Discovery of an insulating parent phase in single-layer $\mathrm{Fe}\mathrm{Se}/\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_3$ films

The nature of the parent compound is intimately connected to the doping-induced emergence of superconductivity and the pairing mechanism in high-temperature superconductors. In high-temperature cuprate superconductors, the parent phase is believed to be a Mott insulator and superconductivity is realized by doping the Mott insulator. In iron-based superconductors, on the other hand, the parent compounds discovered so far are all bad metals which are either antiferromagnetic or nematic. Here we report direct spectroscopic evidence that the parent phase of the single-layer $\mathrm{FeSe}/{\mathrm{SrTiO}}_3$ (FeSe/STO) films is insulating. It represents a case in the iron-based superconductors in which the parent compound is an insulator. We also find that the single-layer FeSe/STO films exhibit a distinct doping evolution to superconductivity that is similar to doping a Mott insulator in cuprate superconductors. These observations provide key insights in understanding the record high- $T_C$ superconductivity in single-layer FeSe/STO films.

Figure 1

Electronic structure evolution of a pure single-layer FeSe/STO film with increasing electron doping during vacuum annealing. (a) Spectral weight distribution as a function of momentum for the pure single-layer FeSe/STO film at different annealing sequences A1–A7. The condition for each annealing sequence is shown in Supplemental Fig. S4 [28]. The images are obtained by integrating the spectral weight over an energy window [$-0.02$, 0.01] eV with respect to the Fermi level. (b)–(d) Band structure at different annealing sequences for the momentum cuts across $\mathrm{\Gamma }$ (b), ${\mathrm{M}}_2$ (c), and ${\mathrm{M}}_3$ (d). The location of the three cuts are marked in the rightmost panel of (a). We note that the weak signal of a holelike band in the initial sample [leftmost panel of (b)] is from the tiny remnant second-layer FeSe islands; it disappears after the annealing sequence A2. (e) Electron doping level for each annealing sequence estimated from the area of the electron pockets near M point. (f),(g) EDCs at $\mathrm{\Gamma }$ (f) [marked by arrow in the rightmost panel in (b)] and at the Fermi momentum $k_R$ around M point (g) [marked by arrow in the rightmost panel in (c)] for different annealing sequences.

Figure 2

Electronic structure evolution of a superconducting single-layer FeSe/STO film upon Se deposition. The initial sample is a perfect superconducting single-layer FeSe/STO film that is similar to the one in Fig. S1(g1) [28]. We performed four consecutive Se depositions, named as DS1–DS4; each Se deposition is 15 s while keeping the sample at 30 K. (a) Spectral weight distribution as a function of momentum at different Se deposition sequences. The images are obtained by integrating the spectral weight over an energy window [$-0.02$, 0.01] eV with respect to the Fermi level. (b)–(d) Band structure at different Se deposition sequences for the momentum cuts across $\mathrm{\Gamma }$ (b), ${\mathrm{M}}_2$ (c), and ${\mathrm{M}}_3$ (d). The location of the three cuts are marked in the leftmost panel of (a). The images are the second derivative of the original data with respect to the energy. (e) Electron doping level determined from the electronlike Fermi pocket area near M point for deposition sequences DS0–DS3. The value for DS4 is estimated from the deposition procedure. (f),(g) EDCs at $\mathrm{\Gamma }$ (f) [marked by arrow in the leftmost panel in (b)] and at the Fermi momentum $k_R$ around M point (g) [marked by arrow in the leftmost panel in (c)] for different Se deposition sequences. (h) The energy position variation of the holelike band top at $\mathrm{\Gamma }$ point as a function of electron doping level. The data are from (b) and also from the single-layer FeSe/STO film in Fig. 1 (Annealing no. 1) and Fig. S1(b) (Annealing no. 2) [28]. (i) Integrated intensity of the photoemission spectra (EDCs) at the Fermi momentum [$k_R$ as marked in the leftmost panel of (b)] near M point. The intensity represents the peak area after subtracting the background as shown in the top-left inset. It also includes data from Fig. 1 and Fig. S1 [28].

Figure 3

Insulating gap evolution with the increase of electron doping in single-layer FeSe/STO film revealed by STM/STS. The sample preparation and the vacuum annealing process mimic those in Fig. 1. The initial perfect single-layer FeSe/STO film was prepared at $450{}^{\circ }\mathrm{C}$ and annealed at $530{}^{\circ }\mathrm{C}$. Then the sample was deposited Se on the surface at room temperature and annealed in four consecutive sequences Q1–Q4: $300{}^{\circ }\mathrm{C}$ for 2 h (Q1), $350{}^{\circ }\mathrm{C}$ for 2 h (Q2), $400{}^{\circ }\mathrm{C}$ for 2 h (Q3), and $480{}^{\circ }\mathrm{C}$ for 2 h (Q4). (a) Atomically resolved STM images of the single-layer FeSe regions for the four annealing sequences measured at 4.2 K. The corresponding STS curves ($dI/dV$) are shown in (b). The arrows in each spectrum indicate the range of the insulating gap.

Figure 4

Electronic phase diagram for the single-layer FeSe/STO films. The figure also includes some data at high electron doping from Refs. [10, 25]. The parent phase is insulating. The insulating gap shrinks with increasing doping and vanishes near $\sim 0.09$ $e$/Fe where there is an insulator-superconductor crossover [25]. The insulting gap size is extracted by tracking the peak position of the EDCs at the Fermi momentum around M point [$k_R$ as marked in the rightmost panel of Fig. 1]. The error bars represent the uncertainty in getting the peak position. Further increase of electron doping leads to an increase of the superconducting gap size and the corresponding $T_C$ [10]. Since the evolution of band structure with doping does not follow a rigid band shift and the route of doping evolution to superconductivity is similar to that of doping a Mott insulator in cuprates, we name the area where the doping level is below the threshold ($\sim 0.06$ $e$/Fe) “Mott-like insulator.” The top inset shows schematically the Fermi surface topology change from insulating region to superconducting region.