Nanoscale chemical phase separation in FeTe${}_{0.55}$Se${}_{0.45}$ as seen via scanning tunneling spectroscopy

Atomically resolved structural and electronic properties of FeTe${}_{1-x}$Se${}_x$ ($x$ $=$ 0 and 0.45) have been studied with scanning tunneling microscopy/spectroscopy (STM/STS). In contrast to the extreme flatness of the Te-terminated FeTe surface, nanoscale chemical phase separation between Te and Se atoms is unambiguously revealed on the surface of FeTe${}_{0.55}$Se${}_{0.45}$. A statistical counting of the two kinds of atoms has the same ratio as that in the bulk. Remarkably, there is no electronic phase separation seen in the tunneling spectroscopy. This indicates that the optimally doped superconductor is chemically inhomogeneous but electronically homogeneous, in contrast to many correlated electron materials.

Figure 1

(a) Bulk crystal structure of FeTe${}_{1-x}$Se${}_x$ and the cleavage position marked by arrow. (b) 3D view of a constant-current STM topographic image of the cleaved FeTe surface showing large atomically flat terraces and steps by using a sample bias $V$ $=$ 800 mV and tunneling current $I$ $=$ 200 pA. Zoom-in (c) filled-state and (d) empty-state STM topographic images on the same location of cleaved FeTe surface (59 Å $\times$ 59 Å) with the tunneling current $I$ $=$ 500 pA. (e) Line profile for the dashed line and (f) histogram of imaged atom heights in (c). The solid curve is the fitting result to a Gaussian distribution.

Figure 2

(a), (b) Constant-current STM topographic images (111 Å $\times$ 111 Å) of FeTe${}_{0.55}$Se${}_{0.45}$ surface with different bias voltages but identical tunneling current $I$ of 1.3 nA. (c) Line profile across different patches of atoms and (d) line profiles on both “bright” and “dark” atom patches marked in panel (a).

Figure 3

(a)–(d) Histograms of atom heights corresponding to the bias-dependent STM topographic images. The solid curves are the results by fitting to two Gaussian distributions. In each panel, the yellow (I) (light gray) and green (II) (dark gray) peaks correspond to “bright” and “dark” atoms in the image, respectively, and the blue (darkest gray) curve is the envelope sum of two peaks.

Figure 4

(a) A 128 Å $\times$ 128 Å constant-current STM topographic image on the FeTe${}_{0.55}$Se${}_{0.45}$ surface. $I=1.5$ nA, $V=-100$ mV. (b) Tunneling spectra taken at several Te and Se sites indicated in (a). Curves are vertically shifted for clarity. (c) Averaged dI/dV spectra at Te and Se sites on FeTe${}_{0.55}$Se${}_{0.45}$ surface in (a) and on FeTe surface. The spectra are also shifted for clarity, with the zero value of dI/dV marked by solid lines. (d) The corresponding $d^{2}I/{\mathit{dV}}^2$ curves acquired by numerical differentiation of the measured dI/dV spectra in (c). All the data were taken at 80 K.