It is shown that in the two-dimensional attractive Hubbard model, the mean-field phase transition is replaced by a renormalized classical regime of fluctuations where a pseudogap opens up in the single-particle spectral weight. It is argued that this pseudogap and precursors of the ordered state quasiparticles can occur only in strongly anisotropic quasi-two-dimensional materials. This precursor phenomenon differs from preformed local pairs. Further, while critical antiferromagnetic fluctuations would also lead to a pseudogap in the repulsive model, there are some important differences with thr superconducting case. (C) 1998 Elsevier Science Ltd. All rights reserved.

1 aVilk, Y M1 aAllen, S1 aTouchette, H1 aMoukouri, S1 aChen, L1 aTremblay, A.-M., S. uhttps://www.physique.usherbrooke.ca/pages/node/728801168nas a2200145 4500008004100000245009200041210006900133300001600202490000800218520067600226100001400902700001200916700002400928856007000952 1994 eng d00a2-particle Self-consistent Theory For Spin and Charge Fluctuations In the Hubbard-model0 a2particle Selfconsistent Theory For Spin and Charge Fluctuations a2235–22360 v2353 aA theory which is self-consistent at the two-particle level is presented for both spin and charge fluctuations in the Hubbard model. It is in quantitative agreement with Monte Carlo data at least up to intermediate coupling (U similar to 8t) It includes both short-wavelength quantum renormalization effects, and long-wavelength thermal fluctuations which can destroy long-range order in two dimensions. This last effect leads to a small energy scale, as often observed in high temperature superconductors. The theory is conserving, satisfies the Pauli principle and includes three-particle correlations necessary to account for the incipient Mott transition.

1 aVilk, Y M1 aChen, L1 aTremblay, A.-M., S. uhttp://www.sciencedirect.com/science/article/pii/092145349492339601808nas a2200145 4500008004100000245012700041210006900168300001600237490000700253520129500260100001501555700001201570700002401582856005601606 1994 eng d00aComparisons Between Monte-carlo Simulations and A Simple Crossing-symmetrical Approach To the Hubbard-model At Low-density0 aComparisons Between Montecarlo Simulations and A Simple Crossing a4106–41180 v493 aA simple crossing-symmetric approximation for the fully reducible vertex is compared with Monte Carlo simulations of the two-dimensional Hubbard model. Up to quarter-filling, in the intermediate coupling regime, accuracies better than 10% are obtained for several static correlation functions, including spin and charge, as well as the pairing channels most widely studied in the context of high-T(c) superconductivity. The accuracy is generally better for the pairing channels. The results shed light on the applicability of the renormalized generalized-random-phase-approximation scheme, its relation to Fermi-liquid theory, and on the regime where nontrivial effects may appear in pairing channels. The approximation under study consists in assuming that for parallel spins the fully reducible particle-particle vertex vanishes, while for antiparallel spins it is equal to the T matrix. The fully reducible particle-hole vertex is then obtained from the latter vertex by using crossing symmetry. This simple approximation is not conserving but it preserves global symmetries. It suggests that Monte Carlo results for the two-dimensional Hubbard model in small systems at low density and intermediate coupling can be interpreted using a weakly correlated Fermi-liquid picture.

1 aDaré, A M1 aChen, L1 aTremblay, A.-M., S. uhttps://www.physique.usherbrooke.ca/pages/node/724201028nas a2200145 4500008004100000245014500041210006900186300001600255490000800271520049600279100001500775700001200790700002400802856005600826 1994 eng d00aCorrelation-functions of the Hubbard-model At Low-density In A Crossing-symmetrical Approximation - Comparisons With Monte-carlo Simulations0 aCorrelationfunctions of the Hubbardmodel At Lowdensity In A Cros a1413–14140 v1943 aThe accuracy of a simple crossing-symmetric approximation for the fully reducible vertex is tested by comparisons of the spin, charge, and pairing correlations with those obtained by Monte Carlo simulations of the two-dimensional Hubbard model. The approximation under study consists in assuming that for parallel spins the fully reducible vertex vanishes, while for anti-parallel spins it is equal to the T-matrix. Up to quarter-filling, accuracies better than 10% are obtained.

