Far-infrared Reflectivity Spectra Research Articles

Probing NaCl at High Pressure through Optical Studies and Ab Initio Calculations

The optical constants of sodium chloride in a wide pressure range were determined from the analysis of the reflectance and transmittance spectra of a minute quantity of NaCl powder placed in diamond anvil cells. The so-called “reststrahlen band” dominates the far-infrared reflectance spectra shifting from 150 cm–1 to 500 cm–1 at 100 GPa. For the 0–17.5 GPa pressure range, measurements allow accurate determination of both transverse and longitudinal mode frequencies. Higher pressure measurements reveal the B1 → B2 structural transition around 30 GPa and provide frequencies for the transverse and longitudinal modes. This spectroscopic signature on a sample smaller than 100 μm using light of a wavelength close to this dimension was observed, thanks to the high brilliance synchrotron source. In addition, ab initio calculations performed for the 0–200 GPa range predict the TO and LO frequencies. They are validated by the excellent agreement with the experiment.

Low-temperature infrared dielectric function of hyperbolic α -quartz

We report the infrared dielectric properties of $\alpha$-quartz in the temperature range from $1.5\ \mathrm{K}$ to $200\ \mathrm{K}$. Using an infrared free-electron laser, far-infrared reflectivity spectra of a single crystal $y$-cut were acquired along both principal axes, under two different incidence angles, in S- and P-polarization. These experimental data have been fitted globally for each temperature with a multioscillator model, allowing to extract frequencies and damping rates of the ordinary and extraordinary, transverse and longitudinal optic phonon modes, and hence the temperature-dependent dispersion of the infrared dielectric function. The results are in line with previous high-temperature studies, allowing for a parametrized description of all temperature-dependent phonon parameters and the resulting dielectric function from $1.5\ \mathrm{K}$ up to the $\alpha$-$\beta$-phase transition temperature, $T_C = 846\ \mathrm{K}$. Using these data, we predict remarkably high quality factors for polaritons in $\alpha$-quartz's hyperbolic spectral region at low temperatures.

NdNb1-x(Mg1/4W3/4)xO4 (0.02≤x≤0.06) solid solution characterized by infrared spectrum and complex chemical theory

The dependence of the bond characters and crystal structure on microwave dielectric properties of NdNb1-x (Mg1/4W3/4)xO4 (x = 0.02–0.06) solid solution ceramics is investigated. X-ray diffraction and Rietveld refinement analysis both indicate a formation of single monoclinic phase in the whole range. Based on complex chemical bond theory, theoretical dielectric constant of the complex crystal is slightly lower than observed one, which is consistent with the analysis of infrared reflectivity spectra. The Q × f (quality factor) values are associated with the total lattice energy and average grain size. Increment of Nb-site bond energy generates a more stable structure, which makes τf (temperature coefficient of resonant frequency) continuously move towards zero. Besides, far-infrared reflectivity spectra illustrates that low-frequency modes play primary role of intrinsic dielectric properties. Excellent microwave dielectric properties and an optimal combination are: εr = 19.59, Q × f = 50339 GHz, τf = −39.62 ppm/°C at x = 0.04.

Determining the Concentration of Free Electrons in n-InSb from Far-Infrared Reflectance Spectra with Allowance for Plasmon–Phonon Coupling

Contactless nondestructive testing is a means for determining the concentration of free electrons N in indium antimonide (InSb) samples from far-infrared reflectance spectra recorded at room temperature. A computer program capable of determining the characteristic wave number from the Kramers–Kronig relation is developed. The calculated calibration dependence makes it possible to determine the electron concentration from the known characteristic wave number. It is shown that this dependence is described by a cubic polynomial. In the calculations, the energy dependence of the electron effective mass is taken into account. It is established that, in determining the electron concentration, account must be taken of plasmon–phonon coupling, specifically at N ≤ 5 × 1017 cm–3. The systematic error introduced into the determination of N by disregard of plasmon–phonon coupling is estimated. The software elaborated here makes it possible to calculate the electron concentration N from experimental reflectance spectra and to store and process the results. The software is tested by the example of the reflectance spectrum of heavily doped n-InSb.

