Dielectric Tensor Research Articles

Optimization of a Weyl-semimetal-based near-field heat transfer system

Near-field radiative heat transfer (NFRHT) is inherently limited by Lorentz reciprocity, which can be partially mitigated through the application of an external magnetic field. However, recent studies have discovered that Weyl semimetals (WSMs) possess the ability to achieve nonreciprocal surface plasmon polariton modes without relying on an external magnetic field. Building upon the nonreciprocal nature of WSMs, this study investigates NFRHT between two WSM slabs and compares different WSM materials. The primary focus of the comparison lies in analyzing the heat flux under varying emitter and receiver temperatures. Three of the selected WSM materials exhibit similar performance in NFRHT, showing sensitivity to a decrease in the receiver temperature due to their low dielectric tensor components. In contrast, Eu2IrO7 displays distinctive characteristics, being sensitive to changes in the emitter temperature as a result of its higher dielectric tensor components. Additionally, a reassessment of the nonreciprocity in Co2MnGa, Eu2IrO7 and Co3Sn2S2 is conducted. Furthermore, Bayesian optimization algorithms are employed to optimize parameters and address the challenge of low thermal radiation observed in existing WSM NFRHT systems. The results highlight the achievement of an optimal WSM material that achieves an impressive heat flux of 5.29 × 108 (W/m2) at an emitter temperature of 310 K and a receiver temperature of 290 K. Notably, this heat flux surpasses that of Co2MnGa by 2.5 times in the context of NFRHT. The discoveries presented in this study promises to offer practical implications for the development of nonreciprocal thermal energy management systems.

Dielectric and optical properties of gas hydrates and ice: <i>ab initio</i> simulation results

The dielectric tensors εik(ω), optical absorption spectra R(ω), reflection spectra a(ω) and the density of electronic states N(E) have been calculated for the lattices of gas hydrates with the structure sI and sH, as well as for ice Ih and III. The dependence of the dielectric permittivity on the type of guest molecules in the sI hydrate is considered.

Effect of different pseudopotentials on the phonon frequencies, dielectric constant, and Born effective charge of SnSe and SnSe2 nanostructures: A density functional perturbation theory study

The choice of the pseudopotentials (PPs) is a crucial step towards an accurate atomistic calculation of physical properties of 2D materials. In this study, based on Density-Functional Perturbation Theory (DFPT), we investigated the dependency of the phonon properties such as phonon frequencies, dielectric constant, Born effective charge, and vibrational modes of SnSe and SnSe2 monolayers to the different types of PPs. On the basis of Generalized Gradient Approximation (GGA) using PBE functional, we computed eight phonon calculations for both lattices in combinations with four different PPs including Normconversing (NC) FHI/HGH, Ultrasoft (USPP), and Standard Solid-State (SSSP), while other parameters like cut-off energy, k-points mesh, and convergence threshold were kept fixed. However, the phonon properties of acoustic and optical phonons were better predicted by USPP and SSSP for phonon dispersion of SnSe and SnSe2, respectively. Regarding the dielectric constant and Bohr effective charge calculations, our results showed that SnSe and SnSe2 structures possess the dielectric tensor of 8.9 and 6.6 along the in-plane direction by using USPP calculations. Our outcomes also revealed less sensitivity of SnSe2 polarization to the type of PPs by applying the electric field. This study recommends USPP and SSSP to further phonon analysis of other semiconductors.

Strain and electric field induced low-symmetry states in Pb0.1Sr0.9TiO3

Polarization rotation and occurrence of monoclinic and triclinic states were investigated in Pb0.1Sr0.9TiO3. For this composition, the rhombohedral ground state may coexist with a tetragonal state close in free energy and become sensitive to electric and mechanical stimuli. The polarization path under an electric field and biaxial strain involves a monoclinic state, while for uniaxial strain the transition involves both monoclinic and triclinic states. Longitudinal and shear piezoelectric constants are enhanced in the induced monoclinic and tetragonal states, respectively. Low-temperature secondary peaks in the literature-reported dielectric spectra can be explained as polycrystalline average of dielectric tensor components of the low-symmetry structures.

