Molecular-beam epitaxy and robust superconductivity of stoichiometric FeSe crystalline films on bilayer graphene

We report on molecular beam epitaxy growth of stoichiometric and superconducting FeSe crystalline thin films on double-layer graphene. Layer-by-layer growth of high-quality films has been achieved in a well-controlled manner by using Se-rich condition, which allow us to investigate the thickness-dependent superconductivity of FeSe. In situ low-temperature scanning tunneling spectra reveal that the local superconducting gap in the quasiparticle density of states is visible down to two triple layers for the minimum measurement temperature of 2.2 K, and that the transition temperature $T_{\mathrm{c}}$ scales inversely with film thickness.

Figure 1

Crystal structure of $\beta$-FeSe.

Figure 2

Substrate temperature-dependent film morphology (a–d, V $=$ 2.5 V, I $=$ 0.1 nA, 200 × 200 nm${}^2$) and the corresponding atomically resolved STM images (e–h, V $=$ 1 mV, $I$ $=$ 0.1 nA, 5 × 5 nm${}^2$) of the as-grown FeSe films at various substrate temperatures. (a,e) 180 °C, (b,f) 220 °C, (c,g) 390 °C, (d,h) 450 °C.

Figure 3

(a) High-resolution STM image ($V$ $=$ 6 mV, $I$ $=$ 0.1 nA, 5 × 5 nm${}^2$) taken on the Se-rich $\sqrt{5}\times \sqrt{5}$ surface. The film thickness is 15 TL, and the substrate temperature for obtaining the film is 220 °C. (b) $\mathit{dI}/\mathit{dV}$ conductance spectra in the Se-rich (upper curve) and stoichiometric (lower curve) FeSe films, Set point: $V$ $=$ 0.5 V, $I$ $=$ 0.1 nA. The upper curve shifts upward (3.8 a.u.) for clarity. Inset: Low-energy $\mathit{dI}/\mathit{dV}$ spectrum showing the superconducting gap in the stoichiometric 8 TL FeSe film measured at 3.0 K. Set point: $V$ $=$ 10 mV, $I$ $=$ 0.1 nA. (c) High-resolution STM image after annealing the Se-rich $\sqrt{5}\times \sqrt{5}$ surface (like in part (a) at 450 °C ($V$ $=$ −1 mV, $I$ $=$ 0.1 nA, 5 × 5 nm${}^2$). Two Se vacancies are observed.

Figure 4

(a,b) Consecutive STM images ($V$ $=$ 2.5 V, $I$ $=$ 0.1 nA) showing the displacement and rotation of an FeSe island. Image size: (a) 145 × 100 nm${}^2$, (b) 145 × 145 nm${}^2$.

Figure 5

(a,c) A series of normalized tunneling conductance spectra on (a) 8 TL and (b) 2 TL FeSe films. Insets: Temperature-dependent zero bias conductance (ZBC) for (a) 8 TL and (b) 2 TL FeSe films. The bias modulation is set at 0.1 mV. (c) Superconducting transition temperature $T_{\mathrm{c}}$ vs the inverse of the film thickness $d$.