S.G. Magalhaes

Inverse freezing in the cluster fermionic spin glass model with a transverse field

The inverse freezing (IF) transition is studied with a cluster fermionic Ising spin glass model in the presence of a transverse magnetic field ?. The model considers a disordered infinite-range interaction among magnetic moments of the clusters with a short-range ferromagnetic intracluster coupling J0. The intercluster disorder is treated within a mean-field theory by using the framework of one-step replica symmetry breaking and the static approximation. This treatment results in an effective model that is computed by means of an exact diagonalization method. In particular, in this fermionic problem, the chemical potential ? and the ? introduce cluster charge fluctuations and quantum spin flipping, respectively. The results indicate an IF transition for a certain range of ?. When the short-range interaction increases, the IF is still found. However, the intensity of J0 can affect the charge fluctuation, introducing a second-order IF transition. On the other hand, the enhancement of ? destroys the inverse freezing.

Spin glass induced by infinitesimal disorder in geometrically frustrated kagome lattice

We propose a method to study the magnetic properties of a disordered Ising kagome lattice. The model considers small spin clusters with infinite-range disordered couplings and short-range ferromagnetic (FE) or antiferromagnetic interactions. The correlated cluster mean-field theory is used to obtain an effective single-cluster problem. A finite disorder intensity in FE kagome lattice introduces a cluster spin-glass (CSG) phase. Nevertheless, an infinitesimal disorder stabilizes the CSG behavior in the geometrically frustrated kagome system. Entropy, magnetic susceptibility and spin–spin correlation are used to describe the interplay between disorder and geometric frustration (GF). We find that GF plays an important role in the low-disorder CSG phase. However, the increase of disorder can rule out the effect of GF.

Inverse transitions in the Ghatak–Sherrington model with bimodal random fields

The present work studies the Ghatak–Sherrington (GS) model in the presence of a longitudinal magnetic random field (RF) hi following a bimodal distribution. The model considers a random bond interaction Ji,j which follows a Gaussian distribution with mean J0/N and variance J2/N. This allows us to introduce the bond disorder strength parameter J/J0 to probe the combined effects of disorder coming from the random bond and the discrete RF over unusual phase transitions known as inverse transitions (ITs). The results within a mean field approximation indicate that these two types of disorder have completely distinct roles for the ITs. They indicate that bond disorder creates the necessary conditions for the presence of inverse freezing, or even inverse melting, depending on the bond disorder strength, while the RF tends to enforce mechanisms that destroy the ITs.

Pseudogap and the specific heat of high Tc superconductors: a Hubbard model in a n-pole approximation

In this work the specific heat of a two-dimensional Hubbard model, suitable to discuss high-

Specific heat of a non-local attractive Hubbard model

T_c

The Sherrington–Kirkpatrick model in the presence of a bimodal random field: a one-step replica-symmetry-breaking approach

superconductors (HTSC), is studied taking into account hopping to first (

Magnetic properties of correlated one-band metals: the effect of the Roth band shift in the single site approximation

t

Inverse freezing in the Hopfield fermionic Ising spin glass with a transverse magnetic field

) and second (

Ferromagnetism in two band metals: The very strong coupling limit☆

t_2

Random field Ising model in a random graph

) nearest neighbors. Experimental results for the specific heat of HTSCs, for instance, the YBCO and LSCO, indicate a close relation between the pseudogap and the specific heat. In the present work, we investigate the specific heat by the Greens function method within a