基于密度泛函理论的单层氢化石墨烯特性分析

本文采用密度泛函理论的第一性原理方法,研究了不同尺寸H-graphene的稳定性、HOMO-LU-MO能隙以及电子激发态.研究结果表明,对于C_(16)H_(10)、C_(30)H_(14)、C_(48)H_(18)、C_(70)H_(22)、C_(96)H_(26)、C_(126)H_(30)计算的比结合能,C_(126)H_(30)相比C_(16)H_(10)的比结合能增长23.9%,且比结合能随着H-graphene尺寸扩大而增加,意味着稳定性不断提高.通过对HOMO-LUMO能隙分析发现,在较小尺寸的H-graphene中,由于量子效应起主要作用,因此出现了较大的HOMO-LUMO能隙,且随着H-graphene团簇尺寸的增加,能隙逐渐缩小可以看出,对于无限大的H-graphene团簇中,HOMO-LUMO能隙无限趋近于零(相当于零带隙),其电子性质与纯石墨烯相似.通过分析C_(16)H_(10)、C_(30)H_(14)、C_(48)H_(18)、C_(70)H_(22)激发态以及了吸收光谱,发现随着尺寸的扩大,吸收光谱发生红移,为石墨烯在电子器件领域的应用提供理论基础.     

Third-order elastic moduli of single-layer graphene

In the framework of the Keating model with allowance made for the anharmonic constant of the central interaction between the nearest neighbors μ, analytical expressions have been obtained for three third-order independent elastic constants c _ijk (μ, ζ) of single-layer graphene, where ζ = (2α − β)/(4α + β) is the Kleinman internal displacement parameter and α and β are the harmonic constants of the central interaction between the nearest neighbors and the noncentral interaction between the next-nearest neighbors, respectively. The dependences of the second-order elastic constants on the pressure p have been determined. It has been shown that the moduli c ₁₁ and c ₂₂ differently respond to the pressure. Therefore, graphene is isotropic in the harmonic approximation, whereas the inclusion of anharmonicity leads to the appearance of the anisotropy.     

Dynamical theory of polariton amplifiers

We present the theory of the dynamics of the polariton amplifier in the region of small polariton densities. We give an analytical solution for the polariton condensate density matrix and show that the formation of a coherent quantum state is possible. Once the condensate is formed, the coherence becomes macroscopically long living. Polariton amplifier represents, therefore, an optical memory element, where the input weak coherent signal can be amplified and kept.     

Dynamical spin polarization in single-layer graphene

In the present work the dynamical behavior of $\mathrm{\pi }\mathrm{-}\mathrm{electronic}$ spin in graphene is investigated. The $\mathrm{\pi }\mathrm{-}\mathrm{electron}$ is under the influence of a normal uniform magnetic field and the Rashba spin–orbit coupling. Introducing a Casimir operator, we show that the governing Hamiltonian and, consequently, the time-evolution matrix is block-diagonal. We then proceed to calculate the temporal behavior of different spin components, when it is initially in-plane polarized. Our calculations show that the spin is dynamically polarized in a plane normal to the graphene sheet and follows the patterns of collapse-revivals. The dependence of amplitudes as well as the collapse-revivals’ periods on the external field and the Rashba spin–orbit coupling is also reported.     

Spin relaxation in single-layer and bilayer graphene

We investigate spin relaxation in graphene spin valves and observe strongly contrasting behavior for single-layer graphene (SLG) and bilayer graphene (BLG). In SLG, the spin lifetime (τ(s)) varies linearly with the momentum scattering time (τ(p)) as carrier concentration is varied, indicating the dominance of Elliot-Yafet (EY) spin relaxation at low temperatures. In BLG, τ(s) and τ(p) exhibit an inverse dependence, which indicates the dominance of Dyakonov-Perel spin relaxation at low temperatures. The different behavior is due to enhanced screening and/or reduced surface sensitivity of BLG, which greatly reduces the impurity-induced EY spin relaxation.     

Molecular dynamics simulation of compression of single-layer graphene

The compression of a single-layer graphene sheet in the “zigzag” and “armchair” directions has been investigated using the molecular dynamics method. The distributions of the xy and yx stress components are calculated for atomic chains forming the graphene sheet. A graphene sheet stands significant compressive stresses in the “zigzag” direction and retains its integrity even at a strain of ～0.35. At the same time, the stresses which accompany the compressive deformation of single-layer graphene in the “armchair” direction are more than an order in magnitude lower than corresponding characteristics for the “zigzag” direction. A compressive strain of ～0.35 in the “armchair” direction fractures the graphene sheet into two parts.     

