Magnetotransport of single crystalline NbAs

We report transport measurement in zero and applied magnetic field on a single crystal of NbAs. Transverse and longitudinal magnetoresistance in the plane of this tetragonal structure does not saturate up to 9 T. In the transverse configuration ($H \parallel c$, $I \perp c$) it is 230,000 \% at 2 K. The Hall coefficient changes sign from hole-like at room temperature to electron-like below $\sim$ 150 K. The electron carrier density and mobility calculated at 2 K based on a single band approximation are 1.8 x 10$^{19}$ cm$^{-3}$ and 3.5 x 10$^{5}$ cm$^2$/Vs, respectively. These values are similar to reported values for TaAs and NbP, and further emphasize that this class of noncentrosymmetric, transition-metal monopnictides is a promising family to explore the properties of Weyl semimetals and the consequences of their novel electronic structure.Comment: accepted in J. Phys.: Condens. Matte

Nuclear magnetic resonance investigation of the heavy fermion system Ce2_2CoAl7_7Ge4_4

We present nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) measurements performed on single crystalline \ccag{}, a member of a recently discovered family of heavy fermion materials Ce$_2M$Al$_7$Ge$_4$ ($M$ = Co, Ir, Ni, or Pd). Previous measurements indicated a strong Kondo interaction as well as magnetic order below $T_M = 1.8$ K. Our NMR spectral measurements show that the Knight shift $K$ is proportional to the bulk magnetic susceptibility $\chi$ at high temperatures. A clear Knight shift anomaly ($K \not\propto \chi$) is observed at coherence temperatures $T^* \sim 17.5$ K for $H_0 \parallel \hat{c}$ and 10 K for $H_0 \parallel \hat{a}$ at the ${}^{59}$Co site, and $T^* \sim 12.5$ K at the ${}^{27}$Al(3) site for $H_0 \parallel \hat{a}$ characteristic of the heavy fermion nature of this compound. At high temperatures the ${}^{59}$Co NMR spin-lattice relaxation rate $T_1^{-1}$ is dominated by spin fluctuations of the 4$f$ local moments with a weak metallic background. The spin fluctuations probed by ${}^{59}$Co NMR are anisotropic and larger in the basal plane than in the $c$ direction. Furthermore, we find $(T_1TK)^{-1} \propto T^{-1/2}$ at the ${}^{59}$Co site as expected for a Kondo system for $T > T^*$ and $T> T_K$. ${}^{59}$Co NQR \slrr{} measurements at low temperatures indicate slowing down of spin fluctuations above the magnetic ordering temperature $T_M \sim 1.8$ K. A weak ferromagnetic character of fluctuations around $\mathbf{q}=0$ is evidenced by an increase of $\chi T$ versus $T$ above the magnetic ordering temperature. We also find good agreement between the observed and calculated electric field gradients at all observed sites