Effects of K and Ca doping on twin boundary energy of cupperate superconductors

Highlights: • Ab initio simulation for twin boundary energy in YBCO and doped YBCO system for the 1st time. • Local charge transfer and charge redistribution on the twin planes. • Structural and electronic properties of YBKCO and YCBCO systems. • Effect of Ca and K doping on twin boundary. • This explain the effect of twin boundaries in agreement with experimental data. • Structural and electronic properties of twined YBKCO and YCBCO systems. - Abstract: Ab-initio calculations under GGA approximation have been employed to find out the effect Ba substitution 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 distribution 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 empirical reports of decreasing the critical temperature (T_c) by increasing the K(Ca) content. Although, the K doping reduces the γ value in YBa_2–xK_xCu₃O₇ system, after calcium doping it remains more or less unchanged. In addition, reduction of the carrier density occurs at twin boundary in CuO₂ layer for the substituted system with respect to the untwined YBCO system. Our results would be noticeable in conjunction with the experimentally reported twinned and alkali substituted superconductive properties of the YBCO samples.