329 research outputs found
Does the Heisenberg model describe the multimagnon spin dynamics in antiferromagnetic CuO layers ?
We compute the absorption spectrum for multimagnon excitations assisted by
phonons in insulating layered cuprates using exact diagonalization in clusters
of up to 32 sites. The resulting line shape is very sensitive to the underlying
magnetic Hamiltonian describing the spin dynamics. For the usual Heisenberg
description of undoped Cu-O planes we find, in accordance with experiment, a
two-magnon peak followed by high energy side bands. However the relative weight
of the side bands is too small to reproduce the experiment. An extended
Heisenberg model including a sizable four-site cyclic exchange term is shown to
be consistent with the experimental data.Comment: To appear in Physical Review Letter
Exact Jastrow-Slater wave function for the one-dimensional Luttinger model
We show that it is possible to describe the ground state of the Luttinger
model in terms of a Jastrow-Slater wave function. Moreover, our findings reveal
that one-particle excitations and their corresponding dynamics can be
faithfully represented only when a Jastrow factor of a similar form is applied
to a coherent superposition of many Slater determinants. We discuss the
possible relevance of this approach for the theoretical description of
photoemission spectra in higher dimensionality, where the present wave function
can be straightforwardly generalized and can be used as a variational ansatz,
that is exact for the 1D Luttinger model.Comment: 10 pages, one figure, to appear in Phys. Rev.
Correlated geminal wave function for molecules: an efficient resonating valence bond approach
We show that a simple correlated wave function, obtained by applying a
Jastrow correlation term to an Antisymmetrized Geminal Power (AGP), based upon
singlet pairs between electrons, is particularly suited for describing the
electronic structure of molecules, yielding a large amount of the correlation
energy. The remarkable feature of this approach is that, in principle, several
Resonating Valence Bonds (RVB) can be dealt simultaneously with a single
determinant, at a computational cost growing with the number of electrons
similarly to more conventional methods, such as Hartree-Fock (HF) or Density
Functional Theory (DFT). Moreover we describe an extension of the Stochastic
Reconfiguration (SR) method, that was recently introduced for the energy
minimization of simple atomic wave functions. Within this extension the atomic
positions can be considered as further variational parameters, that can be
optimized together with the remaining ones. The method is applied to several
molecules from Li_2 to benzene by obtaining total energies, bond lengths and
binding energies comparable with much more demanding multi configuration
schemes.Comment: 20 pages, 5 figures, to be published in the Journal of Chemical
Physic
Theoretical constraints for the magnetic-dimer transition in two-dimensional spin models
From general arguments, that are valid for spin models with sufficiently
short-range interactions, we derive strong constraints on the excitation
spectrum across a continuous phase transition at zero temperature between a
magnetic and a dimerized phase, that breaks the translational symmetry. From
the different symmetries of the two phases, it is possible to predict, at the
quantum critical point, a branch of gapless excitations, not described by
standard semi-classical approaches. By using these arguments, supported by
intensive numerical calculations, we obtain a rather convincing evidence in
favor of a first-order transition from the ferromagnetic to the dimerized phase
in the two-dimensional spin-half model with four-spin ring-exchange
interaction, recently introduced by A.W. Sandvik et al. [Phys. Rev. Lett. 89,
247201 (2002)].Comment: 7 pages and 5 figure
Alleviation of the Fermion-sign problem by optimization of many-body wave functions
We present a simple, robust and highly efficient method for optimizing all
parameters of many-body wave functions in quantum Monte Carlo calculations,
applicable to continuum systems and lattice models. Based on a strong
zero-variance principle, diagonalization of the Hamiltonian matrix in the space
spanned by the wav e function and its derivatives determines the optimal
parameters. It systematically reduces the fixed-node error, as demonstrated by
the calculation of the binding energy of the small but challenging C
molecule to the experimental accuracy of 0.02 eV
Two spin liquid phases in the spatially anisotropic triangular Heisenberg model
The quantum spin-1/2 antiferromagnetic Heisenberg model on a two dimensional
triangular lattice geometry with spatial anisotropy is relevant to describe
materials like and organic compounds like
{-(ET)Cu(CN)}. 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 between the intra-chain nearest neighbor coupling and the
inter-chain one . 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.Comment: 35 pages, 24 figures, 3 table
Gaussian quantum Monte Carlo methods for fermions
We introduce a new class of quantum Monte Carlo methods, based on a Gaussian
quantum operator representation of fermionic states. The methods enable
first-principles dynamical or equilibrium calculations in many-body Fermi
systems, and, combined with the existing Gaussian representation for bosons,
provide a unified method of simulating Bose-Fermi systems. As an application,
we calculate finite-temperature properties of the two dimensional Hubbard
model.Comment: 4 pages, 3 figures, Revised version has expanded discussion,
simplified mathematical presentation, and application to 2D Hubbard mode
UV finiteness of 3D Yang-Mills theories with a regulating mass in the Landau gauge
We prove that three-dimensional Yang-Mills theories in the Landau gauge
supplemented with a infrared regulating, parity preserving mass term are
ultraviolet finite to all orders. We also extend this result to the
Curci-Ferrari gauge.Comment: 6 page
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