Infrared-active Phonons Research Articles

Raman, infrared and x-ray diffraction study of phase stability in La1−xBaxMnO3 doped manganites

Formation of different crystallographic phases and their characteristic Raman and infrared spectra are studied in the manganite system La1−xBaxMnO3 for x in the range of 0⩽x⩽1, and with variations in oxygen stoichiometry. Synthesis of the end member LaMnO3 in pure argon environment leads to the formation of Jahn–Teller distorted, antiferromagnetic orthomanganite. While the observed Raman modes in this compound are primarily due to internal vibrations of MnO6 octahedra, the infrared (IR) spectra show an absorption edge in addition to the IR active phonons. The oxygen rich LaMnO3 is rhombohedral and has fewer zero-wave-vector phonon modes. In the barium substituted compositions with 0<x⩽0.25, a single phase rhombohedral compound of decreasing rhombicity is formed. A further increase in x leads to the ideal cubic perovskite structure for which a factor group analysis yields no Raman active and three IR active phonons of F1u symmetry. The compound with x=0.35, shows faint Raman modes of hexagonal BaMnO3 (P63mc noncentrosymmetric group) whose presence is not seen in x-ray diffraction data. For x>0.35, all three techniques used here show the formation of a two-phase mixture of La0.65Ba0.35MnO3 and hexagonal BaMnO3. An attempt has been made to correlate the progressive increase in symmetry of the crystal structure, shift of IR absorption edge to lower energies, and disappearance of phonon modes with x and δ with the reported electrical and magnetic behavior of this hole-doped manganite.

The infrared properties of Bi 2.07Sr 1.8CuO 6+ δ and Bi 2Sr 1.6La 0.4CuO 6+ δ superconductors

The infrared reflectivity of the bismuth-based high- T c superconductors Bi 2.07Sr 1.8CuO 6+ δ (2201) with T c =7 K and Bi 2Sr 1.6La 0.4CuO 6+ δ (La2201) with T c =30 K have been measured between 25 and 8000 cm −1 at temperatures between 5 and 300 K. The temperature- and frequency-dependent far-infrared part of the spectrum is analyzed. We discuss the influence of La presence on the infrared-active phonons.

Infrared-active phonon line due to a phase transition at around 6 K inαII−NPPy(TCNQ)2

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Lattice dynamics of fluoride scheelites: II. Inelastic neutron scattering in and modelization

A general view of the fluoride scheelite lattice dynamical properties is proposed. The symmetry properties of the vibrations and the symmetry-adapted eigenvectors of the scheelite structure have been determined for the most important directions of the Brillouin zone, namely , and . The low-frequency phonon spectrum along these three directions has been measured by inelastic neutron scattering. The corresponding data, together with the measured sound velocities and the Raman- and infrared-active phonon frequencies, are described in the framework of a rigid ion model. The average error between experimental results and calculated frequencies is less than 5%. The most important normal modes of vibration and the one-phonon density of states are deduced.

Temperature dependence of the infrared properties of Bi 2.1− xPb xSr 2Ca 2Cu 3O 10+δ ( x = 0.3, 0.4 and 0.5)

We have measured the reflectivity of high-temperature superconducting Bi 2.1− x Pb x Sr 2Ca 2Cu 3O 10+δ ceramics for x = 0.3, 0.4 and 0.5 in the frequency range 25–8000 cm −1 for various temperatures above and below T c. A Kramers-Kronig analysis was performed. We discuss the influence of Pb presence on infrared-active phonons.

Isotope effects on the optical phonons ofYBa2Cu3O7: Eigenvector and infrared charge determination

Using Raman spectroscopy and far-infrared ellipsometry we have investigated several ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$ ceramics which were made from different isotopically pure Cu or Ba precursors. The observed shifts of Raman- and infrared-active phonons reveal insight into their relative Ba and Cu normal mode content. This allows us to examine mode eigenvectors obtained from lattice dynamical calculations. Our Raman results demonstrate that the 121 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ and 153 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ ${\mathrm{A}}_{\mathrm{g}}$ phonons are only very weakly mixed and represent almost pure Ba and Cu vibrations, respectively, whereas they suggest considerable Ba-Cu-O mixing in the case of the ${\mathrm{B}}_{2\mathrm{g}}$ and ${\mathrm{B}}_{3\mathrm{g}}$ phonons at 140 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ and 142 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$. The analysis of the infrared-active ${\mathrm{B}}_{1\mathrm{u}}$ modes yields a substantial contribution of Cu to the 150 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ and the 280 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ phonons and of Ba to the 150 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ mode. Making use of proposed eigenvectors, which are shown to be compatible with the observed isotope shifts, we determine effective charges for all ions from measured infrared oscillator strengths on a ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$ crystal. We find that the proposed eigenvectors are compatible with earlier experiments of the oxygen isotope effect as well as the replacement of Y by rare earth elements.

