Spin fluctuations and superconductivity in powders of Fe${}_{1+x}$Te${}_{0.7}$Se${}_{0.3}$ as a function of interstitial iron concentration

Using neutron inelastic scattering, we investigate the role of interstitial iron on the low-energy spin fluctuations in powder samples of Fe${}_{1+x}$Te${}_{0.7}$Se${}_{0.3}$. We demonstrate how combining the principle of detailed balance along with measurements at several temperatures allows us to subtract both temperature-independent and phonon backgrounds from $S(Q,\omega )$ to obtain purely magnetic scattering. For small values of interstitial iron [$x=0.009\left(3\right)$], the sample is superconducting ($T_c=14$ K) and displays a spin gap of 7 meV peaked in momentum at wave vector ${\mathbf{q}}_{\mathbf{0}}=(\pi ,\pi )$ consistent with single-crystal results. On populating the interstitial iron sites, the superconducting volume fraction decreases and we observe a filling in of the low-energy magnetic fluctuations and a decrease of the characteristic wave vector of the magnetic fluctuations. For large concentrations of interstitial iron [$x=0.048\left(2\right)$] where the superconducting volume fraction is minimal, we observe the presence of gapless spin fluctuations at a wave vector of ${\mathbf{q}}_{\mathbf{0}}=(\pi ,0)$. We estimate the absolute total moment for the various samples and find that the amount of interstitial iron does not change the total magnetic spectral weight significantly, but rather has the effect of shifting the spectral weight in $Q$ and energy. These results show that the superconducting and magnetic properties can be tuned by doping small amounts of iron and are suggestive that interstitial iron concentration is also a controlling dopant in the Fe${}_{1+x}$Te${}_{1-y}$Se${}_y$ phase diagram in addition to the Te/Se ratio.

Figure 1

(a) Phase diagram derived and presented in Ref. 18. (b) Superconducting volume fraction as a function of interstitial iron concentration for a fixed $y=0.3$.

Figure 2

(a),(b) Measured intensity on DCS at $T=2$ and 85 K. (c),(d) $S(Q,E)$ at these temperatures; the temperature-independent background is presented in (e).

Figure 3

(a) Total $S(Q,E)$ derived using detailed balance and after background subtraction for Fe${}_{1.009}$,Te${}_{0.7}$Se${}_{0.3}$. (b) Phonon background obtained from the method described in the text. (c) Subtraction illustrating the magnetic scattering. The analysis fails at the largest values of $Q$ and lowest energies.

Figure 4

(a)–(d) Contours of magnetic intensity derived for the samples described in the text. The momentum positions of $\mathbf{q}=(\pi ,0)$ and $\mathbf{q}=(\pi ,\pi )$ are indicated by dashed lines.

Figure 5

False-color contour maps of the temperature evolution of the spin fluctuations are illustrated in the left-hand panels. Momentum integrating energy scans are presented for the same temperatures in the right-hand panels.

Figure 6

The first momentum in energy as a function of momentum transfer is illustrated for the interstitial iron concentrations investigated. The solid curves are fits to the Hohenberg-Brinkman sum rule described in the text.

Figure 7

The momentum integrated intensity is plotted at $T=2$ K.

Figure 8

(a) Mean energy position as defined in the text. (b) Peak position of the magnetic spectrum in momentum. (c) Total integrated intensity in energy and momentum as a function of interstitial iron concentration. All of the data is presented for $T=2$ K.