Measurement of an Enhanced Superconducting Phase and a Pronounced Anisotropy of the Energy Gap of a Strained FeSe Single Layer in $\mathrm{FeSe}/\mathrm{Nb}:{\mathrm{SrTiO}}_{3}/{\mathrm{KTaO}}_{3}$ Heterostructures Using Photoemission Spectroscopy

Measurement of an Enhanced Superconducting Phase and a Pronounced Anisotropy of the Energy Gap of a Strained FeSe Single Layer in $FeSe / Nb : {SrTiO}_{3} / {KTaO}_{3}$ Heterostructures Using Photoemission Spectroscopy

R. Peng, X. P. Shen, X. Xie, H. C. Xu, S. Y. Tan, M. Xia, T. Zhang, H. Y. Cao, X. G. Gong, J. P. Hu, B. P. Xie, and D. L. Feng

Phys. Rev. Lett. 112, 107001 – Published 11 March 2014

Abstract

Single-layer FeSe films with an extremely expanded in-plane lattice constant of $3.99 \pm 0.02 Å$ are fabricated by epitaxially growing $FeSe / Nb : {SrTiO}_{3} / {KTaO}_{3}$ heterostructures and studied by in situ angle-resolved photoemission spectroscopy. Two elliptical electron pockets at the Brillouin zone corner are resolved with negligible hybridization between them, indicating that the symmetry of the low-energy electronic structure remains intact as a freestanding single-layer FeSe, although it is on a substrate. The superconducting gap closes at a record high temperature of 70 K for the iron-based superconductors. Intriguingly, the superconducting gap distribution is anisotropic but nodeless around the electron pockets, with minima at the crossings of the two pockets. Our results place strong constraints on current theories.

Received 11 October 2013

DOI:https://doi.org/10.1103/PhysRevLett.112.107001

Authors & Affiliations

R. Peng^1,2, X. P. Shen^1,2, X. Xie^1,2, H. C. Xu^1,2, S. Y. Tan^1,2, M. Xia^1,2, T. Zhang^1,2, H. Y. Cao^1,3, X. G. Gong^1,3, J. P. Hu^4,5, B. P. Xie^1,2,*, and D. L. Feng^1,2,†

¹State Key Laboratory of Surface Physics, Department of Physics, Fudan University, Shanghai 200433, China
²Advanced Materials Laboratory, Fudan University, Shanghai 200433, People’s Republic of China
³Key Laboratory for Computational Physical Sciences (MOE), Fudan University, Shanghai 200433, China
⁴Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100080, People’s Republic of China
⁵Department of Physics, Purdue University, West Lafayette, Indiana 47907, USA

^*bpxie@fudan.edu.cn
^†dlfeng@fudan.edu.cn

Article Text (Subscription Required)

Click to Expand

Supplemental Material (Subscription Required)

Click to Expand

References (Subscription Required)

Click to Expand

Issue

Vol. 112, Iss. 10 — 14 March 2014

Reuse & Permissions

Access Options

Author publication services for translation and copyediting assistance advertisement

