Observation of an isosceles triangular electronic structure around the excess iron atoms in Fe${}_{1+\delta }$Te

We present scanning tunneling microscopy and spectroscopy studies around an individual excess Fe atom, working as a local perturbation, in the parent material of the iron-chalcogenide superconductor Fe${}_{1+\delta }$Te. Spectroscopic imaging reveals an isosceles triangular electronic structure around the excess Fe atoms. Its spatial symmetry reflects the underlying bicollinear antiferromagnetic spin state and the structural monoclinic symmetry. These findings provide important clues to understanding the role of the excess Fe atoms, which complicate the understanding of the phenomena occurring in iron-chalcogenide materials.

Figure 1

(a) and (b) Schematic illustrations of the crystal structure of Fe${}_{1+\delta }$Te. In these figures, light blue, dark blue, black, and red spheres represent Te atoms above the Fe layer [Te(1)], Te atoms below the Fe layer [Te(2)], Fe atoms in the Fe layer [Fe(1)], and excess interstitial Fe atoms [Fe(2)], respectively. In (b), the difference between the atomic heights of Te(1) and Fe(2) from the Fe(1) layer (${\delta }_1$) is determined by using the previous neutron diffraction measurement on Fe${}_{1.05}$Te at 5 K in Ref. 33. The difference between the ionic radii of Te(1) and Fe(2) is shown as ${\delta }_2$ (see text). $R_{\mathrm{Fe}\left(1\right)}$, $R_{\mathrm{Fe}\left(2\right)}$, and $R_{\mathrm{Te}}$ are the ionic radii of Fe(1), Fe(2), and Te atoms, respectively. (c) STM image taken at $V_{\mathrm{B}}$ $=$ $+$500 mV and $I_{\mathrm{set}}$ $=$ 500 pA on an 86 Å $\times$ 86 Å field of view (FOV). The white arrows indicate the crystal $a$ and $b$ axes. The inset shows tunneling spectra averaged over an area far from the excess Fe atoms (black circles) and over ten Fe(2) sites (red diamonds). (d) Magnified STM image for the 15.8 Å $\times$ 14.5 Å FOV indicated by the black box in (c). (e) Zero-bias conductance map [G(r, 0 meV)] for the same FOV as in (d). In (d) and (e), the light blue, black, and red spheres correspond to Te(1), Fe(1), and Fe(2) atoms, respectively. Atomic sites near excess Fe atoms are labeled A–F.

Figure 2

(a)–(c) STM images taken at $V_{\mathrm{B}}$ $=$ $+$25, $+$500, and –500 mV, respectively. (d)–(f) Histograms of the values of $\Delta Z$ $=$ ($Z_1$ $+$ $Z_2$) – ($Z_3$ $+$ $Z_4$) at $V_{\mathrm{B}}$ $=$ $+$25, $+$500, and –500 mV, respectively, where $Z_i$ ($i$ $=$ 1,2,3,4) is the STM signal at the Te(1)${}_i$ site designated in (b).

Figure 3

(a)–(f) Conductance maps taken at energies from –20 to –330 meV. (g)–(h) Conductance maps taken at energies from $+$20 to $+$330 meV. All maps were taken under the same conditions ($V_{\mathrm{B}}$ $=$ $+$500 mV and $I_{\mathrm{set}}$ $=$ 500 pA) and on the same FOV as in Figs. 1d and 1e.

Figure 4

In the main panel, the tunneling spectra averaged over ten B (out of the triangular) and D (inside) sites as shown in the inset. Inset indicates a typical STM image including a single excess Fe atom.

Figure 5

(a) and (b) Schematic pictures of the monoclinic and tetragonal structures viewed along the crystal $c$ axis. In (a), for reasons of clarity, the angle between the $a$ and $c$ axes ($\beta$) is exaggerated to 80${}^{\circ }$, which is smaller than the actual angle (89.2${}^{\circ }$). The distances between the excess Fe site and the neighboring atomic sites (A–F) are labeled as $d_{\mathrm{A}}$–$d_{\mathrm{F}}$. In the actual crystal structure, $d_{\mathrm{A}}$ $=$ 2.801 Å $>$ $d_{\mathrm{c}}$ $=$ 2.763 Å $>$ $d_{\mathrm{F}}$ $=$ 2.733 Å, and $d_{\mathrm{B}}$ $=$ 2.707 Å $>$ $d_{\mathrm{D}}$ $=$ 2.702 Å. (c) and (d) Spin configurations of the bicollinear AFM order in the Fe(1) layer. For clarity, the spins adjacent to the excess Fe(2) atoms are emphasized.