Far-infrared optical properties of antiferromagnetic SmTiO3

The polarized reflectance of nominally stoichiometric SmTiO3 has been measured in the far-infrared at temperatures above and below the antiferromagnetic Néel temperature TN of 50 K. The appearance of a feature near 120 cm-1 when the electric field vector is polarized along the b axis correlates with the onset of magnetic ordering in the Ti and Sm sublattices, suggesting it is likely due to a magnetic transition. Its large linewidth and relatively high-energy scale point to a two-magnon process. Because the RTiO3 family of compounds (where R is a rare earth or Y) is susceptible to doping by vacancies on the R site, the nominally stoichiometric sample examined has a small, but finite background conductivity of the order 20 Ω-1cm-1 at low frequencies. This background conductivity, like that along the semiconducting c axis in underdoped cuprate superconductors, appears to be of an incoherent nature with a magnitude below Mott’s minimum metallic conductivity. At temperatures below TN there is evidence that the continuum interacts strongly with a nearby allowed phonon mode causing considerable asymmetry. The correlation between the onset of magnetic ordering and the appearance of asymmetry in the allowed phonon mode suggests that this continuum, to which the mode appears to be strongly coupled, may be of magnetic origin.