Band structure and optical properties of the TlInTe2 chain compound

This paper reports on the results of calculations of the band structure, the effective masses of holes and electrons, the spectral and polarization dependences of the imaginary and real parts of the complex permittivity, the effective number of valence electrons, and the characteristic loss function for the TlInTe₂semiconductor compound over a wide spectral range. The calculations are performed using the pseudopotential method. The refractive indices of TlInTe₂ crystals are determined, and the available theoretical and experimental data are analyzed and compared.     

Anisotropic properties and conduction mechanism of TlInSe2 chain semiconductor

TlInSe₂ chain crystals were prepared using the modification of the Bridgman technique. The grown crystals were identified by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDX), and X-ray diffraction (XRD). We investigate the anisotropy of transport properties for the first time for TlInSe₂ crystals. Temperature dependence of the dc electrical conductivity, Hall coefficient, Hall mobility, and charge carrier concentration were investigated in the temperature range 184–455 K. The conduction mechanism of TlInSe₂ crystals was studied, and measurements revealed that the dc behavior of the grown crystals can be described by Mott’s variable range hopping (VRH) model in the low temperature range, while it is due to thermoionic emission of charge carriers over the chain boundaries above 369 K. The Mott temperature, the density of states at the Fermi level, and the average hopping distance are estimated in the two crystallographic directions. The temperature dependence of the ac conductivity and the frequency exponent, s, is reasonably well interpreted in terms of the correlated barrier-hopping CBH model.     

Band structure and optical properties of MnO

Both the valence and lowest conduction bands of MnO have been performed by the LCAO method thanks to the use of localized orbitals obtained in a self-consistent manner. The imaginary part of the relative dielectric function has been computed by evaluating separately the various band-to-band transitions. The absorption edge has been attributed to 2p → 3d transitions.     

Band structure of the superconducting MgB2 compound and modeling of related ternary systems

Band structure of a novel superconductor—magnesium diboride—is studied by the self-consistent FP-LMTO method. Density of states near the Fermi level of MgB₂ and its electronic properties are governed by the metal-like boron 2p orbitals in the planar network of boron atoms. The modification of the band structure of MgB₂ upon doping the boron (with Be, C, N, and O substitutional impurities) and the magnesium (with Be, Ca, Li, and Na substitutional impurities) sublattices or upon the introduction of structural vacancies (boron nonstoichiomety) is analyzed. The electronic structures of MgB₂ and hypothetical CaB₂ are also studied as functions of pressure.     

Band structure and lattice vibration properties of III-P ternary alloys

Electronic band structure, optical and vibrational properties of zinc-blende GaP_xSb_1−x and GaAs_1−xP_x ternary alloys are obtained from pseudo-potential calculations. Comparisons are made with the available experimental values and with data obtained in previous theoretical studies. These comparisons show generally good agreement between the present results and experiment. The direct and indirect band-gap energies, the transverse effective charge, and the longitudinal and transversal optical phonon energies show a non-linear behavior with varying the composition x. The ionicity of the materials of interest is discussed in terms of the antisymmetric gap.     

Band structure and optical transitions in the Hg3Se2Cl2 crystals

First-principles calculations of the band structure for Hg₃Se₂Cl₂ crystal were performed by means of density functional theory (DFT). The exchange and correlation potential was described within a framework of the local density approximation based on exchange-correlation energy optimization to calculate the total energy. DOS/PDOS and valence charge distribution were studied in details. Absorption near the fundamental edge was found to be due to indirect and direct allowed transitions. For device applications based on Hg₃Se₂Cl₂ crystal, understanding these fundamental issues is highly important and essential. It should be noted that optoelectronic applications of Hg₃Se₂Cl₂ are caused by coexistence of the large polarized Hg cations and a huge contribution of an harmonic phonon subsystem caused by anions.     

Band structure and optical properties of alkali metal halides

Optical spectra, photoemission spectra, and photoconductivity spectra of alkali metal halides are analyzed using energy band structure calculations and selection rules for direct and indirect transitions. The main feature of the band structure of MeN₃ (Me: Na, K, Rb, Cs) as proposed by the authors is the presence of two conductivity bands, one anionic and the other cationic in nature. A weak dispersion of the valence subband indicates that phonons may yield a significant contribution to the observed spectra. All of the optical and photoemission spectra so far reported for the metal azides may be explained on the basis of the proposed band model.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 1, pp. 3–7, January, 1995.     

