Quantum spin liquids (QSLs) occur when the magnetic moments of a material act like a liquid, remaining disordered even at ultra-low temperatures. Their highly entangled nature should give rise to exotic physical phenomena that could have applications in data storage and memory, and developments in QSLs may also help in the understanding of high temperature superconductivity.

​​A group of researchers from across Europe has developed a complex material called EDT-BCO and shown that it acts as a QSL. The structure of EDT-BCO is a combination of two lattices. The first is made of the EDT molecules arranged in a triangular lattice, each with spin-1/2 that forms a magnetic dipole.

When molecules are positioned in this way, it is impossible to place a third molecule onto the lattice whilst maintaining a favourable (antiparallel) alignment of the dipole compared to its two neighbours (see right​). This maintains disorder in the dipoles, and is the reason why many quantum spin liquids are based on triangular structures.​

In this material, the layers of EDT are separated by layers of BCO molecular rotors, which were designed and synthesized by the group of Patrick Batail at Université d'Angers (CNRS). “These rotors give the material its unique properties", explains Péter Szirmai from Ecole Polytechnique Fédérale de Lausanne (EPFL). Their work, published in PNAS, used muons at ISIS and the Swiss Muon Source, alongside other measurements including NMR and ESR, to show that the random rotational positions of the BCO rotor molecules maintains disorder in the material, even at ultra-low temperatures.

​By maintaining disorder in the material, the elusive QSL state can be formed. This discovery, that materials with built-in disorder can cause a QSL to form, opens up new avenues for designing other novel structures with similar QSL properties.

Bálint Náfrádi from László Forró's laboratory at EPFL, explains; "Beyond the superb demonstration of the QSL state, our work is highly relevant, because it provides a tool to obtain additional QSL materials via custom-designed functional rotor molecules."

​Further information

The full paper can be found online at DOI: 10.1073/pnas.2000188117