Enhancement of the London penetration depth in pnictides at the onset of SDW order under superconducting dome

TL;DR

This paper explains the peak in London penetration depth observed near optimal doping in pnictides as a quantum critical point related to the onset of spin-density-wave order, influenced by critical magnetic fluctuations.

Contribution

It demonstrates that magnetic critical fluctuations near a QCP cause the enhancement of the London penetration depth in pnictides, contrasting with simpler Fermi liquid behavior.

Findings

Peak in (x) near optimal doping linked to QCP

Critical magnetic fluctuations renormalize (x)

Pnictides differ from Galilean invariant Fermi liquids in this context

Abstract

Recent measurements of the doping dependence of the London penetration depth \lambda(x) at low temperatures in clean samples of isovalent BaFe_2[As_(1-x)P_x]_2 at T<<Tc [Hashimoto et al., Science 336, 1554 (2012)] revealed a peak in \lambda(x) near optimal doping x=0.3. The observation of the peak at T<<Tc, points to the existence of the quantum critical point (QCP) beneath the superconducting dome. We associate such a QCP with the onset of a spin- density-wave order and show that the renormalization of \lambda(x) by critical magnetic fluctuations, gives rise to the observed feature. We argue that the case of pnictides is conceptually different from a one-component Galilean invariant Fermi liquid, for which correlation effects do not cause the renormalization of the London penetration depth at T=0.