dHvA effect of Nb in low fields down to H_c2 Comment on the dHvA effect in the superconducting mixed state

The dHvA effect experiments on Nb have been performed in low fields down to H_c2 using a standard field modulation method. The signal amplitude becomes much smaller below H_c3 than that expected from a straight extension of the Dingle plot from the higher fields and is below the noise level around and below H_c2. The rapid decrease of the amplitude below H_c3 is simply understood as arising from the screening of the modulation field and is expected to be observed for other homogeneous type-II superconductors. The present observation is inconsistent with those reported for materials such as NbSe₂, where the same or comparable amplitudes to those in the normal state are detected below H_c2 by the field modulation method.