1 aDaré, A M1 aChen, L1 aTremblay, A.-M., S. uhttps://www.physique.usherbrooke.ca/pages/node/724100627nas a2200181 4500008003900000245012700039210007000166260000900236300001000245490003200255100001800287700001600305700001200321700001400333700002400347700001800371856005600389 1994 d00aQuantum Monte Carlo simulations for a model of high-tem¬perature superconductors: effect of next-nearest-neighbor hopping0 aQuantum Monte Carlo simulations for a model of hightem¬perature c1994 a78-850 vUniversity of Toronto Press1 aVeilleux, A F1 aDaré, A -M1 aChen, L1 aVilk, Y M1 aTremblay, A.-M., S.1 aRoss, John, W uhttps://www.physique.usherbrooke.ca/pages/node/756001091nas a2200145 4500008004100000245006400041210006300105300001800168490000700186520064600193100001400839700001200853700002400865856005600889 1994 eng d00aTheory of Spin and Charge Fluctuations In the Hubbard-model0 aTheory of Spin and Charge Fluctuations In the Hubbardmodel a13267–132700 v493 aA self-consistent theory of both spin and charge fluctuations in the Hubbard model is presented. It is in quantitative agreement with Monte Carlo data at least up to intermediate coupling (U approximately 8t). It includes both short-wavelength quantum renormalization effects, and long-wavelength thermal fluctuations, which can destroy long-range order in two dimensions. This last effect leads to a small energy scale, as often observed in high-temperature superconductors. The theory is conserving, satisfies the Pauli principle, and includes three-particle correlations necessary to account for the incipient Mott transition.

1 aVilk, Y M1 aChen, L1 aTremblay, A.-M., S. uhttps://www.physique.usherbrooke.ca/pages/node/728900867nas a2200145 4500008004100000245010600041210006900147300001800216490000700234520037500241100001200616700001300628700002400641856005600665 1993 eng d00aFlux-quantization In Rings For Hubbard (attractive and Repulsive) and T-j-like Hamiltonians - Comment0 aFluxquantization In Rings For Hubbard attractive and Repulsive a a15316–153180 v473 aIt is shown for three models with strong correlations that the value of the total spin of the ground state of finite-size rings with two fermions (holes or electrons) can change as a function of magnetic flux PHI. It is concluded that the magnetic flux periodicity may be used as a test of binding only if one also checks for changes in spin quantum numbers.

1 aChen, L1 aMei, C J1 aTremblay, A.-M., S. uhttps://www.physique.usherbrooke.ca/pages/node/723801601nas a2200145 4500008004100000245009600041210006900137300001800206490000700224520111700231100001501348700001201363700002401375856005601399 1993 eng d00aMagnetic Neutron-scattering From 2-dimensional Lattice Electrons - the Case of La2-xsrxcuo40 aMagnetic Neutronscattering From 2dimensional Lattice Electrons t a15217–152410 v473 aIt is found that the one-band Hubbard model, in the weak- to intermediate-coupling regime, can account qualitatively for magnetic-neutron-scattering experiments in the paramagnetic phase of La2-xSrxCuO4 when second-neighbor hopping is included. However, the peak positions, which in two dimensions are determined mostly by the band structure, cannot agree quantitatively with the experimental results when concentration-independent band parameters are used. More importantly, while the energy scale of roughly 150 K seen in the experiments can come from second-neighbor hopping, it arises most naturally if one is very close to a magnetic instability. The proximity to a magnetic instability can be checked experimentally by measuring the relative size of the lattice equivalent of 2k(F) anomalies that appear closer to the origin in wave-vector space. Such lattice-2k(F) anomalies would allow magnetic neutron scattering to become a spectroscopic tool for the two-dimensional Fermi surface. Finally, exact results are also given for the imaginary part of the Lindhard function on the square lattice.

