Valleys Of Graphene Research Articles

Observation of unconventional splitting of Landau levels in strained graphene

In strained graphene, lattice deformation can create pseudo-magnetic fields and result in zero-field Landau level-like quantization. In the presence of an external magnetic field, valley-polarized Landau levels are predicted to be observed because the pseudo-magnetic fields are of opposite directions in the K and K' valleys of graphene. Here, we present experimental spectroscopic measurements by scanning tunneling microscopy of strained graphene on Rh foil. We direct observed valley splitting of the Landau level induced by the coexistence of the pseudo-magnetic fields and external magnetic fields. The observed result paves the way to exploit novel electronic properties in graphene through the combination of the pseudo-magnetic fields and the external magnetic fields.

Single-valley engineering in graphene superlattices

The two inequivalent valleys in graphene preclude the protection against inter-valley scattering offered by an odd-number of Dirac cones characteristic of Z2 topological insulator phases. Here we propose a way to engineer a chiral single-valley metallic phase with quadratic crossover in a honeycomb lattice through tailored \sqrt{3}N *\sqrt{3}N or 3N *3N superlattices. The possibility of tuning valley-polarization via pseudo-Zeeman field and the emergence of Dresselhaus-type valley-orbit coupling are proposed in adatom decorated graphene superlattices. Such valley manipulation mechanisms and metallic phase can also find applications in honeycomb photonic crystals.

Gate-tunable valley-filter based on suspended graphene with double magnetic barrier structures

We theoretically investigate the effects of strain-induced pseudomagnetic fields on the transmission probability and the ballistic conductance for Dirac fermion transport in suspended graphene. We show that resonant tunneling through double magnetic barriers can be tuned by strain in the suspended region. The valley-resolved transmission peaks are apparently distinguishable owing to the sharpness of the resonant tunneling. With the specific strain, the resonant tunneling is completely suppressed for Dirac fermions occupying the one valley, but the resonant tunneling exists for the other valley. The valley-filtering effect is expected to be measurable by strain engineering. The proposed system can be used to fabricate a graphene valley filter with the large valley polarization almost 100%.

Mechanically and optically controlled graphene valley filter

We theoretically investigate the valley-dependent electronic transport through a graphene monolayer modulated simultaneously by a uniform uniaxial strain and linearly polarized light. Within the Floquet formalism, we calculate the transmission probabilities and conductances of the two valleys. It is found that valley polarization can appear only if the two modulations coexist. Under a proper stretching of the sample, the ratio of the light intensity and the light frequency squared is important. If this quantity is small, the electron transport is mainly contributed by the valley-symmetric central band and the conductance is valley unpolarized; but when this quantity is large, the valley-asymmetric sidebands also take part in the transport and the valley polarization of the conductance appears. Furthermore, the degree of the polarization can be tuned by the strain strength, light intensity, and light frequency. It is proposed that the detection of the valley polarization can be realized utilizing the valley beam splitting. Thus, a graphene monolayer can be used as a mechanically and optically controlled valley filter.

Low-energy resonant scattering from hydrogen impurities in graphene

We study the electronic transport properties of graphene with covalently bonded hydrogen impurities. Our measurements reveal low-energy resonant scattering processes within the transport for each charge carrier type. The observed resonances exhibit a strong energy dependence and are accompanied by a sharp increase in the scattering cross section. The ability to observe the scattering resonances was found to depend on the amount of disorder introduced into the graphene through the bonding of hydrogen. The results are shown to be in agreement with a theory regarding low-energy resonant scattering off a short-range impurity in graphene that takes into account both intravalley and intervalley scattering. Theory dictates that the observed resonances are the result of the formation of quasibound states of the Dirac fermions in graphene due to a divergence in one or more of the scattering lengths for the short-range hydrogen impurity potential. We anticipate our experimental results to have implications in graphene valley physics as well as graphene chemical modification, scattering, and localization theories.

Coupled spin and pseudomagnetic field in graphene nanoribbons

A pseudomagnetic field becomes an experimental reality after the observation of zero-field Landau level-like quantization in strained graphene, but it is not expected that the time-reversal symmetric pseudomagnetic fields will have any effect on the spin degree of freedom of the charge carriers. Here, we demonstrate that spin-orbit coupling (SOC) could act as a bridge between the pseudomagnetic field and spin. In quantum spin Hall (QSH) states, the direction of the spin of edge states is tied to their direction of motion because of the SOC. The pseudomagnetic field affects the clockwise and counterclockwise edge currents of the QSH states and consequently lifts the degenerate edge states of opposite spin orientation. Because of opposite signs of the pseudomagnetic field in two valleys of graphene, the one-dimensional charge carriers at the two opposite edges have different group velocities, and in some special cases, the edge states can only propagate at one edge of the nanoribbon, and the group velocity at the other edge becomes zero.

Graphene Valley Filter Using a Line Defect

With its two degenerate valleys at the Fermi level, the band structure of graphene provides the opportunity to develop unconventional electronic applications. Herein, we show that electron and hole quasiparticles in graphene can be filtered according to which valley they occupy without the need to introduce confinement. The proposed valley filter is based on scattering off a recently observed line defect in graphene. Quantum transport calculations show that the line defect is semitransparent and that quasiparticles arriving at the line defect with a high angle of incidence are transmitted with a valley polarization near 100%.

Valley filter and valley valve in graphene

The potential of graphene for carbon electronics rests on the possibilities offered by its unusual band structure to create devices that have no analogue in silicon-based electronics1,2. Conduction and valence bands in graphene form conically shaped valleys, touching at a point called the Dirac point. There are two inequivalent Dirac points in the Brillouin zone, related by time-reversal symmetry. Intervalley scattering is suppressed in pure samples3,4,5. The independence and degeneracy of the valley degree of freedom suggests that it might be used to control an electronic device6, in much the same way as the electron spin is used in spintronics7 or quantum computing8. A key ingredient for ‘valleytronics’ would be a controllable way of occupying a single valley in graphene, thereby producing a valley polarization. Here we propose such a valley filter, based on a ballistic point contact with zigzag edges. The polarity can be inverted by local application of a gate voltage to the point contact region. Two valley filters in series may function as an electrostatically controlled valley valve, representing a zero-magnetic-field counterpart to the familiar spin valve.

Intervalley Scattering, Long-Range Disorder, and Effective Time-Reversal Symmetry Breaking in Graphene

We discuss the effect of certain types of static disorder, like that induced by curvature or topological defects, on the quantum correction to the conductivity in graphene. We find that when the intervalley scattering time is long or comparable to tau(phi), these defects can induce an effective time-reversal symmetry breaking of the Hamiltonian associated to each one of the two valleys in graphene. The phenomenon suppresses the magnitude of the quantum correction to the conductivity and may result in the complete absence of a low-field magnetoresistance, as recently found experimentally. Our work shows that a quantitative description of weak localization in graphene must include the analysis of new regimes, not present in conventional two-dimensional electron gases.