T. Lemberger | Searcharxiv

arXiv2013

Robustness of the Berezinskii-Kosterlitz-Thouless Transition in Ultrathin NbN Films near the Superconductor-Insulator Transition

Occurrence of the Berezinskii-Kosterlitz-Thouless (BKT) transition is investigated by superfluid density measurements for two-dimensional (2D) disordered NbN films with disorder level very close to a superconductor-insulator transition (SIT). Our data show a robust BKT transition even near this 2D disorder-tuned quantum critical point (QCP). This observation is in direct contrast with previous data on deeply underdoped quasi-2D cuprates near the SIT. As our NbN films approach the QCP, the vortex-core energy, an important energy scale in the BKT transition, scales with the superconducting gap, not with the superfluid density, as expected within the standard 2D-XY model description of BKT physics.

cond-mat.supr-con↗

arXiv2003

The Cuprate Pseudogap: Competing Order Parameters or Precursor Superconductivity

In this paper we compare two broad classes of theories for the pseudogap in cuprate superconductors. The comparison in made in reference to measurements of the superfluid density, $ρ_s(T,x)$, in $YBa_2CuO_{7- δ}$ films having a wide range of stoichiometries, $δ$, or, hole doping, $x$. The theoretical challenge raised by these (and previous) data is to understand why the T-dependence of $ρ_s(T,x)$ is insensitive to the fermionic excitation gap $Δ(T,x)$, which opens in the normal state and persists into the superconducting state, when presumably $ρ_s(T)$ is governed, at least in part, by fermionic excitations. Indeed, $ρ_s(T,x)$ seems to have a BCS-like dependence on $T_c(x)$, which, although not unexpected, is not straightforward to understand in pseudogapped superconductors where $T_c(x)$ and the excitation gap have little in common. Here, we contrast "extrinsic" and "intrinsic" theoretical approaches to the pseudogap and argue that the former (for example, associated with a competing order parameter) exhibits more obvious departures from BCS-like $T$ dependences in $ρ_s(T)$ than approaches which associate the pseudogap with the superconductivity itself. Examples of the latter are Fermi liquid based schemes as well as a pair fluctuation mean field theory. Thus far, the measured behavior of the superfluid density appears to argue against an extrinsic interpretation of the pseudogap, and supports instead its intrinsic origin.

cond-mat.supr-con↗