We have used ion-irradiation to damage the (001) surfaces of SmB_6 single crystals to varying depths, and have measured the resistivity as a function of temperature for each depth of damage. We observe a reduction in the residual resistivity with inc
reasing depth of damage. Our data are consistent with a model in which the surface state is not destroyed by the ion-irradiation, but instead the damaged layer is poorly conducting and the initial surface state is reconstructed below the damage. This behavior is consistent with a surface state that is topologically protected.
We report the phase diagram of the doping series U_2Pt_xRh_(1-x)C_2, studied through measurements of resistivity, specific heat and magnetic susceptibility. The Neel temperature of U_2RhC_2 of ~ 22 K is suppressed with increasing Pt content, reaching
zero temperature close to x=0.7, where we observed signatures of increased quantum fluctuations. In addition, evidence is presented that the antiferromagnetic state undergoes a spin-reorientation transition upon application of an applied magnetic field. This transition shows non-monotonic behaviour as a function of x, peaking at around x=0.3. Superconductivity is observed for x>=0.9, with T_c increasing with increasing x. The reduction in T_c and increase in residual resistivity with decreasing Pt content is inconsistent with the extension of the Abrikosov-Gorkov theory to unconventional superconductivity.
The easily tuned balance among competing interactions in Kondo-lattice metals allows access to a zero-temperature, continuous transition between magnetically ordered and disordered phases, a quantum-critical point (QCP). Indeed, these highly correlat
ed electron materials are prototypes for discovering and exploring quantum-critical states. Theoretical models proposed to account for the strange thermodynamic and electrical transport properties that emerge around the QCP of a Kondo lattice assume the presence of an indefinitely large number of itinerant charge carriers. Here, we report a systematic transport and thermodynamic investigation of the Kondo-lattice system CeNi$_{2-delta}$As$_2$ ($delta$$thickapprox$0.28) as its antiferromagnetic order is tuned by pressure and magnetic field to zero-temperature boundaries. These experiments show that the very small but finite carrier density of $sim$0.032 $e^-$/f.u. in CeNi$_{2-delta}$As$_2$ leads to unexpected transport signatures of quantum criticality and the delayed development of a fully coherent Kondo lattice state with decreasing temperature. The small carrier density and associated semi-metallicity of this Kondo-lattice material favor an unconventional, local-moment type of quantum criticality and raise the specter of Nozi`{e}res exhaustion idea that an insufficient number of conduction-electron spins to separately screen local moments requires collective Kondo screening.
