Electrical control of intervalley scattering in graphene via the charge state of defects

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Electrical control of intervalley scattering in graphene via the charge state of defects

Baoming Yan, Qi Han, Zhenzhao Jia, Jingjing Niu, Tuocheng Cai, Dapeng Yu, and Xiaosong Wu

Phys. Rev. B 93, 041407(R) – Published 11 January 2016

Abstract

We study the intervalley scattering in defected graphene by low-temperature transport measurements. The scattering rate is strongly suppressed when defects are charged. This finding highlights “screening” of the short-range part of a potential by the long-range part. Experiments on calcium-adsorbed graphene confirm the role of a long-range Coulomb potential. This effect is applicable to other multivalley systems, provided that the charge state of a defect can be electrically tuned. Our result provides a means to electrically control valley relaxation and has important implications in valley dynamics in valleytronic materials.

Received 22 June 2015
Revised 26 November 2015

DOI:https://doi.org/10.1103/PhysRevB.93.041407

Physics Subject Headings (PhySH)

Magnetoresistance Valleytronics

Graphene

Raman spectroscopy

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Baoming Yan, Qi Han, Zhenzhao Jia, Jingjing Niu, Tuocheng Cai, Dapeng Yu, and Xiaosong Wu^*

State Key Laboratory for Artificial Microstructure and Mesoscopic Physics, Peking University, Beijing 100871, China and Collaborative Innovation Center on Quantum Matter, Beijing 100871, China

^*xswu@pku.edu.cn

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Issue

Vol. 93, Iss. 4 — 15 January 2016

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Images

Figure 1
(a) Raman spectrum measured after introducing defects by ${Ar}^{+}$ bombardment. Pronounced $D$ peak is observed with $I_{D} / I_{G} \sim 0.2$ , corresponding to a defect concentration of $1.9 \times 10^{11} {cm}^{- 2}$ . (b) Conductivity vs gate voltage at different temperatures. For comparison, the conductivity of a typical pristine graphene sample at 4.5 K is plotted as a dashed line. Separated data points at 1 K and 2 K are taken from the magnetoconductivity data at $B = 0$ and linked by lines as a guide to the eye.
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Figure 2
Intervalley scattering in defected graphene. (a) Low-field magnetoconductivity $Δ σ$ of defected graphene (dots) at various gate voltages at $T = 1$ K. Data are shifted in $y$ for clarity. Solid lines are best fits of the experimental data to Eq. (2). Different colors of shading represent the different charge states of defects (see the text), light pink for the charged state and light green for the neutral state. (b) Intervalley scattering times obtained by fitting are plotted as a function of the gate voltage. On the charged side, the error bars represent the uncertainty in the fitting to Eq. (2), while on the neutral side, they represent the upper bound ( $2 τ$ ; see the text) and the lower bound ( $τ$ ) of $τ_{i}$ .
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Figure 3
Schematic diagram for defect scattering. (a) Density of states of defected graphene. (i) When the Fermi level is in the conduction band (positive gate voltage), defects are neutral, (ii) while they become charged when the Fermi level is in the valance band (negative gate voltage). (b) Carriers scattered off a neutral defect and a charged one.
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Figure 4
Transport experiments of Ca-doped graphene. (a) Conductivity versus gate voltage at increasing coverage of Ca adatoms. Different colors denote different Ca coverage. (b) The gate dependence of conductivity for a Ca coverage of 0.24% at temperatures from 1 K to 10 K at steps of 1 K. Inset: Schematic diagram of Ca adatoms on graphene. Ca adatoms are assumed to be at the hollow site [23, 31]. (c) Low-field magnetoconductivity data (dots) for a Ca coverage of 0.24% at various gate voltages from $- 40$ to 70 V relative to the Dirac point voltage ( $T = 150$ mK). The black curve is the conductivity before Ca deposition (pristine). Solid lines are best fits of experimental data to Eq. (2). (d) Intervalley scattering time $τ_{i}$ obtained by fitting to Eq. (2) is plotted as a function of gate voltage. The black dashed line denotes $τ_{i}$ of pristine graphene in (c).
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