Tuning the Pseudospin Polarization of Graphene by a Pseudomagnetic Field

Georgi, A.; Nemes-Incze, Péter; Carrillo-Bastos, Ramon; Faria, Daiara; Kusminskiy, Silvia Viola; Zhai, Dawei; Schneider, Martin; Subramaniam, Dinesh; Mashoff, T.; Freitag, Nils M.; Liebmann, Marcus; Pratzer, M.; Wirtz, Ludger; Woods, Colin R.; Gorbachev, Roman; Cao, Yang; Новоселов, К. С.; Sandler, Nancy; Morgenstern, Markus

doi:10.1021/acs.nanolett.6b04870

Cited by 130 publications

(130 citation statements)

References 89 publications

(261 reference statements)

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“…Deforming the graphene lattice introduces an effective gauge field A in the low energy Dirac spectrum [1,2], causing a pronounced sublattice polarization [3][4][5][6]. One can associate a pseudomagnetic field (PMF) with this gauge field, B s ¼ ∇ × A.…”

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Graphene Nanobubbles as Valley Filters and Beam Splitters

et al. 2016

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The low energy band structure of graphene has two inequivalent valleys at K and K points of the Brillouin zone. The possibility to manipulate this valley degree of freedom defines the field of valleytronics, the valley analogue of spintronics. A key requirement for valleytronic devices is the ability to break the valley degeneracy by filtering and spatially splitting valleys to generate valley polarized currents. Here we suggest a way to obtain valley polarization using strain-induced inhomogeneous pseudomagnetic fields (PMF) which act differently on the two valleys. Notably, the suggested method does not involve external magnetic fields, or magnetic materials, as in previous proposals. In our proposal the strain is due to experimentally feasible nanobubbles, whose associated PMFs lead to different real space trajectories for K and K electrons, thus allowing the two valleys to be addressed individually. In this way, graphene nanobubbles can be exploited in both valley filtering and valley splitting devices, and our simulations reveal that a number of different functionalities are possible depending on the deformation field.

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confidence: 99%

Graphene Nanobubbles as Valley Filters and Beam Splitters

et al. 2016

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“…The DoS in the opposite valley is half that of pristine graphene. Since the PMF also gives rise to a pronounced sublattice polarization [61][62][63][64][65], a special state is created for the zeroth Landau level. This state is both valley and sublattice polarized, meaning that only one sublattice and one valley contributes to the density of states.…”

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Valley-polarized quantum transport generated by gauge fields in graphene

2017

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We report on the possibility to simultaneously generate in gaphene a bulk valley-polarized dissipative transport and a quantum valley Hall effect by combining strain-induced gauge fields and real magnetic fields. Such unique phenomenon results from a "resonance/anti-resonance" effect driven by the superposition/cancellation of superimposed gauge fields which differently affect time reversal symmetry. The onset of a valley-polarized Hall current concomitant to a dissipative valley-polarized current flow in the opposite valley is revealed by a e 2 /h Hall conductivity plateau. We employ efficient linear scaling Kubo transport methods combined with a valley projection scheme to access valley-dependent conductivities and show that the results are robust against disorder.In graphene and other two-dimensional materials, degenerate valleys of energy bands, well-separated in momentum space, constitute a discrete degree of freedom for low-energy carriers, provided intervalley mixing is negligible (case of long range disorder). Such valley degree of freedom can then be seen as a nonvolatile information carrier, provided that it can be coupled to external probes. Similarly to research in graphene spintronics [1,2], valleytronics and controlling the valley degree of freedom in graphene and other 2D materials has attracted a considerable attention [3][4][5], and many studies have investigated different options to realize valley polarized current or to filter electrons with a given valley polarization [6][7][8][9][10][11][12]. In presence of broken inversion symmetry, the valley index is also predicted to play a similar role as the spin degree of freedom in phenomena such as Hall transport, magnetization, optical transition selection rules, and chiral edge modes [13][14][15][16]. Recently, it has been shown that for modest levels of strain, graphene can also sustain a classical valley Hall effect that can be detected in nonlocal transport measurements [17]. All this stimulates the search for efficient control of valley dynamics by magnetic, electric, and optical means, which would form the basis of valley based information processing.On the other hand, it was soon realized that nonuniformly strained graphene can be modeled by the inclusion of a gauge field in the effective Hamiltonian (though it is not Berry-like). Such gauge field preserves time reversal symmetry, but induces pseudomagnetic fields (PMFs) of opposite signs in the two valleys forming the low-energy electronic bandstructure [18]. In particular, mechanical deformations aligned along three main crystallographic directions were predicted to generate strong gauge fields, acting effectively as an uniform magnetic field, opening the possibility to trigger a pseudomagnetic quantum Hall effect for strained superlattices [19]. Experimentally, evidences of PMFs with values varying from few tens up to hundreds of Tesla have been reported [20,21]. Finally, Gorbachev and coworkers recently measured intriguingly large nonlocal resistance signals in a situation where the alig...

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“…Figure 2c,d, for example, shows the energy position of the Dirac point, ED, as a function of applied gate voltage, which could be extracted from the conductance minimum in dI/dV measurements of graphene deposited on SiO 2 [34]. An STM tip has been recently used to strain a graphene sample locally, in the form of a small Gaussian bump, and at the same time to map the imbalance of the local density of states (LDOS) at the sublattice level, demonstrating the pseudospin polarization by a pseudomagnetic field [48].…”

Section: Scanning Tunneling Microscopymentioning

confidence: 99%

Advanced Scanning Probe Microscopy of Graphene and Other 2D Materials

Musumeci

2017

Crystals

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Two-dimensional (2D) materials, such as graphene and metal dichalcogenides, are an emerging class of materials, which hold the promise to enable next-generation electronics. Features such as average flake size, shape, concentration, and density of defects are among the most significant properties affecting these materials' functions. Because of the nanoscopic nature of these features, a tool performing morphological and functional characterization on this scale is required. Scanning Probe Microscopy (SPM) techniques offer the possibility to correlate morphology and structure with other significant properties, such as opto-electronic and mechanical properties, in a multilevel characterization at atomic-and nanoscale. This review gives an overview of the different SPM techniques used for the characterization of 2D materials. A basic introduction of the working principles of these methods is provided along with some of the most significant examples reported in the literature. Particular attention is given to those techniques where the scanning probe is not used as a simple imaging tool, but rather as a force sensor with very high sensitivity and resolution.

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Tuning the Pseudospin Polarization of Graphene by a Pseudomagnetic Field

Cited by 130 publications

References 89 publications

Graphene Nanobubbles as Valley Filters and Beam Splitters

Graphene Nanobubbles as Valley Filters and Beam Splitters

Valley-polarized quantum transport generated by gauge fields in graphene

Advanced Scanning Probe Microscopy of Graphene and Other 2D Materials

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