Phase Competition And Spin Dynamics In High-Temperature Cuprate Superconductors

The discovery of the high-Tc superconductivity in cuprates in 1986 opened a new window to search the high-temperature superconductors both experimentally and theo-retically.Understanding the nature of high-temperature superconductivity is one of the most challenging issues in condensed matter physics due to the strongly correlations in these compounds.Though the considerable progress had been made,some fundamen-tal issues remain elusive,for example the mechanism of high-Tc superconductivity,the origin of pseudogap,as well as the relation between them.In this thesis,the phase competition between the pseudogap and superconductivity and the spin excitation in cuprates are focused within the framework of the extended t-J model.In Chapter 1,we briefly review the superconductivity and cuprates,togather with some relevant experimental techniques discussed in the thesis.The theoretical model and methods are introduced in Chapter 2,including the mean-field theory and the vari-ational Monte Carlo method.In Chapter 3,the phase competition between pesudogap and superconductivity is presented.Based on the Landau theory and the realistic microscopic t-J-V model,we find a robust back-bending behavior of characteristic temperature of pesudogap(T*)below the superconducting dome,in agreement with the phase diagram proposed by recent ARPES measurements.Near the quantum critical point,the resultant quasi-particle spectrum and the Raman response exhibit evident temperature anomaly.This anomalous temperature behavior undergoes a two-step thermal evolution dominated by the superconducting gap and the pseudogap,respectively,providing a simple expla-nation on the observed anomalous temperature evolution both in ARPES and Raman experiments.Our results imply that the revised phase diagram is likely to take place in high-temperature superconductors.In Chapter 4,we study the spin excitation of cuprates based on the variational Monte Carlo method.This parameter-free method treats the no-double occupancy con-straint exactly,and thus provides the unbiased evaluations on the spin dynamics of various mean-field trial wavefunction.Our numerical results provide the compelling evidence that the low branch of the hour-glass shaped magnetic dispersion originates from the strong antiferromagnetic background,while the upper branch is related to the periodic state like stripe.We conclude that the lower branch is also a collective mode.The pairing density wave state may suppress the commensurate spin excitation,in agreement with the recent inelastic neutron scattering measurements.The conclusions are summarized in Chapter 5,together with some discussions and prospect.