Thermopower and Nernst effect in graphene in a magnetic field

We report measurements of the thermopower S and Nernst signal S-yx in graphene in a magnetic field H. Both quantities show strong quantum oscillations vs the gate voltage V-g. Our measurements for Landau levels of index n not equal 0 are in quantitative agreement with the edge-current model of Girvin and Jonson (GJ). The inferred off-diagonal thermoelectric conductivity alpha(xy) is consistent in magnitude with GJ's prediction. At the Dirac point (n=0), however, the width of the peak in alpha(xy) is very narrow. We discuss the physical meaning of the peak width and the puzzle raised by alpha(xy) for states at the Dirac point.