Oxygen Breathing Mode Research Articles

Observing structural distortions in complex oxides by x-ray photoelectron diffraction

The observation of structural distortions in complex oxides is becoming more important in order to explain their macroscopic behavior. X-ray photoelectron diffraction is an excellent technique to study the crystal structure and potentially observe oxygen distortions in an element-specific fashion. Its surface-sensitive character opens up the possibility to perform experiments on ultra-thin films without probing the substrate underneath. For BaBiO3 thin films, for example recent experimental results point towards a suppression of an oxygen breathing mode with decreasing film thickness resulting in a decreased energy band gap. As another example, a nontrivial electronic band structure is predicted for [111]-bilayers of LaMnO3. However, the likely presence of a Jahn–Teller distortion would suppress the topological phase. Here, the working principle of x-ray photoelectron diffraction is described and the data analysis required afterwards is discussed. X-ray photoelectron diffraction interference patterns are obtained for various thicknesses of BaBiO3 and LaMnO3 thin films. Experimental results are compared with multiple-scattering simulations. Although, indications for the structural distortions are observed, we discuss limitations of the technique in quantifying the displacement of the oxygen atoms.

Thickness-Dependent Band Gap Modification in BaBiO3.

The material BaBiO is known for its insulating character. However, for thin films, in the ultra-thin limit, metallicity is expected because the oxygen octahedra breathing mode will be suppressed as reported recently. Here, we confirm the influence of the oxygen breathing mode on the size of the band gap. The electronic properties of a BaBiO thickness series are studied using in-situ scanning tunneling microscopy. We observe a wide-gap ( > 1.2 V) to small-gap ( ≈ 0.07 eV) semiconductor transition as a function of a decreasing BaBiO film thickness. However, even for an ultra-thin BaBiO film, no metallic state is present. The dependence of the band gap size is found to be coinciding with the intensity of the Raman response of the breathing phonon mode as a function of thickness.

Ionic radii correlative study on LaFe0.5Cr0.5O3 and YbFe0.5Cr0.5O3magnetic double perovskites

Comparative study on the iso-structural compounds LaFe0.5Cr0.5O3 (LFCO) and YbFe0.5Cr0.5O3 (YFCO) are carried out to investigate the structural, optical and magnetic properties. Rietveld refinement and the derived structural parameters disclose the extent of distortion exhibited by the ions substituted in the orthorhombic lattice. Raman analysis describes the genuine differences in the phonon modes. Contradictory Raman shift observed in the oxygen breathing mode at ~679 cm−1 denotes the existence of the multi-valence state in LFCO than YFCO. EPR and XPS analysis validates the multi-valence state of Cr3+/4+/6 ions present in the LFCO compound. The optical transition between the Cr3+/Fe3+ ions is studied by diffuse reflectance spectroscopy, where the direct bandgap energy (Eg) is 2.05 and 2.08 eV, respectively. Magnetic measurement reveals complex magnetic Phenomena such as magnetisation reversal and spin reorientation transition, illustrating a contrast in the magnetic interaction present due to La3+(non-magnetic) and Yb3+ (magnetic) ion and with the interplay of Cr3+/Fe3+ ion between their sublattices.

Orbital facilitated charge transfer originated phonon mode in Cr‐substituted PrFeO3: A brief Raman study

AbstractWe report the possible origin of orbital facilitated new Raman mode in orthorhombic perovskite PrFe1‐xCrxO3. For this purpose, resonance and power‐dependent Raman spectroscopy measurements have been carried out to understand the origin of local oxygen breathing mode in the mixed Fe–Cr orthorhombic perovskites through the orbital mediated charge transfer mechanism. It has been observed that multi‐phonon scattering is highly sensitive to the presence of mixed Fe–Cr ions and is absent in pure iso‐structural vacuum annealed PrFeO3. The Raman spectroscopy results infer a scaling in the intensity of octahedral breathing Raman mode around 660 cm−1, which is attributed to the lattice rearrangement due to the transfer of an electron from Cr3+(d3) to Fe3+(d5) and results to d2–d6 (Cr4+–O−2–Fe2+) configuration. The interactions of d2–d6 configuration are different from d3–d5 configuration, which leads to the octahedral lattice rearrangement and activates the new Raman mode. The charge transfer phenomenon is supported through first‐principle calculations and valence band spectroscopy.

