Ingredients for the electronic nematic phase in FeSe revealed by its anisotropic optical response

Abstract

The origin of the anisotropy in physical quantities related to a symmetry-broken (nematic) electronic state is still very much debated in high-temperature superconductors. FeSe at ambient pressure undergoes a structural, tetragonal-to-orthorhombic phase transition at Ts 90 K without any magnetic ordering on further cooling, which leads to an ideal electronic nematicity. Our unprecedented optical results provide evidence that the low-energy excitation spectrum in the nematic phase is shaped by an important interplay of the anisotropic Drude weight and scattering rate. In the zero-frequency limit though, the temperature dependence of the anisotropic scattering rate plays the dominant role and, combined with the nematic order parameter as evinced from the high energy optical response, accounts for the anisotropic dc resistivity. This favours the scattering by anisotropic spin fluctuations as the prominent candidate in governing the properties of the nematic phase.

Optical anisotropy in optimally doped iron-based superconductor

“…Therefore, an anisotropy in the charge dynamics due to (p-induced) nematicity [24][25][26][27] seems to be a generic feature in 122 iron-based superconductors even into the optimally-doped regime, similar to the dc transport properties. 15 Somehow peculiar with respect to the previously investigated underdoped 122 materials and even FeSe, [24][25][26][27][28][29] the optical anisotropy in the optimally K-doped 122 compound occurs at the much lower FIR energy scales, relevant to the superconducting gap(s) (Figs. 1a-c, 2f and 3c-e), thus excluding the involvement of bands deep into the electronic structure and possibly implying a less prominent impact of orbital ordering in the optimally doped than in the underdoped regime.…”

“…However, the optical anisotropy at ω 0 in FeSe displays a rather sudden drop on approaching T s from low T. We conclude that the anticipated pinduced lattice distortion above T s in FeSe may be less strongly or not obviously bound to the electronic structure as in other ironbased superconductors. 28,29 Common to both families of Counderdoped 122 and FeSe is nonetheless the persistence even at T < T c of the optical anisotropy evinced in the nematic phase, so that superconductivity develops in an electronically polarized state. [26][27][28][29] Overall, the anisotropy in the optical response at midinfrared energy scales unfolded in Ba(Fe 1−x Co x ) 2 As 2 and FeSe signals the impact of the nematic phase to all iron d orbitals (squeezed in energy by correlation effects) and highlights the relevance of the orbital degree of freedom, as advanced by photoemission data.…”

“…28,29 Common to both families of Counderdoped 122 and FeSe is nonetheless the persistence even at T < T c of the optical anisotropy evinced in the nematic phase, so that superconductivity develops in an electronically polarized state. [26][27][28][29] Overall, the anisotropy in the optical response at midinfrared energy scales unfolded in Ba(Fe 1−x Co x ) 2 As 2 and FeSe signals the impact of the nematic phase to all iron d orbitals (squeezed in energy by correlation effects) and highlights the relevance of the orbital degree of freedom, as advanced by photoemission data. 31,[39][40][41][42][43][44] Such an anisotropic optical response at mid-infrared energy scales originates from the split of the relevant bands of the order of only a few tens of meV.…”

“…Complementary to the dc transport data mostly affected by the energy scales close to the Fermi level, optical spectroscopy proves to be a powerful experimental tool in order to reveal the fingerprints of the nematic phase on the whole electronic structure. [21][22][23][24][25][26][27][28][29] In our experiment, we use uniaxial and in-situ tunable applied stress (p), which acts as a conjugate field to the orthorhombic distortion and allows circumventing sample twinning below T s (see Supplementary Information). Above T s , stress as external symmetry-breaking field induces a finite value of the orthorhombic distortion.…”

Near-Infrared Reflectivity of Superconducting FeSe Thin Films

“…In particular, because we have obtained the high quality and relatively large-size single crystal FeSe thin film samples, it now becomes possible to examine the basic physical properties of FeSe thin films via optical measurements. As a matter of fact, the optical properties of FeSe thin films or bulk materials have been measured via conventional optical experiments [14]- [17]. Nakajima et al [16] reported the optical properties of FeSe film on CaF 2 substrate.…”