Electromagnetism without Electrons: A Brief History of Thermal Hall Effect

The past decade has witnessed the rise of the thermal Hall measurement as a sensitive probe of transport properties in solids. Experiments performed on a wide range of materials, such as magnetic insulators, spin ice, kagome spin liquids with both ferromagnetic and antiferromagnetic exchange interactions, a quantum paraelectric, and even high-Tc cuprates, showed the existence of thermal Hall transport phenomena caused by neutral excitations. There is little doubt that an era of electromagnetism without electrons has dawned. This review covers a brief and somewhat personal account of the theory and the experimental developments of the thermal Hall effect as a new discipline of condensed matter physics over the past decade.