Thermodynamic evidence for bulk charge-density wave rigidity in NbSe3

We show that the split wave form of the magnetic quantum oscillations obser ved in NbSe3 cannot be attributed to the Zeeman splitting of the Landau lev els. Instead, the behavior of the pronounced second harmonic is consistent with the "frequency doubling" effect that occurs when the oscillatory chemi cal potential interferes with a commensurate charge-density wave. Since the charge-density waves in NbSe3 are most definitely incommensurate, this low temperature rigidity must be caused by pinning mechanisms that involve the bulk of the sample.