Universal optical conductance of graphite

We find experimentally that the optical sheet conductance of graphite per graphene layer is very close to (pi/2)e2/h, which is the theoretically expected value of dynamical conductance of isolated monolayer graphene. Our calculations within the Slonczewski-Weiss-McClure model explain well why the interplane hopping leaves the conductance of graphene sheets in graphite almost unchanged for photon energies between 0.1 and 0.6 eV, even though it significantly affects the band structure on the same energy scale. The f-sum rule analysis shows that the large increase of the Drude spectral weight as a function of temperature is at the expense of the removed low-energy optical spectral weight of transitions between hole and electron bands.     

Near-edge x-ray absorption fine-structure investigation of graphene

We report the near-edge x-ray absorption fine-structure (NEXAFS) spectrum of a single layer of graphite (graphene) obtained by micromechanical cleavage of highly ordered pyrolytic graphite on a SiO2 substrate. We utilized a photoemission electron microscope to separately study single-, double-, and few-layers graphene samples. In single-layer graphene we observe a splitting of the pi resonance and a clear signature of the predicted interlayer state. The NEXAFS data illustrate the rapid evolution of the electronic structure with the increased number of layers.     

Spectrum of pi electrons in graphene as a macromolecule

We report the exact solution of the spectral problem for a graphene sheet framed by two armchair- and two zigzag-shaped boundaries. The solution is found for the pi electron Hamiltonian and gives, in particular, a closed analytic expression of edge-state energies in graphene. It is shown that the lower symmetry of graphene, in comparison with C6h of 2D graphite, has a profound effect on the graphene band structure. This and other results obtained have far-reaching implications for the understanding of graphene electronics. Some of them are briefly discussed.     

Rashba effect in the graphene/ni(111) system

We report on angle-resolved photoemission studies of the electronic pi states of high-quality epitaxial graphene layers on a Ni(111) surface. In this system the electron binding energy of the pi states shows a strong dependence on the magnetization reversal of the Ni film. The observed extraordinarily large energy shift up to 225 meV of the graphene-derived pi band peak position for opposite magnetization directions is attributed to a manifestation of the Rashba interaction between spin-polarized electrons in the pi band and the large effective electric field at the graphene/Ni interface. Our findings show that an electron spin in the graphene layer can be manipulated in a controlled way and have important implications for graphene-based spintronic devices.     

拉伸作用对单层石墨片电子能隙的影响

基于紧束缚方法,在考虑最近邻相互作用的情况下,研究了拉伸锯齿型边和扶手型边单层石墨的能带结构,得到了两种类型单层石墨片的π电子能带及带隙与拉力的解析关系式.通过数值计算能够发现：拉力不但使单层石墨产生带隙,而且带隙随着拉力的增大而变宽,并且锯齿型比扶手型的带隙更易变宽. 关键词： 单层石墨片 拉伸形变 电子能隙     

基于光子晶体异质结构的磁可调石墨烯多带吸收

石墨烯在光电子学领域具有广泛应用,但石墨烯的吸收率较低限制了其在某些方面的应用。为了改善单层石墨烯的吸收特性,在前人研究的基础上,设计了石墨烯和光子晶体异质结构构成的复合结构。利用4×4传输矩阵法研究了外磁场、费米能量和设计波长等参数对石墨烯吸收特性的影响。结果表明,所设计的光学结构使石墨烯既保持了原有的宽吸收带,还增加了数目可调的窄吸收带。由于考虑到磁光效应,石墨烯的吸收特性表现出一定的磁圆二色性。对于各吸收带,通常情况下左旋圆偏振光的吸收率要大于右旋圆偏振光的吸收率。但调节外磁场和费米能量,可使各吸收带具有99%以上的吸收,在一定的条件下,还可实现近完美的100%吸收。研究结果为光电子学领域中基于石墨烯的相关器件的设计提供了参考。     

锌-牛血清白蛋白的电荷转移谱带研究

研究了1：1锌－牛血清白蛋白体系的差示吸收光谱,使用文献数据计算了组氨酸咪唑基的三个π轨道的光学电负性,利用这些数据归属了Zn^2 离子与牛血清白蛋白所形成的配合物的两条电荷转移谱带,推测它们分别为白蛋白组氨酸咪唑基的π2,π3,轨道向Zn(Ⅱ)离子的LMCT跃迁。     

Klein backscattering and Fabry-Pérot interference in graphene heterojunctions

We present a theory of quantum-coherent transport through a lateral p-n-p structure in graphene, which fully accounts for the interference of forward and backward scattering on the p-n interfaces. The backreflection amplitude changes sign at zero incidence angle because of the Klein phenomenon, adding a phase pi to the interference fringes. The contributions of the two p-n interfaces to the phase of the interference cancel with each other at zero magnetic field, but become imbalanced at a finite field. The resulting half-period shift in the Fabry-Pérot fringe pattern, induced by a relatively weak magnetic field, can provide a clear signature of Klein scattering in graphene. This effect is shown to be robust in the presence of spatially inhomogeneous potential of moderate strength.     