Structural, optical, elastic, thermoelectric and thermodynamic properties of the IrMn material: A DFT study

In this work, we present a study of the structural, optical, elastic, thermoelectric and thermodynamic properties of iridium manganese (IrMn) using the code “WIEN2k” with the full potential linearized augmented plane wave (FP-LAPW) method. The lattice constants of the IrMn compound are calculated by the generalized gradient approximation with Perdew–Burke–Ernzerhof (GGA-PBE). Indeed, we have studied the magnetism exhibited by the compound IrMn. We found that the antiferromagnetic phase of this material is the most stable phase. The magnetic moment, plasma frequency, bandgap, elastic constants, and electronic and magnetic properties of this material are deduced and discussed. We have shown that the antiferromagnetic phase is the most stable with a minimum energy value of around −1034947.477[Formula: see text]eV. We optimized the [Formula: see text] ratio and found that its value is approximately [Formula: see text] 0.923133. On the other hand, the absorption and reflectivity coefficients, the optical conductivity and the dielectric tensor are studied. In addition, the thermo-electric and thermodynamic properties have been illustrated. The IrMn compound could be useful for different applications of spintronics.

The magnetocaloric, magnetic, optical, and thermoelectric properties of EuB6 compound: DFT and Monte Carlo study

AbstractThe hexaboride EuB6 has been proven to have qualities that theoretically travel from being electronic to being magneto‐caloric. Under GGA‐PBE and GGA + SOC + U schemes, first‐principles calculations have been used to examine the electronic band structure and density of states. The magnetic moment and the interaction constant have each been calculated using DFT. Additionally, the thermoelectric properties are considered within the particular transport restrictions in order to determine the dimensionless figure of merit (ZT). The extremum of the ZT for EuB6 is determined by using the Seebeck, electrical, thermal, and lattice conductivity coefficients, which are computed. At 1800 K, a maximum ZT of 0.22 is discovered. The computed values of optical conductivity, electron energy loss, absorption coefficient, dielectric tensor, refractive index, and extinction coefficient throughout the range of 0–13 eV anticipate the optical considerations of such an alloy. The infrared spectrum is that where this chemical is active. The critical temperature (Tc) has been determined using the Metropolis algorithm and the Monte Carlo simulation. The material exhibits a ferromagnetic to paramagnetic phase transition, as indicated by temperature‐dependent magnetization. The magnetocaloric efficiency of this material can be measured using the magnetic entropy change (−ΔSm) and the relative cooling power.

Achieving controllable multifunctionality through layer sliding

Dynamical variation of physical properties in a controllable fashion provides exciting possibilities to obtain multifunctional materials. In this work, layer-sliding is employed to modify the structural, interfacial electronic and optical properties of unintercalated and Mg-intercalated two-dimensional (2D) van der Waals heterostructure (vdW-HS) consisting of buckled silicene and hexagonal boron nitride (hBN). The most stable stacking configuration of silicene over hBN is screened out and then intercalated with Mg at the interface. Dynamical-dependent changes in the properties of vdW-HS are performed by sliding silicene over hBN monolayer in the absence and presence of the intercalant. Layer-sliding is carried out in equal length intervals, and various parametric quantities related to the physical characteristics of the vdW-HS are repeatedly calculated and compared. Apart from various parametric quantities, stability of unintercalated and Mg-intercalated vdW-HS is also checked by means of relative total energies, binding energies and vdW gaps along the sliding pathway. Comparison of binding energies shows that the un-slided, half-slided, and fully-slided Mg-intercalated vdW-HS are 1.52, 1.44 and 1.42 eV more stable than the unintercalated vdW-HS. Opening of a small band gap of 12, 31 and 28 meV for un-slided, half-slided and fully-slided unintercalated vdW-HS, respectively, is worth mentioning. To study the interfacial electronic behavior, planar average charge density difference (Δρ) and charge transfer (ΔQ) are also calculated and varied via layer-sliding. Further, we calculated diverse optical spectra such as the complex dielectric function (DF), electron energy loss function [L(ω)], diagonal components of dielectric tensor [ε(iω)], refractive index [n(ω)], extinction coefficient [k(ω)], absorption coefficient [α(ω)], and reflectivity [R(ω)] for un-slided, half-slided and fully-slided unintercalated and Mg-intercalated vdW-HS. Interestingly, the polarization and energy losses have been reduced in the case of Mg-intercalated vdW-HS. The suggested layer-sliding method can be established as a general scheme for bringing multifunctionality into a layered material.

