Strain induced modulation of superconducting gap in Fe(Se,Te)

Fe-based high-temperature superconductors often combine superconductivity, structural distortions, and electronic nematicity, yet their interplay is not always evident. For FeTe_1-x Se_x , electronic nematicity develops near the critical composition (x≈0.45), coinciding with the maximum superconducting transition temperature. Spectroscopic-imaging STM measurements revealed that superconducting coherence peaks are locally reduced in regions of strong static nematic order, while nanoscale strain can pin nematic fluctuations and promote their localization [1]. These findings demonstrate direct competition between superconductivity and nematicity, with strain acting as a key tuning parameter of the electronic ground state. While previous work showed a reduction of the coherence peaks, the gap size itself did not seem affected, though this is difficult to determine at 4.5 K due to significant thermal broadening. In our experiments, performed at 300 mK, we observe that the superconducting gap can in fact be entirely suppressed along strain lines, whereas unstrained regions display a well-defined, fully formed gap. Specifically, the gap size is modulated by the spacing between strain lines, with shorter distances leading to smaller gaps. By applying the Lawler–Fujita method [2], we determine the strength and orientation of the surface strain to establish a relation between strain and superconductivity.

1. Zhao H. et al. Nat. Phys. 17, 903–908 (2021)

2. Lawler M. et al. Nature 466, 347–351 (2010)