POLARIZATION AND FREQUENCY-POLARIZATION DEPENDENCES OF THREE-PHOTON INTERBAND LINEAR-CIRCULAR DICHROISM IN SEMICONDUCTORS OF CUBIC SYMMETRY

The polarization and frequency-polarization dependences of the linear-circular dichroism and light absorption coefficients in semiconductors of cubic symmetry, caused by vertical three-photon optical transitions between the states of the spin-orbit splitting and conduction bands, are calculated. KEY WORDS: three-photon optical transitions, spin-orbit splitting band, conduction band, linear-circular dichroism, light absorption, semiconductor.

The misconception in graphene’s dispersion energy simulations

This study aims to find equations and simulations that satisfy the characteristics of graphene’s energy dispersion and identify misconceptions that may occur. Here we give students nine articles about graphene’s dispersion energy. They were asked to identify the equations, parameters, and software used in each of the articles. The assignment was then to make the distribution of the data in a spreadsheet. The parameters used were the lattice constant of 2.46 Å, the range of the k wave function for the x and y axes of -2πa to 2πa, and the interval for each range of 0.1. Each equation is divided into two parts, E(+) and E(-). The analysis was carried out by making a slice in the middle of the x and y axes, as well as the main and off-diagonals. Graphene has Dirac points where the band gap is zero. This means that there is no distance or very small distance between the valence and conduction bands. From this activity, it can be concluded that Rozhkov (2016) has the equations and simulations that best satisfy graphene’s dispersion energy. Misconceptions occur in almost all existing equations and simulations.

Spectrum of localized states in fermionic chains with defect and adiabatic charge pumping

In this paper, we study the localized states of a generic quadratic fermionic chain with finite-range couplings and an inhomogeneity in the hopping (defect) that breaks translational invariance. When the hopping of the defect vanishes, which represents an open chain, we obtain a simple bulk-edge correspondence: the zero-energy modes localized at the ends of the chain are related to the roots of a polynomial determined by the couplings of the Hamiltonian of the bulk. From this result, we define an index that characterizes the different topological phases of the system and can be easily computed by counting the roots of the polynomial. As the defect is turned on and varied adiabatically, the zero-energy modes may cross the energy gap and connect the valence and conduction bands. We analyze the robustness of the connection between bands against perturbations of the Hamiltonian. The pumping of states from one band to the other allows the creation of particle–hole pairs in the bulk. An important ingredient for our analysis is the transformation of the Hamiltonian under the standard discrete symmetries, C, P, T, as well as a fourth one, peculiar to our system, that is related to the existence of a gap and localized states.

Doping limits in p-type oxide semiconductors

The ability to dope a semiconductor depends on whether the Fermi level can be moved into its valence or conduction bands, on an energy scale referred to the vacuum level. For oxides, there are various suitable n-type oxide semiconductors, but there is a marked absence of similarly suitable p-type oxides. This problem is of interest not only for thin-film transistors for displays, or solar cell electrodes, but also for back-end-of-line devices for the semiconductor industry. This has led to a wide-ranging search for p-type oxides using high-throughput calculations. We note that some proposed p-type metal oxides have cation s-like lone pair states. The defect energies of some of these oxides were calculated in detail. The example SnTa2O6 is of interest, but others have structures more closely based on perovskite structure and are found to have more n-type than p-type character. Graphic abstract

Conduction Band Engineering of Half-Heusler Thermoelectrics Using Orbital Chemistry

Semiconducting half-Heusler (HH, XYZ) phases are promising thermoelectric materials owing to their versatile electronic properties. Because the valence band of half-Heusler phases benefit from the valence band extrema at several high-symmetry points in the Brillouin zone (BZ), it is possible to engineer better p-type HH materials through band convergence. However, the thermoelectric studies of n-type HH phases have been lagging behind since the conduction band minimum is always at the same high-symmetry point (X) in the BZ, giving the impression that there is little opportunity for band engineering. Here we study the n-type orbital diagram of 69 HHs, and show that there are two competing conduction bands with very different effective masses actually at the same X point in the BZ, which can be engineered to be converged. The two conduction bands are dominated by the d orbitals of X and Y atoms, respectively. The energy offset between the two bands depends on the difference in electron configuration and electronegativity of the X and Y atoms. Based on the orbital phase diagram, we provide the strategy to engineer the conduction band convergence by mixing the HH compounds with the reverse band offsets. We demonstrate the strategy by alloying VCoSn and TaCoSn. The V0.5Ta0.5CoSn mixture presents the high conduction band convergence and corresponding significantly larger density-of-states effective mass than either VCoSn or TaCoSn. Our work indicates that analyzing the orbital character of band edges provides new insight into engineering thermoelectric performance of HH compounds.

Electric Field Induced Twisted Bilayer Graphene Infrared Plasmon Spectrum

In this work, we investigate the role of an external electric field in modulating the spectrum and electronic structure behavior of twisted bilayer graphene (TBG) and its physical mechanisms. Through theoretical studies, it is found that the external electric field can drive the relative positions of the conduction band and valence band to some extent. The difference of electric field strength and direction can reduce the original conduction band, and through the Fermi energy level, the band is significantly influenced by the tunable electric field and also increases the density of states of the valence band passing through the Fermi level. Under these two effects, the valence and conduction bands can alternately fold, causing drastic changes in spectrum behavior. In turn, the plasmon spectrum of TBG varies from semiconductor to metal. The dielectric function of TBG can exhibit plasmon resonance in a certain range of infrared.

Heavy Thallium Based Fluoroperovskite TlAF3 (A = Ge, Sn and Pb) Compounds: A Computational Investigation

Combination of heavy elements in forming a stable system leads to enhancement in effective atomic number making it desirable in many applications such as detection and shielding of radiation. We present our theoretical investigations on new Thallium based heavy fluoroperovskites TlAF3 (A = Ge, Sn and Pb). The study is carried out to explore the structural, elastic, electronic, and optical properties through the Density Functional Theory (DFT) using the Full-Potential Linearized Augmented Plane Wave (FP-LAPW) method implemented in WIEN2k. Generalized Gradient Approximation with consideration of electronic correlation effects (GGA+U) was employed for calculations. The lattice constants deduced from the optimization curves were found to be in the range of 4.00 Å to 4.85 Å. Elastic properties were obtained from the calculated elastic constants. From band structure calculations, it is evident that the bandgaps range from 0.84 to 1.89 eV. All the studied compounds exhibit indirect bandgap nature. Fluorine atom contributes significant number of electronic states in valence and conduction bands of all studied compounds. The optical response in terms of refractive index, extinction coefficient, optical conductivity, reflectivity, and absorption coefficients are calculated and discussed in the energy range of (0-20) eV. The properties of compounds in this study are being reported for the first time.