Resonance Raman scattering in graphene: Probing phonons and electrons

In this work, by using different laser excitation energies, we obtain important electronic and vibrational properties of mono- and bi-layer graphene. For monolayer graphene, we determine the phonon dispersion near the Dirac point for the in-plane transverse optical (iTO) mode. This result is compared with recent calculations that take into account electron–electron correlations for the phonon dispersion around the K point. For bilayer graphene we extract the Slonczewski–Weiss–McClure band parameters and compare them with recent infrared measurements. We also analyze the second-order feature in the Raman spectrum for trilayer graphene.     

Infrared absorption spectra of few-layer graphenes studied by first principles calculations

The infrared (IR) absorption spectra of the undoped, the hole- and electron-doped few-layer graphene (FLG) with layer number of N=1,2,3 have been calculated using the density functional theory in the local density approximation. It is found that in contrast with the featureless optical spectrum of the undoped monolayer graphene, the undoped AB-stacking bilayer and trilayer graphenes exhibit interesting rich IR spectra, e.g., the peaks and jumps in their IR spectra, which are caused by the coupling between different layers. And clear characteristic peaks, lying at different energies, exist in the IR spectra of the hole- or electron-doped bilayer and trilayer graphenes due to the asymmetrical band structures. Beside, based upon their different IR spectra, a powerful experimental tool has been proposed to identify accurately the layer number and doping type of the FLGs.     

The low energy electronic band structure of bilayer graphene

We employ the tight binding model to describe the electronic band structure of bilayer graphene and we explain how the optical absorption coefficient of a bilayer is influenced by the presence and dispersion of the electronic bands, in contrast to the featureless absorption coefficient of monolayer graphene. We show that the effective low energy Hamiltonian is dominated by chiral quasiparticles with a parabolic dispersion and Berry phase 2π. Layer asymmetry produces a gap in the spectrum but, by comparing the charging energy with the single particle energy, we demonstrate that an undoped, gapless bilayer is stable with respect to the spontaneous opening of a gap. Then, we describe the control of a gap in the presence of an external gate voltage. Finally, we take into account the influence of trigonal warping which produces a Lifshitz transition at very low energy, breaking the isoenergetic line about each valley into four pockets.     

Band structures of symmetrical graphene superlattice with cells of three regions

We study the electronic band structures of massless Dirac fermions in symmetrical graphene superlattice with cells of three regions. opening gaps and additional Dirac points. Finally, we inspect the potential effect on minibands, the anisotropy of group velocity and the energy bands contours near Dirac points. We also discuss the evolution of gap edges and cutoff region near the vertical Dirac points.     

Ti-O共掺杂石墨烯的电子结构和光学性质的理论研究

本文利用基于密度泛函理论的第一性原理方法研究了本征石墨烯和不同掺杂浓度下Ti-O共掺杂石墨烯的电子结构和光学性质,并讨论了其内部的微观机制.研究结果表明:本征石墨烯是一种零带隙材料,狄拉克点在费米能级面上,其在紫外光区的光吸收强度较强.Ti-O共掺杂石墨烯可以很好的打开石墨烯的带隙和提高石墨烯的光催化强度,Ti18-O18@G模型费米能级附近的态密度主要由C-p轨道、Ti-d轨道和O-p轨道杂化而成.Ti18-O18@G模型在可见光区的吸收谱强度最大,主要归因于其内部晶格畸变、带隙被打开和杂质能带的出现,这些因素可以促进电子空穴对的产生和分离,从而使石墨烯在可见光区的光催化能力得到增强.本研究结果可为开发高催化活性的石墨烯提供理论依据.     

Artificial graphenes: Dirac matter beyond condensed matter

After the discovery of graphene and of its many fascinating properties, there has been a growing interest for the study of “artificial graphenes”. These are totally different and novel systems that bear exciting similarities with graphene. Among them are lattices of ultracold atoms, microwave or photonic lattices, “molecular graphene” or new compounds like phosphorene. The advantage of these structures is that they serve as new playgrounds for measuring and testing physical phenomena that may not be reachable in graphene, in particular the possibility of controlling the existence of Dirac points (or Dirac cones) existing in the electronic spectrum of graphene, of performing interference experiments in reciprocal space, of probing geometrical properties of the wave functions, of manipulating edge states, etc. These cones, which describe the band structure in the vicinity of the two connected energy bands, are characterized by a topological “charge”. They can be moved in the reciprocal space by appropriate modification of external parameters (pressure, twist, sliding, stress, etc.). They can be manipulated, created or suppressed under the condition that the total topological charge be conserved. In this short review, I discuss several aspects of the scenarios of merging or emergence of Dirac points as well as the experimental investigations of these scenarios in condensed matter and beyond.     

