Fig. 1

Tuning superconductivity in LiFeAs by uniaxial strain. a Ball model of the LiFeAs unit cell under positive uniaxial strain along the [110] direction. Dashed open circles represent the atomic positions in the unstrained unit cell, red and blue arrows indicate strain direction along [110] and [100]. b Fermi surface of unstrained LiFeAs. Arrows indicate nesting vectors Q between hole bands at the zone centre and electron bands at the zone corner. c Topographic image of LiFeAs strained along [100] (setpoint conditions V_s = 20 mV, I_s = 50 pA, T = 4 K). A blue arrow indicates the strain direction. Scale bar = 4.3 nm. Inset, sample holder for in-situ strain tuning (without sample). An arrow indicates the direction of strain. d dI/dV spectra obtained at the position marked with a cross in (c) with strain along [100] at different voltages V_strain applied to the piezo stack, showing the superconducting gap (V_s = 14 mV, I_s = 0.5 nA, T = 4.2 K, spectra are normalised at V = 14 mV). Brown-red dashed vertical lines indicate the positions of the coherence peaks for the spectrum obtained at V_strain = −300 V, black dashed vertical lines for unstrained LiFeAs. e Plot of gap size Δ_SC versus V_strain extracted from d (see also Supplementary Note 1, Supplementary Figure S7). The number near each data point indicates the order in which the spectra were taken. Error bars of the gap size are obtained from the non-linear least squares fit to the experimental data in d and represent the 1σ confidence interval. For unstrained LiFeAs, Δ_SC = 5.8 meV at T = 4.2 K, outside the range of the graph. f dI/dV spectra on samples strained along [110]. Spectra from bottom to top are shown in order of increasing strain. The bottom curve is for an unstrained crystal. All spectra are normalised at V = 15 mV and vertically offset for clarity. Vertical lines indicate the energy of the coherence peaks for the unstrained crystal. Horizontal lines in d, f indicate zero conductance for each of the spectra