Strain-induced shifts of the infrared-active phonon of cubic boron nitride.

Recent calculations by Fahy [Phys. Rev. B 51, 12 873 (1995)] have produced values of strain coefficients for the infrared-active phonon of cubic BN. We discuss these results and show that they are not fully consistent. Furthermore, we comment on their general trends in comparison with other crystals of the diamond-zinc-blende family.

THz electromagnetic emission by coherent infrared-active phonons.

Coherent phonons are excited in single-crystal tellurium by above-band-gap excitation with femtosecond laser pulses. Coherent infrared-active lattice vibrations give rise to the emission of electromagnetic waves at the phonon frequency. The excitation mechanism is based on the ultrafast buildup of a photo-Dember field and the coupling of a polar lattice mode to this field. The dynamics of the Dember field is obtained from a numerical simulation of the electron-hole plasma dynamics close to the crystals surface. This effect itself gives rise to a broadband emission of THz radiation from highly absorbing semiconductor surfaces. Details of the generation and the outcoupling of the radiation are discussed. \textcopyright{} 1996 The American Physical Society.

Infrared properties of CuO-superconductors

Infrared measurements on CuO-superconductors with →E ∥ c are discussed. In particular the renormalization of infrared-active phonons by charge fluctuations in doublelayer systems and the influence of Josephson coupling on the dynamical conductivity in c-direction are investigated. It is shown that measurements on tilted layer systems can give valuable information also.

Symmetry-dependent phonon interactions in YBa 2Cu 4O 8 superconductors: a Raman and infrared spectroscopic study

Superconductivity induced renormalization of Raman- and infrared-active optical phonons is studied for undoped and T c-reduced Pr- and Zn-substituted double-chain YBa 2Cu 4O 8 materials. The magnitude of the phonon frequency and the linewidth variation upon cooling below T c is found to be dependent on the mode symmetry; odd- end even-parity phonons respond qualitatively different to the reduction of T c. Assuming phonon coupling to the excitations participating in the formation of the superconducting gap, these experimental facts provide evidence for the gap anisotropy of YBa 2Cu 4O 8.

Infrared-active phonons and the superconducting gap of Tc-reduced double-chain YBa2Cu4O8 superconductors.

We report a far-infrared reflectivity study in the temperature range 10--300 K of the double-chain ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{4}$${\mathrm{O}}_{8}$ superconductor with Zn and Sr substituting for Cu and Ba, respectively. Superconductivity-induced self-energies of the transverse optical ${\mathit{B}}_{1\mathit{u}}$(z) phonon were used to estimate the position of the superconducting gap. Evidence for a shift of the gap to lower energies upon decreasing the superconducting transition temperature ${\mathit{T}}_{\mathit{c}}$ was found for Zn- as well as for Pr-doped materials. A comparison of the linewidth changes of the plane-oxygen phonon (at \ensuremath{\sim}300 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$) with the results of a strong-coupling calculation yields the gap-to-${\mathit{T}}_{\mathit{c}}$ ratio 2${\mathrm{\ensuremath{\Delta}}}_{0}$/${\mathit{kT}}_{\mathit{c}}$=(8.2\ifmmode\pm\else\textpm\fi{}1.0) for ${\mathrm{YBa}}_{2}$(${\mathrm{Cu}}_{3.96}$${\mathrm{Zn}}_{0.04}$)${\mathrm{O}}_{8}$ which is close to 2${\mathrm{\ensuremath{\Delta}}}_{0}$/${\mathit{kT}}_{\mathit{c}}$=(6.6\ifmmode\pm\else\textpm\fi{}0.2) reported for (${\mathrm{Pr}}_{0.37}$${\mathrm{Y}}_{0.63}$)${\mathrm{Ba}}_{2}$${\mathrm{Cu}}_{4}$${\mathrm{O}}_{8}$. For Sr in place of Ba in Y(${\mathrm{Ba}}_{2\mathrm{\ensuremath{-}}\mathit{z}}$${\mathrm{Sr}}_{\mathit{z}}$)${\mathrm{Cu}}_{4}$${\mathrm{O}}_{8}$, for which ${\mathit{T}}_{\mathit{c}}$ remains essentially the same within a broad range of compositions (0\ensuremath{\le}z\ensuremath{\le}0.6), no change of the temperature behavior of the phonon and thus no variation of the position of the superconducting gap was observed. These experimental findings confirm that the picture of superconductivity-induced changes in the phonon parameters holds in the double-chain compound, similarly to the single-chain material. We also report the effects of these dopants on the phonon anomaly which has been attributed to a spin gap.

