Universal features in the photoemission spectroscopy of high temperature superconductors

摘要

The energy gap for electronic excitations is one of the most importantcharacteristics of the superconducting state, as it directly refects thepairing of electrons. In the copper-oxide high temperature superconductors(HTSCs), a strongly anisotropic energy gap, which vanishes along high symmetrydirections, is a clear manifestation of the d-wave symmetry of the pairing.There is, however, a dramatic change in the form of the gap anisotropy withreduced carrier concentration (underdoping). Although the vanishing of the gapalong the diagonal to the square Cu-O bond directions is robust, the dopingdependence of the large gap along the Cu-O directions suggests that its originmight be different from pairing. It is thus tempting to associate the large gapwith a second order parameter distinct from superconductivity. We useangle-resolved photoemission spectroscopy (ARPES) to show that the two-gapbehavior, and the destruction of well defined electronic excitations, are notuniversal features of HTSCs, and depend sensitively on how the underdopedmaterials are prepared. Depending on cation substitution, underdoped sampleseither show two-gap behavior or not. In contrast, many other characteristics ofHTSCs, such as the domelike dependence of Tc on doping, long-lived excitationsalong the diagonals to the Cu-O bonds, energy gap at the antinode (crossing ofthe underlying Fermi surface and the (pi, 0)-(pi, pi) line) decreasingmonotonically with doping, while persisting above Tc (the pseudogap), arepresent in all samples, irrespective of whether they exhibit two-gap behavioror not. Our results imply that universal aspects of high Tc superconductivityare relatively insensitive to differences in the electronic states along theCu-O bond directions.