Phonon-Mediated Tunneling into Graphene

Wehling, T. O.; Grigorenko, Ilya; Lichtenstein, A. I.; Balatsky, A. V.

doi:10.1103/physrevlett.101.216803

Cited by 81 publications

(117 citation statements)

References 22 publications

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“…This is also different from band-structure-dependent tunnelling in silicon 20 because the wave-vector dependence seen here is a result of inelastic excitations that couple the unique electronic band structure and phonon spectrum of graphene. A recent calculation 21 by Wehling et al confirms this general interpretation through density functional theory. They find significant mixing of the graphene σ and π electron bands due to the out-ofplane phonon mode at K/K , resulting in a strongly enhanced inelastic electron tunnel current.…”

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confidence: 56%

Giant phonon-induced conductance in scanning tunnelling spectroscopy of gate-tunable graphene

et al. 2008

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The honeycomb lattice of graphene is a unique two-dimensional system where the quantum mechanics of electrons is equivalent to that of relativistic Dirac fermions 1,2 . Novel nanometre-scale behaviour in this material, including electronic scattering 3,4 , spin-based phenomena 5 and collective excitations 6 , is predicted to be sensitive to charge-carrier density. To probe local, carrier-density-dependent properties in graphene, we have carried out atomically resolved scanning tunnelling spectroscopy measurements on mechanically cleaved graphene flake devices equipped with tunable back-gate electrodes. We observe an unexpected gap-like feature in the graphene tunnelling spectrum that remains pinned to the Fermi level (E F ) regardless of graphene electron density. This gap is found to arise from a suppression of electronic tunnelling to graphene states near E F and a simultaneous giant enhancement of electronic tunnelling at higher energies due to a phonon-mediated inelastic channel. Phonons thus act as a 'floodgate' that controls the flow of tunnelling electrons in graphene. This work reveals important new tunnelling processes in gate-tunable graphitic layers.Graphene provides an ideal platform for the local study of high-mobility two-dimensional (2D) electrons because it can be fabricated on top of an insulating substrate. The availability of a back-gate electrode makes graphene the first gate-tunable 2D system directly accessible to scanning probe measurement (Fig. 1a). Previous experiments have demonstrated the power of scanning tunnelling microscopy (STM) to probe the local electronic structure of graphene grown epitaxially on SiC (refs 7-9). That system, however, cannot be easily gated, and questions remain as to the influence of the SiC substrate on the graphene layer 6,10 . Mechanically cleaved graphene is a desirable alternative to graphene grown on SiC because it can be readily gated and placed on wellcontrolled substrates (Fig. 1a), thus making it useful for extracting intrinsic graphene properties.The STM topography of a gated graphene flake device is shown in Fig. 2a. Corrugations with a lateral dimension of a few nanometres and a vertical dimension of ∼1.5Å (r.m.s. value over a 60 × 60 nm 2 area) are observed, probably due to roughness in the underlying SiO 2 (refs 11,12). The graphene honeycomb lattice can be clearly resolved on top of the surface corrugation, as seen more clearly in Fig. 2b. We explored the local electronic structure of these graphene flake devices using dI /dV measurements at zero gate voltage, as shown in Fig. 2c. Strikingly, the spectrum shows a ∼130 mV gap-like feature centred at the Fermi energy, E F , as opposed to the linear density of states that might be expected from elastic tunnelling to a Dirac cone. A local minimum in the tunnelling conductance spectrum can also be seen at V D = −138 mV, making the spectrum asymmetric about E F . Close examination of the low-bias spectrum (Fig. 2c, inset) reveals that the tunnelling conductance does not go to absolute zero in the gap...

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confidence: 56%

Giant phonon-induced conductance in scanning tunnelling spectroscopy of gate-tunable graphene

et al. 2008

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“…This gap is a characteristic signature of electrons tunneling into graphene, as revealed previously in STM studies for graphene on SiC, 15 SiO 2 , 16 BN, 17 and Pt, 18 and described by ab-initio calculation of the tunnelling density of states. 19 Indeed, at low bias (< 60 mV), only elastic tunneling paths are enabled near the graphene's K points at E F . Due to the particular band structure of graphene this leads to a quenching of the injected current (Fig.…”

