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Title: Direct imaging of electron transfer and its influence on superconducting pairing at FeSe/SrTiO 3 interface

The exact mechanism responsible for the significant enhancement of the superconducting transition temperature (T c) of monolayer iron selenide (FeSe) films on SrTiO 3 (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 T c of the films while minimally changing the carrier density. Furthermore, this increase in T c 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.
Authors:
ORCiD logo [1] ; ORCiD logo [2] ; ORCiD logo [3] ;  [4] ; ORCiD logo [5] ; ORCiD logo [4] ;  [6] ; ORCiD logo [5] ; ORCiD logo [4]
  1. Pennsylvania State Univ., University Park, PA (United States); Harbin Institute of Technology, Shenzhen (People's Republic of China)
  2. Massachusetts Inst. of Technology (MIT), Cambridge, MA (United States); Brookhaven National Lab. (BNL), Upton, NY (United States)
  3. Pennsylvania State Univ., University Park, PA (United States); Massachusetts Inst. of Technology (MIT), Cambridge, MA (United States)
  4. Pennsylvania State Univ., University Park, PA (United States)
  5. Brookhaven National Lab. (BNL), Upton, NY (United States)
  6. Massachusetts Inst. of Technology (MIT), Cambridge, MA (United States)
Publication Date:
Report Number(s):
BNL-205703-2018-JAAM
Journal ID: ISSN 2375-2548
Grant/Contract Number:
SC0012704
Type:
Accepted Manuscript
Journal Name:
Science Advances
Additional Journal Information:
Journal Volume: 4; Journal Issue: 3; Journal ID: ISSN 2375-2548
Publisher:
AAAS
Research Org:
Brookhaven National Laboratory (BNL), Upton, NY (United States)
Sponsoring Org:
USDOE Office of Science (SC), Basic Energy Sciences (BES) (SC-22)
Country of Publication:
United States
Language:
English
Subject:
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND SUPERFLUIDITY
OSTI Identifier:
1439295