Quantum spin liquids

Besides the introduction of nonthermal control parameters and quenched disorder as mentioned in quantum criticality, another route to suppress magnetic order is through intrinsic geometrical frustration without chemical disorder. In some systems, the arrangement of spins makes it impossible to find an energetically favorable configuration even down to zero temperature. Common examples are nearest-neighbor interactions on the kagome and triangular lattices in two dimensions (2D), and on the pyrochlore lattice in three dimensions (3D). A quantum spin liquid (QSL) is a solid material that describes the above systems in which the liquid-like magnetic state persists down to zero temperature. Two key features rigorously defining a QSL are long-range entanglement and the associated fractional spin excitations with non-Abelian statistics. Both features are important ingredients for quantum computation which is the next frontier in cutting-edge science. In 1980s, QSLs have been also suggested to explain the high-temperature superconductivity.

For the real materials, although two-dimensional kagome-lattice ZnCu₃(OH)₆Cl₂ and triangular lattice YbMgGaO₄ have been considered to be QSL candidates since neither exhibits long range magnetic order and both show spin excitation continua in neutron scattering measurements at low temperatures. However, existence of site chemical disorder in both systems complicates the interpretation of the data. With this in mind, it is important to look for other QSL candidates with minimum magnetic and nonmagnetic chemical disorder. High-quality single crystalline samples without disorder are important to start with, because disorder of any variety often leads to spin freezing and induces new low-energy excitations. These excitations behave also like a spin liquid, but without the hallmark of a QSL carrying fractional spin excitations.