Nonreciprocal Transport Reveals a Critical Boundary between Josephson and Pancake Vortex Dynamics

Highly anisotropic superconductors host distinct vortex excitations whose roles in transport can be diffcult to separate. In Bi₂Sr₂CaCu₂O_8+δ, an in-plane magnetic field generates Josephson vortices between CuO2 layers, whereas a finite out-of-plane field component nucleates pancake vortices in the superconducting planes. Although tilted magnetic fields are known to produce rich mixed vortex configurations, identifying which vortex subsystem dominates dissipation remains challenging with conventional transport. Here we show that nonreciprocal transport provides a symmetry-sensitive route to this problem in mechanically exfoliated Bi₂Sr₂CaCu₂O_8+δ flakes. Using second-harmonic transport under an ac current drive, we find a pronounced nonlinear response near Tc that flips sign upon reversing the in-plane magnetic field, while such sign-reversing nonreciprocal response is not resolved for out-of-plane fields. The nonlinear response is concentrated in the resistive superconducting transition region. We discuss its connection to Josephson-vortex dynamics qualitatively, using the field-orientation dependence and the competition between thermal fluctuations and pinning. Upon introducing a small out-of-plane tilt, the nonreciprocal signal abruptly collapses at a critical angle of about 2°-2.5°. The superconducting critical field also shows a rapid suppression when the field is tilted away from the ab plane, and its angular dependence remains broadly consistent with the anisotropic Ginzburg-Landau description. The much narrower collapse of the nonreciprocal response therefore cannot be attributed simply to an anomalous deviation of the superconducting critical field from conventional angular scaling. This sharp angular selectivity is consistent with a change in the vortex dynamics contributing to nonlinear transport and supports nonreciprocity as a symmetry-sensitive probe of vortex dynamics in mixed states.