Interband Optical Conductivity Research Articles

Microscopic analysis of relaxation behavior in nonlinear optical conductivity of graphene

We produce a general formalism to study the interband dynamical optical conductivity in the nonlinear regime of graphene in the presence of a quantum bath comprising phonons and electrons. When a quantum solid of graphene is subjected to an intense electric field in the optical frequency range, the observation of a nonlinear response is facilitated by formulating a quantum master equation of the density operator associated with the Hamiltonian encapsulated in a spin-boson model of dissipative quantum statistical mechanics. Our results reveal the nonlinear steady-state regime’s population inversion and decoherence. The present method enables us to investigate further the nonlinear interband optical conductivity of pristine and gapped graphene characterized by a single dimensionless parameter at finite temperatures. Different bath spectra’ effects on phonons and electrons are examined in detail. The temperature dependence of conductivity reveals that changing temperature can enable us to make a transition from the linear to the nonlinear regime for fixed optical field parameters. Interestingly, a fascinating switching-like behavior is observed for the low-temperature optical conductivity of the gapped graphene while we vary the energy gap as well as the frequency of the externally applied field. Although our general formulation can address a variety of nonequilibrium responses of the two-band system, it also facilitates a connection with the phenomenological modeling of nonlinear optical conductivity.

Optical conductivity and linear dichroism in monolayer C[formula omitted]N

We theoretically study the optical properties of monolayer C3N within the framework of linear response theory by using a two band effective k⋅p Hamiltonian. We find that both the effective masses and optical conductivities are anisotropic arising from the anisotropic band structure at the M point. Although the inter-band coupling only exists in the armchair (x-)direction, the effective mass along the armchair direction is larger than that along the zigzag (y-)direction. Interestingly, the absorption part of the optical conductivity Re σxx excited by linearly polarized light along the armchair direction is an order of magnitude larger than Re σyy excited by linearly polarized light along the zigzag direction, resulting in a strong linear dichroism, because the inter-band optical conductivity is dominated by inter-band coupling which only exists in the armchair direction. Finite doping can adjust the optical conductivities i.e., Re σxx and Re σyy and modulate the linear dichroism, but the effects of n-type doping and p -type doping are different because of the electron–hole asymmetry. Our results provide further understanding of the electronic state of monolayer C3N and maybe useful to design polarized optical devices based on it.

Interband optical conductivity in electromagnetic field modulated strained black phosphorene

Black phosphorene (BP) has been widely investigated for its anisotropic and unique photoelectric properties. Strain, voltage and so on are commonly used to modulate the energy band structure and accordingly its photoelectric characteristics. In this study, we consider the energy band structure of BP in the vertical magnetic field, electric field, and in-plane/out-of-plane strains by using the tight-binding approximate Hamiltonian. The anisotropic frequency-dependent interband optical conductivity (IOC) of BP is investigated by using the Kubo formula in these modulation factors. Inherent asymmetry in band dispersion along the armchair (AC) direction and the zigzag (ZZ) direction leads to anisotropic IOC. The introduction of a vertical magnetic field induces band splitting, thereby generating multiple interband transition channels. In this case, the IOC along both the AC direction and the ZZ direction exhibits three peaks around the original peak position, and the magnitudes of the peaks are also modulated. With the increase of in-plane strain (from –20% to 20%), the band gap increases monotonically, and both the position and magnitude of the peaks vary with band gap changing. However, the band gap of BP undergoes a non-monotonic change under out-of-plane strain (from –20% to 20%), which is different from the change under in-plane strain. The band gap reaches a minimum value when a tensile strain of 12% is applied. Along the AC direction, the modulation of the IOC by in-plane strain is opposite to the modulation of out-of-plane strain (<i>ε</i><sub><i>z</i></sub> < 12%), indicating a competitive effect when triaxial strains are applied. Along the ZZ direction, in-plane strain primarily modulates the peak magnitude, while out-of-plane strain effectively modulates not only the peak position but also the peak magnitude obviously. The modulation of the IOC by forward and reverse electric fields are symmetrical. The coefficient for the peak position shift due to the vertical electric field is 1/2 in the AC direction and 1/10 in the ZZ direction. By integrating various modulation factors, we achieve versatile control over the energy band and IOC of BP, providing theoretical support for the application of BP in optoelectronic devices.

