Strain derivative of thermoelectric properties as a sensitive probe for nematicity

Abstract

The nematic instability 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 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 TS in all the elasto-transport measurements and we provide evidences that nematicity must be band selective.

Ubiquitous enhancement of nematic fluctuations across the phase diagram of iron based superconductors probed by the Nernst effect

“…We point out, as is discussed in Ref. 24, that an anisotropic scattering time can be expected to play a further important role in these systems.…”

“…Clearly, this observation implies that elastoresistivity and Nernst effect probe similar, yet distinct aspects of the nematic fluctuations, where apparent differences are likely related to subtleties of the considered systems. This conclusion is corroborated by recent elasto-Seebeck and elasto-Nernst data which possess a somewhat different coupling to the nematic susceptibility than the elastoresistivity [24]. In this context it is important to consider that all the elasto-transport properties, including elastoresistivity, crucially depend on the electron-lattice coupling, since, by definition, they represent the response of an electronic system to an external uniaxial strain.…”

“…In contrast, in the magnetically ordered orthorhombic phase either large positive or negative contributions occur in the Nernst effect. In the case of nematic order, an interpretation of these low-temperature contributions is elusive since a strong in-plane anisotropy, uncontrolled twinning and possible additional contributions from highly mobile Dirac-like fermions need to be considered [22][23][24]. On the other hand, the strong additional increase which is well recognizable near T c of the superconducting samples is well-known to result from vortex motion and therefore is of no further interest in the present study.…”

Absence of nematic instability in LiFeAs

“…One very successful experimental approach to investigate nematic properties in FeSCs is measuring the strain-susceptibility of the resistivity η = d d ∆ρ ρ . With this elastoresistance technique, Chu et al [18] were able to establish the temperature-dependent behavior of η as a proof of the electronic origin of nematicity in FeSCs and a measure of the strength of nematic fluctuations, which has since then been confirmed und used by other research groups [19][20][21][22][23]. In this work, we investigate nematic fluctuations in LiFeAs using elastoresistance measurements on undoped, electron-doped and hole-doped LiFeAs.…”

Elasto-Raman scattering: arsenic optical phonon as a probe of nematicity in BaFe$_2$As$_2$

“…Initially limited to transport measurements [1][2][3][11][12][13], techniques under tunable uniaxial strain have been recently implemented and leveraged to study several superconducting materials, with in the case of Fe SC a particular focus on accessing nematic degrees of freedom [14][15][16][17][18][19][20][21][22][23]. However few of these techniques are symmetry resolved and as such they do not directly access the nematic order parameter.…”