Lattice dynamics of the hybrid improper ferroelectrics (Ca,Sr)3Ti2O7

The structure and lattice dynamics of the hybrid improper ferroelectric compound ${\mathrm{Ca}}_{3\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{Ti}}_{2}{\mathrm{O}}_{7}$ (for $x=0,0.6,$ and 0.9) have been studied with a combination of diffraction, inelastic scattering experiments, Raman spectroscopy, and calorimetry measurements, as well as first-principles simulations. Using inelastic neutron scattering, we have measured the phonon density of states (DOS) for $x=0.9$, which revealed a strong broadening but little change in phonon energies on heating from 10 K to 728 K across the ferroelectric phase transition temperature, ${T}_{FE}$. Using inelastic x-ray scattering, the momentum-resolved phonon dispersions were measured from 80 K to 950 K on a single crystal (for $x=0.6$), and also revealed a strong phonon broadening but a small energy shift for acoustic modes on heating across ${T}_{FE}$. Our Raman measurements (for $x=0.6$) showed robust rotational and oxygen breathing modes but soft tilt modes, consistent with previous measurements on similar compounds. Our density functional calculations achieve good agreement with both the phonon DOS and dispersions measured. We did not observe any unusual quadratic dispersion for $c$-polarized transverse acoustic modes, at odds with a recently predicted quasi-2D character, for either undoped ($x=0$) or doped ($x=0.6$) compounds.

Band gap tuning and orbital mediated electron–phonon coupling in HoFe1−xCrxO3 (0 ≤ x ≤ 1)

We report on the evidenced orbital mediated electron–phonon coupling and band gap tuning in HoFe1−xCrxO3 (0 ≤ x ≤ 1) compounds. From the room temperature Raman scattering, it is apparent that the electron-phonon coupling is sensitive to the presence of both the Fe and Cr at the B-site. Essentially, an Ag like local oxygen breathing mode is activated due to the charge transfer between Fe3+ and Cr3+ at around 670 cm−1, this observation is explained on the basis of Franck-Condon mechanism. Optical absorption studies infer that there exists a direct band gap in the HoFe1−xCrxO3 (0 ≤ x ≤ 1) compounds. Decrease in band gap until x = 0.5 is ascribed to the broadening of the oxygen p-orbitals as a result of the induced spin disorder due to Fe3+ and Cr3+ at B-site. In contrast, the increase in band gap above x = 0.5 is explained on the basis of the reduction in the available unoccupied d-orbitals of Fe3+ at the conduction band. We believe that above results would be helpful for the development of the optoelectronic devices based on the ortho-ferrites.

Model of the Electron-Phonon Interaction and Optical Conductivity ofBa1−xKxBiO3Superconductors

We investigate the physical properties of the Ba(1-x)K(x)BiO(3) compounds with a focus on the optical properties. Results from the simple Holstein model, describing a single band coupled to an oxygen breathing mode with parameters derived from first principles calculations, are in excellent agreement with a broad range of experimental information. It accounts for an insulating parent compound at x=0, with a direct (optical) and an indirect gap, and a metal insulator transition around x=0.38. Strong electron-phonon coupling leads to spectral weight redistribution over a frequency scale much larger than the characteristic phonon frequency and to strongly anharmonic phonons. We find that the metallic phase in the vicinity of phase boundary is close to the polaronic regime.

