—Courtesy of Professor Elbio Dagotto
Scientists working at the UT Physics Department and Oak Ridge National Laboratory recently published a manuscript in the prestigious journal Nature Communications* containing a theoretical prediction of the magnetic dynamical structure factor S(q, ω) (see figure) corresponding to a family of materials that is receiving considerable attention in the condensed matter physics community. The team used powerful computational techniques to study "BaFe2Se3 ladders." These are materials dominated by one-dimensional substructures with irons located in their atomic crystal arrangement in what looks like ladders, with long legs (lines of irons) joined by short iron rungs. This one dimensional iron geometry allowed for the calculations, which employed powerful algorithms and computers, to be nearly exact, leading to accurate predictions for future experiments. This scientific work was carried about by Postdocs Jacek Herbrych and Alberto Nocera, Graduate Student Nitin Kaushal, ORNL Staff Member Gonzalo Alvarez, and Physics Professors Adriana Moreo and Elbio Dagotto.
More specifically, the effort employed a technique called Density Matrix Renormalization Group and a model Hamiltonian with several active 3d orbitals, as demanded by having iron, located in the middle of the periodic table, as the main atom in the structure. The calculation unveiled exotic features, such as an acoustic mode with intensity in only half the Brillouin zone in momentum space (see horizontal axis in figure), and a novel optical mode at approximately 0.1 eV in energy (denoted as ω in the vertical axis). The acoustic mode corresponds to collective excitations of the magnetic state, which happened to involve a peculiar arrangement with four iron spins up followed by four spins down repeated regularly. The optical mode defines a novel intra-atomic excitation involving the Hund coupling, related to the famous Hund rule of atomic physics in quantum mechanics. Up to now, this material is only known in "powder" form but when single crystals become available, the predictions can be confirmed by using neutron scattering techniques at ORNL. A merit of the calculation that impressed the Nat. Commun. editors and reviewers is how difficult the effort is computationally, due to the multi orbital and dynamical nature of the problem. Only the local vast computers resources at UT (such as the Advanced Computing Facility) and at ORNL allowed for such a project to be carried out successfully, highlighting the unique characteristics of the scientific environment at the university and the national laboratory.
* J. Herbrych, N. Kaushal, A. Nocera, G. Alvarez, A. Moreo, and E. Dagotto, "Spin dynamics of the block orbital-selective Mott phase," Nature Communications 9, 3736 (2018). DOI: https://doi.org/10.1038/s41467-018-06181-6