Electronic Raman scattering of superconducting YBa2Cu3Oy single crystals

TheA _1g andB _1g low-energy Raman continua of YBa₂Cu₃O _y (Y123) single crystals, withy=7.0, 6.99, and 6.93, have been investigated. It is found that the peak frequency of theA _1g continuum is equal to 310±10 cm^–1 and independent of oxygen concentration fory in the above range. The central frequency of the broad peak in theB _1g continuum, however, shifts from about 470 cm^–1 fory7.0 to 550 cm^–1 fory6.93. Thus, a relatively small change in oxygen concentration results in a significant redistribution of the states contributing to theB _1g continuum. Assuming the low-energy portions of the continua are electronic in origin, the Raman spectra have been calculated and the results compared to the experimental spectra. It is suggested that the Raman continua arise, at least in part, from scattering across a spin fluctuation-induced pseudogap.     

Raman scattering studies of ultrathin-layer YBa2Cu3O7/PrBa2Cu3O7 superlattices

We present Raman scattering studies ofc-oriented ultrathin-layer superconducting (YBa₂Cu₃O₇) _m /(PrBa₂Cu₃O₇) _n superlattices. For the superlattice with (m=2,n=1) sequence, Raman spectra reveal a new line in the spectral region around 320 cm^–1. It is interpreted as a mode representing a combination of IR optical phonons of the Y-sublayers with an admixture of aB _1g type Raman active vibration in the Pr sublayers. This new line, which is similar to those from the interior of the Brillouin zone of the original lattice, does not exhibit superconductivity-induced self-energy effects, although its counterpart in the pure substance does. No additional line is found in the (m=1,n=2) superlattice in the same region, supporting our interpretation for the (m=2,n=1) sample.     

Investigation of the temperature dependence of electron and phonon Raman scattering in single crystal YBa2Cu3O6.952

Detailed Raman-scattering measurements have been performed on high-quality YBa₂Cu₃O_6.952 single crystal (T _c =93 K, T _c =0.3 K). A sharp (FWHM 7.2 cm^–1 at 70 K and 10.0 cm^–1 at 110 K) 340 cm^–1phonon mode has been observed inB _1g polarization. An electronic scattering peak at 500 cm^–1 in theB _1g polarization extends down to 250 cm^–1. These FWHM values determine the upper limit of the homogeneous linewidth of the phonon and electronic excitations. The start of the electronic spectral function renormalization and of the 340 cm^–1 mode anomalies (frequency softening, linewidth sharpening, and intensity increase) have been observed to occur approximately 40 K aboveT _c . The 340 cm^–1 mode Fano shape analysis has been performed and the temperature dependences of the Fano shape parameters have been estimated. All 340 cm^–1 mode anomalies have been explained by the electronic spectral function renormalization.This work was supported by Swedish Natural Sciences Research Council (G.B. and L.B.) and by the National Science Foundation (DMR 91-20000) through the Science and Technology Center for Superconductivity (G.B. and M.V.K.).     

Spectroscopic studies of hydrogen related defects in CVD diamond

Thin diamond films prepared by the hot filament chemical vapour deposition (HFCVD) method at various deposition pressures have been characterized using a variety of spectroscopic techniques. Interpretation of the spectral details have provided useful information about the nature of the films. Deposition pressure appears to affect the quality of the diamond films which is reflected in terms of the position and width of the characteristic Raman peak of diamond. Raman spectra of the films prepared at low deposition pressures showed the presence of a sharp peak at ∼1332 cm⁻¹ characteristic of theT _2g mode of diamond. The study of the effect of deposition pressure on the diamond growth, shows that in the range between 20 torr and 60 torr, there is little effect on the width and the shift of the 1332 cm⁻¹ Raman peak. However, at higher pressures the peak showed a blue shift and was considerably broadened. These studies indicate the development of strain in the lattice due to the introduction of unetched hydride layer, at higher deposition pressures, as well as distortions in the lattice leading to partial lifting of the degeneracy of theT _2g mode. A broad band corresponding to the non-diamond phase (which exists at the grain boundaries, interface or as inclusions inside the grain), which can be attributed to the effect of hydrogen impurity creeping into the lattice at higher deposition pressures is also observed. SEM and XRD patterns have confirmed the dominance of diamond phase in these films.     

