Negative Dielectric Function Research Articles

Electronic, Elastic, Optical, and Thermodynamic Properties Study of Ytterbium Chalcogenides Using Density Functional Theory

In this study, the structural, electronic, optical, elastic, and thermodynamic properties of Ytterbium chalcogenides YbX (X = S, Se and Te) were computed within the first principles using generalized gradient approximation (GGA) as implemented in the pseudopotential plane wave approach. The equilibrium total energy for YbX (X = S, Se, and Te) was calculated as a function of the energy cutoff, k-point grid, and lattice parameter. An optimized lattice parameter of 5.6, 5.66, and 6.136 Å were calculated for YbS, YbSe, and YbTe, respectively. The energy band gaps of YbS, YbSe, and YbTe computed are 1.14, 1.32, and 1.48 eV, respectively. In addition, the low band gap (less than 3 eV) for ytterbium chalcogenides indicated that they may have potential applications in photovoltaic cells and laser diodes. Moreover, the negative dielectric function value for a certain frequency range indicates that these compounds are suitable for specific optical and microwave circuit applications. The result of elastic and thermodynamic property computation reveals that ytterbium chalcogenides are mechanically and thermodynamically stable, which can be useful in a variety of electronic device applications.

Ab-initio calculation of halogen elements effect on the behavior of new double perovskite Cs2InSbX6 (X = F, Cl, Br, I): 1st part structural and optoelectronic properties

In recent years, there has been a growing fascination with lead-free perovskite materials, particularly in the realms of solar energy and photonics. It is imperative to conduct comprehensive theoretical investigations of these materials to effectively guide experimental pursuits. In this research, we harnessed the power of density functional theory (DFT) calculations, employing the Wien2k code, to delve into the distinctive characteristics of double-perovskite halides Cs2InSbX6 (X = F, Cl, Br, I). The viability and stability of their cubic structures were meticulously assessed, taking into consideration essential metrics such as the tolerance factor, octahedral factor and formation energy. Notably, the pronounced influence of spin–orbit coupling necessitated gap energy adjustments, achieved through a dual-pronged approach utilizing the generalized Perdew–Burke–Ernzerhof gradient approximation (PBE-GGA) and the modified Becke-Johnson (mBJ) exchange potential. The intricacies of band structure and state density were scrutinized using the GGA + mBJ method. The comprehensive findings elucidate that these compounds manifest fundamental electronic bandgaps spanning a range from 0.26 to 2.67 eV. Furthermore, they exhibit remarkably low carrier effective masses and a substantial degree of band dispersion. Of significance is the profound influence of halogen atom substitutions on their optical properties. Notably, a high absorption coefficient in the ultraviolet spectrum and the presence of a negative real dielectric function provide strong evidence for the adoption of metallic properties within a specific energy range. These compelling outcomes underscore the potential of these materials as highly promising candidates for a diverse array of photonic applications.

Relaxing Graphene Plasmon Excitation Constraints Through the Use of an Epsilon-Near-Zero Substrate

Graphene plasmons have attracted significant attention due to their tunability, potentially long propagation lengths and ultracompact wavelengths. However, the latter characteristic imposes challenges to light-plasmon coupling in practical applications, generally requiring sophisticated coupling setups, extremely high doping levels and/or graphene nanostructuring close to the resolution limit of current lithography techniques. Here, we propose and theoretically demonstrate a method for alleviating such a technological strain through the use of a practical substrate whose low and negative dielectric function naturally enlarges the graphene polariton wavelength to more manageable levels. We consider silicon carbide (SiC), as it exhibits a dielectric function whose real part is between \(-1\) and 0, while its imaginary part remains lower than 0.05, in the 951 to 970 cm\(^{-1}\) mid-infrared spectral range. Our calculations show hybridization with the substrate’s phonon polariton, resulting in a polariton wavelength that is an order of magnitude longer than obtained with a silicon dioxide substrate, while the propagation length increases by the same amount.