Interpretation of temperature dependence of the in-plane electrical resistivity in YBa 2Cu 4O 8: Electron–phonon approach

In this paper, we undertake a quantitative analysis of observed temperature-dependent in-plane normal state electrical resistivity of single crystal YBa 2Cu 4O 8. The analysis is within the framework of classical electron–phonon i.e., Bloch-Gruneisen model of resistivity. It is based on the inherent acoustic (low frequency) phonons ( ω ac ) as well as high frequency optical phonons ( ω op ), the contributions to the phonon resistivity were first estimated. The optical phonons of the oxygen breathing mode yields a relatively larger contribution to the resistivity compared to the contribution of acoustic phonons. Estimated contribution to in-plane electrical resistivity by considering both phonons i.e., ω ac and ω op , along with the zero-limited resistivity, when subtracted from single crystal data infers a quadratic temperature dependence over most of the temperature range [80 ⩽ T ⩽ 300]. Quadratic temperature dependence of ρ diff. = [ ρ exp − { ρ 0 + ρ e − ph (= ρ ac + ρ op )}] is understood in terms of electron–electron inelastic scattering. The relevant energy gap expressions within the Nambu-Eliashberg approach are solved imposing experimental constraints on their solution (critical temperature T c ). It is found that the indirect-exchange formalism provides a unique set of electronic parameters [electron–phonon ( λ ph ), electron-charge fluctuations ( λ pl ), electron–electron ( μ) and Coulomb screening parameter ( μ *)] which, in particular, reproduce the reported value of T c .

Electron-lattice interactions in the perovskiteLaFe0.5Cr0.5O3characterized by optical spectroscopy andLDA+Ucalculations

We use resonance Raman scattering (incident photon energies between 1.8 and 4.13 eV), $\text{LDA}+\text{U}$ calculations, spectroscopic ellipsometry, and oblique IR reflectivity to characterize the strong electron-phonon interactions in the disordered perovskite ${\text{LaFe}}_{0.5}{\text{Cr}}_{0.5}{\text{O}}_{3}$. When the photon energy coincides with a Cr to Fe Mott-Hubbard transfer gap around 2.4 eV the electron-phonon interaction is manifested by a Franck-Condon effect with exceptional first- and higher order scattering of a local oxygen breathing mode. At higher incident energies we observe a superposition of Franck-Condon scattering and Fr\"ohlich interaction induced infrared active longitudinal optical two-phonon scattering activated mainly by O to Fe charge transfer. Our results establish ${\text{LaFe}}_{0.5}{\text{Cr}}_{0.5}{\text{O}}_{3}$ as a model compound for research on electron-phonon interactions in strongly correlated complex systems and show that Franck-Condon scattering in complex solids is not limited to Jahn-Teller active compounds.

Electron-phonon interactions in perovskites containing Fe and Cr studied by Raman scattering using oxygen-isotope and cation substitution

