First-principles calculation of the superconducting transition in (formula presented) within the anisotropic Eliashberg formalism

We present a study of the superconducting transition in (formula presented) using the ab initio pseudopotential density-functional method, a fully anisotropic Eliashberg equation, and a conventional estimate for (formula presented) Our study shows that the anisotropic Eliashberg equation, constructed with ab initio calculated momentum-dependent electron-phonon interaction and anharmonic phonon frequencies, yields an average electron-phonon coupling constant (formula presented) a transition temperature (formula presented) and a boron isotope-effect exponent (formula presented) The calculated values for (formula presented) (formula presented) and (formula presented) are in excellent agreement with transport, specific-heat, and isotope-effect measurements, respectively. The individual values of the electron-phonon coupling (formula presented) on the various pieces of the Fermi surface, however, vary from 0.1 to 2.5. The observed (formula presented) is a result of both the raising effect of anisotropy in the electron-phonon couplings and the lowering effect of anharmonicity in the relevant phonon modes.