The stripe state at 1/8 Ba doping hosts optimal superconductivity in La-214 cuprates under low in-plane stress

The cuprate system La 2− x Ba x CuO 4 (LBCO) exhibits a pronounced sensitivity to in-plane uniaxial stress, particularly near the 1/8 doping anomaly, where stripe order strongly suppresses bulk superconductivity. While previous studies have focused on compositions close to 0.125, the commensurate x =0.125 phase remains largely unexplored under symmetry-selective lattice tuning. Here, we combine muon-spin rotation ( μ SR), AC susceptibility, and electrical resistivity to investigate superconductivity, spin-stripe order, and structural response in LBCO-0.125 under in-plane uniaxial stress applied 45 ∘ to the Cu-O bond direction. Complementary resistivity measurements on x =0.115 and 0.135 track the evolution across both sides of the anomaly. We observe a giant enhancement of the bulk superconducting transition temperature in LBCO-0.125, increasing from 5 K to 37 K under 0.5 GPa. While the onset temperature of spin-stripe order decreases only modestly, the magnetic volume fraction is reduced by about a factor of two, with local order preserved. Simultaneously, the resistivity peak associated with the LTT phase is fully suppressed across all dopings. These results demonstrate that suppression of the LTT phase and reduction of the static spin-stripe-ordered volume fraction are crucial for the development of optimal three-dimensional superconductivity. Strikingly, the composition x =0.125, with the most robust stripe stability and the lowest ambient-pressure T c develops the highest T c under stress, reaching a zero-resistance state at 37 K and an onset of the superconducting transition as high as 46 K. Uniaxial stress suppresses the lattice anisotropy that pins stripes, promoting a more spatially uniform superconducting state. These findings provide important experimental constraints on the microscopic interplay between stripe order and superconductivity and are consistent with one possible scenario in which static stripe order primarily limits global phase coherence.