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.     

The Nodal SDW Gap and the Superconducting Gap in?BaFe2?x Co x As2

The spin density wave (SDW) transition in BaFe₂As₂ and the superconducting transition in BaFe_1.84Co_0.16As₂ were investigated by Raman scattering. The symmetries of the nodal SDW gap at 400?cm^?1 and the superconducting gap at 75?cm^?1 are both?B _2g. The superconducting coherent peak energy is smaller than the gap energy of the hole pocket, indicating that the peak is the resonant peak in the S _?? superconductor. The superconducting symmetry is given by B _2g in the orbital combination and A _1g (S _??) in the momentum space. The exchange interaction energies are estimated from the two-magnon peak.     

Raman and infrared studies of cupric oxide

Polarized Raman and fourier-transform infrared (FTIR) measurements have been made on a single crystal of CuO. Group theory predicts nine vibrations of which three (A _g , 2B _g ) are Raman-active and six (3A _u , 3B _u ) are infrared-active. We have observed three Raman modes at 296 (A _g ), 346 (B _g ¹ ) and 636 (B _g ² ) cm^?1. We have also observed six infrared modes at 146 (B _u ³ ), 164 (A _u ² ), 355 (A _u ³ ), 480 (B _u ¹ ), 542 (?) and 603 (B _u ² ) cm^?1. The normal frequencies and eigenvectors have been calculated using Wilson’s FG method; a good fit between theory and experiment has been obtained.     

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, ΔT _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.     

Synthesis of carbon nano-fibers on p-Si having improved temperature sensing capability

Synthesis of an innovative material for temperature sensor based on carbon nano-fibers (CNFs) on p-Si substrates has been demonstrated. The CNF films were characterized by SEM, Raman and FTIR studies. First order Raman spectra indicated a G band at ～1597 cm^?1 corresponding to the E_2g tangential stretching mode of an ordered graphitic structure with sp² hybridization and a D band located ～1350 cm^?1 originated from disordered carbon. Gold fingers were deposited on the p-Si/CNF surface for resistance measurement. Temperature sensing properties were also investigated critically. Resistance changes with temperature (ΔR/R) in p-Si/CNF films are found to be significantly large 30–60% Very stable, reproducible and improved temperature sensing properties would make this material superior to commonly available temperature sensors.     

Effect of Co doping on the structure,optical and magnetic properties of LaAlO3 thin films

LaAl_1?x Co _x O₃ (x = 0, 0.05 and 0.10) thin films were fabricated on quartz substrates by sol–gel method. X-ray diffraction data indicate that all thin films belong to perovskite LaAlO₃, and there is no secondary phase. Two obvious Raman peaks are observed in the Raman spectra, and the 113 cm^?1 peak is assigned to A₁ mode of perovskite LaAlO₃ while the 696 cm^?1 peak is caused by the Co–O stretching vibration. The band gap of the films decreases from 5.66 to 5.40 eV with the Co composition increasing from 0 to 10 %. The magnetization of the films was investigated, and it enhances significantly with increase of the Co content.     

Local structure and electron spin resonance of copper-doped SrTiO3 ceramics

We report X-ray diffraction and electron spin resonance (ESR) measurements of the effect of SrTiO₃ ceramics doping using Cu²⁺ ions. ESR measurements reveal two kinds of Cu²⁺ centers in weakly (0.2–0.5 mol% Cu) doped SrTiO₃. Both kinds of centers have been attributed to Cu²⁺ at octahedral Ti sites and possibly associated either with a nearest-neighboring oxygen vacancy (center #1) or some other positively charged defect (center #2). The ESR spectra of the above centers are described by the following spin Hamiltonian parameters: g _‖ = 2.263(1), g _⊥ = 2.041(1), A _‖ = 170(1) × 10^?4 cm^?1, A _⊥ = 27(1) × 10^?4 cm^?1 (center #1) and g _‖ = 2.334(1), g _⊥ = 2.059(1), A _‖ = 137(1) × 10^?4 cm^?1, A _⊥ ≈ 0(1) × 10^?4 cm^?1 (center #2). For copper concentration larger than 2 mol%, the antiferromagnetic SrCu₃Ti₄O₁₂ (SCTO) phase has been detected by both X-ray diffraction and ESR. Its volume increases with increase of Cu concentration reaching about 17 % at Cu doping of 20 mol%. The composite SrTiO₃–SCTO ceramics exhibits substantial magnetocapacitance effect, which could be enhanced by electrostriction of SrTiO₃.     

