ar X iv : c on d - m at / 9 60 20 54 v 1 9 F eb 1 99 6 Phase Separation in the Hubbard Model

ar X iv : c on d - m at / 9 60 20 27 v 1 6 F eb 1 99 6 Néel and singlet RVB orders in the t - J model

The Néel and the singlet RVB orders of the t-J model in a 2D square lattice are studied in the slave-boson mean-field approximation. It is shown that the Néel order parameter takes the maximum value at the finite temperature and disappear at the lower temperature for a certain range of doping. It is also shown that the Néel and the singlet RVB orders coexist at low temperature. This suggests th...

ar X iv : c on d - m at / 9 60 31 90 v 1 2 9 M ar 1 99 6 The Four - Dimensional XY Spin Glass

ar X iv : c on d - m at / 9 90 64 41 v 1 3 0 Ju n 19 99 FINITE TEMPERATURE TRANSPORT IN INTEGRABLE QUANTUM MANY BODY SYSTEMS

Recent developments in the analysis of finite temperature dissipationless transport in inte-grable quantum many body problems are presented. In particular, we will discuss: (i) the role played by the conservation laws in systems as the spin 1/2 Heisenberg chain and the one-dimensional Hubbard model, (ii) exact results obtained using the Bethe ansatz method on the long time decay of current corr...

ar X iv : c on d - m at / 9 60 51 64 v 1 2 7 M ay 1 99 6 Influence of electron phonon interaction on superexchange

We investigate the influence of electron-phonon coupling on the superex-change interaction of magnetic insulators. Both the Holstein-Hubbard model where the phonons couple to the electron density, as well as an extended Su, Schrieffer, Heeger model where the coupling arises from modulation of the overlap integral are studied using exact diagonalization and perturba-tive methods. In all cases fo...

ar X iv : d g - ga / 9 60 20 02 v 1 1 F eb 1 99 6 Associativity properties of the symplectic sum

In this note we apply a 4-fold sum operation to develop an associativity rule for the pairwise symplectic sum. This allows us to show that certain diffeomorphic symplectic 4-manifolds made out of elliptic surfaces are in fact symplectically deformation equivalent. We also show that blow-up points can be traded from one side of a symplectic sum to another without changing the symplectic deformat...