${\mathrm{V}}^{3+}$ an absorption dip was obtained due to resonant scattering. A frequency resolution of about 3% of the energy gap was achieved.