Gap opening in single-layer graphene in the presence of periodic scalar and vector potentials within the continuum model

Gap opening at the Dirac point of the single-layer graphene with periodic scalar and vector potentials has been theoretically investigated under the continuum model. The symmetry analysis indicates that the two-fold degeneracy at the Dirac point can be lifted when the potentials break both the chiral symmetry and the time-reversal symmetry. With the perturbation theory, we derive an analytical expression (gap equation) for gap opening at the Dirac point. Furthermore, the bandgap from the gap equation agrees well with the exact result, when the applied potentials are weak.     

Raman mapping of a single-layer to double-layer graphene transition

We report on confocal Raman spectroscopy on a few-layer graphene flake. Adjacent single- and double-layer graphene sections allow mapping the transition in vibrational and electronic properties to a second stacked graphene sheet and with it a weak interlayer coupling. Most prominently the width of the D' peak doubles upon going from a single to a double layer, which can be explained within the double-resonant Raman model. The intensities of the G and G' lines decrease at the crossover to a single layer. Contrary to the G' line the G peak position shifts to higher wave numbers, however, not uniformly over the entire section: its frequency fluctuates spatially. The Raman map of the D line intensity shows a non-zero contribution at the boundaries of the flake and the individual sections, which can be attributed either to defects and disorder or to the breakdown of translational symmetry, whereas within the flake no D line signal is detected.     

Quantum critical scaling in graphene

We show that the emergent relativistic symmetry of electrons in graphene near its quantum critical point (QCP) implies a crucial importance of the Coulomb interaction. We derive scaling laws, valid near the QCP, that dictate the nontrivial magnetic and charge response of interacting graphene. Our analysis yields numerous predictions for how the Coulomb interaction will be manifested in experimental observables such as the diamagnetic response and electronic compressibility.     

Computer simulation of thin nickel films on single-layer graphene

The energy, mechanical, and transport properties of nickel films on a single-layer graphene sheet in the temperature range 300 K ≤ T ≤ 3300 K have been investigated using the molecular dynamics method. The stresses generated in the plane of the metallic film are significantly enhanced upon deposition of another nickel film on the reverse side of the graphene sheet. In this case, the self-diffusion coefficient in the film plane above 1800 K, in contrast, decreases. An appreciable temperature elongation per unit length of the film also occurs above 1800 K and dominates in the “zigzag” direction of the graphene sheet. The vibrational spectra of the nickel films on single-layer graphene for horizontal and vertical displacements of the Ni atoms have very different shapes.     

One-atom-thick reflectors for surface plasmon polariton surface waves on graphene

Inspired by its analog from classic optics, we introduce one-atom-thick reflectors for infrared (IR) surface plasmon polaritons (SPP) surface waves based on graphene. Using numerical simulations, we first present the simple case of one-atom-thick straight-line mirror and then show how a one-atom-thick parabolic reflector (mirror) can be envisioned for focusing guided SPP waves on the graphene.     

Statistical measures and the Klein tunneling in single-layer graphene

Statistical complexity and Fisher–Shannon information are calculated in a problem of quantum scattering, namely the Klein tunneling across a potential barrier in graphene. The treatment of electron wave functions as masless Dirac fermions allows us to compute these statistical measures. The comparison of these magnitudes with the transmission coefficient through the barrier is performed. We show that these statistical measures take their minimum values in the situations of total transparency through the barrier, a phenomenon highly anisotropic for the Klein tunneling in graphene.     

Optical conductivity of single-layer graphene induced by temporal mass-gap fluctuations

We consider the dynamics of charge carriers in single-layer graphene that are subject to random temporal fluctuations of their mass gap. The optical conductivity is calculated by incorporating the quantum-stochastic time evolution into the standard linear-response (Kubo) theory. We find that, for an intermediate range of frequencies below the average gap size, electron transport is enhanced by fluctuations. At the same time, in the limit of high as well as low frequencies, the conductivity is suppressed as the variance of gap fluctuations increases. In particular, the dc conductivity is always suppressed by a random temporal mass with nonvanishing mean value and vanishes in the zero-temperature limit. Our results are complementary to those obtained recently for static random-gap disorder in finite-size systems.     

