[Sangwoo Sim, Heejun Yang and Prof. Je-Geun Park] Sizable Suppression of Thermal Hall Effect upon Isotopic Substitution in SrTiO3
We report measurements of the thermal Hall effect in single crystals of both pristine and isotopically substituted strontium titanate. We discovered a two orders of magnitude difference in the thermal Hall conductivity between SrTi16O3 and 18O-enriched SrTi18O3 samples. In most temperature ranges, the magnitude of thermal Hall conductivity (kxy) in SrTi18O3 is proportional to the magnitude of the longitudinal thermal conductivity (kxx), which suggests a phonon-mediated thermal Hall effect. However, they deviate in the temperature of their maxima, and the thermal Hall angle ratio (|kxy/kxx|) shows anomalously decreasing behavior below the ferroelectric Curie temperature Tc ~25 K. This observation suggests a new underlying mechanism, as the conventional scenario cannot explain such differences within the slight change in phonon spectrum. Notably, the difference in magnitude of thermal Hall conductivity and rapidly decreasing thermal Hall angle ratio in SrTi18O3 is correlated with the strength of quantum critical fluctuations in this displacive ferroelectric. This relation points to a link between the quantum critical physics of strontium titanate and its thermal Hall effect, a possible clue to explain this example of an exotic phenomenon in non-magnetic insulating systems.
참여 연구원: 심상우, 양희준, 김하림, Matt Coak, Mitsuru Itho, Yukio Noda, 박제근