Theoretical model for the superconducting and magnetically ordered borocarbides

We present a theory of superconductivity in the presence of a general magne tic structure in a form suitable For the description of complex magnetic ph ases encountered in borocarbides. The theory, complemented with some detail s of the band structure and with the magnetic phase diagram, may explain th e nearly reentrant behavior and the anisotropy of the upper critical field of HoNi2B2C. The onset of the helical magnetic order depresses superconduct ivity via the reduction of the interaction between phonons and electrons ca used by the formation of magnetic Bloch states. At mean field level, no add itional suppression of superconductivity is introduced by the incommensurab ility of the helical phase.