Condensed Matter Seminar, Piers Coleman, Rutgers, "SmB6: Strange insulator at the brink of superconductivity?"

Mon, May 22, 2017, 1:15 pm
PCTS Seminar Room

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 allow 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.

[*]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.

[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 1701.06582 (2017).