Observation of the Yamaji effect in a cuprate superconductor

The pseudogap state in high-T_c cuprates, known through its partial gapping of the Fermi surface, is believed to hold the key to understanding the origin of Planckian relaxation and quantum criticality. However, the nature of the Fermi surface gapping in the pseudogap state has remained a fundamental open question. Here, we report the observation of the Yamaji effect in angular dependent magnetoresistivity measurements above the superconducting temperature T_c of an underdoped cuprate. This observation provides direct evidence of closed Fermi surface pockets. The small area of the observed Fermi surface pocket, approximately 1.3% of the Brillouin zone area, indicates Fermi surface reconstruction above Tc in the pseudogap state. Such reconstruction is all the more surprising given the absence of evidence for long-range broken translational symmetry, and suggests an unconventional reconstruction mechanism in the pseudogap state of the cuprates.