Electronic structure and optical properties of superconducting compounds ScGa3 and LuGa3

The optical properties of bulk AuCu3-type ordered superconducting binary compounds ScGa3 and LuGa3 have been experimentally investigated for the first time in the wide spectral range and explained in terms of the band structure. First-principles calculations of electronic densities of states and optical conductivities are performed within the framework of LSDA+U method using the LMTO-ASA software package. The measured dielectric functions spectra for both materials exhibit a strong absorption region above ∼ 1[Formula: see text]eV, which are in a reasonable agreement with the calculated metal-like energy structures of compounds. Features of quantum light absorption are discussed on the basis of a comparative analysis of experimental and theoretical spectra of interband optical conductivity. Plasma and relaxation frequencies of conduction electrons are determined.

The Electronic Structure and Optical Spectroscopy of ErNi2Mnx Compounds

The electronic structure and optical properties of nonstoichiometric ErNi2Mnx compounds (with х = 0, 0.5, 1) have been studied. Spin-polarization calculations of the total and partial densities of electron states have been performed in terms of DFT + U method with a correction for strong electronic correlations in the 4f shell of Er in the approximation of ErNi2 – xMnx solid-solution. The peculiarities of transformations of the densities of electron states Have been determined depending on the manganese content. The optical properties of these compounds have been studied over a wide wave length range. The calculated interband optical conductivity spectra have been compared with the dependences obtained experimentally. The origin of the quantum absorption of light is discussed. The plasma and relaxation frequencies of current carriers have been determined.

Investigation of semi-metallic properties of GdSb and TbSb compounds

The results of the electronic structure calculations and the optical properties study of antiferromagnetic compounds GdSb and TbSb are presented. In the spectral range 0.078 – 5.6 eV, the energy dependences of the real and imaginary parts of the complex dielectric permittivity are investigated by ellipsometric method. The anomalous behavior of spectral characteristics in the infrared region is associated with the semi-metallic properties of these materials. Based on the performed calculations of the band structure, the energy dependences of the interband optical conductivity spectra are discussed.

Graphene as an inhomogeneously broadened two-level saturable absorber.

We show that the inter-band optical conductivity of graphene follows a dependence on intensity that is characteristic of inhomogeneously broadened saturable absorbers, and we obtain a simple formula for the saturation intensity. We compare our results with those from more exact numerical calculations and selected sets of experimental data, and obtain good agreement for photon energies much larger than twice the chemical potential.

Tunable absorptance by the magnetic field in multilayer black phosphorene dielectric structures

The magnetic field effect on the interband optical conductivity of black phosphorene (BP) is considered in the investigation of the absorptance of BP-based multilayer dielectric structures using the transfer matrix method. The BP interband optical conductivity is anisotropic and shows multi-peaks in the presence of the magnetic field because of the splitting of energy levels. The peak position of the absorptance is tuned via the magnetic field strength. In the energy region (0.5 eV - 1.7 eV), the maximum absorptance in the armchair direction is larger than that in the zigzag (ZZ) direction which depends on the angle of incidence and the dielectric structure. But the increased rate of the maximum absorptance in the presence of a magnetic field in the ZZ direction is quite obvious and increased about 10 times. An increased number of BP/Si layers makes the optical absorption in both directions much stronger.