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confidence: 99%

Graphene-Passivated Nickel as an Oxidation-Resistant Electrode for Spintronics

et al. 2012

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We report on graphene-passivated ferromagnetic electrodes (GPFE) for spin devices. GPFE are shown to act as spin-polarized oxidation-resistant electrodes. The direct coating of nickel with few layer graphene through a readily scalable chemical vapour deposition (CVD) process allows the preservation of an unoxidized nickel surface upon air exposure. Fabrication and measurement of complete reference tunneling spin valve structures demonstrates that the GPFE is maintained as a spin polarizer and also that the presence of the graphene coating leads to a specific sign reversal of the magnetoresistance. Hence, this work highlights a novel oxidation-resistant spin source which further unlocks low cost wet chemistry processes for spintronics devices.Information storage is today mainly based on magnetism, with hundreds of millions of hard drives sold every year, 1,2 and further growth is expected driven by the proliferation of enormous data centers for online "cloud" computing. Spintronics is at the heart of this nonvolatile data-storage revolution, with ferromagnetic memory elements acting as basic building blocks: spin sources or analyzers. The efficiency of spin polarized electrodes, based on ferromagnetic metals like nickel and cobalt, thereby heavily relies on the quality of the interfaces at play. A major challenge for device processing and integration is to prevent detrimental corrosion and oxidation of the ferromagnetic metals in use. To date, this severely

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“…2a. (A somewhat related effect has been suggested in tunneling from a metal tip to graphene [40,41].) The relevant phonon energy ω θ increases with the rotation angle, Fig.…”

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confidence: 99%

Phonon-Mediated Interlayer Conductance in Twisted Graphene Bilayers

Perebeinos¹,

Tersoff²,

Avouris³

2012

Phys. Rev. Lett.

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Conduction between graphene layers is suppressed by momentum conservation whenever the layer stacking has a rotation. Here we show that phonon scattering plays a crucial role in facilitating interlayer conduction. The resulting dependence on orientation is radically different than previously expected, and far more favorable for device applications. At low temperatures, we predict diodelike current-voltage characteristics due to a phonon bottleneck. Simple scaling relationships give a good description of the conductance as a function of temperature, doping, rotation angle, and bias voltage, reflecting the dominant role of the interlayer beating phonon mode.Graphene has generated broad excitement both for fundamental science and for its potential applications in technology [1]. Attention has turned increasingly to bilayer and few layer graphene [2], because of their scientific richness and their promise in technological applications involving band-gap opening by an external electric field, high current-carrying capacity, and electro-optical coupling [3][4][5][6][7][8][9][10][11][12]. Interlayer conductance is crucial in almost all such applications. However, most fabrication methods lead to "twisted" layer stacking, with a random angle of rotation between layers. As a result, momentum conservation in certain respects "decouples" the layers [13][14][15][16][17][18][19]. Strictly speaking, the layers do couple, as evidenced by velocity renormalization [20][21][22][23][24]. But the states near the Dirac point in one layer largely decouple from those near the rotated Dirac point in the other layer [17][18][19]25], suppressing interlayer current [19].Understanding the interlayer conduction has proven surprisingly subtle and difficult. Calculations of the interlayer conductance to date have required some phenomenological lifetime broadening, with the conductance depending on this broadening [19]. In the limit of small carrier scattering the incoherent transport picture will fail, and it becomes problematic to even define an interlayer conductance independent of the external contacts [19]. With a realistic scattering lifetime the conductance is well defined, but it's calculated value is extraordinarily sensitive to even small details of the matrix-element modeling [19,25]. Finite conductance is predicted even for twisted stacking [19] due to Umklapp electronic coupling [18], but the coupling decays exponentially with the size of the rotational supercell [17][18][19]25]. (Here "supercell" denotes the primitive cell of the Bravais lattice of the commensurate rotated bilayer.) This would pose a serious obstacle to many device applications, where some reliable lower bound on the interlayer conductivity is essential. Experimentally, some interlayer transport is observed [26,27], but the mechanism remains unclear. Extrinsic scattering mechanisms could also relax momentum conservation, especially for defect-rich substrates.Here we show that essentially all of the conceptual and computational problems of twisted bilayers are res...

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Phonon-Mediated Tunneling into Graphene

Cited by 81 publications

References 22 publications

Giant phonon-induced conductance in scanning tunnelling spectroscopy of gate-tunable graphene

Giant phonon-induced conductance in scanning tunnelling spectroscopy of gate-tunable graphene

Graphene-Passivated Nickel as an Oxidation-Resistant Electrode for Spintronics

Phonon-Mediated Interlayer Conductance in Twisted Graphene Bilayers

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