Two-dimensional superconducting diode effect in topological insulator/superconductor heterostructure

Abstract: The superconducting diode effect (SDE) is characterized by the nonreciprocity of Cooper-pair motion with respect to current direction. In three-dimensional (3D) materials, SDE results in a critical current that varies with direction, making the effect distinctly observable: the material exhibits superconductivity in one direction while behaving as a resistive metal in the opposite direction. However, in genuinely two-dimensional (2D) materials, the critical current density is theoretically zero, leaving the manifestation of SDE in the 2D limit an intriguing challenge. Here, we present the observation of SDE in a heterostructure composed of the topological insulator Bi$_2$Te$_3$ and the iron based superconductor Fe(Se,Te) $-$a candidate for topological superconductor$-$ where superconductivity is confined to the 2D limit. The observed I-V characteristics reveal nonreciprocity in the vortex-creep regime, where finite voltages arise due to the two-dimensional nature of superconductivity. Furthermore, our 2D film demonstrates abrupt voltage jumps, influenced by both the current flow direction and the transverse magnetic field direction. This behavior resembles that of 3D materials but, in this case, is driven by the vortex-flow instability, as illustrated by voltage controlled S-shaped I-V curves. These results underscore the pivotal role of vortex dynamics in SDE and provide new insights into the interplay between symmetry breaking and two-dimensionality in topological insulator/superconductor systems.