48,034 research outputs found
Mean-field theories of Hubbard and t-J models
No full textWe report novel results obtained for the Hubbard and t-J models by various mean-field approximations
Lower Bounds for the Ground-State Energies of the 2D Hubbard and t-J Models
No full textWe present simple lower bounds on the ground-state energy of the two-dimensional (2D) Hubbard and t-J models for arbitrary values of band filling and coupling constant. For the Hubbard model we derive two types of bounds, both based on decomposing the model Hamiltonian into a sum of sub-Hamiltonians. For a decomposition into local cluster sub-Hamiltonians, we perform a generalized Legendre transform on previously derived bounds for the grand-canonical potential. For a decomposition into spin-up and spin-down parts, previous results on the spinless Falicov-Kimball model may be used to obtain bounds for the Hubbard model, generalizing a result of Langer and Mattis to arbitrary filling. For the 2D t-J model we have only considered the decomposition into clusters. The 1D Hubbard model is used as a test case. The bounds may be improved by diagonalizing the Hamiltonian for larger clusters
Similarities between the t-J and Hubbard models in weakly correlated regimes
No full textWe present a comparative study of the Hubbard and models far
away from half-filling. We show that, at such fillings the
Hamiltonian can be seen as an effective model of the repulsive Hubbard
Hamiltonian over the whole range of correlation strength. Indeed, the
range of the Hubbard model can be
mapped onto the finite range of the
model, provided that the effective exchange parameter J is
defined variationally as the local singlet-triplet excitation energy.
In this picture the uncorrelated limit U=0 is associated with the
super-symmetric point and the infinitely correlated
limit with the usual J=0 limit. A numerical comparison
between the two models is presented using different macroscopic and
microscopic properties such as energies, charge gaps and bond orders
on a quarter-filled infinite chain. The usage of the Hamiltonian
in low-filled systems can therefore be a good alternative to the
Hubbard model in large time-consuming calculations
The 2-site Hubbard and t-J models
No full textThe fermionic and bosonic sectors of the 2-site Hubbard model have been exactly solved by means of the equation of motion and Green's function formalism. The exact solution of the t-J model has been also reported to investigate the low-energy dynamics. We have successfully searched for the exact eigenoperators, and the corresponding eigenenergies, having in mind the possibility to use them as an operatorial basis on the lattice. Many local, single-particle, thermodynamical and response properties have been studied as functions of the external parameters and compared between the two models and with some numerical and exact results. It has been shown that the 2-site Hubbard model already contains the most relevant energy scales of the Hubbard model: the local Coulomb interaction U and the spin-exchange one J = 4t 2/U. As a consequence of this, for some relevant properties (kinetic energy, double occupancy, energy, specific heat and entropy) and as regards the metal-insulator transition issue, it has resulted possible to almost exactly mime the behavior of larger systems, sometimes using a higher temperature to get a comparable level spacing. The 2-site models have been also used as toy models to test the efficiency of the Green's function formalism for composite operators. The capability to reproduce the exact solutions, obtained by the exact diagonalization technique, gives a firm ground to the approximate treatments based on this formalism. © 2003 Springer-Verlag Berlin/Heidelberg
Hubbard model versus t-J model : The one-particle spectrum
Get PDFSubmitted to: Phys. Rev., B
Abstract: The origin of the apparent discrepancies between the one-particle spectra of the Hubbard and t-J models is revealed: Wavefunction corrections, in addition to the three-site terms, should supplement the bare t-J. In this way a quantitative agreement between the two models is obtained, even for the intermediate- values appropriate for the high-Tc cuprate superconductors. Numerical results for clusters of up to 20 sites are presented. The momentum dependence of the observed intensities in the photoemission spectra of Sr2CuO2Cl2 are well described by this complete strong-coupling approach.
