Dynamic Spin-Lattice Coupling and Nematic Fluctuations in NaFeAs

Abstract

We use inelastic neutron scattering to study acoustic phonons and spin excitations in single crystals of NaFeAs, a parent compound of iron-pnictide superconductors. NaFeAs exhibits a tetragonalto-orthorhombic structural transition at T s ≈ 58 K and a collinear antiferromagnetic order at T N ≈ 45 K. While longitudinal and out-of-plane transverse acoustic phonons behave as expected, the in-plane transverse acoustic phonons reveal considerable softening on cooling to T s and then harden on approaching T N before saturating below T N . In addition, we find that spin-spin correlation lengths of low-energy magnetic excitations within the FeAs layer and along the c axis increase dramatically below T s and show a weak anomaly across T N . These results suggest that the electronic nematic phase present in the paramagnetic tetragonal phase is closely associated with dynamic spin-lattice coupling, possibly arising from the one-phonon-two-magnon mechanism.

Local orthorhombic lattice distortions in the paramagnetic tetragonal phase of superconducting NaFe1−xNixAs

“…Since our neutron Larmor diffraction measurements were carried out using polarized neutron beam produced by an Heusler monochromator, which has an energy resolution of about ∆E ≈ 1.0 meV [28,29], the local orthorhombic distortions captured in our measurements are either static or fluctuating slower than a time scale of τ ∼ /2∆E ∼ 0.3 ps, where is the reduced Planck constant [41,42]. One possible origin of such slow fluctuations may be in-plane transverse acoustic phonons that exhibit significant softening in the paramagnetic tetragonal phase when approaching a nematic instability [43]. Future neutron scattering experiments with energy resolutions much better than ∆E ≈ 1 meV are desirable to separate the static and slowly fluctuating contributions.…”

Weaker nematic phase connected to the first order antiferromagnetic phase transition in SrFe2As2 compared to BaFe2As2Tam1, Wang2, Zhang3 et al. 2019Phys. Rev. BSelf Cite9280View full textAdd to dashboardCite

“…Although recent neutron pair distribution function and Lamor diffraction experiments on dif-ferent classes of iron pnictides including Sr 1−x Na x Fe 2 As 2 [35] and NaFe 1−x Ni x As [36] reveal clear evidence for local orthorhombic lattice distortions in temperatures well above T s , these local lattice distortions are evenly distributed along the two orthorhombic lattice directions and therefore not expected to induce resistivity anisotropy. The clear presence of resistivity [12,13], spin excitation [19][20][21][22], and orbital population anisotropy [37] in the paramagnetic phase of iron pnictides can arise from the interaction of applied uniaxial pressure with nematic susceptibility and associated spin excitations through magnetoelastic coupling [38]. The applied uniaxial pressure should be mostly sensitive to low energy spin excitations and acoustic phonons, and have little impact to high energy spin excitations.…”

Orbital selective spin waves in detwinned NaFeAs

“…In addition, spin waves broaden and split in the transverse direction with increasing energy, and merge into the Q = (1, 1) position where they remain somewhat anisotropic at E = 120 ± 10 meV. In previous inelastic neutron scattering work on twinned NaFeAs, low-energy spin excitations are enhanced dramatically below T s instead of T N due to spin-lattice coupling [53]. To determine whether spin excitation anisotropy at Q strong and Q weak in NaFeAs is also controlled by the nematic phase below T s , we summarize in Fig.…”