Layer-sliding-mediated controllable synthetic strategy for the preparation of multifunctional materials

Strategies capable of inducing multifunctionality in materials in a controllable manner are highly desirable. In this work, we have brought multifunctionality to a two-dimensional (2D) van der Waals heterostructure (vdW-HS) consisting of a layer of puckered silicene and indium selenide (InSe) through layer-sliding. Electronic, optical and interfacial properties of silicene/InSe vdW-HS have been varied dynamically by sliding silicene over InSe. The aforementioned layer-sliding is carried out in a gradual manner without and with Mg-intercalation. The buckling parameter relating to the corrugation of silicene drops from 0.438 Å to 0.351 Å on Mg-intercalation, whereas in the case of InSe buckling remains unaffected. The calculated band structure for un-intercalated vdW-HS shows a small band gap opening 205/210 meV with LDA/GGA functional) along high symmetry K direction in the first Brillion zone. The widening of the bandgap makes it suitable for optoelectronics and strain sensor devices, where a small band gap (0.1–1 eV) is required. To look inside into the interface of the vdW-HS, various interfacial electronic parameters such as planar average charge density difference (Δρ), dipole charge transfer (ΔQ) and dipole moment (Δµ) have been determined. It is found that the charge accumulation peaks have appeared near the silicene layer and the charge is transferred from silicene to the InSe layer in both un-intercalated and Mg-intercalated vdW-HSs. Further, we investigated the different optical parameters including real [ε1(ω)] and imaginary [ε2(ω)] parts of dielectric function (DF), energy loss function [L(ω)] and diagonal components of dielectric tensor [ε (iω)] for un-intercalated and Mg-intercalated vdW-HSs. The anisotropic analysis shows that the computed in-plane components significantly dominate the out-of-plane component of the DF, which show minor change as the sliding proceeds. The study endorses that multifunctional electronic 2D materials can be achievable by employing the layer-sliding dynamical strategy.

Hyperbolic Polaritons in Topological Nodal Ring Semimetals.

In mirror-symmetric systems, there is a possibility of the realization of extended gapless electronic states characterized as nodal lines or rings. Strain induced modifications to these states lead to the emergence of different classes of nodal rings with qualitatively different physical properties. Here we study optical response and the electromagnetic wave propagation in type I nodal ring semimetals, in which the low-energy quasiparticle dispersion is parabolic in momentum k_{x} and k_{y} and is linear in k_{z}. This leads to a highly anisotropic dielectric permittivity tensor in which the optical response is plasmonic in one spatial direction and dielectric in the other two directions. The resulting normal modes (polaritons) in the bulk material become hyperbolic over a broad frequency range, which is furthermore tunable by the doping level. The propagation, reflection, and polarization properties of the hyperbolic polaritons not only provide valuable information about the electronic structure of these fascinating materials in the most interesting region near the nodal rings but also pave the way to tunable hyperbolic materials with applications ranging from anomalous refraction and waveguiding to perfect absorption in ultrathin subwavelength films.

Vectorial inverse scattering for dielectric tensor tomography: overcoming challenges of reconstruction of highly scattering birefringent samples.

Many important microscopy samples, such as liquid crystals, biological tissue, or starches, are birefringent in nature. They scatter light differently depending on the polarization of the light and the orientation of the molecules. The complete characterization of a birefringent sample is a challenging task because its 3 × 3 dielectric tensor must be reconstructed at every three-dimensional position. Moreover, obtaining a birefringent tomogram is more arduous for thick samples, where multiple light scattering should also be considered. In this study, we developed a new dielectric tensor tomography algorithm that enables full characterization of highly scattering birefringent samples by solving the vectoral inverse scattering problem while accounting for multiple light scattering. We proposed a discrete image-processing theory to compute the error backpropagation of vectorially diffracting light. Finally, our theory was experimentally demonstrated using both synthetic and biologically birefringent samples.

