Thermodynamics of the two-dimensional Falicov-Kimball model: a classical Monte Carlo study

The two-dimensional Falicov-Kimball (FK) model is analyzed using Monte Carlo method. In the case of concentrations of both itinerant and localized particles equal to 0.5 we determine temperature dependence of specific heat, charge density wave susceptibility and density-density correlation function. In the weak interaction regime we find a first order transition to the ordered state and anomalous temperature dependence of the correlation function. We construct the phase diagram of half-filled FK model. Also, the role of next-nearest-neighbor hopping on the phase diagram is analyzed. Lastly, we discuss the density of states and the spectral functions for the mobile particles in weak and strong interaction regime.Comment: 15 pages, RevTe

Collective Excitations of Supersymmetric Plasma

Collective excitations of N = 1 supersymmetric electromagnetic plasma are studied. Since the Keldysh-Schwinger approach is used, not only equilibrium but also non-equilibrium plasma, which is assumed to be ultrarelativistic, is under consideration. The dispersion equations of photon, photino, electron and selectron modes are written down and the self-energies, which enter the equations, are computed in the Hard Loop Approximation. The self-energies are discussed in the context of effective action which is also given. The photon modes and electron ones appear to be the same as in the usual ultrarelativistic plasma of electrons, positrons and photons. The photino modes coincide with the electron ones and the selectron modes are as of free relativistic massive particle.Comment: 14 pages, typos corrected, Phys. Rev. D in prin

Ising t-J model close to half filling: A Monte Carlo study

Within the recently proposed doped-carrier representation of the projected lattice electron operators we derive a full Ising version of the t-J model. This model possesses the global discrete Z_2 symmetry as a maximal spin symmetry of the Hamiltonian at any values of the coupling constants, t and J. In contrast, in the spin anisotropic limit of the t-J model, usually referred to as the t-J_z model, the global SU(2) invariance is fully restored at J_z=0, so that only the spin-spin interaction has in that model the true Ising form. We discuss a relationship between those two models and the standard isotropic t-J model. We show that the low-energy quasiparticles in all three models share the qualitatively similar properties at low doping and small values of J/t. The main advantage of the proposed Ising t-J model over the t-J_z one is that the former allows for the unbiased Monte Carlo calculations on large clusters of up to 10^3 sites. Within this model we discuss in detail the destruction of the antiferromagnetic order by doping as well as the interplay between the AF order and hole mobility. We also discuss the effect of the exchange interaction and that of the next nearest neighbour hoppings on the destruction of the AF order at finite doping. We show that the short-range AF order is observed in a wide range of temperatures and dopings, much beyond the boundaries of the AF phase. We explicitly demonstrate that the local no double occupancy constraint plays the dominant role in destroying the magnetic order at finite doping. Finally, a role of inhomogeneities is discussed.Comment: 24 pages, 10 figure

Correlations in hexagonal lattice systems : application to carbon nanotubes

We present exact diagonalization studies of two-dimensional electron gas on hexagonal lattice. Using Lanczos method we analyze the influence of the Coulomb correlations on the density of states and spectral functions. Choosing appropriate boundary conditions we simulate the geometry of a single wall carbon nanotube. In particular, integration over the boundary condition in one direction and summation in the other one allows us to perform cluster calculations for a tube-like system with a finite diameter and infinite length

Next-nearest-neighbor hopping in the Falicov-Kimball model

Results of Monte Carlo simulations for the spinless Falicov-Kimball model with the next-nearest-neighbor hopping are presented. We find the critical value of the next-nearest-neighbor hopping integral, below which the low temperature configuration of the localized particles is the same as in the presence of only the nearest-neighbor hopping. Beyond this critical value the localized particles form horizontal or vertical stripes

Hofstadter butterfly for a finite correlated system

We investigate a finite two-dimensional system in the presence of external magnetic field. We discuss how the energy spectrum depends on the system size, boundary conditions and Coulomb repulsion. On one hand, using these results we present the field dependence of the transport properties of a nanosystem. In particular, we demonstrate that these properties depend on whether the system consists of even or odd number of sites. On the other hand, on the basis of exact results obtained for a finite system we investigate whether the Hofstadter butterfly is robust against strong electronic correlations. We show that for sufficiently strong Coulomb repulsion the Hubbard gap decreases when the magnetic field increases.Comment: 7 pages, 5 figures, revte

Studies of ignition behaviour of biomass particles in a down-fire reactor for improving co-firing performance

To realize large percentage biomass co-firing with coal in existing coal-fired boilers, the combustion behaviour of biomass is expected to be similar or comparable to that of coal. When co-firing with coal, biomass is not necessarily to be ground as fine as the dedicated coal particles due to its higher reactivity. With aim of achieving promising performance of co-firing with dedicated coal particles, the determination of suitable particle size of biomass becomes important. The paper investigates experimentally the ignition behaviour of three biomass materials in a down-fire reactor associated with thermogravimetric analyser (TGA). TGA results showed that the devolatilization process is accelerated by the presence of oxygen, but failed to identify the impacts of particle size on the ignition behaviour of biomass. However, the ignition testing results based in the down-fire reactor clearly showed that ignition delay time of a large biomass particle is longer than that of smaller one. In addition, being injected into the furnace, the softwood particles take a longer residence time to be ignited than the straw particles at same sizes, which agrees well with their reactivity analysis in TGA. Moreover, the ignition test results suggested that the ignition mechanism of biomass could be alternated from homogeneous to the heterogeneous ignition when the furnace temperature is increasing; at high enough furnace temperatures, the ignition predictably occurs at the particle surface without requiring the start of devolatilization. The results quantitatively demonstrate the effects of particle size on the ignition delay time of biomass , which, together with the transport phenomena and surrounding atmosphere, can contribute to control the biomass combustion profile and co-firing performance

Studies of ignition behaviour of biomass particles in a down-fire reactor for improving co-firing performance

To realize large percentage biomass co-firing with coal in existing coal-fired boilers, the combustion behaviour of biomass is expected to be similar or comparable to that of coal. When co-firing with coal, biomass is not necessarily to be ground as fine as the dedicated coal particles due to its higher reactivity. With aim of achieving promising performance of co-firing with dedicated coal particles, the determination of suitable particle size of biomass becomes important. The paper investigates experimentally the ignition behaviour of three biomass materials in a down-fire reactor associated with thermogravimetric analyser (TGA). TGA results showed that the devolatilization process is accelerated by the presence of oxygen, but failed to identify the impacts of particle size on the ignition behaviour of biomass. However, the ignition testing results based in the down-fire reactor clearly showed that ignition delay time of a large biomass particle is longer than that of smaller one. In addition, being injected into the furnace, the softwood particles take a longer residence time to be ignited than the straw particles at same sizes, which agrees well with their reactivity analysis in TGA. Moreover, the ignition test results suggested that the ignition mechanism of biomass could be alternated from homogeneous to the heterogeneous ignition when the furnace temperature is increasing; at high enough furnace temperatures, the ignition predictably occurs at the particle surface without requiring the start of devolatilization. The results quantitatively demonstrate the effects of particle size on the ignition delay time of biomass , which, together with the transport phenomena and surrounding atmosphere, can contribute to control the biomass combustion profile and co-firing performance