Effect of the chemical pressure on superconductivity and SDW in undoped and 15%F doped La1-yYyFeAsO compounds

TL;DR

This study investigates how substituting yttrium for lanthanum in LaFeAsO compounds affects their structural, magnetic, and superconducting properties, revealing that chemical pressure influences these phenomena but disorder also plays a significant role.

Contribution

It demonstrates that partial yttrium substitution modifies lattice parameters and enhances superconductivity, highlighting the combined effects of chemical pressure and structural disorder.

Findings

Yttrium substitution shrinks lattice parameters by 1.8% and 1.7%.

Chemical pressure lowers SDW temperature and increases Tc.

Maximum Tc of 40.2 K achieved at 50% yttrium substitution.

Abstract

We present a study concerning the partial substitution of yttrium at the lanthanum site of the undoped LaFeAsO and superconducting LaFeAsO0.85F0.15 compounds. We prepared samples with a nominal yttrium content up to 70% producing simultaneous shrinkage of both the a- and c-lattice parameters by 1.8% and 1.7%, respectively. The chemical pressure provided by the partial substitution with this smaller ion size causes a lowering of the spin density wave temperature in the undoped compounds, as well as an increase of the superconducting transition temperatures in the doped ones. The 15% fluorine-doped samples reach a maximum critical temperature of 40.2 K for the 50% yttrium substitution. Comparison with literature data indicates that chemical pressure cannot be the only mechanism which tunes drastically both TSDW and Tc in 1111 compounds. Our data suggest that structural disorder induced by the partial substitution in the La site or by doping could play an important role as well.