Bi2Se3 is one of a handful of known topological insulators. Here we show that copper intercalation in the van der Waals gaps between the Bi2Se3 layers, yielding an electron concentration of ~ 2 x 10^20cm-3, results in superconductivity at 3.8 K in Cu
xBi2Se3 for x between 0.12 and 0.15. This demonstrates that Cooper pairing is possible in Bi2Se3 at accessible temperatures, with implications for study of the physics of topological insulators and potential devices.
Photoemission experiments have shown that Bi$_2$Se$_3$ is a topological insulator. By controlled doping, we have obtained crystals of Bi$_2$Se$_3$ with non-metallic conduction. At low temperatures, we uncover a novel type of magnetofingerprint signal
which involves the spin degrees of freedom. Given the mm-sized crystals, the observed amplitude is 200-500$times$ larger than expected from universal conductance fluctuations. The results point to very long phase breaking lengths in an unusual conductance channel in these non-metallic samples. We discuss the nature of the in-gap conducting states and their relation to the topological surface states.
The large magnetic anisotropy in the layered ferromagnet Fe_{1/4}TaS_2 leads to very sharp reversals of the magnetization $bf M$ at the coercive field. We have exploited this feature to measure the anomalous Hall effect (AHE), focussing on the AHE co
nductivity $sigma^A_{xy}$ in the inelastic regime. At low temperature T (5-50 K), $sigma^A_{xy}$ is T-independent, consistent with the Berry-phase/Karplus-Luttinger theory. Above 50 K, we extract an inelastic AHE conductivity $sigma^{in}_{xy}$ that scales as the square of $Deltarho$ (the T dependent part of the resistivity $rho$). The term $sigma^{in}_{xy}$ clarifies the T dependence and sign-reversal of the AHE coefficient R_s(T). We discuss the possible ubiquity of $sigma^{in}_{xy}$ in ferromagnets, and ideas for interpreting its scaling with $(Deltarho)^2$. Measurements of the magnetoresistance (MR) reveal a rich pattern of behavior vs. T and field tilt-angle. We show that the 2 mechanisms, the anisotropic MR effect and field-suppression of magnons, account for the intricate MR behavior, including the bow-tie features caused by the sharp reversals in $bf M$.