Instantons, colloids and convergence of the 1/N expansion for the homogeneous electron gas

We investigate non-perturbative corrections to the large N expansion of the homogeneous electron gas. These are associated with instanton solutions to the effective action of the plasmon field. We show that, although the large field behavior of that action dominates the quadratic bare Coulomb term, there are no solutions at large field, and consequently none at large density. We argue that solutions would exist at low density if the large N theory had a Wigner crystal (WC) phase. However, we argue that this is not the case. Together with the implied convergence of the large N expansion, this implies that the homogeneous electron gas with N component spins and a Coulomb interaction scaling like 1∕N can only have a WC phase below a curve in the plane of N and density, which asymptotes to zero density at infinite N. We argue that for systems with a semi-classical expansion for order parameter dynamics, and a first order quantum transition between fluid and crystal phases, there are instantons associated with the decays of meta-stable fluid and crystal phases in the appropriate regions of the phase diagram. We argue that the crystal will decay into one or more colloidal or bubble phases (Kivelson and Spivak, 2006) rather than directly into the fluid. It is possible that the bubble phases remain stable all the way to the density where the crystal solution disappears. The bubbles of fluid will expand as the density is raised and gradually convert the system into a fluid filled with chunks of crystal. The transition to a translationally invariant phase is likely to be second order. Unfortunately, the HEG does not have a crystal phase at large N, where these semi-classical ideas could be examined in detail. We suggest that the evidence for negative dielectric function at intermediate densities for N=2 is an indicator of this second order transition. While the infinite N limit does not have a negative dielectric function at any density, it is possible that the closed large N equation for the plasmon two point function, derived in Banks (2018) might capture at least the qualitative features of the second order transition.

Epsilon‐Negative Active Composites: Loss‐Free and Amplifying Plasmonic Materials

The main issue restricting the applications of plasmonics is the intrinsic losses in metals. In order to mitigate the effects of losses, plasmonic systems embedded with gain layers have extensively been investigated over the years. However, the ultimate solution is to use loss‐free or amplifying epsilon‐negative materials. This work reveals that the goal can be achieved by using gain composite materials containing metal nanoparticles as building blocks for plasmonics. By controlling the gain coefficient of the host medium and the filling factor of metal nanoparticles, one can significantly change the magnitude of the negative effective dielectric function while maintaining gain. Their superior performances are evidenced by the evaluations of quality factors for localized and propagating plasmons, and also the detailed studies of spasing behaviors of the composite nanoparticles and hyperbolic responses of their multilayered structures. The findings presented in this work introduce a new paradigm of plasmonics.

Dynamic depolarization in plasmonic metal nanoparticles

At very low photon energies most metals have a very large and negative dielectric function. For the response of a metal nanoparticle to an external field in this limit, this means that the particular choice of metal does not matter and the localized surface plasmon energy mainly depends on the shape and size of the particle. Here, we present a theoretical framework to describe this situation and unearth the interplay between the depolarization factor of the problem at hand and the dielectric function of the particle. Available experimental results compare favorably with our theoretical framework.

High-Temperature Superconductivity in a Hyperbolic Geometry of Complex Matter from Nanoscale to Mesoscopic Scale

While it was known that High Temperature Superconductivity appears in cuprates showing complex multiscale phase separation due to inhomogeneous charge density wave (CDW) order, the spatial distribution of CDW domains remained an open question for a long time, because of the lack of experimental probes able to visualize their spatial distribution at mesoscale, between atomic and macroscopic scale. Recently scanning micro X-ray Diffraction (SmXRD) revealed CDW crystalline electronic puddles with a complex fat-tailed spatial distribution of their size. In this work we have determined and mapped the anisotropy of the Charge Density Waves (CDW) puddles in HgBa2CuO4+y (Hg1201) single crystal. We discuss the emergence of high temperature superconductivity in the interstitial space with hyperbolic geometry, that opens a new paradigm for quantum coherence at high temperature where negative dielectric function and interference between different pathways can help to raise the critical temperature.