We use temperature-dependent inelastic light scattering to study the origin of the strong multiphonon scattering of a local oxygen breathing mode present in the mixed $B$-site orthorhombic (space group $Pnma$) perovskite ${\text{LaFe}}_{0.5}{\text{Cr}}_{0.5}{\text{O}}_{3}$ but absent in isostructural ${\text{LaFeO}}_{3}$ and ${\text{LaCrO}}_{3}$. It is seen that the multiphonon scattering is critically sensitive to the presence of both Fe and Cr ions on the $B$ site. These results support our interpretation that the multiphonon scattering is activated by local electron-phonon interactions according to the Franck-Condon picture following an Fe-Cr charge transfer. Further, $^{18}\text{O}$ substitution is performed on the $x=0$, 0.04, and 0.5 compounds and clearly shows that all modes appearing above the first-order phonon-scattering region in these compounds originate from higher-order oxygen stretching vibrations. In particular this is the case for the strong second-order scattering dominating the scattering response in ${\text{LaFeO}}_{3}$. Accordingly we propose that these modes are generated by infrared-active longitudinal optical (IR LO) two-phonon and combination scattering activated by Fr\ohlich interaction. For $x=0.02$ and 0.04 the characteristic IR LO two-phonon and Franck-Condon multiphonon-scattering profiles mix. We also study the influence of isovalent cation substitution and Sr doping in $A{\text{Fe}}_{0.5}{\text{Cr}}_{0.5}{\text{O}}_{3}$ ($A=\text{La}$, Nd, and Gd) and ${\text{La}}_{1\ensuremath{-}y}{\text{Sr}}_{y}{\text{Fe}}_{0.5}{\text{Cr}}_{0.5}{\text{O}}_{3\ensuremath{-}\ensuremath{\delta}}$ ($y=0$, 0.16, and 0.5) on the strong electron-phonon coupling present in ${\text{LaFe}}_{0.5}{\text{Cr}}_{0.5}{\text{O}}_{3}$. The Franck-Condon effect in ${\text{LaFe}}_{0.5}{\text{Cr}}_{0.5}{\text{O}}_{3}$, is not significantly affected by isovalent $A$-site substitution, despite the increasing orthorhombic distortion associated with decreasing $A$-site ionic radii. On the contrary, aliovalent Sr doping causes a rapid decrease in the Franck-Condon scattering. This shows that the strong electron-phonon coupling in these compounds is highly sensitive to local lattice and electronic decoherence but insensitive to global lattice distortions. Finally, a preliminary assignment of the ${A}_{g}$ and ${B}_{2g}$ phonon modes in $A{\text{Fe}}_{0.5}{\text{Cr}}_{0.5}{\text{O}}_{3}$ ($A=\text{La}$, Nd, and Gd) is made based on the present observations and published results for ${\text{LaCrO}}_{3}$ and $A{\text{MnO}}_{3}$. The modes associated with oxygen octahedral tilt and bending vibrations are heavily influenced by the magnitude of the orthorhombic distortion.

Franck-Condon higher order lattice excitations in theLaFe1−xCrxO3(x=0, 0.1, 0.5, 0.9, 1.0) perovskites due to Fe-Cr charge transfer effects

First and higher order lattice excitations in the $B$-site disordered perovskites $\mathrm{La}{\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Cr}}_{x}{\mathrm{O}}_{3}$ ($x=0$, 0.1, 0.5, 0.9, and 1) and ${\mathrm{La}}_{0.835}{\mathrm{Sr}}_{0.165}{\mathrm{Fe}}_{0.5}{\mathrm{Cr}}_{0.5}{\mathrm{O}}_{3\ensuremath{-}\ensuremath{\delta}}$ are investigated using temperature dependent and polarized inelastic light scattering [$\ensuremath{\lambda}=515\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ $(2.41\phantom{\rule{0.3em}{0ex}}\mathrm{eV})$ and $676\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ $(1.83\phantom{\rule{0.3em}{0ex}}\mathrm{eV})$] on oriented crystallites. A peak at approximately $2.4\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ in the imaginary part of the dielectric function of $\mathrm{La}{\mathrm{Fe}}_{0.5}{\mathrm{Cr}}_{0.5}{\mathrm{O}}_{3}$ is assigned to a charge transfer from ${\mathrm{Fe}}^{3+}$ $({d}^{5})$ to ${\mathrm{Cr}}^{3+}$ $({d}^{3})$ ions, coupled with the appearance of an intense ${A}_{g}$-like mode at approximately $700\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ in the Raman data. This excitation is identified as a symmetric oxygen breathing mode activated by the Fe-Cr charge transfer through an orbital coupling mechanism. Higher order scattering (up to seventh order) of the intrinsic Raman active symmetric breathing mode is also explained by an orbital-mediated electron-phonon coupling, similar to the Franck-Condon effect observed in the Jahn-Teller active-perovskite-structured manganite $\mathrm{La}\mathrm{Ma}{\mathrm{O}}_{3}$. These results show that the Franck-Condon mechanism is a more common mechanism for resonant higher order scattering in solids than previously believed and propose the $\mathrm{La}{\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Cr}}_{x}{\mathrm{O}}_{3}$ system as a model system for electron-phonon coupling and higher order Raman scattering in solids.

