Superconductivity, Kondo physics and magnetic order: Tuning the groundstate in the La$_{1-x}$Ce$_x$FeSiH solid solution through the interplay between $3d$ and $4f$ correlated electrons

TL;DR

This study explores how varying cerium concentration in La$_{1-x}$Ce$_x$FeSiH tunes the ground state from superconductivity to magnetic order, revealing complex interplay between $3d$ and $4f$ electrons, Kondo physics, and heavy Fermi liquid behavior.

Contribution

It introduces a comprehensive phase diagram showing the evolution of superconductivity, Kondo effects, and magnetic order in La$_{1-x}$Ce$_x$FeSiH as a function of cerium content, highlighting the interplay of correlated electrons.

Findings

Superconductivity exists at low cerium concentrations ($x \,\leq\, 0.20$).

Kondo effects and heavy Fermi liquid behavior emerge at intermediate to high cerium levels.

Magnetic order appears at high cerium concentrations ($x \,\geq\, 0.85$).

Abstract

We report a study of the La$_{1-x}$Ce$_x$FeSiH solid solution ($0 \leq x \leq 1$), a family of intermetallic hydrides of ZrCuSiAs-type structure, with space group $P4/nmm$. For low cerium concentrations $x \leq 0.20$, we observe the presence of superconductivity, which originates from the correlated $3d$ electrons of iron. The superconducting regime is progressively suppressed by the cerium substitution. For moderate cerium concentration $0.07 \leq x \leq 0.50$, we observe evidence of the single-ion Kondo effect and no magnetic phase transition down to 2 K. For $0.07 \leq x \leq 0.20$, the single-ion Kondo effect coexists with a superconducting ground state at low temperatures. From $x > 0.50$, we observe signatures of Kondo coherence and a heavy Fermi liquid regime at low temperature. Finally, at high cerium concentration $x \geq 0.85$, we observe signatures of magnetic ordering at low temperatures. We discuss our results by introducing temperature scales related to superconductivity, the Kondo effect, and magnetic order, which permits building a rich phase diagram temperature versus cerium content $x$. This shows that using the cerium concentration $x$ as a unique control parameter, we can explore the Kondo entanglement between correlated $3d$ and $4f$ electrons, which suggests an unusual change between the superconducting state related to the $3d$ electrons and the Kondo coherent state involving both $3d$ and $4f$ electrons.