Effect of temperature on thermal expansion and anharmonicity in Cu2ZnSnS4 thin films grown by co-sputtering and sulfurization

Copper zinc tin sulfide (CZTS, made from the earth abundant and non-toxic materials) is a quaternary semiconducting compound which has received increasing interest for solar cells applications. In this study, CZTS thin film has been grown by co-sputtering Cu, Zn and Sn metal targets and sulfurizing it in H₂S. XRD, SEM, EDS, XRF and optical studies show that these films are suitable for solar cell applications. The temperature-dependent Raman spectroscopic study on CZTS thin ?lm was carried out in the temperature range of 80–450 K. A decrease in the intensity of “A” mode Raman peaks, shift of Raman peaks towards lower frequency and increase in the line width have been observed with increase in temperature. Raman “A” mode shifts from 337 cm⁻¹ at 80 K to 329 cm⁻¹ at 450 K. The peak at 288 cm⁻¹ disappears when the measurement was taken at 450 K. Based on experimental results and fittings of anharmonic equation; it has been shown that these effects are due to thermal expansion and interaction of the phonon with other phonons which arise at higher temperature. These phonons interact resulting in damping of the “A” mode phonon intensity. It was concluded that in order to truly analyze multicomponent compounds, Raman studies should be carried out at low temperature.     

Raman temperature dependence analysis of carbon-doped titanium dioxide nanoparticles synthesized by ultrasonic spray pyrolysis technique

Carbon-doped titanium dioxide nanoparticles were prepared by ultrasonic spray pyrolysis technique using titanium tetra-ethoxide as a precursor and glucose as a dopant. The as-synthesized nanoparticles were then characterized using high resolution transmission electron microscopy for the structural properties and temperature dependence Raman spectroscopy for the optical properties. High resolution transmission electron microscopy analysis shows that the ultrasonic synthesized carbon-doped titanium dioxide nanoparticles have an interplanar d-spacing of 0.352 nm, a value close to 0.374 nm of the pure undoped anatase titanium dioxide (bulk). The fast Fourier transform (FFT) of the selected electron diffraction images, of the selected areas, display the fact that only the main diffraction (reflection) plane of Miller indices (101) in titanium dioxide is responsible for diffraction pattern. Raman spectroscopy confirms the titanium dioxide polymorph to be anatase with the intense phonon frequency at 153 cm⁻¹ blue-shifted from 141 cm⁻¹ due to both carbon doping and particle size decrease. The temperature dependence analysis of the spectra shows an initial linear dependence of the Raman shift for the E _g(1) mode at 152.7 cm⁻¹ with increase in temperature up to a critical temperature 450 °C, after which we observe a decrease with increase in temperature. The other Raman modes [B _1g and E _g(3)] exhibit a different temperature dependence in that the B _1g displays a somewhat sinusoidal behavior and the E _g(3) mode shows a linear decrease of Raman shift with an increase in temperature. Temperature dependence analysis of peak width for the E _g(1) indicates the peak width of the as prepared nanoparticle to be 20 cm⁻¹ which is far much larger than that for single crystal of 7 cm⁻¹ at room temperature.     

Sol-gel synthesis of lead-free (K0.5Bi0.5)0.4Ba0.6TiO3 nanorods

A kind of lead-free ferroelectric nanorods, (K_0.5Bi_0.5)_0.4Ba_0.6TiO₃, has been prepared by the sol-gel process. The phase formation, structure and morphological analyses of (K_0.5Bi_0.5)_0.4Ba_0.6TiO₃ were investigated by XRD, FTIR, Raman and TEM. The results revealed that single-crystalline (K_0.5Bi_0.5)_0.4Ba_0.6TiO₃ nanorods with width around 80 to 120 nm, and length around 200-300 nm were obtained by calcining dried gels at 800 °C for 2 h. Raman analysis of (K_0.5Bi_0.5)_0.4Ba_0.6TiO₃ nanorods indicated that A₁(TO₂) and E(TO) mode incorporated into one broad peak at around 285 cm^− 1, which can be attributed to the cation disorder (Bi, K, Ba) on the 12-fold coordinated A site of ABO₃ structure.     

