Strain and High Temperature Superconductivity: Unexpected Results from Direct Electronic Structure Measurements in Thin Films

Abstract

Angle-resolved photoemission spectroscopy reveals very surprising strain-induced effects on the electronic band dispersion of epitaxial La(2-x)Sr(x)CuO(4-delta) thin films. In strained films we measure a band that crosses the Fermi level (E(F)) well before the Brillouin zone boundary. This is in contrast to the flat band reported in unstrained single crystals and in our unstrained films, as well as in contrast to the band flattening predicted by band structure calculations for in-plane compressive strain. In spite of the density of states reduction near E(F), the critical temperature increases in strained films with respect to unstrained samples. These results require a radical departure from commonly accepted notions about strain effects on high temperature superconductors, with possible general repercussions on superconductivity theory.

Electron-Phonon Interaction in the High- Cuprates in the Framework of the Van Hove Scenario

“…Labbé and Bok [5] proposed the Van Hove scenario for the cuprates, using electron-phonon interaction and predicted an anomalous isotope effect. The presence of saddle points (or VHs) near the Fermi level has been confirmed by many experiments, in particular by Angular Resolved Photoemission Spectroscopy (ARPES) [6,7] in different compounds.…”

Strain-engineering Mott-insulating La2CuO4

“…Before fitting our results, it is useful to consider the ratios U / t , t ′/ t and t ″/ t ′ for single-layer cuprate systems. In-plane strain will enhance oxygen- p to copper- d orbital hybridisations and hence the effective nearest-neighbour hopping t in the one-band Hubbard model description 21 . This trend can be calculated from approximate numerical methods such as density functional theory (DFT) [see Fig.…”

In-situ angle-resolved photoemission spectroscopy of copper-oxide thin films synthesized by molecular beam epitaxy

“…Moreover, if an atomic-layer-by-layer 1 (ALL-MBE) synthesis mode is achieved, one can terminate the film growth at a desired atomic layer, as well as study the effects of epitaxial strain, proximity effects in heterostructures, etc. But this comes at the expense of substantial technical sophistication and challenges, and hence just a few systems in which complex-oxide MBE synthesis is integrated under ultra-high vacuum (UHV) conditions with ARPES 2,3,4,5,6,7,8 and/or STM 9 , have been built and operated so far.…”