Many of the exciting properties of strongly correlated materials are
intricately linked to quantum critical points in their phase diagram. This
includes phenomena such as high temperature superconductivity, unconventional
superconductivity in heavy fermion materials, as well as exotic nematic states
in Sr3βRu2βO7β. One of the experimentally most successful pathways to
reaching a quantum critical point is tuning by magnetic field allowing studies
under well-controlled conditions on ultra-clean samples. Yet, spectroscopic
evidence of how the electronic states change across a field-tuned quantum phase
transition, and what the importance of quantum fluctuations is, is not
available so far. Here we show that the surface layer of Sr2βRuO4β is an
ideal two-dimensional model system for a field-tuned quantum phase transition.
We establish the existence of four van Hove singularities in close proximity to
the Fermi energy, linked intricately to checkerboard charge order and
nematicity of the electronic states. Through magnetic field, we can tune the
energy of one of the van Hove singularities, with the Lifshitz transition
extrapolated at ~32T. Our experiments open up the ability to directly study
spectroscopically the role of quantum fluctuations at a field-tuned quantum
phase transition in an effectively 2D strongly correlated electron material.
Our results further have implications for what the leading instability in
Sr2βRuO4β is, and hence for understanding the enigmatic superconductivity
in this material.Comment: 31 pages, 4 figure