Spin fluctuations, electron-phonon coupling and superconductivity in near-magnetic elementary metals; Fe,Co,Ni and Pd

Abstract

An investigation of possibilities for superconductivity mediated by spin fluctuations in some elementary metals is motivated by the recent discovery of superconductivity in the hcp high-pressure phase of iron. The electronic structure, the electron-phonon coupling ($λ_{ph}$) and the coupling due to spin-fluctuations ($λ_{sf}$) are calculated for different phases and different volumes for four elementary metals. The results show that such possibilities are best for systems near, but on the non-magnetic side of, a magnetic instability. Fcc Ni, which show stable magnetism over a wide pressure range, is not interesting in this respect. Ferro- and antiferro-magnetic fluctuations in hcp Fe contribute to a relatively strong coupling in the pressure range where superconductivity is observed. The absence of fluctuations at large q-vectors makes fcc Pd only moderately interesting despite its large exchange enhancement for q=0. Fcc Co at high pressure ($\sim$ 0.5 Mbar) behaves as an improved version of Pd, where the fluctuations extend to larger q. The estimations of T$_C$, which reproduce the experimental situation in Fe quite well, suggest a measurable T$_C$ for the high-pressure phase of fcc Co, while the estimate is lower for the ambient-pressure phase of fcc Pd.