KCo2As2 : A new portal for the physics of high-purity metals

Abstract

High-quality single crystals of ${\mathrm{KCo}}_{2}{\mathrm{As}}_{2}$ with the body-centered tetragonal ${\mathrm{ThCr}}_{2}{\mathrm{Si}}_{2}$ structure were grown using KAs self flux. Structural, magnetic, thermal, and electrical transport properties were investigated. No clear evidence for any phase transitions was found in the temperature range 2--300 K. The in-plane electrical resistivity $\ensuremath{\rho}$ versus temperature $T$ is highly unusual, showing a ${T}^{4}$ behavior below 30 K and an anomalous positive curvature up to 300 K, which is different from the linear behavior expected from the Bloch-Gr\"uneisen theory for electron scattering by acoustic phonons. This positive curvature has been previously observed in the in-plane resistivity of high-conductivity layered delafossites such as ${\mathrm{PdCoO}}_{2}$ and ${\mathrm{PtCoO}}_{2}$. The in-plane $\ensuremath{\rho}(T\ensuremath{\rightarrow}0)=0.36\phantom{\rule{0.16em}{0ex}}\ensuremath{\mu}\mathrm{\ensuremath{\Omega}}$ cm of ${\mathrm{KCo}}_{2}{\mathrm{As}}_{2}$ is exceptionally small for this class of compounds. The material also exhibits a magnetoresistance at low $T$ which attains a value of about 40% at $T=2$ K and magnetic field $H=80$ kOe. The magnetic susceptibility $\ensuremath{\chi}$ of ${\mathrm{KCo}}_{2}{\mathrm{As}}_{2}$ is isotropic and about an order of magnitude smaller than the values for the related compounds ${\mathrm{SrCo}}_{2}{\mathrm{As}}_{2}$ and ${\mathrm{BaCo}}_{2}{\mathrm{As}}_{2}$. The $\ensuremath{\chi}$ increases above 100 K, which is found from our first-principles calculations to arise from a sharp peak in the electronic density of states just above the Fermi energy ${E}_{\mathrm{F}}$. Heat capacity ${C}_{\mathrm{p}}(T)$ data at low $T$ yield an electronic density of states $N({E}_{\mathrm{F}})$ that is about 36% larger than predicted by the first-principles theory. The ${C}_{\mathrm{p}}(T)$ data near room temperature suggest the presence of excited optic vibration modes, which may also be the source of the positive curvature in $\ensuremath{\rho}(T)$. Angle-resolved photoemission spectroscopy measurements are compared with the theoretical predictions of the band structure and Fermi surfaces. Our results show that ${\mathrm{KCo}}_{2}{\mathrm{As}}_{2}$ provides a new avenue for investigating the physics of high-purity metals.