Far-infrared Reflectivity Spectra Research Articles

Mixed vibrational modes of PbTe nanocrystallites

Far-infrared reflectance (20–700 cm−1) and Raman spectra of porous PbTe were studied. The spectral dependences found significantly differ from those observed in the case of bulk single-crystal PbTe. Aside from the transverse (TO) and longitudinal (LO) optical phonon modes, additional vibrational modes were detected in the spectra, whose frequencies are close to those in a bulk material. Their number and spectral positions are in satisfactory agreement with the results of calculations carried out to determine the vibrational mode frequencies for nanocrystallites taking into account boundary conditions at their surface. The macroscopic continuum model allows calculation of the frequencies of the mixed phonon modes in quantum dots by the simultaneous consideration of atomic displacements and the accompanying change in the electrostatic potential. It was found experimentally that the detected modes are mixed (LO-TO), as was predicted by theory. The data of atomic-force microscopy allowed us to estimate the average radius of quantum dots, which form the nanorelief of the porous PbTe layer.

Optical properties of SrSe thin films grown by molecular beam epitaxy

Absorption, reflection and Raman scattering measurements have been carried out for the optical properties of a SrSe thin film grown by molecular beam epitaxy on a BaF2 (111) substrate. The film quality and lattice constant have been assessed by x-ray diffraction, where the good quality of the SrSe thin film is evident in a full width at half maximum value of 310 arcsec. Temperature-dependent indirect band gaps are obtained with the aid of temperature-dependent absorption measurements from 16 to 300 K on the SrSe thin film. By fitting the far-infrared reflection spectra, transverse optical phonon reflection bands in the binary SrSe thin film have been assigned, which are further verified by temperature-dependent Raman scattering measurements.

Far-infrared analysis of lattice vibrations in ZnSe/ZnCdSe superlattices

The results of the first measurements of far-infrared lattice reflection spectra of ZnSe/ZnCdSe superlattices grown on a GaAs substrate by molecular-beam epitaxy are presented. It is shown by a dispersion analysis of the experimental spectra measured at temperatures 300 and 10 K that, in the superlattices under study, instead of two TO modes corresponding to two different layers ZnSe and ZnCdSe of the superlattice only one TO mode is observed which has a frequency between the frequencies of the two layers. The existence of a single transverse mode in the lattice reflectivity spectrum of the ZnSe/ZnCdSe superlattice has been explained by a convergence of the vibrational modes under the effect of internal elastic strains.

Observation of low energy optical phonon in far-infrared spectra of β-phase in CuI

Far-infrared reflectivity spectra of a sintered CuI have been measured as a function of temperature through the γ-, β- and α-phase. Three bands were observed near 120, 70 and 30 cm −1 in the β(wurtzite-type)-phase. A reststrahlen band shows a rapid broadening with increasing temperature. For the latter two bands, the possibility for the LA-mode at the A-point and the E 2-mode at the zone center is discussed, and the relationship between the frequency of low energy optical phonon mode and the ionic activation energy of the α-phase is established.

ＩＴＯ（Ｉｎｄｉｕｍ Ｔｉｎ Ｏｘｉｄｅ）単結晶成長および電気・光学的性質の研究

The demand for the transparent conductors or semi-conductors in the visible region, which have good transparency and electric conductivity, is increasing with the development of electro-optical devices. Recently, ITO films with main element of In2O3 doped with low concentration of SnO2 have been prepared. However, there are few reports about the single crystal of In2O3- SnO2 (ITO). The aim of this study is to know the defect structure minutely, which is typical in these oxide materials. ITO single crystals were prepared by the flux method. The structure was characterized by X-ray diffraction analysis, and free carriers were evaluated through the dispersion analysis of far-infrared reflection spectra.