Orientation studies by N.M.R. spectroscopy using a lyotropic liquid crystal solvent

Excitation profile of resonance enhanced Raman lines, due to totally symmetric motions of soluble cis-polyacetylene, show a resolved Frank-Condon structure which reproduces the one in the electronic absorption spectrum. A model calculation which uses two excited electronic states coupled by intra-band vibronic interaction, three vibrational modes, homogeneous and inhomogeneous broadening and a small contribution from pre-resonance states, has been worked out to fit the experimental excitation profiles in the framework of the band model approach like that used for metals in solid state language.     

旋转双层石墨烯的电子结构

本文将第一性原理和紧束缚方法结合起来, 研究了层间不同旋转角度对双层石墨烯的电子能带结构和态密度的影响. 分析发现, 旋转双层石墨烯具有线性的电子能量色散关系, 但其费米速度随着旋转角度的减小而降低. 进一步研究其电子能带结构发现, 不同旋转角度的双层石墨烯在M点可能会出现大小不同的的带隙, 而这些能隙会增强双层石墨烯的拉曼模强度, 并由拉曼光谱实验所证实. 通过对比双层石墨烯的晶体结构和电子态密度, 发现M点处带隙来自于晶体结构中的“类AB堆垛区”. 关键词： 旋转双层石墨烯 第一性原理 紧束缚 电子结构     

Bound states of Dirac fermions in monolayer gapped graphene in the presence of local perturbations

In graphene,conductance electrons behave as massless relativistic particles and obey an analogue of the Dirac equation in two dimensions with a chiral nature.For this reason,the bounding of electrons in graphene in the form of geometries of quantum dots is impossible.In gapless graphene,due to its unique electronic band structure,there is a minimal conductivity at Dirac points,that is,in the limit of zero doping.This creates a problem for using such a highly motivated new material in electronic devices.One of the ways to overcome this problem is the creation of a band gap in the graphene band structure,which is made by inversion symmetry breaking(symmetry of sublattices).We investigate the confined states of the massless Dirac fermions in an impured graphene by the short-range perturbations for "local chemical potential" and "local gap".The calculated energy spectrum exhibits quite different features with and without the perturbations.A characteristic equation for bound states(BSs) has been obtained.It is surprisingly found that the relation between the radial functions of sublattices wave functions,i.e.,f_m~+(r),g_m~+(r),and f_m~-(r),g_m~-(r),can be established by SO(2) group.     

High-energy limit of massless Dirac fermions in multilayer graphene using magneto-optical transmission spectroscopy

We have investigated the absorption spectrum of multilayer graphene in high magnetic fields. The low-energy part of the spectrum of electrons in graphene is well described by the relativistic Dirac equation with a linear dispersion relation. However, at higher energies (>500 meV) a deviation from the ideal behavior of Dirac particles is observed. At an energy of 1.25 eV, the deviation from linearity is approximately 40 meV. This result is in good agreement with the theoretical model, which includes trigonal warping of the Fermi surface and higher-order band corrections. Polarization-resolved measurements show no observable electron-hole asymmetry.     

Electronic structures and optical absorption of multilayer graphenes

We study the electronic structures and the optical absorption spectra of the multilayer graphenes in the effective mass approximation. We decompose the Hamiltonian of graphene with an arbitrary thickness into smaller subsystems effectively identical to monolayer or bilayer graphene, and express the optical spectrum as a summation over the subsystems. We include the full band parameters which compose the bulk graphite, and closely study their effects on the band structure. We found that the particular band parameters destroying the electron–hole symmetry can affect the optical spectrum through shift of the absorption edge.     

