Enhancement of the in-fieldJcofMgB2viaSiCl4doping

Abstract

We present the following results. (1) We introduce a doping source for ${\text{MgB}}_{2}$, liquid ${\text{SiCl}}_{4}$, which is free of C, to significantly enhance the irreversibility field $({H}_{\text{irr}})$, the upper critical field $({H}_{c2})$, and the critical current density $({J}_{c})$ with a little reduction in the critical temperature $({T}_{c})$. (2) Although Si can not be incorporated into the crystal lattice, a significant reduction in the $a$-axis lattice parameter was found, to the same extent as for carbon doping. (3) Based on the first-principles calculation, it is found that it is reliable to estimate the C concentration just from the reduction in the $a$-lattice parameter for C-doped ${\text{MgB}}_{2}$ polycrystalline samples that are prepared at high sintering temperatures, but not for those prepared at low sintering temperatures. Strain effects and magnesium deficiency might be reasons for the $a$-lattice reduction in non-C or some of the C-added ${\text{MgB}}_{2}$ samples. (4) The ${\text{SiCl}}_{4}$-doped ${\text{MgB}}_{2}$ shows much higher ${J}_{c}$ with superior field dependence above 20 K compared to undoped ${\text{MgB}}_{2}$ and ${\text{MgB}}_{2}$ doped with various carbon sources. (5) We introduce a parameter, RHH $({H}_{c2}/{H}_{\text{irr}})$, which can clearly reflect the degree of flux-pinning enhancement, providing us with guidance for further enhancing ${J}_{c}$. (6) It was found that spatial variation in the charge-carrier mean free path is responsible for the flux-pinning mechanism in the ${\text{SiCl}}_{4}$ treated ${\text{MgB}}_{2}$ with large in-field ${J}_{c}$.