Strain controlled valley filtering in multi-terminal graphene structures

Milovanović, S. P.; Peeters, F. M.

doi:10.1063/1.4967977

Cited by 70 publications

(76 citation statements)

References 28 publications

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“…Different routes have been suggested to create valley polarization in graphene [13][14][15][16][17], relying on nanoribbons or constrictions [17][18][19][20][21], interplays between external fields [22][23][24][25], spin-orbit coupling [26,27], or spatial or temporal combinations of gating and magnetic fields [28][29][30]. However, an experimental verification has proven to be challenging as practical and effective methods to manipulate the valleys in realistic setups still need to be established.…”

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confidence: 99%

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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“…Their interpretation proposes that the formation of bulk topological valley currents is intertwined with the presence of a Berry curvature generated by the mass term [23], a scenario which is under questioning [24,25]. Accordingly to date, despite the wealth of theoretical proposals of valley dependent effects [26][27][28][29][30][31][32][33][34][35][36][37][38][39][40], experimental fingerprints of PMF on quantum transport and unambiguous demonstration of a valley Hall effect in graphene remain elusive.Here, we predict that once the electronic structure of Dirac fermions embeds a strain-related gauge field, it is possible to fine-tune the superposition of an external real magnetic field to reach a resonant effect, where the sum of valley-dependent effective magnetic fields either sum up or cancel each other. This results in a remarkable valley-polarized quantum transarXiv:1705.09085v2 [cond-mat.mes-hall] 1 Jul 2017…”

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confidence: 99%

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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“…Our work provide a promising and effective way to confirm the function of the valley filter. Although our study is based on the BPC valley filter, the idea of detecting valley polarized current by AR can be hopefully extended to other schemes of valley filter [26][27][28][29][30][31][32]. This is reasonable by noting that the electron pairing occurs between ±K valleys, while the valley filter filters one of them.…”

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confidence: 99%

Probing the valley filtering effect by Andreev reflection in a zigzag graphene nanoribbon with a ballistic point contact

2017

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Ballistic point contact (BPC) with zigzag edges in graphene is a main candidate of a valley filter, in which the polarization of the valley degree of freedom can be selected by using a local gate voltage. Here, we propose to detect the valley filtering effect by Andreev reflection. Because electrons in the lowest conduction band and the highest valence band of the BPC possess opposite chirality, the inter-band Andreev reflection is strongly suppressed, after multiple scattering and interference. We draw this conclusion by both the scattering matrix analysis and the numerical simulation. The Andreev reflection as a function of the incident energy of electrons and the local gate voltage at the BPC is obtained, by which the parameter region for a perfect valley filter and the direction of valley polarization can be determined. The Andreev reflection exhibits an oscillatory decay with the length of the BPC, indicating a negative correlation to valley polarization.

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Strain controlled valley filtering in multi-terminal graphene structures

Cited by 70 publications

References 28 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

Probing the valley filtering effect by Andreev reflection in a zigzag graphene nanoribbon with a ballistic point contact

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