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 tetragonal-to-orthorhombic structural transition at $T_s\approx 58$ K and a collinear antiferromagnetic (AF) order at $T_N\approx 45$ 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 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.
Highlights
Spin waves and phonons are two fundamental quasiparticles in a solid describing propagating disturbance of the ordered magnetic moment and lattice vibrations, respectively [1]
Low-energy acoustic phonons near the nuclear-zone-center Γ point in NaFeAs can interact with the low-energy spin excitations near the AF zone-center M point at QAF in reciprocal space without direct phonon-spin excitation crossing [Figs. 1(b) and 1(c), Figs. 2–5]. Since both the in-plane transverse acoustic (IPTA) phonons [Fig. 1(b)] and spin excitations show dramatic anomaly across Ts, our results indicate that the magnon-phonon interaction may be responsible for the low-temperature electronic nematic phase in NaFeAs [13,14,15,16,17,18,19,20,21,22,23,24,25,26]
Since the IPTA phonon mode dynamically changes the nearest-neighbor Fe-Fe distance, which is directly coupled to the nearest-neighbor magnetic exchange coupling, the reduced scattering phase space below Ts explains the recovery of the phonon softening at low temperatures in NaFeAs
Summary
Spin waves (excitations) and phonons are two fundamental quasiparticles in a solid describing propagating disturbance of the ordered magnetic moment and lattice vibrations, respectively [1]. The dynamic lattice vibrations (phonons) interacting with time-dependent spin excitations may create energy gaps in the magnon dispersion at the nominal intersections of the magnon and phonon modes [1], as seen in the insulating noncollinear antiferromagnet ðY; LuÞMnO3 [7]. Since both the in-plane transverse acoustic (IPTA) phonons [Fig. 1(b)] and spin excitations show dramatic anomaly across Ts, our results indicate that the magnon-phonon interaction may be responsible for the low-temperature electronic nematic phase in NaFeAs [13,14,15,16,17,18,19,20,21,22,23,24,25,26]. Instead of a pure spin [14,15,16,17] or lattice (orbital) [31,32,33,34,35,36] degree of freedom, dynamic spinlattice coupling is important for the electronic nematic phase and anisotropic electronic properties in the paramagnetic state of NaFeAs
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