  In an electric field, the flow of electrons in a solid produces an entropy
current in addition to the familiar charge current. This Peltier effect
underlies all thermoelectric refrigerators. The upsurge in thermoelectric
cooling applications has led to a search for more efficient Peltier materials
and to renewed theoretical interest in how electron-electron interaction may
enhance the thermopower $Q$ of materials such as the transition-metal oxides
\cite{Mahan,Beni,Kotliar,Chaikin}. An important factor in this enhancement is
the electronic spin entropy, which is predicted \cite{Chaikin,Kwak,KwakChaikin}
to dominate the entropy current. Here we report evidence for such suppression
in the layered oxide $\rm Na_xCo_2O_4$, and present evidence that it is a
strong-correlation effect.
