Surface orbital order and chemical potential inhomogeneity of the iron-based superconductor $\mathrm{Fe}{\mathrm{Te}}_{0.55}{\mathrm{Se}}_{0.45}$ investigated with special STM tips

The atomically clean surface of the iron-based superconductor $\mathrm{Fe}{\mathrm{Te}}_{0.55}{\mathrm{Se}}_{0.45}$ is investigated by low-temperature scanning tunneling microscope (STM) with different tip apex states. We show that the scattering channel between the $\mathrm{\Gamma }$ and $X$/$Y$ points of the sample's Brillouin zone can be visualized clearly by a supersharp STM tip. Furthermore, by manipulating a single Fe atom onto the tip apex, signatures of the orbital nature of the subsurface Fe layer of $\mathrm{Fe}{\mathrm{Te}}_{0.55}{\mathrm{Se}}_{0.45}$ can be identified. By preparing a charged tip state, the intrinsic spatial inhomogeneity of the chemical potential of $\mathrm{Fe}{\mathrm{Te}}_{0.55}{\mathrm{Se}}_{0.45}$ can be revealed. As a result, three different types of vortex bound states originating from locally varying topological properties of the $\mathrm{Fe}{\mathrm{Te}}_{0.55}{\mathrm{Se}}_{0.45}$ surface are observed by scanning tunneling spectroscopy.

Figure 1

Atomic-resolution STM images and corresponding FFT maps before and after tip modification. (a), (e) Constant-current STM topography in the vicinity of two Fe impurities before (a) and after (e) tip modification. Scale bar: 1 nm. (b), (f) FFT maps of (a) and (e). $X$ and $Y$ denote the first-order spots of the reciprocal lattice. The second-order spots can only be seen clearly in (f). (c), (g) $dI/dV$ maps taken at 1 mV for the same regions as shown in (a), (e) before (c) and after (g) tip modification. A quasiparticle interference pattern can be seen in both (c) and (g). Scale bar: 1 nm. (d), (h) FFT maps of (c) and (g). After tip modification, the intensity of the spots at points $X$ and $Y$ is greatly enhanced in (h) compared to that in (d) before the tip modification. Tunneling parameters in (a), (c), (e), and (g): $V_{\mathrm{stab}}=-10\mathrm{mV}$, $I_{\mathrm{stab}}=600\mathrm{pA}$, $V_{\mathrm{osc}}=0.03\mathrm{mV}$.

Figure 2

Imaging Fe orbital states with a special STM tip. (a), (b) Atomically resolved constant-current STM topography image before (a) and after (b) the tip has been modified by the adsorption of a single Fe atom at its apex. A stripe pattern coexisting with the square lattice of the Te/Se lattice is visible in (b). (c) Atomic-resolution constant-current STM topography image of another area obtained with the same tip as used in (b). The direction of a special periodic pattern is highlighted by a green arrow labeled with $k$. Tunneling parameters in (a)–(c): $V=-10\mathrm{mV}$, $I=400\mathrm{pA}$. (d) FFT map of (c). New peaks labeled $K$ and $K^{\prime }$ appear beside ±$X$ and ±$Y$ which reflect the Te/Se square lattice. (e) STM image of the same area as shown in (c) obtained with the same sample bias of −10 mV, but different tunneling current 1 nA. (f) STM image of the same area as shown in (e) with the same tunneling current 1 nA but a different sample bias of −40 mV. (g) Top-view model of the Fe(Te,Se) crystal. (h) Bonds between Te/Se and Fe atoms. Green/yellow corners and intersections represent the top/bottom Te/Se atoms. Purple corners and intersections represent the interlayer Fe atoms. Fe-Fe bonds are shown as purple lines between the Fe atoms. Scale bar in (a)–(c), (e), and (f): 1 nm.

Figure 3

Charging effects revealed by a special STM tip. (a)–(d) Constant-current STM images of the same region with the same sample bias of −10 mV, but a different tunneling current of 50 pA in (a), 100 pA in (b), 200 pA in (c), and 400 pA in (d). The shapes of the islandlike features with sharp boundaries are changing with tunneling current. The atomic Te/Se square lattice can also be resolved in (b)–(d). Scale bar: 2 nm. (e)–(h) Constant-current STM images of the same region with the same tunneling current of 200 pA, but a different sample bias of −10 mV in (e), −20 mV in (f), −30 mV in (g), and −40 mV in (h). The contrast between the islandlike features and the surroundings decreases with larger negative sample bias but the shapes of the islands change little. Scale bar: 2 nm.

Figure 4

Three typical vortex bound states. (a), (b) $dI/dV$ map and STS line cuts of vortex $A$. A total of 76 $dI/dV$ spectra are presented in (b) taken along the white dashed line with a length of 15 nm shown in (a). A vortex bound state at zero energy with no spatial dispersion can be recognized. (c), (d) $dI/dV$ map and STS line cuts of vortex $B$. A total of 81 $dI/dV$ spectra are presented in (d) taken along the white dashed line with a length of 16 nm shown in (c). Within vortex $B$, nondispersive CBS can be identified. (e), (f) $dI/dV$ map and STS line cuts of vortex $C$. A total of 81 $dI/dV$ spectra are presented in (f) taken along the white dashed line with a length of 17 nm shown in (e). Within the center of vortex $C$, the bound state has an energy of 0.17 mV. The energy of this bound state shifts to zero energy near the boundary of the vortex. Tunneling parameters in (a)–(f): $V_{\mathrm{stab}}=35\mathrm{mV}$, $I_{\mathrm{stab}}=100\mathrm{pA}$, $V_{\mathrm{osc}}=0.03\mathrm{mV}$. Scale bar in (a), (c), and (e): 2 nm.