High-pressure phase diagrams of FeSe1-xTex: correlation between suppressed nematicity and enhanced superconductivity

Abstract

The interplay among magnetism, electronic nematicity, and superconductivity is the key issue in strongly correlated materials including iron-based, cuprate, and heavy-fermion superconductors. Magnetic fluctuations have been widely discussed as a pairing mechanism of unconventional superconductivity, but recent theory predicts that quantum fluctuations of nematic order may also promote high-temperature superconductivity. This has been studied in FeSe_1-xS_x superconductors exhibiting nonmagnetic nematic and pressure-induced antiferromagnetic orders, but its abrupt suppression of superconductivity at the nematic end point leaves the nematic-fluctuation driven superconductivity unconfirmed. Here we report on systematic studies of high-pressure phase diagrams up to 8 GPa in high-quality single crystals of FeSe_1-xTe_x. When Te composition x(Te) becomes larger than 0.1, the high-pressure magnetic order disappears, whereas the pressure-induced superconducting dome near the nematic end point is continuously found up to x(Te) ≈ 0.5. In contrast to FeSe_1-xS_x, enhanced superconductivity in FeSe_1-xTe_x does not correlate with magnetism but with the suppression of nematicity, highlighting the paramount role of nonmagnetic nematic fluctuations for high-temperature superconductivity in this system.

Fig. 1. Evolution of structural parameters and resistivity with Te composition in single crystals of FeSe_1−xTe_x at ambient pressure.

a–c Lattice parameters and Te composition x(Te) determined by X-ray diffraction (XRD). Lattice constants c (a) and a (b) as well as the chalcogen height from the Fe plane (c) are shown as a function of x(Te). Solid line represents the linear x(Te) dependence (Vegard’s law). d Temperature dependence of the in-plane resistivity ρ normalized by the value at 200 K for 0 ≤ x(Te) ≲ 0.50. Each curve is shifted vertically for clarity. The resistive anomalies associated with the nematic transition temperature T_s (black arrows) are determined by the minimum (0 ≤ x(Te) ≲ 0.22) or the maximum (0.28 ≲ x(Te) ≲ 0.50) of the derivative curves dρ(T)/dT. e XRD intensity as a function of the scattering angle 2θ near the (220) Bragg peak measured at several temperatures for x ≈ 0.45. Each curve is shifted vertically for clarity. The black solid and dashed lines indicate the result of the two-peak fitting for the data at 13.3 K. f The orthorhombicity δ = (a_o − b_o)/(a_o + b_o) estimated from the two-peak fitting of (hk0) Bragg peaks for x(Te) ≈ 0.07, 0.20, 0.37, and 0.45 (circles, left axis) and the split width of (hkl) Bragg peaks for x(Te) ≈ 0.07 and 0.20 (triangles, right axis) as a function of temperature. We also plot additional width Δσ of single peak near the transition (crosses, right axis), estimated by the standard deviation σ of the single Gaussian fitting subtracted by the value at the maximum temperature measured for each sample.

Fig. 2. Temperature versus Te composition phase diagram of FeSe_1−xTe_x at ambient pressure.

a Nematic and superconducting transition temperatures as a function of x(Te). The blue and red circles represent the nematic (T_s) and superconducting (T_c) transition temperatures, respectively, determined by the resistivity measurements. The light blue triangles represent T_s, determined by the splitting of the Bragg peaks in the XRD measurements. The black line is a least squares x(Te)-linear fit to the T_s data from the resistivity measurements. The color shades for the nematic and superconducting (SC) states are the guides to the eyes. Error bars represent the uncertainty in determining T_s from the data in Fig. 1f. b The same as in a, but the temperature range is 0–20 K. c Dependence of ρ(200 K)/ρ(15 K) on x(Te) extracted from the resistivity data.

Fig. 3. Evolution of the temperature dependence of resistivity with pressure.

Temperature dependence of resistivity in FeSe_1−xTe_x below 100 K at different pressures up to 8 GPa for x(Te) ≈ 0.04 (a), 0.06 (b), 0.10 (c), 0.14 (d), 0.18 (e), 0.21 (f), 0.38 (g), and 0.50 (h). The data are vertically shifted for clarity. The resistive anomalies at transition temperatures T_s (blue), T_m (green), and T_c (red) are indicated by arrows.

Fig. 4. Temperature–pressure phase diagrams in FeSe_1−xTe_x.

Pressure dependence of T_c, T_s, and T_m indicated by red, blue, and green circles, respectively, for x(Te) ≈ 0.04 (a), 0.06 (b), 0.10 (c), 0.14 (d), 0.18 (e), 0.21 (f), 0.38 (g), and 0.50 (h). The color shades for the nematic, spin density wave (SDW), and superconducting (SC) states are the guides to the eyes. The error of pressure for P < 2 GPa is relatively large (see error bars for 1 GPa) compared to higher pressures. The errors of T_s are estimated from the least squares fit in Fig. 2a. i Three-dimensional electronic phase diagram, temperature versus pressure and Te concentration x(Te), of FeSe_1−xTe_x, combined with the reported T–P–x(S) phase diagram of FeSe_1−xS_x (0 ≤ x(S) ≲ 0.17). The surface plot shows T_c and the purple and white circles represent T_s and T_m, respectively. The red circles represent T_m of FeSe. The gray and purple shadowed areas indicate the magnetic and nematic phases, respectively.