Room temperature excitation continuum in the Rare-Earth Magnetic Insulator RInO₃

Mott insulators have the capacity to support various unique ground states, and one of the most intriguing systems among them is a quantum spin liquid that comprises highly entangled spins. It is established that such an entangled system can host novel non-local excitations, like the continuum excitation known as a spinon. Using terahertz time-domain spectroscopy, we have detected a continuum excitation in the magnetic insulators RInO₃ (where R stands for Tb³⁺ and Gd³⁺). This continuum excitation exhibits optical conductivity proportional to the square of frequency, even at room temperature. Furthermore, a Fano distortion observed in the lowest optical phonon mode suggests a pronounced interaction with the continuum excitation. In the case of TbInO₃, the proportional frequency-dependent optical conductivity remains robust and is unaffected by external magnetic fields of up to 7 T, persisting at low temperatures as low as 1.5 K. Conversely, the continuum excitation in GdInO₃ diminishes as the temperature decreases. Our results propose the existence of an emergent charge excitation, even in a wide-bandgap Mott insulator, and hint at the potential for achieving a highly entangled many-body state at room temperature.