Supercondutividade na presença de correlações que podem gerar pseudogap

• Autor(a) principal: Sampaio, Leonardo Fernandes
• Data de Publicação: 2019
• Tipo de documento: Tese
• Idioma: por
• Título da fonte: Manancial - Repositório Digital da UFSM
• Texto Completo: http://repositorio.ufsm.br/handle/1/31398
• topic: Pseudogap; Funções de Green; Interação coulombiana; Modelo de Hubbard de uma banda; Supercondutividade; Green's functions; Coulomb interaction; Superconductivity; Hubbard model; CNPQ::CIENCIAS EXATAS E DA TERRA::FISICA

Since the discovery of superconductivity at high temperatures in 1986, the problem of correlations in superconductivity has received a lot of attention, but it remains as an open problem in condensed matter physics. In this work, superconductivity with di erent symmetries is considered in an one-band Hubbard model. Besides the local repulsive interaction, the studied model also presents an attractive potential, which is responsible for the superconductivity. This study was conducted for superconductors with s, dx2�����y2 , and s-extended symmetry. The main objective is to investigate the role of correlations in the normal and superconducting properties of the model. To treat the model, we used the Green's functions technique together with a n-pole approximation. The n-pole approximation was considered in two versions, the original version (R-O) proposed by Roth (Phys. Rev 184 (1969)) and a modi ed version (R-M) (IJMPB 25 (2011)), which describes most appropriate antiferromagnetic correlations associated with the model. In the R-M version, antiferromagnetic correlations may give rise to a pseudogap in the density of states of the normal state. The pseudogap has been a central point in the study of cuprate superconductors because it is believed to be directly associated with the mechanisms responsible for superconductivity in these systems. More speci cally, we investigated how the correlations responsible for the pseudogap scenario a ect the superconducting critical temperature for the di erent symmetries. The obtained results show that superconductors with di erent symmetries are a ected di erently by the correlations. In general, the critical transition temperature Tc is higher in the R-M approximation compared to that of the R-O approximation. Compared to mean eld results, the Tc 's obtained in the n-pole approximations are higher than the mean eld ones. For s symmetry, in the RM approximation, the increasing of the interaction U, suppresses the superconductivity in the region of occupations close to NT = 1; 0 (with NT < 1; 0) and favors superconductivity in an intermediate region of occupation. In this case, it was observed an increasing of the maximum Tc. For the symmetry dx2�����y2 , in the RM approximation, we observe a behavior for Tc as a function of U, similar to that one of the symmetry s, however, in this case the maximum Tc decreases with U. The s extended symmetry presents a di erent behavior for the Tc. The superconductivity occurs in the low occupancy region, in the hole doping regime, and in the region of high occupations, in the electron doping regime. However, when U increases, superconductivity also occurs for occupations close to NT = 1; 0 . A more detailed analysis of the band structure has shown that the increase in the superconducting region when U is increased, is directly linked to the antiferromagnetic correlations, which deform the energy bands. Thus, in general, we can conclude that the antiferromagnetic correlations that cause a pseudogap in the density of states can signi cantly a ect the superconducting critical temperature.