Interplay magnetism and temperature in the large-demensional limits of the Hubbard and t-J models
No full textWe describe a theory for the finite-temperature properties of the infinite dimensional (d"#infinity#) half-filled Hubbard model and the t-J model. The work presented extends the local moment approach (LMA) to include the effects of temperature. We start by investigating the t-J model, which provides the leading strong coupling asymptotics to the Hubbard model. The only relevant energy scale, apart from temperature T, is the cost to flip a spin in the exchange field of the neighbouring spins. By identifying the temperature dependence of the spin-flip cost w_p, we are able to obtain an exact solution to the d"#infinity# t-J model. For the Hubbard model we first consider a scenario where T enters solely implicitly via the thermal disorder of the local moment state, which we describe at unrestricted Hartree-Fock level. The low-energy scale for spin-flip excitations is identified via an RPA treatment of the transverse spin polarization propagator. Dynamical coupling of single-particle processes to the spin-excitations yields a renormalized self-consistent description of the self-energy. This is shown to be asymptotically exact in strong coupling and results describing the thermal evolution of the spectra are discussed for moderate to strong Interaction strengths. By translating the diagrams for the RPA-propagator and the self-energy into the imaginary-frequency formalism we include explicit temperature effects via Fermi functions. The imaginary-frequency expressions are then analytically continued to obtain the related retarded real-frequency expressions. We also derive the correct form of the dynamical conductivity on the d"#infinity# Bethe lattice. Results for finite-temperature single-particle spectra and conductivities are then presented and particular attention given to the thermal evolution of these quantities and the finite-temperature insulator to metal crossover. (author)SIGLEAvailable from British Library Document Supply Centre-DSC:D203926 / BLDSC - British Library Document Supply CentreGBUnited Kingdo
Transformation of the Hubbard Hamiltonian to the t-J Hamiltonian using the Raleigh- Schrodinger Perturbation Expansion
No full textThe t-J model is deduced from the Hubbard model using Rayleigh-Schrodinger perturbation theory. The exchange is antiferromagnetic as found by other workers
Magnetic Phases and Generalized t-J Models in Doped Mott-Hubbard Insulators
No full textWe review some recent results obtained for the dynamics of a single hole and for the ground states at finite hole doping in t-J model. Next, we address the role of orbital degeneracy in doped Mott-Hubbard insulators and show examples of effective strong coupling models which include the orbital degrees of freedom. These new t-J models have interesting phase diagrams, with the new magnetic phases stabilized by a competition between magnetic energy and excitonic excitations. It is argued that the doped holes always bind to the excitons and that the new phases identified on the mean-field level give rise to local distortions of the lattice. We conclude that realistic t-J models derived from the electronic structure of particular compounds may be successfully applied for understanding both the observed magnetic ground states, and the results of photoemission experiments, as we have demonstrated recently for NiO
Measurement of the ratio of prompt χ c to J / ψ production in pp collisions at √s = 7 TeV
Get PDFThe prompt production of charmonium χ c and J / ψ states is studied in proton-proton collisions at a centre-of-mass energy of √s = 7 TeV at the Large Hadron Collider. The χ c and J / ψ mesons are identified through their decays χ c → J / ψ γ and J / ψ → μ + μ - using 36 pb - 1 of data collected by the LHCb detector in 2010. The ratio of the prompt production cross-sections for χ c and J / ψ, σ (χ c → J / ψ γ) / σ (J / ψ), is determined as a function of the J / ψ transverse momentum in the range 2 < p T J / ψ < 15 GeV / c. The results are in excellent agreement with next-to-leading order non-relativistic expectations and show a significant discrepancy compared with the colour singlet model prediction at leading order, especially in the low p T J / ψ region
Unambiguous relationship between the Hubbard, t-J and d-p models in One-dimension based on the Luttinger liquid Theory
Get PDFWe examine the one-dimensional (1D) d–p model in comparison with typical 1D models such as the 1D Hubbard model and the 1D t–J model using the numerical diagonalization method combined with the Luttinger liquid theory. We calculate the spin velocity vσ, the charge velocity vρ and the Luttinger liquid parameter Kρ for each model. Using these parameters, a relationship between the models is obtained unambiguously. We find that the d–p model can be described by the Hubbard model in the wide parameter region, while it can be described by the t–J model only in the strong coupling limit
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