Semimetallic Structure Research Articles

Insights into the Prediction Structural, Electronic, Optic, Elastic, and Phonon Properties of Half-Heusler Compound LiAgSe via Density Functional Theory

The structural, electronic, optic, elastic and dynamic features of LiAgSe half-Heusler structure are studied by using first principle calculations. LiAgSe half-Heusler compound is examined with the Generalized Gradient Approximation using the Density Functional Theory. The Quantum Espresso simulation program is preferred to investigate its structural, electronic and dynamic features. The ABINIT simulation program is preferred to investigate its elastic and optic properties. The electronic band structure graph of the LiAgSe crystal formed as a result of the calculation shows that this crystal has a semi-metallic structure. Optic properties such as, complex dielectric constant, extinction coefficient, reflectivity, for the volume of LiAgSe are calculated and plotted. In this study, elastic constants, Poisson's ratio and Debye Temperature values of LiAgSe half-Heusler crystal are determined. Apart from these, phonon dispersion curve graph is obtained. It has been calculated that the LiAgSe half-Heusler crystal is not dynamically stable in the ground state. However, when applied a pressure under nearly 16.396 GPa the crystal becomes stable.

Lattice dynamics and electronic properties of Heusler alloys Li2AlX (X=Ga, In): A comparison study

The lattice parameters, bulk modulus, first derivative of the bulk modulus, electronic band structures, phonon dispersion curves and phonon density of states calculations for Li2AlGa and Li2AlIn Heusler alloys are performed and compared in this study using density functional theory within the generalized gradient approximation. Computed lattice parameters display a good agreement with the literature. Obtained electronic band structures of both Heusler alloys show that they are in semi-metallic structure. Phonon dispersion curves and the phonon density of states graphs are also obtained in order to study the lattice dynamics of these Heusler alloys. It is noticed that Li2AlGa and Li2AlIn Heusler alloys are dynamically stable in the ground state.

Electron transport properties of graphene nanoribbons with Gaussian deformation

Gaussian deformation in graphene structures exhibits an interesting effect in which flower-shaped confinement states are observed in the deformed region [Carrillo-Bastos et al., Phys. Rev. B 90 041411 (2014)]. To exploit such a deformation for various applications, tunable electronic features including a bandgap opening for semi-metallic structures are expected. Besides, the effects of disorders and external excitations also need to be considered. In this work, we present a systematic study on quantum transport of graphene ribbons with Gaussian deformation. Different levels of deformation are explored to find a universal behavior of the electron transmission. Using a tight-binding model in combination with Non-Equilibrium Green Functions formalism, we show that Gaussian deformation influences strongly the electronic properties of ribbons in which the electron transmission decreases remarkably in high energy regions even if small deformations are considered. Interestingly, it unveils that the first plateau of the transmission of semi-metallic armchair ribbons is just weakly affected in the case of small deformations. However, significant large Gaussian bumps can induce a strong drop of this plateau and a transport gap is formed. The transmission at the zero energy is found to decrease exponentially with increasing the size of the Gaussian bump. Moreover, the gap of semi-conducting ribbons is enlarged with large deformations. The opening or the widening of the transport gap in large deformed armchair structures is interpreted by a formation of a three-zone behavior along the transport direction of the hopping profile. On the other hand, a transport gap is not observed in zigzag ribbons regardless of the size of Gaussian bumps. This behavior is due to the strong localization of edge states at the energy point E = 0...

Galvanomagnetic Properties of Cobalt Monosilicide and Alloys Based on It

The Hall coefficient and conductivity of cobalt-monosilicide CoSi, as well as Co1 –xFexSi and Co1 –xNixCo alloys with iron and nickel contents of up to 8 and 5 at %, respectively, are investigated. The temperature dependences of the Hall coefficient and conductivity are measured in the temperature range of 77–800 K. Theoretical interpretation of the experimental dependences is based on two different models of the electronic structure of the compound: a simple two-band semimetallic structure with a small overlap of isotropic parabolic bands and an ab initio electronic structure, containing topological features with multiply degenerate intersections of bands near the Fermi energy.

