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SmB6 : Skyrme insulator on the brink of superconductivity

Piers Coleman


Work done in collaboration with Onur Erten, Po-Yao Chang and Alexei Tsvelik,
supported by US Department of Energy Grants FG02-99ER45790 and DE-AC02-98CH10886

SmB6, an ultra-narrow gap insulator discovered more than 50 years ago, poses a paradox. On the one hand, current theory and some experiments indicate that SmB6 is likely a topological insulator. On the other hand, this system has been long known to display a large linear specific heat[1] and recent dHvA measurements[2] suggest that it contains a Fermi surface which responds to the Lorentz force, but is insulating. Optical measurements also reveal that this insulator is an AC conductor, with a large ingap ac conductivity and also exhibits a thermal conductivity that is proportional to the applied field[3], a feature reminiscent of an unpinned vortex lattice. These paradoxical results, if true, suggest an very unusual kind of insulator. I’ll argue that the apparent presence of a Fermi surface of nominally neutral quasiparticles which nevertheless respond to a Lorentz force requires broken Gauge invariance, which normally would imply superconductivity. This leads us to propose the concept of a "Skyrme insulator" : a condensate with a Meissner stiffness which is nevertheless topologically unable to support the quantization of circulation.
The Skyrme insulator theory allows us to understand the linear specific heat of SmB6 in terms of a neutral Majorana Fermi sea, and the theory predicts that in a screened environment at below fields of order a Gauss, SmB6 will develop a Meissner effect.

REFERENCES :

[1] K. Flachbart, M. Reiffers, and S. Janos, Journal of Less Common Metals 88, L11 (1982).
[2] B. S. Tan, et al, Science 349, 287 (2015).
[3] N. J. Laurita, et al, Phys. Rev. B 94, 165154 (2016).
[4] S. Sebastian, APS March meeting , 1603.09681 (2016).
[5 ]Onur Erten, Po-Yao Chang, Piers Coleman and Alexei Tsvelik, arXiv 1701.XXXX to be published (2017).

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