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The Hall effect and resistivity of the carrier doped magnetic semiconductor Fe$_{1-x}$Co$_x$S$_2$ were measured for $0\le x \le 0.16$, temperatures between 0.05 and 300 K, and fields of up to 9 T. Our Hall data indicate electron charge carriers with a density of only 10 to 30% of the Co density of our crystals. Despite the previous identification of magnetic Griffiths phase formation in the magnetic and thermodynamic properties of this system for the same range of $x$, we measure a temperature independent resistivity below 0.5 K indicating Fermi liquid-like transport. We also observe no indication of quantum corrections to the conductivity despite the small values of the product of the Fermi wave vector and the mean-free-path, $1.5 \le k_F\ell \le 15$, over the range of $x$ investigated. This implies a large inelastic scattering rate such that the necessary condition for the observation of quantum contributions to the carrier transport, quantum coherence over times much longer than the elastic scattering time, is not met in our samples. Above 0.5 K we observe a temperature and magnetic field dependent resistivity that closely resembles a Kondo anomaly for $x$ less than that required to form a long range magnetic state, $x_c$. For $x>x_c$, the resistivity and magnetoresistance resemble that of a spin glass with a reduction of the resistivity by as much as 35% in 5 T fields. We also observe an enhancement of the residual resistivity ratio by almost a factor of 2 for samples with $x\sim x_c$ indicating temperature dependent scattering mechanisms beyond simple carrier-phonon scattering. We speculate that this enhancement is due to charge carrier scattering from magnetic fluctuations which contribute to the resistivity over a wide temperature range., Comment: 13 pages including 14 figures

Doping of the band insulator FeS$_2$ with Co on the Fe site introduces a small density of itinerant carriers and magnetic moments. The lattice constant, AC and DC magnetic susceptibility, magnetization, and specific heat have been measured over the $0\le x\le 0.085$ range of Co concentration. The variation of the AC susceptibility with hydrostatic pressure has also been measured in a small number of our samples. All of these quantities show systematic variation with $x$ including a paramagnetic to disordered ferromagnetic transition at $x=0.007\pm 0.002$. A detailed analysis of the changes with temperature and magnetic field reveal small power law dependencies at low temperatures for samples near the critical concentration for magnetism, and just above the Curie temperature at higher $x$. In addition, the magnetic susceptibility and specific heat are non-analytic around H=0 displaying an extraordinarily sharp field dependence in this same temperature range. We interpret this behavior as due to the formation of Griffiths phases that result from the quenched disorder inherent in a doped semiconductor., Comment: 22 pages including 27 figures

We present a feasibility study for loading cold atomic clouds into magnetic traps created by single-wall carbon nanotubes grown directly onto dielectric surfaces. We show that atoms may be captured for experimentally sustainable nanotube currents, generating trapped clouds whose densities and lifetimes are sufficient to enable detection by simple imaging methods. This opens the way for a novel type of conductor to be used in atomchips, enabling atom trapping at sub-micron distances, with implications for both fundamental studies and for technological applications.

Spin glasses are systems whose magnetic moments freeze at low temperature into random orientations without long-range order. It is generally accepted that both frustration and disorder are essential ingredients in all spin glasses, so it was surprising that PrAu2Si2, a stoichiometric compound with a well-ordered crystal structure, was reported to exhibit spin glass freezing. In this article, we report on inelastic neutron scattering measurements of the crystal field excitations, which show that PrAu2Si2 has a singlet ground state and that the exchange coupling is very close to the critical value to induce magnetic order. We propose that spin glass freezing results from dynamic fluctuations of the crystal field levels that destabilize the induced moments and frustrate the development of long-range magnetic correlations. This novel mechanism for producing a frustrated ground state could provide a method of testing the concept of `avoided criticality' in glassy systems., Comment: 5 pages, 5 figures

Critical points that can be suppressed to zero temperature are interesting because quantum fluctuations have been shown to dramatically alter electron gas properties. Here, the metal formed by Co doping the paramagnetic insulator FeS$_2$, Fe$_{1-x}$Co$_x$S$_2$, is demonstrated to order ferromagnetically at $x>x_c=0.01\pm0.005$ where we observe unusual transport, magnetic, and thermodynamic properties. We show that this magnetic semiconductor undergoes a percolative magnetic transition with distinct similarities to the Griffiths phase, including singular behavior at $x_c$ and zero temperature., Comment: 10 pages, 4 figures

The conductivity and magnetization of Fe1-xCoxS2 were measured to investigate quantum critical behavior in disordered itinerant magnets. Small x (<0.001) is required to convert insulating iron pyrite into a metal, followed by a paramagnetic-to-ferromagnetic metal transition at x = 0.032+/-0.004. Singular contributions are discovered that are distinct from those at either metal-insulator or magnetic transitions. Our data reveal that disorder and low carrier density associated with proximity to a metal-insulator transition fundamentally modifies the critical behavior of the magnetic transition., Comment: 14 pages 4 figures