Гальваномагнитные свойства моносилицида кобальта и сплавов на его основе

In this work, we study the Hall coefficient and the conductivity of cobalt monosilide CoSi, as well as Co1−xFexSi and Co1−xNixCo alloys with contents up to 8 at% of iron and up to 5 at% of nickel. The temperature dependences of the Hall coefficient and of the conductivity were measured in the temperature range of 77−800K. The theoretical interpretation of the experimental dependencies is based on two different models of the electronic structure of the compound: a simple 2-band semimetallic structure with small overlap of isotropic parabolic bands; and ab initio electronic structure, containing near Fermi energy topological features with multiply degenerate intersections of bands.

Enhancement of the Seebeck effect in bilayer armchair graphene nanoribbons by tuning the electric fields

The Seebeck coefficient in single and bilayer graphene sheets has been observed to be modest due to the gapless characteristic of these structures. In this work, we demonstrate that this coefficient is significantly high in quasi-1D structures of bilayer armchair graphene nanoribbons (BL-AGNRs) thanks to the open gaps induced by the quantum confinement effect. We show that the Seebeck coefficient of BL-AGNRs is also classified into three groups 3p, 3p + 1, 3p + 2 as the energy gap. And for the semiconducting BL-AGNR of width of 12 dimer lines, the Seebeck coefficient is found as high as 707 μV/K and it increases up to 857 μV/K under the impact of the vertical electric field. While in the semimetallic structure of width of 14 dimer lines, the Seebeck coefficient remarkably enhances 14 times from 40 μV/K to 555 μV/K. Moreover, it unveils an appealing result as the Seebeck coefficient always increases with the increase of the applied potential. Such BL-AGNRs appear to be very promising for the applications of the next generation of both electronic and thermoelectric devices applying electric gates.

Spin-orbit semimetals in the layer groups

Recent interest in point and line node semimetals has led to the proposal and discovery of these phenomena in numerous systems. Frequently, though, these nodal systems are described in terms of individual properties reliant on specific space group intricacies or band-tuning conditions. Restricting ourselves to cases with strong spin-orbit interaction, we develop a general framework which captures existing systems and predicts new examples of nodal materials. In many previously proposed systems, the three-dimensional nature of the space group has obscured key generalities. Therefore, we show how within our framework one can predict and characterize a diverse set of nodal phenomena even in two-dimensional systems constructed of three-dimensional sites, known as the "Layer Groups". Expanding on an existing discussion by Watanabe, Po, Vishwanath, and Zaletel of the relationship between minimal insulating filling, nonsymmorphic symmetries, and compact flat manifolds, we characterize the allowed semimetallic structures in the layer groups and draw connections to related three-dimensional systems.

Room‐Temperature Electronic Template Effect of the SmSi(111)‐8×2 Interface for Self‐Alignment of Organic Molecules

This work describes an innovative concept for the development of organized molecular systems based on the template effect of the pre-structured semi-conductive SmSi(111) interface. This substrate is selected because Sm deposition in the submonolayer range leads to a 8x2-reconstruction, which is a well-defined one-dimensional semi-metallic structure. Adsorption of aromatic molecules [1,4-di-(9-ethynyltriptycenyl)-benzene] on SmSi(111)- 8x2 and Si(111)-7x7 interfaces is investigated by scanning tunneling microscopy (STM) at room temperature. Density functional theory (DFT) and semi-empirical (ASED+) calculations define the nature of the molecular adsorption sites of the target molecule on SmSi as well as their self-alignment on this interface. Experimental data and theoretical results are in good agreement.

