Abstract: Whereas single-band theories have long been the dominating paradigm applied to cuprate superconductors, recent theoretical studies have highlighted the implications of orbital hybridization in the cuprates. Hybridization of the dx2-y2 band with the dz2 band increases with reducing apical oxygen height which in turn lowers the superconducting transition temperature. In light of these theoretical predictions, we have performed angle-resolved photoemission spectroscopy (ARPES) studies on the overdoped cuprate La2-xSrxCuO4 with low apical oxygen height. By recording ARPES spectra down to ~ 2 eV below the Fermi level, not only the well-documented dx2-y2 band but also the dz2 band was successfully identified. We clearly observed a hybridization gap between those two bands along the antinodal (π, 0) direction, which is the direct experimental evidence of the orbital hybridization . This is in stark contrast with the nodal (π, π) direction where the opposite mirror symmetry between the dx2-y2 and dz2 band prohibits their hybridization and constitute a two-dimensional type-II Dirac nodal line . By utilizing soft x rays which yield decent momentum resolution in the out-of-plane direction, we further revealed the three-dimensional nature of the Fermi surface originating from the hybridization between the dx2-y2 and dz2 orbitals . Our studies thus shed light on the multi-orbital nature of a cuprate with a wealth of implications about high-temperature superconductivity, band topology, and pseudogap.
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