Suppression of Superconductivity and Nematic Order in Fe_1–ySe_1–xS_x (0 ≤ x ≤ 1; y ≤ 0.1) Crystals by Anion Height Disorder

Abstract

Connections between crystal chemistry and critical temperature T c have been in the focus of superconductivity, one of the most widely studied phenomena in physics, chemistry, and materials science alike. In most Fe-based superconductors, materials chemistry and physics conspire so that T c correlates with the average anion height above the Fe plane, i.e., with the geometry of the FeAs 4 or FeCh 4 (Ch = Te, Se, or S) tetrahedron. By synthesizing Fe 1−y Se 1−x S x (0 ≤ x ≤ 1; y ≤ 0.1), we find that in alloyed crystals T c is not correlated with the anion height like it is for most other Fe superconductors. Instead, changes in T c (x) and tetragonal-to-orthorhombic (nematic) transition T s (x) upon cooling are correlated with disorder in Fe vibrations in the direction orthogonal to Fe planes, along the crystallographic c-axis. The disorder stems from the random nature of S substitution, causing deformed Fe(Se,S) 4 tetrahedra with different Fe−Se and Fe−S bond distances. Our results provide evidence of T c and T s suppression by disorder in anion height. The connection to local crystal chemistry may be exploited in computational prediction of new superconducting materials with FeSe/S building blocks.

Electron correlations in the Hc2 of Fe y Se 1−x S x (0.10 ⩽x⩽ 0.24, y⩾ 0.9)

“…Hence, stronger temperature dependence of γ H outside the nematic region is consistent with the larger gap becoming strongly anisotropic as x is tuned across the NQCP, in agreement with thermal conductivity and specific heat studies [18]. We present T − x phase diagram in figure 4(a), T s and T c are based on the resistivity data (figure 1) and are consistent with a previous report [18,35]. Figure 4(b) shows the slope of the H c2 (T) curve at T c as a function of x.…”

“…Single crystals of Fe y Se 1−x S x (0.10 ⩽ x ⩽ 0.24; y ⩾ 0.9) were grown by the chemical vapor transport method using an eutectic mix of the KCl and AlCl 3 as the transport agent as described elsewhere [33][34][35]. Elemental analysis was obtained from the energy dispersive x-ray spectroscopy (EDX) in JEOL 7600F scanning electron microscopes with about 5% accuracy.…”

“…The average x values and largest deviation from the average were: 0.10 ± 0.01, 0.12 ± 0.02, 0.14 ± 0.02, 0.17 ± 0.02, 0.21 ± 0.03, 0.24 ± 0.02. In addition, there is up to about 10% vacancies on Fe atomic site [35]. X-ray diffraction (XRD) was performed on pulverized crystals at the room temperature by using Cu-K α (λ = 0.15418 nm) radiation in a Rigaku Miniflex powder diffractometer.…”

Mooij Law Violation from Nanoscale Disorder

“…The reflections shift to higher scattering angles with increasing x , consistent the decrease of the unit cell volume. The nematic transition T s in FeSe, detected via the dip anomaly in ρ­( T ), gradually shifts to T c → 0 as S substitutes for Se in the lattice and vanishes above x = 0.18 (Figure l,m); T c increases weakly up to about 11 K for x = 0.1 (Figure n) which is in agreement with previous observations. − The width of the resistivity transition around T c broadens as x increases from 0 and T c reaches its minimum value of 2.1 K for all 0 ≤ x ≤ 1 at x = 0.4 (Figure n,o). For higher S content approaching FeS, T c increases (Figure o), consistent with T c seen in χ­( T )…”

Peculiarities of the nematic transition in FeSe0.675Te0.3S0.025 and its proximity to the quantum critical point

“…Later it became clear that the chemical pressure caused by seleniumsulfur substitution in FeSe 1−x S x did not have the same impact on superconducting properties as the hydrostatic pressure in FeSe. Even for FeSe 1−x S x with 0.2 ≤ x ≤ 1 that have been obtained by the hydrothermal method [11,12], the temperature of the superconducting transition remains below 10 K. It was also demonstrated that disorder suppresses nematicity in FeSe and causes a slight increase in T c [13], which is similar to the behavior of FeSe 1−x S x at low x.…”