Normal State Electronic Structure in Underdoped High-Tc Cuprates

Comprehensive angle-resolved quantum oscillation measurements on YBa₂Cu₃O_6+x in magnetic fields approaching 100 tesla are used to address longstanding problem of the normal state electronic of underdoped high temperature superconducting cuprates. The symmetry of the Fermi surface points uniquely to its reconstruction by biaxial ordering of the charge and bond degrees of freedom.