Influence of electric and magnetic fields andσ-edge bands on the electronic and optical spectra of graphene nanoribbons

The unusual electronic and optical properties of armchair and zigzag graphene nanoribbons (GNRs) subject to in-plane transverse electric and perpendicular magnetic fields have been systematically investigated. Our calculations were carried out within the generalized multi-orbital tight-binding model based on a Hamiltonian which takes into account hopping integrals among the (s, $p_x$, $p_y$, $p_z$) atomic orbitals as well as the external electric and magnetic fields. The electronic structure consists of $\pi$ bands arising from the $p_z$ orbital and $\sigma$ bands originating from the (s, $p_x$, $p_y$) orbitals. The energy bands and optical spectra are diversified by both the nature of the edge of the nanoribbon and strength of the external fields. Armchair GNRs display a width-dependent energy gap in addition to low-energy $\sigma$ bands while the zigzag system has the unfilled flat band with $\pi$ edge states at zero energy and partially filled wide-range $\sigma$ bands. An applied in-plane electric field leads to the splitting of energy bands and shifted Fermi level, thereby enriching the inter-band and intra-band optical conductivities. The interplay between an external magnetic field and the edge geometry gives rise to extraordinary quantized Landau levels and special optical spectra.

Электронная структура и оптические спектры соединений GdFeAl и GdFeSi

Results of investigations of electronic structure and optical properties of GdFeAl and GdFeSi compounds are presented. Spin-plarized density of states and interband optical conductivity spectra were calculated in frame of DFT+U technique with a correction for strong correlation effects in 4f shell of Gd. Optical properties were measured by ellipsometric technique in wavelength interval of 0.22 – 16 μm. Nature of quantum light absorption is discussed on the base of comparative analysis of experimental and calculated spectra. It is shown that main features of frequency dependencies of the optical conductivity are interpret qualitatively by the calculated density of electronic states.

Infrared Optical Conductivity of Bulk Bi2Te2Se

Mid- and near-infrared measurements reveal that the optical conductivity of the three-dimensional topological insulator, Bi2Te2Se, is dominated by bulk carriers and shows a linear-in-frequency increase at 0.5 to 0.8 eV. This linearity might be interpreted as a signature of three-dimensional (bulk) Dirac bands; however, band-structure calculations show that transitions between bands with complex dispersion contribute instead to the inter-band optical conductivity at these frequencies and, hence, the observed linearity is accidental. These results warn against the oversimplified interpretations of optical-conductivity measurements in different Dirac materials.

Spin-splitting effects on the interband optical conductivity and activity of phosphorene

Being able to tune the anisotropic interband transitions in phosphorene at finite temperature offers an enormous amount of possibilities in finding new insights in the optoelectronic community. To contribute to this goal we propose a Zeeman spin-splitting field aiming at absorbing various frequencies of the incident light. Employing the tight-binding Hamiltonian to describe the carrier dynamics and the Kubo formalism to formulate the orientation-dependent interband optical conductivity (IOC) and optical activity of phosphorene we investigate the absorption and scattering mechanisms in phosphorene depending on the Zeeman field strength and optical energy parameters. The optical activity features are characterized by exploring the eccentricity and shift phase of reflected and transmitted electromagnetic waves of the incident light. Different electronic phases in the absence and presence of Zeeman field ultimate different types of interband transitions of which in all cases the IOC along the armchair direction is larger than the zigzag one. However, we observed an irregular (regular) process for IOC with the Zeeman field along the armchair (zigzag) direction, resulting in irregular (regular) absorption and scattering mechanisms. Additionally, a little to no effects for temperature-dependent IOC are provided with the Zeeman field in undoped phosphorene. Further, almost linearly and elliptically polarizations are reported for the transmitted and reflected waves, respectively, indicating that the phosphorene is almost transparent. The emergence of Zeeman spin-splitting effects in optoelectronic properties of phosphorene is pleasant to make it a great potential candidate for logic applications.

