Superconductivity in a disordered metal with Coulomb interactions

Abstract

We study electronic densities of states (DOS) of strongly disordered superconducting thin films of TiN. We find through scanning tunneling microscopy (STM) measurements that the DOS decreases towards the Fermi level in the normal phase obtained by applying magnetic fields. The DOS shows spatial fluctuations whose length scale is related to the energy-dependent decrease in the DOS close to the Fermi level and is similar in normal and superconducting phases. This shows that Coulomb interaction in disordered metals strongly influences the normal DOS and suggests that reduced Coulomb screening leads to spatial variations in the superconducting DOS.

Signatures of two-dimensional superconductivity emerging within a three-dimensional host superconductor

“…superconductivity | superconductor-insulator transition | phase transitions | reduced dimensions | scanning tunneling microscopy and spectroscopy I n the two-dimensional (2D) limit of the superconductorinsulator transition (SIT) (1)(2)(3), there is evidence of emergent electronic granularity in numerical simulations (4)(5)(6) and thin-film experiments (7)(8)(9)(10). It has been recently discovered (11,12) that bulk three-dimensional (3D) BaPb 1−x Bi x O 3 (BPBO) (Fig.…”

“…Scanning tunneling microscopy and spectroscopy (STM/STS) enable a local probe of the electronic properties through measurements of local density of states (LDOS) along with topographic information. This technique has been used to study the LDOS homegeneity in quasi-2D superconductors (SC), including high-T c superconductors (HTSC) (15)(16)(17) and 2D thin-film SC systems (10,(18)(19)(20). STS reports that examine the influence of disorder on the SC state in 3D systems are limited (21,22).…”

Model-independent determination of the gap function of nearly localized superconductors

“…(iii) The measured spatial modulations of the tunneling density of states strongly suggest that also "emergent granularity", i.e. electronic inhomogeneity in structurally effectively homogeneous systems does play a role [10,11]. Unfortunately, the combined effect of changing interactions, phase fluctuations, and localization is presently not understood [1].…”

“…When the QBS occurs in homogeneously disordered systems, all three mechanisms are intertwined: (i) The relevance of the fermionic scenario is documented most convincingly by the recent observation of quantum corrections to optical conductivity up to at least ∼4 eV [9]. (ii) The temperature dependence of the resistivity indicates that phase fluctuations definitely can not be neglected in such systems [10,11]. (iii) The measured spatial modulations of the tunneling density of states strongly suggest that also "emergent granularity", i.e.…”

“…In case of conventional superconductors ∆(ω) can be routinely extracted from the tunneling data [12]. For nearly localized superconductors such data are in fact available from a series of recent papers [10,11,15], but all of them find that the superconducting feature in the density of states develops on top of a normal-state background with a non-constant density of states, presumably due to interaction-induced corrections [4,16]. In such case the standard extraction procedure [12] is not applicable.…”