Enhanced Critical Currents and Irreversibility Fields in YBa$_2$Cu$_4$O$_8$ Films through Ca-Substitution
Year: 2026
Type: preprint
Abstract: The intrinsic carrier concentration of superconducting materials may not be optimal, and different superconducting properties may have distinct optimal doping levels. Here, we show that Ca substitution increases the hole concentration in intrinsically underdoped epitaxial (Y$_{1-x}$Ca$_x$)Ba$_2$Cu$_4$O$_8$ (YCa124) films with $x \leq 0.10$, driving YCa124 toward optimal doping, increasing $T_c$ and $J_c$. At 40 K, 10% Ca doping increases $J_c$ by factors of 2.4 and 10 at 0.03 and 6 T, respectively, compared with the undoped film. The field dependence of $J_c$ follows $J_c \propto B^{-0.5}$, independent of Ca content, indicating that Ca substitution does not alter the dominant pinning mechanism. This field dependence is consistent with pinning by planar Y-125 stacking-fault intergrowths observed by electron microscopy. In addition, Ca substitution slows thermally activated vortex motion (creep) at $T \gtrsim 15$ K, while at lower temperatures the creep rate $S$ shows an unusual decrease with increasing field. Finally, we consider how doping tunes the upper bound on $J_c$, set by the depairing current density $J_d$, and the lower bound on creep, set by the Ginzburg parameter $G_i$. We find positive correlations between $J_c$ and $J_d$, and between $S$ and $G_i$, and compare them with trends across a broad range of superconductors.
Source: arXiv (Cornell University)
Language: English
Subfield: Condensed Matter Physics
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: green
Awards: G1739289980, Influence of quantum critical point on depairing current density and critical current density, G4515799968, MRSEC: UW Molecular Engineering Materials Center, CAREER: Skyrmion-Vortex Interactions in Ferromagnet-Superconductor Heterostructures, 新材料設計指針に基づく元素戦略高温超電導線材の創製, G8874625583
OpenAlex ID: W7214893239