Dielectric response of II–VI semiconductor core–shell ensembles: Study of the lossless optical condition

We theoretically investigate optical properties of II–VI core–shell distribution mixtures made of two type-I sized-nanoshells as a plausible negative dielectric function material. The nonlocal optical response of the semiconductor QD is described by using a resonant excitonic dielectric function, while the shell response is modeled with Demangeot formula. Achieving the zero-loss at an optical frequency ω, i.e., ϵeff=ϵeff′+iϵeff″ with ϵeff′<0 and ϵeff″=0, is of fundamental importance in nanophotonics. Resonant states in semiconductors provide a source for negative dielectric function provided that the dipole strength and the oscillator density are adequate to offset the background. Furthermore, the semiconductor offers the prospect of pumping, either optically or electrically, to achieve a gain mechanism that can offset the loss. We analyse optimal conditions that must be satisfied to achieve semiconductor-based negative index materials. By comparing with II–VI semiconductor quantum dots (QDs) previously reported in the literature, the inclusion of phonon and shell contributions in the ϵeff along with the finite barrier Effective Mass Approximation (EMA) approach, we found similar qualitative behaviours for the ϵeff. The lossless optical condition along with ϵeff′<0 is discussed in terms of sizes, volume fractions and embedding medium of the mixtures׳ distributions. Furthermore, we estimated optical power to maintain nanocrystals density in excited states and this value is less than that previously obtained in II–VI semiconductor QDs.

Spin waves and dielectric softening of polar molecule condensates.

We consider an oblate Bose-Einstein condensate of heteronuclear polar molecules in a weak applied electric field. This system supports a rich quasiparticle spectrum that plays a critical role in determining its bulk dielectric properties. In particular, in sufficiently weak fields the system undergoes a polarization wave rotonization, leading to the development of textured electronic structure and a dielectric instability that is characteristic of the onset of a negative static dielectric function.

A dielectric superfluid of polar molecules

We show that, under achievable experimental conditions, a Bose–Einstein condensate of polar molecules can exhibit dielectric character. In particular, we derive a set of self-consistent mean-field equations that couple the condensate density to its electric dipole field, leading to the emergence of polarization modes that are coupled to the rich quasi-particle spectrum of the condensate. While the usual roton instability is suppressed in this system, the coupling can give rise to a phonon-like instability that is characteristic of a dielectric material with a negative static dielectric function.

On the dielectric function tuning of random metal-dielectric nanocomposites for metamaterial applications

The potential of random metal-dielectric nanocomposites as constituent elements of metamaterial structures is explored. Classical effective medium theories indicate that these composites can provide a tunable negative dielectric function with small absorption losses. However, the tuning potential of real random composites is significantly lower than the one predicted by classical theories, due to the underestimation of the spectral range where topological resonances take place. This result suggests that a random mixture consisting of a metal matrix with embedded isolated dielectric inclusions is a promising design guideline for the fabrication of tunable composites for metamaterial purposes.

Structural and electronic properties of expanding fluid metals

It has been theoretically predicted that interacting electron gas suffers a negativecompressibility when the electron density is sufficiently reduced. Dielectric anomaly isanother expression of compressional instability in which the static dielectric function ofelectron gas changes sign from positive to negative. In a medium with a negativedielectric function, like charges can attract. Expanding fluid alkali metals arethe ideal system in which such instability of electron gas can be probed via thestructural response of ions. We have performed x-ray diffraction and small angle x-rayscattering measurements using synchrotron radiation for expanding fluid rubidiumand cesium. On the basis of the experimental results we discuss the existence ofcompressional instability and dielectric anomaly in three-dimensional electrongas.