Interpretation of Temperature-Dependent Resistivity of La–Pb–MnO3: Role of Electron–Phonon Interaction

The present paper deals with the theoretical investigation of temperature-dependent resistivity of the perovskite manganites La0.78Pb0.22MnO3-δ within the framework of the classical electron–phonon model of resistivity, i.e., the Bloch–Gruneisen model. Due to inherent acoustic (low-frequency) phonons (ωac) as well as high-frequency optical phonons (ωop), the contributions to the electron–phonon resistivity have first been estimated. At low temperatures the acoustic phonons of the oxygen-breathing mode yield a relatively larger contribution to the resistivity as compared to the contribution of optical phonons. Furthermore, the nature of phonons changes around T = 215 K exhibiting a crossover from an acoustic to optical phonon regime with elevated temperature. The contribution to resistivity estimated by considering both phonons, i.e., ωac and ωop, when subtracted from experimental data, infers a T4.5 temperature dependence over most of the temperature range. Deduced T4.5 temperature dependence of ρdiff = [ρexp − {ρ0 + ρe-ph( = ρac + ρop)}] is justified in terms of electron–magnon scattering within the double exchange (DE) process. Within the proposed scheme, the present numerical analysis of temperature dependent resistivity shows similar results as those revealed by experiments

Electrical resistivity in the ferromagnetic metallic state of La-Ca-MnO$\mathsf{_{3}}$: Role of electron-phonon interaction

The temperature-dependent resistivity of the perovskite manganites La1-x Ca x MnO3, with x = 0.33, is theoretically analysed within the framework of the classical electron-phonon model of resistivity, i.e., the Bloch-Gruneisen model. Due to inherent acoustic (low-frequency) phonons ( $\omega_{ac})$ as well as high-frequency optical phonons ( $\omega_{op})$ , the contributions to the resistivity have first been estimated. The acoustic phonons of the oxygen-breathing mode yield a relatively larger contribution to the resistivity compared to the contribution of optical phonons. Furthermore, the nature of phonons changes around T = 167 K exhibiting a crossover from an acoustic to optical phonon regime with elevated temperature. The contribution to resistivity estimated by considering both phonons, i.e. $\omega_{ac}$ and $\omega_{op}$ , when subtracted from thin film data, infers a power temperature dependence over most of the temperature range. The quadratic temperature dependence of $\rho_{\it diff.} = [ \rho_{\exp} . - \{\rho_{0} + \rho_{e\text{-}ph} (= \rho_{ac} + \rho_{op}) \} ]$ is understood in terms of electron-electron scattering. Moreover, in the higher temperature limit, the difference can be varies linearly with T 4.5 in accordance with the electron-magnon scattering in the double exchange process. Within the proposed scheme, the present numerical analysis of temperature dependent resistivity shows similar results as those revealed by experiment.

Flux phase as a dynamic Jahn-Teller phase: Berryonic matter in the cuprates

There is considerable evidence for some form of charge ordering on the hole-doped stripes in the cuprates, mainly associated with the low-temperature tetragonal phase, but with some evidence for either charge density waves or a flux phase, which is a form of dynamic charge-density wave. These three states form a pseudospin triplet, demonstrating a close connection with the E X e dynamic Jahn-Teller effect, suggesting that the cuprates constitute a form of Berryonic matter. This in turn suggests a new model for the dynamic Jahn-Teller effect as a form of flux phase. A simple model of the Cu-O bond stretching phonons allows an estimate of electron-phonon coupling for these modes, explaining why the half breathing mode softens so much more than the full oxygen breathing mode. The anomalous properties of $O^{2-}$ provide a coupling (correlated hopping) which acts to stabilize density wave phases.