Mechanical Exfoliation and Characterization of Single‐ and Few‐Layer Nanosheets of WSe2, TaS2, and TaSe2

Single‐ and few‐layer transition‐metal dichalcogenide nanosheets, such as WSe₂, TaS₂, and TaSe₂, are prepared by mechanical exfoliation. A Raman microscope is employed to characterize the single‐layer (1L) to quinary‐layer (5L) WSe₂ nanosheets and WSe₂ single crystals with a laser excitation power ranging from 20 μW to 5.1 mW. Typical first‐order together with some second‐order and combinational Raman modes are observed. A new peak at around 308 cm^?1 is observed in WSe₂ except for the 1L WSe₂, which might arise from interlayer interactions. Red shifting of the A_1g mode and the Raman peak around 308 cm^?1 is observed from 1L to 5L WSe₂. Interestingly, hexagonal‐ and monoclinic‐structured WO₃ thin films are obtained during the local oxidation of thinner (1L–3L) and thicker (4L and 5L) WSe₂ nanosheets, while laser‐burned holes are found during the local oxidation of the WSe₂ single crystal. In addition, the characterization of TaS₂ and TaSe₂ thin layers is also conducted.     

Residual stresses and Raman shift relation in anatase TiO2 thin film

Anatase TiO₂ film (100-1000 nm thick) grown on glass, sapphire (0001), and Si (100) substrates by pulsed dc-magnetron reactive sputtering were evaluated for stress and strain analysis using Raman spectroscopy and curvature measurement techniques. The X-ray analysis revealed that films prepared for this study were purely anatase, and the measurements indicate that the film exhibit that (101) is the preferred growth orientation of the crystallites, especially for the film thicker than 100 nm. Curvature measurements and Raman spectroscopy, with 514.5 nm excitation wavelength, phonon line shift were used for stress analysis. A comparison between Raman lineshapes and peak shifts yields information on the strain distribution as a function of film thickness. The measurements of residual stresses for crystalline anatase TiO₂ thin film showed that all thin film were under compressive stress. A correlation between Raman shifts and the measured stress from the curvature measurements was established. The behavior of the anatase film on three different substrates shows that the strain in film on glass has a higher value compared to the strain on sapphire and on silicon substrates. The dominant 144 cm^− 1E_g mode in anatase TiO₂ clearly shifts to a higher value by 0.45-5.7 cm^− 1 depending on the type of substrate and film thickness. The measurement of the full width at half maximum values of 0.59-0.80 (2θ°) for the anatase (101) peaks revealed that these values are greater than anatase powder 0.119 (2θ°) and this exhibits strong crystal anisotropy with thermal expansion.     

Preparation and magnetic study of a new iridium (V) perovskite: LaLi0.5Ir0.5O3

A new Ir(V) oxide LaLi_0.5Ir_0.5O₃ with an ordered orthorhombic perovskite structure has been prepared. Its lattice parameters are a = 5.63, b = 5.58 and c = 7.87 Å. The possible space group is Pmm2. Ir(V) has been characterized magnetically with a low spin configuration t⁴_2ge⁰_g (³T_1g ground term) and a large spin-orbit coupling constant (7485 ± 30 cm^?1).     

Temperature dependent Raman scattering of the epitaxial Sr1.9Ca0.1NaNb5O15 film

Epitaxial ferroelectric Sr_1.9Ca_0.1NaNb₅O₁₅ (SCNN) thin films have been fabricated successfully by pulsed laser deposition on (100)MgO substrates. Temperature dependency of the vibrational modes of these films was investigated for the first time using Raman scattering technique. For temperatures ranging between 50 °C and 450 °C, two strong and broad A₁(TO) phonons around 238 and 608 cm^− 1, and one weak B₁(TO) phonon around 142 cm^− 1 were observed. Changes in the temperature dependency of the peak position, the full-width at half maximum and integrated intensity of the Raman modes were observed in different temperature ranges. These changes are attributed to the structural phase transitions between 200 °C and 325 °C for the SCNN films as well as their relaxor properties.     

