We report the hydroxide (OH ad ) and oxide (O ad ) experimental electroadsorption free energies, their dependences on pH, and their correlations to the oxygen evolution reaction (OER) electrocatalysis on RuO 2 (110) surface. The Sabatier principle predicts that catalyst is most active when the intermediate stabilization is moderate, not too strong such that the bound intermediate disrupts the subsequent catalytic cycle, nor too weak such that the surface is ineffective. For decades, researchers have used this concept to rationalize the activity trend of many OER electrocatalysts including RuO 2 , which is among the state-of-the-art OER catalysts. In this article, we report an experimental assessment of the Sabatier principle by comparing the oxygen electroadsorption energy to the OER electrocatalysis for the first time on RuO 2 . We find that the OH ad and O ad electroadsorption energies on RuO 2 (110) depend on pH and obey the scaling relation. However, we did not observe a direct correlation between the OH ad and O ad electroadsorption energies and the OER activity in the comparative analysis that includes both RuO 2 (110) and IrO 2 (110). Our result raises a question of whether the Sabatier principle can describe highly active electrocatalysts, where the kinetic aspects may influence the electrocatalysis more strongly than the electroadsorption energy, which captures only the thermodynamics of the intermediates and not yet kinetics.

We employ reactive molecular-beam epitaxy to synthesize the metastable perovskite SrIrO3 and utilize in situ angle-resolved photoemission to reveal its electronic structure as an exotic narrowband semimetal. We discover remarkably narrow bands which originate from a confluence of strong spin-orbit interactions, dimensionality, and both in-and out-of-plane IrO6 octahedral rotations. The partial occupation of numerous bands with strongly mixed orbital characters signals the breakdown of the single-band Mott picture that characterizes its insulating two-dimensional counterpart, Sr2IrO4, illustrating the power of structure-property relations for manipulating the subtle balance between spin-orbit interactions and electron-electron interactions. PACS numbers: Keywords:The combination of strong spin-orbit interactions (SOIs) with electron-electron correlations has recently been predicted to realize a variety of novel quantum states of matter, including topological Mott insulators [1, 2], quantum spin Hall, quantum anomalous Hall, and axion insulators [3-6], Weyl semimetals [7], and even high temperature superconductors [8]. Although typically viewed to be disparate properties, the recent discovery that SOI can sufficiently enhance the effective role of electron correlations to stabilize a Mott-like insulating state in the quasi-two-dimensional 5d transition metal oxide Sr 2 IrO 4 [9, 10] has opened a new frontier for exploring the rare interplay between these 2 degrees of freedom. Its three-dimensional perovskite analogue, SrIrO 3 , has been theoretically proposed as a key building block for engineering topological phases at interfaces and in superlattices [3,11,12]. In bulk, SrIrO 3 is believed to lie in close proximity to a metal-insulator transition [13,14], yet little is known of its electronic structure to date.Here we reveal the momentum-resolved electronic structure of SrIrO 3 using a combination of reactive oxide molecular-beam epitaxy (MBE) and in situ angleresolved photoemission spectroscopy (ARPES). Our measurements uncover an exotic semimetallic ground state, hosting an unusual coexistence of heavy holelike and light electronlike bands. Contrary to conventional expectations that increased coordination leads to broader bands in higher-dimensional materials [13], we find that the bandwidths of SrIrO 3 are, instead, narrower than its insulating two-dimensional counterpart. By combining first-principles calculations with spectroscopic measurements, we uncover the surprising interplay of spinorbit interactions, dimensionality, and octahedral rotations which drives the narrow-band, semimetallic state in SrIrO 3 . Our results indicate that subtle changes in the structure and rotation angles should drive substantial changes in the electronic structure and physical properties of SrIrO 3 . This highlights the important structure-property relationships in correlated quantum materials [15,16], much like in ferroelectrics and multiferroics [17]. This Letter also underscores that simple toy models which neglect these str...

Superconductivity is among the most fascinating and well-studied quantum states of matter. Despite over 100 years of research, a detailed understanding of how features of the normal-state electronic structure determine superconducting properties has remained elusive. For instance, the ability to deterministically enhance the superconducting transition temperature by design, rather than by serendipity, has been a long sought-after goal in condensed matter physics and materials science, but achieving this objective may require new tools, techniques and approaches. Here, we report the transmutation of a normal metal into a superconductor through the application of epitaxial strain. We demonstrate that synthesizing RuO2 thin films on (110)-oriented TiO2 substrates enhances the density of states near the Fermi level, which stabilizes superconductivity under strain, and suggests that a promising strategy to create new transition-metal superconductors is to apply judiciously chosen anisotropic strains that redistribute carriers within the low-energy manifold of d orbitals.

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