The asymmetric Fermi surface of Bi2201

High-resolution angle-resolved photoemission spectroscopy (ARPES) performed on the single-layered cuprate (Pb_1−y,Bi_y)₂Sr_2−xLa_xCuO₆+_δ (Bi2201) reveals a 6-10% difference in the nodal k_F vectors along the ΓY and ΓX directions. This asymmetry is notably larger than the 2% orthorhombic distortion in the CuO₂ plane lattice constants determined using X-ray crystallography from the same samples. First principles calculations indicate that crystal-field splitting of the bands lies at the root of the k_F asymmetry. Concomitantly, the nodal Fermi velocities for the ΓY quadrant exceed those for ΓX by 4%. Momentum distribution curve widths for the two nodal dispersions are also anisotropic, showing identical energy dependencies, bar a scaling factor of ∼ 1.17± 0.05 between ΓY and ΓX. Consequently, the imaginary part of the self-energy is found to be 10-20% greater along ΓY than ΓX. These results emphasize the need to account for Fermi surface asymmetry in the analysis of ARPES data on Bi-based cuprate high temperature superconductors such as Bi2201. To illustrate this point, an orthorhombic tight-binding model (with twofold in-plane symmetry) was used to fit ARPES Fermi surface maps spanning all four quadrants of the Brillouin zone, and the ARPES-derived hole-doping (Luttinger count) was extracted. Comparison of the Luttinger count with one assuming four-fold in-plane symmetry strongly suggests the marked spread in previously-reported Fermi surface areas from ARPES on Bi2201 results from the differences in k_F along ΓY and ΓX. Using this analysis, a new, linear relationship emerges between the hole-doping derived from ARPES (p_ARPES) and that derived using the Presland (p_Presland) relation such that p_ARPES = p_Presland + 0.11. The implications for this difference between the ARPES- and Presland-derived estimates for p are discussed and possible future directions to elucidate the origin of this discrepancy are presented.