Electronic structure and magnetic phase transition of hexagonal FeSe thin films studied by photoemission spectroscopy

Hexagonal FeSe thin films were grown on $\mathrm{SrTi}{\mathrm{O}}_3$ substrates and the temperature and thickness dependence of their electronic structures were studied. The hexagonal FeSe is found to be metallic, with a Fermi surface consisting of six elliptical electron pockets. With decreased temperature, parts of the bands shift downward to high binding energy while some bands shift upward to $E_{\mathrm{F}}$. The shifts of these bands begin around 300 K and saturate at low temperature, indicating a magnetic phase-transition temperature of about 300 K. With increased film thickness, the Fermi surface topology and band structure show no obvious change. Our paper reports the electronic structure of hexagonal FeSe, and shows that the possible magnetic transition is driven by large-scale electronic structure reconstruction.

Figure 1

Crystal and electronic structure of 1-UC hexagonal FeSe. (a) Schematic structure of the FeSe films on the STO substrate. (b) Hexagonal NiAs-type structure. (c) LEED pattern of 1-UC hexagonal FeSe thin film. (d) Fermi surface topology extract from (c) and (e). (e) Photoemission intensity map at $E_{\mathrm{F}}$ integrated over [$E_{\mathrm{F}}-10\mathrm{meV},E_{\mathrm{F}}+10\mathrm{meV}$]. (f), (g) Photoemission intensity plots around Γ and $M$ points, respectively. (h) Photoemission intensity plot along the Γ-$M$ direction [cut 3 in (e)]. All the ARPES data were collected at 30 K.

Figure 2

Temperature dependence of the low-energy band structure for 1-UC hexagonal FeSe. (a1–a8) Temperature dependence of the band structure around the Γ point, [cut 1 in Fig. 1]. (b1–b8) Temperature dependence of the band structure around the $M$ point, [cut 4 in Fig. 1]. (c) Temperature dependence of the EDCs at the Γ point; the peak position marked by the red triangle corresponds to the band top of ${\alpha }^{\prime }$. (d) Temperature dependence of the EDCs at the $M$ point, the peak position marked by the red triangle correspond to the band bottom of β.

Figure 3

Temperature dependence of the Fermi surface and valence-band structure for 1-UC hexagonal FeSe. (a1), (a2) Photoemission intensity map at 30 and 290 K, respectively. (b1), (b2) Photoemission intensity plots around the Γ point at 30 and 380 K, respectively. (c1), (c2) Photoemission intensity plots around the $M$ point at 30 and 380 K, respectively. (d) Temperature dependence of the EDCs at Γ point; the red triangle marks the band top of ${\alpha }^{\prime }$ and the blue triangle marks the band position of η. (e) Temperature dependence of the EDCs at the $M$ point; the blue triangle marks the band bottom of β and the red triangle marks the band position of ε.

Figure 4

Thickness dependence of the Fermi surface and valence-band structure for hexagonal FeSe at 30 K. (a)–( c) Photoemission intensity map of 1-, 2-, and 6-UC hexagonal FeSe, respectively. (d)–(f) Photoemission intensity plots around the Γ point of 1-, 2-, and 6-UC hexagonal FeSe, respectively. (g) Thickness dependence of the EDCs at the Γ point; the black triangle marks the band bottom of ν and the red circle marks the band position of μ.