Investigations of the Spin Hamiltonian Parameters for the Cu2+ Sites in PrBa2Cu3O6+x and Pr0.5Er0.5Ba2Cu3O6+x

The spin Hamiltonian parameters (the anisotropic g factors g _∥ and g _⊥ and the hyperfine structure constants) for the Cu²⁺ sites in PrBa₂Cu₃O_6+x and Pr_0.5Er_0.5Ba₂Cu₃O_6+x are theoretically investigated using the high order perturbation formulas of these parameters for a 3d⁹ ion in tetragonally elongated octahedra. In these formulas, the tetragonal field parameters are determined from the superposition model and the local structures of the Cu²⁺ sites. The theoretical spin Hamiltonian parameters are in good agreement with the observed values for both systems, and the results show improvements as compared with those based on various adjustable covalency coefficients in the previous work. The larger hyperfine structure constants for PrBa₂Cu₃O_6+x than those for Pr_0.5Er_0.5Ba₂Cu₃O_6+x can be attributed to the weaker covalency due to the relatively longer Cu²⁺– O^[`2]\mathrm{O}^{\bar{2}} bonds in the former.     

Investigation of Gd Addition Added on Magnetic and Structural Properties of Bi1.8Pb0.35Sr1.9Ca2.1Cu3GdxOy Superconductors by ac Susceptibility

This study reports the effect of Gd addition on magnetic and structural properties of Bi_1.8Pb_0.35Sr_1.9Ca_2.1Cu₃Gd _x O _y superconductor with x=0, 0.1, 0.2, 0.3, 0.4 and 0.5 by means of ac susceptibility measurements at various ac fields (ranging from 270 to 1352 A/m) and scanning electron microscopy (SEM) images. Critical onset (T_c^on)T_{\mathrm{c}}^{\mathrm{on}}) and loss peak temperatures (T _p) were qualitatively estimated from the ac susceptibility curves. The peak temperature at zero ac-magnetic field (T _p0) and intergrain critical current densities (J _c) were theoretically calculated from the ac susceptibility plots via the critical state models. The results show that peak temperatures and critical current densities were found to decrease with increasing Gd addition. Moreover, using a self-field approximation together with J _c dependence on temperature, the characteristic length (L _c) associated with the pinning force is estimated to be approximately the same as the average grain size (R _g) of the pinning center because of the linear decrease in J _c with increasing temperature. Surface morphology and grain connectivity of the samples were also obtained to degrade with increase in the Gd addition from SEM investigations.     

New glasses within the Tl2O-Ag2O-TeO2 system: Thermal characteristics, Raman spectra and structural properties

Within the Tl₂O-Ag₂O-TeO₂ system, a large glass-forming domain was evidenced and is presented for the fist time. Densities, glass transition (T_g) and crystallization (T_c) temperatures of the relevant glasses were measured. A structural approach of these glasses as functions of the composition was performed using Raman scattering. The Raman spectra were analysed in terms of the structural modifications induced by the Tl₂O and Ag₂O modifiers. It has clearly evidenced a phase separation inherent in tellurite glasses with low valence cations (as Tl⁺ and Ag⁺). The glasses would be constituted of two phases only: one of pure TeO₂ and one of pure ortho-tellurite M₂TeO₃ (M = Ag, Tl) with the statistically mixed Ag-Tl cationic composition.     

Microstructure and microwave dielectric properties of Ba(Mg1/3Nb2/3)O3–MgO composite ceramics with nano-silica added

Ba(Mg_1/3Nb_2/3)O₃–MgO composite ceramics were prepared by solid-phase method. The ceramic exhibited the 1:2 ordered structure. By adding a proper amount of MgO, the permittivity decreased rapidly compared with other Ba(Mg_1/3Nb_2/3)O₃-based ceramics. The Q?×?f values of the samples were greatly improved by nano-silica. Raman spectra showed that the permittivity and Q?×?f value showed a strong correlation with Raman shift and full width at half maximum of the A_1g(O) phonon mode, respectively. The Raman shift of A_1g(O) was consistent with the variation trend of permittivity. And the full width at half maximum of the A_1g(O) phonon mode had a negative correlation with the Q?×?f value. The results showed that upon adding 1.5 wt% nano-silica to the ceramics, the ceramics sintered at 1550 °C for 5 h had the lowest Raman shift and the narrowest full width at half maximum, achieving the best microwave dielectric properties: ε_r?=?22.22, Q?×?f?=?80,436 GHz, τ_f?=?–?5.89 ppm/°C.

