Fig. 3

The Hall resistivity ρ_xy for PdCoO₂ a and PtCoO₂ b at nine temperatures between 2 K and 300 K, offset for clarity. In all cases the field H is applied parallel to the crystallographic c axis, perpendicular to the conducting planes. A pronounced decrease in gradient is seen at low fields, with a characteristic field width that grows rapidly with temperature. This is illustrated in panels c and d, showing color scale plots of dρ_xy /dH as a function of field and temperature, produced by interpolating numerical derivatives of the data shown in panels a and b. The large decrease in gradient is preceded by a much weaker increase in gradient seen in, for example, the 100 K data for fields between 6 and 9 T in panels a and b, and the black bands in panels c and d