Metallic Surface States Probed Within the Microwave Skin Depth of the Putative Topological Insulator YBiPt Compound

Abstract

Electron Spin Resonance (ESR) experiments of diluted Nd${}^{3+}$ ions in the claimed topological insulator (TI) YBiPt are reported. Powdered samples with grain size from $\approx$ 100 $\hat{I}¼$m to $\approx$ 2,000 $\hat{I}¼$m were investigated. At low temperatures, 1.6 K $\lesssim$ \emph{T} $\lesssim$ 20 K, the X-band ($9.4$ GHz) ESR spectra show a \emph{g}-value of 2.66(4) and a Dysonian resonance lineshape which shows a remarkably unusual temperature, concentration, microwave power and particle size dependence. These results indicate that metallic and insulating behavior coexist within a skin depth of $\hat{I}´\approx$ 15 $\hat{I}¼$m. Furthermore, the Nd${}^{3+}$ spin dynamics in YBiPt are consistent with the existence of a \emph{phonon-bottleneck process} which allows the energy absorbed by the Nd${}^{3+}$ ions at resonance to reach the thermal bath via the conduction electrons in the metallic surface states of YBiPt. These results are discussed in terms of the claimed topological semi-metal properties of YBiPt.

8 pages, 10 figures