Momentum-resolved superconducting energy gaps of Sr₂RuO₄ from quasiparticle interference imaging.

Abstract

Sr₂RuO₄ has long been the focus of intense research interest because of conjectures that it is a correlated topological superconductor. It is the momentum space (<i>k</i>-space) structure of the superconducting energy gap [Formula: see text] on each band <i>i</i> that encodes its unknown superconducting order parameter. However, because the energy scales are so low, it has never been possible to directly measure the [Formula: see text] of Sr₂RuO₄ Here, we implement Bogoliubov quasiparticle interference (BQPI) imaging, a technique capable of high-precision measurement of multiband [Formula: see text] At <i>T</i> = 90 mK, we visualize a set of Bogoliubov scattering interference wavevectors [Formula: see text] consistent with eight gap nodes/minima that are all closely aligned to the [Formula: see text] crystal lattice directions on both the α and β bands. Taking these observations in combination with other very recent advances in directional thermal conductivity [E. Hassinger <i>et al.</i>, <i>Phys. Rev. X</i> 7, 011032 (2017)], temperature-dependent Knight shift [A. Pustogow <i>et al.</i>, <i>Nature</i> 574, 72-75 (2019)], time-reversal symmetry conservation [S. Kashiwaya <i>et al.</i>, <i>Phys. Rev B</i>, 100, 094530 (2019)], and theory [A. T. Rømer <i>et al.</i>, <i>Phys. Rev. Lett.</i> 123, 247001 (2019); H. S. Roising, T. Scaffidi, F. Flicker, G. F. Lange, S. H. Simon, <i>Phys. Rev. Res.</i> 1, 033108 (2019); and O. Gingras, R. Nourafkan, A. S. Tremblay, M. Côté, <i>Phys. Rev. Lett.</i> 123, 217005 (2019)], the BQPI signature of Sr₂RuO₄ appears most consistent with [Formula: see text] having [Formula: see text] [Formula: see text] symmetry.