Orbital quantization in a system of edge Dirac fermions in nanoperforated graphene

The dependence of the electric resistance R of nanoperforated graphene samples on the position of the Fermi level E _F, which is varied by the gate voltage V _g, has been studied. Nanoperforation has been performed by irradiating graphene samples on a Si/SiO₂ substrate by heavy (xenon) or light (helium) ions. A series of regular peaks have been revealed on the R(V _g) dependence at low temperatures in zero magnetic field. These peaks are attributed to the passage of E _F through an equidistant set of levels formed by orbitally quantized states of edge Dirac fermions rotating around each nanohole. The results are in agreement with the theory of edge states for massless Dirac fermions.     

Effects of annealing on localized states in amorphous Ge films

Annealing behaviors of the activation energy for the electrical conduction E_σ, that for the thermoelectric power E_s, the optical gap E^opt_g, and the spin density in evaporated amorphous Ge are investigated. E_σ is independent of E_s and E^opt_g, and the rates of variation of E^opt_g and E_s with annealing temperature are connected by △E^opt_g = 2.5△E^opt_s. It is suggested that the position of the Fermi level does not change with annealing in contrast with amorphous Si, and the edge of the localized tail state shifts with annealing.     

Abnormal electronic transport in disordered graphene nanoribbon

We investigate the conductivity σ of graphene nanoribbons with zigzag edges as a function of Fermi energy E_F in the presence of the impurities with different potential range. The dependence of σ(E_F) displays four different types of behavior, classified to different regimes of length scales decided by the impurity potential range and its density. Particularly, low density of long range impurities results in an extremely low conductance compared to the ballistic value, a linear dependence of σ(E_F) and a wide dip near the Dirac point, due to the special properties of long range potential and edge states. These behaviors agree well with the results from a recent experiment by Miao et al. [Science 317 (2007) 1530 (SOM)].     

Comparison of the texture functions describing the grain alignment in NdFeB magnets

Two types of Gaussian distribution function (`θ type’ and `tan θ type') describing the degree of grain alignment in sintered NdFeB magnets have been compared in the distribution coefficient σ (or σ_g), the distribution probability P(θ) and the grain alignment dependence of coercivity. The results show that when the grain alignment is good (the ratio of remanence-to-saturation polarization J_r/J_s⩾0.90), σ(σ_g) and P(θ) for the two types of Gaussian functions have similar variation tendencies, the calculated values of normalized coercivity based on the starting field theory are basically the same and are consistent with experiments. When the grain alignment is not good (J_r/J_s⩾0.80), the variation tendencies of σ and P(θ) are different. In addition, according to `tan θ type’ Gaussian function, the theoretical values of the normalized coercivity based on the starting field theory are still consistent with the experiments, but according to `θ type’ Gaussian function, the theoretical values seriously deviate from the experiments. This means that the `tan θ type’ Gaussian function is a better texture function for describing the grain alignment.     

Effect of kink-rounding barriers on step edge fluctuations

The effect that an additional energy barrier E_kr for step adatoms moving around kinks has on equilibrium step edge fluctuations is explored using scaling arguments and kinetic Monte Carlo simulations. When mass transport is through step edge diffusion, the time correlation function of the step fluctuations behaves as C(t)=A(T)t^1/4. At low temperatures the prefactor A(T) shows Arrhenius behavior with an activation energy (E_det+3?)/4 if E_kr<? and (E_det+E_kr+2?)/4 if E_kr>?, where ? is the kink energy and E_det is the barrier for detachment of a step adatom from a kink. We point out that the assumption of an Einstein relation for step edge diffusion has lead to an incorrect interpretation of step fluctuation experiments, and explain why such a relation does not hold. The theory is applied to experimental results on Pt(1 1 1) and Cu(1 0 0).     

