Twin-Boundary Engineering in FeTe _{1– <i>x</i>} Se _<i>x</i> Superconductor

Defect engineering has long been a universal and effective way for tailoring the physical properties of quantum systems. Among various types of defects, twin boundaries represent a structurally coherent and low-energy class. However, the role of twin-boundary defects in two-dimensional (2D) superconducting systems remains largely unexplored, primarily due to challenges in obtaining high-quality 2D twin crystals. Unlike other defect structures that can be introduced through postprocessing, the formation of 2D twin boundaries generally relies on precisely controlled synthesis strategies. In this study, we employed a salt-assisted chemical vapor deposition (CVD) method to successfully synthesize high-quality FeTe1–xSex twin crystals, a promising iron-based superconductor. Low-temperature quantum transport measurements revealed distinct electrical transport behaviors across twin boundaries. Notably, we observed pronounced nonreciprocal transport behavior in this noncentrosymmetric quantum system, which can be attributed to in-plane polarization at twisted twin boundaries. Furthermore, we demonstrated that the synergistic contribution of vortex motion and paraconductivity enhances this nonreciprocal effect, thus affecting the superconducting transition. Our findings provide key insights into the impact of twin boundaries with inversion-symmetry breaking in governing nonreciprocal transport, establishing a promising platform for exploring unconventional superconductivity.