High conductivity of ultrathin nanoribbons of SrRuO3 on SrTiO3 probed by infrared spectroscopy

SrRuO3 (SRO) is a perovskite increasingly used in oxide-based electronics both for its intrinsic metallicity, which remains unaltered in thin films and for the ease of deposition on dielectric perovskites like SrTiO3, (STO) to implement SRO/STO microcapacitors and other devices. In order to test the reliability of SRO/STO also as high-current on-chip conductor, when the SRO dimensions are pushed to the nanoscale, here we have measured the electrodynamic properties of arrays of nanoribbons, fabricated by lithography starting from an ultrathin film of SRO deposited on a STO substrate. The nanoribbons are 6 or 4 nm thick, 400, 200 and 100 nm wide and 5 mm long. The measurements have been performed by infrared spectroscopy, a non-contact weakly perturbing technique which also allows one to separately determine the carrier density and their scattering rate or mobility. Far-infrared reflectivity spectra have been analyzed by Rigorous Coupled-Wave Analysis (RCWA) and by an Effective Medium Theory, obtaining consistent results. With the radiation polarized along the nanoribbons, we obtain a carrier density similar to that of a flat film used as reference, which in turn is similar to that of bulk SRO. Moreover, in the nanoribbons the carrier scattering rate is even smaller than in the unpatterned film by about a factor of 2. This shows that the transport properties of SRO deposited on STO remain at least unaltered down to nanometric dimensions, with interesting perspectives for implementing on-chip nano-interconnects in an oxide-based electronics. When excited in the perpendicular direction, the nanoribbons appear instead virtually transparent to the radiation field, as predicted by RCWA.

Phonon characteristics, crystal structures and intrinsic properties of non-stoichiometric Ba1+xWO4 ceramics

Scheelite-type non-stoichiometric Ba1+xWO4 (0 ≤ x ≤ 0.04) ceramics were synthesized by a conventional solid-state reaction method. X-ray diffraction patterns showed that the main phase is a tetragonal scheelite structure with space group I41/a. The vibrational modes were obtained from Raman spectra, which were assigned and illustrated. The intrinsic properties were obtained by Clausius-Mosotti equation (C–M) and the damping equation, and there is a positive relationship between the FWHM values of the Bg(Ba) modes and the dielectric losses (calculated by the damping equation), as well as a negative correlation between the Raman shifts of the Eg (near 355 cm−1) and the dielectric constants (deduced by C–M equation). The far-infrared reflectivity spectra were analyzed by the four-parameter semi-quantum (FPSQ) model to calculate the intrinsic properties of the samples, which agreed well with those values obtained by the measured data.

Phonon characteristics and dielectric properties of BaMoO4 ceramic

BaMoO4 ceramic was prepared using a conventional solid-sintering method. It was observed that the sample is a pure BaMoO4 with a tetragonal scheelite structure by analyzing X-ray diffraction data. Scanning electron microscopy characterized the dense sample with uniform grains. The phonon modes were analyzed by Raman and Far-infrared reflection spectra, and the phonon characteristics were studied. The intrinsic properties of the sample were calculated by the four-parameter semi-quantum model as well as the Clausius-Mosotti & damping equations, and the results were of εr = 9.388 and tanδ = 4.760 × 10−4, εr = 9.798 and tanδ = 6.445 × 10−4, respectively, which agrees well with the experimental values (εr = 9.84, tanδ = 5.0 × 10−4). The contributions to dielectric properties of each mode were investigated, and the results indicate that the external mode (Eu) yield greatest contribution to intrinsic permittivity and loss.

Far-infrared reflectivity spectra of nanotwinned GaV4Se8

ABSTRACT GaV Se is one of the lacunar spinel materials able to host magnetic skyrmionic textures. Earlier investigations at temperatures below its Jahn–Teller ferroelectric phase transition disclosed a nanoscale ferroelectric domain separated by 109 domain walls. Here, we have investigated the impact of this nanoscale twinning on polar phonon modes. We report dielectric and reflectivity spectra of GaV Se crystal in the phonon frequency region, derived from the ab initio predicted phonon frequencies, eigenvectors and Born-effective charges. Reflectivity spectra of the nanotwinned crystal, calculated using exact expressions for the effective macroscopic dielectric response of a perfect regular twin, are compared with the predicted reflectivity spectra of single-domain crystal. A characteristic spectral feature caused by the nanoscale twinning has been identified. While this feature is not a well-defined geometric resonance, it may still be accessible in experiments.

Crystal structure characteristics, intrinsic properties, and vibrational spectra of non-stoichiometric Ca1+xWO4 ceramics

Non-stoichiometric Ca1+xWO4 ceramics were determined in a composition range of −0.04 ≤ x ≤ 0.04 by a conventional solid-state method. X-ray diffraction patterns show that the main phase is a tetragonal scheelite structure. Scanning electron microscopy images exhibited dense and uniform grains. The vibrational modes were assigned and illustrated in particular. A positive correlation exists between the full-width at half-maximum values of the Bg(Ca) Raman modes and the intrinsic losses simulated according to the damping factor and also a negative correlation between the Raman shifts of the ν4 (Bg) modes and the dielectric constants calculated by the Clausius-Mossotti (C-M) equation. The intrinsic properties of the samples were calculated by the four-parameter semi-quantum (FPSQ) model based on the far-infrared reflectivity spectra, which agreed well with those values obtained by the C-M and damping equations. At x = −0.01, the Ca0.99WO4 ceramic sintered at 1150 °C possesses the intrinsic properties of εr = 10.62 and tan δ = 4.62 × 10−4 based on C-M and damping equations and εr = 10.19 and tan δ = 2.35 × 10−4 deduced from the FPSQ model.

