Disordered Bilayer Graphene Research Articles

Quasiparticle and transport properties of disordered bilayer graphene

Quasiparticle and transport properties of disordered bilayer graphene

Energy Relaxation and Cooling in Impure Bilayer Graphene at Low Temperatures

Understanding the physical mechanism of heat dissipation is a matter of increasing concern from the perspective of waste heat management in nanostructure-based devices. The impurity-driven electron–phonon scattering is a significant channel for energy relaxation in graphene and its bilayer. The recently developed Keldysh formalism for single-layer graphene (SLG) is extended to investigate the transport phenomena due to phonon interaction via the deformation potential in disordered bilayer graphene (BLG) at low temperatures within the impure limit, q l ≪ 1 . It is observed that in the BLG system also, the temperature dependence of the relaxation rate and the cooling power are affected by the occurrence of disorder and screening, with the temperature exponent of pure BLG T 4 modified to T 3 in impure BLG. A comparison with the temperature and mean free path exponents of SLG reveals that the exponents in BLG turn out to be the same, but they both differ from the conventional 2DEG. However, the magnitude of the scattering rate and cooling power is more pronounced in BLG in a low-temperature regime. As the magnitude of the relaxation rate and the cooling rate varies with impurity, the impurity-assisted electron–phonon scattering has rich implications on hot carrier transport in BLG devices.

Anomalous ballistic transport in disordered bilayer graphene: A Dirac semimetal induced by dimer vacancies

We report anomalous quantum transport features in bilayer graphene in presence of a random distribution of structural vacancies. By using an efficient real-space Kubo-Greenwood transport methodology, the impact of a varying density of dimer versus non-dimer vacancies is investigated in very large scale disordered models. While non-dimer vacancies are shown to induce localization regimes, dimer vacancies result in an unexpected ballistic regime whose energy window surprisingly enlarges with increasing impurity density. Such counterintuitive phenomenon is explained by the formation of an effective linear dispersion in the bilayer bandstructure, which roots in the symmetry breaking effects driven by dimer vacancies, and provides a novel realization of Dirac semimetals in high dimension.

Theory of Valley Hall Conductivity in Bilayer Graphene

The valley Hall conductivity, having opposite signs between the K and K′ valleys, is calculated in disordered bilayer graphene in the presence of gate electric field. Numerical calculations are performed within a self-consistent Born approximation for scatterers with Gaussian potential and for charged impurities. The results show that the valley Hall conductivity is much enhanced as compared to that in the ideal case without scatterers, remains appreciable in the presence of large disorder, and exhibits double-peak structure near zero energy.

Inhomogenous Electronic Structure, Transport Gap, and Percolation Threshold in Disordered Bilayer Graphene

The inhomogenous real-space electronic structure of gapless and gapped disordered bilayer graphene is calculated in the presence of quenched charge impurities. For gapped bilayer graphene, we find that for current experimental conditions the amplitude of the fluctuations of the screened disorder potential is of the order of (or often larger than) the intrinsic gap Δ induced by the application of a perpendicular electric field. We calculate the crossover chemical potential Δ(cr), separating the insulating regime from a percolative regime in which less than half of the area of the bilayer graphene sample is insulating. We find that most of the current experiments are in the percolative regime with Δ(cr)≪Δ. The huge suppression of Δ(cr) compared with Δ provides a possible explanation for the large difference between the theoretical band gap Δ and the experimentally extracted transport gap.

Conductance of bilayer graphene in the presence of a magnetic field: Effect of disorder

We investigate the electronic transport properties of unbiased and biased bilayer graphene nanoribbon in n-p and n-n junctions subject to a perpendicular magnetic field. Using the non-equilibrium Green's function method and the Landauer-B\"{u}ttiker formalism, the conductance is studied for the cases of clean, on-site, and edge disordered bilayer graphene. We show that the lowest Hall plateau remains unchanged in the presence of disorder, whereas asymmetry destroys both the plateaus and conductance quantization. In addition, we show that disorder induces an enhancement of the conductance in the n-p region in the presence of magnetic fields. Finally, we show that the equilibration of quantum Hall edge states between distinctively doped regions causes Hall plateaus to appear in the regime of complete mode mixing.

Electronic properties of disordered bilayer graphene

Based on the Anderson tight-binding model, the electronic properties of disordered bilayer graphene are investigated. Our numerical results show that electrons in bilayer graphene with monolayer disorder (MDBG) exhibit a peculiar behavior that is quite different from that in bilayer graphene with bilayer disorder (BDBG). With the increase of disorder, BDBG becomes an insulating disordered two-dimensional (2D) electron system. In the case of monolayer disorder, however, the energy spectrum has strongly localized tail states and a stable extended central state separated by persistent mobility edges which are independent of the disorder strength. This indicates that a metal–insulator transition occurs in the monolayer disorder structure. This is similar to the case in an order–disorder separated quantum film. The results could offer useful information for understanding and manipulating the electronic properties of disordered bilayer graphene.

Anderson transition in disordered bilayer graphene

Employing the kernel polynomial method (KPM), we study the electronic properties of thegraphene bilayers with Bernal stacking in the presence of diagonal disorder, within thetight-binding approximation and nearest neighbor interactions. The KPM method enablesus to calculate local density of states (LDOS) without the need to exactly diagonalize theHamiltonian. We use the geometrical averaging of the LDOS at different lattice sites as acriterion to distinguish the localized states from extended ones. We find that this modelundergoes an Anderson metal–insulator transition at a critical value of disorder strength.

Electron delocalization in bilayer graphene induced by an electric field

Electronic localization is numerically studied in disordered bilayer graphene with an electric-field-induced energy gap. Bilayer graphene is a zero-gap semiconductor, in which an energy gap can be opened and controlled by an external electric field perpendicular to the layer plane. We found that, in the smooth disorder potential not mixing the states in different valleys ($K$ and ${K}^{\ensuremath{'}}$ points), the gap opening causes a phase transition at which the electronic localization length diverges. We show that this can be interpreted as the integer quantum Hall transition at each single valley, even though the magnetic field is absent.