Correlation-Driven Electron-Hole Asymmetry in Graphene Field Effect Devices
Nicholas Dale, Ryo Mori, M. Iqbal Bakti Utama, Jonathan D. Denlinger, Conrad Stansbury, Claudia G. Fatuzzo, Sihan Zhao, Kyunghoon Lee, Takashi Taniguchi, Kenji … 展开作者
Nicholas Dale, Ryo Mori, M. Iqbal Bakti Utama, Jonathan D. Denlinger, Conrad Stansbury, Claudia G. Fatuzzo, Sihan Zhao, Kyunghoon Lee, Takashi Taniguchi, Kenji Watanabe, Chris Jozwiak, Aaron Bostwick, Eli Rotenberg, Roland J. Koch, Feng Wang, Alessandra Lanzara
Electron-hole asymmetry is a fundamental property in solids that can determine the nature of quantum phase transitions and the regime of operation for devices. The observation of electron-hole asymmetry in graphene and recently in the phase diagram of bilayer graphene has spurred interest into whether it stems from disorder or from fundamental interactions such as correlations. Here, we report an effective new way to access electron-hole asymmetry in 2D materials by directly measuring the quasiparticle self-energy in graphene/Boron Nitride field effect devices. As the chemical potential moves from the hole to the electron doped side, we see an increased strength of electronic correlations manifested by an increase in the band velocity and inverse quasiparticle lifetime. These results suggest that electronic correlations play an intrinsic role in driving electron hole asymmetry in graphene and provide a new insight for asymmetries in more strongly correlated materials.
Electron-electron interaction and electron-hole asymmetry in bilayer graphene (Supporting Materials)
我们报告了使用Shubnikov-de Haas振荡的温度依赖性精确测量高质量双层石墨烯中的有效质量m*。在0.7 x 10^12/cm ^2 < n < 4.1 x 10^12 /cm ^2的密度范围内，空穴质量m*_h和电子质量m*_e都随着密度的增加而增加，表明能带的双曲线性质。空穴质量m*_h大约比电子质量m*_e大20-30%。紧束缚计算提供了一个很好的描述的电子-空穴的不对称性，并产生一个准确的测量层间跳跃参数v_4 = 0.063。与单粒子值相比，m*_h和m*_e均被大幅抑制，为双层石墨烯能带结构中电子-电子相互作用的强重整化效应提供了清晰且前所未有的证据。
Electron-electron interaction and electron-hole asymmetry in bilayer graphene
我们报告了使用Shubnikov-de Haas振荡的温度依赖性精确测量高质量双层石墨烯中的有效质量m*。在0.7 x 10^12/cm ^2 < n < 4.1 x 10^12 /cm ^2的密度范围内，空穴质量m*_h和电子质量m*_e都随着密度的增加而增加，表明能带的双曲线性质。空穴质量m*_h大约比电子质量m*_e大20-30%。紧束缚计算提供了一个很好的描述的电子-空穴的不对称性，并产生一个准确的测量层间跳跃参数v_4 = 0.063。与单粒子值相比，m*_h和m*_e均被大幅抑制，为双层石墨烯能带结构中电子-电子相互作用的强重整化效应提供了清晰且前所未有的证据。
Observations of electron-hole asymmetry in transport through graphene devices at high magnetic field challenge prevalent models of the graphene quantum Hall effect. Here, we study this asymmetry both in conventional magnetotransport and in scanning gate microscopy maps measured in an encapsulated graphene constriction. We reveal that the presence of upstream modes and local doping in the vicinity of electrical contacts leads to a totally different picture of topological breakdown for electrons and holes, explaining the observed asymmetry.
