Low-temperature magnetic and transport properties of layered (Sr,Ca)xCoO2 single crystals

Sr0.35CoO2 and Ca0.38CoO2 single crystals have been successfully prepared by a low-temperature ion exchange of layered NaxCoO2 precursors. Both samples are isostructural to NaxCoO2 with the layered triangular structure. Low-temperature susceptibility, specific heat, and resistivity are measured. For both samples, the magnetic susceptibility shows a Curie-Weiss behavior at high temperatures and exhibits a broad peak at low temperatures, characteristic of two-dimensional antiferromagnetic fluctuations. For Sr0.35CoO2 crystal, the resistivity is metallic in the whole temperature range and the specific heat below 10 K can be well described by a sum of the electronic and lattice contributions. In the case of Ca0.38CoO2 crystal, the resistivity exhibits a metallic behavior above 25 K, while a transition from metal to insulator for rho(c) below 25 K is observed. The low-temperature specific heat for Ca0.38CoO2 crystal can be well fitted by a sum of the electronic, lattice, and exponential contributions. A gap feature in electronic excitation spectra is observed, and this opening of the gap may account for the metal to insulator transition in resistivity measurement.