High-density carriers at a strongly coupled interface between graphene and a three-dimensional topological insulator

A. Zalic, T. Dvir, and H. Steinberg

Phys. Rev. B 96, 075104 – Published 2 August 2017

Abstract

We report on a strongly coupled bilayer graphene- ${Bi}_{2} {Se}_{3}$ device with a junction resistance of less than 1.5 $k Ω μ m^{2}$ . This device exhibits unique behavior at the interface, which cannot be attributed to either material in absence of the other. We observe quantum oscillations in the magnetoresistance of the junction, indicating the presence of well-resolved Landau levels due to hole carriers of unknown origin with a very large Fermi surface. These carriers, found only at the interface, could conceivably arise due to significant hole doping of the bilayer graphene with charge transfer on the order of $2 \times 10^{13} {cm}^{- 2}$ , or due to twist-angle-dependent miniband transport.

Received 8 January 2017

DOI:https://doi.org/10.1103/PhysRevB.96.075104

Physics Subject Headings (PhySH)

Electrical conductivity Fermi surface

Graphene Topological insulators

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

A. Zalic, T. Dvir, and H. Steinberg

Racah Institute of Physics, The Hebrew University, Jerusalem 91904, Israel

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Issue

Vol. 96, Iss. 7 — 15 August 2017

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Images

Figure 1
(a), (b) False color SEM images of Devices 1,2, with BLG flake (blue) on the bottom and ${Bi}_{2} {Se}_{3}$ flake (red) on top. Black arrow marks crystallographic orientation of ${Bi}_{2} {Se}_{3}$ determined by EBSD measurement. (c), (d) Zigzag, armchair orientations between the BLG lattice (gray, top layer only shown) and ${Bi}_{2} {Se}_{3}$ (black, bottom Se layer only shown) in Device 1. The airmchair orientation is at a nearly commensurate angle of $33^{\circ}$ . (e) Schematic illustration of measurement configuration. Current is induced between the source electrode on the TI and the drain electrode on the BLG, and voltage is measured between the source and the drain ( $V_{S D}$ ), and between the source and the probe ( $V_{S P}$ ). Gate voltage is applied between the Si back gate and the device.
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Figure 2
(a) $d I / d V$ vs $V_{S D}$ of Device 2 (weakly coupled), tracing the linear DOS of the ${Bi}_{2} {Se}_{3}$ surface state. The Dirac point, where the DOS reaches a minimum, is located at $V_{S D} \approx - 230 mV$ . (b) $d I / d V$ vs $V_{S P}$ of Device 1 (strongly coupled). The trace is over a narrow range of $V_{S P}$ and does not exhibit any notable feature other than a low bias conductance suppression.
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Figure 3
Quantum oscillations at the junction of Device 1 in a perpendicular magnetic field. (a) $d V_{S P} / d V_{g}$ (color scale) vs $V_{g}$ and $B$ . Dispersing features may be traced to two intersecting Landau fans, originating at gate voltages of $\sim 8$ V and $\sim 294$ V. (b) Fits to the data in (a), extrapolated to the fan origins. (c) $d V_{S P} / d B$ vs $V_{g}$ and $B$ , highlighting the fast oscillating Landau fan originating at $\sim 294$ V. (d) Fourier amplitudes of $V_{S P}$ (linear background subtracted) vs $V_{g}$ and frequency $F$ in $1 / B$ . $F$ is proportional to the area of the Fermi surface in momentum space $A_{k}$ , which may be translated to the hole charge carrier density $n$ (right-hand $y$ axis) assuming known Landau level degeneracy (here we take $g = 4$ for BLG, see text).
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Figure 4
(a)–(c) Visualization of measured $A_{k}$ , depicted as circular, in comparison with expected dispersions of (a) pristine BLG, (b) pristine ${Bi}_{2} {Se}_{3}$ , and (c) highly hole-doped BLG. Electron bands are shown in red, hole bands in blue. Gate voltage is roughly translated to an energy scale using typical dispersion parameters discussed in the text. $A_{k}$ perimeters at the extremal gate voltages of $\pm 35$ V are marked in black, and the intermediate dispersion is shaded gray. (d) Real-space moiré pattern between graphene and ${Bi}_{2} {Se}_{3}$ lattices at a $33^{\circ}$ twist angle. The black line shows the SL wavelength of $\approx 4$ nm. (e) Momentum-space illustration of BLG (red), ${Bi}_{2} {Se}_{3}$ (blue), and SL (gray) BZs at a $33^{\circ}$ twist angle, with the BLG K,K' points marked in red and green, respectively, and the ${Bi}_{2} {Se}_{3} Γ$ point marked in blue (f) Closeup of the central SLBZ. BLG (red/green, holes) and ${Bi}_{2} {Se}_{3}$ (blue, electrons) Fermi pockets repeat with SL periodicity. A possible hole pocket equal in area to the measured $A_{k}$ exists in the second BZ of the ${Bi}_{2} {Se}_{3}$ dispersion, outlined with a black dotted line.
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