Mott insulators represent an intriguing category of materials that, in theory, should conduct electricity due to their structural characteristics but instead exhibit insulating properties. These materials harbor electrons with strong correlations, resulting in complex many-body states characterized by unconventional excitations.
Traditionally, scientists believed that the unconventional excitations in Mott insulators could only manifest at low temperatures. However, a research team comprising experts from Yonsei University, Rutgers University, KAIST, and other South Korean institutes recently uncovered evidence of these unconventional charge carriers at room temperature. This breakthrough, published in Nature Physics, pertains to the Mott insulator terbium indium oxide (TbInO3), distinguished by its triangular lattice structure.
Professor Jae Hoon Kim and Professor Eun-Gook Moon, key contributors to this study, explained, “In magnetic materials, as temperature drops, electron spins begin to interact, eventually leading to ordered states, such as ferromagnetic or antiferromagnetic configurations. However, in a quantum spin liquid, the spins remain disordered even at extremely low temperatures, owing to macroscopic quantum entanglement among them.”
A quantum spin liquid, a concept initially proposed by Nobel laureate Philip W. Anderson in 1973, characterizes a Mott insulator that resembles a liquid in its behavior but lacks magnetic order. As these materials were classified as Mott insulators, it was commonly assumed that quantum spin liquids did not interact with external electromagnetic fields, making it impossible to detect their unconventional excitations through optical measurements at room temperature.
“In the early 2010s, Professor Patrick Lee’s team at MIT proposed that a subset of quantum spin liquids might indirectly ‘sense’ external electromagnetic fields, and they predicted that optical conductivity would be proportional to the square of the frequency at low frequencies,” elaborated Professors Kim and Moon. “Despite a decade of effort, candidate materials displaying this distinct characteristic remained elusive.”
In their recent study, Kim, Moon, and their collaborators aimed to challenge the established belief that exotic excitations only manifest in Mott insulators at low temperatures. To achieve this, they grew high-quality single crystals of the Mott insulator TbInO3 in Prof. Sang-Wook Cheong’s laboratory at Rutgers University, using the laser floating-zone growth technique.
TbInO3 was specifically chosen due to previous research by Prof. Cheong and colleagues, which had already identified unique characteristics indicative of quantum spin liquid behavior in this material through neutron scattering techniques. After growing their TbInO3 samples at Rutgers, the researchers analyzed them at Yonsei University in South Korea, employing terahertz time-domain spectroscopy techniques.
During these experiments, Prof. Kim and his team at Yonsei University observed that the alternating current terahertz conductivity in the material precisely followed the square of the frequency of light, even at room temperature. Professor Eun-Gook Moon at KAIST subsequently developed a series of theoretical explanations to elucidate these unexpected experimental findings.
“Our most significant discovery is the identification of unconventional charge carriers, composed of a substantial number of quantum spins,” emphasized Professors Kim and Moon. “Contrary to the prevailing belief that insulators lack low-energy charge carriers, we have substantiated their existence by measuring optical conductivity proportional to the square of the light frequency. Remarkably, these charge carriers remain coherent and operative up to room temperature.”
Professors Kim, Moon, Cheong, and their collaborators have experimentally demonstrated that unconventional charge carriers can exist in Mott insulators at room temperature. This breakthrough is poised to facilitate further experiments and theoretical investigations, advancing our understanding of the underlying physics behind these observations.
“One plausible scenario is that these charge carriers originate from a subset of quantum spin liquids proposed by Professor Lee,” the researchers surmised. Prof. Kim suggests that quantum spin liquids inherently harbor highly entangled macroscopic states, potentially contributing to the development of fault-tolerant quantum computers that can operate at room temperature, a vision endorsed by colleague So He.
“In the broader context, we plan to extend our research to other indium oxide compounds and explore similar phenomena,” the researchers concluded.