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Anomalous Magnetothermopower in a Metallic Frustrated Antiferromagnet

We report the temperature $T$ and magnetic field $H$ dependence of the thermopower $S$ of an itinerant triangular antiferromagnet PdCrO$_2$ in high magnetic fields up to 32 T. In the paramagnetic phase, the zero-field thermopower is positive with a value typical of good metals with a high carrier density. In marked contrast to typical metals, however, $S$ decreases rapidly with increasing magnetic field, approaching zero at the maximum field scale for $T >$ 70 K. We argue here that this profound change in the thermoelectric response derives from the strong interaction of the 4$d$ correlated electrons of the Pd ions with the short-range spin correlations of the Cr$^{3+}$ spins that persist beyond the Néel ordering temperature due to the combined effects of geometrical frustration and low dimensionality.