SNS shows novel spin waves in iron chalcogenide Fe_1.05Te

Measurements conducted at the Spallation Neutron Source at ORNL have determined exchange couplings in the mag- netically ordered chalcogenide Fe_1.05Te, a nonsuperconducting member of the iron-based family of superconductors. The results show the exchange pairings thought to be related to superconductivity occur in a next-nearest-neighbor ordering of atoms, rather than in nearest-neighbor order as in a previously studied iron arsenide.

Researchers from the University of Tennessee, ORNL, and other institutions collaborated to study the spin waves in Fe_1.05Te. Superconductivity in iron-based superconductors emerges in proximity to magnetic order; similarly, many researchers think superconductivity in the copper oxides is driven by magnetic fluctuations. The magnetic order in Fe_1.05Te is considerably different from the related iron pnictide superconducting system, raising the question of whether magnetism can in both cases still drive the superconductivity in iron-based superconductors.

To answer that question, the exchange coupling energies in the Fe_1.05Te system were charac- terized using the ARCS instrument at SNS and other instruments to measure the spin waves (Fig. 1) and fitting the waves to a Heisenburg Hamiltonian. The dispersion of the spin waves in this sys- tem is novel and very different from that in the previously measured iron pnictide CaFe₂As₂. Fitted exchange couplings of J_1a =-17.5 ± 5.7, J_1b = -51.0 ± 3.4, J₂ = J_2a ≈ J_2b = 21.7 ± 3.5, J₃ = 6.8 ± 2.8 meV were found.

Although the nearest-neighbor couplings are very different (opposite in sign) from those in the pnictides, the next-nearest-neighbor couplings are isotropic and very similar. It has been shown theoretically that such an antiferromagnetic next-nearest-neighbor interaction can lead to the su- perconducting pairing symmetry observed in these systems. Therefore, the discovery of a universal next-nearest-neighbor coupling in these systems suggests superconductivity in both these classes of iron-based superconductors has a common magnetic origin that is intimately associated with the antiferromagnetic next-nearest-neighbor exchange couplings.

The paper was recently accepted for publication in Physical Review Letters.

“Spin waves in the (π, 0) magnetically ordered iron chalcogenide Fe_1.05Te” by O. J. Lipscombe, G. F. Chen, C. Fang, T. G. Perring, D. L. Abernathy, A. D. Christianson, T. Egami, N. Wang, J. P. Hu, and P. Dai, Phys. Rev. Lett. (2011, in press)

Contact: Pengcheng Dai, 865-974-1509, daip@ornl.gov

Read the full article.