Far-infrared reflection spectra of PbSrSe thin filmsgrown by molecular beam epitaxy

We report the far-infrared (FIR) reflection measurements of Pb1-xSrxSethin films with different Sr concentrations grown by molecular beam epitaxy on BaF2 substrates. Sr composition and lattice parameter have been assessed by x-ray diffraction. By fitting the FIR reflection spectra and by comparison with the results of binary PbSe and SrSe thin films, both PbSe-like and SrSe-like optical phonon reflection bands in ternary PbSrSe thin films have been identified. Also, the carrier concentration and mobility of PbSrSe thin films have been extracted from the theoretical calculation of the reflection spectra, and are found to be in agreement with reported results from Hall measurements.

Temperature dependences of phonon and mobile-ion spectra in superionic conductor CuBr

The temperature dependence of far-infrared reflectivity spectra in sintered CuBr has been measured from 18 to 570 K. Phonon and mobile-ion parameters are evaluated by the fitting procedure with the factored form of the dispersion relation. A strong fourth order anharmonicity is assigned. Estimated values of optical dielectric constant and optical ionic mobility μ op decrease with increasing temperature within a limited temperature region. For μ op value, the effects of mobile ion scattering by optical phonons and of the expansion of interionic distance are discussed with respect to the phonon damping constant and the ionic character of the interionic force.

Far-Infrared Optical Response of Neutral-Ionic Phase Transition in an Organic Charge-Transfer Complex

Infrared optical spectra have been investigated for a charge-transfer complex of tetrathiafulvalene and 2-bromo-3,5,6-trichloro- $p$-benzoquinone, which undergoes a neutral-ionic ( $N\ensuremath{-}I$) transition at ${T}_{c}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}68\mathrm{K}$. With the decrease of temperature toward ${T}_{c}$, the far-infrared reflectivity spectrum is gradually transformed into a Drude-like high reflectance band edging around $100{\mathrm{cm}}^{\ensuremath{-}1}$, and the optical conductivity shows an anomalous peak around $10{\mathrm{cm}}^{\ensuremath{-}1}$. The far-infrared anomaly is related to the gigantic dielectric response just above ${T}_{c}$, which are both discussed in terms of dynamics of $N\ensuremath{-}I$ domain walls with fractional charge.

Enhanced interionic forces above Tc in ABX 3-type superionic conductors

In order to investigate interionic forces in ABX 3-type superionic conductors, we measure far-infrared reflectivity spectra of CsPbCl 3 in the temperature range from 40 to 381 K. Four phonon bands are observed in the low temperature region, and a contribution of the fourth order anharmonicity to the phonon damping constant is suggested. A rapid increase and decrease in bindings between the frame ions, and between the mobile and frame ions at T c, respectively, are suggested from the long-range force f L, and short-range force f S, that are estimated from the two high frequency bands in CsPbCl 3 and in Ag 3SX (X=Br, I), using the point charge model. These forces are related to several relationships for the ion conduction.

Far-infrared spectra of Pb 1− xMn xTe alloys

Far-infrared reflectivity spectra of Pb 1− x Mn x Te (0.0001⩽ x⩽0.1) single crystals were measured in the 10–250 cm −1 range at room temperature. The analysis of the far-infrared spectra was made by a fitting procedure based on the model of coupled oscillators. In spite of the strong plasmon–LO phonon interaction, we found that the long wavelength optical phonon modes of these mixed crystals showed an intermediate one–two mode behavior.

C-Axis Optical Response in the Static Stripe Ordered Phase of the Cuprates

The c-axis far-infrared reflectivity spectra in the superconducting state were investigated for La1.85-yNd(y)Sr0.15CuO4. The Josephson plasma edge rapidly shifts towards lower frequency with increasing y, and almost disappears when y exceeds a critical value ( y(c) approximately 0.12) above which the low-temperature-tetragonal (LTT) deformation switches on, stabilizing the spin/charge ordered stripe phase. In the vicinity of y(c), a "normal state" reentrant behavior was observed with lowering temperature below Tc. This clearly demonstrates that the static stripe order pinned by the LTT distortion suppresses the interlayer phase coherence.

