Second-harmonic voltage response in high-temperature superconductor Bi₂Sr₂CaCu₂O8+δ at zero magnetic field

The second-harmonic voltage response in superconductors represents an unconventional transport phenomenon that signals critical current asymmetry and potential superconducting diode effects. In this work, we systematically investigate this response in optimally doped cuprate high-temperature superconductor Bi 2 Sr 2 CaCu 2 O 8+δ (BSCCO) under zero magnetic field. Across 20-64 K, a sharp second-harmonic voltage extremum (V peak 2ω ) emerges near the critical current, reaching ~85 μV, providing evidence of intrinsic critical current asymmetry in zero-field BSCCO. Through numerical simulations correlating V peak 2ω with diode efficiency η , we identify a superconducting diode effect with maximum efficiency | η |≈ 3% at 48 K. Our work delivers experimental evidence of intrinsic critical current asymmetry in zero-field BSCCO, while pioneering second-harmonic voltage analysis as a transformative diagnostic technique for emergent nonreciprocal quantum phenomena. This approach enables quantitative assessment of superconducting diode efficiencies in regimes where conventional transport fails, accelerating the realization of high-temperature superconducting integrated circuits.