E.J. Calegari

Superconductivity in an extended Hubbard model with attractive interaction

In this work, a two-dimensional one-band Hubbard model is investigated within a two-pole approximation. The model presents a non-local attractive potential U (U < 0) that allows the study of d-wave superconductivity and also includes hopping up to second-nearest neighbors. The two-pole scheme has been proposed to improve the Hubbard-I approximation. The analytical results show a more complex form for the gap Δ(T), when compared to the one obtained in the latter approximation. Indeed, new anomalous correlation functions associated with the superconductivity are involved in the calculation of Δ(T). Numerical results in a range of temperatures are presented. Moreover, the structure of the quasiparticle bands and the topology of the Fermi surface are studied in detail in the normal state. Connections with some experimental results are also included.

Unusual Magnetic Field-Dependence of the Electronic Dispersion Relations in a Hidden Ordered Phase.

and orbital density wave state in the underscreened Anderson Lattice Model. The spin-flip part of the Hund’s rule coupling stabilizes a spontaneous spin-dependent mixing of 5f quasiparticle bands that, in the normal state, have pure orbital characters. The transition breaks the spin-rotational invariance and leads to an asymmetric pseudo-gap which forms in the density of states. When a magnetic field is applied, the electronic dispersion relations become dependent on the relative orientation of the field and the spontaneously-chosen quantization axis. We argue that this may result in the magnetic susceptibility of the low-temperature phase becoming anisotropic, but without the development of a static magnetization.

A minimal model for “Hidden Order” in URu2Si2

We present a bare bones description of a model which may lead to a Hidden Order Phase. We use a variational approximation and find that spin-rotational invariance, periodic translational invariance and gauge invariance are broken at low temperatures. The breaking of spin-rotational invariance leads to the development of an anisotropy in the susceptibility.

Pseudogap and the specific heat of high Tcsuperconductors

Abstract The specific heat of a two dimensional repulsiveHubbard model with local interaction is investi-gated. We use the two-pole approximation whichexhibits explicitly important correlations that aresources of the pseudogap anomaly. The interplaybetween the specific heat and the pseudogap is themain focus of the present work. Our self consistentnumerical results show that above the occupationn T ≈ 0.85, the specific heat starts to decrease dueto the presence of a pseudogap in the density ofstates. We have also observed a two peak struc-ture in the specific heat. Such structure is robustwith respect to the Coulomb interaction U but it issignificantly affected by the occupation n T . A de-tailed study of the two peak structure is carried outin terms of the renormalized quasi-particle bands.The role of the second nearest neighbor hopping onthe specific heat behavior and on the pseudogap, isextensively discussed. 1 Introduction The one-band Hubbard model with repulsive lo-cal interaction [1] is one of the simplest but sig-

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-

Interplay Between Condensation Energy, Pseudogap, and the Specific Heat of a Hubbard Model in a n-Pole Approximation

T_c

Signatures of broken spin-rotational invariance in the “Hidden Ordered” compound URu2Si2?

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

Specific heat of a non-local attractive Hubbard model

t

Many-body effects in high-Tc materials: anomalous properties in the pseudogap region

) and second (

Unusual magnetic field-dependence of a possible hidden order phase

t_2