Infrared modulated reflectance spectra of n and p type gallium antimonide and n type indium antimonide

We have observed the modulated reflectance spectra of n and p type GaSb at 300, 80, and 5 K from 0.56 to 2 eV. The modulated reflectance of intrinsic n type InSb was measured at 80 K from 0.2 to 2 eV. The “dry sandwich” vapor deposition technique was used to make the electroreflectance (ER) samples. The low-temperature spectrum of the undoped p type GaSb sample shows three peaks at the band edge that could be associated with transitions from the top of the valence band, the light (0.903 eV) and heavy (1.014eV) hole state Fermi levels to the conduction band. The energies of the observed peaks are in agreement with the Fermi level determination from Hall effect and Faraday rotation measurements. This modulation mechanism is based on band population effects. The ER signal of InSb under flatband condition at 80 K has five half oscillations at the direct band gap. The contribution of piezoelectric strain to ER is present since the dc bias required to achieve flatband condition is different at the band gap than at E₁. The ER signal corresponding to the direct gap energy E₀ and to the spin-orbit energy E₀ + Δ₀ was determined in the n and p type samples of GaSb at different temperatures. We have measured the intrinsic energy gap in GaSb at room temperature. E_g = 0.74 eV. The corresponding spin-orbit splitting was found to be Δ₀ = 0.733 ± 0.002 eV.     

Edge-modified zigzag-shaped graphene nanoribbons: Structure and electronic properties

Control of the band gap of graphene nanoribbons is an important problem for the fabrication of effective radiation detectors and transducers operating in different frequency ranges. The periodic edge-modified zigzag-shaped graphene nanoribbon (GNR) provides two additional parameters for controlling the band gap of these structures, i.e., two GNR arms. The dependence of the band gap E _g on these parameters is investigated using the π-electron tight-binding method. For the considered nanoribbons, oscillations of the band gap E _g as a function of the nanoribbon width are observed not only in the case of armchair-edge graphene nanoribbons (as for conventional graphene nanoribbons) but also for zigzag GNR edges. It is shown that the change in the band gap E _g due to the variation in the length of one GNR arm is several times smaller than that due to the variation in the nanoribbon width, which provides the possibility for a smooth tuning of the band gap in the energy spectrum of the considered graphene nanoribbons.     

Optical properties of amorphous CuInSe2

Optical absorption spectra of polycrystalline and amorphous CuInSe₂ thin films were measured at room temperature in the photon energy range from 0.8 to 2.1 eV. In amorphous CuInSe₂ the absorption coefficient follows the relation α(hv) = A(hv?E₀)/hv characteristic of optical transitions between extended states in both the valence and conduction band. The optical gap of E₀ = 1.38 ± 0.01 eV is larger than the fundamental gap energy of E_g = 1.01 ± 0.01 eV in crystalline CuInSe₂. A comparison of the results for CuInSe₂ with those for ZnSe is given.     

Atomic vibrational dynamics of amorphous Fe-Mg alloy thin films

The atomic vibrational dynamics of ⁵⁷Fe in 800-Å thick amorphous Fe_xMg_1−x alloy thin films (0.3≤x≤0.7) has been investigated at room temperature by nuclear resonant inelastic X-ray scattering (NRIXS) of 14.4125 keV synchrotron radiation. The amorphous phase has been successfully stabilized by codeposition of Fe and Mg in ultrahigh vacuum onto a substrate held at −140 °C during deposition. The amorphous structure of the samples was confirmed by X-ray diffraction and conversion electron Mössbauer spectroscopy. The ⁵⁷Fe-projected partial vibrational density of states, g(E), has been obtained from the measured NRIXS vibrational excitation probability, together with thermodynamic quantities such as the probability of recoilless absorption (f-factor), the average kinetic energy per Fe atom, the average force constant, and the vibrational entropy per Fe atom. A plot of g(E)/E² versus E proves the existence of non-Debye-like vibrational excitations with a peak at E_bp∼3-5 meV (boson peak). Both the boson peak height and E_bp were found to depend linearly on the composition x. Above the boson peak, g(E)/E² exhibits an exponential decrease.     

Electronic transport properties of liquid InSn20 alloy

In this article, temperature dependences of the electrical conductivity σ and the absolute thermoelectric power S of InSn20 wt% melt were investigated. Abnormal phenomena were observed on both σ–T and S–T curves within certain temperature range, suggesting a temperature-induced liquid structural change occurred in the melt. Combined with the data of the mean neighbor distance r ₁ obtained by the prior X-ray diffraction, according to Faber–Ziman theory, the temperature functioning patterns of the density of states N(E_F ) and its gradient value dN(E_F )/dE were deduced. The results reflect that the electronic structure of the melt at Fermi level also changes during the transition.     

