Recent progress on magnetic-field studies on quantum-spin-liquid candidates

Quantum spin liquids (QSLs) represent a novel state of matter in which quantum fluctuations prevent the conventional magnetic order from being established, and the spins remain disordered even at zero temperature. There have been many theoretical developments proposing various QSL states. On the other hand, experimental movement was relatively slow largely due to limitations on the candidate materials and difficulties in the measurements. In recent years, the experimental progress has been accelerated. In this topical review, we give a brief summary of experiments on the QSL candidates under magnetic fields. We arrange our discussions by two categories: i) Geometrically-frustrated systems, including triangular-lattice compounds YbMgGaO₄ and YbZnGaO₄, κ-(BEDT-TTF)₂Cu₂(CN)₃, and EtMe₃Sb[Pd(dmit)₂]₂, and the kagomé system ZnCu₃(OH)₆Cl₂; ii) the Kitaev material α-RuCl₃. Among these, we will pay special attention to α-RuCl₃, which has been intensively studied by ours and other groups recently. We will present evidence that both supports and rejects the QSL ground state for these materials, based on which we give several perspectives to stimulate further research activities. (topical review)