Observation of two-dimensional bulk electronic states in the superconducting topological insulator heterostructure ${\mathbf{Cu}}_{x}{(\mathbf{PbSe})}_{\mathbf{5}}{({\mathbf{Bi}}_{\mathbf{2}}{\mathbf{Se}}_{\mathbf{3}})}_{\mathbf{6}}$: Implications for unconventional superconductivity

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Observation of two-dimensional bulk electronic states in the superconducting topological insulator heterostructure ${Cu}_{x} {(PbSe)}_{5} {({Bi}_{2} {Se}_{3})}_{6}$ : Implications for unconventional superconductivity

K. Nakayama, H. Kimizuka, Y. Tanaka, T. Sato, S. Souma, T. Takahashi, Satoshi Sasaki, Kouji Segawa, and Yoichi Ando

Phys. Rev. B 92, 100508(R) – Published 28 September 2015

Abstract

We have performed angle-resolved photoemission spectroscopy (ARPES) on ${Cu}_{x} {(PbSe)}_{5} {({Bi}_{2} {Se}_{3})}_{6}$ [(CPSBS); $x = 1.47$ ], a superconductor derived from a topological insulator heterostructure, to elucidate the electronic states relevant to the occurrence of possible unconventional superconductivity. Upon Cu intercalation into the parent compound ${(PbSe)}_{5} {({Bi}_{2} {Se}_{3})}_{6}$ , we observed a distinct energy shift of the bulk conduction band due to electron doping. Photon-energy-dependent ARPES measurements of CPSBS revealed that the observed bulk band forms a cylindrical electronlike Fermi surface at the Brillouin-zone center. The two-dimensional nature of the bulk electronic states puts strong constraints on the possible topological character of the superconducting state in CPSBS.

Received 2 April 2015

DOI:https://doi.org/10.1103/PhysRevB.92.100508

Authors & Affiliations

K. Nakayama¹, H. Kimizuka¹, Y. Tanaka¹, T. Sato¹, S. Souma², T. Takahashi^1,2, Satoshi Sasaki^3,*, Kouji Segawa^3,†, and Yoichi Ando^3,‡

¹Department of Physics, Tohoku University, Sendai 980-8578, Japan
²WPI Research Center, Advanced Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan
³Institute of Scientific and Industrial Research, Osaka University, Ibaraki, Osaka 567-0047, Japan

^*Present address: School of Physics and Astronomy, University of Leeds, Leeds LS2 9JT, United Kingdom.
^†Present address: Department of Physics, Kyoto Sangyo University, Kyoto 603-8555, Japan.
^‡Present address: Institute of Physics II, University of Cologne, Köln 50937, Germany.

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Vol. 92, Iss. 10 — 1 September 2015

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Figure 1
(a)–(d) Comparison of near- $E_{F}$ EDCs around the $\bar{Γ}$ point [(a) and (c)] and corresponding ARPES intensity [(b) and (d)]. The data in (a)–(d) were taken on different domains of the cleaved surface of pristine PSBS. All the data have been obtained at $T = 30 K$ with 60-eV photons. Black and blue dots in (a) [as well as black and blue curves in (b)] are a guide to the eyes to trace the dispersions of bulk conduction band and gapped Dirac-cone bands, respectively. (e) Schematic of the cleaved surface of PSBS. (f) and (g) Near- $E_{F}$ EDCs and corresponding ARPES intensity plot, respectively, for CPSBS measured with $h ν = 60 eV$ . (h) Schematic of the cleaved surface of CPSBS.
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Figure 2
(a) EDCs around the $\bar{Γ}$ point for CPSBS measured at $T = 30 K$ with $h ν = 21 eV$ . (b) Second-derivative intensity of (a) plotted as a function of binding energy and wave vector. (c) Same as (b), but measured with $h ν = 60 eV$ . (d) and (e) ARPES intensity map at $E_{F}$ and 0.28-eV binding energy, respectively, for CPSBS plotted as a function of the in-plane wave vector. The intensity was obtained by integrating the second-derivative EDCs within $\pm 10 meV$ centered at $E_{F}$ or 0.28 eV. (f) and (g) Second-derivative ARPES intensity around the $\bar{Γ}$ point for PSBS at $h ν = 21$ and 60 eV, respectively. (h) Schematic of the photoemission process for $h ν = 21$ and 60 eV. (i) and (j) Near- $E_{F}$ band dispersions obtained soon after cleaving and $\sim 10 h$ after cleaving, respectively, at $h ν = 15 eV$ . (k) Comparison of the EDCs at the $\bar{Γ}$ point for fresh and aged surfaces of CPSBS. (l) Momentum distribution curve at $E_{F}$ extracted from the raw ARPES intensity of (j). Arrows indicate the Fermi wave vectors of ${CB}_{1 st}$ and band A.
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Figure 3
(a) Second-derivative ARPES intensity around the $\bar{Γ}$ point for CPSBS at various photon energies plotted as a function of binding energy and wave vector. ${CB}_{1 st}$ and ${CB}_{2 nd}$ denote the conduction bands stemming from the topmost 1-QL ${Bi}_{2} {Se}_{3}$ unit and the second ${Bi}_{2} {Se}_{3}$ unit located beneath the PbSe layer, respectively. (b) Comparison of the EDC at the $\bar{Γ}$ point measured with various photon energies. (c) Photon-energy dependence of the band dispersion for ${CB}_{2 nd}$ in CPSBS extracted from the second-derivative intensities. Filled circles and triangles represent the results for the fresh and aged surfaces, respectively. Detailed procedures used to extract the data points for the aged surface are presented in Figs. 2–2 and the Supplemental Material [31]. The red curve is a guide for the eyes to trace the band dispersion. The band dispersion for PSBS measured with $h ν = 21 eV$ is also plotted by open diamonds. (d) Schematic of the band dispersion and the Fermi surface in the topmost and the second ${Bi}_{2} {Se}_{3}$ units in CPSBS.
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Physical Review B

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Observation of two-dimensional bulk electronic states in the superconducting topological insulator heterostructure ${Cu}_{x} {(PbSe)}_{5} {({Bi}_{2} {Se}_{3})}_{6}$ : Implications for unconventional superconductivity

K. Nakayama, H. Kimizuka, Y. Tanaka, T. Sato, S. Souma, T. Takahashi, Satoshi Sasaki, Kouji Segawa, and Yoichi Ando

Phys. Rev. B 92, 100508(R) – Published 28 September 2015

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Physical Review B

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Observation of two-dimensional bulk electronic states in the superconducting topological insulator heterostructure Cux(PbSe)5(Bi2Se3)6: Implications for unconventional superconductivity

K. Nakayama, H. Kimizuka, Y. Tanaka, T. Sato, S. Souma, T. Takahashi, Satoshi Sasaki, Kouji Segawa, and Yoichi Ando

Phys. Rev. B 92, 100508(R) – Published 28 September 2015

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Observation of two-dimensional bulk electronic states in the superconducting topological insulator heterostructure ${Cu}_{x} {(PbSe)}_{5} {({Bi}_{2} {Se}_{3})}_{6}$ : Implications for unconventional superconductivity