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.     

β型锑烯热电输运性质的第一性原理研究

利用第一性原理与半经典玻尔兹曼方程，计算并分析β型锑烯的声子色散、声子群速度、声子弛豫时间、晶格热导率及不同温度下的塞贝克系数、电导率和电子热导率随化学势的变化；结果表明：β型锑烯由于非平面六角结构，三支声学声子在Γ点附近均呈线性变化；声学声子对整个晶格热导率的贡献高达96.68%，而光学声子仅仅占到3.32%；由于较大的声光带隙（a-o gap）导致LA支在声子群速度和弛豫时间中占据主导地位，从而增大了LA支声子对整个热导的贡献；热电优值随温度的升高而增大，在费米面附近其绝对值最大可达0.275.     

Electronic Correlation Effects on the Optical Properties of SiC,BeO and Boron Nitride Nanotubes and Monolayers

We present numerical calculation of the impact of electron-electron interaction on the behavior of density of states and optical properties of BeO,SiC and Boron-Nitride nanotubes and sheets.Hubbard model hamiltonian is applied to describe the dynamics of electrons on the lattice structure of theses compounds.The excitation spectrum of the system in the presence of local electronic interactions has been found using mean Seld approach.We find the band gap width in both optical absorption and density of states reduces with local Hubbard electronic interaction parameter.The absorption spectra exhibits the remarkable peaks,mainly owing to the divergence behavior of density of states and excitonic effects.Also we compare optical absorption frequency behavior of BeO,SiC and Boron-Nitride nanotubes with each other.Furthermore we investigate the optical properties of BeO and SiC sheets.A novel feature of optical conductivity of these structures is the decrease of frequency gap in the optical spectrum due to electronic interaction.     

Electronic Correlation Effects on the Optical Properties of SiC,BeO and Boron Nitride Nanotubes and Monolayers

We present numerical calculation of the impact of electron-electron interaction on the behavior of density of states and optical properties of BeO, SiC and Boron-Nitride nanotubes and sheets. Hubbard model hamiltonian is applied to describe the dynamics of electrons on the lattice structure of theses compounds. The excitation spectrum of the system in the presence of local electronic interactions has been found using mean field approach. We find the band gap width in both optical absorption and density of states reduces with local Hubbard electronic interaction parameter. The absorption spectra exhibits the remarkable peaks, mainly owing to the divergence behavior of density of states and excitonic effects. Also we compare optical absorption frequency behavior of BeO, SiC and Boron-Nitride nanotubes with each other. Furthermore we investigate the optical properties of BeO and SiC sheets. A novel feature of optical conductivity of these structures is the decrease of frequency gap in the optical spectrum due to electronic interaction.     

Validity of the Franck-Condon principle in the optical spectroscopy: optical conductivity of the Fröhlich polaron

The optical absorption of the Fr?hlich polaron model is obtained by an approximation-free diagrammatic Monte Carlo method and compared with two new approximate approaches that treat lattice relaxation effects in different ways. We show that: (i) a strong coupling expansion, based on the Franck-Condon principle, well describes the optical conductivity for large coupling strengths (alpha > 10); (ii) a memory function formalism with phonon broadened levels reproduces the optical response for weak coupling strengths (alpha < 6) taking the dynamic lattice relaxation into account. In the coupling regime 6 < alpha < 10, the optical conductivity is a rapidly changing superposition of both Franck-Condon and dynamic contributions.     

Optical properties of gapped graphene structure driven by electron electron interaction

We analyze the effects of on-site electronic coulomb repulsion U on the optical absorption and density of states of a graphene like structure with two different sublattice on-site energies in the context of Hubbard model. Mean field approximation has been implemented in order to find excitation spectrum of electronic system. Antiferromagnetic long range ordering has been considered as the ground state of model Hamiltonian. We find that the band gap in both optical conductivity and density of states decreases with strength of coulombic interaction. The absorption spectra of the graphene like structure as a nanoscale system exhibit the prominent peaks, mainly owing to the divergent density of states and excitonic effects.     