Energy of bonding an adsorbed atom with a single-layer graphene

The change in the local density of states δρ _g of a single-sheet graphene due to adsorption of a single atom has been calculated in the framework of the M model proposed earlier. The dependence of the local density of states δρ _g on the position of the adatom energy level ɛ _a with respect to the Dirac point and other parameters of the problem has been analyzed. It has been shown that the largest changes in the local density of states δρ _g are caused by adatoms with the ɛ _a levels lying in the vicinity of the Dirac point, so that the minimum density of states of graphene remains equal to zero. An analytical expression has been derived for the energy of bonding W _ads of the adatom with graphene. The obtained estimates of the bonding energy W _ads in the weak and strong adatom-substrate bonding regimes are presented. Atoms of alkali metals and halogens have been considered as specific adsorbates.     

Vibration analysis of defected and pristine triangular single-layer graphene nanosheets

This paper investigates the vibration behavior of pristine and defected triangular graphene sheets; which has recently attracted the attention of researchers and compare these two types in natural frequencies and sensitivity. Here, the molecular dynamics method has been employed to establish a virtual laboratory for this purpose. After measuring the different parameters obtained by the molecular dynamics approach, these data have been analyzed by using the frequency domain decomposition (FDD) method, and the dominant frequencies and mode shapes of the system have been extracted. By analyzing the vibration behaviors of pristine triangular graphene sheets in four cases (right angle of 45-90-45 configuration, right angle of 60-90-30 configuration, equilateral triangle and isosceles triangle), it has been demonstrated that the natural frequencies of these sheets are higher than the natural frequency of a square sheet, with the same number of atoms, by a minimum of 7.6% and maximum of 26.6%. Therefore, for increasing the resonance range of sensors based on 2D materials, non-rectangular structures, and especially the triangular structure, can be considered as viable candidates. Although the pristine and defective equilateral triangular sheets have the highest values of resonance, the sensitivity of defective (45,90,45) triangular sheet is more than other configurations and then, defective (45,90,45) sheet is the worst choice for sensor applications.     

Influence of defects on the conductivity of graphene within the effective theory approach

The results of the Monte Carlo simulations of graphene with structural defects are presented. The calculations are performed within an effective quantum field theory with non-compact 3 + 1-dimensional Abelian gauge field and 2 + 1-dimensional Kogut-Susskind fermions. It was found that defects shift the semimetal-insulator phase transition point towards higher values of a substrate permittivity.     

Magnetoresistance of single-layer graphene under the conditions of short-range potential scattering

The magnetoresistance of single-layer graphene on a Si/SiO₂ substrate is measured in the temperature range of 2.5?C150 K. It is found that, at high enough temperatures and away from the Dirac point, the resistance varies as the square root of the magnetic field. This agrees with a recent theoretical calculation of the magnetoresistance for the case of charge-carrier scattering by defects characterized by a short-range potential.     

Quantum theory of the polariton of the bounded semiconductor

The quantization of the polariton field is carried out for the semiconductor bounded by the surface. The orthonormalized set of the normal mode of the vacuum-semiconductor system is explicitly given for the case of the excitonic polarization field with the standard form of the spatial dispersion and the additional boundary condition of the elastic type. The scheme is employed to interpret the several experiments: optical reflectance and transmission, resonant Raman and Brillouin scattering, and the induced absorption of the polariton. Particularly, a recent experiment on CuCl which shows the existence of correlation among the polaritons and reflected light is discussed in view of the quantum picture.     

Numerical simulation of graphene in a magnetic field within the effective field theory

Monte Carlo simulation of graphene in an external magnetic field perpendicular to the plane of graphene has been reported. The calculations have been performed using the effective quantum field theory with a noncompact (3 + 1)-dimensional Abelian gauge field and (2 + 1)-dimensional Kogut-Susskind fermions. It has been revealed that the external magnetic field shifts the semimetal-insulator phase transition point toward higher dielectric constants of the substrate. The phase diagram of the semimetal-insulator phase transition has been plotted in the (dielectric constant of the substrate-magnetic field) plane.