Highly anisotropic Dirac cones in epitaxial graphene modulated by an island superlattice

We present a new method to engineer the charge carrier mobility and its directional asymmetry in epitaxial graphene by using metal cluster superlattices self-assembled onto the moiré pattern formed by graphene on Ir(111). Angle-resolved photoemission spectroscopy reveals threefold symmetry in the band structure associated with strong renormalization of the electron group velocity close to the Dirac point giving rise to highly anisotropic Dirac cones. We further find that the cluster superlattice also affects the spectral-weight distribution of the carbon bands as well as the electronic gaps between graphene states.     

石墨烯中新奇量子物态的研究

石墨烯特殊的晶格结构和能带结构赋予了它独特的电学性质. 近年来, 分数量子霍尔态、 魔角石墨烯中的 关联绝缘体态和超导态等现象的发现不断证明着石墨烯是一种理想的二维模型体系, 可用于实现一系列新奇的量子物态, 对石墨烯中新奇量子物态的探测和调控也一直是凝聚态物理领域的前沿研究热点之一. 本文将系统地介绍近年来石墨烯中对称性破缺量子物态的研究进展, 包括平带中强关联量子物态的研究以及谷赝自旋调控的研究, 并介绍一种在纳米尺度、 单电子精度上探测二维材料体系简并度及对称性破缺态的普适方法, 希望为相关领域的研究人员提供参考和借鉴.     

Equilateral-triangular shape in 14C

An equilateral-triangular shape of three alpha clusters surrounded by excess neutrons is suggested for 14C, based on the molecular-orbit model. It is found that the attractive interaction between an excess neutron and an alpha particle stabilizes the K(pi)=0(+) and 3(-) rotational bands, which demonstrates an equilateral-triangular symmetry. This K(pi)=3(-) band at 3 MeV below the 10Be+alpha threshold energy corresponds to the experimentally observed band built on top of the second 3(-) state. A positive-parity rotational band (0(+), 2(+), 4(+)) arises similarly. These two bands suggest a molecular 3-alpha structure stabilized by the excess neutrons and can be viewed as a realization of the alpha crystallization in the dilute nuclear medium.     

Electronic structure and resonant X-ray emission spectra of carbon shells of iron nanoparticles

The electronic structure of carbon shells of carbon encapsulated iron nanoparticles carbon encapsulated Fe@C has been studied by X-ray resonant emission and X-ray absorption spectroscopy. The recorded spectra have been compared to the density functional calculations of the electronic structure of graphene. It has been shown that an Fe@C carbon shell can be represented in the form of several graphene layers with Stone-Wales defects. The dispersion of energy bands of Fe@C has been examined using the measured C Kα resonant X-ray emission spectra.     

Raman fingerprint of doping due to metal adsorbates on graphene

The properties of single-layer graphene are strongly affected by metal adsorbates and clusters on graphene. Here, we study the effect of a thin layer of chromium (Cr) and titanium (Ti) metals on chemical vapor deposition (CVD)-grown graphene by using Raman spectroscopy and transport measurements. The Raman spectra and transport measurements show that both Cr and Ti metals affect the structure as well as the electronic properties of the CVD-grown graphene. The shift of peak frequencies, intensities and widths of the Raman bands are analyzed after the deposition of metal films of different thickness on CVD-grown graphene. The shifts in G and 2D peak positions indicate the doping effect of graphene by Cr and Ti metals. While p-type doping was observed for Cr-coated graphene, n-type doping was observed for Ti-coated graphene. The doping effect is also confirmed by measuring the gate voltage dependent resistivity of graphene. We have also found that annealing in Ar atmosphere induces a p-type doping effect on Cr- or Ti-coated CVD-grown graphene.     