U-perturbation treatment of heavy fermion systems

Heavy fermion systems are described by the periodic Anderson Model (PAM), i.e. a lattice of localized, highly correlatedf-electron states hybridized with the delocalized states of a conduction band. We treat the PAM within the second orderU perturbation theory around the non-magnetic Hartree-Fock solution (U on site Coulomb correlation between thef-electrons). This treatment has the advantage that Fermi liquid relations (Luttinger theorem) are automatically fulfilled. Thef-electron selfenergy and spectral function are calculated for different temperatures, and, for the symmetric PAM, we obtain single-particle peaks near toE _f andE _f +U and in addition many-particle (Kondo) resonance peaks near to the chemical potential (E _f baref-electron energy). The resonance peaks are strongly temperature dependent and vanish on a characteristic temperature scaleT _K. For the symmetric PAM and a constant on-site hybridization the Fermi energy falls into a hybridization gap. A second, smaller characteristic temperature scaleT _coh (coherence temperature), on which the hybridization gap vanishes, is observed within this approach. For the non-symmetric PAM (i.e.E _f andE _f +U not symmetric around the chemical potential) we obtain a similar behaviour, but the single-particle peaks are no longer at the correct positionsE _f andE _f +U. The proper behaviour for the symmetric PAM but less satisfactory behaviour for the non-symmetric PAM can be understood from the fact that only for the symmetric PAM the exactly solvable limit of a vanishing hybridization is reproduced within this approach.     

The polarization of protons from the 2H(d, p)3H reaction for deuteron energies below 1 MeV

The angular distribution of proton polarization P^γ' (θ) from the ${}^2\text{H}\text{(}\text{d,}\text{p}\text{)}{}^3\text{H}$ reaction has been measured at 975 keV deuteron energy. Moreover, the energy dependence of P^γ′(E_d) was measured at 45°(lab) for deuteron energies between 250 and 975 keV. The values of σ₀(θ)P^γ' (θ) were fitted in terms of an associated Legendre polynomial expansion. The measured energy dependence of P^γ' (E_d) has been analyzed in terms of barrier-penetration parameters.     

Total photonuclear absorption cross section for Pb and for heavy nuclei in the Δ-resonance region

The partial sum σ⁽²⁾(E_γ) = Σ_i?2 σ(γ, in…), representing the inclusive cross section for all reaction channels in which at least two neutrons are emitted, has been measured with a quasimonochromatic photon beam obtained by the in-flight annihilation of monoenergetic positrons, and neutron multiplicity counting. These experimental results, taken with photon energies E_γ from 145 up to 440 MeV for Pb and with photon energies E_γ= 235 MeV and 330 MeV for Al, Cu, Zr, Sn, Ho, Ta and U, are subsequently used to determine the total photonuclear absorption cross section σ(tot: E_γ) and to study the dependence upon the mass number A of the normalized cross section σ(tot: E_γ)/A. These results are then compared with other information on the total photonuclear absorption cross section.     

Energy gap modulation of graphene nanoribbons by F termination

Under the generalized gradient approximation (GGA), the electronic properties are studied for the F-terminated graphene nanoribbons (GNRs) with either zigzag edge (ZGNRs) or armchair edge (AGNRs) by using the first-principles projector augmented wave potential within the density function theory (DFT) framework. The results show that an edge state appears at the Fermi level E_F in the broader F-terminated ZGNRs, but does not appear in all the F-terminated AGNRs due to their dimerized C-C bonds at edge. The density of states (DOS) and projected DOS (PDOS) analyses show that the F-terminated ZGNRs are metallic and have a sharp peak at the Fermi level when the width is large enough. In contrast, the AGNRs are always semiconductors independent of their width. The charge density contours analyses shows that the C-F bond is an ionic bond due to a much stronger electronegativity of the F atom than that of the C atom. However, all kinds of the C-C bonds display a typical nonpolar covalent bonding feature.     

