Plasmon modes in MLG-2DEG heterostructures: Temperature effects

We calculate the plasmon frequency and damping rate in a double-layer system made of monolayer graphene and GaAs quantum well at finite temperature using the random-phase-approximation dielectric function and taking into account the inhomogeneity of the dielectric background of the system. We show that the temperature, interlayer correlation parameters and dielectric background inhomogeneity affect significantly the plasmon frequencies and damping rates of the system. At low temperatures, acoustic (optical) plasmon frequency increases (decreases) with the increase of temperature. We also find that damping rates of both plasmon modes increase remarkably compared to the zero-temperature case.     

Plasmon modes in double-layer gapped graphene at zero temperature

Plasmon dispersions and damping rate of plasma oscillations in a double-layer gapped graphene made of two parallel mono layer gapped graphene sheets grown on dielectric separation are calculated within random-phase-approximation at zero temperature. By using long wavelength limit expansion, analytical expressions for optical and acoustic plasmon frequencies have been formed, and the formulae demonstrate that the considerable difference in analytical form for plasmon frequencies comes from the factor depending on the band gap, compared to gapless situation. Numerical results show that only large band gap decreases remarkably plasmon frequencies of two modes in the range of large wave vector. Acoustic plasmon branch becomes shorter than that in case of zero gap while optical one seems independent with small band gap. In addition, interlayer separation and carrier density affect on collective excitations and damping rate when taking into account the band gap quite similarly to those in case of zero gap.     

The exact solution of the mutator model with directed mutations,linear fitness function,and finite genome length

In this study, we considered the model by Beckman and Loeb [Proc. Natl. Acad. Sci. U.S.A. 103 (2006) 14140] for the mutator phenomena. We construct an infinite population Crow-Kimura model with a mutator gene, directed mutations, a linear fitness function, and a finite genome length. We solved analytically the dynamics of the model using the generating function method. Such models provide realistic predictions for finite population sizes and have been widely discussed recently. The analytical formulas provided can be used to calculate the advantage of the mutator mechanism for the accumulation of mutations in cancer biology.     

Temperature effect on plasmon dispersions in double-layer graphene systems

We investigate the plasmon dispersion relation and damping rate of a double-layer graphene system consisting of two separated monolayer graphenes with no interlayer tunneling at finite temperature. We use the temperature dependent RPA dielectric function which is valid for graphene systems to obtain the plasmon frequencies and damping rates at different temperatures, interlayer correlation parameters and electron densities and then compare them with those obtained from the zero temperature calculations. Our results show that by increasing the temperature, the plasmon frequencies decrease and the decay rate increases. Furthermore, we find that the behavior of a double-layer graphene system at small and large correlation parameters is different from the conventional double-layer two-dimensional electron gas system. Finally, we obtain that in a density imbalanced double-layer graphene system, the acoustic plasmons are more affected by temperature than the equal electron densities one.     

二维二元碳化硅纳米结构的等离激元激发

基于含时密度泛函理论,研究两种构型的二维二元碳化硅（SiC）纳米结构的等离激元激发.SiC纳米结构有两个等离激元共振带.由于键长的改变,相对于硅烯纳米结构和石墨烯纳米结构,SiC纳米结构的两个等离激元共振带分别发生蓝移和红移.一种SiC纳米结构的等离激元共振激发依赖于边界的构型和纳米结构形貌;另一种SiC纳米结构的等离激元共振激发对于边界构型和纳米结构形貌的依赖性降低.沿不同方向激发时,等离激元共振特性基本相同.     

Toward a theory of plasma waves in bilayer graphene

An expression for the longitudinal dielectric permeability of a nondegenerated electron gas in bilayer graphene is derived. In the calculations, the single-band low-energy approximation is used for the electron spectrum. The dispersion relation and the damping rate of plasma waves in bilayer graphene are found.     

Ab-initio calculation of optical properties of AA-stacked bilayer graphene with tunable layer separation

Density functional theory based calculations revealed that optical properties of AA-stacked bilayer graphene are anisotropic and highly sensitive to the interlayer separation. In the long wave length limit of electromagnetic radiation, the frequency dependent response of complex dielectric function becomes vanishingly small beyond the optical frequency of 25.0 eV. Besides, static dielectric constant shows a saturation behaviour for parallel polarization of electric field vector when interlayer spacing is greater than 2.75 Å. As a consequence, an appropriate modification of effective fine structure constant is observed as a function of layer separation. Moreover, the bilayer systems are highly transparent in the optical frequency range of 7.0–10.0 eV. The electron energy loss function exhibits two different in-plane collective (plasmon) excitations and a single out of plane plasmon excitation. The spectral nature of different frequency dependent optical properties is observed to be very similar to that of the monolayer pristine graphene apart from their exact numerical values.     

