Infrared-active Phonons Research Articles

Optical conductivity of rattling phonons in type-I clathrateBa8Ga16Ge30

A series of infrared-active optical phonons have been detected in type-I clathrate Ba$_8$Ga$_{16}$Ge$_{30}$ by terahertz time-domain spectroscopy. The conductivity spectra with the lowest-lying peaks at 1.15 and 1.80 THz are identified with so-called rattling phonons, i.e., optical modes of the guest ion Ba$^{2+}(2)$ with $T_{1u}$ symmetry in the oversized tetrakaidecahedral cage. The temperature dependence of the spectra from these modes are totally consistent with calculations based on a one-dimensional anharmonic potential model that, with decreasing temperature, the shape becomes asymmetrically sharp associated with a softening for the weight to shift to lower frequency. These temperature dependences are determined, without any interaction effects, by the Bose-factor for optical excitations of anharmonic phonons with the nonequally spaced energy levels.

Femtosecond laser excitation of coherent optical phonons in ferroelectricLuMnO3

We have used femtosecond pump-probe spectroscopy to excite and probe coherent optical phonon vibrations in single crystals of hexagonal ferroelectric ${\text{LuMnO}}_{3}$. An optical phonon mode of ${A}_{1}$ symmetry was coherently excited with 25 fs pump-laser pulses $(\ensuremath{\lambda}\ensuremath{\approx}800\text{ }\text{nm})$. The phonon mode, involving Lu ion motion along the $c$ axis, was identified as the soft mode driving the ferroelectric transition. The excitation mechanism was determined to be purely displacive in nature due to resonant excitation of a narrow intra-atomic ${d}_{xy,{x}^{2}\ensuremath{-}{y}^{2}}\ensuremath{\rightarrow}{d}_{3{z}^{2}\ensuremath{-}{r}^{2}}$ transition in Mn. The lifetime of the $\text{Mn}\text{ }{d}_{xy,{x}^{2}\ensuremath{-}{y}^{2}}\ensuremath{\rightarrow}{d}_{3{z}^{2}\ensuremath{-}{r}^{2}}$ excitation was measured to be 0.8 ps. A remarkable reversal of the sign of the oscillation amplitude ($\ensuremath{\pi}$ phase shift) of the reflectivity curve was observed upon comparing longitudinal-optical (LO) with transverse-optical (TO) mode geometries. The phase reversal is attributed to the macroscopic electric depolarization field accompanying infrared-active longitudinal phonon modes but absent in TO modes. In addition to the direct effect of the ion motion on the optical properties, which is the same in LO and TO modes, the longitudinal depolarization field of the LO mode gives rise to an additional modulation of the refractive index via the linear electro-optic effect which dominates the optical response.

Electron-phonon interaction and spectral weight transfer inFe1−xCoxSi

A comprehensive ellipsometric study was performed on Fe$_{1-x}$Co$_{x}$Si single crystals in the spectral range from 0.01 eV to 6.2 eV. Direct and indirect band gaps of 73 meV and 10 meV, respectively, were observed in FeSi at 7 K. One of four infrared-active phonons that is energetically close to the direct absorption edge is coupled both to the electrons and to the low-energy phonon. This is evident from asymmetry in the phonon line shape and a reduction of its frequency when the absorption edge shifts across the phonon energy due to the temperature dependence of the direct band gap. As the temperature increases, the indirect gap changes sign, which manifests as a transition from a semiconductor to a semimetal. The corresponding gain of the spectral weight at low energies was recovered within an energy range of several eV. The present findings strongly support the model indicating that Fe$_{1-x}$Co$_{x}$Si can be well described in an itinerant picture, taking into account self-energy corrections.

Infrared spectra of the low-dimensional quantum magnetSrCu2(BO3)2: Measurements andab initiocalculations

The reflectance of the insulating quasi-two-dimensional quantum magnet ${\text{SrCu}}_{2}{({\text{BO}}_{3})}_{2}$ has been examined over a wide temperature and frequency range for light polarized parallel ($a$ axis) and perpendicular ($c$ axis) to the copper- and boron-oxygen sheets. The spectra have been measured for temperatures below the structural phase transition ${T}_{s}=395\text{ }\text{K}$ for both polarizations; above ${T}_{s}$ a limited study of the in-plane properties was undertaken in the far-infrared region only. Several new modes appear in the reflectance just below ${T}_{s}$ along the $a$ and $c$ axes, while others are visible only for $T⪡{T}_{s}$. Below ${T}_{s}$, the intensity of some of the new modes displays little or no temperature dependence, while the intensity of some vibrations increases dramatically with decreasing temperature. Ab initio calculations have been performed for the room-temperature phase using density-functional theory, and the frequencies and atomic characters of the infrared-active phonons at the zone center were obtained using the direct method. The agreement between the calculated and experimentally observed frequencies is quite good, and assignments of the modes are discussed. The vibrational features that are observed only at low temperature appear to be magnetic in origin.

