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A conserving T-matrix approximation is applied to the two-dimensional attractive Hubbard model in the low-density regime. A set of self-consistent equations is solved in the real-frequency domain to avoid the analytic continuation procedure. By tuning the chemical potential the particle density was varied in the limits 0.01 < n < 0.18. For the value of the attractive potential U=8t the binding energy of pairs monotonically decreases with increasing n, from its zero-density limit 2.3t and vanishes at a critical density n=0.19. A pairing-induced pseudogap in the single-particle density of states is found at low densities and temperatures., Comment: 5 pages, 4 figures, accepted for publication in Phys. Rev. Lett

A brief summary of the formulation of QCD at finite chemical potental, $\mu$, is presented. The failure of the quenched approximation to the problem is reviewed. Results are presented for dynamical simulations of the theory at strong and intermediate couplings. We find that the problems associated with the quenched theory persist: the onset of non-zero quark number does seem to occur at a chemical potential $\approx { {m_{\pi}} \over 2}$. However analysis of the Lee-Yang zeros of the grand canonical partition function in the complex fugacity plane, ($e^{\mu/T}$), does show signals of critical behaviour in the expected region of chemical potential. Results are presented for a simulation at finite density of the Gross-Neveu model on a $16^3$ lattice near to the chiral limit. Contrary to our simulations of QCD no pathologies were found when $\mu$ passed through the value $m_{\pi}/2}$., Comment: 14 pages, Latex, 18 eps figures, Review for Tsukuba workshop

We study QCD at non-zero quark density, zero temperature, infinite coupling using the Glasgow algorithm. An improved complex zero analysis gives a critical point \mu_c in agreement with that of chiral symmetry restoration computed with strong coupling expansions, and monomer-dimer simulations. We observe, however, two unphysical critical points: the onset for the number density \mu_o, and \mu_s the saturation threshold, coincident with pathological onsets observed in past quenched QCD calculations. An analysis of the probability distributions for particle number supports our physical interpretation of the critical point \mu_c, and offers a new intepretation of \mu_o, which confirms its unphysical nature. The perspectives for future lattice QCD calculations of the properties of dense baryonic matter are briefly discussed., Comment: 36 pages, RevTeX, 13 eps figures

A Quantum Monte Carlo calculation of dynamical spin susceptibility in the half-filled 2D Hubbard model is presented for temperature $T=0.2t$ and an intermediate on-site repulsion $U=4t$. Using the singular value decomposition technique we succeed in analytically continuing the Matsubara Green's function into the real frequency domain and in deriving the spectral representation for the longitudinal and transverse spin susceptibility. The simulation results, while contradicting the random-phase approximation prediction of antiferromagnetic long-range order at this temperature, are in agreement with an extension of a self-consistent renormalization approach of Moriya. The static susceptibility calculated using this technique is qualitatively consistent with the $\omega \rightarrow 0 $ simulation results., Comment: 4 pages, Revtex, encoded figs.uu file with 3 figures enclosed

A finite size scaling is applied to the Yang-Lee zeros of the grand canonical partition function for the 2-D Hubbard model in the complex chemical potential plane. The logarithmic scaling of the imaginary part of the zeros with the system size indicates a singular dependence of the carrier density on the chemical potential. Our analysis points to a second-order phase transition with critical exponent ${1\over \delta}={1\over 2}\pm {1\over 3}$ which leads to a divergence of the electronic susceptibility. We interprete these results as reflecting singular behaviour of the density of states in the quasiparticle spectrum., Comment: 8 pages, Revtex

The singular value decomposition technique is used to reconstruct the electronic spectral weight function for a half-filled Hubbard model with on-site repulsion $U=4t$ from Quantum Monte Carlo data. A two-band structure for the single-particle excitation spectrum is found to persist as the lattice size exceeds the spin-spin correlation length. The observed bands are flat in the vicinity of the $(0,\pi),(\pi,0)$ points in the Brillouin zone, in accordance with experimental data for high-temperature superconducting compounds., Comment: 4 pages, Revtex.

We present a new technique for a numerical analysis of the phase structure of the 2D Hubbard model as a function of the hole chemical potential. The grand canonical partition function for the model is obtained via Monte Carlo simulations. The dependence of the hole occupation number on the chemical potential and the temperature is evaluated. These calculations, together with a study of the Yang-Lee zeros of the grand canonical partition function, show evidence of a phase transition at zero temperature and particle density below half-filling. The binding energy of a pair of holes is calculated in the low temperature regime and the possibility for pairing is explored., Comment: 33 pages, GUTPA/91/11-1

The behaviour of the chiral condensate in QCD is investigated by means of a study of the distribution of the zeros of the partition function in the complex quark mass plane. Simulations are performed at fixed temperature on three different spatial volumes at $\beta=5.04$ and at $\beta=4.9$ and $\beta=5.2$ on a $4^4$ lattice. Evidence is found for a chirally related transition at non-zero quark mass in the intermediate coupling region for $\beta < 5.2 $ but superimposed upon a smooth behaviour for the condensate. The critical mass at which this transition is found is only weakly dependent on the spatial volume and decreas with decreasing temperature., Comment: 22 pages, GUTPA/91/11-2