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Tunneling to the STM tip is argued to happen primarily due to the d x z/ d y z states in the sample, with the justification that d x y states associated with the γ-band have lobes that lie in the plane, while d x z/ d y z states have lobes pointing out of the surface plane towards the tip (see also Fig. 18 argued that the gap observed in tunneling corresponds to that on the 1D d x z/ d y z bands, with the d x y band not contributing to tunneling spectra but still exhibiting a sizeable gap due to proximity coupling. In scanning tunneling microscopy (STM) measurements, the situation has been less clear, and not all bands found in the bulk have been detected so far: Firmo et al. This vHs has not only an important role in the properties of Sr 2RuO 4, but also of the bilayer and trilayer ruthenates, where the van Hove singularity has been suggested to be the origin of the metamagnetic behavior 17.
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The γ-band has a van Hove singularity (vHs) which in the bulk is ~14 meV above the Fermi energy 14, but whose energy depends sensitively on small structural changes 15 with significant consequences for the superconductivity 16. The electronic structure in the bulk of Sr 2RuO 4 near the Fermi energy is well-known to consist of weakly hybridized 1D sheets ( α and β) of d x z/ y z character, as well as a 2D d x y sheet ( γ) that hybridizes with both. Significant progress has recently been made towards achieving this goal. STM is a more appropriate tool, which due to its very high energy resolution that can be achieved at low temperatures and the ability to obtain information about the momentum- and phase-resolved structure of the superconducting gap through quasi-particle interference (QPI) imaging 12, 13 promises to resolve the most pressing questions about the superconducting properties of Sr 2RuO 4. However, the energy scales involved, such as the transition temperature of only 1.5 K in Sr 2RuO 4, or the temperature at which the metamagnetic transitions in Sr 3Ru 2O 7 occur, are beyond the capabilities of current ARPES instruments. In principle, direct measurement of the superconducting gap by, e.g., angular resolved photoemission spectroscopy (ARPES), could provide important guidance, as it did in the cuprates. Recently, several new experimental results 7, 8, 9, 10 have called into question the NMR results on which the traditional triplet pairing scenario was based 11, providing evidence for spin-singlet rather than triplet pairing and leading to a renaissance in the quest to identify the exact pairing state of this fascinating material.
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Much of the interest in the community centered on the possibility of chiral p-wave pairing, but the compound has also attracted attention simply because of its structural similarity to the cuprates, Fermi liquid behavior at low temperatures, and the availability of very clean samples with high-quality surfaces. Strontium Ruthenate, Sr 2RuO 4, has played a leading role in discussions of unconventional superconductivity since its discovery almost three decades ago 1, 2, 3, 4, 5, 6.
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We trace its dispersion and demonstrate from a model calculation accounting for the full vacuum overlap of the wave functions that its detection is facilitated through the octahedral rotations in the surface layer. Here, we determine the electronic structure of the van Hove singularity in the surface layer of Sr 2RuO 4 by quasi-particle interference imaging. Tiny structural distortions move the van Hove singularity across the Fermi energy with dramatic consequences for the physical properties. The superconductivity as well as the properties of the multi-layered compounds of the ruthenate perovskites are strongly influenced by a van Hove singularity in proximity of the Fermi energy. While for many years it was thought to be the best candidate for a chiral p-wave superconducting ground state, desirable for topological quantum computations, recent experiments suggest a singlet state, ruling out the original p-wave scenario. The single-layered ruthenate Sr 2RuO 4 is one of the most enigmatic unconventional superconductors.