Structure of Li 3X (X = N, P, As) superionic conductors: X-ray diffraction and FTIR studies

Structural studies of the layered superionic conductors Li 3N and Li 3P and the semimetallic conductor Li 3As are reported. X-ray diffraction measurements show that these compounds crystallize in a hexagonal structure with a P6/ mmm space group. The lattice dimensions expand anisotropically as the size of the anion increases. The temperature dependence of the Fourier transform infrared (FTIR) absorption spectra of Li 3X compounds have been studied in the spectral range 20–700 cm −1. The complex dielectric response was analyzed using Lorentzian-oscillator fits. The frequency and damping of the infrared-active phonons are determined as well as the permittivity constants. These data illustrate the change in lattice dynamics in Li 3X compounds. The deduced infrared effective charge and electronic polarizability indicate the chemical bonding modification for the Li 3X compounds. The relationship between polarizability and conduction is addressed.

Interaction of axial infrared-active phonons with electronic excitations and phonon-mediated charge transfer in YBa 2Cu 3O 7− x

We report on an infrared study of the resonance interaction between phonons and electronic excitations in an YBa 2Cu 3O 7 single crystal. Our results show that two transverse axial phonon modes ( ω≅300 cm −1 and ω≅550 cm −1) are strongly coupled to the c-polarized electronic continuum resulting in Fano-type line shapes at temperatures below T c. The calculated dimensionless coupling constants λ j of these modes are 0.3 and 0.45, respectively. The estimated value of the total coupling constant λ≅3 is high enough to account for the high T c value. A dramatic increase in the bare oscillator strength of these phonons in the superconducting phase of YBa 2Cu 3O 7 is interpreted in terms of phonon-mediated charge transfer between CuO 2 plane and CuO chain through the BaO plane.

Infrared activity in elemental crystals.

In a previous paper, Zallen [Phys. Rev. 173, 824 (1968)] reported a group-theoretical analysis of the competition between unit-cell complexity and crystal symmetry in determining the presence or absence of infrared-active phonons in an elemental crystal. Here we correct an error in that paper's treatment of certain hexagonal space groups. Our results modify the minimum-complexity condition for infrared activity: For 228 of the 230 space groups, a necessary and sufficient condition for the existence of symmetry-allowed infrared-active modes in an elemental crystal is the presence of three or more atoms in the primitive unit cell. The two exceptional space groups are P6/mmm (${\mathit{D}}_{6\mathit{h}}^{1}$) and P${6}_{3}$/mmc (${\mathit{D}}_{6\mathit{h}}^{4}$); for each of these symmetries, there exists one structure with four atoms per cell and no infrared modes. The P${6}_{3}$/mmc structure includes, as special cases, Lonsdaleite (or ``wurtzite silicon'') as well as a c-axis-aligned hcp arrangement of diatomic molecules which is relevant to models of solid molecular hydrogen at high pressure.

Frequency shifts and linewidth changes of infrared-active phonons in double-layered high-temperature superconductors

We calculate frequency shifts and changes in linewidths of infrared-active phonons within a shell model for the bare phononic system coupled to an electronic double-layer structure with inter-layer charge transfer. The theoretical concept is applied to YBa 2Cu 3O 7 yielding a good description of experimental results in the normal state as well as at the transition to the superconducting state.

Metastability of atomic phases of nitrogen.

The metastability of a recently proposed candidate for a polymeric structure for nitrogen is examined using linear response theory to calculate the phonon dispersion throughout the Brillouin zone. It is found that the structure is mechanically stable at pressures below 360 GPa. In contrast, nitrogen in the simple-cubic structure is found to be unstable over the same pressure range. Calculations of Raman- and infrared-active phonon frequencies are presented for the metastable phase.

Infrared spectra of optical phonons in semiconducting La 2CuO 4+ x single crystals

Far-infrared reflectivity spectra R( v) in polarization E ⊥ C and DC-conductivity σ DC have been studied in a semiconducting phase of La 2CuO 4+ x single crystals with various oxygen contents. The spectrum of infrared-active optical phonons has been obtained for stoichiometric La 2CuO 4. It has been shown that the phonon spectrum should be analyzed in a framework of the orthorhombic crystal structure at T≤300 K. Extra lines have been found in the phonon spectrum upon doping the crystals with oxygen. The results of studying the temperature dependence of R( v) and σ DC allow one to interpret these features as vibrational modes, which are active in the Raman scattering spectrum and displayed in the IR spectrum due to their interaction with self-localized holes.

