Composite Fermions and the Energy Gap in the Fractional Quantum Hall Effect

The energy gaps for the fractional quantum Hall effect at filling fractions 1/3, 1/5, and 1/7 have been calculated by variational Monte Carlo using Jain's composite fermion wave functions before and after projection onto the lowest Landau level. Before projection there is a contribution to the energy gaps from the first excited Landau level. After projection this contribution vanishes, the quasielectron charge becomes more localized, and the Coulomb energy contribution increases. The projected gaps agree well with previous calculations, lending support to the composite fermion theory.Comment: 12 pages, Revtex 3.0, 2 compressed and uuencoded postscript figures appended, NHMFL-94-062

Dispersion of a single hole in the t-J model

The dispersion of a single hole in the t-J model obtained by the exact result of 32 sites and the results obtained by self-consistent Born approximation and the Green function Monte Carlo method can be simply derived by a mean-field theory with d-RVB and antiferromagnetic order parameters. In addition, it offers a simple explanation for the difference observed between those results. The presence of the extended van Hove region at (pi,0) is a consequence of the d-RVB pairing independenct of the antiferromagnetic order. Results including t' and t" are also presented and explained consistently in a similar way.Comment: LaTex file, 5 pages with 5 embedded eps figure

From antiferromagnetism to d-wave superconductivity in the 2D t-J model

We have found that the two dimensional t-J model, for the physical parameter range J/t = 0.4 reproduces the main experimental qualitative features of High-Tc copper oxide superconductors: d-wave superconducting correlations are strongly enhanced upon small doping and clear evidence of off diagonal long range order is found at the optimal doping \delta ~ 0.15. On the other hand antiferromagnetic long range order, clearly present at zero hole doping, is suppressed at small hole density with clear absence of antiferromagnetism at \delta >~ 0.1.Comment: 4 pages, 5 figure

Spatially homogeneous ground state of the two-dimensional Hubbard model

We investigate the stability with respect to phase separation or charge density-wave formation of the two-dimensional Hubbard model for various values of the local Coulomb repulsion and electron densities using Green-function Monte Carlo techniques. The well known sign problem is particularly serious in the relevant region of small hole doping. We show that the difference in accuracy for different doping makes it very difficult to probe the phase separation instability using only energy calculations, even in the weak-coupling limit ($U=4t$) where reliable results are available. By contrast, the knowledge of the charge correlation functions allows us to provide clear evidence of a spatially homogeneous ground state up to $U=10t$.Comment: 7 pages and 5 figures. Phys. Rev. B, to appear 200