April 2016: We publish in Nature Materials a study of electron-phonon interactions in the SrTiO3(001) two-dimensional electron liquid.
The band insulator strontium titanate, SrTiO3, widely used as substrate for growing oxides films, is a highly fascinating material in its own right. Most notably, it becomes superconducting upon doping with less than 1 excess electron per 104 Ti ions. This implies an exceptionally strong attractive interaction among dilute carriers, a fascinating property of electrons in certain quantum solids that is not understood to date. Moreover, when interfaced with other materials, SrTiO3 plays a crucial role in creating novel physical properties. For instance, combined with a thin layer of LaAlO3, stoichiometric SrTiO3 can host a 2D electron liquid that displays superconductivity and is already used as a platform for the realization of next-generation electronic devices. A second example is the appearance of superconductivity above the temperature of liquid nitrogen in a single monolayer of FeSe grown on STO. This critical temperature is higher than in any iron-based bulk material suggesting a key-role of the SrTiO3 substrate.
In this work, scientists from PSI, Diamond Light Source and the University of Geneva teamed up to investigate why SrTiO3 is so special. Using high-resolution Angle-Resolved Photoemission Spectroscopy (ARPES), a uniquely powerful technique for visualizing the motion of electrons in crystalline solids, they studied how electrons interact with vibrations of the periodic lattice of ions as they propagate through SrTiO3. To this end, they first prepared the bare surface of SrTiO3 in a way to host a 2D electron liquid (2DEL) with tunable carrier density. Intriguingly, the ARPES measurements on this 2DEL revealed that at low carrier density, electrons are always accompanied by a dynamic lattice deformation. Together with this lattice deformation, they form a new composite quasiparticle called Fröhlich polaron, which still moves in a band-like fashion through the solid, although with an increased effective mass. Intriguingly, the long ranged interaction between electrons and phonons (quantized lattice vibrations) that creates Fröhlich polarons has long been considered favorable for superconducting pairing. However, it proved difficult to observe directly. Increasing the carrier density, the team could further monitor how the Fröhlich polarons dissociate into electrons and phonons that interact only weakly and over short distances. These findings provide for the first time a microscopic basis for understanding why superconductivity in different SrTiO3 based systems appears at a certain carrier density and disappears again as more carriers are added.
Wang et al. Tailoring the nature and strength of electron–phonon interactions in the SrTiO3(001) 2D electron liquid. Nature Materials (2016). DOI: 10.1038/NMAT4623