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

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

Comment on "T-dependence of the magnetic penetration depth in unconventional superconductors at low temperatures: Can it be linear?"

We show that the clean superconductor with line of gap nodes is not in conflict with the Nernst theorem. The answer to the question in the title of the Schopohl-Dolgov paper in Phys. Rev. Lett. 80 (1998) 4761 (cond-mat/9802264) is yes.Comment: Comment to the paper by Schopohl and Dolgov in Phys. Rev. Lett. 80 (1998) 4761 (cond-mat/9802264), RevTex file, 1 page, no figures, typos are corrected, submitted to Phys. Rev. Let

Theory of Optical Orientation in n-Type Semiconductors

Time resolved measurements of magnetization in n-GaAs have revealed a rich array of spin decoherence processes, and have shown that fairly long lifetimes (\sim 100 ns) can be achieved under certain circumstances. In time-resolved Faraday rotation and time-resolved Kerr rotation the evolution of the magnetization can be followed as a function of temperature, applied field, doping level and excitation level. We present a theory for the spin relaxation in n-GaAs based on a set of rate equations for two interacting thermalized subsystems of spins: localized states on donor sites and itinerant states in the conduction band. The conduction band spins relax by scattering from defects or phonons through the D'yakonov-Perel' mechanism, while the localized spins relax by interacting with phonons (when in an applied field) or through the Dzyaloshinskii-Moriya interaction. In this model, numerous features of the data, including puzzling temperature and doping dependences of the relaxation time, find an explanation.Comment: 4 pages, 2 figures; revised version has a more complete discussion of the Elliott-Yafet and spin-phonon decay mechanism

Superconductivity in the Two-Dimensional tt-JJ Model at Low Hole Doping

By combining a generalized Lanczos scheme with the variational Monte Carlo method we can optimize the short- and long-range properties of the groundstate separately. This allows us to measure the long-range order of the groundstate of the $t$-$J$ model as a function of the coupling constant $J/t$, and identify a region of finite d-wave superconducting long-range order. With a lattice size of 50 sites we can reliably examine hole densities down to 0.16.Comment: 12 pages and 4 PostScript figures, ReVTeX 3.0, ETH-TH/94-1