Images

Figure 1
(a) Schematic illustration of the heterostructure. (b) Cubic unit cell of ${KTaO}_{3}$ substrate. (c) X-ray diffraction reciprocal space map of the heterostructure around the (103) Bragg reflections. (d) Thickness-dependent photoemission intensity maps at the Fermi energy ( $E_{F}$ ) for $FeSe / Nb : {SrTiO}_{3} / {KTaO}_{3}$ . The intensity was integrated over a window of [ $E_{F} - 10 meV$ , $E_{F} + 10 meV$ ]. (e) In-plane lattice of FeSe as a function of FeSe thickness deduced from the Brillouin zone (BZ) size measured by ARPES. The data of $FeSe / Nb : {SrTiO}_{3}$ are reproduced from Ref. [5].
Reuse & Permissions
Figure 2
(a) Photoemission intensity map of ${FeSe}^{X}$ compared with that of ${FeSe}^{M}$ , which is reproduced from Ref. [5]. The intensity was integrated over a window of [ $E_{F} - 10 meV$ , $E_{F} + 10 meV$ ]. The measured Fermi surface sheets are shown by the dashed curves. (b) Fourfold symmetrized sketch of the Fermi surface sheets observed in (a) for ${FeSe}^{X}$ . Cuts #1, #2, #3, and #4 are indicated in the BZ. (c) Calculated band structure and Fermi surface of single-layer FeSe with the in-plane lattice constant of 3.989 Å. An extra $0.12 e^{-}$ per Fe is included by shifting the chemical potential [23]. (d1) Photoemission intensity along cut #1 through $Γ$ as indicated in (b) and (d2) the corresponding energy distribution curves (EDCs). (e1) Photoemission intensity along cut #2, (e2) the corresponding MDCs, and (e3) MDC at $E_{F}$ fitted by Lorentzian peaks. (f1)–(f3) Same as (e1)–(e3) but along cut #3. (g1)–(g3) Same as (e1)–(e3) but along cut #4. (h) Temperature dependence of the symmetrized EDC at the Fermi crossing of $γ_{1}$ band along cut #2 for ${FeSe}^{X}$ . The gap is obtained following the standard fitting procedure described in Ref. [28]. The original data, the fitted results, and gap positions are shown in black dots, red curves, and red arrows, respectively. (i) Superconducting gap versus temperature for ${FeSe}^{X}$ compared with that of ${FeSe}^{M}$ , which is reproduced from Ref. [5].
Reuse & Permissions
Figure 3
(a) Fermi surface sheets around $M 1$ and the definition of cut #1, cut #2, and polar angle $φ$ . (b),(c) Symmetrized EDCs for sample $A$ along cut #1 and #2, respectively. The $k_{F}$ ’s of $γ_{1}$ are shown by the thicker curves, with the polar angle of 274° and 224°, respectively. (d) Symmetrized EDCs at $k_{F}$ ’s of $γ_{1}$ band for sample $A$ , with the momentums counterclockwise along $γ_{1}$ pocket as indicated by the polar angles. (e) Same as (d) but measured on sample $B$ .
Reuse & Permissions
Figure 4
(a) Gap distribution of the $γ_{1}$ pocket in polar coordinates for sample $A$ , where the radius represents the gap size and the polar angle represents $φ$ . The gap is estimated through an empirical fit [28], and the error bars come from the standard deviation of the fitting process. Solid black curve shows the fitting by $Δ = Δ_{0} + Δ_{1} | \cos (2 φ) |$ , with $Δ_{0} = 8.52 meV$ and $Δ_{1} = 8.69 meV$ . (b) Same as (a) but measured on sample $B$ . The black curve shows the fitting results with $Δ_{0} = 8.41 meV$ and $Δ_{1} = 5.73 meV$ . (c) Illustration for the sign-changing $s$ -wave pairing symmetry around $M$ in recent models, and it is arranged in $A_{1 g}$ symmetry on the four zone corners around $Γ$ [21, 22, 32].
Reuse & Permissions

Physical Review Letters

Measurement of an Enhanced Superconducting Phase and a Pronounced Anisotropy of the Energy Gap of a Strained FeSe Single Layer in FeSe/Nb:SrTiO3/KTaO3 Heterostructures Using Photoemission Spectroscopy

R. Peng, X. P. Shen, X. Xie, H. C. Xu, S. Y. Tan, M. Xia, T. Zhang, H. Y. Cao, X. G. Gong, J. P. Hu, B. P. Xie, and D. L. Feng

Phys. Rev. Lett. 112, 107001 – Published 11 March 2014

Abstract

Authors & Affiliations

Article Text (Subscription Required)

Supplemental Material (Subscription Required)

References (Subscription Required)

Issue

Access Options

Authorization Required

Other Options

Download & Share

Images

Figure 1

Figure 2

Figure 3

Figure 4

Log In

Search

Article Lookup

Paste a citation or DOI

Enter a citation

Measurement of an Enhanced Superconducting Phase and a Pronounced Anisotropy of the Energy Gap of a Strained FeSe Single Layer in $FeSe / Nb : {SrTiO}_{3} / {KTaO}_{3}$ Heterostructures Using Photoemission Spectroscopy