Symmetry Properties of Single-Walled BC(2)N Nanotubes.

The symmetry properties of the single-walled BC2N nanotubes were investigated. All the BC2N nanotubes possess nonsymmorphic line groups. In contrast with the carbon and boron nitride nanotubes, armchair and zigzag BC2N nanotubes belong to different line groups, depending on the index n (even or odd) and the vector chosen. The number of Raman- active phonon modes is almost twice that of the infrared-active phonon modes for all kinds of BC2N nanotubes.

Dipole-active optical phonons inYTiO3: Ellipsometry study and lattice-dynamics calculations

The anisotropic complex dielectric response was accurately extracted from spectroscopic ellipsometry measurements at phonon frequencies for the three principal crystallographic directions of an orthorhombic $(Pbnm)$ ${\text{YTiO}}_{3}$ single crystal. We identify all 25 infrared-active phonon modes allowed by symmetry $7{B}_{1u}$, $9{B}_{2u}$, and $9{B}_{3u}$ polarized along the $c$, $b$, and $a$ axes, respectively. From a classical dispersion analysis of the complex dielectric functions $\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{ϵ}(\ensuremath{\omega})$ and their inverses $\ensuremath{-}1/\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{ϵ}(\ensuremath{\omega})$, we define the resonant frequencies, widths, and oscillator strengths of the transverse-optical (TO) and longitudinal-optical phonon modes. We calculate eigenfrequencies and eigenvectors of ${B}_{1u}$, ${B}_{2u}$, and ${B}_{3u}$ normal modes and suggest assignments of the TO phonon modes observed in our ellipsometry spectra by comparing their frequencies and oscillator strengths with those resulting from the present lattice-dynamics study. Based on these assignments, we estimate dynamical effective charges of the atoms in the ${\text{YTiO}}_{3}$ lattice. We find that in general, the dynamical effective charges in ${\text{YTiO}}_{3}$ lattice are typical for a family of perovskite oxides. By contrast to a ferroelectric ${\text{BaTiO}}_{3}$, the dynamical effective charge of oxygen related to a displacement along the $c$ axis does not show the anomalously large value. At the same time, the dynamical effective charges of Y and $ab$ plane oxygen exhibit anisotropy, indicating a strong hybridization along the $a$ axis.

A first-principles study of enhanced dielectric responses in Ti and Ce doped HfO2

In this letter, the dielectric response in Ti and Ce doped hafnia (HfO2) has been investigated by density functional theory calculations based on pseudopotentials, plane wave basis, and local density approximation. It is found that Ti doped samples give greater dielectric response than Ce doped HfO2, with Hf0.5Ti0.5O2 having the highest response. The Born effective charges on atoms and infrared-active phonon modes have been studied; soft phonon modes cause enhancement in the responses. The materials remain insulating upon doping of HfO2 and are of potential importance for applications as gate oxide.

Electronic and optical properties of orthorhombic and hexagonal phases of CsMgH3: First-principles calculations

Abstract The band structure, chemical bonding, optical and Raman properties of the low pressure orthorhombic ( lpo ) and high pressure hexagonal ( hph ) phases of CsMgH 3 are extensively studied using first-principles calculations based on the density-functional theory. Total energy calculations are performed to understand the structural stability of the two polymorphs under pressure. The phase transformation of orthorhombic to hexagonal is calculated to occur at 0.8 GPa. Moreover, the current band structure calculation suggests that the lpo - and hph -CsMgH 3 are insulators with indirect and direct energy gaps of 3.3 and 3.6 eV, respectively. A detailed study of the electronic density of states and the charge density redistribution reveals the ionic/covalent mixing bonding features between Mg and H atom for both phases. Optical response calculation suggests that the imaginary part of dielectric function spectra is assigned to be the interband transition. Furthermore, the other optical properties of the lpo - and hph -CsMgH 3 are also presented. From the calculation of the zone-center ( Γ ) phonon frequencies, eigenvectors, and the dynamical matrix § ( Γ ), we obtained irreducible representations of allowed Raman-active and infrared-active phonons for both structures.

