Seminář ÚFKL: Dirk van der Marel

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Dirk van der Marel (Département de Physique de la Matière Quantique Université de Genève):
The mechanism of superconducting pairing in doped strontium titanate

Abstrakt:

Strontium titanate is a semiconducting material, which can be tuned to become ferro-electric by substituting the heavier ¹⁸O isotope for the natural ¹⁶O isotope. When doped with electrons, the material becomes a good metal with a resistivity that rises with temperature proportional to T². The doped material is superconducting at low temperatures, even down to doping levels where the average distance between the conduction electrons is more than 200 Angstrom. This state of affairs has lead to many speculations as to the mechanism that binds the electrons together in Cooper-pairs. Among the many proposals, the interplay between ferro-electricity and superconducting pairing has been particularly prominent in recent years and has lead to the prediction of a very large increase of superconducting T_c with ¹⁸O isotope substitution. While experimental studies in various laboratories have confirmed this prediction, the role of soft ferroelectric modes as a pairing glue has been challenged on the basis of symmetry selection rules for the relevant electron-phonon matrix elements. We have studied this phenomenon experimentally using optical spectroscopy. We show that pairing mediated by soft ferroelectric modes is in fact possible, but involves two-phonon exchange processes, which are allowed by symmetry. The coupling parameters are derived from the observed "charged phonon" effect in SrTiO₃, and are found to have the correct magnitude to explain the superconducting T_c mediated by the exchange of pairs of transverse optical phonons.