# Broken time-reversal symmetry in superconducting Pr$_{1-x}$La$_{x}$Pt$_{4}$Ge$_{12}$

## TL;DR

This study reveals broken time-reversal symmetry in the superconducting state of Pr$_{1-x}$La$_{x}$Pt$_{4}$Ge$_{12}$ alloys, with evidence of multiband superconductivity and complex order parameters, varying with composition.

## Contribution

It provides systematic experimental evidence of broken TRS and multiband superconductivity in Pr$_{1-x}$La$_{x}$Pt$_{4}$Ge$_{12}$, highlighting the evolution of superconducting properties with alloy composition.

## Key findings

- Broken TRS observed below T_c for x<0.5
- Multiband superconductivity indicated by H_{c2}(T)
- Single specific heat jump at T_c in all alloys

## Abstract

The superconducting state of the filled skutterudite alloy series Pr$_{1-x}$La$_{x}$Pt$_{4}$Ge$_{12}$ has been systematically studied by specific heat, zero-field muon spin relaxation ($\mu$SR), and superconducting critical field measurements. An additional inhomogeneous local magnetic field, indicative of broken time-reversal symmetry (TRS), is observed in the superconducting states of the alloys. For $x \lesssim 0.5$ the broken-TRS phase sets in below a temperature $T_m$ distinctly lower than the superconducting transition temperature $T_c$. For $x \gtrsim 0.5$ $T_m \approx T_c$. The local field strength decreases as $x \to 1$, where LaPt$_{4}$Ge$_{12}$ is characterized by conventional pairing. The lower critical field $H_{c1}(T)$ of PrPt$_{4}$Ge$_{12}$ shows the onset of a second quadratic temperature region below $T_q \sim T_m$. Upper critical field $H_{c2}(T)$ measurements suggest multiband superconductivity, and point gap nodes are consistent with the specific heat data. In Pr$_{1-x}$La$_{x}$Pt$_{4}$Ge$_{12}$ only a single specific heat discontinuity is observed at $T_c$, in contrast to the second jump seen in PrOs$_{4}$Sb$_{12}$ below $T_c$. These results suggest that superconductivity in PrPt$_{4}$Ge$_{12}$ is characterized by a complex order parameter.