Two-component physics of cuprates and superconductor–insulator transitions

Abstract

We show that superconductor–insulator transitions (SIT) may arise due to a charge densitywave instability induced by the over-screened Coulomb interaction modified due todecreasing carrier density. Within this new insulating state a novel quasiparticle entity, amicroscopic Coulomb bubble (CB), emerges. These bubbles are embedded inside thesuperconductor and form nuclei of the new insulating state. The growth of a bubble isterminated by the Coulomb force and each bubble has a quantized charge and a fluctuatingphase. When bubbles first appear they are covered by superfluid liquid due to a proximityeffect and are invisible. However, when the carrier density decreases further, the bubbles’size increases and the superconducting proximity inside the bubbles vanishes. Theinsulating state arises via a percolation of insulating islands originating insidethe CBs, which form a giant percolating cluster that prevents the flow of theelectrical supercurrent through the system. We also show the formation of twogroups of charge carriers in these compounds associated with free and localizedstates. The localized component arises due to Coulomb clumps. Our results arecompletely consistent with the two-component picture of cuprates deduced earlier byGorkov and Teitelbaum (GT) (Gorkov and Teitelbaum 2006 Phys. Rev. Lett. 97247003, Gorkov and Teitelbaum 2008 J. Phys.: Conf. Ser. 108 012009) from theanalysis of Hall effect data and the ARPES spectra. These CBs induce nanoscalesuperstructures observed in scanning tunneling microscope (STM) experiments (Gorkovand Teitelbaum 2008 J. Phys.: Conf. Ser. 108 012009, Pan et al 2001 Nature 413 282–5,Dubi et al 2007 Nature 449 876–9, Gomes et al 2007 Nature 447 569, Lee et al 2006Nature 442 546, McElroy et al 2005 Science 309 1048, Zhu et al 2006 Phys. Rev.Lett. 97 177001) and responsible for the pseudogap and Nernst effect in HTSC.