Role of Antiferromagnetic Fluctuations in High Temperature Superconductivity

Will-be-set-by-IN-TECHSoon after this study we [5] proposed an improved slave-boson theory which fundamentally differs from these approaches in that a term involving coupling between the spin and charge degrees of freedom or simply spin-charge coupling appears in our rigorous slave-boson treatment of the t-J Hamiltonian.The resulting effective mean field Lagrangian reveals coupling between the spin (spinon) paring order, ∆ f and the charge (holon) pairing order, ∆ b .As a consequence the Cooper pairing order is satisfactorily seen to be a composite of these two order parameters, ∆ f and ∆ b to allow for the bose condensation of the Cooper pairs rather than the single-holon bose condensation or the double-holon bose condensation.Accordingly this theory has led to successful reproductions of not only the monotonously decreasing spin gap temperature but also the long-waited dome-shaped structure of the superconducting transition temperature in the phase diagram.Further other important physical observations such as the boomerang behavior of superfluid weight, the peak-dip-hump structure of optical conductivity and both the temperature and doping dependence of spectral functions are reproduced in agreement with observations [6].For the sake of self-containment we will first review our earlier proposed slave-boson theory[5] of the t-J Hamiltonian which reveals the spin-charge coupling mentioned above.Earlier it was shown by others that inclusion of the t ′ term in the t-J Hamiltonian leads to satisfactory descriptions of the electronic structure of high T C cuprates[7-11] and the enhancement of pairing correlation resulting in an increasing trend of T C in the overdoped region in the phase diagram for the choice of t ′ /t < 0, e.g., t ′ /t = -0.3[12,13].It is, thus, of great interest to see how its inclusion affects the entire structure of the phase diagram which includes the pseudogap temperature.At present there has been no study which addresses the role of the diagonal hopping t ′ on the spin gap temperature, T * .S u c h study is needed to find whether there exists any relation between T * and T C or the spin gap phase and the superconducting phase.In this regard we would like to draw attention to the fact that the observed phase diagrams of high T C cuprate samples (e.g., LSCO and BSCCO samples) reveal that higher the T * , higher the T C as earlier discussed by Oda et al.[14]This suggests that the two energy or temperature scales, T * and T C are no longer independent of each other.Thus one of our main objectives is to study how the pseudogap or spin gap temperature, T * and the superconducting transition temperature, T C are correlated and show that such correlation arises owing to the presence of the short-range antiferromagnetic (AF) spin fluctuations of the shortest possible correlation length involved with the spin pairing correlations.For a concerted, self-consistent study, we use a predicted phase diagram to calculate magnetic susceptibility and discuss two important observations made by the inelastic neutron scattering (INS) measurements, namely the temperature dependence of the magnetic resonance peak[17] and the linear scaling behavior between the magnetic peak resonance energy, E res and the superconducting transition temperature [18].From this study we show that the short-range AF spin fluctuations are directly responsible for the magnetic susceptibility observed by the INS measurements mentioned here. Theory: U(1) slave boson representation of the t-J HamiltonianIn the present study we limit ourselves to the derivation of the U(1) slave boson representation of the t-J Hamiltonian.We refer details of its derivation to Appendix A. In Appendix B a brief exposure of the SU(2) approach is made in association with the U(1) representation.Here only a rudimentary description is presented by introducing the next-nearest neighbor hopping or 2 Superconductors -Properties, Technology, and Applications

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