Ferroelectricity, superconductivity and vortex-fluctuations at oxide interfaces.

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The two dimensional electron liquid formed at oxide interfaces is a laboratory for studying electronic properties in tunable correlated system where various degrees of freedom of the interfacial constituents can be combined. The electronic properties of such interface are sensitive to symmetry changes imposed by the interface itself and to various phase transitions in the constituent materials. In this seminar I will present our recent study of various polar oxides/SrTiO₃-based interfaces.

The triangular lattice of the (111) LaAlO₃/SrTiO₃ interface is reflected in a six-fold anisotropic magnetoresistance [1].  Superconductivity is observed in a dome-shaped region in the carrier density – temperature phase diagram. The superconducting transition temperature follows the same behavior as the spin-orbit interaction suggesting a link between these quantities [2].

Upon removing carriers by gate voltage this interface is driven into a highly insulating regime where the sheet resistance is significantly larger than the quantum one. We use this interface to study the superconductor-to-insulator transition as a function of electrostatic gate and magnetic field. This crystalline interface surprisingly exhibits very strong features observed previously only in amorphous systems. These features persist deep into the insulating state. We identify a new magnetic field scale, H_pairing, where superconducting fluctuations are muted and find a lengthscale x_ins interpreted as the size of the vortex fluctuation in the insulating state. Our findings suggest that vortex fluctuation excitations and Cooper pair localization are responsible for the observed superconductor to insulating transition and that these excitations surprisingly persist deep into the insulating state. [3]

Finally, we study the case where a small amount of Sr atoms in SrTiO₃ is replaced with calcium. This gives rise to a ferroelectric phase transition below a Curie temperature that increases with the level of Ca substitution. We discuss the various effects of ferroelectricity on the superconducting state. [4]  

[1] P.K. Rout, I. Agireen, E. Maniv, M. Goldstein, Y. Dagan Physical Review B 95 (24), 241107

[2] P.K. Rout, E. Maniv, Y. Dagan, Physical review letters 119 (23), 237002

[3] M. Mograbi et al. Accepted for publication in arXiv:1805.09574

[4] G. Tuvia P. K. Rout et al. To be published.