Influence of magnetic ion doping on structural,optical, magnetic and hyperfine properties of nanocrystalline SnO<Subscript>2</Subscript> based dilute magnetic semiconductors

This article reports the structural, optical and magnetic properties of transition metal (Ni, Co, Mn and Fe) doped SnO₂ nanoparticles prepared by modified Pechini sol–gel method. From the X-ray diffraction studies, it is obvious that all the synthesized samples show a phase purity of rutile tetragonal crystal structure of SnO₂. The morphology was studied and the particle sizes were estimated from the field emission scanning electron microscopy. From photoluminescence spectra, we observed emission due to the presence of singly ionized oxygen vacancies. Raman spectroscopy shows dominant peaks at 644 and 782 cm^?1 which were ascribed to A_1g and B_2g modes of the rutile structure. Isomer shifting due to dopant addition and large quadrupole splitting due to surface defects were observed in Mössbauer spectra. All the samples show ferromagnetic ordering up to 1 T. The relatively stronger ferromagnetic nature in Fe and Co doped SnO₂ is due to the strong p–d exchange interaction. In case of Ni and Mn doped SnO₂ samples, the lack of carrier-mediated interaction due to its inherent semiconducting nature reduces the total magnetic moment observed in these samples. The exchange coupling depends on the dopant type and its concentration.     

Superconducting gap in Pr x Y1−x Ba2Cu4O8 and YBa2−y Sr y Cu4O8 probed by infrared phonon self-energies

We report a reflectivity study of thez-polarized TO-phonons of Pr _x Y_1?x Ba₂Cu₄O₈ and YBa_2?y Sr _y Cu₄O₈ alloys in the temperature range 10–300 K. Anomalies of the frequency and linewidth of the plane-oxygen vibration atω～300 cm^?1 due to the opening of the superconducting gap are found to occur upon crossing the superconducting transition temperatureT _c . Phonon self-energy effects are strongly dependent onT _c , providing evidence for a relative shift of the gap with respect to the energy of phonon.     

Infrared emission of single-crystal calcite under broadband short-wavelength excitation

Our results demonstrate that broadband excitation of an oriented calcite single crystal with the radiation from a cw xenon lamp (incident power density of ～1 W/cm²) gives rise to secondary IR emission from the crystal surface. We have measured the IR emission spectrum of a calcite single crystal in two orientations in the range 500–1500 cm^?1 with a resolution of ～1 cm^?1 using a Fourier transform IR spectrometer. The spectrum shows sharp lines arising from IR-active (polar) vibrational transitions in the calcite crystal and can be interpreted in trems of a two-photon decay of a nonpolar vibrational state (A _1g mode) effectively excited through broadband visible pumping. We analyze the ways of raising the efficiency of the conversion of primary (visible) to secondary IR radiation. In particular, this might be achieved using photonic crystals and an excitation frequency near the maximum in their density of states. The new type of IR emission spectroscopy, taking advantage of broadband visible or UV excitation of molecular vibrations in bulk dielectric inorganic materials or materials infiltrated into the pore space of globular photonic crystals, appears to have considerable promise.     

