Transport and tunnelling studies in Bi2Sr2CaCu2O8+δ

Abstract

Here we address various aspects of the electronic density of states (DoS) of high-temperature superconductors (HTS). The DoS contains information about various correlations which are present and is a fundamental parameter in describing many physical properties. We discuss the existence of the normal-slate gap, or pseudogap in the electronic density states of the high-temperature superconducting (HTS) cuprates, describing the present state of understanding of the pseudogap and existing experimental techniques for studying it. We fit a pseudogap model, based upon existing Angle-Resolved Photoemission Spectroscopy (ARPES) results, to entropy data for the HTS cuprate compounds YBa2Cu3O7-δ and Bi2Sr2CaCu2O8+δ; as well as to magnetic spin susceptibility data for Yi-xCaxBa2Cu3O7-δ. This data is used to construct maps of the electronic contribution, Sel, to the entropy, electronic specific heat coefficient γel, and magnetic spin susceptibility Xs across the entire phase diagram, which indirectly constitute maps of the DoS. Our findings suggest that this data is indeed consistent with the pseudogap shown by ARPES; and also that Sel is approximately linearly related to Xs by the Wilson ratio aw for nearly-free electrons. Such a Fermi liquid-like result is surprising in a system which is evidently strongly correlated. Yurgens, Krasnov, et al have studied the DoS in single crystals of Bi2Sr2CaCuO8+δ through interlayer c-axis tunnelling experiments. We have laid the foundations for doing the same for Bi2rSr2CaCu2O8+δ crystals in which Cu sites have been substituted by scattering impurities such as Zn (non-magnetic) and Co (magnetic). We here describe our progress in the microfabrication of complex mesa structures for doing this, and the techniques we have used to successfully resolve most of the associated practical problems.