Band structure and optical transitions in semiconducting double-wall carbon nanotubes

The electronic structure of semiconducting double-wall carbon nanotubes (CNTs) is calculated using the linearized augmented cylindrical wave method. The consideration is performed in the framework of the local density functional theory and the muffin-tin (MT) approximation for the one-electron Hamiltonian. The electronic spectrum of a double-wall CNT is determined by the free motion of electrons in the interatomic space of the two cylindrical layers, scattering by the MT spheres, and tunneling through the classically impenetrable region. Calculated results for double-wall CNTs of the (n, 0)@(n′, 0) zigzag type indicate that the shift of the band-gap width depends on whether n and n′ are divided by 3 with a remainder of 1 or 2. It is found that, regardless of the type of the inner tube, the energy gap E _g of the outer tube decreases by 0.15–0.22 eV if the tube belongs to the sequence n = 2 (mod 3). For the outer tubes of the sequence n = 1 (mod 3), the shifts of the band gap ΔE _g are always negative ?0.15 ≤ ΔE _g ≤ ?0.05 eV. In both cases, the shifts ΔE _g weakly oscillate rather than decrease in going to tubes of a larger diameter d. For the inner tubes, the changes in the band gap ΔE _g are more sensitive to the diameter. At 10 ≤ n ≤ 16, the shifts ΔE _g are positive and the maximum value of ΔE _g equals 0.39 and 0.32 for the sequences n = 2 (mod 3) and n = 1 (mod 3), respectively. In going to the inner tubes of a larger diameter, ΔE _g rapidly drops and then oscillates in the range from ?0.05 to 0.06 eV. The calculated results indicate that the shifts of the optical band gaps in core and shell tubes upon the formation of double-wall CNTs are significant, which must hinder the identification of double-wall CNTs by optical methods. On the other hand, the obtained results open up possibilities for a more detailed classification of double-wall nanotubes.     

Band spectrum and optical absorption in n-CdSnAs2

Using the crystal splitting (CS) method in perturbation theory, the energy spectrum of the valence zone and the conductivity zone is calculated in the neighborhood of the point of the Brillouin zone for the semiconductor CdSnAs₂. The parameters necessary for calculating the spectrum were found from analysis of the edge of optical absorption in n-CdSnAs₂. Using the found values of the parameters, the effective masses at the point and the density of states are calculated.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 6, pp. 28–34, June, 1978.In conclusion, we thank N. A. Zakharov for the calculation of the energy spectrum and wave functions of InAs.     

Band structure and optical properties of antimony-sulfobromide: density functional calculation

The electronic structure, linear, and non-linear optical properties of ferroelectric-semiconductor SbSBr are investigated in the non-polar (paraelectric) and polar (ferroelectric) phase, using the density functional methods in the generalized gradient approximation. The electronic band structure obtained shows that SbSBr has an indirect forbidden gap of 2.16 and 2.21 eV in the paraelectric and ferroelectric phase, respectively. The linear photon-energy dependent dielectric functions and some optical functions, such as absorption and extinction coefficients, refractive index, energy-loss function, reflectivity, and optical conductivity in both phases and photon-energy dependent second-order susceptibilities in the ferroelectric phase are calculated. Moreover, some important optical parameters, such as the effective number of valence electrons and the effective optical dielectric constant, are calculated in both phases.      

Band structure,optical properties,and compton profile of copper

A self-consistent calculation of the band structure of copper has been performed using a basis of gaussian orbitals and a local exchange-correlation potential. Results are also presented for the optical conductivity and the Compton profile.     

