Does the Two-Dimensional t-J Model have Hole Pockets?

We have calculated the high temperature series for the momentum distribution function n_k of the 2D t-J model to 12th order in inverse temperature. By extrapolating the series to T=0.2J we investigate the possibility of hole pockets in the t-J model. We find no indication of hole pockets at an electron density of n=0.9 with J/t=0.5 or J/t=1.0.Comment: 2 pages, 2 figures. Contribution to Spectroscopies of Novel Superconductors 97, Cape Cod, M

Violation of Luttinger's Theorem in the Two-Dimensional t-J Model

We have calculated the high temperature series for the momentum distribution function n_k of the 2D t-J model to 12th order in inverse temperature. By extrapolating the series to T=0.2J we searched for a Fermi surface of the 2D t-J model. We find that three criteria used for estimating the location of a Fermi surface violate Luttinger's Theorem, implying the 2D t-J model does not have an adiabatic connection to a non-interacting model.Comment: 4 pages, 5 figures. Version with grayscale figures available upon reques

Theory of Electron Spin Relaxation in n-Doped Quantum Wells

Recent experiments have demonstrated long spin lifetimes in uniformly n-doped quantum wells. The spin dynamics of exciton, localized, and conduction spins are important for understanding these systems. We explain experimental behavior by invoking spin exchange between all spin species. By doing so we explain quantitatively and qualitatively the striking and unusual temperature dependence in (110)-GaAs quantum wells. We discuss possible future experiments to resolve the pertinent localized spin relaxation mechanisms. In addition, our analysis allows us to propose possible experimental scenarios that will optimize spin relaxation times in GaAs and CdTe quantum wells.Comment: Small corrections made. Accepted to Phys. Rev. B. 8 pages, 5 figure

Large physical spin approach for strongly correlated electrons

We present a novel approach for a systematic large--spin expansion of the $t$-$J$ Hamiltonian which enables us to work without the constraint of no double occupancy. In our scheme we can perform the large--spin limit ensuring that the low energy spin excitations are in {\em exact} correspondence with the physical excitations of the $s={1\over2}$ Hilbert space. As a consequence, we expect a smooth dependence of the physical quantities on the expansion parameter $1/ s$. As a first application of the method we study the case of a single hole in a N\'eel background. A systematic expansion in fluctuations about this stable solution indicates that by increasing $t/J$ the quasiparticle weight strongly depends on the momentum carried by the hole. Results, obtained on small lattice sizes, are found in excellent agreement with exact diagonalization data.Comment: RevTeX 3.0 + 2 postscript figures appended (search for FIG1a and FIG1b ) revised, to appear in Phys. Lett.

Limits on Phase Separation for Two-Dimensional Strongly Correlated Electrons

From calculations of the high temperature series for the free energy of the two-dimensional t-J model we construct series for ratios of the free energy per hole. The ratios can be extrapolated very accurately to low temperatures and used to investigate phase separation. Our results confirm that phase separation occurs only for J/t greater than 1.2. Also, the phase transition into the phase separated state has Tc of approximately 0.25J for large J/t.Comment: 4 pages, 6 figure

Phase diagram of the two-dimensional t--J model at low doping

The phase diagram of the planar t--J model at small hole doping is investigated by finite size scaling of exact diagonalisation data of NXN clusters (up to 26). Hole-droplet binding energies, compressibility and static spin and charge correlations are calculated. Short range antiferromagnetic correlations can produce attractive forces between holes leading to a very rich phase diagram including a liquid of d-wave hole pairs (for $J/t\gtrsim 0.2$), a liquid of hole droplets (quartets) for larger J/t ratios ($J/t\gtrsim 0.5$) and, at even larger coupling J/t, an instability towards phase separation.Comment: 3 pages, latex, 5 postscript figures, uuencode