Evolution of ground state and upper critical field inR1−xGdxNi2B2C(R=Lu,Y): Coexistence of superconductivity and spin-glass state

Abstract

We report effects of local magnetic moment, ${\text{Gd}}^{3+}$, doping $(x\ensuremath{\lesssim}0.3)$ on superconducting and magnetic properties of the closely related ${\text{Lu}}_{1\ensuremath{-}x}{\text{Gd}}_{x}{\text{Ni}}_{2}{\text{B}}_{2}\text{C}$ and ${\text{Y}}_{1\ensuremath{-}x}{\text{Gd}}_{x}{\text{Ni}}_{2}{\text{B}}_{2}\text{C}$ series. The superconducting transition temperature decreases and the heat capacity jump associated with it drops rapidly with Gd doping; qualitative changes with doping are also observed in the temperature-dependent upper critical field behavior, and a region of coexistence of superconductivity and spin-glass state is delineated on the $x\text{\ensuremath{-}}T$ phase diagram. The evolution of superconducting properties can be understood within Abrikosov-Gor'kov theory of magnetic impurities in superconductors taking into account the paramagnetic effect on upper critical field with additional contributions particular for the family under study.