c-axis magnetotransport in CeCoIn${}_5$

Abstract

We present the results of out-of-plane electrical transport measurements on the heavy fermion superconductor CeCoIn${}_5$ at temperatures from 40 mK to 400 K and in magnetic field up to 9 T. For $T<$ 10 K transport measurements show that the zero-field resistivity ${\rho }_c$ changes linearly with temperature and extrapolates nearly to zero at 0 K, indicative of non-Fermi-liquid (nFL) behavior associated with a quantum critical point (QCP). The longitudinal magnetoresistance (LMR) of CeCoIn${}_5$ for fields applied parallel to the c-axis is negative and scales as $B/(T+T^{\ast })$ between 50 and 100 K, revealing the presence of a single-impurity Kondo energy scale $T^{\ast }\sim 2$ K. Beginning at 16 K a small positive LMR feature is evident for fields less than 3 tesla that grows in magnitude with decreasing temperature. For higher fields the LMR is negative and increases in magnitude with decreasing temperature. This sizable negative magnetoresistance scales as $B{}^2/T$ from 2.6 K to roughly 8 K, and it arises from an extrapolated residual resistivity that becomes negative and grows quadratically with field in the nFL temperature regime. Applying a magnetic field along the c-axis with B $>$ B${}_{c2}$ restores Fermi-liquid behavior in ${\rho }_c(T)$ at $T$ less than 130 mK. Analysis of the $T{}^2$ resistivity coefficient's field-dependence suggests that the QCP in CeCoIn${}_5$ is located \emph{below} the upper critical field, inside the superconducting phase. These data indicate that while high-$T$ c-axis transport of CeCoIn${}_5$ exhibits features typical for a heavy fermion system, low-$T$ transport is governed both by spin fluctuations associated with the QCP and Kondo interactions that are influenced by the underlying complex electronic structure intrinsic to the anisotropic CeCoIn${}_5$ crystal structure.