1,721,131 research outputs found
SPIN-CHARGE DECOUPLING AND THE GREENS-FUNCTION OF ONE-DIMENSIONAL MOTT INSULATORS
No full textAn exact expression for the Green's function G(k,omega) of one hole in the U --> infinity one-dimensional Hubbard model is derived from the known Bethe-ansatz solution of the model. Due to the spin-charge decoupling of the one-hole excitation spectrum and the Fermi-like statistics of the spin excitations, G shows branch-cut singularities at omega=+/-2 sink and a nontrivial dependence on the momentum of the hole. As a result, the hole can propagate although the quasiparticle weight vanishes
Phase separation in the large-spin t-J model
No full textWe investigate the phase diagram of the two-dimensional t-J model using a recently developed technique that allows one to solve the mean-field model Hamiltonian with a variational calculation. The accuracy of our estimate is controlled by means of a small parameter 1/q, analogous to the inverse spin magnitude 1/s employed in studying quantum spin systems. The mathematical aspects of the method and its connection with other large-spin approaches are discussed in detail. In the large-q limit the problem of strongly correlated electron systems turns into the minimization of a total-energy functional. We have performed this optimization numerically on a finite but large L x L lattice. For a single hole the static small-polaron solution is stable except for small values Of J, where polarons of increasing sizes have lower energy. At finite doping we recover phase separation above a critical J and for any electron density, showing that the Emery et al. picture represents the semiclassical behavior of the t-J model. Quantum fluctuations are expected to be very important, especially in the small-J-small-doping region, where Phase separation may also be suppressed
Spin-Wave Theory On Finite Lattices: Application To The J1-J2 Heisenberg Model
No full textWe present a new method for a systematic spin-wave expansion for the quantum fluctuations of a generic spin Hamiltonian in a finite lattice, where the inverse spin magnitude 1/S is a well-defined expansion parameter. The first two leading contributions of the spin-spin correlation function are evaluated for the J1-J2 Heisenberg model. Very good agreement between our finite-size predictions and the exact diagonalization and Monte Carlo results is found for J2/J1 < 0.2 and S = 1/2, thus confirming the existence of antiferromagnetic long-range order in this J region. For J2/J1 > 0.3 the expansion is poorly converging, suggesting a possible breakdown of the spin-wave approximation. Here our calculation seems consistent with a possible spin liquid ground state
ONE-HOLE GREEN-FUNCTION, MOMENTUM DISTRIBUTION AND QUASI-PARTICLE WEIGHT OF THE U-]INFINITY-1D HUBBARD-MODEL
No full textTwo spin liquid phases in the spatially anisotropic triangular Heisenberg model
No full textThe quantum spin-1/2 antiferromagnetic Heisenberg model on a two dimensional triangular lattice geometry with spatial anisotropy is relevant to describe materials like Cs2CuCl4 and organic compounds like {κ-(ET)2Cu2(CN)3}. The strength of the spatial anisotropy can increase quantum fluctuations and can destabilize the magnetically ordered state leading to non conventional spin liquid phases. In order to understand these intriguing phenomena, quantum Monte Carlo methods are used to study this model system as a function of the anisotropic strength, represented by the ratio J′/J between the intra-chain nearest neighbor coupling J and the inter-chain one J′. We have found evidence of two spin liquid regions. The first one is stable for small values of the coupling J'/J \alt 0.65, and appears gapless and fractionalized, whereas the second one is a more conventional spin liquid with a small spin gap and is energetically favored in the region 0.65\alt J'/J \alt 0.8. We have also shown that in both spin liquid phases there is no evidence of broken translation symmetry with dimer or spin-Peirls order or any broken spatial reflection symmetry of the lattice. The various phases are in good agreement with the experimental findings, thus supporting the existence of spin liquid phases in two dimensional quantum spin-1/2 systems
Benchmark study of an auxiliary-field quantum Monte Carlo technique for the Hubbard model with shifted-discrete Hubbard-Stratonovich transformations
No full textWithin the ground-state auxiliary-field quantum Monte Carlo technique, we introduce discrete Hubbard-Stratonovich transformations (HSTs) that are also suitable for spatially inhomogeneous trial functions. The discrete auxiliary fields introduced here are coupled to local spin or charge operators fluctuating around their Hartree-Fock values. The formalism can be considered a generalization of the discrete HSTs by J. E. Hirsch [Phys. Rev. B 28, 4059 (1983)PRBMDO0163-182910.1103/PhysRevB.28.4059] or a compactification of the shifted-contour auxiliary-field Monte Carlo formalism by N. Rom et al. [Chem. Phys. Lett. 270, 382 (1997)CHPLBC0009-261410.1016/S0009-2614(97)00370-9]. An improvement of the acceptance ratio is found for a real auxiliary field, while an improvement of the average sign is found for a purely imaginary auxiliary field. Efficiencies of the different HSTs are tested in the single-band Hubbard model at and away from half filling by studying the staggered magnetization and energy expectation values, respectively
Many-body effects on polarization and dynamical charges in a partly covalent polar insulator
No full textOne-hole Green function, momentum distribution and quasiparticle weight of the U to infinity 1D Hubbard model
No full textStarting from the known Lieb and Wu solution of the one-dimension-Hubbard model in the U to infinity limit, the authors show how the spin-charge decoupling of the elementary excitations is responsible for several peculiar features in one-particle properties, such as momentum distribution, quasiparticle weight and the Green function. In particular they analyse in detail the structure of the one-hole Green function at half-filling, which has not been previously calculated by field theory methods due to the breakdown of conformal invariance. A rich structure is found with branch cut singularities at omega =+or-2 sin k but no simple poles. The non-trivial dependence on the momentum of the hole allows for hole propagation although the analytic structure of G(k, omega ) is quite different from that usually characterizing band insulators. These results provide a precise characterization of one-dimensional Mott insulators. The relationship between the branch cuts of the Green function and the finite-size scaling of the quasiparticle weight is also discussed together with its implications for the analysis of numerical data
Finite Drude weight for one-dimensional low-temperature conductors
No full textWe apply well established finite temperature Quantum Monte Carlo techniques to one dimensional Bose systems with soft and hardcore constraint, as well as to spinless fermion systems. We give clear and robust numerical evidence that, as expected, no superfluid density for Bosons or Meissner fraction for fermions. is possible at {\em any} non zero temperature in one dimensional interacting Bose or fermi lattice models, whereas a finite Drude weight is generally observed in gapless systems, in partial disagreement to previous expectations
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