Unusual Nernst effect and spin density wave precursors in superconducting $\rm\bf LaFeAsO_{1-x}F_x$

TL;DR

This study investigates the Nernst effect in LaFeAsO_{1-x}F_x}, revealing unusual normal state properties, SDW precursors, and doping-dependent behaviors that shed light on the complex physics of iron-based superconductors.

Contribution

It provides the first systematic analysis of the Nernst effect across different doping levels in LaFeAsO_{1-x}F_x, highlighting the presence of SDW precursors and unusual normal state responses.

Findings

Huge negative Nernst coefficient in parent compound with SDW order

Enhanced Nernst signal at underdoping indicating SDW precursors

Normal state Nernst response at optimal doping with Sondheimer cancellation

Abstract

The Nernst effect has recently proven as a sensitive probe for detecting unusual normal state properties of unconventional superconductors. Here we present a systematic study of the Nernst effect of the iron pnictide superconductor \laf with a particular focus on its evolution upon doping. For the parent compound we observe a huge negative Nernst coefficient accompanied with a severe violation of the Sondheimer cancellation in the spin density wave (SDW) ordered state. Surprisingly, an unusual and enhanced Nernst signal is also found at underdoping ($x=0.05$) despite the presence of bulk superconductivity and the absence of static magnetic order, strongly suggestive of SDW precursors at $T\lesssim150$ K. A more conventional normal state Nernst response is observed at optimal doping ($x=0.1$) where it is rather featureless with a more complete Sondheimer cancellation.