Observation of a phase transition at 55 K in single-crystal CaCu1.7As2

Abstract

We present the structural, magnetic, thermal and $ab$-plane electronic transport properties of single crystals of ${\mathrm{CaCu}}_{1.7}{\mathrm{As}}_{2}$ grown by the self-flux technique that were investigated by powder x-ray diffraction, magnetic susceptibility $\ensuremath{\chi}$, isothermal magnetization $M$, specific heat ${C}_{\mathrm{p}}$, and electrical resistivity $\ensuremath{\rho}$ 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 ${\mathrm{ThCr}}_{2}{\mathrm{Si}}_{2}$-type structure with $\ensuremath{\sim}15%$ vacancies on the Cu sites as previously reported, corresponding to the composition ${\mathrm{CaCu}}_{1.7}{\mathrm{As}}_{2}$. The $\ensuremath{\chi}(T)$ data are diamagnetic, anisotropic, and nearly independent of $T$. The $\ensuremath{\chi}$ is larger in the $ab$ plane than along the $c$ axis, as also observed previously for ${\mathrm{SrCu}}_{2}{\mathrm{As}}_{2}$ and for pure and doped BaFe${}_{2}$As${}_{2}$. The ${C}_{\mathrm{p}}(T)$ and $\ensuremath{\rho}(T)$ data indicate metallic $sp$-band character. In contrast to the $\ensuremath{\chi}(T)$ and ${C}_{\mathrm{p}}(T)$ data that do not show any evidence for phase transitions below 300 K, the $\ensuremath{\rho}(T)$ data exhibit a sharp decrease on cooling below a temperature ${T}_{\mathrm{t}}=54$--56 K, depending on the crystal. The $\ensuremath{\rho}(T)$ data show no hysteresis on warming and cooling through ${T}_{\mathrm{t}}$ and the transition thus appears to be second order. The phase transition may arise from spatial ordering of the vacancies on the Cu sublattice. The ${T}_{\mathrm{t}}$ is found to be independent of $H$ for $H\ensuremath{\le}8$ T. A positive magnetoresistance is observed below ${T}_{\mathrm{t}}$ that increases with decreasing $T$ and attains a value in $H=8.0$ T of 8.7$%$ at $T=1.8$ K.