From mixed valence to the Kondo lattice regime

Many heavy fermion materials are known to cross over from the Kondo lattice regime to the mixed valence regime or vice versa as a function of pressure or doping. We study this crossover theoretically by employing the periodic Anderson model within the framework of the dynamical mean field theory. Changes occurring in the dynamics and transport across this crossover are highlighted. As the valence is decreased (increased) relative to the Kondo lattice regime, the Kondo resonance broadens significantly, while the lower (upper) Hubbard band moves closer to the Fermi level. The resistivity develops a two peak structure in the mixed valence regime: a low temperature coherence peak and a high temperature 'Hubbard band' peak. These two peaks merge, yielding a broad shallow maximum upon decreasing the valence further. The optical conductivity likewise exhibits an unusual absorption feature (shoulder) in the deep mid-infrared region, which grows in intensity with decreasing valence. The involvement of the Hubbard bands in dc transport and of the effective f-level in the optical conductivity are shown to be responsible for the anomalous transport properties. A two-band hybridization-gap model, which neglects incoherent effects due to many-body scattering, commonly employed to understand the optical response in these materials is shown to be inadequate, especially in the mixed valence regime. Comparison of theory with experiment carried out for (a) dc resistivities of CeRhIn(5), Ce(2)Ni(3)Si(5), CeFeGe(3) and YbIr(2)Si(2), (b) pressure dependent resistivity of YbInAu(2) and CeCu(6), and (c) optical conductivity measurements in YbIr(2)Si(2) yields excellent agreement.     

Scattering mechanisms and anomalous conductivity of heavily N-doped 3C-SiC in ultraviolet region

Using the first-principles density functional method, we investigate the band structures and conductivity spectra for N-doped 3C-SiC. It is found that conductivity peaks of heavily N-doped 3C-SiC are observed in the ultraviolet (UV), visible and infrared (IR) regions while the peaks can be only seen in the UV region for 3C-SiC. In the UV region, the conductivity peaks of 3C-SiC are obviously higher than those of N-doped 3C-SiC. According to the data of band structures, we calculate the ionized impurity scattering, inter-carrier scattering and neutral impurity scattering. The calculation results show that the scattering by incomplete ionization N to electrons and inter-carrier scattering have large effect on the conductive behavior of heavily N-doped 3C-SiC at room temperature. In the UV region, the conductivity of 3C-SiC depends on long-wavelength optical wave scattering, which has a longer relaxation time than that inter-carrier scattering and neutral scattering. This is the reason of anomalous conductivity of N-doped 3C-SiC in the UV region.     

闪锌矿结构AlSb空位缺陷性质的第一性原理研究

采用基于密度泛函理论(DFT)第一性原理的平面波超软赝势方法，计算含有锑空位和铝空位的电子结构，发现空位引起周围原子弛豫，晶体结构发生畸变。在此基础上研究了空位缺陷对闪锌矿型AlSb体系电子结构的影响，结果表明，铝空位缺陷使锑化铝费米能级降低，带隙变窄；而锑空位缺陷则使锑化铝费米能级升高，带隙变窄，同时，锑化铝的半导体类型由p型变为n型。对光学性质的研究发现，由于空位的引入使其邻近原子电子结构发生变化，使得空位缺陷系统光学性质的变化主要集中在低能量区域。     

闪锌矿结构AlSb空位缺陷性质的第一性原理研究

采用基于密度泛函理论(DFT)第一性原理的平面波超软赝势方法,计算含有锑空位和铝空位的AlSb电子结构,发现空位引起周围原子弛豫,晶体结构发生畸变.在此基础上研究了空位缺陷对闪锌矿型AlSb体系电子结构的影响,结果表明,铝空位缺陷使锑化铝费米能级降低,带隙变窄;而锑空位缺陷则使锑化铝费米能级升高,带隙变窄,同时,锑化铝的半导体类型由p型变为n型.对光学性质的研究发现,由于空位的引入使其邻近原子电子结构发生变化,使得空位缺陷系统光学性质的变化主要集中在低能量区域.     

Spin relaxation and band excitation of a dipolar Bose-Einstein condensate in 2D optical lattices

We observe interband transitions mediated by dipole-dipole interactions for an array of 1D quantum gases of chromium atoms, trapped in a 2D optical lattice. Interband transitions occur when dipolar relaxation releases an energy larger than the lattice band gap. For symmetric lattice sites, and a magnetic field parallel to the lattice axis, we compare the measured dipolar relaxation rate with a Fermi golden rule calculation. Below a magnetic field threshold, we obtain an almost complete suppression of dipolar relaxation, leading to metastable 1D gases in the highest Zeeman state.     

