Hidenori TAKAGI
In conventional magnetic materials, interactions between the spins lead to a phase transition from a high-temperature disordered state to a magnetically ordered state as the temperature is lowered. The transition is typically accompanied by singularities in the thermodynamic observables at the transition point, and spontaneous symmetry breaking and a reduction of the spin entropy to zero as the system enters a unique ground state. However, the spin entropy can also be released without any symmetry breaking, down to zero temperature, by forming a collective quantum spin state with long-range quantum entanglement. This exotic state of matter is called a quantum spin liquid. A goal of condensed matter physics is to discover new quantum phases formed by the ensemble of interacting spins and charges in solids. The QSL is perhaps one of the most exotic quantum phases known so far partly because of the nontrivial elementary excitations and has been attracting the attention of condensed matter scientists for several decades.
In 2006 a theoretical breakthrough in the field of QSLs was reported. Alexei Kitaev proposed a simple new model that is exactly solvable and that gives a QSL ground state, in which the spins fractionalize into emergent quasiparticles — Majorana fermions1. Soon after, a spin-orbital Jeff = 1/2 Mott insulator was identified in a complex 5d iridium oxide. This led to a theoretical proposal for the realization of the Kitaev model using Jeff = 1/2 pseudo-spins in an iridate, and initiated a search for the QSL state and the hidden Majorana fermions in a family of iridium and ruthenium compounds2. I am going to talk about the rapid progress in the materialization Kitaev QSL as well as the hunting of Majorana fermions through the detection of unusual heat transport.
[2] H.Takagi, T. Takayama, G. Jackeli, G. Khaliullin, S. E. Nagler, Nat. Rev. Phys. 1, 264–280 (2019)