In this paper, we study the quantum properties of a bilayer graphene with (asymmetry) line defects. The localized states are found around the line defects. Thus, the line defects on one certain layer of the bilayer graphene can lead to an electric transport channel. By adding a bias potential along the direction of the line defects, we calculate the electric conductivity of bilayer graphene with line defects using Landauer-Büttiker theory, and show that the channel affects the electric conductivity remarkably by comparing the results with those in a perfect bilayer graphene. This one-dimension…
By using the real-space Green-Kubo formalism we study numerically the electron transport properties of low-fluorinated graphene. At low temperatures the diffuse transport regime is expected to be prevalent, and we found a pronounced electron-hole asymmetry in electrical conductivity as a result of quasi-resonant scattering on the localized states. For the finite temperatures in the variable-range hopping transport regime the interpretation of numerical results leads to the appearance of local minima and maxima of the resistance near the energies of the localized states. A comparison with the e…
Using the method developed in the companion paper, we construct the effective continuum theories for two different microscopic tight binding models of the twisted bilayer graphene at the twist angle of $1.05^\circ$, one Slater-Koster based and the other ab-initio Wannier based. The energy spectra obtained from the continuum theory -- either for rigid twist or including lattice relaxation -- are found to be in nearly perfect agreement with the spectra from the tight binding models when the gradient expansion is carried out to second order, demonstrating the validity of the method. We also analy…
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We derive an effective Hamiltonian at low energies for bilayer graphene when Fermi velocity manufactured on each layer is different of the velocity measured in pristine graphene. Based on the effective Hamiltonian, we investigate the influence of Fermi velocity asymmetry on the band structure of trigonally warped bilayer graphene in the presence of interlayer applied bias. In this case, the Fermi line at low energies is still preserved its threefold rotational symmetry appearing as the three pockets. Furthermore, the interlayer asymmetry in Fermi velocities leads to an indirect band gap which …
Under a magnetic field perpendicular to an monolayer graphene, the existence of a two-dimensional periodic scatter array can not only mix Landau levels of the same valley for displaying split electron-hole Hofstadter-type energy spectra, but also couple two sets of Landau subbands from different valleys in a bilayer graphene. Such a valley mixing effect with a strong scattering strength has been found observable and studied thoroughly in this paper by using a Bloch-wave expansion approach and a projected $2\times 2$ effective Hamiltonian including interlayer effective mass, interlayer coupling…
We report strong electron-phonon coupling in magic-angle twisted bilayer graphene (MA-TBG) obtained from atomistic description of the system including more than 10000 atoms in the moire supercell. Electronic structure, phonon spectrum, and electron-phonon coupling strength lambda are obtained before and after atomic-position relaxation both in and out of plane. Obtained lambda is very large for MA-TBG, with lambda > 1 near the half-filling energies of the flat bands, while it is small (lambda ~ 0.1) for monolayer and unrotated bilayer graphene. Significant electron-hole asymmetry occurs in the…
We investigate the magnetic minibands of a heterostructure consisting of bilayer graphene (BLG) and hexagonal boron nitride (hBN) by numerically diagonalizing a two-band Hamiltonian that describes electrons in BLG in the presence of a moire potential. Due to inversion-symmetry breaking characteristic for the moire potential, the valley symmetry of the spectrum is broken, but despite this, the zero-energy Landau level in BLG survives, albeit with reduced degeneracy. In addition, we derive effective models for the low-energy features in the magnetic minibands and demonstrate the appearance of se…
Accurate determination of electron-hole asymmetry and next-nearest neighbor hopping in graphene
次近邻跳跃项t'决定了石墨烯中几种现象的大小和重要性，这些现象包括由于断裂的键引起的自掺杂和在t'存在下不再完美的克莱因隧穿。理论估计的t'变化很大，而一些现有的测量，通过使用偏振分辨磁谱已经发现了令人惊讶的大t'，接近或甚至超过最高的理论值。在这里，我们报告了通过使用高质量电容设备对石墨烯中的态密度进行的专门测量。态密度表现出明显的电子-空穴不对称性，随能量线性增加。该行为产生约-0.30 eV +-15%的t'，与理论估计的高端一致。我们讨论了电子-电子相互作用在确定t'和概述的现象，可以影响这样一个大的值的t'的作用。