Band structures of a slowly rotating dipolar Bose-Einstein condensate with a quantized vortex along a one-dimensional optical lattice

We derive the effective Gross-Pitaevskii equation for a slowly rotating dipolar Bose-Einstein condensate (BEC) with a quantized vortex along a one-dimensional optical lattice and calculate its band structures. The band structure of a slowly rotating BEC in a lattice becomes interesting when dipole-dipole interaction (DDI) is involved. Under rotation, a dipolar rotating term emerges from the DDI potential. The dipolar rotating term makes a BEC with an attractive DDI more stable than one with a repulsive DDI. The dipolar rotating term changes and generalizes the definition for the type of BEC, which cannot be simply determined by an s-wave scattering length or an effective contact interaction term. The dipolar rotating term also makes the band structure fascinating and tunable. A so-called swallowtail band structure, i.e., a multi-valued solution due to nonlinear interaction, can either elongate or shrink as the band index increases, in contrast to a non-rotating dipolar BEC system with a monotonic dependence. With the dipolar rotating term, various band structures as well as an attractive BEC without collapse can be easily achieved. We demonstrate that a rotating dipolar BEC system subject to an optical lattice combines features of a crystal and a superfluid and promises wide applications.     

Anisotropy of light propagation in thin opal films

Thin opal films are prepared by crystallization in a moving meniscus, and their optical transmission spectra are recorded in polarized light and studied. It is shown that the anisotropy of light propagation in the films is unambiguously related to the photonic band structure of opal and depends on the angle of incidence, the orientation of the incidence plane with respect to the opal lattice, and the wavelength and polarization of the incident light. Azimuthal diagrams of transmitted polarized light are constructed in the range of photonic band gaps of three orders for oblique incidence of a light beam. The anisotropy is found to vary with the light wave-length independently in perpendicular polarizations. A model of the band structure of opal wherein opal is represented as an fcc lattice of close-packed spheres adequately describes the optical transmission of opal films only in the range of the first-order photonic band gap.     

纤锌矿结构AlInN电子及光学性质的第一性原理研究

本文通过基于密度泛函理论的第一性原理,研究了纤锌矿结构Al1-xInxN在不同In浓度下的稳固结构,以及电子和光学性质的变化规律。研究表明,AlInN不同In浓度的晶格结构都很稳定,说明AlInN的兼容性很好。晶格常数随In浓度的增大不断增大,而混晶的带隙则不断减小。并且随In浓度的增大,混晶在紫外光区的吸收系数、反射系数及折射率增大,吸收边、吸收峰和反射峰蓝移,且这两个峰的峰值减小。AlInN的吸收、反射和折射率曲线在Eg处出现峰值行为,此Eg处的峰值大小随In浓度的增加而增大。当In浓度达到87.5%时,混晶AlInN在紫外光区的吸收、反射和折射能力均达到最强,表明此时的掺杂效果最好。     

纤锌矿结构AlInN电子及光学性质的第一性原理研究

本文通过基于密度泛函理论的第一性原理,研究了纤锌矿结构Al1-xInxN在不同In浓度下的稳固结构,以及电子和光学性质的变化规律。研究表明,AlInN不同In浓度的晶格结构都很稳定,说明AlInN的兼容性很好。晶格常数随In浓度的增大不断增大,而混晶的带隙则不断减小。并且随In浓度的增大,混晶在紫外光区的吸收系数、反射系数及折射率增大,吸收边、吸收峰和反射峰蓝移,且这两个峰的峰值减小。AlInN的吸收、反射和折射率曲线在Eg处出现峰值行为,此Eg处的峰值大小随In浓度的增加而增大。当In浓度达到87.5%时,混晶AlInN在紫外光区的吸收、反射和折射能力均达到最强,表明此时的掺杂效果最好。     

Pressure effect on electronic,elastic and optical properties of Eu:CaF2 crystal: a first-principles study

The structure, electronic structure, elastic and optical properties of pure CaF₂ and Eu:CaF₂ are investigated by performing the generalized gradient approximation in the frame of density functional theory. The obtained lattice constant, band structure, elastic constants and imaginary part of dielectric function of pure CaF₂ agree well with the experimental and other theoretical results. These properties of Eu:CaF₂ are also calculated, and it follows that Eu doping leads to the crystal structure distorted, as well as the spin polarization of the system. Especially an impurity band is introduced in the bandgap, above the Fermi level, in the band structure of the Eu:CaF₂, narrowing the bandgap. In addition, it is observed that the optical peaks of Eu:CaF₂ shift towards the longer wavelength side, and the peak intensities drop compared with pure CaF₂. Furthermore, Eu doping induces a series of new peaks in the optical properties, which is probably caused by the transitions of the 4f electrons of the Eu atoms. It is worth mentioning that the effect of pressure on the optical properties of Eu:CaF₂ is also studied, we can hold that the optical peaks will have a blueshift.     

