OPTICAL MASS IN THE T-J MODEL

Twisted boundary conditions are used to study the behavior of the optical mass in the two-dimensional t-J model. A 4 x 4 torus with two holes is shown to exhibit a paramagnetic behavior under certain conditions for J/t < 0.7. However, it is shown that by averaging the charge stiffness over the test fluxes (i.e., the boundary conditions) one can reduce finite-size effects and extract the optical mass relevant to a macroscopic system. For small J/t, the optical mass appears to deviate from the (larger) quasiparticle effective mass defined by the quasiparticle dispersion relation. Our numerical work also reveals large weight in the optical conductivity at frequencies characteristic of the spin fluctuations. Finally, we note that no sign of flux quantization characteristic of pairing is observed in this torus geometry for J/t < 0.65.