Enhanced low-energy magnetic excitations evidencing the Cu-induced localization in the Fe-based superconductor ${\mathrm{Fe}}_{0.98}{\mathrm{Te}}_{0.5}{\mathrm{Se}}_{0.5}$

We have performed inelastic neutron scattering measurements on optimally doped ${\mathrm{Fe}}_{0.98}{\mathrm{Te}}_{0.5}{\mathrm{Se}}_{0.5}$ and 10% Cu-doped ${\mathrm{Fe}}_{0.88}{\mathrm{Cu}}_{0.1}{\mathrm{Te}}_{0.5}{\mathrm{Se}}_{0.5}$ to investigate the substitution effects on the spin excitations in the whole energy range up to 300 meV. It is found that substitution of Cu for Fe enhances the low-energy spin excitations ($\le 100\mathrm{meV}$), especially around the (0.5, 0.5) point, and leaves the high-energy magnetic excitations intact. In contrast to the expectation that Cu with spin 1/2 will dilute the magnetic moments contributed by Fe with a larger spin, we find that the 10% Cu doping enlarges the effective fluctuating moment from 2.85 to 3.13 ${\mu }_{\mathrm{B}}/\mathrm{Fe}$, although there is no long- or short-range magnetic order around (0.5, 0.5) and (0.5, 0). The presence of enhanced magnetic excitations in the 10% Cu doped sample which is in the insulating state indicates that the magnetic excitations must have some contributions from the local moments, reflecting the dual nature of the magnetism in iron-based superconductors. We attribute the substitution effects to the localization of the itinerant electrons induced by Cu dopants. These results also indicate that the Cu doping does not act as electron donor as in a rigid-band shift model, but more as scattering centers that localize the system.

Figure 1

Constant-energy contour maps and line cuts of the magnetic excitations at 100 K for ${\mathrm{Fe}}_{0.88}{\mathrm{Cu}}_{0.1}{\mathrm{Te}}_{0.5}{\mathrm{Se}}_{0.5}$ and ${\mathrm{Fe}}_{0.98}{\mathrm{Te}}_{0.5}{\mathrm{Se}}_{0.5}$. Panels (a)–(f) and (g)–(l) present constant-energy contours for Cu10 and Cu0 samples, respectively. The measurements in panels (a) and (g) were carried out on HYSPEC and the data were symmetrized to be fourfold symmetric, while the data on ARCS in the other panels were folded along the $H$ axis to the $K\ge 0$ side and then the folded data were duplicated on the $K\le 0$ side to represent the fourfold symmetry. The streaks across the excitations in panels (g)–(j) were due to the lack of detector coverage wherein. Panels (m)–(r) are linear cuts of the spin excitations through ($-0.5$, 0.5) along the [110] direction, illustrated as the dashed line in panel (a), at the corresponding energies labeled in the same column. The errors through data points represent one standard deviation throughout the paper. Solid lines through data are fitted with Gaussian functions. The data integration range for the linear cuts are $H=K=\pm 0.025$ rlu.

Figure 2

Dispersions for ${\mathrm{Fe}}_{0.88}{\mathrm{Cu}}_{0.1}{\mathrm{Te}}_{0.5}{\mathrm{Se}}_{0.5}$ (Cu10) and ${\mathrm{Fe}}_{0.98}{\mathrm{Te}}_{0.5}{\mathrm{Se}}_{0.5}$ (Cu0). Left and right panels are contour maps of the dispersions presented by energy evolutions of the intensity along ($-0.5\pm \delta , 0.5\pm \delta$) for Cu10 and Cu0 samples, respectively. The integration range along the $\left[1\overline{1}0\right]$ direction is 0.056 rlu. For each sample, only the dispersion on one side of ($-0.5$, 0.5) has been presented, with the other side folded and averaged for both panels. The data points on top of the contour maps are the peak positions extracted from the fittings with Gaussian functions to the constant-$E$ scans such as those shown in Figs. 1–1. The vertical error bars stand for the energy binning ranges used for making the fittings, and the horizontal bars are the fitting errors. Solid and dashed lines through data are guides to the eye, illustrating the dispersions for Cu10 and Cu0, respectively.

Figure 3

Energy dependence of the $\mathit{Q}$-integrated dynamical spin-spin correlation function $S\left(E\right)$ for ${\mathrm{Fe}}_{0.88}{\mathrm{Cu}}_{0.1}{\mathrm{Te}}_{0.5}{\mathrm{Se}}_{0.5}$ and ${\mathrm{Fe}}_{0.98}{\mathrm{Te}}_{0.5}{\mathrm{Se}}_{0.5}$. Integration of $S\left(E\right)$ covers an area of one Brillouin zone ($-1\le \mathrm{H}\le 0,0\le \mathrm{K}\le 1$). The normalized data from different spectrometers and incident energies are consistent with each other in the overlapping regimes, indicating the reliability of the normalizations. Horizontal error bars stand for the energy binning range, and vertical error represents one standard deviation. Solid lines through data are guides to eye. The dashed line is plotted by subtracting intensities of Cu0 from Cu10, indicating the extra intensities induced by the Cu substitution.