Effect o K and Ca doping on twin boundary energy of cupperate superconductors

چکیده مقاله

Ab-initio calculations under GGA approximation have been employed to find out the effect Ba substitu- tion by K and Ca on the structural and electronic properties twined and untwined YBCO system. In this regard, the twin boundary energy, γ, and impact of the substitution on the boundary’s charge distribu- tion have been of special consideration. Our results show that despite the structural changes the presence of K (Ca) modifies substantially density of levels at the Fermi level, which could be responsible for em- pirical reports of decreasing the critical temperature (T c ) by increasing the K(Ca) content. Although, the K doping reduces the γvalue in YBa 2 –x K x Cu 3 O 7 system, after calcium doping it remains more or less unchanged. In addition, reduction of the carrier density occurs at twin boundary in CuO 2 layer for the substituted system with respect to the untwined YBCO system. Our results would be noticeable in con- junction with the experimentally reported twinned and alkali substituted superconductive properties of the YBCO samples. ©