We report electron-hole conduction asymmetry in monolayer graphene. Previously, it has been claimed that electron-hole conduction asymmetry is due to imbalanced carrier injection from metallic electrodes. Here, we show that metallic contacts have negligible impact on asymmetric conduction and may be either sample or device-dependent phenomena. Electrical measurements show that monolayer graphene based devices exhibit suppressed electron conduction compared to hole conduction due to the presence of donor impurities which scatter electrons more efficiently. This can be explained by the relativis…
The conductivity and Hall effect were measured in CVD-grown monolayer graphene as a function of the gate voltage, $V_{g}$, at temperatures down to $T$ = 2 K and in magnetic fields up to $B$ = 8 T. The minimal conductivity was observed at positive $V_{g}$ which shows the position of the charge neutrality point, $V_{NP}$. With decreasing $T$, $V_{NP}$ first decreases, but stop to decrease at low $T$. Hysteresis of conductivity shows similar behavior: it decreases with decreasing $T$ and disappears at low $T$. A significant asymmetry was observed at low density of charge carriers $|n|=(n,p)$: mob…
Graphene/hexagonal boron nitride (G/$h$-BN) heterostructures offer an excellent platform for developing nanoelectronic devices and for exploring correlated states in graphene under modulation by a periodic superlattice potential. Here, we report on transport measurements of nearly $0^{\circ}$-twisted G/$h$-BN heterostructures. The heterostructures investigated are prepared by dry transfer and thermally annealing processes and are in the low mobility regime (approximately $3000~\mathrm{cm}^{2}\mathrm{V}^{-1}\mathrm{s}^{-1}$ at 1.9 K). The replica Dirac spectra and Hofstadter butterfly spectra a…
Spontaneous orbital magnetism observed in twisted bilayer graphene (tBG) on nearly aligned hexagonal boron nitride (BN) substrate builds on top of the electronic structure resulting from combined G/G and G/BN double moire interfaces. Here we show that tBG/BN commensurate double moire patterns can be classified into two types, each favoring the narrowing of either the conduction or valence bands on average, and obtain the evolution of the bands as a function of the interlayer sliding vectors and electric fields. Finite valley Chern numbers $\pm 1$ are found in a wide range of parameter space wh…
We examine the possibility of using graphene nanoribbons (GNRs) with directly substituted chromium atoms as spintronic device. Using density functional theory, we simulate a voltage bias across a constructed GNR in a device setup, where a magnetic dimer has been substituted into the lattice. Through this first principles approach, we calculate the electronic and magnetic properties as a function of Hubbard U, voltage, and magnetic configuration. By calculating of the total energy of each magnetic configuration, we determine that initial antiferromagnetic ground state flips to a ferromagnetic s…
Graphitic nitrogen-doped graphene is an excellent platform to study scattering processes of massless Dirac fermions by charged impurities, in which high mobility can be preserved due to the absence of lattice defects through direct substitution of carbon atoms in the graphene lattice by nitrogen atoms. In this work, we report on electrical and magnetotransport measurements of high-quality graphitic nitrogen-doped graphene. We show that the substitutional nitrogen dopants in graphene introduce atomically sharp scatters for electrons but long-range Coulomb scatters for holes and, thus, graphitic…
Integrating high-mobility graphene grown by chemical vapor deposition (CVD) into spin transport devices is one of the key tasks in graphene spintronics. We use a van der Waals pickup technique to transfer CVD graphene by hexagonal boron nitride (hBN) from the copper growth substrate onto predefined Co/MgO electrodes to build inverted spin valve devices. Two approaches are presented: (i) a process where the CVD-graphene/hBN stack is first patterned into a bar and then transferred by a second larger hBN crystal onto spin valve electrodes and (ii) a direct transfer of a CVD-graphene/hBN stack. We…
We propose a directional switching effect in a metallic device. To such end we exploit a graphene-based device with a three-terminal geometry in the presence of a magnetic field. We show that unidirectional charge and valley currents can be controlled by the Fermi energy and the magnetic field direction in the active device. Interestingly, unidirectional transport of charge and valley is generated between two-terminals at the same bias voltage. Furthermore, we quantify the valley depolarization as a function of disorder concentration. Our results open a way for active graphene-based valleytron…
