No abstract

Nematic fluctuations and order play a prominent role in material classes such as the cuprates 1 , some ruthenates 2 or the iron-based compounds [3][4][5][6] and may be interrelated with superconductivity [7][8][9][10][11] . In iron-based compounds 12 signatures of nematicity have been observed in a variety of experiments. However, the fundamental question as to the relevance of the related spin 13 , charge 9,14 or orbital 8,15,16 fluctuations remains open. Here, we use inelastic light (Raman) scattering and study Ba(Fe 1−x Co x ) 2 As 2 (0 ≤ x ≤ 0.085) for getting direct access to nematicity and the underlying critical fluctuations with finite characteristic wavelengths 17-21 . We show that the response from fluctuations appears only in B 1g (x 2 − y 2 ) symmetry (1 Fe unit cell). The scattering amplitude increases towards the structural transition at T s but vanishes only below the magnetic ordering transition at T SDW < T s , suggesting a magnetic origin of the fluctuations. The theoretical analysis explains the selection rules and the temperature dependence of the fluctuation response. These results make magnetism the favourite candidate for driving the series of transitions.The magneto-structural phase transition is among the most thoroughly studied phenomena in iron-based materials. When Fe is substituted by Co in BaFe 2 As 2 the structural transformation at T s precedes the magnetic ordering at T SDW < T s (ref. 22). The nematic phase between T s and T SDW is characterized by broken C 4 symmetry but preserved O(3) spin rotational symmetry (no magnetic order). Nematic fluctuations are present even above T s in the tetragonal phase, as has been demonstrated by both elastoresistance measurements 4,6 and studies of the elastic constants 23,24 . In strained samples, one observes orbital ordering in the photoemission spectra 25 and electronic nematicity by transport 4,26 . However, it is rather difficult to derive the dynamics and critical momentum typical for the underlying fluctuations and to identify which of the ordering phenomena drives the instabilities.Raman scattering provides experimental access to all types of dynamic nematicity although only the charge sector has been studied in more detail 9,14,27,28 . However, also in the case of spindriven nematic order the technique can play a prominent role for coupling to a two-spin operator whereas a four-spin correlation function is the lowest order contribution to the neutron crosssection 5 . We exploit this advantage here and study the low-energy Raman response of Ba(Fe 1−x Co x ) 2 As 2 experimentally and interpret the results in terms of a microscopic model for a spin-driven nematic

We show that electronic Raman scattering affords a window into the essential properties of the pairing potential V(k,k') of iron-based superconductors. In Ba0.6K0.4Fe2As2 we observe band dependent energy gaps along with excitonic Bardasis-Schrieffer modes characterizing, respectively, the dominant and subdominant pairing channel. The d(x(2)-y(2)) symmetry of all excitons allows us to identify the subdominant channel to originate from the interaction between the electron bands. Consequently, the dominant channel driving superconductivity results from the interaction between the electron and hole bands and has the full lattice symmetry. The results in Rb(0.8)Fe(1.6)Se(2) along with earlier ones in Ba(Fe(0.939)Co(0.061))(2)As(2) highlight the influence of the Fermi surface topology on the pairing interactions.

A detailed knowledge of the gap structure for the Fe-pnictide superconductors is still rather rudimentary, with several conflicting reports of either nodes, deep gap minima, or fully isotropic gaps on the Fermi surface sheets, both in the k x − k y plane and along the c−axis. In this paper we present considerations for electronic Raman scattering which can help clarify the gap structure and topology using different light scattering geometries. Using density functional calculations for the Raman vertices, it is shown that the location of the gap minima may occur on loops stretching over a portion of the c-axis in Ba(Fe 0.94 Co 0.06 ) 2 As 2 .