Strong Magnetic Fields at the Crossroads of Superconductivity, Quantum Criticality and Fermi Surface Reconstruction in the Cuprates*

A central question in high-$T_{\rm c}$ superconductivity concerns whether the pseudogap hosts a normal metallic ground state in the absence of superconductivity. High field measurements have thus far been reported the suppression of superconductivity to yield either a metallic pseudogap ground state, characterized by properties such as magnetic quantum oscillations, magnetotransport and signatures in the thermal conductivity, or a vortex liquid ground state characterized by properties such as diamagnetism. In this talk, I will present electrical transport measurements in both pulsed magnetic fields extending to 90~T and static magnetic fields of up to 45~T, over several decades in temperature and current density and over a wide range of YBa$_2$Cu$_3$O$_{6+x}$ hole dopings, performed in collaboration with Yu-Te Hsu, Mate Hartstein, Alexander J. Davies, Mun K. Chan, Juan Porras, Toshinao Loew, Sofia V. Taylor, Hsu Liu, Mathieu Le Tacon, Huakun Zuo, Jinhua Wang, Zengwei Zhu, Gilbert G. Lonzarich, Bernhard Keimer, and Suchitra E. Sebastian. We find evidence for a previously concealed robust superconducting state up to very high magnetic fields. Our high field measurements reveal an unusual pseudogap state in which seemingly metallic properties likely arise from the nodal region of minimal or suppressed superconducting gap, while the antinodal regions of maximal superconducting gap are dominated by large amplitude pairing correlations.