An enduring question in correlated systems concerns whether superconductivity is favoured at a quantum critical point (QCP) characterised by a divergent quasiparticle effective mass. Despite such a scenario being widely postulated in high Tc cuprates and invoked to explain non-Fermi liquid transport signatures, experimental evidence is lacking for a critical divergence under the superconducting dome. We use ultra-strong magnetic fields to measure quantum oscillations in underdoped YBa2Cu3O6+x, revealing a dramatic doping-dependent upturn in quasiparticle effective mass at a critical metal-insulator transition beneath the superconducting dome. Given the location of this QCP under a plateau in Tc in addition to a postulated QCP at optimal doping, we discuss the intriguing possibility of two intersecting superconducting subdomes, each centred at a critical Fermi surface instability.
@article{arxiv.0910.2359,
title = {Metal-insulator quantum critical point beneath the high Tc superconducting dome},
author = {Suchitra E. Sebastian and N. Harrison and M. M. Altarawneh and C. H. Mielke and Ruixing Liang and D. A. Bonn and W. N. Hardy and G. G. Lonzarich},
journal= {arXiv preprint arXiv:0910.2359},
year = {2011}
}