Giant negative phonon drag thermopower in pure bismuth

In contrast with previous reports, an important negative phonon drag hump is observed in a large binary bismuth single crystal. Without questioning the theory, this result is compared with the existing data showing the extreme sensitivity of phonon drag to size in the liquid helium range. This, together with the marked anisotropy of the transport properties, should be taken into account to explain the apparent discrepancy between the published experimental data. It is suggested that the phonon drag hump is negative for an infinite sample.