Study on Band Structure of YbB_6 and Analysis of Its Optical Conductivity Spectrum

The electronic structure of YbB6 crystal was studied by means of density functional （GGA ＋ U） method. The calculations were performed by FLAPW method. The high accurate band structure was achieved. The correlation between the feature of the band structure and the Yb-B6 bonding in YbB6 was analyzed. On this basis, some optical constants of YbB6 such as reflectivity, dielectric function, optical conductivity, and energy-loss function were calculated. The results are in good agreement with the experiments. The real part of the optical conductivity spectrum and the energy-loss function spectrum were analyzed in detail. The assignments of the spectra were carried out to correlate the spectral peaks with the interband electronic transitions, which justify the reasonable part of previous empirical assignments and renew the missed or incorrect ones.

Optical conductivity of twisted bilayer graphene near the magic angle

We theoretically study the band structure and optical conductivity of twisted bilayer graphene(TBG) near the magic angle considering the effects of lattice relaxation. We show that the optical conductivity spectrum is characterized by a series of peaks associated with the van Hove singularities in the band structure, and the peak energies evolve systematically with the twist angle. Lattice relaxation effects in TBG modify its band structure, especially the flat bands, which leads to significant shifts of the peaks in the optical conductivity. These results demonstrate that spectroscopic features in the optical conductivity can serve as fingerprints for exploring the band structure, band gap, and lattice relaxation in magic-angle TBG as well as identifying its rotation angle.

Drude Weight, Optical Conductivity of Two-Dimensional Hubbard Model at Half Filling

We study the Drude weight D and optical conductivity of the two-dimensional （2D） Hubbard model at half filling with staggered magnetic flux （SMF）. When SMF being introduced, the hopping integrals are modulated by the magnetic flux. The optical sum rule, which is related to the mean kinetic energy of band electrons, is evaluated for this 2D Hubbard Hamiltonian. Our present result gives the dependence of the kinetic energy, D and the optical conductivity on SMF and U. At half filling D vanishes exponentially with system size. We also find in the frequency dependence of the optical conductivity, there is δ-function peak at ω ≈ 2｜m｜U and the incoherent excitations begin to present themselves extended to a higher energy region.

Optical conductivity of ABA-stacked trilayer graphene

The optical conductivity of a trilayer graphene is studied using the Kubo-Greenwood formula. We calculate the real part of the diagonal optical conductivity of an ABA-stacked trilayer graphene with different Fermi energies. The optical conductivity arises from interband matrix elements of the electric current operator involving the transitions from the occupied states to the unoccupied ones. We study the dependence of the real part of the diagonal optical conductivity on the photon energy, and the role of the transitions.

Optical conductivity as a probe of a hidden Fermi-liquid behavior in BaFe_(1.904) Ni_(0.096) As_2

We measured the infrared reflectivity of BaFe1.904Ni0.096As2 single crystal from room temperature down to 20 K. Two Drude terms and a Lorentz term well describe the real part of the optical conductivity σ1 （ω）. We fit the reciprocal of static optical conductivity 1/σ1（0） by the power law ρ （T）=ρo＋ATn with n= 1.6. The ＂broad＂ Drude component exhibits an incoherent background with a T-independent scattering rate 1/τb, while the other ＂narrow＂ one reveals a T-quadratic scattering rate 1/τn, indicating a hidden Fermi-liquid behavior in BaFe1.904Nio.096As2 compound.

Optical Conductivity of Impurity-Doped Parabolic Quantum Wells in an Applied Electric Field

The optical conductivity of impurity-doped parabolic quantum wells in anapplied electric field is investigated with the memory-function approach, and the analyticexpression for the optical conductivity is derived. With characteristic parameters pertaining toGaAs/Ga_(1-x)Al_xAs parabolic quantum wells, the numerical results are presented. It is shown that,the smaller the well width, the larger the peak intensity of the optical conductivity, and the moreasymmetric the shape of the optical conductivity; the optical conductivity is more sensitive to theelectric field, the electric Geld enhances the optical conductivity; when the dimension of thequantum well increases, the optical conductivity increases until it reaches a maximum value, andthen decreases.