Anisotropic Optical Properties of Monolayer Aligned Single‐Walled Carbon Nanotubes

The 2D materials are the fundamental building blocks for modern optoelectronics and photonics. Optically anisotropic monolayers give even more flexibility in device design and performance. However, the random orientation of optical axes in the large‐scale samples prevents anisotropic monolayers from widespread use. The alternative structure is a monolayer of aligned single‐walled carbon nanotubes (SWCNTs) with an anisotropic dielectric tensor. Herein, aligned SWCNTs monolayer anisotropic optical constants in a broad spectral range (250–1700 nm) are measured for the first time. It is discovered that it has a large birefringence of Δn ≈ 0.2 and a high dichroism of Δk ≈ 0.4. Moreover, it is demonstrated that aligned SWCNTs monolayer optical response can be described by an effective medium approximation using the graphene dielectric function. In addition, it gives a universal approach for a determination of carbon concentration in nanotubes structures. It also applies for other types of carbon nanotubes, such as multi‐walled and randomly oriented carbon nanotubes arrays. Hence, in the results, aligned SWCNTs monolayer optical constants are added to the optical anisotropy database, which facilitates the longstanding challenge of using 1D structures in two dimensions, and a rapid characterization method for carbon nanotubes is provided.

First-principles property assessment of hybrid formate perovskites.

Hybrid organic-inorganic formate perovskites, AB(HCOO)3, are a large family of compounds that exhibit a variety of phase transitions and diverse properties, such as (anti)ferroelectricity, ferroelasticity, (anti)ferromagnetism, and multiferroism. While many properties of these materials have already been characterized, we are not aware of any study that focuses on the comprehensive property assessment of a large number of formate perovskites. A comparison of the properties of materials within the family is challenging due to systematic errors attributed to different techniques or the lack of data. For example, complete piezoelectric, dielectric, and elastic tensors are not available. In this work, we utilize first-principles density functional theory based simulations to overcome these challenges and to report structural, mechanical, dielectric, piezoelectric, and ferroelectric properties of 29 formate perovskites. We find that these materials exhibit elastic stiffness in the range 0.5-127.0GPa; highly anisotropic linear compressibility, including zero and even negative values; dielectric constants in the range 0.1-102.1; highly anisotropic piezoelectric response with the longitudinal values in the range 1.18-21.12 pC/N; and spontaneous polarizations in the range 0.2-7.8 μC/cm2. Furthermore, we propose and computationally characterize a few formate perovskites that have not been reported yet.

Dirac points, new photonic band gaps, and effect of magnetically induced transparency in dichroic cholesteric liquid crystals with wavelength-dependent magneto-optical activity parameter.

We investigated the properties of cholesteric liquid crystals (CLCs) being in an external static magnetic field directed along the helix axis. We considered a dichroic CLC, that is, CLC with parameters ReΔ=Reɛ_{1}-Reɛ_{2}/2=0 and ImΔ=Imɛ_{1}-Imɛ_{2}/2≠0, where ɛ_{1,2} are the principal values of the local dielectric permittivity tensor. We have shown that in the case of the wavelength dependence of the magneto-optic activity parameter, new features appear in the optics of dichroic CLCs, in particular, in this case new Dirac points appear. Dirac points are points where there is an intersection of any two wave vector curves (they degenerate) and a linear law of the wave vector dependence on the frequency near these points. And moreover, at some Dirac points photonic band gaps (PBGs) appear; at others, lines of magnetically induced transparency (MIT), that is, a full transmission band appears, in an absorbing medium. In this case a polarization-sensitive transmission band appears too. At certain values of the helix pitch of the CLC and of the magnitude of the external magnetic field, three PBGs of different nature appear: a transmittance band, two narrow lines of MIT, and at others a broadband MIT, etc. This system is nonreciprocal, and the nonreciprocity changes over a wide range. It is observed both for reflection and transmittance and for absorption. The soft-matter nature of CLCs and their response to external influences lead to easily tunable multifunctional devices that can find a variety of applications. They can be applied as tunable narrow-band or broadband filters and mirrors, a highly tunable broad/narrow-band coherent perfect absorber, transmitter, ideal optical diode, and in other devices.

