Non-conservation of density of states in Bi₂Sr₂CaCu₂O_y: Coexistence of pseudogap and superconducting gap

The tunneling spectra obtained within the (a,b)-plane of Bi₂Sr₂CaCu₂O_y (Bi2212) for temperatures below and above the critical temperature (T_c) are analyzed. We find that the tunneling conductance spectra for the underdoped compound in the superconducting state do not follow the conservation of states rule. There is a consistent loss of states for the underdoped Bi2212 implying an underlying depression in the density of states (DOS) and hence the pseudogap near the Fermi energy (E_F). Such an underlying depression can also explain the peak-dip-hump structure observed in the spectra. Furthermore, the conservation of states is recovered and the dip-hump structure disappears after normalizing the low-temperature spectra with that of the normal state. We argue that this is a direct evidence for the coexistence of a pseudogap with the superconducting gap.