Singular behavior observed in the low temperature electronic transport and thermodynamic properties of Mn dopedNa0.68CoO2

Abstract

We report the electronic transport, magnetoresistance (MR), specific heat, and magnetic susceptibility of the doped compound Na0.68Co1-xMnxO2. The resistivity of the samples increases with Mn doping and a resistivity upturn at low temperatures was shown for samples with Mn exceeding 4%. This resistivity upturn can be suppressed by an external magnetic field, leading to a remarkable negative MR. The specific heat of the undoped samples can be fitted by C=gamma T+beta T-3, while the specific heat of the doped samples does not follow this behavior. Analysis shows that for samples with higher doping, the electrical resistivity (rho), the MR (M), and the specific heat divided by temperature (C/T) all exhibit similar behavior at low temperatures. They can be fitted by rho(T)=rho(0)+aT(2)+b ln T, M=M-0+a'T-2+b' ln T, and C/T=gamma+beta T-2+alpha ln T, respectively. The magnetic susceptibility (chi) of the doped samples follows the Curie-Weiss law chi=chi(0)+C/(T-theta) at high temperatures, while it exhibits chi proportional to T-lambda behavior (-1 <lambda < 0) at low temperatures. Mn doping also weakens the magnetic field dependence of the magnetic susceptibility at low temperatures. This work suggests that magnetic fluctuations are enhanced in the Mn doped cobalt oxides, leading to possible quantum critical behavior.