Unconventional Hall effect in oriented ${\mathrm{Ca}}_3{\mathrm{Co}}_4{\mathrm{O}}_9$ thin films

Transport properties of the good thermoelectric misfit oxide ${\mathrm{Ca}}_3{\mathrm{Co}}_4{\mathrm{O}}_9$ are examined. In-plane resistivity and Hall resistance measurements were made on epitaxial thin films which were grown on $c$-cut sapphire substrates using the pulsed laser deposition technique. Interpretation of the in-plane transport experiments relates the substrate-induced strain in the resulting film to single crystals under very high pressure $(\sim 5.5\mathrm{GPa})$ consistent with a key role of strong electronic correlation. They are confirmed by the measured high temperature maxima in both resistivity and Hall resistance. While holelike charge carriers are inferred from the Hall effect measurements over the whole investigated temperature range, the Hall resistance reveals a nonmonotonic behavior at low temperatures that could be interpreted with an anomalous contribution. The resulting unconventional temperature dependence of the Hall resistance seems thus to combine high temperature strongly correlated features above $340\mathrm{K}$ and an anomalous Hall effect at low temperature, below $100\mathrm{K}$.

Figure 1

(Color online) Longitudinal resistivity of a representative ${\mathrm{Ca}}_3{\mathrm{Co}}_4{\mathrm{O}}_9$ thin film. The inset displays the temperature region where $\rho -{\rho }_0$ versus $T^2$ is linear. $T^{*}$ notes a crossover above which the resistivity no longer varies as $T^2$.

Figure 2

(Color online) Single crystal (circles) and extrapolated thin film (squares) values for the coefficients of the metallike resistivity as a function of pressure: $A$, the Fermi liquid transport coefficient (upper panel) and $T^{*}$, the transport crossover temperature (lower panel). The strain-induced pressure was extrapolated from the values of $T^{*}$. The two dotted lines are guides to the eyes.

Figure 3

(Color online) Hall effect measurements of the ${\mathrm{Ca}}_3{\mathrm{Co}}_4{\mathrm{O}}_9$ thin films. The transverse resistivity $\left({\rho }_{xy}\right)$ versus magnetic field is shown for values above $100\mathrm{K}$ (upper panel) and below $100\mathrm{K}$ (lower panel).

Figure 4

(Color online) Temperature dependence of the Hall resistance $({\rho }_{xy}∕H)$. A broad maximum is observed at high temperatures (inset).

Figure 5

(Color online) The Hall resistance $({\rho }_{xy}∕H)$ values of the thin film versus inverse temperature. A linear relationship is observed below $100\mathrm{K}$ as ${\rho }_{xy}∕H=2.6\times {10}^{-9}(1+132∕T)$.