Researchers used the sharp tip of a scanning tunneling microscope to press very slightly on a single layer of iron selenide (FeSe) grown on strontium titanate (SrTiO3). That tiny, controlled strain pushed an electron band called dz2 closer to the energy level where electrons can form Cooper pairs. As a result, the superconducting energy gap increased in two clear steps, from 17.8 milli‑electronvolts to 23.6 meV for the main gap — an increase of about 32 percent.

The team changed the tip-sample distance by varying the tunneling current in the microscope and read out the local electronic spectrum with scanning tunneling spectroscopy (STS). At large tip distance (low current) they saw the usual U-shaped superconducting gap. As the tip moved closer (current increased from about 100 picoamperes up to several nanoamperes), the gap became larger. The gap evolution showed two stages. Below about 1.25 nanoamperes the gap rose slowly. Above that current there was a sharper, stronger increase in gap size. The inner and outer gap features rose by roughly 39% and 32% overall, respectively.

To understand why the gap changed, the authors tracked features in the STS spectra that they associate with specific iron d-orbital bands. Orbitals are simply different shapes that an electron’s probability cloud can take around an atom; the labels dz2, dxy and dxz/yz name those shapes. The dz2 feature normally lies far below the energy that matters for pairing. Under tip-induced tensile strain the dz2 feature moved up in energy toward the Fermi level. At larger strain it mixed, or hybridized, with a relatively flat dxy band near −80 meV. That band reconstruction coincided with the second, stronger jump in the superconducting gap. The authors also report that Fermi wave vectors stayed the same, which argues the effect comes from reshaping and renormalizing bands and correlations, not from adding or removing charge carriers.