Critical spin fluctuations and the origin of nematic order in Ba(Fe1−xCox)2As2

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

Charge-induced nematicity in FeSe

“…It is unlikely to be from an Azlamazov-Larkin-type contribution of the fluctuations of the stripe magnetic state (24,27,37,38) because, below T S , inelastic neutron scattering and NMR data suggest an enhancement of low energy spin fluctuations (4,6,14), whereas we observe a shift of spectral weight of χ b ″ðωÞ to higher frequencies. It is also unlikely that the feature is an interband transition, because χ b ″ðωÞ does not show any gap at low frequencies above T S (Supporting Information).…”

“…Here, we investigate the nature of nematicity in FeSe by using the unique ability of electronic Raman scattering to selectively probe the dynamics of electronic nematic degrees of freedom without lattice effects (22)(23)(24)(25)(26)(27). We unravel the presence of critical charge nematic fluctuations in the tetragonal phase that signals the presence of a d-wave Pomeranchuk instability of the Fermi surface (28).…”

Dynamical susceptibility of a near-critical nonconserved order parameter and quadrupole Raman response in Fe-based superconductors

“…As T approaches T s , the peak position moves to a smaller frequency and the slope at → 0 increases. Several explanations of the peak have been presented [16,[34][35][36][37]. Within our theory, this peak is a direct consequence of Eq.…”

“…This interaction does not directly affect χ (q = 0, ), but it does contribute to χ (q → 0, = 0) [16,18,36] and therefore gives an additional contribution to the difference between ξ 0 and ξ . It also affects the momentum dependence of D(q, ) at < v F q, and eventually cuts off the critical behavior at T = T s [34,41].…”

Indication of subdominant d -wave interaction in superconducting CaKFe4As4

“…Thus, the argument that the strongest peak in the B 1g spectra results from the γ band can be discarded. Since in contrast to Ba(Fe 1−x Co x ) 2 As 2 [21,22], NaFe 1−x Co x As [23], or FeSe [24,25] there is no nematic phase and related fluctuations in CaKFe 4 As 4 , an interpretation of the observation in B 1g symmetry in terms of nematic fluctuations would be far fetched.…”