Signatures of quantum criticality in the thermopower of Ba(Fe(1-x)Co(x))2As2

TL;DR

This paper shows that thermopower measurements can detect spin fluctuations near quantum critical points in Ba(Fe(1-x)Co(x))2As2, revealing their role in superconductivity.

Contribution

It introduces thermopower as a sensitive probe for spin fluctuations near quantum criticality in Fe-based superconductors, linking thermopower behavior to spin dynamics and superconducting regions.

Findings

S/T increases near the SDW QCP

Maximal S/T near critical x_c ~ 0.05

Thermopower is influenced by critical spin fluctuations

Abstract

We demonstrate that the thermopower (S) can be used to probe the spin fluctuations (SFs) in proximity to the quantum critical point (QCP) in Fe-based superconductors. The sensitivity of S to the entropy of charge carriers allows us to observe an increase of S/T in Ba(Fe(1-x)Co(x))2As2 close to the spin-density-wave (SDW) QCP. This behavior is due to the coupling of low-energy conduction electrons to two-dimensional SFs, similar to heavy-fermion systems. The low-temperature enhancement of S/T in the Co substitution range 0.02 < x < 0.1 is bordered by two Lifshitz transitions, and it corresponds to the superconducting region, where a similarity between the electron and non-reconstructed hole pockets exists. The maximal S/T is observed in proximity to the commensurate-to-incommensurate SDW transition, for critical x_c ~ 0.05, close to the highest superconducting T_c. This analysis indicates that low-T thermopower is influenced by critical spin fluctuations which are important for the superconducting mechanism.