Quantum Cafe: Kamran Behnia

Title: Superconductivity, Charge Transport and Phonon Hydrodynamics in Strontium Titanate

Abstract:
Long-range ferroelectric order is aborted in strontium titanate because of quantum fluctuations. As a consequence, the low-temperature static electric permittivity becomes extremely large and the effective Bohr radius of the order of a micron. This is why removing a tiny fraction of oxygen atoms turns the system to a dilute metal, which has both a sharp Fermi surface and a superconducting instability. Substituting strontium with calcium stabilizes a long-range ferroelectric order in Sr1-xCaxTiO3 coexisting with metallicity and its superconducting instability in a narrow window of doping. As the carrier concentration is increased, the ferroelectric order is eventually destroyed by a quantum phase transition and the superconducting critical temperature is enhanced. Recent analysis points to the failure of Boltzmann-Drude picture of conductivity in explaining charge transport of this dilute metal. Thermal transport in the insulator detects the Poiseuille flow of phonons in a narrow temperature window as a consequence of strong coupling between the ferroelectric soft mode and acoustic phonons.