Microwave dielectric properties of (Zr0.8Sn0.2)TiO4 ceramics doped with WO3

Effects of WO3 on the microwave dielectric properties and infrared reflectivity spectra of (Zr0.8Sn0.2)TiO4 were investigated. Microwave dielectric properties of the specimens were enhanced by addition of WO3 up to 1.0 mol.%, which is due to the decrease of the oxygen vacancies and the increase of grain growth. They deteriorate above 2.0 mol.% of WO3, which is due to inhibition of grain growth and formation of pores at the grain boundaries. The specimens with 1.0 mol.% WO3 and 0.35 wt.% (B2O3Li2O) sintered at 1300°C for 5 h showed the dielectric constant of 38.1, Q.f values of 57,021GHz, and the temperature coefficient of resonance frequency of-4.8 ppm/°C. The intrinsic microwave losses in the specimens were evaluated from the far-infrared reflectivity spectra.

Raman scattering and infrared reflectivity in [(InP)5(In0.49Ga0.51As)8]30 superlattices

We have measured far-infrared and infrared reflectivity as well as Raman scattering in an [(InP)5(In0.49Ga0.51As)8]30 superlattice grown by molecular beam epitaxy. A numerical model for calculating the reflectivity coefficient for complex systems which includes superlattice, buffer layer, and substrate has been developed. The far-infrared reflectivity spectra consists of the superlattice confined and interface modes as well as the modes from the buffer layer (In0.49Ga0.51As) and the substrate (InP). In the infrared spectral range above 1000 cm−1 we observe only interference fringes from the buffer layer. A good agreement between calculated and experimental spectra is achieved. The folded longitudinal acoustic phonon doublet appears at about 39 cm−1 in the Raman scattering spectra. The frequency agrees well with a continuum model calculation. In the optical phonon spectral region we observe confined modes corresponding to both constituents. The modes representing vibrations of atoms at both interfaces: InP/InGaAs (230 cm−1 mode) and InGaAs/InP (240 and 260 cm−1 modes) have also been observed. The geometrical parameters of the sample, obtained from the fitting of the reflectivity data, agree well to the values of the layer thickness obtained by double crystal x-ray diffraction.

Temperature dependence of dielectric and phonon properties in superionic conductor CuI

Roles of interionic ( I) forces on the mobile defects are investigated by measuring far-infrared reflectivity (FIR) spectra of sintered CuI as a function of temperature ( T) below 300 K. Anomalous increase was observed for the phonon damping constant γ ph and long-range Coulomb force f L. From this T-dependence and small increase of effective charge with T, we point out the effective role of the ionic character of the I force on ionic conduction.

Small band gap and high ionic conduction in Cu 2S

Far-infrared reflectivity spectra of Cu 2S have been measured from 20 to 700 cm −1 at several temperatures through the superionic transition temperature T c. Many bands and two broad bands were observed at low and high temperatures, respectively. From the spectra, we determined optical dielectric constant ε ∞, the angular frequencies of longitudinal (LO) and transverse (TO) optical phonons, and transverse effective charge. Based on these values, we suggest significant role of small band gap for high ionic conduction with small activation energy and the possibility of lattice distortion arising from an enhanced effective charge. Anomalous temperature dependence has been found for ε ∞ and TO–LO splitting.

Observation of the Josephson plasma reflectivity edge in the infrared region in Bi-based superconducting cuprates