Effective electromagnetic wave properties of disordered stealthy hyperuniform layered media beyond the quasistatic regime

Disordered stealthy hyperuniform dielectric composites exhibit novel electromagnetic wave transport properties in two and three dimensions. Here, we carry out the first study of the electromagnetic properties of one-dimensional 1D) disordered stealthy hyperuniform layered media. From an exact nonlocal theory, we derive an approximation formula for the effective dynamic dielectric constant tensor ε e (k q ,ω) of general 1D media that is valid well beyond the quasistatic regime and apply it to 1D stealthy hyperuniform systems. We consider incident waves of transverse polarization, frequency ω, and wavenumber k q . Our formula for ε e (k q ,ω), which is given in terms of the spectral density, leads to a closed-form relation for the transmittance T. Our theoretical predictions are in excellent agreement with finite-difference time-domain (FDTD) simulations. Stealthy hyperuniform layered media have perfect transparency intervals up to a finite wavenumber, implying no Anderson localization, but non-stealthy hyperuniform media are not perfectly transparent. Our predictive theory provides a new path for the inverse design of the wave characteristics of disordered layered media, which are readily fabricated, by engineering their spectral densities.

Investigation of electronic and optical properties of alkali atom doped CuInSe2 using density functional theory

In this study, the effect of alkali metal (Rb, Cs) doping on the electronic structure of CuInSe2 chalcopyrite have been investigated. The electronic structure of pure and doped chalcopyrites has been interpreted in terms of energy bands and density of states. The doping of Rb and Cs increases the band gap of CuInSe2 from 0.81 eV and attains its maximum value of 1.16 eV with 25% doping of Rb at Cu site. The forbidden gap of doped compounds is found to be suitable for optoelectronic and photovoltaic applications. Therefore, the investigations of various optical properties such as, dielectric tensors, absorption, reflection and refraction spectra, for Cu1−xAxInSe2 (A = Rb, Cs; x = 0, 0.125 and 0.25) compounds are performed to understand the optical performance of all these compounds. The imaginary part of dielectric tensor of pure and doped CuInSe2 are explained with the help of the various inter-band transitions. The refractive index for CuInSe2 is found to be 2.60 which reduces to 2.40 and 2.53 for Cu0.75Rb0.25InSe2 and Cu0.75Cs0.25InSe2 compounds, respectively. All investigations for Cu1−xAxInSe2 (A = Rb, Cs; x = 0, 0.125 and 0.25) compounds have been carried out using density functional theory. Present study shows that doping of Rb and Cs enhances the optoelectronic response of CuInSe2 for its utilization in photovoltaic and optoelectronic applications.

Thermal phonon modulation of III-nitride semiconductors under strong electric fields

Self-heating effects in high-power III-nitride semiconductor electronics and optoelectronic devices limit their practical applications, while phonon modulation can be used to alleviate heat dissipation problems. In this paper, we report our efforts to suppress macroscopic polarization fields when applying strong electric fields, which affect the phonon frequency. We investigate alterations in phonon frequencies for two materials, 2H-GaN and 2H-AlN, for various electric field strengths ranging from 0.00 V/Å to 0.07 V/Å and 0.00 V/Å to 0.18 V/Å, respectively. We also analyze the changes in the macroscopic dielectric tensor and the BORN effective charge tensor. The calculation results reveal that the vibration frequencies of the longitudinal-optical (LO) A1(LO) mode are decreased by approximately 2 cm−1 for 2H-GaN and 3 cm−1 for 2H-AlN. Our simulation results are consistent with the Raman spectroscopy results of Si-doped 2H-GaN specimens illuminated with 325 nm excitation for a variety of carrier concentrations. This study contributes to a better understanding of the factors influencing phonon behavior, thus offering new methods for optimizing the thermal regulation and dissipation patterns of III-nitride semiconductor materials and devices.