The molecular structure ordering and orientation of the metallophthalocyanine CoPc,ZnPc, CuPc,and MgPc thin layers deposited on silicon substrate,as studied by micro-Raman spectroscopy

In this article, we present orientation study of metallophthalocyanine (MPcs) (CoPc, ZnPc, CuPc, and MgPc) thin films deposited on silicon substrate. The MPc’s thin layers were obtained by the quasi-molecular beam evaporation. The micro-Raman scattering spectra of MPc’s thin films were investigated in the spectral range 550–1650 cm⁻¹ using 488 nm excitation wavelength. Raman scattering studies were performed at room temperature before and after annealing process. Annealing process of thin layers was carried out at 200 °C for 6 h. From polarized Raman spectra using surface Raman mapping, the information on polymorphic phase of MPc’s layers has been obtained. The chosen Raman modes A_1g and B_1g are connected with different polymorphic phases of MPc (α and β form) thin layers. Moreover, the obtained results showed the influence of the annealing process on the ordering of the molecular structure. Following the annealing process, it was observed arrangement of the thin layers structure being revealed in Raman spectra. The obtained results indicate that the annealing process has a significant influence on the structure of thin layers being under study.     

Enhanced multiferroic and dielectric properties of Sr2+-doped BiFe0.94(Mn0.04Cr0.02)O3 thin films

Sr²⁺-doped B_1?xSr_xFe_0.94(Mn_0.04Cr_0.02)O₃ (B_1?xSr_xFMC, x = 0.00, 0.05 and 0.09) thin films were prepared on FTO/glass (SnO₂: F) substrates by using a sol–gel spin-coating method. X-ray diffraction (XRD), Rietveld refined XRD data and Raman scattering spectra illustrate a structural evolution from trigonal (R3c: H) to tetragonal (P4) occurs in B_1?xSr_xFMC thin films with the increase of Sr²⁺ concent. Superiorly multiferroic and dielectric properties were obtained in the B_1?xSr_xFMC thin films, e.g., large remanent polarization value, a high dielectric constant (P _r = 139.21 μC/cm² and ε_r = 396.7 for x = 0.09) and large saturation magnetization (M _s = 2.08 emu/cm³ for x = 0.05). The leakage current density of B_1?xSr_xFMC thin films is increased with increasing Sr²⁺ concentration, wherein the leakage current density of all the B_1?xSr_xFMC films is of the order of magnitude of 10^?5 A/cm², which are still lower than that in the pure BFO film (10^?3 A/cm²).     

Morphology and surface structure of nanocarbon allotropes: A comparative study

Different carbon allotropes, including vulcan carbon, multiwall carbon nanotubes, graphene, and nanodiamonds, were processed by chemical purification and treated in a mixture of H₂SO₄–HNO₃. The materials were characterized by infrared and Raman spectroscopy as well as by scanning and transmission electron microscopy. Oxidative differences are indicated by Raman through the G band (～1570 cm^?1), D band (～1340 cm^?1), and G’ band (～2684 cm^?1). The crystal size (L_a) and purity, relative to the amorphous carbonaceous material, were studied as well, along with the morphological changes induced by the treatments.     

Copper doped zinc oxide Y2BaZnO5: ERS and optical investigation

In copper doped Y₂BaZnO₅ oxides, copper exhibits a distorted square pyramidal coordination which is consistant with the values of g and A tensors obtained from O band ERS spectrum for a sample containing about 1 % Cu. Three values for g and A are observed, g₁ = 2.049₅, g₂ = 2.051₅, g₃ = 2.275, $\text{¦}\text{A}{}_1\text{¦ = 13 10}{}^{\mathrm{?4}}\text{cm}{}^{\mathrm{?1}}$, $\text{¦}\text{A}{}_2\text{¦ = 10 10}{}^{\mathrm{?4}}\text{cm}{}^{\mathrm{?1}}$ and $\text{¦}\text{A}{}_3\text{¦ = 147.5 10}{}^{\mathrm{?4}}\text{cm}{}^{\mathrm{?1}}$. Since $\text{g}{}_1\text{?}\text{g}{}_2$ an approximate C_4v point symmetry can be assumed for copper. The electronic spectrum shows three bands at 11700, 14500 and 20500 cm^?1 which can be assigned to the transitions A₁ → B₁, B₂ → B₁ and E → B₁ respectively. The orbital reduction parameters are calculated and the bonding covalency is discussed.     

