Strong Spin-Orbit Coupling Effects on the Fermi Surface of Sr$_{2}$RuO$_{4}$

The Fermi surface of Sr$_{2}$RuO$_{4}$ was studied by a wide variety of probes, establishing this material as the first complex oxide for which the de Haas-van Alphen bulk transport technique [1] and surface- sensitive angle-resolved photoemission spectroscopy (ARPES) [2] have arrived at a precise quantitative agreement. This result was obtained by exploiting temperature as an empirical cleaving parameter in suppressing the photoemission intensity associated with the reconstructed surface of the material [2]. On the basis of STM experiments [3], we have been able to show that this is a consequence of a temperature-dependent increase in the surface density of defects at the mesoscopic scale, and might be used as an effective mean to gain bulk-representative information by ARPES on unstable oxide surfaces. By comparing these bulk ARPES results to first-principle calculations, we provide evidence for the importance of spin-orbit coupling effects [4]. Subtle Fermi surface modifications are observed whenever the bands are nearly degenerate; most importantly, however, spin-orbit coupling induces a strong momentum dependence, normal to the RuO$_2$ planes, for both orbital and spin character. These findings have profound implications for the understanding of unconventional superconductivity in Sr$_{2}$RuO$_{4}$.\\[4pt] [1] Bergemann {\it et al.}, PRL {\bf 84}, 2662 (2000).\\[0pt] [2] A. Damascelli {\it et al.}, PRL {\bf 85}, 5194 (2000).\\[0pt] [3] Y. Pennec {\it et al.}, PRL {\bf 101}, 216103 (2008).\\[0pt] [4] M.W. Haverkort {\it et al.}, PRL {\bf 101}, 026406 (2008).