Broken Time Reversal Symmetry in Superconducting Pr1-xCexPt4Ge12

TL;DR

This study investigates how Ce doping affects broken time-reversal symmetry in Pr$_{1-x}$Ce$_{x}$Pt$_4$Ge$_{12}$ superconductors, revealing a linear decrease of spontaneous magnetic fields with doping and linking Pr interactions to TRS breaking.

Contribution

It provides the first detailed analysis of Ce doping's impact on TRS in this superconductor, showing the suppression of TRS with increased Ce concentration and the role of Pr-Pr interactions.

Findings

Broken TRS decreases linearly with Ce doping

TRS disappears near x ≈ 0.4 where superconductivity vanishes

Pr-Pr interactions are linked to TRS breaking

Abstract

We report results of zero-field muon spin relaxation experiments on the filled-skutterudite superconductors~Pr$_{1-x}$Ce$_{x}$Pt$_4$Ge$_{12}$, $x = 0$, 0.07, 0.1, and 0.2, to investigate the effect of Ce doping on broken time-reversal symmetry (TRS) in the superconducting state. In these alloys broken TRS is signaled by the onset of a spontaneous static local magnetic field~$B_s$ below the superconducting transition temperature. We find that $B_s$ decreases linearly with $x$ and $\to 0$ at $x \approx 0.4$, close to the concentration above which superconductivity is no longer observed. The (Pr,Ce)Pt$_4$Ge$_{12}$ and isostructural (Pr,La)Os$_4$Sb$_{12}$ alloy series both exhibit superconductivity with broken TRS, and in both the decrease of $B_s$ is proportional to the decrease of Pr concentration. This suggests that Pr-Pr intersite interactions are responsible for the broken TRS\@. The two alloy series differ in that the La-doped alloys are superconducting for all La concentrations, suggesting that in (Pr,Ce)Pt$_4$Ge$_{12}$ pair-breaking by Ce doping suppresses superconductivity. For all $x$ the dynamic muon spin relaxation rate decreases somewhat in the superconducting state. This may be due to Korringa relaxation by conduction electrons, which is reduced by the opening of the superconducting energy gap.