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Diverse magnetic and electronic properties of halogen-adsorbed silicene are investigated by the first-principles theoretical framework, including the adatom-diversified geometric structures, the atom-dominated energy bands, the spatial spin density distributions, the spatial charge density distributions and its variations, and the spin- and orbital-projected density of states. Also, such physical quantities are sufficient to identify similar and different features in the double-side and single-side adsorptions. The former belongs to the concentration-depended finite gap semiconductors or p-type metals, while the latter display the valence energy bands with/without spin-splitting intersecting with the Fermi level. Both adsorption types show the halogen-related weakly dispersed bands at deep energies, the adatom-modified middle-energy sigma bands, and the recovery of low-energy pi bands during the destruction of the halogen concentrations. Such feature-rich band structures can be verified by the angle-resolved photoemission spectroscopy experiment.

A systematic study is conducted on the fluorination-enriched essential properties of 2D graphene and 1D graphene nanoribbons using the first-principles method. The combined effects, which arise from the significant chemical bonds in C-C, F-C and F-F bonds, the finite-size quantum confinement, and the edge structure, can greatly diversify geometric structures, electronic properties and magnetic configurations. By the detailed analyses, the critical orbital hybridizations in determining the essential properties are accurately identified from the atom-dominated energy bands, the spatial charge distributions, and the orbital-projected density of states. The top-site F-C bonds, with the multi-orbital hybridizations, create the non-uniform buckled honeycomb lattice. There exist the C-, F- and (C, F)-dominated energy bands. Fluorinated graphene belongs either to the p-type metals (with/without the ferromagnetic spin arrangement) or to the large-gap semiconductors (without magnetism), depending on the concentration and distribution of adatoms. Specially, fluorinated graphene nanoribbons, with armchair/zigzag edge, presents five kinds of spin-dependent properties, covering the non-magnetic and ferromagnetic metals, non-magnetic semiconductors, and anti-ferromagnetic semiconductors with/without the spin splitting. The various band-edge states in 2D and 1D systems appear as the rich and unique structures in density of states. Part of theoretical predictions are consistent with the experimental measurements, and the others are worthy of the further examinations. Also, the fluorination-created diverse properties clearly indicate the high potentials in various applications that will be discussed in detail, e.g., electronic and spintronic nanodevices., Comment: arXiv admin note: text overlap with arXiv:1702.08639

The feature-rich electronic and magnetic properties of fluorine-doped graphene nanoribbons are investigated by the first-principles calculations. They arise from the cooperative or competitive relations among the significant chemical bonds, finite-size quantum confinement and edge structure. There exist C-C, C-F, and F-F bonds with the multi-orbital hybridizations. Fluorine adatoms can create the p-type metals or the concentration- and distribution-dependent semiconductors, depending on whether the $\pi$ bonding is seriously suppressed by the top-site chemical bonding. Furthermore, five kinds of spin-dependent electronic and magnetic properties cover the non-magnetic and ferromagnetic metals, the non-magnetic semiconductors, and the anti-ferromagnetic semiconductors with/without the spin splitting. The diverse essential properties are clearly revealed in the spatial charge distribution, the spin density, and the orbital-projected density of states.

The alkali-adsorbed graphene nanoribbons exhibit the feature-rich electronic and magnetic properties. From the first-principles calculations, there are only few adatom-dominated conduction bands, and the other conduction and valence bands are caused by carbon atoms. A lot of free electrons are revealed in the occupied alkali- and carbon-dependent conduction bands. Energy bands are sensitive to the concentration, distribution and kind of adatom and the edge structure, while the total linear free carrier density only relies on the first one. These mainly arise from a single $s-2p_z$ orbital hybridization in the adatom-carbon bond. Specifically, zigzag systems can present the anti-ferromagnetic ordering across two edges, ferromagnetic ordering along one edge and non-magnetism, being reflected in the edge-localized energy bands with or without spin splitting. The diverse energy dispersions contribute many special peaks in density of states. The critical chemical bonding and the distinct spin configuration could be verified from the experimental measurements.