IR-active optical phonons in Pnma-1, Pnma-2 and phases of

Infrared-active phonons in LaMnO3+δ were studied by means of the reflection and transmission spectroscopy from 50 to 800cm-1 at room temperature. Powder and ceramic samples of the phases of Pnma-2(δ=0.02),Pnma-1(δ=0.08) and R3¯c(δ=0.15) were investigated. In addition, energies of the dipole-active phonons in Pnma-2, Pnma-1 phases were obtained by lattice-dynamics calculations. The transformations of IR-active phonons with the increase in δ in the sequence of Pnma-2, Pnma-1, R3¯c are discussed.

Analysis of Dielectric Response of TiO2 in Terahertz Frequency Region by General Harmonic Oscillator Model

The terahertz range dielectric response of polycrystalline TiO2 (rutile) ceramics and rutile single crystals were investigated by analyzing infrared-active phonons by both Fourier-transform far-infrared (FT-FIR) reflectivity measurement and terahertz time-domain spectroscopy (THz-TDS). The dielectric functions at room temperature measured by THz-TDS are well expressed using the general harmonic oscillator model. We found that the imaginary part of the dielectric permittivity of the TiO2 ceramic is threefold higher than that of the rutile single crystal at 300 K. The large dielectric loss is attributed to the additional damping of the A2u mode or/and an increase in the phonon damping. The low-temperature dielectric loss of the rutile single crystals is exceedingly low. However, TiO2 ceramics shows little temperature dependence. These results indicate that extrinsic scattering significantly affects the dielectric loss of the TiO2 ceramics in the THz region.

Magnetoelastic coupling inDyMn2O5via infrared spectroscopy

We investigated the infrared-active phonons in ${\text{DyMn}}_{2}{\text{O}}_{5}$ as a function of temperature and magnetic field in order to elucidate the magnetic ordering-induced lattice distortions. Spin-lattice coupling in this geometrically frustrated multiferroic is evidenced by small frequency shifts of several phonon modes at various magnetic ordering temperatures, vibrational frequency shifts between 60 and 65 K from which we extract large coupling constants, and magnetic-field dependence of numerous phonons. We analyze these results in terms of local lattice distortions and discuss the consequences for the ferroelectric polarization and magnetodielectric effect.

Structural and optical studies of high dielectric constant(Na0.5A0.5)Cu3Ti4O12 (A = La and Bi)

We report x-ray powder diffraction and temperature-dependent infrared reflectivity measurements of(Na0.5La0.5)Cu3Ti4O12 and(Na0.5Bi0.5)Cu3Ti4O12 in orderto investigate the origin of their lower room-temperature dielectric constants in comparison with the giantvalue of CaCu3Ti4O12. Substituting Ca with Na/La or Na/Bi is found to decrease all Ti–O–Ti angles by theTiO6 octahedra tilts, resulting in an increase of the local structural disorder on the Na/La andNa/Bi compounds. Further, several infrared-active phonon modes show a broadening intheir linewidths, reflecting that the coherency in these vibrational modes is degraded bydisorder. Additionally, the lowest-frequency mode of the Ca material is observed tostrengthen dramatically at low temperatures, but to a lesser extent in the Na/La andNa/Bi compounds. These results suggest the important role of the local structuraldisorder on the anomalous low-frequency dielectric response in these materials.

Pressure-induced changes in the optical properties of quasi-one-dimensionalβ−Na0.33V2O5

The pressure-induced changes in the optical properties of $\beta$-Na$_{0.33}$V$_2$O$_5$ single crystals at room temperature were studied by polarization-dependent Raman and far-infrared reflectivity measurements under high pressure. From the changes in the Raman- and infrared-active phonon modes in the pressure range 9 - 12 GPa a transfer of charge between the different V sites can be inferred. The importance of electron-phonon coupling in the low-pressure regime is discussed.

