Evidence of precursor orthorhombic domains well above the electronic nematic transition temperature in Sr(Fe_1−x Co _x )₂As₂

Abstract

Raman scattering, synchrotron x-ray diffraction, specific heat, resistivity and magnetic susceptibility measurements were performed in Sr(Fe 1−x Co x ) 2 As 2 [x = 0.20(3)] single crystals with superconducting critical temperature T c = 22 K and two additional transitions at 132 and 152 K observed in both specific heat and resistivity data. A quasielastic Raman signal with B 2g symmetry (tetragonal cell) associated with electronic nematic fluctuations is observed. Crucially, this signal shows maximum intensity at T nem ∼ 132 K, marking the nematic transition temperature. X-ray diffraction shows evidence of coexisting orthorhombic and tetragonal domains between T nem and T o ∼ 152 K, implying that precursor orthorhombic domains emerge over an extended temperature range above T nem . While the height of the quasielastic Raman peak is insensitive to T o , the temperature-dependence of the average nematic fluctuation rate indicates a slowing down of the nematic fluctuations inside the precursor orthorhombic domains. These results are analogous to those previously reported for the LaFeAsO parent oxypnictide (Kaneko et al 2017 Phys. Rev. B 96 014506). We propose a scenario where the precursor orthorhombic phase may be generated within the electronically disordered regime (T > T nem ) as long as the nematic fluctuation rate is sufficiently small in comparison to the optical phonon frequency range. In this regime, the local atomic structure responds adiabatically to the electronic nematic fluctuations, creating a net of orthorhombic clusters that, albeit dynamical for T > T nem , may be sufficiently dense to sustain long-range phase coherence in a diffraction process up to T o .

Electron–phonon coupling enhancement and displacive magnetostructural transition in SrCr2 As2 under magneto-Raman spectroscopy

“…The 122 system is one of the IBSC that has the ThCr 2 Si 2 tetragonal structure at room temperature, where antiferromagnetism and superconductivity coexist and the highest T c occurs when the Néel temperature T N is close to zero, showing the relevance of quantum criticality in the system [2]. Widely studied compounds in this class are AFe 2 As 2 (A = Sr, Ba, Ca), which can achieve the superconducting state under Co-substitution (and similar metals) in the iron site [3][4][5][6] or under pressure [8][9][10]. In fact, substitution in the three sites triggers superconductivity [11][12][13].…”

Fluctuations and pairing in Fe-based superconductors: light scattering experiments

“…For LaFeAsO and Sr(Fe 1−x Co x ) 2 As 2 the analysis arrives at slightly different conclusions for the temperature dependence but similar ones for the origin of the uctuations [35,44]. In both compounds the maxima of the uctuation response do not coincide with the structural transition temperature, and in Sr(Fe 0.8 Co 0.2 ) 2 As 2 the uctuations do not disappear right below the magnetic ordering temperature.…”

“…Several detailed studies of uctuations in both pnictides and chalcogenides were presented more recently in references [30,33,35,40,44,49,53,116,175,176]. As discussed in section 4.8, one and/or two uctuations may contribute to the Raman spectra via distinct scattering processes.…”

“…This procedure requires an extrapolation of χ ′′ B1g (Ω, T) to zero energy and the selection of a high-energy cutoff since χ ′′ B1g (Ω → ∞, T) ≈ c where c is a constant. The method was applied to the analysis of the data of BFCA [30,116] (see next paragraph) and subsequently of Eu/Sr122 [33,44], BKFA [176], and Na111 [49]. Thorsmølle et al demonstrated the scaling of the static susceptibility obtained from Raman scattering and NMR data for Na111.…”