Synopsis: The spin-1/2 atoms in a so-called quantum spin liquid form a liquid-like magnetic state in which spins never order as a unit. The phase has been linked to certain forms of superconductivity and might be useful for quantum computing. But quantum spin liquids have been observed in only a handful of materials. Theorists at the University of Tokyo suggest looking for them in a new kind of system: crystals known as metal-organic frameworks (MOFs), in which spin-bearing metal ions are linked by organic molecules.
The team focused on a spin liquid model, proposed by Alexei Kitaev in 2006, that is considered attractive for quantum computing. Here, spin-1/2 atoms are arranged in a honeycomb lattice, and each spin couples to its three neighbors. These couplings “ask” the spin to align along three different directions, making ordering impossible. Some iridium-based oxides are thought to fit this description, though experimentalists have so far found evidence for the Kitaev spin liquid in only one compound, α-RuCl3-RuCl3 (which does not contain iridium).
Departing from the focus on inorganic solids, Masahiko Yamada and his colleagues proposed a MOF whose metal ions (such as ruthenium) are each surrounded by a “cage” of oxygen ions. These cages would be connected by organic ligands to form six-sided rings. Based on density-functional-theory calculations, the team estimates that the spins in this honeycomb MOF, which chemists are trying to grow, would form a Kitaev quantum spin liquid below 1 K. The researchers also suggest that, since MOFs can grow in a variety of structures, chemists might be able to synthesize an exotic 3D version of the Kitaev phase.
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