Infrared optical anisotropy of diluted magneticGa1−xMnxN∕c-sapphire epilayers grown with a GaN buffer layer by metalorganic chemical vapor deposition

Optical anisotropy of hexagonal ${\mathrm{Ga}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x}\mathrm{N}$ ($x$ from 0.0% to 1.5%) epitaxial films grown on $c$-plane sapphire substrates has been investigated using far- and mid-infrared $s$- and $p$-polarized reflectance spectra at oblique incidence (at 10\ifmmode^\circ\else\textdegree\fi{}, 22\ifmmode^\circ\else\textdegree\fi{}, and 32\ifmmode^\circ\else\textdegree\fi{}, respectively). The experimental data at room temperature can be well reproduced simultaneously in the measured frequency region of $200--2000\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ $(5--50\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{m})$, which was based on a four-phase layered system using a $4\ifmmode\times\else\texttimes\fi{}4$ matrix method [M. Schubert, Phys. Rev. B 53, 4265 (1996)]. The lattice vibrations perpendicular and parallel to the optic $c$ axis (${E}_{1}$ and ${A}_{1}$ modes) were expressed by Lorentz oscillator dielectric function model. There was a striking absorption dip at the ${A}_{1}$ phonon frequency in $p$-polarized reflectance spectra due to the optical anisotropy. These infrared-active phonon parameters were obtained with uniaxial dielectric tensor. It was found that the ${A}_{1}$ longitudinal-optical phonon frequency linearly increases with the Mn composition for the diluted magnetic semiconductor epilayers. The broadening values of the ${A}_{1}$ phonon changed from 3.6 $(\ifmmode\pm\else\textpm\fi{}1.3)$ to 9.0 $(\ifmmode\pm\else\textpm\fi{}1.6)\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$, showing the high film crystal quality. Moreover, the ordinary and extraordinary dielectric functions ${\ensuremath{\epsilon}}_{\ensuremath{\perp}}$ and ${\ensuremath{\epsilon}}_{\ensuremath{\Vert}}$ of the epilayers were determined. It indicated that ${\ensuremath{\epsilon}}_{\ensuremath{\Vert}}$ was larger than ${\ensuremath{\epsilon}}_{\ensuremath{\perp}}$ for the ${\mathrm{Ga}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x}\mathrm{N}$ films in the reststrahlen region, which can be ascribed to slight structural degradation of the wurtzite lattice.

Ab Initiocalculations of phonon splitting in antiferromagneticZnCr2O4

The full zone-center optical phonon spectrum and the exchange coupling constant $J$ of the uniform collinear antiferromagnetic (AFM) ordered phase of $\mathrm{Zn}{\mathrm{Cr}}_{2}{\mathrm{O}}_{4}$ are calculated within density functional theory using the local spin density approximation (LSDA), the LSDA plus Hubbard $U$ $(\mathrm{LSDA}+U)$, and the spin-polarized generalized gradient approximation $(\ensuremath{\sigma}\text{\ensuremath{-}}\mathrm{GGA})$. The AFM ordering is found to induce splittings in the infrared-active phonon modes, confirming the importance of spin-phonon coupling in $\mathrm{Zn}{\mathrm{Cr}}_{2}{\mathrm{O}}_{4}$. The $\ensuremath{\sigma}\text{\ensuremath{-}}\mathrm{GGA}$ and the $\mathrm{LSDA}+U$ give magnitudes of phonon frequencies close to experiment, while the LSDA frequencies are slightly softer. However, only the $\mathrm{LSDA}+U$ exchange constant and phonon splittings are consistent with experiment and previous calculations. A correspondence between the exchange constant and the splittings for the infrared-active phonon modes is found.

Stripe order and vibrational properties ofLa2NiO4+δforδ=2∕15: Measurements andab initiocalculations

The optical properties of the static charge- and stripe-ordered material ${\mathrm{La}}_{2}\mathrm{Ni}{\mathrm{O}}_{4+\ensuremath{\delta}}$ for $\ensuremath{\delta}=\frac{2}{15}$ have been measured over a wide frequency and temperature range for light polarized within the $a\text{\ensuremath{-}}b$ planes and along the $c$ axis. Below the charge-ordering temperature, ${T}_{\mathrm{co}}\ensuremath{\simeq}319\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, a charge gap opens and the electronic background, upon which four strong infrared-active phonons are superimposed, drops towards zero. As the temperature decreases, many new spectral features are observed in response to the ordering of interstitial oxygen as well as the formation of a superlattice due to the charge order in the $\mathrm{Ni}{\mathrm{O}}_{2}$ planes. In particular, the prominent mode at $354\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ splits into three components; while the frequencies do not shift below the magnetic-ordering transition at ${T}_{\mathrm{m}}=110\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, there is a transfer of oscillator strength in response to the change in registry of the charge stripes with respect to the underlying lattice. Ab initio calculations have been performed using density-functional theory, and the phonon dispersion curves were obtained using the direct method. Likely assignments of the new modes activated by stripe order are discussed. In some crystals, two antiresonances are observed in the conductivity for $T\ensuremath{\simeq}{T}_{\mathrm{co}}$, which change to a resonant character for $T\ensuremath{\lesssim}{T}_{\mathrm{m}}$; these modes are shown to be due to longitudinal optic $c$-axis modes which appear as a result of surface misorientation.