Examining the validity of the two-dimensional conical model to describe the three-dimensional ZrTe5

Understanding the low-energy excitation state in three-dimensional layered compound ZrTe5 remains a challenging problem in the study of novel topological materials. Recently, a two-dimensional conical model was proposed to explain the experimental optical spectroscopy in the 3D ZrTe5, Martino et al. [Phys. Rev. Lett. 122, 217402 (2019)]. Motivated by this work, in this paper, we perform a systematic theoretical study on the optical conductivity of this model in both cases without and with an external magnetic field to further demonstrate the validity of this model and to recover new physics.We find that there exist completely different characteristics for optical conductivity along different directions, due to anisotropic low-energy excitations in this two-dimensional conical model. Specifically, for the interband optical conductivity, we find asymptotic dependence on the optical frequency as Re(\sigma_x)\sim\omega^{1/2} and Re(\sigma_z)\sim{\omega}^{3/2}, which are universal both in the gapped insulator phase and Weyl semimetal phase. For the magneto-optical conductivity, on the contrary, Re(\sigma^B_{x/z}) shows distinct signatures in the gapped insulator phase and Weyl semimetal phase, which can help distinguish the two phases. Our results, to be verified in future experiments, could provide more insights into the understanding of the topological nature of ZrTe5.

Plasmonic mode coupling in graphene-based photonic crystals

Abstract Plasmonic bands in 1D graphene-based photonic crystals have recently been reported for transverse magnetic polarization in the low THz regime. In this work, we demonstrate that plasmon coupling, giving rise to plasmonic bands, is also present for transverse electric polarization at IR frequencies corresponding to the region of interband transitions in graphene. Due to the doping level dependence of graphene interband optical conductivity, plasmonic band region can be shifted in the IR frequency regime for different doping levels. We have analyzed graphene-based photonic crystals with a homogeneous and periodic doping level sequence along crystal growth direction in the unit cell; our results prove that plasmonic band opening is obtained for these two different graphene doping sequences, in the frequency region such that imaginary optical conductivity turns negative, displaying different levels of attenuation due interband absorption processes. Finally, we show that TE plasmonic modes in a finite multilayer can be excited by attenuated total reflection technique in Otto configuration.

Electronic Structure and Optical Properties of the FeAl2 Compound

The electronic structure and optical properties of the binary intermetallic compound FeAl2 have been studied. Spin-polarized calculations of the electronic structure have been performed and the magnetic moments of atoms have been determined. Using the method of ellipsometry, the optical characteristics of material have been measured in a spectral range of 0.22–15 μm. It is shown that the experimental interband optical conductivity of the compound is satisfactorily interpreted based on the calculated density of electronic states.

Electronic and optical properties of HoNi5 and HoNi4Si: An LSDA + U study

We investigated the effect of Si substitution on structural, electronic and optical properties of [Formula: see text]-type [Formula: see text] alloys. The optimized lattice constants and internal cell parameters are in agreement with the available data. We found that the valence band is mainly dominated by Ni-3d states in the energy range 0–4 eV below the [Formula: see text], whereas conduction band is contributed by spin-down Ho-[Formula: see text] states and lies about 2 eV above the [Formula: see text]. Substitution of Si atoms for Ni decreases the total magnetic moment from 6.308 [Formula: see text]/f.u. [Formula: see text] to 4.052 [Formula: see text]/f.u. [Formula: see text], whereas magnetic moments on [Formula: see text]-ions increase from 3.92 [Formula: see text]/atom [Formula: see text] to 4 [Formula: see text]/atom [Formula: see text]. On the other hand, induced moment on Ni[Formula: see text]-ions decrease rapidly to a negligibly small value. By the use of charge density estimates, we found that Ho-5d, Ni-3d and Si-3p orbitals are mainly involved in bonding and there is a weak hybridization between Ni-3d and Si-3p orbitals which varies with substitution of Si for Ni. Furthermore, the complex role played by Ho-[Formula: see text] electrons is also investigated, they are found to be partly involved in metallic bondings as well as in the intra-atomic charge transfer from [Formula: see text] to [Formula: see text] states in [Formula: see text]-ions. The calculated interband optical conductivity spectra reproduce the main features of experimental spectra well and also reveal their metallic nature.

