ANISOTROPY OF ELECTRON-PHONON INTERACTION AND SUPERCONDUCTING ENERGY GAP IN NIOBIUM

Abstract

Publisher Summary Despite a great deal of effort devoted to the study of the electron–phonon interaction, our knowledge of anisotropy effects is rather meager. Much of our experimental information comes from tunneling experiments that measure the electron–phonon spectral function and from inversion of empirical T equations. In transition metals the comparison of theory, to a large extent, reaches its natural limit. This chapter presents data on the anisotropy of the electron–phonon coupling in Nb. Accurate and complete de Haas–van Alphen (dHvA) measurements and sophisticated band theory of the Fermi surface are done. In addition to obtaining information on the electron–phonon interaction, a great deal about anisotropic superconducting properties is also inferred. The direct connection between the energy gap and the electron–phonon coupling strength is also discussed. The data that supports a multiple gap model for Nb is also interpreted. Two gaps of widely different magnitudes that exist in Nb are used to explain tunneling, heat capacity, ultrasonic attenuation, and thermal conductivity data.