Absence of Long-Wavelength Nematic Fluctuations in LiFeAs

Abstract

We investigated long-wavelength nematic fluctuations in an Fe-based superconductor LiFeAs near q=(0.05,0,0) by measuring temperature-dependent renormalization of acoustic phonons through inelastic neutron scattering. We found that the phonons have conventional behavior, as would be expected in the absence of electronic nematic fluctuations. This observation implies that either electronphonon coupling is too weak to see any effect or that nematic fluctuations are not present. Keywords: pnictide, superconductor, nematic, INSJust like the cuprates, iron pnictides and iron chalcogenites become superconducting when antiferromagnetic "parent" compounds are doped 1 . The parent compounds as well as low-doped superconducting ones undergo magnetic and structural phase transitions at temperatures T N and T s respectively. As a function of decreasing temperature, the system first becomes paramagnetic orthorhombic phase below T S , and then orders antiferromagnetically below T N with T N ≤T S . A lot of recent research focused on the role of electronic nematicity, where electronic properties of the system occur preferentially along one of two otherwise degenerate perpendicular directions in the paramagnetic orthorhombic state.2 Spin nematic order occurs when magnetic fluctuations break the C4 rotational symmetry but magnetic order does not form 3 . Magnetoelastic coupling 4,5 of the lattice to nematic fluctuations is responsible for orthorhombic distortion at zero and low doping. This spin-phonon coupling changes some phonon frequencies across the magnetic ordering transition, but the effect is much smaller than predicted.

Lattice, Charge, and Spin Phase Inhomogeneity in Complex-Striped Quantum Matter

“…In the field of the local atomic structure in iron based superconductors the double-well for the Fe-As bond fluctuations [28] and related lattice instabilities have been detected by EXAFS [29]. Scaling of the stress and temperature dependence of the optical anisotropy in Ba(Fe 1−x Co x ) 2 As 2 , the absence of long-wavelength nematic fluctuations in LiFeAs [30] and the vortex structure [31] and anisotropic superconducting gaps in Ba[Fe(Ni)] 2 As 2 . [32] have been observed and the europium valence state in related compound EuCo 2 As 2 [33] has been investigated.…”

Evidence for bosonic mode coupling in electron dynamics of LiFeAs superconductor*

“…The second possibility is the electron coupling to collective magnetic excitations. Because there is no static magnetic ordering or long-range orbital ordering observed in LiFeAs, [41,47,48] the energy scale of ∼ 55 meV cannot be induced by electron coupling with magnons. The third possibility is the electron coupling with multiple phonons; in this case, the feature at ∼ 55 meV may be understood.…”