# Strain-derivative of thermoelectric properties: a sensitive probe for nematicity

## TL;DR

This study demonstrates that thermoelectric transport measurements under strain reveal the nematic order parameter and fluctuations in iron-based superconductors, highlighting the roles of anisotropic scattering, Fermi surface distortions, and orbital effects.

## Contribution

It introduces the strain-derivative of thermoelectric coefficients as a novel probe for nematicity, emphasizing the importance of both scattering and orbital effects in nematic phenomena.

## Key findings

- Thermoelectric transport is highly sensitive to strain-induced nematicity.
- Nematic fluctuations follow a Curie-Weiss law above the structural transition.
- Nematicity is band-selective and involves both scattering and Fermi surface distortions.

## Abstract

The nematic instability of iron-based superconductors is an undebatable ingredient of the physics of iron-based superconductors. Yet, its origin remains enigmatic as it involves a fermiology with an intricate interplay of lattice-, orbital- and spin- degrees of freedom. It is well known that thermoelectric transport is an excellent probe for revealing even subtle signatures of instabilities and pertinent fluctuations. In this paper, we report a strong response of the thermoelectric transport properties of two underdoped 1111 iron-based superconductors to a vanishingly small strain. By introducing the strain-derivative of the Seebeck and the Nernst coefficients, we provide a novel description of the nematic order parameter, proving the existence of an anisotropic Peltier-tensor beside an anisotropic conductivity-tensor. Our measurements reveal that the transport nematic phenomenology is the result of the combined effect of both an anisotropic scattering time and Fermi surface distortions, pointing out that in a realistic description, abreast of the spin-fluctuations also the orbital character is a fundamental ingredient. In addition, we show that nematic fluctuations universally relax in a Curie-Weiss fashion above T_S in all the elasto-transport measurements and we provide evidences that nematicity must be band-selective.