Se p 19 99 Underlying Pairing States of High T c Superconductivity

In this talk, I present a microscopic theory I proposed recently to describe high-Tc superconductivity in cuprates. I show that coherent pairing states consisting of extended singlet Cooper pairs and triplet π pairs can manifest both the Mott insulating antiferromagnetic order and the d-wave superconducting order. From this configuration of coherent pairing states, I can describe both the single electron properties and the low energy collective excitations in high Tc superconductivity in the same framework. The quasiparticle can be derived directly with respect to the coherent pairing states. While, the low-lying quantum fluctuations associated with spin-wave and charge excitation can be investigated from the path integral of the coherent pairing states. Discovery of high-Tc superconductivity in copper-oxides reveals very attractive, but also extremely complicated, new phenomena in strongly correlated electron systems[1]. One of the most striking phenomena is that cuprates undergo a transition from the Mott insulating antiferromagnetic (AF) order to the d-wave superconducting (dSC) order under dopings. Theorists have tried various different mechanisms, such as the theory of resonating valence bond (RVB) states of singlet pairs introduced by Anderson a decade ago[2] and the SO(5) unified theory of AF and dSC order parameters proposed by S. C. Zhang recently[3], to explain this metalinsulator transition. However, a microscopic description of this AF to dSC transition has not yet been completed. In this talk, I report a microscopic theory I proposed recently to describe high-Tc superconductivity in cuprates[4]. In this theory, I show that coherent pairing states consisting of extended singlet Cooper pairs and triplet π pairs can manifest both the Mott insulating antiferromagnetic order and the d-wave superconducting order. At half-filling, this configuration describes an insulating AF ordering arisen from the mixing of singlet and triplet pairs. Upon doping with holes the d-wave pairing gap appears, and above a certain level cuprates become superconducting. In this AF to dSC transition, the nature of no-double-occupied-sites and the presence of triplet π pairs play an important role. Based on the square lattice structure of layered copper-oxides, the cuprate ground state |Ψ〉 may be obtained by projecting out all states of no double occupied sites from the following generalized pairing state |Φ〉: |Φ〉 = ′ k exp { η1(k)c † k↑c † −k↓ + η2(k)c † k+Q↑c † −k+Q↓ + η3(k)c † k↑c † −k+Q↓ + η4(k)c † k↓c † −k+Q↑ + η5(k)c † k↑c † −k+Q↑ + η6(k)c † k↓c † −k+Q↓ − H.c } |0〉, (1) ∗Talk presented in 1999 Taiwan International Conference on Superconductivity (TICS’99) & The Sixth Workshop on Low Temperature Physics (WLTP6) held on August 17-20, 1999 in Kenting, Taiwan