KCo${}_2$As${}_2$: A New Portal for the Physics of High-Purity Metals

Abstract

High-quality single crystals of KCo${}_2$As${}_2$ with the body-centered tetragonal ThCr${}_2$Si${}_2$ structure were grown using KAs self flux. Structural, magnetic, thermal, and electrical transport were investigated. No clear evidence for any phase transitions was found in the temperature range 2 to 300 K. The in-plane electrical resistivity $\rho$ 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-Grüneisen 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 PdCoO${}_2$ and PtCoO${}_2$. The in-plane $\rho (T\to 0)=0.36\mu \Omega$ cm of KCo${}_2$As${}_2$ is exceptionally small for this class of compounds. The material also exhibits a nearly linear magnetoresistance at low $T$ which attains a value of about 40% at $T=2$K and magnetic field $H=80$ kOe. The magnetic susceptibility $\chi$ of KCo${}_2$As${}_2$ is isotropic and about an order of magnitude smaller than the values for the related compounds SrCo${}_2$As${}_2$ and BaCo${}_2$As${}_2$. The $\chi$ 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 $\rho (T)$. Our results show that KCo${}_2$As${}_2$ provides a new avenue for investigating the physics of high-purity metals.

13 pages, 14 figures