1 aBénard, P1 aChen, L1 aTremblay, A.-M., S. uhttps://www.physique.usherbrooke.ca/pages/node/722901003nas a2200145 4500008004100000245009600041210006900137300001400206490000700220520052300227100001500750700001200765700002400777856005600801 1993 eng d00aNeutron-scattering Measurements As A Test of Theories of High-temperature Superconductivity0 aNeutronscattering Measurements As A Test of Theories of Hightemp a589–5920 v473 aIt is shown that the Hubbard model in the intermediate-coupling regime can qualitatively explain neutron-scattering experiments in La2-xSrxCuO4 only if there are strong magnetic fluctuations in the system. By contrast, the marginal-Fermi-liquid approach explains the data without appealing at all to strong magnetic fluctuations. It is shown that the strength of the magnetic fluctuations can be estimated by detecting incommensurate peaks located near the zone center using neutron-scattering experiments.

1 aBénard, P1 aChen, L1 aTremblay, A.-M., S. uhttps://www.physique.usherbrooke.ca/pages/node/722801513nas a2200133 4500008004100000245009100041210006900132300001400201490000600215520105200221100001201273700002401285856007001309 1992 eng d00aDeterminant Monte-carlo For the Hubbard-model With Arbitrarily Gauged Auxiliary Fields0 aDeterminant Montecarlo For the Hubbardmodel With Arbitrarily Gau a547–5600 v63 aMonte Carlo methods for the Hubbard model rely on a Hubbard-Stratonovich (HS) decomposition (auxiliary field method) to perform importance sampling on classical variables. Freedom in the choice of the local HS fields can be formally seen as a gauge choice. While the choice of gauge does not influence observable quantities, it may influence intermediate quantities in the calculation, such as the famous "fermion sign", and it may also influence the efficiency with which the algorithm explores phase space. The effect of arbitrary gauge choices on both aspects of the algorithm are investigated. It is found that in the single spin-flip determinantal approach, certain gauges lead to a better exploration of phase space. This improvement is demonstrated, in the intermediate coupling regime, by histograms which for the first time show the behavior expected from grand canonical simulations. It is also found that the improved phase space exploration can in practice offset the apparent disadvantage of a smaller fermion sign.

1 aChen, L1 aTremblay, A.-M., S. uhttps://www.worldscientific.com/doi/abs/10.1142/S021797929200032300572nas a2200145 4500008003900000245015000039210006900189260002800258100001200286700001900298700001200317700002400329700001700353856005600370 1991 d00aThe influence of spin fluctuations on the temperature dependance of the magnetic susceptibility and nuclear relaxation in high Tc superconductors0 ainfluence of spin fluctuations on the temperature dependance of aZurich, Suissec08/19911 aLi, T W1 aBourbonnais, C1 aChen, L1 aTremblay, A.-M., S.1 aBrinkmann, D uhttps://www.physique.usherbrooke.ca/pages/node/757901053nas a2200157 4500008004100000245005900041210005600100300001400156490000700170520059700177100001200774700001900786700001000805700002400815856005600839 1991 eng d00aMagnetic-properties of the 2-dimensional Hubbard-model0 aMagneticproperties of the 2dimensional Hubbardmodel a369–3720 v663 aMonte Carlo simulations of the magnetic structure factor of the two-dimensional Hubbard model are in qualitative agreement with the slave-boson approach and, in the low-temperature intermediate-coupling limit, they are in even better quantitative agreement with the random-phase approximation, as long as a renormalized repulsion U is used. This renormalization comes from maximally crossed diagrams, which account for two-body short-range correlations. One of the consequences is that Stoner ferromagnetism is not a generic property of the two-dimensional one-band Hubbard model.

1 aChen, L1 aBourbonnais, C1 aLi, T1 aTremblay, A.-M., S. uhttps://www.physique.usherbrooke.ca/pages/node/723700574nas a2200181 4500008003900000245005800039210005800097260002000155300001200175490004800187100002400235700001300259700001200272700001500284700001200299700002500311856005600336 1990 d00aMonte Carlo Method for Strongly Interacting Electrons0 aMonte Carlo Method for Strongly Interacting Electrons aMontréalc1990 a197-2100 vProceedings of Supercomputing Symposium '901 aTremblay, A.-M., S.1 aBoily, C1 aChen, L1 aNelisse, H1 aReid, A1 aPelletier, Dominique uhttps://www.physique.usherbrooke.ca/pages/node/7559