Fermi-liquid behavior and characteristic temperature-dependent susceptibility in clean single crystal RuO$_2$

The magnetic nature of the altermagnet candidate RuO$_2$ remains under debate. It has been recently shown from quantum oscillations and angle-resolved photoemission spectroscopy that the high-quality RuO$_2$ bulk single crystal is a paramagnetic metal. Here we report the specific heat and magnetic susceptibility in ultraclean RuO$_2$ single crystals with residual resistivity ratio up to 1200. The electronic specific heat, the magnetic susceptibility, and the 𝑇$^2$ coefficient in resistivity point to a weakly correlated three-dimensional Fermi-liquid state, as supported by the Wilson and Kadowaki-Woods ratios. The magnetic susceptibility increases with temperature and is phenomenologically fitted with an inclusion of 𝑇⁢ln⁡(𝑇/𝑇$_0$) over a wide temperature range up to 400 K. In contrast, the energy dependence of the density of states and thermal activation of quasiparticles lead to a decrease with temperature. Such characteristic temperature dependence, similar to that observed in other 𝑑-electron metals, is attributable to an enhanced orbital contribution arising from lattice-expansion-induced changes in the band structure.