Theoretical study of p-d hybridization in Co perovskite

The real-space recursion method and unrestricted Hartree-Fock approximation have been applied to calculate the density of states of various Co perovskite, CeCoO₃, SrCoO₃ and Sr_1−xCe_xCoO₃. We have studied the magnetically ordered states of these Co perovskites in an enlarged double cell, and find its various magnetic structures due to the occupancy of 3d band and its interaction with neighboring Co ions. In this study, we have studied the p-d hybridization of the three Co perovskites, we find t_2g electrons are localized and the flat e_g band is responsible for the itinerant behavior, and although the rare earth elements itself contribute little to the DOS at the Fermi energy, the DOS at Fermi energy and the magnetic moment changed consequently because of different valence of Co ions in these compounds and p-d hybridization effect is very important.     

The energy dependence of fusion in the 9Be+28Si system

The angular distributions of the energy spectra of the light charged particles (p, d and α) from the ⁹Be + ²⁸Si reaction have been measured in the energy range 12 ≦ E_lab ≦ 30 MeV. The particle evaporation spectra and the angular distributions were analyzed with a spin dependent statistical model. Angular distributions of ⁹Be ions elastically scattered on ²⁸Si have been measured at the energies 12 MeV, 17 MeV, 23 MeV and 30 MeV and were analysed, together with previously measured cross sections, with the optical model. The fusion cut-off angular momentum l_fus(E), the fusion cross section σ_fus(E) and the ratio σ_fus/σ_R^OM(E) were deduced. The excitation function for fusion was analyzed with the Glas and Mosel model. The parameters obtained from the fusion excitation function were compared with the corresponding ones from the ⁹Be + ²⁸Si optical-model interaction potential.     

Electronic properties of TiC(100) and polar TiC(111) surfaces

The energy bands of films of TiC have been calculated using the linear-combination-of-atomic-orbitals method with parameters obtained by a fit to the bulk band structure. The Madelung potentials and charge redistribution have been determined self-consistently. For the neutral TiC(100) surface, the density of states (DOS) is similar to that of the bulk. For the non-neutral Ti-covered TiC(111) surface, Ti 3d-derived surface states appear around the Fermi energy E_F. The long-range electric field produced by the polar surfaces is screened by the charge redistribution, and the polar surfaces are stabilized. Characteristic features of TiC(111) compared to other surfaces of TiC are attributed to the high surface DOS at E_F.     

On the visibility of electron-electron interaction effects in field emission spectra

One of the most convenient methods to obtain information about the energy distribution function of electrons in conducting materials is the measurement of the energy resolved current j(ω) in field emission (FE) experiments. Its high energy tail j_>(ω) (above the Fermi edge) contains invaluable information about the nature of the electron-electron interactions inside the emitter. Thus far, j_>(ω) has been calculated to second order in the tunnelling probability, and it turns out to be divergent toward the Fermi edge for a wide variety of emitters. The extraction of the correlation properties from real experiments can potentially be obscured by the eventually more divergent contributions of higher orders as well as by thermal smearing around E_F. We present an analysis of both factors and make predictions for the energy window where only the second order tunnelling events dominate the behaviour of j_>(ω). We apply our results to the FE from Luttinger liquids and single-wall carbon nanotubes.     

Neutron induced reaction cross-section of115In around 14 MeV

A systematic investigation was carried out on¹¹⁵In to determine the contribution of different reactions to the total non-elastic cross-section in the 13.43 and 14.84 MeV incident neutron energy range. All the major components ofσ _NE were measured with exception of theσ ^g(n, n′) cross section. An analytical expression is recommended to estimate theσ _NE data as a function of mass number at E_n=14.1 MeV. By the knowledge ofσ _NE, the energy dependence ofσ ^g(n, n′) could be deduced. The isomeric cross section ratios both for (n, 2n) and (n, n′) processes were also determined in the given energy range. The present experiment proves the dependence ofσ ^m/(σ ^g+σ ^m) ratio on the spin value (I _m) of the isomeric state in the (n, 2n) reaction. Excitation functions of the (n, 2n), (n, n′) (n, p) and (n∶ p, α) reactions calculated by STAPRE code show good agreement with the experimental results.     

Absorption of Ni-centres in alkali chlorides

Absorption spectra of Ni²⁺ doped NaCl, KCl, and RbCl were measured in the spectral range from 55,000 to 5,000 cm^?1. The bands in the UV region are ascribed to the transition 3t _1u(σ, π)→3e _g(σ) of NiCl₆ complex ion. The connection of the intensity of charge transfer andd-d transitions has been discussed.