Dissipationless Anomalous Hall Current in the Ferromagnetic Spinel CuCr2_2Se4−x_{4-x}Brx_x

In a ferromagnet, an applied electric field $\bf E$ invariably produces an anomalous Hall current ${\bf J}_H$ that flows perpendicular to the plane defined by $\bf E$ and $\bf M$ (the magnetization). For decades, the question whether ${\bf J}_H$ is dissipationless (independent of the scattering rate), has been keenly debated without experimental resolution. In the ferromagnetic spinel CuCr$_2$Se$_{4-x}$Br$_x$, the resistivity $\rho$ (at low temperature) may be increased 1000 fold by varying $x$(Br), without degrading the $\bf M$. We show that ${\bf J}_H/E$ (normalized per carrier, at 5 K) remains unchanged throughout. In addition to resolving the controversy experimentally, our finding has strong bearing on the generation and study of spin-Hall currents in bulk samples.Comment: 7 pages, 6 figure

Charge-ordering, commensurability and metallicity in the phase diagram of layered Na(x)CoO(2)

The phase diagram of non-hydrated Na(x)CoO(2) has been determined by changing the Na content x using a series of chemical reactions. As x increases from 0.3, the ground state goes from a paramagnetic metal to a charge-ordered insulator (at x=1/2) to a `Curie-Weiss metal' (around 0.70), and finally to a weak-moment magnetically ordered state (x>0.75). The unusual properties of the state at 1/2 (including particle-hole symmetry at low T and enhanced thermal conductivity) are described. The strong coupling between the Na ions and the holes is emphasized.Comment: 4 pages with 3 figures, changed conten