Superconducting Tc and Electron-Phonon Coupling in Nb to 132 GPa: Magnetic Susceptibility at Megabar Pressures

We have measured superconducting Tc using a highly sensitive magnetic susceptibility technique to megabar pressures (>100GPa). We observed anomalies in Tc(P) for Nb at 5--6 GPa and 60--70 GPa, at which pressure Tc increases by 0.7 K and decreases by about 1 K, respectively. In contrast, Tc in Ta remains nearly constant up to 45 GPa. We suggest that the anomalies in Nb arise from stress-sensitive electronic topological transitions. Between 70 and 132 GPa, Tc for Nb drops continuously to 4.7 K, which is related to the decrease in density of states at the Fermi level with increasing pressure.