Directional dichroism in the paramagnetic state of multiferroics: a case study of infrared light absorption in Sr2CoSi2O7 at high temperatures

• Main Authors: Viirok, J, Nagel, U, Rõõm, T, Farkas, D, Balla, P, Szaller, D, Kocsis, V, Tokunaga, Y, Taguchi, Y, Tokura, Y, Bernáth, B, Kamenskyi, D. L, Kézsmárki, I, Bordács, S, Penc, K
• Format: Journal Article
• Language: English
• Published: 26-09-2018
• Subjects: Physics - Materials Science; Physics - Strongly Correlated Electrons
• DOI: 10.48550/arxiv.1809.10207

Phys. Rev. B 99, 014410 (2019) The coexisting magnetic and ferroelectric orders in multiferroic materials
give rise to a handful of novel magnetoelectric phenomena, such as the
absorption difference for the opposite propagation directions of light called
the non-reciprocal directional dichroism (NDD). Usually these effects are
restricted to low temperature, where the multiferroic phase develops. In this
paper we report the observation of NDD in the paramagnetic phase of Sr2CoSi2O7
up to temperatures more than ten times higher than its N\'eel temperature (7 K)
and in fields up to 30 T. The magnetically induced polarization and NDD in the
disordered paramagnetic phase is readily explained by the single-ion
spin-dependent hybridization mechanism, which does not necessitate correlation
effects between magnetic ions. The Sr2CoSi2O7 provides an ideal system for a
theoretical case study, demonstrating the concept of magnetoelectric spin
excitations in a paramagnet via analytical as well as numerical approaches. We
applied exact diagonalization of a spin cluster to map out the temperature and
field dependence of the spin excitations, as well as symmetry arguments of the
single ion and lattice problem to get the spectrum and selection rules.