Magnetically enhanced THz generation by self-focusing laser in VA-MCNTs

Magnetically enhanced terahertz (THz) radiations are generated on account of the self-focusing of the laser beam in the bunch of anharmonic Vertically Aligned Metallic Carbon Nanotubes (VA-MCNTs) embedded on the non-conductive sapphire or silicon on sapphire (SOS) substrate. The high-power Gaussian laser beam gets self-focused in the bunch of VA-MCNTs as the initial power of the propagating beam is greater than its critical power. The resulting laser beam interacts with the bunch of VA-MCNTs and as a result, the electrons of MCNTs experience a nonlinear ponderomotive force to show oscillatory behavior with resonant nonlinear transverse velocity. It produces the nonlinear current which drives the THz radiation generation. Enhanced THz generation is noticed in the regions where self-focusing becomes stronger. We have observed that an applied magnetic field, anharmonic behavior of MCNTs, self-focusing, and dimensions of MCNTs also pave the way for the enhancement of the normalized THz amplitude. The anisotropic behavior of the dielectric tensor in the presence of an externally applied static magnetic field also helps to enhance the THz amplitude. The results shown (by the beautiful graphs and well supported by the numerical simulation) in the present scheme indicate that the bunch of VA-MCNTs can play a diverse and significant role in the important applications of THz medical photonics by varying the values of various parameters. The emitted THz radiation has the ability to detect changes in the DNA of human beings because the frequency of the emitted radiation is observed to lie in the frequency region of molecular spectra of DNA and the corresponding energy of THz radiation is not high enough to damage the DNA by ionization.

A first principle investigations for electronic, optical and mechanical response of novel Ba2AgIO6 perovskite

The double perovskite compound, Ba2AgIO6 has been studied through density functional theory to explore its mechanical, electronic, and optical properties. To examine the mechanical stability of the compound, elastic constants, Young’s modulus, bulk modulus and shear modulus have been computed. The computed electronic properties show the direct band gap semiconducting nature of the studied perovskite compound. The optical properties of Ba2AgIO6 are interpreted with the help of energy dependent dielectric tensor, absorption, reflection, refraction, and energy loss spectra. From the present study it is found that Ba2AgIO6 is suitable for the various photovoltaic and optoelectronic applications.

Anisotropic excitonic photocurrent in β−Ga2O3

Polarization-dependent photocurrent spectra are measured on a (001) $\ensuremath{\beta}\text{\ensuremath{-}}{\mathrm{Ga}}_{2}{\mathrm{O}}_{3}$ Schottky photodetector, where the linear polarization of light is rotated within the ab plane. Three spectral peaks at 4.92, 5.15, and 5.44 eV are observed that vary in intensity with the optical polarization direction. The peak transition energies are consistent with excitons previously reported in $\ensuremath{\beta}\text{\ensuremath{-}}{\mathrm{Ga}}_{2}{\mathrm{O}}_{3}$ due to interband transitions modified by the valence-band $p$-orbital anisotropy and the electron-hole Coulombic attraction. The measured polarization dependence of the photocurrent matches our predictions based on electromagnetic simulations of anisotropic absorption using the complex dielectric function tensor extracted from previous ellipsometry studies. These results illustrate the dominance of excitonic absorption and photocurrent in $\ensuremath{\beta}\text{\ensuremath{-}}{\mathrm{Ga}}_{2}{\mathrm{O}}_{3}$ both below and above the band gap, demonstrate a combined theoretical/experimental understanding of anisotropic photocarrier generation, and validate previous atomistic band-structure calculations in this low-symmetry ultrawide-band-gap semiconductor.

A point-to-point convolutional neural network for reconstructing electromagnetic parameters of multiple cavities scattering with inhomogeneous anisotropic media

We propose a point-to-point convolutional neural network (CNN) model to reconstruct the electromagnetic parameters of multiple cavities filled with inhomogeneous anisotropic media. The input of the model is a grid matrix of electromagnetic field magnitude solved by Petrov–Galerkin finite element interface method, and the output is a distribution matrix of the electromagnetic parameters of the cavities. The uniform non-body-fitted mesh is set in the computational domain, which does not need to be updated repeatedly at the complex interfaces of different media. Compared with the standard finite element, this mesh reduction method effectively saves the cost of mesh generation. Level set functions are applied to capture these arbitrarily shaped interfaces. The matrix coefficients caused by the dielectric constant and permeability tensors of anisotropic media can also be handled with this method. The point-to-point CNN model effectively reconstructs the value and the distribution of electromagnetic parameters in the cavities, also predicting the shape and the position of the interfaces. Three optimized schemes are then proposed to improve reconstruction accuracy. Numerical results show that the size of the convolution kernel and the number of fully connected layers are the crucial parameters in determining interfaces and reducing the number of artefacts.