Electronic and magnetic properties of hydrogenated graphene nanoflakes

We investigated the energetic stability, electronic, and magnetic properties of hydrogenated graphene nanoflakes (GNFs) by using density-functional theory (DFT). Hydrogenated GNFs were found to be the stable heterojunction structures. As the increase of H coverage, a transition of a small-gap semiconductor to wide-gap semiconductor occurs, accompanied with a nonmagnetic (with the coverage χ=0$\chi =0$) → magnetic (with the coverage 0<χ<1$0<\chi <1$) → nonmagnetic (with the coverage χ=1$\chi =1$) transfer for hexagonal nanoflakes and magnetic (with the coverage 0?χ<1$0?\chi <1$) → nonmagnetic (with the coverage χ=1$\chi =1$) transfer for triangular nanoflakes. The efficacious tune of band gaps and the magnetic moments on these nanoflakes by hydrogenation offers an effectual avenue for the applications of C-based nanomagnets.     

Noble metals induced magnetic properties of graphene

The adsorption energies, stable configurations, electronic structures, and magnetic properties of the graphene with noble metal (NM=Pt, Ag, and Au) atom adsorption were investigated using first-principles density-functional theory. It is found that the bridge site is the most stable adsorption site for the Pt adatom; the Ag adatom can be stabilized almost equally at the bridge or the top site, while the Au adatom prefers to be adsorbed at top site. The Pt-graphene interaction is stronger than the interaction of Ag-graphene and Au-graphene, since the Pt atom has an unsaturated electronic d-shell (d⁹s¹). While there is no net magnetic moment for the Pt adatom, the Ag and Au adatoms still exhibit magnetic character on the graphene. The magnetic moments of the NM-graphene systems may be quenched (e.g., Pt-graphene), reduced (e.g., Ag-graphene) or not changed (e.g., Au-graphene) as compared with the values before adsorption. Therefore, the magnetic character of the adatom-graphene system can be turned by adsorbing different NM atoms on the graphene.     

Transport properties of mesoscopic graphene rings

Based on a recursive Green's function method, we investigate the conductance of mesoscopic graphene rings in the presence of disorder, in the limit of phase coherent transport. Two models of disorder are considered: edge disorder and surface disorder. Our simulations show that the conductance decreases exponentially with the edge disorder and the surface disorder. In the presence of flux, a clear Aharonov-Bohm conductance oscillation with the period Φ₀ (Φ₀=h/e) is observed. The edge disorder and the surface disorder have no effect on the period of AB oscillation. The amplitudes of AB oscillations vary with gate voltage and flux, which is consistent with the previous results. Additionally, ballistic rectification and negative differential resistance are observed in I-V curves, with on/off characteristic.     

Shot noise in magnetic field modulated graphene superlattice

We investigate the shot noise properties in a monolayer graphene superlattice modulated by N parallel ferromagnets deposited on a dielectric layer. It is found that for the antiparallel magnetization configuration or when magnetic field is zero the new Dirac-like point appears in graphene superlattice. The transport is almost forbidden at this new Dirac-like point, and the Fano factor reaches its maximum value 1/3. In the parallel magnetization configuration as the number of magnetic barriers increases, the shot noise increases. In this case, the transmission can be blocked by the magnetic–electric barrier and the Fano factor approaches 1, which is dramatically distinguishable from that in antiparallel alignment. The results may be helpful to control the electron transport in graphene-based electronic devices.     

Transport properties of Dirac electrons in graphene based double velocity-barrier structures in electric and magnetic fields

Using transfer matrix method, transport properties in graphene based double velocity-barrier structures under magnetic and electric fields are numerically studied. It is found that velocity barriers for the velocity ratio (the Fermi velocity inside the barrier to that outside the barrier) less than one (or for the velocity ratio greater than one) have properties similar to electrostatic wells (or barriers). The velocity barriers for the velocity ratio greater than one significantly enlarge the resonant tunneling region of electrostatic barriers. In the presence of magnetic field, the plateau width of the Fano factor with a Poissonian value shortens (or broadens) for the case of the velocity ratio less than one (or greater than one). When the Fermi energy is equal to the electrostatic barrier height, for different values of the velocity ratio, both the conductivities and the Fano factors remain fixed.     

Transport properties in a line defect superlattice of graphene

It was recently reported that a kind of graphene line defect can be fabricated in a controllable experimental way. In the present work we theoretically investigate the band structure and the electronic transport properties of a graphene superlattice formed by embedding periodically line defects in the graphene lattice. Based on the calculated results, we suggest that such a superlattice can be used as a quantum wire array which can carry much larger current than a single graphene nanoribbon. A remarkable advantage of this superlattice over other quantum wires is that the electronic transport in it is insensitive to scattering effects except that the scattering potential range is smaller than the graphene lattice constant. Moreover, we find that the anisotropy of the Dirac cone presented in this superlattice has a nontrivial influence on the universal minimal conductivity and the sub-Poissonian shot noise of graphene.     

