Evolution of electronic structure upon Cu doping in the topological insulator Bi${}_{2}$Se${}_{3}$

Evolution of electronic structure upon Cu doping in the topological insulator Bi $_{2}$ Se $_{3}$

Y. Tanaka, K. Nakayama, S. Souma, T. Sato, N. Xu, P. Zhang, P. Richard, H. Ding, Y. Suzuki, P. Das, K. Kadowaki, and T. Takahashi

Phys. Rev. B 85, 125111 – Published 12 March 2012

Abstract

We have performed angle-resolved photoemission spectroscopy of Cu $_{x}$ Bi $_{2}$ Se $_{3}$ as a function of Cu doping ( $x = 0.0$ –0.25) to investigate the doping-induced evolution of the electronic structure. We found that the topological surface state is preserved even in the heavy-doping region ( $x$ $=$ 0.25), indicative of the robustness of the surface state against doping and impurities. The estimated carrier concentration is far smaller than that expected from a simple intercalation picture, and saturates at $x \sim 0.1$ where superconductivity emerges. This indicates that the carrier concentration responsible for superconductivity is dominated by a subtle balance between two competing processes of electron and hole doping.

Received 10 December 2011

DOI:https://doi.org/10.1103/PhysRevB.85.125111

Authors & Affiliations

Y. Tanaka¹, K. Nakayama¹, S. Souma², T. Sato¹, N. Xu³, P. Zhang³, P. Richard³, H. Ding³, Y. Suzuki^4,5, P. Das^4,5, K. Kadowaki^4,5, and T. Takahashi^1,2

¹Department of Physics, Tohoku University, Sendai 980-8578, Japan
²WPI Research Center, Advanced Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan
³Beijing National Laboratory for Condensed Matter Physics, and Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China
⁴Graduate School of Pure and Applied Sciences, University of Tsukuba, Tsukuba 305-8573, Japan
⁵WPI-MANA(NIMS) and CREST-JST

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Vol. 85, Iss. 12 — 15 March 2012

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Images

Figure 1
(a) Bulk and surface Brillouin zones (BZs) of Cu $_{x}$ Bi $_{2}$ Se $_{3}$ . (b) Bulk BZ in the $k_{x}$ - $k_{z}$ plane. (c) Near- $E_{F}$ ARPES intensity around the $\bar{Γ}$ point of pristine Bi $_{2}$ Se $_{3}$ at $T = 30$ K plotted as a function of in-plane wave vector and $E_{B}$ measured at various $k$ cuts [around the $k_{x} = 0$ region of the solid curves in (b)]. SS, VB, and CB denote surface state, valence band, and conduction band, respectively. The cut crossing the $Γ$ point of the bulk BZ ( $h ν = 19$ eV) is indicated by a blue curve in (b). We used an inner potential value of $V_{0} = 9.7$ eV to calculate the $k_{z}$ value. (d) and (e) Cu-doping dependence of (d) near- $E_{F}$ ARPES intensity around the $Γ$ point and (e) corresponding energy distribution curves in Cu $_{x}$ Bi $_{2}$ Se $_{3}$ ( $x = 0.0, 0.1, 0.15$ , and 0.25) measured at $h ν = 19$ eV. $E_{DP}$ denotes the energy position of the Dirac point. Blue dashed curves in (e) are a guide to eyes to trace the surface-band dispersions.Reuse & Permissions
Figure 2
(a) Cu-doping dependence of the chemical potential ( $μ$ ) shift estimated from the energy shift of the Dirac point ( $E_{DP}$ ), the conduction-band (CB) bottom, and the valence-band (VB) energy at the $Γ$ point. Previously reported results for $E_{DP}$ [9] are also shown for comparison. A purple dashed curve is a guide to eyes to trace the energy shift. (b) Near- $E_{F}$ ARPES intensity as a function of wave vector and $E_{B}$ of Cu $_{x}$ Bi $_{2}$ Se $_{3}$ ( $x = 0.25$ ) measured with $h ν = 19$ and 51 eV. (c) Comparison of the surface-state (SS) and bulk band dispersions in Cu $_{x}$ Bi $_{2}$ Se $_{3}$ ( $x = 0.0, 0.1, 0.15, 0.25$ ) estimated by tracing the peak position in energy distribution curves at $T = 30$ K. The band dispersions are shifted so as to align the Dirac point. The energy position of the chemical potential $μ$ for each composition is shown by a dashed line.Reuse & Permissions
Figure 3
(a) ARPES intensity map at $E_{F}$ of Cu $_{x}$ Bi $_{2}$ Se $_{3}$ ( $x = 0.25$ ) as a function of two-dimensional wave vector measured at $h ν = 19$ eV. The intensity at $E_{F}$ has been obtained by integrating the ARPES spectra within $\pm$ 5 meV with respect to $E_{F}$ . Red and blue circles show experimental Fermi vectors ( $k_{F}$ 's) of the surface and bulk conduction bands, respectively, estimated by tracing the peak maxima in the momentum distribution curves (MDCs). The $k_{F}$ points were folded by taking into account the crystal symmetry. (b) MDCs at $E_{F}$ of Cu $_{x}$ Bi $_{2}$ Se $_{3}$ ( $x = 0.25$ ) measured at $h ν = 19$ and 51 eV. Dashed lines in (a) and (b) correspond to the momentum location of $k_{F}$ points along the $k_{x}$ axis. (c) Cu doping dependence of the bulk and surface carrier numbers with respect to each Brillouin zone ( $n_{b}$ and $n_{s}$ ). Filled circles are obtained from the present ARPES experiment and open circles are from a previous study.[9] The black dashed line represents the theoretical $x$ dependence of carrier concentrations when one assumes that all the doped Cu atoms are intercalated into the van der Waals gap between Bi $_{2}$ Se $_{3}$ quintuple layers to produce one electron per one Cu atom. Red and blue dashed lines are guides for the eyes to trace the $x$ dependence of $n_{s}$ and $n_{b}$ , respectively.Reuse & Permissions

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