光器件应用改性Ge的能带结构模型

Ge为间接带隙半导体,通过改性技术可以转换为准直接或者直接带隙半导体.准/直接带隙改性Ge半导体载流子辐射复合效率高,应用于光器件发光效率高;同时,准/直接带隙改性Ge半导体载流子迁移率显著高于Si半导体载流子迁移率,应用于电子器件工作速度快、频率特性好.综合以上原因,准/直接带隙改性Ge具备了单片同层光电集成的应用潜力.能带结构是准/直接带隙改性Ge材料实现单片同层光电集成的理论基础之一,目前该方面的工作仍存在不足.针对该问题,本文主要开展了以下三方面工作:1)揭示了不同改性条件下Ge材料带隙类型转化规律,完善了间接转直接带隙Ge实现方法的相关理论; 2)研究建立了准/直接带隙改性Ge的能带E-k模型,据此所获相关结论可为发光二极管、激光器件仿真模型提供关键参数; 3)提出了准/直接带隙改性Ge的带隙调制方案,为准/直接带隙改性Ge单片同层光电集成的实现提供了理论参考.本文的研究结果量化,可为准/直接带隙改性Ge材料物理的理解,以及Ge基光互连中发光器件有源层研究设计提供重要理论依据.     

InAs/GaSb量子阱的能带结构及光吸收

采用八能带K-P理论以及有限差分方法,研究了沿[001]方向生长的InAs/GaSb二类断带量子阱体系的能带结构、波函数分布和对[110]方向线性偏振光的吸收特性.研究发现,通过改变InAs或GaSb层的厚度,可有效调节该量子阱体系的能带结构及波函数分布.计算结果表明,当InAs/GaSb量子阱的导带底与价带顶处于共振状态时,导带基态与轻空穴基态杂化效应很小,且导带基态与第一激发态的波函数存在较大的重叠,导带基态与第一激发态之间在布里渊区中心处的跃迁概率明显大于导带底与价带顶处于非共振状态时的跃迁概率.研究结果对基于InAs/GaSb二类断带量子阱体系的中远红外波段的新型级联激光器、探测器等光电器件的设计具有重要意义.     

Band structure and optical properties of intrinsic tetragonal dioxides of groups-IV elements

Energy band structure calculations are carried out on three tetragonal dioxides belonging to the IV group (SnO₂, GeO₂, β-PbO₂) with the help of the KKR method. The results obtained are compared with many experimental results in order to try to make a synthesis of the optical properties of these compounds and to relate them to our theoretical results; In fact, the complexity of their unit cell and the difficulty to obtain the energy levels near the energy band gap by optical measurements seem to be responsible for the large discrepancies of results in literature concerning these compounds.Our results have confirmed the presence of the expected Γ⁺₁ and Γ⁺₄ states at the bottom of the conduction band; they have also given a valence band model that does not completely agree with any of the various models of energy levels lying in the vicinity of the energy gap.Moreover, our results seem to prove that no indirect transition exists neither in SnO₂, nor in β-PbO₂. At last, among several results that can be drawn from our calculations, one of the most significant is that the three compounds exhibit a strong dischroism, since the E⊥C transition has a weaker energy than the E∥C transition in SnO₂ and GeO₂, and conversely in β-PbO₂.     

Band structure and dynamic behaviors of Bose-Einstein condensates in Fourier-Synthesized optical lattices

By employing a nonlinear three-mode model, we study the band structure of Bose-Einstein condensates in Fourier-Synthesized optical lattices, where the nonlinearity comes from the mean field treatment of interaction between atoms. In linear case, we present the band structure of the system. It is demonstrated that the energy band structure is strongly dependent on the value of relative phase of the two lattice harmonics. In the nonlinear case, we show that the eigenenergies as the functions of the quasi-momentum have a novel bowl structure in the middle energy level. It is found that there exist four critical values of interaction strength at which the band structure will undergo interesting changes. Furthermore, the stability of the eigenstate is also investigated.     

Band structure effects on the nonlinear optical response of bilayer graphene

Recently, Rabi-like oscillations that occur far from resonance were predicted in monolayer graphene. In bilayer graphene, when the trigonal warping effect is taken into account, this new Rabi frequency shows a zero non-trivial minimum as a function of the strength of the applied electric field in addition to the trivial minimum at zero field. The zero non-trivial minimum occurs where the ‘leg pocket’ of the Fermi surface develops, described in the pioneering work of McCann et al. [Eur. Phys. J. Special Topics 148, 91 (2007)]. Thereafter, the anomalous Rabi frequency varies linearly with the square of the intensity of the applied field consistent with a bilayer system without trigonal warping. It is seen that this anomalous Rabi frequency is affected much more by trigonal warping than the conventional Rabi frequency. The induced current is also significantly affected by the trigonal warping. A fully numerical solution of the optical Bloch equations completely corroborates the analytical findings and provides a basis for the approximation schemes employed.