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

Abstract

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$^\circ$ 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 $T_{\rm c}$ develops the highest $T_{\rm c}$ under stress, reaching a zero-resistance state at 37 K and an onset of the superconducting transition as high as 46 K. This behavior indicates that stripe-related interactions enhance pairing strength, while static stripe order competes with superconductivity primarily at the level of phase coherence rather than pairing itself.

Figures (4)

• Figure 1: Uniaxial Stress Tuning of the LBCO Phase Diagram. (a-b) Crystal Structure Schematics of LBCO Showing the High-Temperature Tetragonal (HTT), Low-Temperature Orthorhombic (LTO), Low-temperature less Orthorhombic (LTLO) and Low-Temperature Tetragonal (LTT) Phases. (c-d) Comparison of the Temperature–Doping Phase Diagram of LBCO at Zero Stress and Under 0.23 GPa In-Plane Stress.
• Figure 2: Superconducting and spin-stripe response under in-plane stress for LBCO-0.125. (a) Schematic view of the uniaxial stress holder for ${\mu}$SR experiments with the sample mounted. The view is shown from the direction of the incoming muon beam. The rectangular crystal is glued along the stress axis. The compressive stress was applied at an angle of 45$^{o}$ to the Cu–O bond direction (denoted as [100]). Hematite pieces are positioned to mask the holder-frame regions exposed to the muon beam, thereby reducing background contributions from the pressure cell and ensuring that the ${\mu}$SR signal predominantly originates from the sample. (b) Temperature dependence of the diamagnetic susceptibility measured under various in-plane uniaxial stress conditions up to 0.32 GPa. (c) The zero-field ${\mu}$SR spectra, recorded at the base temperature 700 mK under various stresses. (d) The temperature dependence of the spin-stripe–ordered volume fraction measured under various in-plane uniaxial stress conditions up to 0.32 GPa.
• Figure 3: Electrical resistivity response of LBCO under in-plane uniaxial stress. (a–c) Temperature dependence of the in-plane electrical resistance, normalized to its value at 100 K, measured under various in-plane uniaxial stress conditions for three different dopings: $x$=0.115 (a), 0.125 (b), and 0.135 (c). The arrow marks the peak originating from the LTT structural phase transition.(d–f) Stress dependence of the superconducting transition temperature, defined by the onset of zero resistance, and of the normal-state resistivity peak height for the three dopings: $x$=0.115 (d), 0.125 (e), and 0.135 (f). For the $x$=0.125 and $x$=0.135 samples, a small hump just above $T_{\rm c}$ remains visible even at the highest applied stress, amounting to only a few percent. However, in panels (e) and (f), it was set to zero.
• Figure 4: Temperature–Stress Phase Diagram for LBCO-0.125. (a)Stress dependence of the spin-stripe ordering temperature $T_{\rm so}$, the 2D superconducting transition temperature $T^{R}_{\rm c,zero}$ defined by the onset of zero in-plane resistance, and the 3D superconducting transition temperature $T^{AC}_{\rm c,mid}$ determined from the midpoint of the diamagnetic susceptibility transition. (b) Stress dependence of the magnetic volume fraction $V_{\rm M}$ and the normal-state resistivity peak height ${\Delta}R_{\rm LTT}$ associated with the LTT phase. (c) Stress dependence of the internal magnetic field $B_{\rm int}$. (d) We compare the superconducting transition temperature $T_{\rm c}(x)$ of LSCO at ambient stress with $T_{\rm c}(x)$ of LBCO under applied uniaxial stress tuned to the condition of optimal superconductivity. The onset temperatures of superconductivity, determined from both susceptibility and resistivity measurements, are shown. This highlights that LBCO near $x$${\simeq}$ 1/8 develops an onset $T_{\rm c}$ that exceeds that of LSCO under ambient conditions. The dashed line marks the $x$=0.125 doping level, at which superconductivity is most strongly suppressed in LBCO under ambient conditions. The data for LSCO is taken from Ref. PhysRevB.40.2254.