The main point of the current work is the theoretical formulation of self-contained graphene-based systems. Bilayer graphene (BLG) and low-layer graphene (FLG) can be made by stacking single sheets of graphene on top of each other.
Historical background
Graphene
We have listed all the allotropes of carbon are three dimensional while the two dimensional allotrope of carbon is absent. Although it was a hypothetical object, two-dimensional carbon is a theoretical basis for studying allotropes of carbon in other dimensions.
Bilayer graphene
Bilayer graphene can also be found in the twisted configuration, in addition to its Bernal stacked configuration, known as twisted bilayer graphene. In the twisted bilayer graph, the two layers show a relative twist or rotation with respect to each other.
Similarities and differences between single and bilayer graphene
At low energy, bilayer graphene is a zero-gap (between valance band and conduction band) semiconductor, but a tunable band gap can be opened by applying a perpendicular electric field [16,20]. The pseudo-spin in single-layer graphene describes the alternation between the sublattices, while in the case of double-layer graphene, the pseudo-spin shows the transposition of the layers [25].
Why is bilayer graphene interesting?
This property of bilayer graphene gives the signature to use it in the electronic devices that can be turned on and off just like a semiconductor. The on/off ratio found in bilayer graphene is high enough – of the order of a million – at low temperatures.
Crystal structure
Single layer graphene
The reciprocal lattice can also be considered as the rotation of 90◦ of direct graphene lattice. The first Brillouin zone is the smallest volume in the reciprocal space bounded by the planes that bisect the vectors to the nearest reciprocal lattice points as in Fig.
Bilayer graphene
The primitive reciprocal lattice vectors~b1 and~b2 can be calculated from the following relationship between primitive direct lattice vectors and primitive reciprocal lattice vectors. Two of the six points, one at each corner, in the first Brillouin zone are nonequivalently denoted by K+ and K−, also denoted by K and K0.
Electronic Band structure
Band structure of graphene
Let us check whether Eq. 1.12) vanishes for a certain value of momentum, it is equivalent to finding the zeros in Eq. 2 e−iKxac−c(iqx−qy) (1.13) Extract an overall factor,−i.e.−iKxac−c, which does not affect the physical results, then we can write Eq. Figure 1.10: Schematic of the electronic band structure of graphene honeycomb lattice.
Band structure of bilayer graphene
- Gap opening in bilayer graphene band structure
- Trigonal warping of bands in bilayer graphene
A more extensive discussion of the control of the gap in bilayer graphene is given in the references [17,28]. In the presence of parameter γ3, the band structure of bilayer graphene becomes anisotropic, it is direction dependent.
Pseudo-spin and Berry phase
Single layer graphene
In quantum mechanics the helicity operator, the projection of the momentum operator along (pseudo). This property states that the states of the system near the Dirac point have well-defined chirality or helicity.
Bilayer graphene
The amplitudes of the electronic wave function at two sublattice locations A1 and B2 can be expressed in terms of the pseudo-spin degree of freedom associated with these amplitudes. The pseudo-spin can thus be represented as a linear combination of up and down states.
Methods of synthesis
The “Scotch Tape Method”
The size of the obtained single-layer graphene ranges from nanometers to several tens of micrometers. The duct tape method of graphene production is the easiest method for research purposes, but not for industrial purposes.
Growth techniques
On the other hand, this method allows us to produce large graphene samples both in size and quantity.
Properties of graphene and bilayer graphene
Electronic properties
Similar to single layer graphene, the behavior of bilayer graphene is also more like a semiconductor. The quantum Hall effect in bilayer graphene shows anomalies observed in single layer graphene.
Optical properties
The study of the optical properties of graphene-based systems is an active field of research interest. Among the several known most important optical properties of graphene, optical conduction is one of them.
Applications of graphene
The Rabi oscillation phenomenon is one of the well-known optical phenomena in quantum optics and also in conventional semiconductors [152]. Among the many experimental tools for studying the optical properties of graphene systems, Raman spectroscopy is one of them.
Motivations
The spin-wave approximation is valid when the frequency of the external field is almost equal to the excitation frequency of the electron-hole pair. We are interested in studying the phenomenon of off-resonance Rabi oscillations in graphene systems when the frequency of the applied field is very large compared to the excitation frequency of the electron-hole pair.
Solution of Bloch equations
- Solution of Bloch equations near resonance, νω ≈ 2 |k| 2m 2
- Solution of Bloch equations far from resonance, ω ω R , 2 |k| 2m 2 52
- Current density of bilayer graphene in RWA regime
- Current density of bilayer graphene in ARWA regime
The compact form of current density in momentum space for bilayer graphene can be written as follows (the average current is of course independent of position). The threshold behavior of current density in off-resonance case is shown in figure 1.
Numerical solution of the problem
In this section, no approximations of any kind are involved, so the results obtained would be reliable, and if they agree with the approximate analytical results, it would provide an immediate confirmation of the approximation methods used. Far from resonance, the slow part of the polarization oscillates exactly with the ARF matching the frequency deduced from numerical simulations, see Fig.
