Fig. 2.

Current-magnitude and temperature dependence of nonreciprocal transport. a The current-magnitude dependence of second harmonic resistance R^2ω at T = 9.5 K, and B = 0.5 K deduced from the data shown in b. The black dotted line is the fitting line. b The magnetic field dependence of R^2ω at T = 9.5 K measured under I = 40, 80, 120, 160, and 200 μA. c The magnetic field dependence of R^2ω/R^ω measured under I = 200 μA at T = 6.9, 7.2, 7.5, 7.8, 8.1, 8.5, 9, 9.5, and 10 K. d The temperature dependence of resistance measured under I = 200 μA. The black curve is the fitting of the Berezinskii–Kosterlitz–Thouless (BKT) transition using Halperin–Nelson formula, $R=R_0\exp \left(-2b{\left(\frac{T_{\mathrm{c}0}-T}{T-T_{\mathrm{BKT}}}\right)}^{0.5}\right)$, where R₀ and b are material parameters. T_c0 is the temperature at which the finite amplitude of the order parameter develops and T_BKT is the BKT transition temperature. The fitting gives the values, T_c0 = 10.7 K and T_BKT = 6.0 K. The blue, green, and red regions correspond to normal, intermediate, and superconducting regions, respectively. e The temperature dependence of γ-value measured under I = 200 μA derived from c. The red points are the measurement on Bi₂Te₃(15 nm)/FeTe(18 nm) sample (denoted as BT(15 nm)/FT). The green point are the measurement on Bi₂Te₃(1.5 nm)/FeTe(18 nm) sample (denoted as BT(1.5 nm)/FT). Note that all the measurements, except for Fig. 1b, and the green curve of Fig. 2e, are done on the BT(15 nm)/FT sample. The purple curve is the fitting of the red points with the formula γ = β${\left(T-T_{\mathrm{BKT}}\right)}^{-1.5}$, where β = 5.3 × 10⁻³ T⁻¹ A⁻¹ m. Note that the BKT model and the fitting is valid only at around T_BKT, which is represented by the solid purple curve. The purple dotted curve is out of the applicable range of theory