The feature-rich electronic excitations of monolayer germanene lie in the significant spin-orbital coupling and the buckled structure. The collective and single- particle excitations are diversified by the magnitude and direction of transferred momentum, the Fermi energy and the gate voltage. There are four kinds of plasmon modes, according to the unique frequency- and momentum-dependent phase diagrams. They behave as two-dimensional acoustic modes at long wavelength. However, for the larger momenta, they might change into another kind of undamped plasmons, become the seriously suppressed modes in the heavy intraband e-h excitations, keep the same undamped plasmons, or decline and then vanish in the strong interband e-h excitations. Germanene, silicene and graphene are quite different from one another in the main features of the diverse plasmon modes.

Essential properties of multilayer graphenes are diversified by the number of layers and the stacking configurations. For an $N$-layer system, Landau levels are divided into $N$ groups, with each identified by a dominant sublattice associated with the stacking configuration. We focus on the main characteristics of Landau levels, including the degeneracy, wave functions, quantum numbers, onset energies, field-dependent energy spectra, semiconductor-metal transitions, and crossing patterns, which are reflected in the magneto-optical spectroscopy, scanning tunneling spectroscopy, and quantum transport experiments. The Landau levels in AA-stacked graphene are responsible for multiple Dirac cones, while in AB-stacked graphene the Dirac properties depend on the number of graphene layers, and in ABC-stacked graphene the low-lying levels are related to surface states. The Landau-level mixing leads to anticrossings patterns in energy spectra, which are seen for intergroup Landau levels in AB-stacked graphene, while in particular, a formation of both intergroup and intragroup anticrossings is observed in ABC-stacked graphene. The aforementioned magneto-electronic properties lead to diverse optical spectra, plasma spectra, and transport properties when the stacking order and the number of layers are varied. The calculations are in agreement with optical and transport experiments, and novel features that have not yet been verified experimentally are presented., Comment: 104 pages and 29 figures

The magneto-optical properties of simple hexagonal graphite exhibit rich beating oscillations, which are dominated by the field strength and photon energy. The former has a strong effect on the intensity, the energy range of the beating and the number of groups, and the latter modulates the total group numbers of the oscillation structures. The single-particle and collective excitations are simultaneously presented in the magnetoreflectance spectra and can be precisely distinguished. For the loss function and reflectance, the beating pattern of the first group displays stronger intensities and broader energy range than other groups. Simple hexagonal graphite possesses unique magneto-optical characteristics that can serve to identify other bulk graphites., Comment: 17 pages, 6 figures

The low-frequency optical excitations of AA-stacked bilayer graphene are investigated by the tight-binding model. Two groups of asymmetric LLs lead to two kinds of absorption peaks resulting from only intragroup excitations. Each absorption peak obeys a single selection rule similar to that of monolayer graphene. The excitation channel of each peak is changed as the field strength approaches a critical strength. This alteration of the excitation channel is strongly related to the setting of the Fermi level. The peculiar optical properties can be attributed to the characteristics of the LL wave functions of the two LL groups. A detailed comparison of optical properties between AA-stacked and AB-stacked bilayer graphenes is also offered. The compared results demonstrate that the optical properties are strongly dominated by the stacking symmetry. Furthermore, the presented results may be used to discriminate AABG from MG, which can be hardly done by STM.

The low-frequency magneto-optical properties of bilayer Bernal graphene are studied by the tight-binding model with four most important interlayer interactions taken into account. Since the main features of the wave functions are well depicted, the Landau levels can be divided into two groups based on the characteristics of the wave functions. These Landau levels lead to four categories of absorption peaks in the optical absorption spectra. Such absorption peaks own complex optical selection rules and these rules can be reasonably explained by the characteristics of the wave functions. In addition, twin-peak structures, regular frequency-dependent absorption rates and complex field-dependent frequencies are also obtained in this work. The main features of the absorption peaks are very different from those in monolayer graphene and have their origin in the interlayer interactions.

The low-frequency magneto-optical absorption spectra of bilayer Bernal graphene are studied within the tight-binding model and gradient approximation. The interlayer interactions strongly affect the electronic properties of the Landau levels (LL's), and thus enrich the optical absorption spectra. According to the characteristics of the wave functions, the low-energy LL's can be divided into two groups. This division results in four kinds of optical absorption peaks with complex optical selection rules. Observing the experimental convergent absorption frequencies close to zero magnetic field might be useful and reliable in determining the values of several hopping integrals. The dependence of the optical absorption spectra on the field strength is investigated in detail, and the results differ considerably from those of monolayer graphene., Comment: 10 pages and 3 figures