Transport properties in a monolayer graphene modulated by the realistic magnetic field and the Schottky metal stripe

Based on the transfer-matrix method, a systematic investigation of electron transport properties is done in a monolayer graphene modulated by the realistic magnetic field and the Schottky metal stripe. The strong dependence of the electron transmission and the conductance on the incident angle of carriers is clearly seen. The height, position as well as width of the barrier also play an important role on the electron transport properties. These interesting results are very useful for understanding the tunneling mechanism in the monolayer graphene and helpful for designing the graphene-based electrical device modulated by the realistic magnetic field and the electrical barrier.     

Dispersion relation for localized magnetic polaritons propagating at the junction of two ferromagnetic/non-magnetic superlattices

Localized magnetic polaritons are investigated in the systems consisting of two magnetic superlattices, coupled by a ferromagnetic contact layer. The general dispersion relation for localized magnetic polaritons are derived in the framework of the electromagnetic wave theory in the Voigt geometry by the ‘transfer’ matrix method. The numerical calculations were carried out for different parameters of the superlattices and contact layer and then discussed.     

Elastic properties of semiconductor a-Si:H/a-SiNx:H quasiperiodic and periodic superlattices

Surface Rayleigh acoustic waves and related elastic properties of quasiperiodic and periodic modulated a-Si:H/a-SiN_x:H superlattices have been studied by means of a light-scattering technique. Changes in the phase velocity of the surface Rayleigh acoustic waves as a function of sublayer (a-Si:H) thickness are interpreted using an effective medium model. Despite the existence of structural mismatch in the bond-length and coordination number and of structural disorder in the bond-angle for Si-N and Si-Si, no appreciable discrepancy between the measured results and the theoretical predictions has been found.     

Fano resonance in graphene anti-dot and periodic graphene anti-dot arrays

We study the electron transport properties of graphene anti-dot and periodic graphene anti-dot arrays using the nonequilibrium Green?s function method and Landauer–Büttiker formula. Fano resonant peaks are observed in the vicinity of Fermi energy, because discrete states coexist with continuum energy states. These peaks move closer to Fermi energy with increasing the width of anti-dots, but move away from the Fermi energy with increasing the length of anti-dots. When N periodic anti-dots exist in the longitude direction, a rapid fluctuation appears in the conductance with varying resonance peaks, which is mainly from the local resonances created by quasibound state. When P periodic anti-dots exist in the transverse direction, P-fold resonant splitting peaks are observed around the Fermi energy, owing to the symmetric and antisymmetric superposition of quasibound states.     

In掺杂ZnO超晶格光学性质的研究

采用基于第一性原理的线性缀加平面波方法, 计算了超晶格ZnO掺In的光学性质. 计算结果表明, 掺入In元素后Fermi能级进入导带, 介电函数、吸收系数、折射率、反射率都在Fermi面附近产生新的跃迁峰. 随着掺杂层数的增多, 跃迁峰发生红移, 当掺杂两层In时跃迁峰峰值最大, 同时吸收系数的吸收边随掺杂层数的增多而逐渐减小. 与In掺杂ZnO超晶胞相比, ZnO超晶格在可见光范围内具有高透过率的特点.     

Eigen spectra and wave functions of the massless Dirac fermions under the nonuniform magnetic fields in graphene

In this research, firstly, by using the new form of Dirac-Weyl equation and the series method with submitting more suitable details, the energy spectrum and wave functions of the massless Dirac fermions are calculated under the inhomogeneous and q-deformed spatially magnetic fields. Although, we discussed about the results of the energy levels, further, we obtained the wave function as the Hessenberg determinant with calculating the elements of it as exact. On the other hand, by using the Mellin-Barnes integral representation and Hurwitz zeta function, we have achieved the thermodynamic physical quantities of the Dirac-Weyl fermions in the absence of a magnetic field for inside of the graphene quantum dot. Finally, our numerical results for the wave functions and probability densities are presented too.     

Electronic transport properties of graphene/Al2O3 (0001) interface

The electronic structure and transport properties of a single layer of graphene (Gr) on α-Al₂O₃ surface are studied using the density functional theory (DFT). We present three models that take into account the atom at the termination of the alumina surface: a) Al atoms, with the center of the Gr hexagon directly over an Al atom; b) Al atoms, with a carbon directly positioned above an Al atom; c) oxygen atoms. Two processes of geometric optimization were used: (i) All the atoms of the supercell were allowed to move in accordance with the BFGS quasi-Newton algorithm; (ii) The atoms of the three topmost layers of the α-Al₂O₃ (0001) slab, including the C atoms, were allowed to move, whereas the atoms of the remaining layers were frozen in their respective atomic bulk positions. The first two models preserve qualitatively the electronic structure of the pristine Gr using the geometric optimization process (i) whereas, in the third model this structure was lost due to a significant charge transfer between the carbon and oxygen atoms irrespective of the optimization procedure. However, models (a) and (b) with the optimization (ii) reveal a p-type semiconducting behavior.     

