Absence of superconductivity in ultrathin layers of FeSe synthesized on a topological insulator

The structural and electronic properties of FeSe ultrathin layers on $\mathrm{B}{\mathrm{i}}_2\mathrm{S}{\mathrm{e}}_3$ have been investigated with a combination of scanning tunneling microscopy and spectroscopy and angle-resolved photoemission spectroscopy. The FeSe multilayers, which are predominantly 3–5 monolayers (MLs) thick, exhibit a hole pocket-like electron band at $\overline{\mathrm{\Gamma }}$ and a dumbbell-like feature at $\overline{M}$, similar to multilayers of FeSe on $\mathrm{SrTi}{\mathrm{O}}_3$. Moreover, the topological state of the $\mathrm{B}{\mathrm{i}}_2\mathrm{S}{\mathrm{e}}_3$ is preserved beneath the FeSe layer, as indicated by a heavily n-doped Dirac cone. Low temperature scanning tunneling spectroscopy does not exhibit a superconducting gap for any investigated thickness down to a temperature of 5 K.

Figure 1

(a) Ultrathin FeSe islands on $\mathrm{B}{\mathrm{i}}_2\mathrm{S}{\mathrm{e}}_3$ exhibit a linear moiré pattern, whereas the $\mathrm{B}{\mathrm{i}}_2\mathrm{S}{\mathrm{e}}_3$ surface exhibits a dislocation network. (b) Apparent height profile taken along marker in (a). The FeSe layers exhibit an apparent interlayer height of 0.57 nm; the apparent height difference between FeSe and $\mathrm{B}{\mathrm{i}}_2\mathrm{S}{\mathrm{e}}_3$ is 0.3 nm. $V_{\mathrm{S}}=500\mathrm{mV},I_{\mathrm{t}}=300\mathrm{pA}$. (c) The atomic lattice of $\mathrm{B}{\mathrm{i}}_2\mathrm{S}{\mathrm{e}}_3$ observed in regions away from FeSe. $V_{\mathrm{S}}=500\mathrm{mV},I_{\mathrm{t}}=500\mathrm{pA},T=5\mathrm{K}$. (d) The FeSe patches exhibit a square lattice. Two different types of defects, A and B, can be identified. $V_{\mathrm{S}}=500\mathrm{mV},I_{\mathrm{t}}=300\mathrm{pA}$. (e) LEED image of the pristine $\mathrm{B}{\mathrm{i}}_2\mathrm{S}{\mathrm{e}}_3$ showing the expected hexagonal pattern. (f) LEED after the deposition of Fe and annealing shows a superposition of several reciprocal lattices: in addition to the hexagonal pattern stemming from $\mathrm{B}{\mathrm{i}}_2\mathrm{S}{\mathrm{e}}_3$, a square lattice appears in three domains each rotated by 120°. We assign these three domains to FeSe. The different domains are marked by arrows in green, orange, and purple.

Figure 2

(a) Photoemission intensity map for FeSe/$\mathrm{B}{\mathrm{i}}_2\mathrm{S}{\mathrm{e}}_3$. The red dashed arrow in the inset hexagon indicates the direction $(\overline{\mathrm{\Gamma }}\text{−}{\overline{M}}_{\mathrm{B}{\mathrm{i}}_2\mathrm{S}{\mathrm{e}}_3})$ of the cut through the surface BZ of $\mathrm{B}{\mathrm{i}}_2\mathrm{S}{\mathrm{e}}_3$. The blue arrow marks an FeSe band feature at $E_{\mathrm{bin}}\approx 0.2\mathrm{eV}$. (b) Fermi surface showing the two topological surface states. Both states display the expected hexagonal warping of the constant energy contour. The inset hexagon shows the orientation of the surface BZ of $\mathrm{B}{\mathrm{i}}_2\mathrm{S}{\mathrm{e}}_3$. The dashed arrow indicates the direction of the cut in (a). (c) Constant-energy contour acquired at $h\nu =53.6\mathrm{eV}$ across a wider range of momenta, by integrating the spectral weight from a binding energy of $+0.055$ to $-0.055\mathrm{eV}$, revealing the orientation of the $\mathrm{B}{\mathrm{i}}_2\mathrm{S}{\mathrm{e}}_3$ BZ (black dashed hexagons) and the BZs of the three domains of FeSe (green, purple, and yellow dashed squares). The small black hexagons indicate the $\overline{\mathrm{\Gamma }}$ and $\overline{\mathrm{\Gamma }}{}_{\mathrm{B}{\mathrm{i}}_2\mathrm{S}{\mathrm{e}}_3}^{\prime }$ points, and the small circles show the positions of the ${\overline{M}}_{\mathrm{FeSe}}$ points.

