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Observation of a Phase Transition at 55 K in Single-Crystal CaCu1.7As2

We present the structural, magnetic, thermal and ab-plane electronic transport properties of single crystals of CaCu1.7As2 grown by the self-flux technique that were investigated by powder x-ray diffraction, magnetic susceptibility chi, isothermal magnetization M, specific heat Cp, and electrical resistivity rho measurements as a function of temperature T and magnetic field H. X-ray diffraction analysis of crushed crystals at room temperature confirm the collapsed tetragonal ThCr2Si2-type structure with \sim 15% vacancies on the Cu sites as previously reported, corresponding to the composition CaCu1.7As2. The chi(T) data are diamagnetic, anisotropic and nearly independent of T. The chi is larger in the ab-plane than along the c-axis, as also observed previously for SrCu2As2 and for pure and doped BaFe2As2. The Cp(T) and rho(T) data indicate metallic sp-band character. In contrast to the chi(T) and Cp(T) data that do not show any evidence for phase transitions below 300 K, the rho(T) data exhibit a sharp increase in slope on cooling below a temperature Tt = 54-56 K, depending on the crystal. The ρ(T) data show no hysteresis on warming and cooling through Tt and the transition appears to be second order. The phase transition may arise from spatial ordering of the vacancies on the Cu sublattice. The Tt is found to be independent of H for H \leq 8 T. A positive magnetoresistance is observed below Tt that increases with decreasing T and attains a value in H = 8.0 T of 8.7% at T = 1.8 K.