Enlargement of omnidirectional electronic gap in the graphene superlattice heterostructures with Gaussian profile potential voltages

A valid method is used to extend the omnidirectional electronic gap (OEG) of Gaussian gapped graphene superlattices (GSLs) heterostructure. The heterostructure consists of two superlattices with different width ratios of potentials. Each superlattice comprises a periodic repetition of a unit cell consisting of 21 layers with the potential voltages varying according to a Gaussian function and another layer with a fixed potential voltage. The potential width ratios of constituent Gaussian gapped GSL are established utilizing the lower and upper energy edges of omnidirectional electronic gap depending on the width ratio of potentials. Moreover, it is shown that the width of OEG of the heterostructure is sensitive to lattice constant, which can be applicable to the development of graphene-based electronics.     

Structural, electronic, and magnetic properties of CrN under high pressure

The structural,electronic and magnetic properties of CrN under high pressure are investigated by first-principles calculations.The antiferromagnetic orthorhombic structure is identified to be the preferred ground state structure.It possesses a bulk modulus of 252.8 GPa and the nonzero magnetic moment of 2.33 μ B per Cr ion,which agree well with the experimental results.CrN undergoes structural and magnetic transitions from an antiferromagnetic rocksalt structure to a non-magnetic Pnma phase at 132 GPa.Under compression,the magnetic moment of the Cr ion reduces rapidly near the equilibrium and phase transition point,and the distribution of the density of states is broadened,but the form of overlap between the orbitals of Cr d and N p remains unchanged.The broadening of the band induces spin flipping,which consequently results in the smaller magnetic moment of the Cr ion.     

Transport properties of graphene/metal planar junction

The transport properties of graphene/metal (Cu(111), Al(111), Ag(111), and Au(111)) planar junction are investigated using the first-principles nonequilibrium Green's function method. The planar junction induce second transmission minimum (TM2) below the Fermi level due to the existence of the Dirac point of clamped graphene. Interestingly, no matter the graphene is p- or n-type doped by the metal substrate, the TM2 always locates below the Fermi level. We find that the position of the TM2 is not only determined by the doping effect of metal lead on the graphene, but also influenced by the electrostatic potential of the metal substrate and the work function difference between the clamped and suspended graphene.     

Conductance of disordered graphene sheets: A real space approach

In this work the conducting properties of graphene lattice (buckled as well as planar) having different concentrations of defects are studied with the help of real space block recursion method introduced by Haydock et al. Since the defects are completely random, reciprocal space based methods which need artificial periodicity are not applicable here. Different resonant states appear because of the presence of topological and local defects which are calculated within the framework of Green function. Except random voids, in all other density of states (DOS) spectra there are signatures of Breit–Wigner and Fano resonance at occupied and unoccupied regime respectively. Although Fano resonance states are not prominent for graphene with random voids, however Stone–Wales (SW) type defect can naturally introduce their resonance states. The appearance of localized states depends strongly on the concentration of defects.     

Spectral properties of quasiperiodic superlattices and study of wave functions

We report the study of the spectral properties of a quasiperiodic superlattice within a tight binding model. Numerical work is carried out using the transfer matrix method. An approximate analytical scheme is used to obtain expressions for the band gaps which explain all the features obtained numerically. Due to the fact that blocks of atoms are repeated quasiperiodically, the gaps are shown to vanish at specific energies. These states have much the same behaviour as the extended states but the amplitude is a quasiperiodic function of the site index. The total number of such extended states are estimated. Since it is known that other states in quasiperiodic systems are critical, these states are expected to exhibit a cross-over behaviour to the critical states as a function of the energy. Multifractal analysis of the quasiperiodic wave function show that it has the same signature as the extended wave function. We briefly comment on the cross-over behaviour.     

Magnetic moment of phonons in graphene induced by a magnetic field

A magnetic field not only changes the electronic structure in graphene but also affects the phonon excitations via the electron-phonon interaction and even enables the phonons to generate magnetism. In this paper, we evaluate the magnetic moment of phonons in graphene using a generating-functional technique. The calculation results indicate that the phonon magnetic moment exists only in a weak magnetic field. The step-like change of the magnetic moment with the magnetic field reflects a macroscopic quantum effect.