Josephson Plasma in RuSr2GdCu2O8

TL;DR

This study investigates Josephson plasma resonances in various Ru-based cuprates, revealing how ferromagnetism influences plasma frequency and coupling, with implications for understanding their superconducting and magnetic interactions.

Contribution

It provides the first detailed analysis of Josephson plasma behavior in RuSr$_{2}$GdCu$_{2}$O$_{8}$ and related compounds, highlighting the effects of ferromagnetism on plasma properties.

Findings

Josephson plasma observed around 8.5 cm$^{-1}$ in ferromagnetic RuSr$_{2}$GdCu$_{2}$O$_{8}$

Plasma frequency increases to 35 cm$^{-1}$ in non-ferromagnetic variants

No transition from 0-phase to π-phase detected at low temperatures

Abstract

Josephson plasma in RuSr$_{2}$GdCu$_{2}$O$_{8}$, Ru$_{1-x}$Sr$_{2}$GdCu$_{2+x}$O$_{8}$ (x = 0.3), and RuSr$_{2}$Eu$_{2-x}$Ce$_{x}$Cu$_{2}$O$_{10}$ (x = 0.5) compounds is investigated by the sphere resonance method. The Josephson plasma is observed in a low-frequency region (around 8.5 cm$^{-1}$ at T $\ll$ $T_{c}$) for ferromagnetic RuSr$_{2}$GdCu$_{2}$O$_{8}$, while it increases to 35 cm$^{-1}$ for non-ferromagnetic Ru$_{1-x}$Sr$_{2}$GdCu$_{2+x}$O$_{8}$ (x = 0.3), which represents a large reduction in the Josephson coupling at ferromagnetic RuO$_{2}$ block layers. The temperature dependence of the plasma does not shift to zero frequency ({\it i.e.} $j_{c}$ = 0) at low temperatures, indicating that there is no transition from the 0-phase to the $\pi$-phase in these compounds. The temperature dependence and the oscillator strength of the peak are different from those of other non-magnetic cuprates, and the origins of these anomalies are discussed.