Magnetic interactions and electronic structure of $$\hbox {Pt}_{2}\hbox {Mn}_{1-x}\hbox {Y}_{x} \hbox {Ga (Y = Cr and Fe)}$$ system: An ab-initio calculation

Structural, optical, electrical conductivity and dielectric relaxation properties of bulk 4-amino-3-mercapto-6-(2-(2-thienyl)vinyl)-1,2,4-triazin-5(4H)-one donor (AMT) are studied. The structure of AMT in its powder form was analysed by X-ray diffraction (XRD), infrared spectroscopy (FT-IR) and atomic force microscopy (AFM). AC measurements (impedance, capacitance and phase angle) are done over the temperature range 303–373 K and in the frequency range from 42 Hz to 5 MHz. Analytical approaches for the experimental results of the σ _AC(ω, T) and the temperature behaviour of the frequency exponent show that the correlated barrier hopping (CBH) model is a good model to explain the AC electrical conductivity of bulk AMT organic semiconductor material. Application of the dielectric modulus formulism gives a simple method for evaluating the activation energy of the dielectric relaxation. The activation energy from the DC conductivity and the relaxation time are quite similar suggesting a hopping mechanism for AMT. The optical band gap of AMT is investigated using spectrophotometric measurement of transmittance at normal incidence of light in the wavelength range 300–1100 nm.     

Effects of three-body interaction on dynamic and static structure factors of an optically-trapped Bose gas

We investigate how three-body interactions affect the elementary excitations and dynamic structure factor of a Bose- Einstein condensate trapped in a one-dimensional optical lattice. To this end, we numerically solve the Gross-Pitaevskii equation and then the corresponding Bogoliubov equations. Our results show that three-body interactions can change both the Bogoliubov band structure and the dynamical structure factor dramatically, especially in the case of the two-body interaction being relatively small. Furthermore, when the optical lattice is strong enough, the analytical results, combined with the sum-rule approach, help us to understand that： the effects of three-body interactions on the static structure Ihctor can be significantly amplified by an optical lattice. Our predictions should be observable within the current Bragg spectroscopy experiment.     

Optical conductivity of spin/lattice polarons in underdoped copper oxides

Optical and spectral properties of carriers in the presence of strong antiferromagnetic correlations and interacting with optical phonon modes are analyzed using Dynamical Mean Field Theory. We interpret the mid-infrared band in σ(ω) in term of mixed spin lattice polaronic excitations which arise from the stabilization of the lattice polaron due to the antiferromagnetic correlations. We compare our results with experimental data in NCCO showing that the doping and temperature dependences of the optical conductivity in this compound is naturally reproduced within a spin/lattice polaronic model.     

Effect of Gd3+ doping on structural,optical and frequency-dependent dielectric response properties of pseudo-cubic BaTiO3 nanostructures

We report on the structural, optical and dielectric characterization of solid state derived, pseudo-cubic nanoscale barium titanates (BTs) with gadolinium (Gd³⁺) as substitutional dopant. Referring to X-ray diffractograms, apart from the BT peaks related to perovskite structure, the non-existence of any additional peaks due to byproducts has revealed that Gd³⁺ has undergone substitutional doping into the BT host lattice. The well-separated BT nanoparticles of typical size ～10–15 nm were observed through electron microscopy studies. Following a direct, allowed type carrier transition (n=1/2), a reduction in the optical band gap value (from 3.28 to 3.255 eV) was observed when the Gd-doping level was varied within 0–7 %. Conversely, the Urbach energy followed an increasing trend, from a value of 0.741 to 1.879 eV. Furthermore, the dielectric constant showed a decreasing tendency with doping content and with increasing frequency. However, in the low-frequency region, the loss tangent (tanδ), which is the combined result of orientational polarization and electrical conduction, was found to be quite high in the doped samples as compared to their un-doped counterpart. The frequency-dependent electrical data were also analyzed in the framework of conductivity and impedance formalisms. In particular, the ac conductivity which varies as ～ω ^s approaches ideal Debye behavior (s→1) for a low Gd level and a higher doping concentration did not show improved dielectric feature of the host. The incorporation of rare-earth (Gd³⁺) ions into the BT host system could greatly manifest dielectric relaxation and carrier conduction mechanisms, in a given frequency range, and thus can find immense scope in miniaturized nanoelectronic elements including ceramic capacitors and transducers.