关键词: 量子自旋液体；量子涨落；磁无序；几何阻挫；Kitaev 自旋液体

Abstract: Quantum spin liquid (QSL) represents a novel state of matter in which strong quantum fluctuations prevent conventional magnetic order from establishing even down to zero temperature. Such an exotic state does not fit into Landau's paradigm, in which a phase can be described with a certain local order parameter, and the phase transition is accompanied with spontaneous symmetry breaking. Most QSLs exhibit topological properties and figuring out a proper way to understand them has been a very active subject in condensed matter physics. Moreover, QSLs have attracted a lot of attention due to the close connection with the mechanism of unconventional superconductors and potential applications in quantum computing. Although the idea of QSLs was proposed four decades ago, and a lot of progress has been made on the theoretical side, experimental advancement in this field is relatively slow as a result of the lack of QSL candidates and difficulty in achieving the extreme conditions. Recently, the progress on the experimental side has been accelerated thanks to the discovery of new QSL candidates and the development of various experimental techniques in characterizing QSLs. In this brief review, we will highlight some recent interesting experimental results obtained on (1) geometrically frustrated QSL candidates, including triangular-lattice compounds YbMgGaO$_4$ and YbZnGaO$_4$, $\kappa$-(BEDT-TTF)$_2$Cu$_2$(CN)$_3$, EtMe$_3$Sb[Pd(dmit)$_2$]$_2$ and kagom\'e-lattice compound ZnCu$_3$(OH)$_6$Cl$_2$; (2) Kitaev QSL candidates, including iridium-based compounds (Na$_2$IrO$_3$与$\alpha$-, $\beta$-, $\gamma$-Li$_2$IrO$_3$) and $\alpha$-RuCl$_3$. In the end, we will make a brief summary and a perspective for future work.

Key words: Quantum spin liquid; quantum fluctuation; disorder; geometrical frustration; Kitaev quantum spin liquid