The quantum spin liquid is a state of matter that can be achieved in a system of interacting quantum spins. The word “liquid“ refers to a disordered state in comparison to a ferromagnetic spin state (as much in the way liquid water is in a disordered state compared to crystalline ice). However, unlike other disordered states, a quantum spin liquid state preserves its disorder to very low temperatures.
Quantum spin liquid properties
Quantum spin liquids may be considered “quantum disordered” ground states of spin systems, in which zero point fluctuations are so strong that they prevent conventional magnetic long-range order.
More interestingly, quantum spin liquids are prototypical examples of ground states with massive many-body entanglement, of a degree sufficient to render these states distinct phases of matter. Their highly entangled nature imbues quantum spin liquids with unique physical aspects, such as non-local excitations, topological properties, and more.
Quantum spin liquids are frequently found in a class of materials known as frustrated magnets. Localized spins are frustrated if there exist competing exchange interactions that can not all be satisfied at the same time, leading to a large degeneracy of the system’s ground state. In a conventional magnet, the interactions between spins result in stable formations, known as their long-range order, in which the magnetic directions of each individual electron is aligned.
In a frustrated magnet, the arrangement of electron spins prevents them from forming an ordered alignment, and so they collapse into a fluctuating, liquid-like state. In a true quantum spin liquid, the electron spins never align and continue to fluctuate even at the very lowest temperatures of absolute zero, at which the spins in other magnetic states of matter would have already frozen.
Based on our everyday experience, we expect matter at low temperatures to freeze solid with the atoms fixed in a regular arrangement. The magnetic moments arising from the spins of the electrons on the atoms in magnetic materials, also come to rest and become rigidly oriented as temperature falls. However, there are some rare exceptions, the orientations of the electronic spins do not remain fixed even at temperatures near absolute zero.
According to conventional understanding, if the interactions are isotropic (where all spin directions are possible), this phenomenon can occur if the spins are arranged in triangular geometries and the interactions between them are antiferromagnetic favoring antiparallel alignment of the spins. For three atoms forming the corners of a triangle, the electronic spin of one atom cannot simultaneously be oriented antiparallel to those on both the other two atoms.
In real materials that contain triangular units coupled by antiferromagnetic interactions, this “frustration“ can prevent the spins from coming to rest in a particular orientation even at absolute zero temperature. Instead, they move collectively like atoms in a liquid. By contrast, ferromagnetic interactions do not give rise to frustration in isotropic magnets because mutually parallel alignment of the spins can always occur. For these reasons, only a few isotropic materials have been proposed as spin liquid candidates.
Read more about this research here: http://iopscience.iop.org/article/10.1088/0034-4885/80/1/016502/ampdf