Figure 3

(a) Photoemission intensity at the Fermi energy for FeSe/$\mathrm{B}{\mathrm{i}}_2\mathrm{S}{\mathrm{e}}_3$, measured with photon energy $h\nu =26\mathrm{eV}$. The ${\overline{M}}_{\mathrm{FeSe}}$ point for one of the FeSe domains is visible in the measurement range and is labeled. The red box encloses an extremely faint dumbbell-like feature at ${\overline{M}}_{\mathrm{FeSe}}$; the inset shows this faint feature with higher contrast. (b) Photoemission intensity acquired at $h\nu =53.6\mathrm{eV}$ along the $\overline{\mathrm{\Gamma }}\text{−}{\overline{X}}_{\mathrm{FeSe}}\text{−}{\overline{M}}_{\mathrm{FeSe}}\text{−}\overline{\mathrm{\Gamma }}$ directions. (c) Curvature plot of (b). The red boxes in (b) and (c) enclose the extremely faint feature at ${\overline{M}}_{\mathrm{FeSe}}$, and the insets show this feature magnified and at higher contrast. Blue arrows mark the location of the feature that is marked with an identical blue arrow in Fig. 2. (d) and (e) Photoemission intensity along the cuts through $\overline{\mathrm{\Gamma }}$ and ${\overline{M}}_{\mathrm{FeSe}}$ that are marked with blue and green lines, respectively, in (a). (f) and (g) Curvature plots obtained from (d) and (e), respectively. Red arrows highlight the location of the feature at ${\overline{M}}_{\mathrm{FeSe}}$. The dashed red arrow in Fig. 3 indicates the lobe of the dumbbell that falls in the second BZ, where photoemission intensity is faint.

Figure 4

(a) A characteristic high-resolution spectrum around the Fermi energy on an FeSe island with a minimum distance of 35 nm to the island edge. No superconducting gap is visible. $V_{\mathrm{S}}=50\mathrm{mV},I_{\mathrm{t}}=500\mathrm{pA},V_{\mathrm{mod}}=0.5\mathrm{mV}$. (b) Large range STS spectra taken on the $\mathrm{B}{\mathrm{i}}_2\mathrm{S}{\mathrm{e}}_3$ (red) and the FeSe. The FeSe spectrum shows a peak at $-225\mathrm{meV}$, the same energy as that of the broad band in Fig 2. The blue spectrum is shifted for better visibility. Parameters, red: $V_{\mathrm{S}}=250\mathrm{mV},I_{\mathrm{t}}=500\mathrm{pA},V_{\mathrm{mod}}=0.5\mathrm{mV}$; blue: $V_{\mathrm{S}}=1\mathrm{V},I_{\mathrm{t}}=500\mathrm{pA},V_{\mathrm{mod}}=4\mathrm{mV}$.

Figure 5

Spectra taken on and close to a type A defect for an FeSe island. The inset shows the locations of the spectra in color code, from red (on the defect) to dark blue (three atomic lengths off the defect). The defect shows a resonance at $-6\mathrm{mV}$. $V_{\mathrm{S}}=50\mathrm{mV},I_{\mathrm{t}}=200\mathrm{pA},V_{\mathrm{mod}}=0.5\mathrm{mV}$.