Density wave at the reconstructed surface of Sr₂RuO₄ and implications for quantum criticality

Many of the exciting properties of strongly correlated electron materials, including high-temperature superconductivity, are intricately linked to proximity to a quantum phase transition, a transition through which a material can be tuned as a function of an external control parameter other than temperature. Here, we propose that the reconstructed surface layer of Sr₂RuO₄ constitutes an ideal material to study the physics of quantum criticality.
Using ultra-low temperature scanning tunneling microscopy, we observe the signature of a density wave in topographic images of the reconstructed surface of Sr₂RuO₄ which is linked with a sharp peak in the tunneling spectra, a few mV below the Fermi energy. We identify this peak as being the surface van Hove singularity (vHs) by comparing differential conductance (dI/dV) maps with a tight-binding model. Magnetic field splits the vHs peak, pushing one branch towards the Fermi energy. At 0T, the dI/dV maps reveal a density wave with the periodicity of the reconstructed surface unit cell at the vHs peak energy. The energy at which this density wave occurs follows the field-split vHs. We discuss possible origins of this density wave as well as possible consequences for quantum criticality.