Raman scattering from single crystal YBa2Cu3O7-δ in a magnetic field

A magneto-Raman study of a YBa₂Cu₃O_7-δ single crystal(T _c = 92 K) was carried out at a resolution of 2 cm^-1 over the temperature range 5–125 K. At temperatures belowT _c we observed a slight narrowing of the 340 cm^-1 Raman mode in a magnetic field of 5.15 T directed parallel to thec-axisof the crystal. The observed magneto-temperature dependences of the Raman continuum intensity measured in the low-frequency region, at ca. 40 cm^-1, indicate contributions of electronic excitations arising from a pair-breaking process. Within the experimental uncertainty, the Raman intensity of the high-frequency continuum was observed to be independent of the magnetic field strength.     

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.     

Boron Embedded in Metal Iron Matrix as a Novel Anode Material of Excellent Performance

Boron, the most ideal lithium‐ion battery anode material, demonstrates highest theoretical capacity up to 12 395 mA h g^?1 when forming Li₅B. Furthermore, it also exhibits promising features such as light weight, considerable reserves, low cost, and nontoxicity. However, boron‐based materials are not in the hotspot list because Li₅B may only exist when B is in atomically isolated/dispersed form, while the aggregate material can barely be activated to store/release Li. At this time, an ingenious design is demonstrated to activate the inert B to a high specific capacity anode material by dispersing it in a Fe matrix. The above material can be obtained after an electrochemical activation of the precursors Fe₂B/Fe and B₂O₃/Fe. The latter harvests the admirable capacity, ultrahigh tap density of 2.12 g cm^?3, excellent cycling stability of 3180 mA h cm^?3 at 0.1 A g^?1 (1500 mA h g^?1) after 250 cycles, and superlative rate capability of 2650 mA h cm^?3 at 0.5 A g^?1, 2544 mA h cm^?3 at 1.0 A g^?1, and 1696 mA h cm^?3 at 2.0 A g^?1. Highly conductive matrix promoted reversible Li storage of boron‐based materials might open a new gate for advanced anode materials.     

Control of particle size in Si3N4 powders prepared by high-pressure carbothermal nitridation

The grain size variation in “unseeded” Si₃N₄ powders, prepared by high-pressure carbothermal nitridation of SiO₂ (in stoichiometric 1∶2 proportions with C), has been studied by means of scanning electron microscopy (SEM) and “Sedigraph” measurements. The size is a function of process parameters, of which the reactant surface area was found to be the most important. Specifically, with an SiO₂ areaA(SiO₂) ≈ 50 m² g^?1 in the reaction mixture, the resulting mean Si₃N₄ particle diameter,d(Si₃N₄), is very sensitive to the carbon surface area,A(C), such that the minimumd(Si₃N₄) ≈ 1 μm was obtained withA(C)=30m²g^?1 and the maximumd(Si₃N₄) ≈ 7μm withA(C)=115m²g^?1. Using mixtures withA(SiO₂)=50m²g^?1 andA(C)=115m²g^?1, a slight dependence ofd(Si₃N₄) on the furnace heating rate was also observed; larger grains (≈ 7 μm) were obtained with 20deg min^?1 than with 2deg min^?1 (≈5μm). The grain size was found to be virtually independent of nitrogen pressure (in the range 0.3–6.5 MPa), annealing temperature (1470–1830°C) and gas flow rate (2–20 l_(stp) min^?1).     

ESR and X-ray Investigations of Deuterium Atoms and Molecules in Impurity-Helium Solids

Impurity-helium solids created by injecting deuterium atoms and molecules into superfluid ⁴He have been studied via electron spin resonance (ESR) and x-ray diffraction methods. We measured the g-factor, the hyperfine constant and the spin-lattice relaxation time of D atoms in D-D₂-He solids. These measurements show that D atoms are mainly stabilized in D₂ clusters. Using an x-ray method we found the size of D₂ clusters to be ～90Å in diameter and the densities of D₂ molecules in the samples to be of order 2.5?10²¹ cm^?3 . The highest average concentration of D atoms achieved in D-D₂-He solids was ～1.5?10¹⁸ cm^?3 . The local concentrations of D atoms within D₂ clusters is found to be large (～2?10¹⁹ cm^?3).