Internal Relations between Crystal Structures and Intrinsic Properties of Nonstoichiometric Ba1+ xMoO4 Ceramics.

Ba1+ xMoO4 (-0.03 ≤ x ≤ 0.03) ceramics were fabricated by a conventional two-step sintering technique. X-ray diffraction patterns show that there appeared new diffraction peaks when x > 0, which were identified as Ba2MoO5. The Rietveld refinement results indicate that the unit cell volume is the largest at x = -0.02, because it has the lowest packing fraction and covalency. The far-infrared reflectivity (IR) spectra were fitted and analyzed for calculating the intrinsic properties, which comply well with the data obtained from microscopic polarizabilities and damping coefficient angle. The proportion of each mode in the dielectric response demonstrates that the Ba-O8 polyhedra have a decisive role on the dielectric properties. And based on the Raman modes, the internal relations of the structural-properties were revealed with the changes of Ba2+ content.

Determining the Free Carrier Density in Cd x Hg1–xTe Solid Solutions from Far-Infrared Reflection Spectra

A new contactless nondestructive technique for determining the free carrier density in single-crystal samples of Cd x Hg1–x Te solid solutions and multilayer epitaxial heterostructures based on them from farinfrared reflection spectra is proposed. The characteristic point and corresponding wavenumber in the room-temperature spectral dependence of the reflectance are determined. The heavy hole density is established using calculated calibration curves. It is shown that in constructing the calibration curves, it is necessary to take into account the interaction of plasma oscillations with longitudinal optical phonons.

Structural properties of Eu3+ doped Gd2Zr2O7 nanopowders: Far-infrared spectroscopy

The Solution Combustion Synthesis (SCS) method was used to prepare nanopowders of europium doped cubic Gd2Zr2O7 nanopowders. The surface of the samples have been investigated using atomic force spectroscopy (AFM) and far-infrared spectroscopy (FIR). Far-infrared reflectivity spectra of Eu3+ doped Gd2Zr2O7 nanopowders were measured at room temperature in spectral region between 80 and 650 cm−1. The Maxwell–Garnet formula was used to model dielectric function of Eu3+ doped Gd2Zr2O7 nanopowders as mixtures of homogenous spherical inclusions in air.

Bifurcations of Coupled Electron-Phonon Modes in an Antiferromagnet Subjected to a Magnetic Field.

We report on a new effect caused by the electron-phonon coupling in a stoichiometric rare-earth antiferromagnetic crystal subjected to an external magnetic field, namely, the appearance of a nonzero gap in the spectrum of electronic excitations in an arbitrarily small field. The effect was registered in the low-temperature far-infrared (terahertz) reflection spectra of an easy-axis antiferromagnet PrFe_{3}(BO_{3})_{4} in magnetic fields B_{ext}∥c. Both paramagnetic and magnetically ordered phases (including a spin-flop one) were studied in magnetic fields up to 30T, and two bifurcation points were observed. We show that the field behavior of the coupled modes can be successfully explained and modeled on the basis of the equation derived in the framework of the theory of coupled electron-phonon modes, with the same field-independent electron-phonon interaction constant |W|=14.8 cm^{-1}.

Theoretical modelling of infrared spectra of the twinned lead zirconate

The THz-frequency range dispersion of the complex dielectric permittivity tensor of antiferroelectric lead zirconate, associated with its multiple weak infrared active phonon modes, is modelled by the oscillator formula with parameters fully determined from the recently developed atomistic shell model. The resulting realistic dielectric function is used to estimate the far-infrared reflectivity spectra of a twinned lead zirconate crystal in the limit of narrow and wide domains. It is shown that in case of the sufficiently narrow domain widths, the effective-medium average permittivity shows additional modes identified as geometric resonances (i.e. extraneous excitations created by material interfaces), possibly distinguishable in suitable experiments.

Lattice Dynamics, Dielectric Constants, and Phase Diagram of Bismuth Layered Ferroelectric Bi3Ti1−xWxNbO9+δ Ceramics

Lattice dynamics and phase transitions of Aurivillius ferroelectric Bi3Ti1−xWxNbO9+δ (BTWN100x, 0 ≤ x ≤ 15%) ceramics have been investigated by temperature‐dependent Raman spectra (80–800 K) and far‐infrared (FIR) reflectance spectra (6–300 K). The frequency, intensity, and line width of phonon modes as well as complex dielectric functions of BTWN100x ceramics have been extracted by fitting Raman and FIR reflectance spectra with the multi‐Lorentzian oscillator model. It was found that the dielectric constants at a certain frequency region become lower with increasing W compositions at room temperature due to the compressive stress on W–O bands. Moreover, the temperature‐dependent behavior of optical phonon modes in BTWN100x ceramics indicates that there are two intermediate phases during the phase transition from paraelectric I4/mmm to ferroelectric A21am phase on cooling. The phase diagram of BTWN100x ceramics as a function of W composition has been improved.