Overscreening mechanism for room temperature superconductivity

My view for room temperature superconductivity and a mechanism for the superconductivity are presented. The common and different points for conventional and high T c copper-oxide superconductors are discussed. In the oxides the conduction-carrier concentrations are relatively small and the highly correlated carriers strongly interact with lattice vibrations. As a result, the dispersion of the in-plan Cu–O bond-stretching longitudinal optical phonon mode is strongly softened near the Brillouin zone boundary, as seen in the experiments of neutron scattering and photoemission. It can be described with a negative dielectric-function that results in an overscreening of the intersite Coulomb interaction due to phonon-induced charge transfer and vibronic electron phonon resonance. The Coulomb interaction renormalized by the overscreening effect works for an attractive interaction between carriers in regions near the zone boundary. The interaction generates the superconductivity with the transition temperature well in excess of 100 K. On the basis of the overscreening mechanism, we propose the following guiding principle for searching materials with superconducting transition temperatures above the room temperature. The materials should have relatively small conduction-carrier concentrations, and thus the systems are of highly correlated. The materials should contain light atoms that have large lattice vibration amplitudes, and the carriers interact strongly with vibrating atoms.

Optical excitation of surface phonon polaritons in silicon carbide by a hole array fabricated by a focused ion beam

Silicon carbide (SiC) is a polar material with a lattice resonance in the thermal infrared causing a wavelength interval with a negative dielectric function. Within this interval SiC can support surface waves. To excite surface waves, i.e., surface phonon polaritons (SPP), the sample has to be structured with a periodic micro-pattern. The possibilities of using a focused ion beam (FIB) for microfabrication of periodic microstructures in silicon carbide (SiC) is investigated. We present optimized parameters for the microfabrication of SiC with a FIB, as well as calculated and experimental optical results confirming the sensitive optical properties of the material required for the surface excitation are not destroyed by the preparation process.

Dielectric, magnetic and optical properties of nanocomposites

Nanocomposites are media made of nanoparticles embedded in a matrix. Depending on the nanoparticle types and shapes, nanocomposites can have different properties. I discuss here dielectric and magnetic composites at low, as well as, high frequencies. At low frequencies, dielectric and magnetic properties can be enhanced by using elongated nanoparticles. At high frequencies various photonic, plasmonic and polaritonic effects can occur in nanocomposites, in particular in those with periodic arrangement of nanoparticles. The negative refraction can be achieved in nanocomposites with simultaneously negative dielectric function and magnetic permeability.

Dispersive contour-path finite-difference time-domain algorithm for modeling surface plasmon polaritons at flat interfaces

We investigate the accuracy of the two-dimensional Finite-Difference Time-Domain (FDTD) method in modelling Surface Plasmon Polaritons (SPPs) in the case of a single metal-dielectric interface and of a thin metal film showing that FDTD has difficulties in the low-group-velocity region of the SPP. We combine a contour-path approach with Z transform to handle both the electromagnetic boundary conditions at the interface and the negative dispersive dielectric function of the metal. The relative error is thus significantly reduced.

Vibronic mechanism of high-Tcsuperconductivity

The dispersion of the in-plane Cu-O bond-stretching longitudinal optical phonon mode in the high-${T}_{c}$ superconducting cuprates shows strong softening with doping near the zone boundary. We suggest that it can be described with a negative electronic dielectric function that results in an overscreening of the intersite Coulomb interaction due to phonon-induced charge transfer and vibronic electron-phonon resonance. We propose that such a strong electron-phonon coupling of specific modes can form a basis for the phonon mechanism of high-temperature superconductivity. With the Eliashberg theory using the experimentally determined electron dispersion and dielectric function, we demonstrate the possibility of superconductivity with the order parameter of the ${d}_{{k}_{x}^{2}\ensuremath{-}{k}_{y}^{2}}$ symmetry and the transition temperature well in excess of 100 K.

Loop-wire medium for investigating plasmons at microwave frequencies

We present numerical simulations and microwave measurements on a loop-wire structure that acts as an effective medium exhibiting a well-defined bulk plasma frequency in the microwave regime, with an effective negative dielectric function below this plasma frequency. The dependence of this plasmonic response on the self-inductance of the constituent wire elements is made explicit. A finite structure, approximately spherical, composed of this inductive medium is studied, and reveals subwavelength surface plasmon resonances below the bulk plasma frequency.