Anomalous Rabi oscillations in multi-layer graphene
- Hamiltonian of multi-layer graphene
- Bloch equations for n-layer graphene
- Solution of Bloch equations
- Current density in n-layer graphene
In this section, the Bloch equations of n-layer graphene are solved using a process similar to solving the Bloch equations of bilayer graphene. This section describes how to solve the Bloch equations for n-layer graphene using the ARWA method discussed earlier.
Summary and conclusions
Effective low-energy Hamiltonian of gapped single layer graphene 73
There are different ways to induce this mass term, such as interaction between graphene sheets and the substrate on which the graphene sheet is deposited, sandwiching a monolayer graphene between a pair of hexagonal boron nitride layers [190] or applying an external electric field perpendicular to the plane of the graphene sheet. An in-plane electric field is applied to the graphene sheet through a vector potential of the form A(t) =~ A(0)e~ −iωt, where A(0) =~ ec(Ax(0) +iAy(0) )) is a complex amplitude of an external driving lane.
This is the main difference between the Bloch equations of single-layer graphene with gaps and without gaps. The Bloch equations in monolayer graphene have already been solved without gaps [184].
It is clear from the left part of this figure that the threshold frequency of the induced current increases in the presence of asymmetry (solid green) compared to that without asymmetry (dotted black) shown in the inset for clarity. The asymmetry parameter does not affect the exponent at the threshold, but has a significant effect on the value of the threshold frequency.
Effect of asymmetry on Rabi oscillations in bilayer graphene
Effect of intra-layer asymmetry on Rabi oscillations
Since ∆ ω, we can therefore see that the conventional Rabi frequencies are almost independent of the intra-layer asymmetry. It is clear from the left part of this figure that the threshold frequency of the induced current increases in the presence of asymmetry (solid green) compared to without asymmetry (dashed black).
Effect of inter-layer asymmetry on Rabi oscillations
The dimensionless ARF for symmetric bilayer graphene shows a zero trivial minimum in the presence of the applied field. The effect of interlayer asymmetry on anomalous Rabi frequency in bilayer graphene at Dirac point of the Brillouin zone is shown in Fig.
Summary and conclusions
This yields the anomalous Rabi frequency of bilayer graphene without trigonal warp, at the Dirac point of the Brillouin zone. The oscillation frequency of the current density with TW is increased compared to without TW.
Effect of twisting on ROs in bilayer graphene
If turn ∆K = 0, we can return the anomalous Rabi frequency of bilayer graphene without twist. It is seen that the anomalous Rabi frequency depends significantly on the curve for weak applied fields (small ωR).
Summary and conclusions
Fermi surface develops, and then varies linearly with the square of the intensity of the applied field corresponding to bilayer graphene without trigonal distortion effect. The electron-phonon interaction is one of the most important fundamental physical problems in any new electronic material.
Acoustic phonon damped AROs in graphene-systems
This is a schematic numerical plot of Γk,R versus|k|(in arbitrary units) for the case of acoustic phonons in bilayer graphene. a threshold value for electron wavenumber kmin zero otherwise. acoustic phonons are ineffective in dephasing AROs in bilayer graphene near the Dirac point. In other words, acoustic phonons play an important role in dephasing AROs in bilayer graphene far from the Dirac point.
Flexural phonon damped AROs in graphene-systems
The following discussion deals with the effect of electron-bending phonon interaction in dephasing AROs in bilayer graphene. This plot shows variation of Γk,R versus |k| (in arbitrary units) for the case of bending phonons in bilayer graphene. have chosenkth= 1 in arbitrary units.
Summary and conclusions
The above study shows ARO damping in the presence of electron-optical phonon interaction in monolayer and bilayer graphene. We want to study the dephasing of ARO in monolayer and bilayer graphene by means of electron-phonon (optical, acoustic and bending phonons) interaction.
Calculation of equilibrium values of polarization and population
Using the definition of f, the above equations can be written in two coupled linear differential equations as given below. Equation (A.3) can be written in the form of two subgrid locations A1 and B2 by expanding one of two indices as follows.
Formulae used to plot Fig. 2.1
Calculation of current density in bilayer gra-phene
Therefore, the expression for the current density in spatial coordinates is the same. Using the Fourier transform to obtain an expression for the current density in Fourier space (momentum space).
Details of numerical solution of Bloch equations, Sec. 2.4
In the presence of an externally applied electric field, the current density in bilayer graphene is given by the following equation. The electron-optical phonon interacting Hamiltonian [226] at one of the Dirac points is given by.
Extraction of slow part of Hamiltonian
Green’s function
Self-energy
This can be expanded, using (G0ΣG)ab =P. Using the extension of eBAe-B we will get the required result. These two conditions are satisfied only for some special ranges of θ, these ranges of θ can be found using the Reduce of Mathematica route, which turns out to be,.
Electron-flexural phonon Hamiltonian
