Nematic magnetoelastic effect contrasted betweenBa(Fe1−xCox)2As2and FeSe

Abstract

To elucidate the origin of nematic order in Fe-based superconductors, we report a Raman scattering study of lattice dynamics, which quantify the extent of C4-symmetry breaking, in BaFe2As2 and FeSe. FeSe possesses a nematic ordering temperature Ts and orbital-related band-energy split below Ts that are similar to those in BaFe2As2, but unlike BaFe2As2 it has no long-range magnetic order. We find that the Eg phonon-energy split in FeSe sets in only well below Ts, and its saturated value is substantially smaller than that in BaFe2As2. Together with reported results for the Ba(Fe1−xCox)2As2 family, the data suggest that magnetism exerts a major influence on the lattice.

Hidden antiferronematic order in Fe-based superconductor BaFe2As2 and NaFeAs above TS

“…At the structural transition temperature T S , ferro-orbital (FO) order with ψ ≡ (n xz − n yz )/(n xz + n yz ) = 0 is driven by electron correlation [1], by which the orthorhombicity φ = (a − b)/(a + b) occurs in proportion to ψ. Above T S , the electronic nematic susceptibility develops divergently [2][3][4][5]. As possible mechanisms of nematicity, both spin-nematic scenarios [6][7][8][9][10][11][12] and the orbital/charge-order scenarios [13][14][15][16][17][18][19][20][21][22][23][24] have been proposed.…”

Raman scattering study of the tetragonal magnetic phase in Sr1−xNaxFe2As2 : Structural symmetry and electronic gap

“…The E g modes can be detected on the ac surface with ac polarizations of the incident and scattered photons. Because these modes are two-fold degenerate in the tetragonal phase and they become non-degenerate B 2g and B 3g modes in the orthorhombic phase, the splitting of the corresponding Raman peaks has been previously used to characterize the C 4 -C 2 nematic transition [ [34][35][36]. As we expect our Sr 1−x Na x Fe 2 As 2 sample to undergo two transitions related to the breaking and recovery of C 4 symmetry, we begin by showing that this is indeed the case from the Raman scattering perspective, and we focus on the low-energy E g mode around 125 cm −1 because its splitting is more prominent.…”

Observation of Ising spin-nematic order and its close relationship to the superconductivity in FeSe single crystals

“…In FeSe superconductors, no antiferromagnetic long-range order was reported to exist in ambient pressure, but the presence of the rotational symmetry breaking in the electronic structure of the iron plane and its implication for superconducting paring have drawn much attention [11][12][13][14][15][16][17]. It has been argued that the tetragonal-to-orthorhombic structure transition at T s ~ 90 K is driven by the ferro-orbital ordering with unequal occupancies of the 3d xz /3d yz orbitals [18,19].…”