Atypical vortex lattice and the magnetic penetration depth in superconducting Sr${}_2$RuO${}_4$ deduced by $\mu$SR

Abstract

The muon spin rotation ($\mu$SR) technique has been applied to determine the behavior of the in-plane magnetic penetration depth (${\lambda }_{ab}$) in the vortex state of the unconventional superconductor Sr${}_2$RuO${}_4$ as a means of gaining insight into its still unknown superconducting order parameter. A recent $\mu$SR study of Sr${}_2$RuO${}_4$ reported a $T$-linear temperature dependence for ${\lambda }_{ab}$ at low temperatures that was not identified in an earlier $\mu$SR study. Here we show that there is no significant difference between the data in the early and recent $\mu$SR studies and both are compatible with the limiting low-temperature ${\lambda }_{ab}\sim T^2$ dependence expected from measurements of the change in ${\lambda }_{ab}(T)$ in the Meissner state by other techniques. However, we argue that at this time there is no valid theoretical model for reliably determining the absolute value of ${\lambda }_{ab}$ in Sr${}_2$RuO${}_4$ from $\mu$SR measurements. Instead, we identify the formation of an unusual square vortex lattice that introduces a new constraint on candidate superconducting order parameters for Sr${}_2$RuO${}_4$.

6 pages, 3 figures