Resistivity, bandstructure and superconductivity of DHCP and FCC La under pressure

Abstract

Electronic and superconducting properties of DHCP and FCC La are studied both by measurements of the electrical resistance as a function of temperature and pressure and by calculations of the band structure, Fermi surface properties and the electron-phonon interaction lambda . The importance of a hydrostatic environment for reliable resistance measurements is stressed. The transport properties of both phases are similar with small and negative values of the pressure coefficient of resistance. In contrast, the band structure calculations show large differences between the two phases, particularly in the pressure dependence, e.g. of the density of states (DOS) and the plasma frequency. Two new remarkable properties of La have been found, adding to previously known anomalies. One is a strong temperature dependence of the pressure coefficient of resistance which decreases with increasing temperature in both phases. The other is a strong pressure dependence of the plasma frequency in the FCC phase, which is larger than in any other element known to us. From these results the pressure dependence of lambda is obtained by two independent methods for each phase. For FCC, both methods give d ln lambda /dp=0.05+or-0.02 GPa-1, while for DHCP, a decrease of lambda with pressure or an almost pressure independent lambda is suggested. These results cannot fully explain the observed Tc(p) in the FCC phase and fail completely in the DHCP phase. The contribution from spin fluctuations is calculated approximately and found to account qualitatively for this discrepancy. In particular, the DOS decreases slowly with pressure in the FCC phase which gives some decrease of the spin fluctuation contribution in this phase, while in the DHCP phase the DOS decreases strongly, leading to suppression of the spin fluctuations under pressure.