     

Synthesis of Nb2O5 nanosheets and its electrochemical measurements

In this study, Nb₂O₅ nanosheets were first synthesized using NbO₂ particles as the precursor via a simple hydrothermal route. The synthesized Nb₂O₅ nanosheets are highly crystalline and their thicknesses are found to be ca. 3–5 nm. Based on the experimental results of XRD, SEM and TEM measurements, a possible mechanism for the formation of nanosheets was discussed. An electrode materials made of the product containing Nb₂O₅ nanosheets shows a larger capacity of 355 mAh g⁻¹ at a current density of 0.1 A g⁻¹. Cyclic measurements indicate that such an electrode exhibits a high reversible charge/discharge capacity and cycling stability. This might be attributed to the intrinsic characteristics of Nb₂O₅ nanosheets.     

Electron–Phonon Interaction and Phonon Renormalization in the Lamellar Cobaltate Na x CoO2

We study theoretically the electron–phonon interaction in Na _x CoO₂. For the A _1g and E _1g phonon modes found in Raman experiments, we calculate the matrix elements of the electron–phonon interaction. Analyzing the feedback effect of the conduction electrons on the phonon frequency ω, we investigate the doping dependence of these two phonon modes. Due to the momentum dependence of the electron–phonon interaction, we find the strongest renormalization of the E _1g mode around the Brillouin zone boundary which should be observed in the neutron scattering. At the same time, the A _1g mode shows the strongest coupling to the conducting electrons around the Γ point and reveals its doping dependence in the Raman experiments. Our results shed light on the possible importance of the electron–phonon interaction in the lamellar sodium cobaltates.     

New features of the Mott-Hubbard insulator gap of parent HTS compounds found by resonance Raman scattering and UV-excited ordinary Raman scattering

Optical absorption at the insulating gap in the parent phase of cuprate superconductors shows a broad exciton-like peak near 1.7 eV, followed by a gradual decrease in absorption persisting 1 eV above the gap. By using ultraviolet laser lines to excite Raman spectra, we have found a Raman peak 0.2 eV below the first absorption peak in insulating cuprates. The Raman peak is much narrower than the absorption peak and hasA _2g symmetry. We assign it to an exciton consisting of a hole transition from Cu to a linear combination of Cud _xy and nearest neighbor Op orbitals. We have also studied the resonance Raman profile for two-magnon Raman scattering in the same samples. We find a sharp resonance feature at about 2.7 eV, and little Raman intensity for photon energies at the 1.7 eV absorption peak. The state created at the peak must therefore be an inappropriate intermediate state for the double spin-flip Raman process.     

Crystal structure and anisotropy of iodine-intercalated Bi2Sr2Can?1CunOx

The electronic state and structural configuration of the intercalated iodine species in stage-1, I-Bi₂Sr₂Ca _n–1Cu _n O _x (n = l, 2), have been studied through polarization-resolved Raman and¹²⁹I Mössbauer spectroscopy. The polarization dependence of the Raman spectra and the Mössbauer measurement confirmed the dominant species to be triiodide ions, I ₃ ^– , with alignment of these linear molecules either along thea- orb-axis in the host crystals. Transport measurements such as thermoelectric power and Hall coefficient clearly indicated that hole carriers are doped into the CuO₂ planes upon intercalation, by whichT _c of the host superconductor is changed. Furthermore, based on resistivity measurements in a magnetic field, we suggest that the iodine intercalation leads to a decrease of the anisotropy both in normal and superconducting states, suppressing the extremely two-dimensional character of the Bi₂Sr₂Ca _n–1Cu _n O _x systems.     

Raman optical properties of Cu(OH)2 nanowires

We have for the first time investigated Raman optical properties of polycrystalline Cu(OH)₂ nanowires with an average diameter of ca. 9 nm and lengths of up to hundreds of micrometers that were synthesized by a simple surfactant-free solution-phase route at ambient temperature. The result indicated that the Raman peak of the as-prepared polycrystalline Cu(OH)₂ nanowires was obviously broadened, and red-shifted 16 cm^− 1 compared with that of bulk Cu(OH)₂ crystals. Based on the microstructure analysis and phonon confinement model of a crystal, we have proposed a rational explanation for the red-shift and broadening of Raman peak.