Eduardo V. Castro

Biased bilayer graphene: Semiconductor with a gap tunable by the electric field effect

We demonstrate that the electronic gap of a graphene bilayer can be controlled externally by applying a gate bias. From the magnetotransport data (Shubnikov-de Haas measurements of the cyclotron mass), and using a tight-binding model, we extract the value of the gap as a function of the electronic density. We show that the gap can be changed from zero to midinfrared energies by using fields of less, approximately < 1 V/nm, below the electric breakdown of SiO2. The opening of a gap is clearly seen in the quantum Hall regime.

Limits on Charge Carrier Mobility in Suspended Graphene due to Flexural Phonons

The temperature dependence of the mobility in suspended graphene samples is investigated. In clean samples, flexural phonons become the leading scattering mechanism at temperature T≳10 K, and the resistivity increases quadratically with T. Flexural phonons limit the intrinsic mobility down to a few m(2)/V s at room T. Their effect can be eliminated by applying strain or placing graphene on a substrate.

Localized States at Zigzag Edges of Bilayer Graphene

We report the existence of zero-energy surface states localized at zigzag edges of bilayer graphene. Working within the tight-binding approximation we derive the analytic solution for the wave functions of these peculiar surface states. It is shown that zero-energy edge states in bilayer graphene can be divided into two families: (i) states living only on a single plane, equivalent to surface states in monolayer graphene and (ii) states with a finite amplitude over the two layers, with an enhanced penetration into the bulk. The bulk and surface (edge) electronic structure of bilayer graphene nanoribbons is also studied, both in the absence and in the presence of a bias voltage between planes.

Algebraic solution of a graphene layer in transverse electric and perpendicular magnetic fields

We present an exact algebraic solution of a single graphene plane in transverse electric and perpendicular magnetic fields. The method presented gives both the eigenvalues and the eigenfunctions of the graphene plane. It is shown that the eigenstates of the problem can be cast in terms of coherent states, which appears in a natural way from the formalism.

Low-Density Ferromagnetism in Biased Bilayer Graphene

We compute the phase diagram of a biased graphene bilayer. The existence of a ferromagnetic phase is discussed with respect to both carrier density and temperature. We find that the ferromagnetic transition is first-order, lowering the value of U relatively to the usual Stoner criterion. We show that in the ferromagnetic phase the two planes have unequal magnetization and that the electronic density is holelike in one plane and electronlike in the other.

Scattering by flexural phonons in suspended graphene under back gate induced strain

Abstract We have studied electron scattering by out-of-plane (flexural) phonon modes in doped suspended graphene and its effect on charge transport. In the free-standing case (absence of strain) the flexural branch shows a quadratic dispersion relation, which becomes linear at long-wavelengths when the sample is under tension due to the rotational symmetry breaking. In the non-strained case, scattering by flexural phonons is the main limitation to electron mobility. This picture changes drastically when strains above u ¯ = 10 − 4 n ( 10 12 cm − 2 ) are considered. Here we study in particular the case of back gate induced strain, and apply our theoretical findings to recent experiments in suspended graphene.

New Type of Vacancy-Induced Localized States in Multilayer Graphene

We demonstrate the existence of a new type of zero energy state associated with vacancies in multilayer graphene that has a finite amplitude over the layer with a vacancy and adjacent layers, and the peculiarity of being quasilocalized in the former and totally delocalized in the adjacent ones. In a bilayer, when a gap is induced in the system by applying a perpendicular electric field, these states become truly localized with a normalizable wave function. A transition from a localized to an extended state can be tuned by the external gate for experimentally accessible values of parameters.

Temperature dependent resistivity in bilayer graphene due to flexural phonons

We have studied electron scattering by out-of-plane (flexural) phonons in doped suspended bilayer graphene. We have found the bilayer membrane to follow the qualitative behavior of the monolayer cousin. In the bilayer, a different electronic structure combine with a different electron-phonon coupling to give the same parametric dependence in resistivity and, in particular, the same temperature (

Bilayer graphene: gap tunability and edge properties

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Charge instabilities and topological phases in the extended Hubbard model on the honeycomb lattice with enlarged unit cell

) behavior. In parallel with the single layer, flexural phonons dominate the phonon contribution to resistivity in the absence of strain, where a density-independent mobility is obtained. This contribution is strongly suppressed by tension, and in-plane phonons become the dominant contribution in strained samples. Among the quantitative differences, an important one has been identified: room-temperature mobility in bilayer graphene is substantially higher than in monolayer graphene. The origin of quantitative differences has been unveiled.