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Spin entropy as the likely source of enhanced thermopower in NaxCo2O4


In an electric field, the flow of electrons in a solid produces an entropy current in addition to the familiar charge current. This is the Peltier effect, and it underlies all thermoelectric refrigerators. The increased interest in thermoelectric cooling applications has led to a search for more efficient Peltier materials and to renewed theoretical investigation into how electron–electron interaction may enhance the thermopower of materials such as the transition-metal oxides1,2,3,4. An important factor in this enhancement is the electronic spin entropy, which is predicted4,5,6 to dominate the entropy current. However, the crucial evidence for the spin-entropy term, namely its complete suppression in a longitudinal magnetic field, has not been reported until now. Here we report evidence for such suppression in the layered oxide NaxCo2O4, from thermopower and magnetization measurements in both longitudinal and transverse magnetic fields. The strong dependence of thermopower on magnetic field provides a rare, unambiguous example of how strong electron–electron interaction effects can qualitatively alter electronic behaviour in a solid. We discuss the implications of our finding—that spin-entropy dominates the enhancement of thermopower in transition-metal oxides—for the search for better Peltier materials.

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Figure 1: The temperature (T) dependence of magnetic and transport properties of single-crystal NaxCo2O4 and electronic states in the Co ions.
Figure 2: The in-plane thermopower Q versus an in-plane H(- T) at selected T.
Figure 3: The relative change in Q versus a transverse field Hc in NaxCo2O4.
Figure 4: The suppression of the thermopower by an in-plane magnetic field in NaxCo2O4.


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We thank S. Hannahs for technical assistance. We acknowledge support from the US National Science Foundation (NSF). Some of the measurements were performed at the US National High Magnetic Field Laboratory, which is supported by the NSF and the state of Florida.

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Correspondence to N. P. Ong.

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Wang, Y., Rogado, N., Cava, R. et al. Spin entropy as the likely source of enhanced thermopower in NaxCo2O4. Nature 423, 425–428 (2003).

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