Phonon Structure in Tunneling in SrTiO3

Structure has been observed at liquid-helium temperatures in tunneling-current—versus—voltage curves for contacts formed on heavily reduced strontium titanate (SrTi${\mathrm{O}}_3$). Such contacts, formed by soldering indium to a freshly polished SrTi${\mathrm{O}}_3$ crystal, behave as backward diodes. Both first- and second-derivative techniques have been used to show that the structure in the tunneling current for forward-biased (metal positive) contacts occurs at voltages corresponding to the energies of the two highest frequency $q=o$ longitudinal optical phonons of pure SrTi${\mathrm{O}}_3$, at twice these energies, and at the sum of these energies. The semiconducting SrTi${\mathrm{O}}_3$ used in these experiments had a superconducting transition temperature of about 0.25°K; however, no change in the $I-V$ characteristics was observed when the SrTi${\mathrm{O}}_3$ became superconducting.