Far-infrared spectroscopy of halogen-bridged mixed-valence platinum-chain solids: Isotope-substitution studies.

Far-infrared transmittance measurements are performed on a series of isotopically labeled samples of the quasi-one-dimensional solids [Pt(en${)}_{2}$][Pt(${\mathrm{en}}_{2}$${\mathit{X}}_{2}$][${\mathrm{ClO}}_{4}$${]}_{4}$, (en)=ethylenediamine and X=Cl, Br, or I. Pronounced shifts upon deuteration of the (en) ligands reveal that several absorption features previously attributed to localized vibrational modes of chain defects are in fact (en) ligand modes. Recognition of these ligand modes resolves the previous ambiguity over the assignment of the infrared-active chain phonons, permitting the unambiguous assignment of the 238.7-${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ feature in the bromide material to the asymmetric stretch (${\ensuremath{\nu}}_{2}$) chain phonon, and strongly suggesting the 184.2-${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ feature in the iodide is the ${\ensuremath{\nu}}_{2}$ phonon of that material. A high-resolution examination of the chloride, prepared with both natural Cl isotopic abundance and with nearly pure $^{35}\mathrm{Cl}$, reveals a Cl isotopic fine structure that allows conclusive identification of the 359.1-${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ feature as the ${\ensuremath{\nu}}_{2}$ chain phonon. Lattice-dynamics calculations using a harmonic-linear-chain model with randomly distributed Cl isotopes yield good agreement with experiment and reveal that isotopic disorder leads to pronounced vibrational localization in PtCl, with the observed fine structure arising from modes residing on a few distinct sequences of isotopes occurring with high probability. The radical difference between the infrared fine structure of PtCl and that previously reported for the Raman-active chain mode is found to result from differences in the dispersion curves for the two phonon branches, and allows indirect determination of these dispersion curves.

Doping states in the two-dimensional three-band Peierls-Hubbard model.

Doping states in a two-dimensional three-band Peierls-Hubbard model for the copper oxides are investigated with inhomogeneous Hartree-Fock (HF) and random-phase approximations. The doping states are sensitive to small changes of interaction parameters because they easily change local energy balance between different interactions around added holes. For the parameter values derived from constrained-density-functional methods for the copper oxides, added holes form isolated small ferromagnetic polar- ons. When the parameters are varied around these values, different types of doping states are obtained: For stronger on-site repulsion at Cu sites, larger ferromagnetic polarons are formed, which are qualitatively different from the small polarons; for stronger nearest-neighbor Cu-O repulsion, polarons are clumped or there occurs phase separation into Cu- and O-hole-rich regions; Intersite electron-lattice coupling rapidly changes the small polarons by quenching a Cu magnetic moment and locally distorting the lattice in an otherwise undistorted antiferromagnetic background. This is regarded as a rapid crossover from a Zhang-Rice singlet to a covalent molecular singlet, and occurs substantially below a critical strength for destruction of the stoichiometric antiferromagnetic state. However intrasite electron-lattice coupling, in contrast to the intersite coupling, does not dramatically affect the hole-doping states. Doping induces modes in magnetic, optical, and vibronic response functions. Local infrared-active phonon modes are induced in infrared absorption spectra for finite electron-lattice coupling. They are correlated with doping-induced particle-hole excitations observed in optical absorption spectra and in magnetic excitation spectra. These doping-induced particle-hole excitations are associated with the local HF eigenstates in the charge-transfer gap. Each doping state has distinctive excitation spectra in the magnetic, optical, and vibronic channels. In particular, the hole-doping states with small polarons have doping-induced, infrared absorption peaks on the low-frequency side of the stoichiometric peak, while the electron-doping states have them on the high-frequency side.

Electronic susceptibility in double-layered high-temperature superconductors: Plasmons and renormalization of infrared-active phonons

We investigate the electronic susceptibility of a periodic double-layer system serving as a model for high-temperature superconductors in the normal and in the superconducting state. Allowing for a hopping of charge carriers between the planes of each double layer yields a strong influence on the acoustic plasmon branch in both states. Frequency shifts and linewidth-effects observed experimentally on infrared-active phonons at the superconducting transition are explained consistently within our model.