Peculiar low-temperature properties of metallicNd2−xCexCuO4caused by interactions between Nd moments and conduction electrons

Low-energy magnetic excitations in ${\mathrm{Nd}}_{2-x}{\mathrm{Ce}}_x{\mathrm{CuO}}_4$ with $0<~x<~0.2$ have been investigated by measurements of the specific heat. For all Ce-doped samples $(x>~0.05),$ a linear term $\gamma T$ was found at very low temperatures $(T\lesssim 0.5\mathrm{K}),$ with ${0.3\mathrm{}\mathrm{J}/\mathrm{K}}^2{\lesssim \mathrm{\gamma }\lesssim 0.6\mathrm{}\mathrm{J}/\mathrm{K}}^2$ per mole Nd for $0.05<~x<\mathrm{}0.14,$ and ${3\mathrm{}\mathrm{J}/\mathrm{K}}^2{\lesssim \mathrm{\gamma }\lesssim 4\mathrm{}\mathrm{J}/\mathrm{K}}^2$ per mole Nd for higher Ce concentrations. In overdoped ${\mathrm{Nd}}_{1.8}{\mathrm{Ce}}_{0.2}{\mathrm{CuO}}_4,$ γ is rapidly suppressed in magnetic fields exceeding 1.5 T. The temperature dependence of the electrical resistance parallel to the ${\mathrm{CuO}}_2$ planes corresponds to that of two-dimensional metals. The magnetoresistance of ${\mathrm{Nd}}_{1.8}{\mathrm{Ce}}_{0.2}{\mathrm{CuO}}_4$ single crystals and $c$-axis-oriented thin films in magnetic fields parallel to the ${\mathrm{CuO}}_2$ planes is monotonously negative at temperatures above 1 K. Below about 500 mK, the magnetoresistance is positive in small fields, it goes through a maximum near 2 T, and it is negative for $B>\mathrm{}2\mathrm{}\mathrm{T}.$ These features are similar to those observed in heavy-fermion systems. In contrast, in ${\mathrm{Pr}}_{1.8}{\mathrm{Ce}}_{0.2}{\mathrm{CuO}}_4,$ with nonmagnetic Pr, the magnetoresistance is negative at all temperatures of our measurements down to 20 mK. This proves unambiguously that interactions between conduction electrons and Nd moments give rise to the low-temperature maximum of the magnetoresistance. Furthermore, a comparison of the magnetoresistance with the modification of the specific heat by magnetic fields supports the interpretation of the large linear term as resulting from the interaction between Nd moments and conduction electrons.