Non-conservation of Density of States in Bi₂Sr₂CaCu₂Oy: Coexistence of Pseudogap and Superconducting gap

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The tunneling spectra obtained within the ab-plane of Bi₂Sr₂CaCu₂Oy (Bi2212) for temperatures below and above the critical temperature (Tc) 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 (EF). 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.