Superconducting FeSe monolayer with millielectronvolt Fermi energy

Iron selenide (FeSe) is an iron-based superconductor which shows unique properties, including strongly anisotropic superconducting gap, paramagnetism in undoped compound, and extremely small Fermi pocket size. In this work, we demonstrate that the sizes of electron and hole pockets in FeSe monolayer become much smaller than those in bulk. The Fermi energy is on the order of a few meV and can be fine tuned by the thickness of graphene layers underneath. Despite the low carrier density, the FeSe monolayers grown on trilayer or multilayer graphene are superconducting. The superconducting gap size is sensitive to the Fermi energy of the hole band. Remarkably, the FeSe monolayer provides the opportunity to study the physics in the crossover regime where the Fermi energy and superconducting gap are comparable to each other.

Figure 1

(a) Schematic of the Brillouin zone and the Fermi surface. In the nematic phase, the electronic structure should be more properly viewed in the two-Fe Brillouin zone. (b) Topographic image (350 $\mathrm{nm}\times 315$ nm) of FeSe islands acquired by using sample bias of $V=3$ V and tunneling current of $I=20$ pA. 1 UC (unit cell) and 2 UC are the areas for monolayer and bilayer FeSe. (c) Atomically resolved STM topography (10 $\mathrm{nm}\times 10$ nm, 0.1 V, 0.1 nA) of the area marked by the white square in (b). (d) Side view of the monolayer FeSe across the step between BLG and TLG on adjacent SiC terraces. (e) Topographic profile along the black dashed line in (b).

Figure 2

(a)–(b) QPI dispersions of the nonsuperconducting area of FeSe monolayer grown on BLG, obtained by taking line cuts from the Fourier transform of energy-dependent normalized conductance images (Supplemental Material Fig. S2 [28]) along $q_x$ and $q_y$, respectively. The dispersions are fitted by the dashed curves. The tunneling spectra of the mappings were taken on a grid of $128\times 128$ pixels for a 85 $\mathrm{nm}\times 85$ nm field of view. Sample bias voltage $V=20$ mV, tunneling current $I=100$ pA, modulation amplitude for the lock-in detection $V_{\text{mod}}=0.2$ mV. (c) Schematic of the band dispersion around $\mathrm{\Gamma }=(0,0)$ point and $\mathrm{X}=(\pi /{\mathrm{a}}_{\text{Fe}}$,0) point. (d) The $\mathrm{d}I/\mathrm{d}V$ spectrum ($V=10$ mV, $I=0.1$ nA, $V_{\text{mod}}=0.1$ mV) of monolayer on BLG. The arrow marks the top of hole pocket. The peaks are caused by the quantum confinement in the lateral direction.

Figure 3

(a) The dipole layer formed between the SiC and FeSe surface. (b) The $dI/dV$ spectrum ($V=10$ mV, $I=0.1$ nA, $V_{\text{mod}}=0.1$ mV) in the superconducting area. (c) A series of $dI/dV$ spectra ($V=5$ mV, $I=0.1$ nA, $V_{\text{mod}}=0.05$ mV) measured along the arrow of 50 nm long in Fig. 1. (d),(e) QPI dispersions of the superconducting area of FeSe monolayer grown on TLG (Supplemental Material Fig. S3 [28]).

Figure 4

(a)–(c) The average $dI/dV$ spectra of FeSe monolayer on TLG. The spectrum in (a) is the average of a line cut taken along a 60 nm long line (64 points evenly distributed along this line). Set point: $V=2$ mV, $I=100$ pA, $V_{\text{mod}}=0.02$ mV. The spectrum in (b) is the average of a line cut taken along a 50 nm long line (32 points evenly distributed along this line). Set point: $V=-4$ mV, $I=100$ pA, $V_{\text{mod}}=0.04$ mV. The spectrum in (c) is the average of a line cut taken along a 48 nm long line (32 points evenly distributed along this line). Set point: $V=10$ mV, $I=100$ pA, $V_{\text{mod}}=0.1$ mV. (d)–(f) The corresponding QPI dispersions acquired in the same areas as the upper panel (Supplemental Material Fig. S6 [28]).

Figure 5

(a),(b) $dI/dV(r,E$) mapping measured on a $50\mathrm{nm}\times 50\mathrm{nm}$ area of FeSe monolayer films. Standing waves are clearly visible. Set point: $V=-21$ mV, $I=0.1$ nA, $V_{\text{mod}}=0.21$ mV. Lock-in oscillation amplitude 0.21 mV. (c) A series of $dI/dV$ spectra along a black arrow of 50 nm long in (a) (32 points evenly distributed along this line). The averaged gap size is 0.28 meV. Set point: $V=-5$ mV, $I=100$ pA, $V_{\text{mod}}=0.05$ mV. (d) $dI/dV$ at 0.28 mV and $-0.28$ mV along the line. The red curve is shifted vertically for clarity. (e) A series of $dI/dV$ spectra along a yellow arrow of 50 nm long in (a) (64 points evenly distributed along this line). Set point: $V=-10$ mV, $I=100$ pA, $V_{\text{mod}}=0.1$ mV. (f) The blue and red curves are the averaged $dI/dV$ spectra of (c) and (e), respectively. The gap size of the blue (red) curve is 0.28 meV (0.35 meV).

Figure 6

(a) QPI dispersion of FeSe bilayer. (b) $dI/dV$ of superconducting FeSe bilayer. Set point: $V=5$ mV, $I=100$ pA, $V_{\text{mod}}=0.05$ mV. The superconducting gap is 1.24 meV.