Observation of the Yamaji effect in a cuprate superconductor

The pseudogap state of high-temperature superconducting cuprates, known for its partial gapping of the Fermi surface above the superconducting transition temperature, is believed to hold the key to understanding the origin of Planckian relaxation and quantum criticality. However, the nature of the Fermi surface in the pseudogap state has remained a fundamental open question. Here we report the observation of the Yamaji effect, which appears as a peak in the c-axis resistivity at a specific angle of the applied magnetic field, in angle-dependent magnetoresistivity measurements above the critical temperature in the cuprate HgBa₂CuO_4+δ. The observation of the Yamaji peak is evidence for small Fermi-surface pockets in the normal state of the pseudogap phase. The small size of the pockets, each estimated to occupy only 1.3% of the Brillouin zone area, is not expected given the absence of long-range broken translational symmetry.