Optical spectroscopy study of the electronic structure ofEu1−xCaxB6

The room temperature optical conductivity ${\ensuremath{\sigma}}_{1}(\ensuremath{\omega})$ of ${\mathrm{Eu}}_{1\ensuremath{-}x}{\mathrm{Ca}}_{x}{\mathrm{B}}_{6}$ has been obtained from reflectivity and ellipsometry measurements for a series of compositions, $0\ensuremath{\leqslant}x\ensuremath{\leqslant}1$. The interband part of ${\ensuremath{\sigma}}_{1}(\ensuremath{\omega})$ shifts continuously to higher frequency as Ca content $x$ increases. Also the intraband spectral weight of ${\ensuremath{\sigma}}_{1}(\ensuremath{\omega})$ decreases rapidly and essentially vanishes for $x\ensuremath{\geqslant}{x}_{c}=0.35$. These results show that the valence band and the conduction band of ${\mathrm{Eu}}_{1\ensuremath{-}x}{\mathrm{Ca}}_{x}{\mathrm{B}}_{6}$ move away from each other such that their band overlap decreases with increasing Ca substitution. As a result, the electronic state evolves from the semimetallic structure of ${\mathrm{EuB}}_{6}$ to the insulating ${\mathrm{CaB}}_{6}$ where the two bands are separated to open a finite gap $(\ensuremath{\simeq}0.25\phantom{\rule{0.3em}{0ex}}\mathrm{eV})$ at the $X$ point of the Brillouin zone.

Observation of magnetic-field-induced semimetal-semiconductor transitions in crossed-gap superlattices by cyclotron resonance.

A transition from semimetallic to semiconducting behavior induced by a magnetic field has been observed in type-II superlattices of InAs/${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{In}}_{\mathit{x}}$Sb by the use of very high magnetic fields, in excess of 100 T. The carrier densities and effective masses were measured by the study of cyclotron resonance using wavelengths in the region 10.6--3.39 \ensuremath{\mu}m. This was used to demonstrate that the zero-point energy associated with the lowest-electron and highest-hole Landau levels was sufficient to uncross the energy bands in long-period, semimetallic structures. For (100)-oriented structures this transition was found to occur in the region of 50--60 T, while for (111)A-oriented samples the uncrossing field was found to move up to the region of 100 T, due to an orientational dependence of the band offset. The effective masses, studied as a function of both photon energy and superlattice period, were found to be in good agreement with the predictions of k\ensuremath{\cdot}p theory.

Band-edge properties of quasi-one-dimensional HgTe-CdTe heterostructures.

We discuss a number of potential advantages offered by materials with very narrow energy gaps in the study of quasi-one-dimensional physics. Besides the expectation of quite large subband splittings, we predict several new phenomena which have no analog in wide-gap structures, such as the opening of a confinement-induced energy gap in semimetallic structures, a strong decrease of the gap with magnetic field, and ballistic conductance per channel in fractional units of ${\mathit{c}}^{2}$/\ensuremath{\pi}\ensuremath{\Elzxh}.

Effective Hamiltonian Describing the Electronic States of Bismuth-Type Crystals. I

An effective Hamiltonian consistent with the crystal symmetry which operates on the p -type Bloch wave function is constructed and solved for the electronic states near the Fermi level at the points T and L in the Brillouin zone of the three semimetallic elements, arsenic, antimony and bismuth. It is shown that the relative displacement of the sublattices of the crystal is responsible for the order of energy levels at T or L while the shear distortion mainly contributes to stabilizing the semimetallic structure. Parameters in the Hamiltonian in cases of arsenic and antimony are determined so as to reproduce the numerical results of the pseudopotential and OPW calculation. With the parameters extrapolated to the case of bismuth, the energy levels at T and L in bismuth are calculated taking into account the spin-orbit interaction. The results are in good agreement with those of the pseudopotential calculation by Golin.

Effective Hamiltonian Describing the Electronic States in Bismuth-Type Crystals. II

An effective Hamiltonian consistent with the crystal symmetry which operates on the p -type Bloch wave function is constructed and solved for the electronic states near the Fermi level at the points T and L in the Brillouin zone of the three semimetallic elements, arsenic, antimony and bismuth. It is shown that the relative displacement of the sublattices of the crystal is responsible for the order of energy levels at T or L while the shear distortion mainly contributes to stabilizing the semimetallic structure. Parameters in the Hamiltonian in cases of arsenic and antimony are determined so as to reproduce the numerical results of the pseudopotential and OPW calculation. With the parameters extrapolated to the case of bismuth, the energy levels at T and L in bismuth are calculated taking into account the spin-orbit interaction. The results are in good agreement with those of the pseudopotential calculation by Golin.