According to thermodynamic laws, entropy should be zero at absolute zero temperature. This is usually realized by forming ordered phases with spontaneously broken symmetry. In the case of magnets, the magnetic moments become arranged in some spacial pattern below a finite transition temperature.  On the other hand, there is another fascinating possibility, which is a formation of a novel quantum liquid state of the magnetic moments.

 

How to construct such a state ? A path breaking proposal was made in 2006 by A. Kitaev. He proposed the very first example of an exactly solvable theoretical model with a quantum spin liquid ground state. It is a model for spin 1/2 moments on 2D honeycomb lattice with bond dependent Ising axis. Interestingly, the spins here can be described by a combination of two different Majorana fermions.

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Can we have this in a real material ?? It was unlikely at the beginning. But further excitements followed when a possible materialization was pointed out by G. Jackeli and G. Khaliullin for spin-orbital-entangled moments on honeycomb iridates. Then, extensive studies have been made for α-Na2IrO3, α-Li2IrO3 and the closely related compound α-RuCl3. Nevertheless, all these candidate materials turned out to have ordered ground states possibly due to the additional interactions other than those in the model. On the other hand, we have finally found the first example in a new honeycomb iridium compound H3LiIr2O6. By using specific heat, magnetic susceptibility and NMR, we have confirmed that the material doesn't show any sign of magnetic ordering down to 0.05 K.  Now the door is open for the next step, where we could hunt for the signature of the strange excitations.  

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For more detail, please see K. Kitagawa, T. Takayama, Y. Matsumoto et al., Nature 554, 341–345 (2018). Press releases are also available here and here (in Japanese).