Hall effect in the mixed state of Nd_1.85Ce_0.15CuO_4-δ

We observe a sign reversal of the Hall resistivity in the mixed state of the n-type superconductor Nd_1.85Ce_0.15CuO_4-δ. Such an anomaly, which has also been observed earlier in the p-type oxide superconductors, has been widely attributed to thermoelectric effects, pinning effects, or else to complicated band structures. Recently, based on the systematics of the sign reversal in a wide class materials, there have been attempts to explain the anomaly as an intrinsic aspect of the vortex motion. The behavior of the Hall resistivity in Nd_1.85Ce_0.15CuO_4-δ serves as a test for some of the models mentioned above. Particularly, a model based on thermoelectric effects which has been suggested to explain the sign reversal in the p-type superconductor YBa₂Cu₃O_7-δ does not predict the sign change in Nd_1.85Ce_0.15CuO_4-δ. The observation of the Hall anomaly in this n-type systems supports the idea that the sign reversal is intrinsically linked to the vortex dynamics in the flux flow state.