High-Resolution Photoemission on Sr2RuO4 Reveals Correlation-Enhanced Effective Spin-Orbit Coupling and Dominantly Local Self-Energies

Abstract

We explore the interplay of electron-electron correlations and spin-orbit coupling in the model Fermi liquidSr2RuO4using laser-based angle-resolved photoemission spectroscopy. Our precise measurement of the Fermi surface confirms the importance of spin-orbit coupling in this material and reveals that its effective value is enhanced by a factor of about 2, due to electronic correlations. The self-energies for theβandγsheets are found to display significant angular dependence. By taking into account the multi-orbital composition of quasiparticle states, we determine self-energies associated with each orbital component directly from the experimental data. This analysis demonstrates that the perceived angular dependence does not imply momentum-dependent many-body effects but arises from a substantial orbital mixing induced by spin-orbit coupling. A comparison to single-site dynamical mean-field theory further supports the notion of dominantly local orbital self-energies and provides strong evidence for an electronic origin of the observed nonlinear frequency dependence of the self-energies, leading to “kinks” in the quasiparticle dispersion ofSr2RuO4.

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Plain Language Summary

Metals have long-lived low-energy excitations known as “quasiparticles”—collective entities that can be pictured as individual electrons dressed up by interactions with other electrons. If these interactions are strong, quasiparticles move slower and their mass can increase to values far beyond the mass of a bare electron. While the existence of such a mass enhancement is well known in many metallic oxides and can be seen in simple specific-heat experiments, its precise origin remains debated even in the most intensely studied materials. Here, we use high-precision electron spectroscopy and new theoretical analysis to investigate the origin of the mass enhancement in the famous oxide superconductorSr2RuO4.

We determine the variation of the mass enhancement along the entire Fermi surface, which reveals a strong dependence on the direction of the quasiparticle momentum. This is often interpreted as a signature of nonlocal interactions that are strong at certain momenta and weak at others. However, using a new method to analyze our data, we show that this momentum dependence is apparent only if the problem is described in a momentum-space basis and disappears almost completely when electronic states are described in a basis of local orbitals.

This finding establishes that electronic orbitals associated with each atom in the lattice continue to have direct physical relevance in an itinerant metal, where electrons travel over thousands of atoms and behave as long-lived waves. It also implies that the high mass of electronic quasiparticles arises predominantly from local electron-electron interactions. Our study further reveals how quasiparticles get gradually “undressed” as their energies become larger, revealing a far richer energy dependence than previously anticipated.