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. 2018 Mar 16;4(3):eaao2682.
doi: 10.1126/sciadv.aao2682. eCollection 2018 Mar.

Direct imaging of electron transfer and its influence on superconducting pairing at FeSe/SrTiO3 interface

Weiwei Zhao  1   2 , Mingda Li  3   4   5 , Cui-Zu Chang  1   4 , Jue Jiang  1 , Lijun Wu  5 , Chaoxing Liu  1 , Jagadeesh S Moodera  4   6 , Yimei Zhu  5 , Moses H W Chan  1
Affiliations

Direct imaging of electron transfer and its influence on superconducting pairing at FeSe/SrTiO3 interface

Weiwei Zhao et al. Sci Adv. .

Abstract

The exact mechanism responsible for the significant enhancement of the superconducting transition temperature (Tc) of monolayer iron selenide (FeSe) films on SrTiO3 (STO) over that of bulk FeSe is an open issue. We present the results of a coordinated study of electrical transport, low temperature electron energy-loss spectroscopy (EELS), and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) measurements on FeSe/STO films of different thicknesses. HAADF-STEM imaging together with EELS mapping across the FeSe/STO interface shows direct evidence of electrons transferred from STO to the FeSe layer. The transferred electrons were found to accumulate within the first two atomic layers of the FeSe films near the STO substrate. An additional Se layer is also resolved to reside between the FeSe film and the TiO x -terminated STO substrate. Our transport results found that a positive backgate applied from STO is particularly effective in enhancing Tc of the films while minimally changing the carrier density. This increase in Tc is due to the positive backgate that "pulls" the transferred electrons in FeSe films closer to the interface and thus enhances their coupling to interfacial phonons and also the electron-electron interaction within FeSe films.

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Figures

Fig. 1
Fig. 1. Superconducting FeSe films on STO substrates.
(A) Super cell of FeSe on top of STO (001), inferred from combined HAADF image and EELS data. (B) The HAADF-STEM image of the 1-UC FeSe film on STO with FeTe capping layers. (C and D) The integrated Ti L3,2 and Fe L3,2 EELS after subtracting background in false color with increasing intensity in the black-blue-green-red sequence. (E) Schematics of the gate-tuned six-terminal Hall bar device of FeSe films on STO with FeTe capping layer. For clarity, Te film on FeTe is not shown. (F) Normalized Rxx versus T for 1-UC films annealed at different temperatures for 2 hours post MBE growth: S1 (550°C), S1′ (500°C), S1″ (400°C), and S1‴ (330°C). (G to J) Normalized Rxx versus T at various backgating voltage Vg for the (G) S1 (1 UC), (H) S2 (2 UC), (I) S3 (8 UC), and (J) S4 (14 UC) films under the optimal annealing condition at 550°C.
Fig. 2
Fig. 2. Atomically resolved STEM-EELS results at 10 K.
(A to C) Core-loss EELS mapping with an energy range between 680 and 740 eV for (A) S1 (1 UC), (B) S3 (8 UC), and (C) S4 (14 UC). (D to F) The zoomed-in images at the interface region in (A) to (C), respectively. The dash lines are shown as guides for eyes. (G) FEFF simulation of the core-loss EELS spectra using the super cell in Fig. 1A. (H) Schematic of work function difference between STO and FeSe. (I) Schematics of screening potential profiles in the FeSe region induced by electron transfer from the proximal STO interface. Because of the finite screening length, Fe’s L3 (2p3/2) and L2 (2p1/2) levels close to the interface bend accordingly, giving a blue shift of the electron energy loss. VB is total potential variation. (J) At the interface, density of states (DOSs) of hole pocket is higher than that of electron pocket. It needs more electrons to fill up the hole pocket as compared with electron pocket for the same EF shift.
Fig. 3
Fig. 3. Hall transport studies on FeSe/STO samples.
(A) Ryx versus μ0H at Vg = 0 V at different temperatures from 300 to 30 K for the S1 (1 UC) film. (B) Ryx versus μ0H at T = 30 K at different gate voltages ranging from −200 to +200 V for the S1 (1 UC) film. (C) RH as function of T at Vg = 0 V for the S1 (1 UC), S2 (2 UC), S3 (8 UC), and S4 (14 UC) films. (D) RH as function of Vg at T = 30 K for the S1 (1 UC), S2 (2 UC), S3 (8 UC), and S4 (14 UC) films. (E) RH as function of T under Vg = 0 V for S1 (1 UC), S1′ (1 UC), S1″ (1 UC), and S1‴ (1 UC) annealed at different temperatures. (F) RH as a function of Vg at T = 30 K for S1 (1 UC), S1′ (1 UC), S1″ (1 UC), and S1‴ (1 UC).
Fig. 4
Fig. 4. The superconducting transition temperature Tc-mid as a function of the Hall coefficient RH at 30 K.
The different samples in our study are represented by blue arrows. The red circles show Tc-mid and RH values, from bottom to top, at Vg’s of −200, −100, 0, 100, and 200 V. The broad pink arrow groups the 1-UC films annealed at different temperatures 550°, 500°, 400°, and 330°C, respectively, and the green arrow groups samples with different thicknesses annealed at 550°C. The slopes of the blue circles (which summarize the backgating effect) are much steeper than the slopes of the green and pink arrows. This means that backgating is particularly effective in enhancing Tc for thin FeSe films with minimal effect in RH. Plots using RH at 40 and 50 K in the Supplementary Materials show similar conclusions.
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