Spin Gap and Resonance at the Nesting Wave Vector in Superconducting ${\mathrm{FeSe}}_{0.4}{\mathrm{Te}}_{0.6}$

Neutron scattering is used to probe magnetic excitations in ${\mathrm{FeSe}}_{0.4}{\mathrm{Te}}_{0.6}$ ($T_c=14\mathrm{K}$). Low energy spin fluctuations are found with a characteristic wave vector ($\frac{1}{2}\frac{1}{2}L$) that corresponds to Fermi surface nesting and differs from ${\mathbf{Q}}_m=(\delta 0\frac{1}{2})$ for magnetic ordering in ${\mathrm{Fe}}_{1+y}\mathrm{Te}$. A spin resonance with $\hslash {\Omega }_0=6.51(4)\mathrm{meV}\approx 5.3k_B{T}_c$ and $\hslash \Gamma =1.25(5)\mathrm{meV}$ develops in the superconducting state from a normal state continuum. We show that the resonance is consistent with a bound state associated with $s_{\pm }$ superconductivity and imperfect quasi-2D Fermi surface nesting.

Figure 1

Spin excitation spectrum as a function of $\mathbf{Q}=(H,H,-1.2)$ and energy at (a) 1.5 K and (b) 30 K. (c) The difference between the 1.5 K and 30 K spectra. The intensity in (c) is multiplied by a factor of 2 so that the same intensity scale at the top is used for (a)–(c). (d) Resolution convolved theoretical intensity difference from a simplified two-band model extracted from ARPES measurements [25]. The insets show the resolution free theoretical intensity difference (left) and the normal state Fermi surface employed (right).

Figure 2

(a) Constant $\mathbf{Q}=(0.46,0.46,0.65)$ scan at 1.5 K and 30 K, measured at BT7. The sample-turned background measured at 30 K is shown by green open circles. (b) The difference intensity of the const-$\mathbf{Q}=(\frac{1}{2}\frac{1}{2}L)$ scans measured at 4.2 and 30 K using SPINS, with and without an applied 7 T magnetic field. The solid line is a guide to the eye. The blue bar indicates the FWHM instrumental energy resolution.

Figure 3

Constant $\hslash \omega =6.5\mathrm{meV}$ scans (a) along the ($HH\frac{1}{2}$) direction, and (b) along the ($\frac{1}{2}\frac{1}{2}L$) direction, showing the quasi-two-dimensionality of the spin resonance. (c)–(d) The difference between the 1.5 K and 30 K scans. In (c) the solid line is a fit to a resolution convolved Lorentzian, and the horizontal bar represents the FWHM of the resolution. In (d) the line is the product of the Fe magnetic form factor squared and the projection of the instrumental resolution volume along $\mathbf{c}$.

Figure 4

The energy scan at $\mathbf{Q}=(0.46,0.46,0.66)$ as a function of temperature indicating association between the spin resonance and superconductivity in ${\mathrm{FeSe}}_{0.4}{\mathrm{Te}}_{0.6}$. The sample-turned background was subtracted from the data. The inset shows the integrated intensity of the resonance between 5 meV and 8 meV as a function of temperature, and the line is a fit to mean field theory with $T_c=14\mathrm{K}$.