Interpretation of temperature-dependent resistivity of electron-doped cuprates

The temperature-dependent normal state resistivity of single-crystal Nd1.85Ce0.15CuO4−δ is theoretically analysed within the framework of the classical electron–phonon model of resistivity, i.e. the Bloch–Gruneisen model. Due to inherent acoustic (low-frequency) phonons (ωac) as well as high-frequency optical phonons (ωop), the contributions to the resistivity have first been estimated. The optical phonons of the oxygen-breathing mode yield a relatively larger contribution to the resistivity compared to the contribution of acoustic phonons. The contribution to resistivity estimated by considering both phonons, i.e. ωac and ωop, along with the zero limited resistivity, when subtracted from single-crystal data, infers a quadratic temperature dependence over most of the temperature range (25 ≤ T ≤ 300 K). The quadratic temperature dependence of ρdiff = [ρexp − {ρ0 + ρe-ph( = ρac + ρop)}] is understood in terms of electron–electron inelastic scattering. The comparison of single-crystal experimental data with the present analysis appears to be favourable.

Interpretation of resistivity of Nd1.85Ce0.15CuO4: Electron-phonon mechanism

The temperature-dependent normal state resistivity of single crystal Nd1.85Ce0.15CuO4 − δ is theoretically analyzed within the framework of classical electron–phonon i.e., Bloch-Gruneisen model of resistivity. For the reason of inherent acoustic (low frequency) phonons (ωac) as well as high-frequency optical phonons (ωop), the contributions to the resistivity were first derived. The optical phonons of the oxygen breathing mode yields a relatively larger contribution to the resistivity compared to the contribution of acoustic phonons. Estimated contribution to in-plane resistivity by considering both phonons, i.e., ωac and ωop, along with the zero-temperature-limited resistivity, when subtracted from single crystal data infers a quadratic temperature dependence over most of the temperature range [25 ≤ T ≤ 300]. Quadratic temperature dependence of ρdiff. = [ρexp. − {ρ0 + ρe–ph (=ρac + ρop)}] is understood in terms of 3D electron–electron inelastic scattering. The comparison of single crystal experimental data appears favorable with the present analysis.

Explanation of Temperature-Dependent Resistivity of Sm-Doped Cuprates by Electron–Phonon Scattering

The resistivity of electron-doped cuprate Sm1.85Ce0.15CuO4 − δ is theoretically analyzed within the framework of electron–phonon i.e., Bloch–Gruneisen (BG) model of resistivity. Characteristic temperatures as the Debye temperature and the Einstein temperature were first derived from an overlap repulsive potential. The optical phonons of the oxygen-breathing mode yield a relatively larger contribution to the resistivity compared to the contribution of acoustic phonons above 220 K. While to that, below this temperature, acoustic phonon is a major cause of resistivity. Estimated contribution to resistivity by considering both phonons i.e., ωac (acoustic phonons) and ωop (optical phonons), along with the zero limited resistivity, when subtracted from single crystal data, infers a quadratic temperature dependence over most of the temperature range (25 ≤ T ≤ 300). Power temperature dependence of ρdiff.{=[ρexp. – (ρ0 + ρe-ph(=ρac + ρop))]} points the contribution of electron–electron inelastic scattering. The present analysis allows us to infer that the single crystal experimental data is well approximated within the framework of BG electron–phonon model of resistivity. Further calculations of superconducting transition temperature and isotope effect exponent from Kresin's strong coupling theory indicates that the electron–phonon interaction plays an important role in the attractive pairing mechanism.

Phonon spectra in the conductance of Andreev junctions on Bi2223 at high pressure

A softening of the high-frequency phonon modes in Bi2223 ( T c=110 K) at high pressure is found. These modes correspond to the oxygen breathing mode and also the Cu–O bending modes. Qualitatively, the result obtained explains the observed rise in R=2 Δ/ kT c( P) ratio in the framework of the strong electron–phonon interaction model. However, the analysis of the R-ratio in the framework of the s+d model has revealed a possibility of explanation of the observed rise in R( P) also for the d-wave pairing.