垂直电场下扭转双层石墨烯光学吸收性质的理论研究

层间扭转角度是对石墨烯物理性质宽波段可调谐的一个新参量.本文采用2°<θ<15°扭转角度下的连续近似模型,获得了不同扭转角度双层石墨烯分别在有、无电场下的能带结构,通过电子-光子相互作用跃迁速率,计算模拟了范霍夫奇点附近电子带内跃迁和带间跃迁所引起的光学吸收谱.结果表明,在无外加电场时,带间跃迁吸收峰的位置随着扭转角度的增大而发生从红外到可见光波段的蓝移,且吸收系数增大,带内跃迁的光学吸收系数相对于带间跃迁高出2个数量级;而存在外加电场时,两个范霍夫奇点在波矢空间的位置发生偏移,带间跃迁吸收峰发生分裂,且两个分裂的吸收峰位置随着电场强度的不断增大而反向行进.上述研究结果对石墨烯材料在光电器件方面的应用有一定指导作用.     

Formation of a quasi-free-standing graphene with a band gap at the dirac point by Pb atoms intercalation under graphene on Re(0001)

The control of the graphene electronic structure is one of the most important problems in modern condensed matter physics. The graphene monolayer synthesized on the Re(0001) surface and then subjected to the intercalation of Pb atoms is studied by angle-resolved photoelectron spectroscopy and low-energy electron diffraction. The intercalation of Pb atoms under graphene takes place when the substrate is annealed above 500°C. As a result of the intercalation of Pb atoms, graphene becomes quasi-free-standing and a local band gap appears at the Dirac point. The band gap changes with the substrate temperature during the formation of the graphene/Pb/Re(0001) system. The band gap is 0.3 eV at an annealing temperature of 620°C and it increases up to 0.4 eV upon annealing at 830°C. Based on our data, we conclude that the band gap is mainly caused by the hybridization of the graphene π state with the rhenium 5d states located near the Dirac point of the graphene π state.     

第一性原理研究B掺杂Mn_4Si_7的电子结构和光学性质

采用基于密度泛函理论的第一性原理计算方法,对未掺杂及B掺杂Mn_4Si_7的电子结构和光学性质进行理论计算.研究结果表明,未掺杂Mn_4Si_7是间接带隙半导体,其禁带宽度为0.786 eV,B掺杂后其禁带宽度下降为0.723 eV. B掺杂Mn_4Si_7是p型半导体材料.未掺杂Mn_4Si_7在近红外区的吸收系数达到10~5 cm~(-1),B掺杂引起Mn_4Si_7的折射率、吸收系数、反射系数及光电导率增加.     

Crossed Andreev reflection in graphene normal-superconductor-normal structures with pseudo-diffusive interfaces

In this Letter graphene normal-superconductor-normal heterostructures are modeled for studying the crossed Andreev reflection. A thin layer of undoped graphene with Fermi energy at the Dirac point at is assumed the interface between superconductor layer and each normal lead. The resulting contribution of the crossed Andreev reflection to the nonlocal conductance equals that of the electron elastic cotunneling. We explain this as another figure of merit for pseudodiffusive conduction at the Dirac point of the undoped layers. Also structures with only one undoped layer at the interface between the superconductor and one of the normal leads, as well as structures in which one of the leads is ferromagnetic, show pseudodiffusive conduction at the Dirac points.     

Density-functional theory study on the electronic properties of laves phase superconductor CaIr_2

In this work we have used density-functional theory methods such as full-potential local orbital minimum basis(FPLO) and ELK-flapw to study the electronic structure of newly discovered Laves phase superconductor CaIr_2.The calculation of density of states(DOS) indicates that the bands near Fermi level are mostly occupied by the d-electrons of iridium.The simulation of de Haas-van Alphen(dHvA) effect has been performed by using Elk code to check the Fermi surface topology.The results show that there exist four Fermi surfaces in CaIr_2,including two electron-type and two hole-type surfaces.The optical response properties of CaIr_2 have been calculated in the dipole-transition approximations combined with including intra-band Drude-like terms.In the optical spectrum σ(ω) shows that the crossover from intraband to inter-band absorption occur near 1.45 eV.Further analysis on the electron energy loss spectra(EELS) matches the conclusion from that of optical conductivity σ(ω).     

