Thermal Conduction Bent by Magnetic Field Found in Insulators

New probe for unknown magnetic state

The temperature dependence found in thermal Hall conductivity and spin susceptibility of the kagomé volborthite These graphs illustrate the temperature dependence of the thermal Hall conductivity, normalized by the temperature and the magnetic field (pink circle; left units), and that of the magnetic susceptibility (gray line; right units). The spin liquid state is formed below the peak temperature of the magnetic susceptibility. The maximum of the thermal Hall conductivity is also observed around the peak temperature, implying a strong connection between the formation of the spin liquid and the thermal Hall conductivity. © 2016 Minoru Yamashita.

A research group at the University of Tokyo and their collaborators have observed a phenomenon similar to the Hall effect—found only in electricity-conducting metal—in a spin liquid state, which, as an insulator, does not carry electricity. This groundbreaking discovery presents a new research method for studying spin liquid states, which are shrouded in mystery. A trajectory of an electron passing through a magnetic field is bent by what is known as the Lorentz force—the force exerted on the particle by the magnetic field—inducing a voltage proportional to the magnitude of the field. This phenomenon, known as the Hall effect, has a variety of applications, from being used to study fundamental physical properties of electrons in metals to being employed in magnetic sensors in smartphones. This effect does not occur in insulators because they do not transmit electricity, and the Lorenz force targeting mobile electrons in electricity-conducting metals acts as its source. However, recent theoretical studies have pointed out that spins in insulators can show a similar Hall effect in magnetic fields, garnering attention among scientists. Spins do not carry electricity, but they can transport heat so the phenomenon is observed as a thermal Hall effect.

In the current study, the research group led by Associate Professor Minoru Yamashita of the Institute of Solid State Physics at the University of Tokyo found the thermal Hall effect in a spin liquid state by adopting the thermal Hall effect created by spins in its investigation of the kagomé volborthite, a type of copper ore. Furthermore, the group showed for the first time that the magnitude of the thermal Hall effect is related to the generation of a spin liquid state, thereby demonstrating that the presence of this phenomenon is closely linked to the formation of an unknown spin liquid.

A spin liquid state remains an uncharted condition where the spins in it share strong correlations with each other, but resist magnetic ordering even at absolute zero (minus 273 degrees Celcius) temperature; it promises to present opportunities for the discovery of phenomena not seen in conventional magnetic materials, but exactly what those properties are remains unclear. This outcome is expected to put forward new methods for studying spin liquids, and help promote further research.

“I never expected to observe a thermal Hall effect in a spin liquid state, so at first I doubted the reliability of the measurements,” says Yamashita. He continues, “It took some time to verify the results, but when we observed that temperatures were dependent on changes in the spin correlation, I became convinced that our findings were correct. I hope we can gain a more detailed understanding of spin liquids by applying our measurement method to various materials.”