Electronic structure of DyRhSn and HoRhSn compounds: band calculations and optical study

In this paper experimentally measured optical properties of intermetallic ternary compounds DyRhSn and HoRhSn are reported together with the spin-polarized calculations of the electronic structure within DFT+U method, accounting for electronic correlation effects in the 4f shell of Dy and Ho. A number of spectral and electronic characteristics are determined. The performed calculations allow one to qualitatively interpret the energy dependencies of the interband optical conductivity extracted from experimental ellipsometry measurements.

Internal exchange interaction of Tb+3 ions and influence of Si substitution on structural, electronic, magnetic and optical properties of YNi4Si -type TbNi5−xSix (x = 0, 1, 2) compounds

We report the FPLAPW + lo study on new YNi4Si-type TbNi5−xSix (x = 0, 1, 2) compounds. The YNi4Si structure is a new structure type, which is an orthorhombic derivative of CaCu5 structure. The optimized lattice constants and internal cell parameters are in agreement with the available data. The calculated band structures show the valence band (VB) is mainly dominated by Ni–3d states in the energy range 0–4 eV below the EF, whereas conduction band (CB) by Tb–4f states about 2 eV above the EF. With the substitution of Si atoms for Ni, total magnetic moments decreases from 8.308 μB/f.u. (x = 0) to 6.052 μB/f.u. (x = 1), whereas magnetic moments on Tb+3ions increase from 5.92 μB/atom (x = 0) to 6 μB/atom (x = 1, 2) and the induced moments on Ni+2(x = 1, 2) ions decrease rapidly to a negligibly small value. Charge density estimates show that Tb-5d, Ni-3d and Si-3porbitals are mainly involved in bondings and there is a weak hybridization between Ni-3d and Si-3porbitals which varies with Si substitution. Furthermore, the complex role played by the Tb-5d1 electrons is also investigated, and found to be partly involved in metallic bondings as well as in intra-atomic charge transfer from 5d1 to 4f8 states. The calculated interband optical conductivity spectra reproduce the main features of the experimental spectra well and also reveal their metallic nature. Frequency-dependent refractive index, η(ω), the reflectivity, R(ω) and the extinction coefficient, κ(ω) for (x = 0, 1, 2) are also calculated for the incident radiation up to 12 eV.

Combined temperature- and magnetic field-induced optical responses of phosphorene

The paper presents a theoretical investigation of the temperature and an out-of-plane Zeeman magnetic field effects on the interband optical conductivity of phosphorene. The electronic states are calculated with the tight-binding Hamiltonian and to compute the optical conductivity, the Kubo formalism is employed. The main result is that the multiple bands due to the perpendicular Zeeman splitting of the energy levels lead to the multiple resonance structures of the optical conductivity. It is found that the strong anisotropic interband optical conductivity of pristine phosphorene keeps the anisotropic feature when the temperature is increased. Importantly, our results show that the optical properties can be externally tuned via a magnetic field. In particular, when the magnetic field is applied, the unique anisotropic property breaks down and there is a little to no sign for temperature- and orientation-dependent transitions. The scientific soundness of findings is useful for practical aspects of phosphorene in spintronic.

Optical properties of Fe–Mn–Ga alloys

The first-principles calculations of the electronic structures and the interband optical conductivity (OC) spectra have been performed for the stoichiometric Fe2MnGa alloy with L21 and L12 types of atomic ordering. The calculated optical properties of Fe2MnGa alloy for the L21 and L12 phases are complemented by the experimental OC spectra for bulk and thin film Fe–Mn–Ga alloy samples near the stoichiometry 2:1:1 with L21 and L12 (for bulks) as well as the body-centered-cubic and face-centered-cubic (for films) structures, respectively. A reasonable agreement between experimental and calculated interband OC spectra was obtained for both phases of the alloy. The experimental data show no significant difference in the OC spectra with respect to the degrees of atomic and magnetic orders of the samples.