Angle-resolved photoemission studies of uniaxial stress-driven Lifshitz transitions in the bulk and surface electronic structure of Sr₂RuO₄
International Max Planck Research School for Chemistry and Physics of Quantum Materials (IMPRS-CPQM)
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In experimental condensed matter physics, the utilisation of momentum-resolved probes has proven valuable in disentangling the underpinning effects driving the formation of rich collective states in quantum materials. Furthermore, the ability to tune relevant features in the electronic structure and control the breaking of particular symmetries, comprises a powerful route to stabilise phases and electronic states that are not available naturally to equilibrium chemistry. In this work, I show how one can simultaneously benefit from both approaches, specifically, with the development of a technique combining angle-resolved photoemission spectroscopy (ARPES) with the application of uniaxial stress. After a thorough discussion of the experimental method, I show its capabilities in the normal state of the unconventional superconductor Sr₂RuO₄, where the application of uniaxial pressure has recently been shown to more than double the transition temperature, leading to a peak in Tc versus strain. We directly visualise how uniaxial stress drives a Lifshitz transition of one of its three Fermi surfaces, which is in close proximity to a van Hove singularity (vHS), and we point to the key role of strain-tuning the vHS to the Fermi level in mediating the peak in Tc. Our measurements also provide stringent constraints for theoretical models of the strain-tuned electronic structure evolution of Sr₂RuO₄. In the bilayer sister compound Sr₃Ru₂O₇, in-plane rotations of the RuO₆ octahedra and the corresponding doubling of the in-plane unit cell turn the vHS into higher (4th) order. Tuning this extended vHS to the Fermi level with large magnetic fields is thought to drive an exotic nematic state to emerge, which exhibits signatures of quantum criticality. Interestingly, the octahedra rotations that characterise Sr₃Ru₂O₇ are also found in the surface layer of Sr₂RuO₄, potentially making such states accessible also at the surface of the single-layer compound. In this work, I show the evolution of the electronic structure at the surface layer of Sr₂RuO₄ under large in-plane uniaxial stress. From ARPES, we show how the induced strain drives a sequence of Lifshitz transitions, fundamentally reshaping the low-energy electronic structure and the rich spectrum of vHSs that the surface layer of Sr₂RuO₄ hosts. From comparison of tight-binding modelling to our measured dispersions, I show that, surprisingly, the strain is accommodated predominantly by bond-length changes rather than modifications of the octahedral tilt and rotation angles, thus shedding new light on the nature of structural distortions at oxide surfaces, and how targeted control of these can be used to tune density of states singularities to the Fermi level, in turn paving the way to the possible realisation of rich collective states at the surface of Sr₂RuO₄.
Thesis, PhD Doctor of Philosophy
Embargo Reason: Embargo period has ended, thesis made available in accordance with University regulations
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