Quantum Oscillations and High Carrier Mobility in the Delafossite ${\mathrm{PdCoO}}_2$

We present de Haas–van Alphen and resistivity data on single crystals of the delafossite ${\mathrm{PdCoO}}_2$. At 295 K we measure an in-plane resistivity of $2.6\mu \Omega \mathrm{cm}$, making ${\mathrm{PdCoO}}_2$ the most conductive oxide known. The low-temperature in-plane resistivity has an activated rather than the usual $T^5$ temperature dependence, suggesting a gapping of effective scattering that is consistent with phonon drag. Below 10 K, the transport mean free path is $\sim 20\mu \mathrm{m}$, approximately ${10}^5$ lattice spacings and an astoundingly high value for flux-grown crystals. We discuss the origin of these properties in light of our data.

Figure 1

Upper panel: torque curves against $H$, at the indicated angles $\theta$ between $\mathbf{H}$ and the $c$ axis, and magnetoresistance of a resistivity sample. The ticks on this curve and the 4.6° torque curve label the cycle number of the difference frequency: the troughs in the resistance oscillations match up with points where the torque oscillation amplitude (and likely magnetic interaction) is largest. Lower panel: Fourier transforms over the field range 8.1–15 T.

Figure 2

Top: observed dHvA frequencies $F$, obtained by Fourier transform over the field range 8.1 to 15 T (circles; darker shading indicates larger amplitude), and a fit (small points). Bottom: the separate harmonic components included in the fit, and their amplitudes. $k_{60}$ and $k_{12,0}$ are obtained from ARPES data [10], and $k_{00}$, $k_{01}$, $k_{02,}$ and $k_{31}$ from the fit. $k_{01}$ gives the overall Yamaji-type form to $F(\theta )$ and $k_{02}$ an asymmetry between the upper and lower frequencies. $k_{31}$ gives an asymmetry between $\theta >0$ and $\theta <0$, which is allowed by the lack of mirror symmetry in the crystal structure. The Fermi surface that results from summing these components is shown at right.

Figure 3

In-plane ${\mathrm{PdCoO}}_2$ resistivity ${\rho }_{ab}(T)$ (top), and $c$-axis resistivity ${\rho }_c(T)$ (bottom), with fits. At low $T$, ${\rho }_c$ has a $T^2$ dependence, while ${\rho }_{ab}$ is nearly temperature-independent. The inset in the top panel illustrates $k_U$, the minimum wave number for umklapp scattering. Partial fit results: (2) $T_0=165\mathrm{K}$. (3) $A_c=1.4\times {10}^{-9}\Omega \mathrm{cm}/{\mathrm{K}}^2$. (4) $A_c=2.1\times {10}^{-9}\Omega \mathrm{cm}/{\mathrm{K}}^2$, $T_0=157\mathrm{K}$.