The dispersion of phonons and the electronic structure of graphene systems can be obtained experimentally from the double-resonance (DR) Raman features by varying the excitation laser energy. In a previous resonance Raman investigation of graphene, the electronic structure was analyzed in the framework of the Slonczewski-Weiss-McClure (SWM) model, considering the outer DR process. In this work we analyze the data considering the inner DR process, and obtain SWM parameters that are in better agreement with those obtained from other experimental techniques. This result possibly shows that there is still a fundamental open question concerning the double resonance process in graphene systems. 63.20.kd, 63.22.Rc, 73.22.Pr In recent years, the physics of monolayer graphene has been thoroughly investigated, unveiling a wealth of interesting and unusual properties, most of which are related to graphene's distinct electronic properties, that consist of a linear and isotropic dispersion of the electronic states around the Fermi level (E F ) near the K point in the Brillouin zone (BZ). Bilayer graphene is also a very interesting material. While in the unbiased bilayer the valence and conduction bands touch each other at the Fermi level, a gap can be opened and tuned, for example, by the application of an external electric field [1-6], which makes this a promising system for the fabrication of nanoelectronic devices. The development of bilayergraphene-based bulk devices depends on the detailed understanding of its electronic properties. Since the unit cell of AB stacked bilayer graphene is the same as that of graphite, one can model the bilayer electronic structure using a tight-binding (TB) model for graphite [7], by adapting the Slonczewski-Weiss-McClure (SWM) parameterization [8, 9] of relevant couplings. There are several theoretical [10][11][12] and experimental [3,[13][14][15][16][17]] studies of theses SWM parameters, but the agreement between the reported values, obtained with different experimental techniques, is not entirely satisfactory.In previous resonance Raman studies of bilayer graphene [15,16] in our group, the dispersion of the G ′ Raman band (also called 2D band) as a function of the laser energy was measured, and the nearest-neighbor hopping parameters γ 0 , γ 1 , γ 3 and γ 4 (shown in Fig. 1 t'FIG. 1: The intra-(γ0 and t ′ ) and inter-layer (γ1, γ3 and γ4) tight-binding parameters in bilayer graphene.were determined. In Ref.16, the fitting included also the in-plane second-neighbor hopping parameter t ′ , which is expected to be of the same order as the out-of-plane nearest-neighbor parameters. The parameter ∆, which represents the difference between the on-site energies of the sublattices A and B, was also taken into account.Group theory analysis for bilayer graphene predicts four distinct DR processes (P 11 , P 22 , P 12 , and P 21 ) along the ΓKM direction, which are illustrated in Fig. 2. The triangularly-shaped isoelectronic curves around the K and K ′ points in Figs. 2(c) and 2(f) are the equi...

We investigate the electronic structure and lattice stability of pristine and functionalized (with either hydrogen or oxygen) α-graphyne systems. We identify lattice instabilities due to soft-phonon modes, and describe two mechanisms leading to gap opening in the Dirac-fermion electronic spectrum of these systems: symmetry breaking, connected with the lattice instabilities, and partial incorporation of an sp 3 -hybrid character in the covalent-bonding network of a buckled hydrogenated α-graphyne lattice that retains the symmetries of the parent pristine α-graphyne. In the case of an oxygen-functionalized α-graphyne structure, each O atom binds asymmetrically to two twofoldcoordinated C atoms, breaking inversion and mirror symmetries, and leading to the opening of a sizeable gap of 0.22 eV at the Dirac point. Generally, mirror symmetries are found to suffice for the occurrence of gapless Dirac cones in these α-graphyne systems, even in the absence of inversion symmetry centers. Moreover, we analyze the gapless and gapped Dirac cones of pristine and functionalized α-graphynes from the perspective of the dispersion relations for massless and massive free Dirac fermions. We find that mirror-symmetry breaking mimics a Dirac-fermion mass-generation mechanism in the oxygen-functionalized α-graphyne, leading to gap opening and to isotropic electronic dispersions with a rather small electron-hole asymmetry. In the hydrogen-functionalized case, we find that carriers show a remarkable anisotropy, behaving as massless fermions along the M-K line in the Brillouin zone and as massive fermions along the Γ-K line.PACS numbers:

In this paper, we investigate the stability and thermoelectric properties of 1 T PdSSe, PdSTe and PdSeTe Janus structures using density functional theory (DFT). All three systems are narrow gap semiconductors with indirect bandgaps of 0.94 eV, 0.33 eV and 0.34 eV respectively. Compared to transition metal dichalcogenide (TMD) monolayers, PdS 2 and PdSe 2 are semiconductors with wider indirect bandgaps of 1.29 eV and 0.69 eV respectively. Phonon dispersion calculations demonstrate that all pristine and Janus structures are mechanically stable despite the presence of negligible negative frequencies around the Γ point in PdSTe and PdSeTe. Inspection of the lattice thermal conductivity (κ L ) shows that these structures are slightly anisotropic in the x and y directions except for PdSe 2 which shows a higher degree of anisotropy. Influenced by the values of κ L , the thermal electronic conductivity (κ e ), the electronic conductivity (σ) and the Seebeck effect (S), the figure of merit along the x (ZT xx )and y (ZT yy ) directions register the largest values in the case of electron doping for the PdSe 2 and PdSeTe 2D crystals. Interestingly, the figures of merit of the Janus structures are larger than their corresponding pristine PdX 2 (X = S, Se) structures. Once synthesized, such information is crucial for the implementation of the PdXY (Y = Se, Te) structures in industrial applications.

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It is well-known that vibronic interactions can be modelled in terms of an effective Hamiltonian incorporating first and second-order reduction factors (RFs), particularly when analysing the spectroscopic properties. Measurements and calculations of the RFs as a function of the strength of vibronic coupling are therefore of much interest. In this paper, we develop a new general method for determining second-order RFs (soRFs) from the strength of the Jahn–Teller (JT) coupling for systems in which electron orbital degeneracy or pseudo-degeneracy exists. These include in particular the fullerene molecule C60, pseudo-Jahn–Teller molecules and impurity centres in crystals. In order to calculate the important soRFs for intermediate to strong coupling, it is necessary to determine non-Condon corrections to the strong coupling values obtained using the Franck–Condon (FC) approximation. This gives an additional contribution to the nuclear polarizability of the system, thus enabling the electrons to follow the nuclear vibrations. These non-Condon corrections are derived using perturbation theory and are found to be inversely proportional to the square of the JT energy. The validity of the approximation is first tested in the cubic JT system due to its relative simplicity. It is found that the results are closer to those obtained earlier by numerical methods than the analytical FC values alone. Results are then presented that are applicable to C60− anions.

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