Low-energy excitations in A -site ordered SmBaMn 2 O 6

The electron in a solid can be considered a bound state of the three independent, fundamental degrees of freedom creating quasiparticles: spinons, carrying the electron spin; plasmons, carrying the collective charge mode; and orbitons, carrying its orbital degree of freedom. These fundamental degrees of freedom could form ordering states in which dynamics or collective motions could occur and manifest as low-energy excitations. The exotic properties that appear in the materials exhibiting these electronic orderings are associated with these low-energy excitations. Although the orbital order (OO) and its coupling to the spin system creates very interesting phenomena, the microscopic origin of OO has been much less explored than other electronic properties as it is very difficult to directly access experimentally. Due to the recent improvement in energy resolution and flux, soft-x-ray resonant inelastic scattering (RIXS) allows for a reexamination of orbital excitations in manganites. Here, we present a study of low-energy excitations in half doped A -site ordered SmBaMn 2 O 6 through a combination of RIXS and soft-x-ray resonant elastic scattering measurements. We confirm the existence of OO at q = (0.25, 0.25, 0) and find various low-energy excitations below 200 meV. While several excitations can be assigned to be of magnetic and phononic origin, a group of excitations between 80 and 200 meV show a temperature dependence closely following that of the OO, making them possible candidates for orbitons.