Towards high temperature holographic superconductors

Abstract

We explore a holographic superconductor model in which a real scalar field is non-minimally coupled to a gauge field. We consider several types of the non-minimal coupling function h($\psi$) including exponential, hyperbolic (cosh), power-law and fractional forms. We investigate the influences of the non-minimal coupling parameter $\alpha$ on condensation, critical temperature and conductivity. We can categorize our results in two groups. In the first group, conductor/superconductor phase transition is easier to occur for larger values of $\alpha$, while in the second group stronger effects of the non-minimal coupling makes the formation of scalar hair harder. Although the real and imaginary parts of conductivity are impressed by different forms of h($\psi$), they follow some universal behaviors such as connecting with each other through Kramers-Kronig relation in low frequency regime or the appearance of gap frequency at low temperatures. We find the best form of forms of non-minimal coupling function that gives us better information in wide range of non-minimal coupling constant and temperature. Choosing the best form of h($\psi$), we construct a family of solutions for holographic conductor/superconductor phase transitions to discover the effect of the hyperscaling violation when the gauge and scalar fields are non-minimally coupled. we find that the critical temperature increases for higher effects of hyperscaling violation $\theta$ and non-minimal coupling constant $\alpha$. By increasing these two parameters, we obtain lower values of condensation which means that conductor/superconductor phase transition will acquire easier. Furthermore, we understand that the hyperscaling violation affects the conductivity $\sigma$ of the holographic superconductors and changes the expected relation in the gap frequency. Some universal behaviors like infinite DC conductivity are observed.