Cavity plasmon polaritons in monolayer graphene

Plasmon polaritons in a new system, a monolayer doped graphene embedded in optical microcavity, are studied here. The dispersion law for lower and upper cavity plasmon polaritons is obtained. Peculiarities of Rabi splitting for the system are analyzed; particularly, role of Dirac-like spinor (envelope) wave functions in graphene and corresponding angle factors are considered. Typical Rabi frequencies for maximal (acceptable for Dirac-like electron spectra) Fermi energy and frequencies of polaritons near polariton gap are estimated. The plasmon polaritons in considered system can be used for high-speed information transfer in the THz region.     

Coulomb bare interactions in inhomogeneous 4-layer graphene structures

We investigate Coulomb bare interactions in 4-layer graphene systems, taking into account the inhomogeneity of dielectric environment. By solving Poisson equation in momentum space, 16 elements of Coulomb potential tensor have been formed with analytical expressions. The formulae illustrate that Coulomb bare interactions in inhomogeneous 4-layer graphene system differ noticeably from those in homogeneous one. Nevertheless, both intra- and interlayer Coulomb potentials become independent with dielectric constants of contacting media, but they are decided by those of dielectric layer surrounding the system in long wavelength approximation. Besides, numerical results demonstrate that Coulomb bare interactions decrease considerably with the increase in carrier density in graphene sheets as well as their separated distance. In all investigations, the inhomogeneity of dielectric background should not be neglected for improvement in the model.     

Anisotropic Intervalley Plasmon Excitations in Graphene

We investigate theoretically the intervalley plasmon excitations(IPEs) in graphene monolayer within the random-phase approximation. We derive an analytical expression of the real part of the dielectric function. We find a lowenergy plasmon mode with a linear anisotropic dispersion which depends on the Fermi energy and the dielectric constant of substrate. The IPEs show strongly anisotropic behavior, which becomes significant around the zigzag crystallographic direction. More interestingly, the group velocity of IPE varies from negative to positive, and vanishes at special energy.     

石墨烯等离激元的光学性质及其应用前景

石墨烯等离激元由于其独特的电学可调性、本征低衰减以及局域光场高度增强等特性, 引起了广泛的关注并迅速成长为一门新的学科分支--石墨烯表面等离激元光子学. 本文介绍了石墨烯等离激元的一些基本性质, 包括色散关系、局域的等离激元和传导的等离激元以及石墨烯等离激元对其周边介电环境的敏感性等. 在此基础上, 进一步介绍了石墨烯等离激元在太赫兹到中红外频段的应用, 比如有源光调制器的一些功能器件和增强的红外光谱探测等.     

Quasi-particle and plasmaron properties in the electron gas

The self-energy function of the degenerate electron gas is studied in an approximation which uses the dielectric function proposed by Singwi, Tosi, Land and Sjölander, and neglects the corresponding vertex corrections. Two contributions to the self-energy are distinguished which arise from the plasmon pole and the particle-hole continuum respectively. Comparison of the results is made with the analogous approximation to the self-energy which uses the RPA dielectric function, and with a further, simplified approximation. Subsequently the properties of the usual quasi-particle and of the plasmaron are calculated. Nummerically, the most significant effect found is a 25% reduction of the plasmaron damping over the RPA result. For the usual quasiparticle the damping rate is found to be increased by some 10% and the spectral weight reduced by 6%.     

Size characteristics of surface plasmons and their manifestation in scattering properties of metal particles

The change of the scattering properties of sodium, gold and silver spherical particles with size is discussed in the context of surface multipolar plasmon resonances. The presented surface plasmon size characteristics are abstracted from the quantity which is observed and deliver multipolar plasmon resonance frequencies and plasmon damping rates in the form of a continuous function of particle radius. The performed analysis of the plasmon dispersion relation is analogous to the problem of surface plasmon localized at a semi-infinite, flat metal/dielectric interface.Correlation between the multipolar plasmon resonance parameters, and the spectroscopic optical properties of conductive nanoparticles appearing as peaks in the measurable light intensities is analyzed. We discuss the fact, that such peaks arise from interference of all the electromagnetic fields contributing to the measured intensity, and not solely to the fields due to surface plasmon multipolar modes.We describe the results of light scattering experiment in orthogonal polarization geometries with use of spontaneously growing sodium droplets. The polarization geometry of the experiment allows for distinct separation of resonant contribution of dipole and quadrupole plasmon TM mode contributions to the measured intensities as a function of size.Predictions concerning size characteristics for dipole and quadrupole plasmons are compared with the results of light scattering experiments using spherical sodium droplets (our results) and gold and silver particles in suspension [other authors: Sönnichsen C, Franzl T, Wilk T, von Plessen G, Feldmann J. Plasmon resonances in large noble-metal clusters. New J Phys 2002; 4:93.1–8; Haiss W, Thanh NTK, Aveyard J, Fernig DG. Determination of size and concentration of gold nanoparticles from UV–vis spectra. Anal Chem 2007; 79:4215–21; Njoki PN, Lim I-IS, Mott D, Park H-Y, Khan B, Mishra S, et al. Size correlation of optical and spectroscopic properties for gold nanoparticles. J Phys Chem C 2007; 111:14664–9; Mock JJ, Barbic M, Smith DR, Schultz DA, Schultz S. Shape effects in plasmon resonance of individual colloidal silver nanoparticles. J Chem Phys 2002; 116:6755–9].     