The far-infrared c-axis optical reflectivity spectra of a heavily Pb-doped Bi2212, ${\mathrm{Bi}}_{1.6}{\mathrm{Pb}}_{0.6}{\mathrm{Sr}}_{1.8}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{y},$ single crystal ${(T}_{c}=65$ K) were measured in the superconducting state. The crystal was found to be carrier overdoped as the optical phonons are weakly screened by the electronic excitations in the far-infrared region. Below ${T}_{c},$ a sharp reflectivity edge was observed, corresponding to the appearance of the Josephson plasma in the superconducting state. The plasma edge frequency reached 40 ${\mathrm{cm}}^{\ensuremath{-}1}$ at 8 K, which is comparable with that observed for other cuprates with much less anisotropy. The c-axis penetration depth ${\ensuremath{\lambda}}_{\mathrm{c}}=12.6\ensuremath{\mu}\mathrm{m}$ was estimated from the unscreened plasma frequency ${\ensuremath{\omega}}_{\mathrm{ps}}^{c}=126.2{\mathrm{cm}}^{\ensuremath{-}1}.$ This ${\ensuremath{\lambda}}_{c}$ value is one order of magnitude smaller than that of the optimally doped pure Bi2212, reflecting a drastic enhancement of the interlayer coupling in Pb-doped Bi2212.

Far-infrared reflectivity spectroscopy of cuprous chloride, cuprous bromide and their mixed crystals

The temperature dependence of the far-infrared reflectivity spectrum of CuCl, CuBr and CuCl1 - xBrx, in the zinc-blende structure, is reported over a wide temperature range (10-300 K). The analysis of the experimental data proves the activity of two polar lattice modes in both CuCl and CuBr: the expected lattice mode labelled and an additional lattice mode labelled . The present work displays, for the first time, the activity of three lattice modes in the far-infrared reflectivity spectrum of CuCl1 - xBrx, from room temperature down to liquid helium temperature. Side-bands due to two-phonon processes are observed in the low-frequency range of the reflectivity spectrum of both pure and mixed crystals. The phonon anomalies observed in both pure and mixed crystals are discussed and explained in agreement with the off-centre model.

Far-infrared conductivity of the PbZr0.52Ti0.48O3 and Cu-doped PbTiO3 ferroelectric thin films

We report far-infrared reflectivity spectra of the PbZr0.52Ti0.48O3 and copper doped PbTiO3 single-crystal films focusing on the electronic properties. The conductivity has been analyzed by the Drude-Lorentz model and in terms of dynamic interaction, which leads to a frequency dependent scattering rate and enhanced low-frequency carrier mass. Introduction of 0.5 mas.% Cu in PbTiO3 decreases effective mass from 3.5 to 1.5 near 200 cm−1 and increases relaxation time compared with the undoped sample.

Far-infrared study of impurity local modes in gallium-doped PbTe

We present far-infrared reflection spectra and results of galvanomagnetic measurements of PbTe single crystals doped with gallium between 10 and 300 K. The analysis of the far-infrared reflection spectra was made by a fitting procedure based on the model of coupled oscillators. Together with the strong plasmon–phonon coupling we obtain three local modes of gallium at about 122, 166 and 192 cm −1. The position of these modes depends of impurity center charge, and their intensity depends of temperature and of gallium concentration. Persistent photoconductivity effect was registered in the sample with 0.4 at.% Ga by galvanomagnetic and far-infrared measurements.

Far-infrared reflectivity spectra of CaTiO 3–Li 1/2Sm 1/2TiO 3 microwave dielectrics

Microwave dielectric properties and far-infrared reflectivity spectra of the (1 − y)CaTiO 3–yLi 1/2Sm 1/2TiO 3 ceramics were investigated in the range of 0.0 ≤ y ≤ 1.0. The dielectric constant, unloaded Q value, and temperature coefficients of the resonant frequency (TCF) of the specimens decreased with increasing Li 1/2Sm 1/2TiO 3. At y = 0.7, a nearly zero TCF value was obtained, with a high dielectric constant (K = 114) and an unloaded Q (Q·f = 3700). Based on analysis of the far-infrared reflectivity spectra from the 50 to 4000 cm −1 range, which were evaluated using Kramers–Kronig analysis and the classical oscillator model, the change in the Q·f value and dielectric constant of the specimens could be explained by intrinsic factors such as ionic polarization and dielectric loss (tan δ j). The relative tendency of calculated dielectric constant and Q·f values agreed well with that of the measured values.