ar X iv : c on d - m at / 9 30 10 23 v 1 2 0 Ja n 19 93 Soluble ‘ Supersymmetric ’ Quantum XY Model

ar X iv : c on d - m at / 9 40 10 05 v 2 1 0 Ja n 19 94 Composite operators for BCS Superconductor

The new form of the composite operator generalizing the Cooper pairs for a BCS superconductor is introduced. The approach is similar to the derivation of the composite operator of the odd-frequency superconductors. The examples of the d x 2 −y 2 −, d xy − and p− wave composite operators for a 2D t − J model are given.

ar X iv : c on d - m at / 9 60 10 94 v 1 2 2 Ja n 19 96 Excitons in insulating cuprates

We study the electronic excitations near the charge-transfer gap in insulating CuO 2 planes, starting from a six-band model which includes p π and d xy orbitals and Cu-O nearest-neighbor repulsion U pd. While the low lying electronic excitations in the doped system are well described by a modified t−J model, the excitonic states of the insulator include hybrid d xy − p π states of A 2g symmetry...

ar X iv : c on d - m at / 9 40 10 66 v 1 2 8 Ja n 19 94 Ground state energy and quasiparticle gaps

ar X iv : c on d - m at / 9 50 11 19 v 1 2 5 Ja n 19 95 Spectral weight function for the half - filled Hubbard model : a singular value decomposition approach

The singular value decomposition technique is used to reconstruct the electronic spectral weight function for a half-filled Hubbard model with on-site repulsion U = 4t from Quantum Monte Carlo data. A two-band structure for the single-particle excitation spectrum is found to persist as the lattice size exceeds the spin-spin correlation length. The observed bands are flat in the vicinity of the ...

ar X iv : c on d - m at / 9 70 62 99 v 1 3 0 Ju n 19 97 Coupled Cluster Treatment of the XY model

We study quantum spin systems in the 1D, 2D square and 3D cubic lattices with nearest-neighbour XY exchange. We use the coupled-cluster method (CCM) to calculate the ground-state energy, the T = 0 sublattice magnetisation and the excited state energies, all as functions of the anisotropy parameter γ. We consider S = 1/2 in detail and give some results for higher S. In 1D these results are compa...