Infrared-active phonons in carbon nanotubes

The aim of the present paper is to identify the main infrared vibrational features of carbon nanotubes. In this goal, infrared experiments have been performed on different well-characterized single-walled carbon nanotubes (SWCNTs) and double-walled carbon nanotubes (DWCNTs) as well as graphite and carbon aerogel. The comparison between the experimental spectra measured on these different samples allows us to identify the infrared-active modes of carbon nanotubes. In SWCNTs, the tangential modes are located around $1590\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ and the radial mode around $860\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$. This latter mode vanishes in the infrared spectrum of DWCNTs. Finally, in the infrared spectra of all the carbon nanotubes investigated, a band around $1200\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ is evidenced and assigned to the $D$-band (disorder-induced band).

First-principles study of vibrational and dielectric properties ofβ−Si3N4

First-principles calculations have been conducted to study the structural, vibrational, and dielectric properties of $\ensuremath{\beta}\text{\ensuremath{-}}{\mathrm{Si}}_{3}{\mathrm{N}}_{4}$. Calculations of the zone-center optical-mode frequencies (including longitudinal-optical/transverse-optical splittings), Born effective charge tensors for each atom, and dielectric constants, using density functional perturbation theory, are reported. The fully relaxed structural parameters are found to be in good agreement with experimental data. All optic modes are identified and agreement of theory with experiment is excellent. The static dielectric tensor is decomposed into contributions arising from individual infrared-active phonon modes. It is found that high-frequency modes mainly contribute to the lattice dielectric constant.

Infrared reflectance spectrum of BN calculated from first principles

Using the linear response theory, vibrational and dielectric properties are calculated for c -BN, w -BN and h -BN. Calculations of the zone-center optical-mode frequencies (including LO–TO splittings) are reported. All optic modes are identified and excellent agreement is found between the theory and experimental results. The static dielectric tensor is decomposed into contributions arising from individual infrared-active phonon modes. It is found that all the structures have a smaller lattice dielectric constants than those of the electronic contributions. Finally, the infrared reflectance spectrums are presented. Our theoretical results indicate that w -BN shows a similar reflectivity spectrum as c -BN. It is difficult to tell apart the wurtzite structure from the zinc blende phase by IR spectroscopy.

Isotope effect on the optical phonons ofYBa2Cu4O8studied by far-infrared ellipsometry and Raman scattering

The phonon spectra of ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{4}{\mathrm{O}}_{8}$ polycrystalline samples with different isotope substitution for Ba, Cu, and O have been studied using far-infrared ellipsometry and Raman scattering. The observed isotope shifts of Raman- and infrared-active phonons provide insight into their relative Ba, Cu, and O normal mode content. This allows us to examine mode eigenvectors obtained from lattice dynamical calculations using ab initio electronic structures based on the full potential linear muffin-tin-orbital within the local density approximation. Our Raman results demonstrate that the $104\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ ${A}_{g}$ phonon is only very weakly mixed, representing almost pure Ba vibrations. Correspondingly the $151\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ phonon represents pure Cu vibrations. The 339, 436, and $498\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ phonon modes involve mainly O vibrations. Considerable $\mathrm{Cu}\text{\ensuremath{-}}\mathrm{O}$ mixing takes place in the case of the ${A}_{g}$ phonons at 257 and $605\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ and for the ${B}_{3g}$ phonon at $314\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$. These results are in good agreement with the ab initio calculations. The far-infrared experiments reveal that the modes with energies of 115, 297, 502, and $592\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ are mixed vibrations of O and Cu atoms. The phonon mode at $130\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ corresponds to a $\mathrm{Ba}\text{\ensuremath{-}}\mathrm{Cu}\text{\ensuremath{-}}\mathrm{O}$ complex whereas the modes at 191 and $357\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ reflect almost pure Cu and O vibrations, respectively.