Gap States of ZnO Thin Films by New Methods:Optical Spectroscopy,Optical Conductivity and Optical Dispersion Energy

The optical reflectance and transmittance spectra in the wavelength range of 300-2500 nm are used to compute the absorption coefficient of zinc oxide films annealed at different post-annealing temperatures 400, 500 and 600°C.The values of the cross point between the curves of the real and imaginary parts of the optical conductivity ɑ_1 and ɑ_1 with energy axis of films exhibit values that correspond to optical gaps and are about 3.25-3.3 eV. The maxima of peaks in plots dR/dλ and dT/dλ versus wavelength of films exhibit optical gaps at about 3.12-3.25 eV.The values of the fundamental indirect band gap obtained from the Tauc model are at about 3.14-3.2 eV. It can be seen that films annealed at 600°C have the minimum indirect optical band gap at about 3.15 eV. The films annealed at 600°C have Urbach＇s energy minimum of 1.38 eV and hence have minimum disorder. The dispersion energy d of films annealed at 500°C has the minimum value of 43 eV.

Optical Conductivity of Graphene Sheet Including Electron-Phonon Interaction

Using an expression of optical conductivity,based on the linear response theory,the Green's function technique and within the Holstein Hamiltonian model,the effect of electron-phonon interaction on the optical conductivity of graphene plane is studied.It is found that the electron-phonon coupling increases the optical conductivity of graphene sheet in the low frequency region due to decreasing quasiparticle weight of electron excitation while the optical conductivity reduces in the high frequency region.The latter is due to role of electrical field's frequency.

Nonlocal optical conductivity of Fermi surface nesting materials

We investigate the nonlocal optical conductivity of Fermi surface nesting materials that support charge density waves(CDWs)or spin density waves(SDWs).The nonlocal optical conductivity contains information on correlations in electron fluids,which could not be accessed by standard optical probes.A half-metal emerges from doping a CDW,and similarly,a spin-valley halfmetal emerges from doping an SDW.Based on the parabolic band approximation,we find that the Drude peak is shifted to a higher frequency and splits into two peaks in the nonlocal optical conductivity.We attribute this to the two Fermi velocities in the half-metal or spin-valley half-metal states.

Optical study on intermediate-valence compounds Yb_(1-x)Lu_xAl_3

We report an optical spectroscopy study on intermediate valence system Ybl-xLuxA13 with x = 0, 0.25, 0.5, 0.75, and 1. The Kondo temperature in the system is known to increase with increasing Lu concentration. Therefore, it is expected that the energy scale of the hybridization gap should increase with increasing Lu concentration based on the periodic Anderson model. On the contrary, we find that the spectral structure associated with the hybridization effect shifts monotonically to lower energy. Furthermore, the Lu substitution results in a substantial increase of the free carrier spectral weight and less pronounced plasma frequency reduction upon lowering temperature. We attribute the effect to the disruption of the Kondo lattice periodicity by the random substitution of Yb by Lu. The work highlights the importance of the lattice periodicity of the rare earth element for understanding the Kondo lattice phenomena.

Distinct edge states and optical conductivities in the zigzag and armchair silicene nanoribbons under exchange and electric fields

Based on the tight binding model, we investigate the low energy bandstructures, edge states, and optical absorptions for the silicene nanoribbons （SiNRs） with different terminations under an in-plane exchange field and/or a perpendicular electric field. We find that the zigzag SiNRs are gapped by the exchange field, but they could reenter the metallic state after the application of the electric field. Contrarily, a certain kind of armchair SiNRs remain gapless even if a weak exchange field is present. Furthermore, the combination of the exchange and electric fields could effectively modulate the penetration length and the components of the edge states in the SiNRs. The corresponding optical conductivities for the SiNRs are also calculated, which show remarkable dependence on the edge types of the SiNRs and the two external fields.

Optical study of charge dynamics in CaCo_2As_2

We present an infrared spectroscopy study of charge dynamics in CaCo_2As_2 single crystal. In this material, the optical conductivity can be described by two Drude components with different scattering rates（1/τ）： a broad incoherent background and a narrow Drude component. By monitoring the temperature dependence, we find that only the narrow Drude component is temperature-dependent and determines the transport properties. Especially a Fermi liquid behavior of carriers is revealed by the T^2 behavior in the dc resistivity ρ_n and scattering rate 1/τ_n, indicating a coherent nature of quasiparticles in the narrow Drude subsystem.