Anomalous Dispersion of LO Phonons inLa1.85Sr0.15CuO4at Low Temperatures

Inelastic neutron scattering measurements of La1.85Sr0.15CuO4 show that a discontinuity in dispersion develops in the middle of the highest energy LO phonon branch at low temperatures. The result suggests dynamic short range unit cell doubling in the CuO2 plane along the direction of the Cu-O bond, such as charge ordering on every other row of oxygen. Such charge ordering is related to, but is different from, the stripe charge ordering observed for nonsuperconducting cuprates. The question of the influence of the electron-lattice interaction in the normal and superconducting electronic properties of high temperature superconductors remains open. Perhaps the most intriguing experimental evidence of a large and anomalous electron-lattice interaction comes from inelastic neutron scattering experiments on various high temperature superconducting materials [1‐ 6]. A signature of these materials is the strong renormalization of longitudinal high frequency (,80 meV) oxygen bond-stretching phonons in the CuO2 plane when holes are doped into the insulating compounds. The doping causes a large decrease in the frequency of the zone boundary s0.5, 0, 0d half-breathing phonon mode (15%‐ 20% softening compared to the insulating compound), indicating that there is a strong coupling of these phonons to the doped holes. This effect seems to be ubiquitous, and has been observed in single-crystal inelastic neutron scattering measurements of La22xSrxCuO4 [1], La2CuO41y [3], YBa2Cu3O61y [4,5], and even Ba12xKxBiO3 [6]. There are also indications of this effect from the observed softening of oxygen phonon bands in inelastic neutron scattering measurements of the phonon density of states in Pb2Sr2sCa,YdCu3O8 [7] and Li11xTi22xO4 [8]. It has proven quite difficult to interpret these results in terms of normal electron-phonon coupling in a metal, especially since the other measured phonon modes are, for the most part, unaffected (especially the oxygen breathing and quadrupole modes). Typical indications of electronphonon coupling, such as Kohn anomalies, seem to be inconsistent with the wave vector of the affected phonon. The Fermi surface should have strong nesting at 2kf which is close to q › s0.5, 0.5d, but not at s0.5, 0d, as demonstrated by the local-density approximation calculation [9] and ARPES measurements [10]. We have chosen to investigate these phonon phenomena more closely and have discovered that the anomalous nature of the bond-stretching branch is even more mysterious at low temperatures. The sample studied consisted of two single crystals of La1.85Sr0.15CuO4 which were comounted in an aluminum can filled with He exchange gas. Both samples were grown by the floating zone method and were obtained from the same batch. The total size of the sample is approximately 3.8 3 1.5 3 0.8 cm 3 . Previ

Superconductivity and normal state resistivity of Ba0.6K0.4BiO3: an optical phonon approach

We discuss the nature of the pairing mechanism and the physical properties associated with the normal as well as the superconducting state of cubic perovskites Ba0.6K0.4BiO3using the strong coupling theory. An interaction potential which includes the Coulomb, electron–optical phonon and electron–plasmon interactions is developed to elucidate the superconducting state. A model dielectric function is constructed with these interactions fulfilling thef-sum rule. The screening parameter (μ*=0.26) infers the poor screening of charge carriers. The electron–optical phonon strength (λ) estimated as 0.98 is consistent with an attractive electron–electron interaction and supports the moderate to strong coupling theory. The superconducting transition temperature of Ba0.6K0.4BiO3is then estimated as 32 K. Ziman's formula of resistivity is employed to analyse and compare this with the temperature-dependent resistivity of a single crystal. The estimated contribution from the electron–optical phonon together with the residual resistivity clearly infers a difference when a comparison is made with experimental data. The subtracted data infer a quadratic temperature dependence in the temperature domain (30≤ T ≤200 K). The quadratic temperature dependence of ρ [=ρexp−(ρ0+ ρe–ph)] is understood in terms of 3D electron–electron inelastic scattering. The presence of these el–el and el–ph interactions allows a coherent interpretation of the physical properties. Analysis reveals that a moderate to strong coupling exists in the Ba0.6K0.4BiO3system and the coupling of electrons with the high-energy optical phonons of the oxygen breathing mode will be a reason for superconductivity. The implications of the above analysis are discussed.