Orbital electronic heat capacity of hydrogenated monolayer and bilayer graphene

The tight-binding Harrison model and Green's function approach have been utilized in order to investigate the contribution of hybridized orbitals in the electronic density of states(DOS) and electronic heat capacity(EHC) for four hydrogenated structures, including monolayer chair-like, table-like, bilayer AA- and finally AB-stacked graphene. After hydrogenation, monolayer graphene and bilayer graphene are behave as semiconducting systems owning a wide direct band gap and this means that all orbitals have several states around the Fermi level. The energy gap in DOS and Schottky anomaly in EHC curves of these structures are compared together illustrating the maximum and minimum band gaps are appear for monolayer chair-like and bilayer AA-stacked graphane, respectively. In spite of these, our findings show that the maximum and minimum values of Schottky anomaly appear for hydrogenated bilayer AA-stacked and monolayer table-like configurations, respectively.     

Nonlocal screening of plasmons in graphene by semiconducting and metallic substrates: first-principles calculations

We investigate the role of substrates on the collective excitations of graphene by using a first-principles implementation of the density response function within the random-phase approximation. Specifically, we consider graphene adsorbed on SiC(0001) and Al(111) as representative examples of a semiconducting and metallic substrate. On SiC(0001), the long wavelength π plasmons are significantly damped although their energies remain almost unaltered. On Al(111), the long wavelength π plasmons are completely quenched due to the coupling to the metal surface plasmon. The strong damping of the plasmon excitations occurs despite the fact that the single-particle band structure of graphene is completely unaffected by the substrates illustrating the nonlocal nature of the effect.     

Stable Pfaffian state in bilayer graphene

Here, we show that the incompressible Pfaffian state originally proposed for the 5/2 fractional quantum Hall states in conventional two-dimensional electron systems can actually be found in a bilayer graphene at one of the Landau levels. The properties and stability of the Pfaffian state at this special Landau level strongly depend on the magnetic field strength. The graphene system shows a transition from the incompressible to a compressible state with increasing magnetic field. At a finite magnetic field of ~10 T, the Pfaffian state in bilayer graphene becomes more stable than its counterpart in conventional electron systems.     

Charge susceptibilities of armchair graphene nanoribbon in the presence of magnetic field

We present the behaviors of both dynamical and static charge susceptibilities of undoped armchair graphene nanoribbon using the Green's function approach in the context of tight binding model Hamiltonian.Specifically,the effects of magnetic field on the the plasmon modes of armchair graphene nanoribbon are investigated via calculating the correlation function of charge density operators.Our results show that the increase of magnetic field makes the high-frequency plasmon mode for both metallic and insulating cases disappear.We also show that low-frequency plasmon mode for metallic nanoribbon appears due to increase of magnetic field.Furthermore,the number of collective excitation modes increases with ribbon width at zero magnetic field.Finally,the temperature dependence of the static charge structure factor of armchair graphene nanoribbon is studied.The effects of both magnetic field and ribbon width on the static charge structure factor are discussed in detail.     

Effect of a nonuniform magnetic field on the Landau states in a biased AA-stacked graphene bilayer

We study the Landau states in the biased AA-stacked graphene bilayer under an exponentially decaying magnetic field along one spatial dimension. The results show that the energy eigenvalues of the system are strongly dependent on the inhomogeneity of the magnetic field and the bias voltage between the graphene layers, and in particular the reordering and mixing of finite Landau states could occur. Moreover, we also demonstrate that the current carrying states induced by the decaying magnetic field propagate vertically to the magnetic-field gradient within the graphene sample and can be further modulated by the bias voltage between the layers.     

Surface Plasmon Polariton Fizeaus Dragging: via Relative Rotational Sensing

A new scheme focusing on the surface plasmon polariton interferometry between the metal and dielectric interface is introduced. The phase shift is measured by using surface plasmon polariton wave, generated at the interface of metallic and dielectric media. The phase shift of SPPs is modi?ed under phase and amplitude control of complex conductivity for interferometry. The control ?elds strongly in?uence the phase shift of SPPs for detection of molecular motion. The phase shift of SPPs is further modi?ed by Plasmon polariton Fizeaus dragging effect. We measure 20%–25%fractional change in delay and their phases shift between two left and right SPPs modes. Our results may have signi?cant applications in sensor interferometer technology.     

Propagation of THz plasmon pulse on corrugated and flat metal surface

We report here experiments on surface plasmon excitation and propagation along corrugated and smooth aluminum surface in the terahertz frequency range. Narrowband plasmon excitation by a subpicosecond terahertz pulse is shown to be a transient process and plasmon propagation sufficiently changes its measured time profile. Plasmon propagation during its excitation and detection changes measured signal. We suggest to use parameters T (plasmon duration) and τ (plasmon lifetime) to describe the narrowband THz plasmon pulse. Plasmon duration and lifetime were defined and plasmon propagation lengths on smooth and corrugated surface were measured. Plasmon propagation length on flat surface turned out to be much smaller than it is predicted by the Drude model.