Suppressed Magnetic Circular Dichroism and Valley-Selective Magnetoabsorption due to the Effective Mass Anisotropy in Bismuth.

We measure the far-infrared reflectivity and Kerr angle spectra on a high-quality crystal of pure semimetallic bismuth as a function of magnetic field, from which we extract the conductivity for left- and right-handed circular polarizations. The high spectral resolution allows us to separate the intraband Landau level transitions for electrons and holes. The hole transition exhibits 100% magnetic circular dichroism; it appears only for one polarization as expected for a circular cyclotron orbit. However, the dichroism for electron transitions is reduced to only 13±1%, which is quantitatively explained by the large effective mass anisotropy of the electron pockets of the Fermi surface. This observation is a signature of the mismatch between the metric experienced by the photons and the electrons. It allows for a contactless measurement of the effective mass anisotropy and provides a direction towards valley polarized magnetooptical pumping with elliptically polarized light.

Far-infrared spectra of mesoporous ZnS nanoparticles

ZnS nanoparticles were synthesized mechanochemically by high-energy milling, with three different milling times (5 min, 10 min and 20 min). Nitrogen adsorption method was used for examining specific surface area and texture of obtained powders. It was found that all samples are completely mesoporous. The optical properties were studied by far-infrared spectroscopy at room temperature in spectral region of 50–600 cm−1. The analysis of the far-infrared reflectivity spectra was made by the fitting procedure. The dielectric function of ZnS nanoparticles is modeled as a mixture of homogenous spherical inclusions in air by the Maxwell-Garnet formula. In the analysis of the far-infrared reflection spectra, appearance of combined plasmon-LO phonon modes (CPPMs) with high phonon damping are observed, which causes decrease of coupled plasmon-phonon frequencies.

Structural and optical properties of CuSe2 nanocrystals formed in thin solid Cu–Se film

This paper describes the structural and optical properties of Cu–Se thin films. The surface morphology of thin films was investigated by atomic force microscopy (AFM) and scanning electron microscopy (SEM). Formation of Cu–Se thin films is concluded to proceed unevenly, in the form of islands which later grew into agglomerates. The structural characterization of Cu–Se thin film was investigated using X-ray diffraction pattern (XRD). The presence of two-phase system is observed. One is the solid solution of Cu in Se and the other is low-pressure modification of CuSe2. The Raman spectroscopy was used to identify and quantify the individual phases present in the Cu–Se films. Red shift and asymmetry of Raman mode characteristic for CuSe2 enable us to estimate nanocrystal dimension. In the analysis of the far-infrared reflection spectra, numerical model for calculating the reflectivity coefficient of layered system, which includes film with nanocrystalite inclusions (modeled by Maxwell-Garnett approximation) and substrate, has been applied.

Novel glass-free low-temperature fired microwave dielectric ceramics: Bi(Ga1/3Mo2/3)O4

Novel glass-free low temperature firing microwave dielectric ceramics, Bi(Ga1/3Mo2/3)O4, with ordered scheelite structure, were prepared through a conventional solid state reaction method. Dense ceramics were prepared at sintering temperatures from 800 to 860°C. The Bi(Ga1/3Mo2/3)O4 compound was crystallized with B-site ordered Scheelite-type structure with space group C2/c. The Bi(Ga1/3Mo2/3)O4 ceramics can be well sintered at about 830°C, exhibiting good microwave dielectric properties of εr~26.1, Qf~49,800GHz, and TCF~−86ppm/°C. Intrinsic dielectric parameters of the Bi(Ga1/3Mo2/3)O4 ceramics were estimated by fitting far-infrared (FIR) reflectance spectra with the classical harmonic oscillator model.

Far-infrared spectroscopy of Zn1−xMnxGeAs2 single crystals: Plasma damping influence on plasmon – Phonon interaction

The interest in thorough description of Zn1–xMnxGeAs2 arises from its suitability for application in the field of non-linear optics. The room temperature far-infrared reflectivity spectra of single crystals Zn1–xMnxGeAs2, where 0 ≤ x ≤ 0.078, were measured in the spectral range from 80 cm−1 to 500 cm−1. The spectra were analyzed by fitting procedure using a dielectric function which includes interaction between a plasmon and two different phonons. The detected phonons are in excellent agreement with the theoretical predictions. The MnAs cluster phonons are detected, as well.