Superconductivity: Obstinate spin liquid

Let’s start the year by being bold – let’s venture into the realm of superconductivity! The reason why I chose to cover this paper, which appeared in Physical Review X towards the end of 2016, is two-fold: on one hand I decided to stretch outside of my comfort zone and see how I would manage, on the other I felt that that this paper deserved some attention, as it basically reports on how something didn’t quite work out in the way it was expected to work out – and that is both bad news and good news, as often in science. Happy New Year and happy reading!

 

Quantum spin liquids – which describe frustrated systems of interacting spins – are thought to play a significant role in the onset of high-temperature superconductivity. Herbertsmithite, a crystal with kagome lattice structure, was recently identified as an ideal two-dimensional spin liquid candidate.

Theoretical studies indicate that electron doping of herbertsmithite should give rise to metallic and superconducting phases, but the synthesis of electron-doped herbertsmithite is a delicate task because of likely distortions to the lattice. Zachary Kelly and collaborators took up the challenge and intercalated lithium atoms with various concentrations between the kagome lattice layers, successfully producing electron-doped herbertsmithite and reporting their characterization of the synthesized material. Neutron powder diffraction measurements and X-ray photoelectron spectroscopy confirmed the introduction in the crystal of a large number of electrons donated by the lithium atoms. Nevertheless, resistance tests showed that doped herbertsmithite remained an insulator. In fact, the team found no evidence of metallic or superconducting states down to a temperature of 1.8 K.

While a single theoretical model could account for the data on temperature- and magnetic-field-dependent properties of the material (such as magnetic susceptibility and specific heat), the physical origins of the observed insulating behaviour are not yet clear. The authors note that lithium ions could be responsible for a level of disorder that sustains Anderson localisation. Another hampering factor might be the low connectivity of the kagome lattice, where vertices each connect to four neighbours only; indeed, doping in frustrated crystal geometries with higher connectivity (as in the case of a two-dimensional triangular lattice) led to a metallic phase. It seems that quantum spin liquids will keep researchers busy for a while.

Phys. Rev. X 6, 041007 (2016)

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