Effect of Oxygen Concentration and Annealing Theatment on the Optical Properties of the Transparent Conductive CdIn_2O_4 Thin Films

Transparent conductive cadmium indium oxide films (CdIn2O4) were prepared by r.f. reactive sputtering from Cd-In alloy targets under an Ar-O2 atmosphere. Electrical conductivity of the order of 105Ω-1.m-1 and the optical transmission as high as 94% are easily attained by postdeposition annealing treatment. The effects of oxygen concentration in the reactive gas mixture and post-deposition annealing treatment on the optical transmittance as well as optical parameters, such as refractive index (n), extinction coefficient (k), real part (ε') and imaginary part (ε') of the dielectric constant, were studied in the visible and near-infrared region. The highfrequency dielectric constant ε∞ the plasma frequency ωP, and the conduction band effective mass mc of different samples were also investigated

Generalization of iterative perturbation theory and coherent potential approximation (ITP+CPA) to double exchange model with orbital degeneracy

A combination of the iterative perturbation theory （ITP） of the dynamical mean field theory （DMFT） and coherentpotential approximation （CPA） is generalized to the double exchange model with orbital degeneracy. The Hubbard interaction and the off-diagonal components for the hopping matrix tij^mn（m ≠ n） are considered in our calculation of spectrum and optical conductivity. The numerical results show that the effects of the non-diagonal hopping matrix elements are important.

Asymmetrical plasmon reflections in tapered graphene ribbons with wrinkle edges

Asymmetrical graphene plasmon reflection patterns are found in infrared near-field images of tapered graphene ribbons epitaxially grown on silicon carbon substrates. Comparing experimental data with numerical simulations, the asymmetry of these patterns is attributed to reflection of plasmons by wrinkled edges naturally grown in the graphene. These graphene wrinkles are additional plasmon reflectors with varying optical conductivity, which act as nanometer scale plasmonic modulators and thus have potential applications in photoelectric information detectors, transmitters, and modulators.

Temperature dependent phonon modes and ionicity of LiGaO_2 single crystal

This paper reports that polarized far-infrared reflectivity measurements have been done on LiGaO2 single crystal along two crystalline taxes at different temperatures. The temperature dependent frequencies of the longitudinal and transverse optical phonon have been obtained from the real part of optical conductivity and the loss function respectively. A small Drude component is observed at frequency below 300 cm^-1 which could arise from Li ions or oxygen deficiencies. The ionicity of LiGaO2 has been studied from the analysis of the Born effective charge of different ions.

Dynamic conductivity modified by impurity resonant states in doping three-dimensional Dirac semimetals

The impurity effect is studied in three-dimensional Dirac semimetals in the framework of a T-matrix method to consider the multiple scattering events of Dirac electrons off impurities. It has been found that a strong impurity potential can significantly restructure the energy dispersion and the density of states of Dirac electrons. An impurity-induced resonant state emerges and significantly modifies the pristine optical response. It is shown that the impurity state disturbs the common longitudinal optical conductivity by creating either an optical conductivity peak or double absorption jumps, depending on the relative position of the impurity band and the Fermi level. More importantly, these conductivity features appear in the forbidden region between the Drude and interband transition, completely or partially filling the Pauli block region of optical response. The underlying physics is that the appearance of resonance states as well as the broadening of the bands leads to a more complicated selection rule for the optical transitions, making it possible to excite new electron-hole pairs in the forbidden region. These features in optical conductivity provide valuable information to understand the impurity behaviors in 3D Dirac materials.

Theoretical study of broadband near-field optical spectrum of twisted bilayer graphene

We theoretically study the broadband near-field optical spectrum of twisted bilayer graphene(TBG)at various twist angles near the magic angle using two different models.The spectrum at low Fermi energy is characterized by a series of peaks that are almost at the same energies as the peaks in the far-field optical conductivity of TBG.When the Fermi energy is near a van Hove singularity,an additional strong peak appears at finite energy in the near-field spectrum,which has no counterpart in the optical conductivity.Based on a detailed calculation of the plasmon dispersion,we show that these spectroscopic features are associated with interband and intraband plasmons,which can provide critical information about the local band structure and plasmonic excitations in TBG.The near-field peaks evolve systematically with the twist angle,so they can serve as fingerprints for identifying the spatial dependent twist angle in TBG samples.Our findings pave the way for future experimental studies of the novel optical properties of TBG in the nanoscale.