Interband thermoluminescence of semiconductors and semiconductor nanocrystals in the near-infrared

The thermally excited luminescence of undoped semiconductors and semiconductor nanocrystals near the band gap is explored by a simple and unconventional experimental technique. Luminescence spectra are obtained at ambient conditions after slightly heating the samples to approximately 100 °C without using any additional electronic or optical means of excitation. In our investigations, bulk GaAs, bulk InP and semiconductor doped glasses are studied. We show that absorption properties and band gap positions obtained directly from emission spectra not only correspond well to those obtained from transmission measurements, but also yield additional information about the role of defects giving rise to emission from within the band gap.     

Electronic structure of the Au-intercalated graphene/Ni(111) surface

We report the electronic structure of the Au-intercalated graphene/Ni(111) surface using angle-resolved photoemission spectroscopy and low energy electron diffraction. The graphene/Ni(111) shows no Dirac cone near the Fermi level and a relatively broad C 1s core level spectrum probably due to the broken sublattice symmetry in the graphene on the Ni(111) substrate. When Au atoms are intercalated between them, the characteristic Dirac cone is completely recovered near the Fermi level and the C 1s spectrum becomes sharper with the appearance of a 10?×?10 superstructure. The fully Au-intercalated graphene/Ni(111) surface shows a p-type character with a hole pocket of ～0.034?Å^?1 diameter at the Fermi level. When the surface is doped with Na and K, a clear energy gap of ～0.4?eV is visible irrespective of alkali metal.     

Dielectric properties of multiband electron systems II. Collective modes

Starting from the tight-binding dielectric matrix in the random phase approximation we examine the collective modes and electron-hole excitations in a two-band electronic system. For long wavelengths (q → 0), for which most of the analysis is carried out, the properties of the collective modes are closely related to the symmetry of the atomic orbitals involved in the tight-binding states. In insulators there are only inter-band charge oscillations. If atomic dipolar transitions are allowed, the corresponding collectivemodes reduce in the asymptotic limit of vanishing bandwidths to Frenkel excitons for an atomic insulator with weak on-site interactions. The finite bandwidths renormalize the dispersion of these modes and introduce a continuum of incoherent inter-band electron-hole excitations. The possible Landau damping of collective modes due to the presence of this continuum is discussed in detail. In conductors the intra-band charge fluctuations give rise to plasmons. If the atomic dipolar transitions are forbidden, the coupling of inter-band collective modes and plasmons tends to zero as q → 0. On the contrary, in dipolar conductors this coupling is strong and nonperturbative, due to the long range monopole-dipole interactions between intra-band and inter-band charge fluctuations. The resulting collective modes are hybrids of intra-band plasmons and inter-band dipolar oscillations. It is shown that the frequency of the lower hybridized longitudinal mode is proportional to the frequency of the transverse dipolar mode when the latter is small. The dielectric instability in a multi-band conductor is therefore characterized by the simultaneous softening of a transverse and a longitudinal mode, which is an important, directly measurable consequence of the present theory.     

Influence of quadrupole–quadrupole-type interaction on the chaotic dynamics of $$\alpha $$-helical proteins

We calculate the electronic band dispersion of graphene monolayer on a two-dimensional transition metal dichalcogenide substrate (GrTMD) around K and \(\mathbf{K}^{\prime }\) points by taking into account the interplay of the ferromagnetic impurities and the substrate-induced interactions. The latter are (strongly enhanced) intrinsic spin–orbit interaction (SOI), the extrinsic Rashba spin–orbit interaction (RSOI) and the one related to the transfer of the electronic charge from graphene to substrate. We introduce exchange field (M) in the Hamiltonian to take into account the deposition of magnetic impurities on the graphene surface. The cavalcade of the perturbations yield particle–hole symmetric band dispersion with an effective Zeeman field due to the interplay of the substrate-induced interactions with RSOI as the prime player. Our graphical analysis with extremely low-lying states strongly suggests the following: The GrTMDs, such as graphene on \(\hbox {WY}_{2}\), exhibit (direct) band-gap narrowing / widening (Moss–Burstein (MB) gap shift) including the increase in spin polarisation (P) at low temperature due to the increase in the exchange field (M) at the Dirac points. The polarisation is found to be electric field tunable as well. Finally, there is anticrossing of non-parabolic bands with opposite spins, the gap closing with same spins, etc. around the Dirac points. A direct electric field control of magnetism at the nanoscale is needed here. The magnetic multiferroics, like \(\hbox {BiFeO}_{3}\) (BFO), are useful for this purpose due to the coupling between the magnetic and electric order parameters.