Electronic and magnetic properties of quasifreestanding graphene on Ni

For the purpose of recovering the intriguing electronic properties of freestanding graphene at a solid surface, graphene self-organized on a Au monolayer on Ni(111) is prepared and characterized by scanning tunneling microscopy. Angle-resolved photoemission reveals a gapless linear pi-band dispersion near K[over] as a fingerprint of strictly monolayer graphene and a Dirac crossing energy equal to the Fermi energy (EF) within 25 meV meaning charge neutrality. Spin resolution shows a Rashba effect on the pi states with a large (approximately 13 meV) spin-orbit splitting up to EF which is independent of k.     

Narrow and Dual-Band Tunable Absorption of a Composite Structure with a Graphene Metasurface

A tunable absorber, composed of a graphene ribbon on two layers of TiO_2-Au between two slabs of dielectric material all on a metal substrate, is designed and numerically investigated. The absorption of the composite structure varies with the geometrical parameters of the structure and the physical parameters of graphene at mid-infrared frequencies. The numerical simulation shows that a near-perfect absorption with single and dual bands can be achieved in a certain frequency range. We also analyze the electric and surface current distributions to study the dual-band absorber. The results show that the absorber can be tuned by the chemical potential and electron-phonon relaxation time of graphene, and electromagnetically induced transparency phenomenon can be obtained. The results of this study may be beneficial in the fields of infrared communication, perfect absorbers,sensors and filters.     

Electron-electron correlation in graphite: a combined angle-resolved photoemission and first-principles study

The full three-dimensional dispersion of the pi bands, Fermi velocities, and effective masses are measured with angle-resolved photoemission spectroscopy and compared to first-principles calculations. The band structure by density-functional theory underestimates the slope of the bands and the trigonal warping effect. Including electron-electron correlation on the level of the GW approximation, however, yields remarkable improvement in the vicinity of the Fermi level. This demonstrates the breakdown of the independent electron picture in semimetallic graphite and points toward a pronounced role of electron correlation for the interpretation of transport experiments and double-resonant Raman scattering for a wide range of carbon based materials.     

NoSo and NoS pi detection as a function of masker bandwidth in normal-hearing and cochlear-impaired listeners

NoSo and NoS pi detection thresholds for a 500-Hz pure-tone signal were measured as a function of masking noise bandwidth in normal-hearing and cochlear hearing-impaired subjects. NoSo and NoS pi critical bands were derived from the bandlimited noise functions. A notched noise measure of the monaural critical band was also obtained for each ear. One hypothesis tested was that an asymmetrical monaural critical band would result in a relatively steep improvement of the NoS pi detection threshold as a function of decreasing masker bandwidth and would, therefore, be associated with a wider binaural critical band. This was hypothesized because the outputs of the left and right auditory filters would be more decorrelated the greater the interaural difference in the monaural critical band. However, as the noise bandwidth was narrowed, the decorrelation would lessen, resulting in a relatively steep improvement in NoS pi detection. Results indicated that the masking level difference (MLD) was smaller and that the monaural critical bands were generally wider in cochlear-impaired listeners. NoSo and NoS pi critical bands were somewhat larger in the cochlear hearing-impaired listeners having relatively wide monaural critical bands. There was a significant correlation between monaural critical band asymmetry and the NoS pi critical band; however, this correlation was insignificant when a control was employed for the critical band in the worse ear. Therefore, the present results did not support a strong association between monaural critical band asymmetry and the width of the NoS pi critical band.     

Dual-band high impedance surface with graphene-based metasurfaces

High impedance surface (HIS) is an electromagnetic band gap (EBG) material that shows magnetic conductor surface properties within its band gap frequencies. The operation bandwidth is corresponding to the frequency range, where the reflection phase varies between +90^∘ and −90^∘. In this paper, a dual-band HIS is investigated based on planar periodic graphene arrays placed on a grounded dielectric substrate. Analytical circuit model of the graphene array together with the transmission line theory is employed to analyze the proposed structure. We demonstrate that the HIS bands can be adjusted by tuning the geometrical parameters of the structure and the Fermi level of graphene. The graphene-based HIS promises future applications in the low-profile, high gain and high efficiency antennas at THz frequencies.     

Band structure and transport property of graphene/h-BN heterostructure under local potentials

We investigate band structure and transport property of lattice-matched graphene/hexagonal boron nitride (h-BN) heterostructure using the tight-binding approach. It shows that local potentials can significantly modify the band structure and the transport property. A method to individually manipulate the edge states by local potentials is proposed, including shifts and other deformations of edge bands. The two-terminal conductance of each layer is quantized but the interlayer conductance is non-quantized due to band mixing. In addition, we explore the Landau level spectrum in graphene/h-BN nanoribbons under both magnetic field and local potentials. The plateaus-like behavior of the interlayer conductance is observed.