Signature of T$_\textrm{c}$ above 111 K in Li-doped (Bi,Pb)-2223 superconductors: synergistic nature of hole concentration, coherence length and Josephson interlayer coupling

Abstract

Understanding the bottleneck to drive higher critical transition temperature $T_\textrm{c}$ plays a pivotal role in the underlying study of superconductors. We systematically investigate the effect of Li$^+$ substitution for Cu$^{2+}$ cations on the $T_\textrm{c}$, hole concentration, coherence length and interlayer coupling, and microstructure in Li-doped Bi$_{1.6}$Pb$_{0.4}$Sr$_2$Ca$_2$Cu$_3$O$_{10 + \delta}$ or (Bi,Pb)-2223 compound. Remarkably, we demonstrate by utilizing a long-time sintering accompanied by a multiple recurrent intermediate stages of calcining and pressing within our renovated solid-state reaction method, the optimal Li-doped (Bi,Pb)-2223 samples achieve the well-enhanced $T_\textrm{c}$ of 111--113.8 K compared with the standard value of 110 K. We evince the superconducting mechanism that the substitution of Li$^{+}$ for Cu$^{2+}$ ions on the CuO$_2$ layers causes augmenting the hole concentrations and promotes the correlation between the overdoped outer and the underdoped inner CuO$_2$ planes, and thus effects improve $T_\textrm{c}$. Following a universal quadratic relation between $T_\textrm{c}$ and hole concentration, a new higher optimal hole concentration is provided. Additionally, by analyzing the Aslamazov-Larkin and Lawrence-Doniach theories on the reliable excess conductivity data near the critical temperature, we observe the strong effect of Li-doping on the system. The coherence length steadily increases versus the Li-doped content, while the Josephson interlayer coupling strength between the CuO$_2$ layers almost remains a constant for the whole series of Li-doping. Our findings establish an insightful roadmap to improve the critical temperature and intrinsic superconducting properties in the Bi-2223 compounds through the doping process.