Particle correlations in a fermi superfluid

We discuss correlations between particles of different momentum in a superfluid fermi gas, accessible through noise measurements of absorption images of the expanded gas. We include two elements missing from the simplest treatment, based on the BCS wavefunction: the explicit use of a conserving approximation satisfying particle number conservation, and the inclusion of the contribution from Cooper pairs at finite momentum. We expect the latter to be a significant issue in the strongly correlated state emerging in the BCS-BEC crossover.Comment: Published versio

Raman scattering in high temperature superconductors : An integrated view

The common features in the Raman data of high temperature superconductors: (the cuprates, bismathates, alkali doped fullerides and some organic superconductors), are analyzed. It was shown that qualitative understanding of the data can be achieved in terms of non-Fermi liquid models for their normal state, with appropiate bag mechanisms for the superconducting state.Comment: To appear in Physica B (1996). Invited talk presented by S. N. Behera, Latex file in revtex style, six figures available on request to first author (e-mail : [email protected]

Effect of van Hove Singularities on a Spin Liquid

We determine the properties and leading instabilities of a spin liquid with a Fermi surface passing near a van Hove singularity. Our study is motivated by recent photoemission experiments on high $T_c$ cuprates in which it is found that for the optimally doped material the experimental Fermi surface passes near a van Hove singularity, while for underdoped materials, a pseudogap in the electron spectral function is formed in the vicinity of the van Hove point. We show theoretically that proximity to the van Hove singularity suppresses the inelastic scattering due to the gauge field and permits the formation of a d-wave RVB state in which the gap exists only near the van Hove points while finite regions of the Fermi surface remain gapless. This $d$-wave pairing provides a natural explanation of the pseudogap observed in photoemission. We also discuss the relation of the pseudogap observed in the spectral function to the pseudogaps observed in the magnetic susceptibility.Comment: 11 pages, 4 Postscript figures in a separate fil

Hubbard Model with L\"uscher fermions

We study the basic features of the two-dimensional quantum Hubbard Model at half-filling by means of the L\"uscher algorithm and the algorithm based on direct update of the determinant of the fermionic matrix. We implement the L\"uscher idea employing the transfer matrix formalism which allows to formulate the problem on the lattice in $(2+1)$ dimensions. We discuss the numerical complexity of the L\"uscher technique, systematic errors introduced by polynomial approximation and introduce some improvements which reduce long autocorrelations. In particular we show that preconditioning of the fermionic matrix speeds up the algorithm and extends the available range of parameters. We investigate the magnetic and the one-particle properties of the Hubbard Model at half-filling and show that they are in qualitative agreement with the existing Monte Carlo data and the mean-field predictions.Comment: 49 pages, Latex, 11 Postscript figures, uses psfrag packag