Infrared transmission study of Pr<Subscript><Emphasis Type="Bold">2</Emphasis></Subscript>CuO<Subscript><Emphasis Type="Bold">4</Emphasis></Subscript> crystal-field excitations

We present an infrared crystal-field study of Pr₂CuO₄ single crystals and thin films. Excitations from the ground state multiplet ³H₄ to the ³H₅, ³H₆, ³F₂ and ³F₃ excited multiplets are observed in both single crystals and thin films. A precise set of crystal-field parameters, that reproduces the energy and the symmetry of the levels, is determined. Received 25 April 2001     

Comparative study of hydrofullerides C<Subscript><Emphasis Type="Bold">60</Emphasis></Subscript>H<Subscript><Emphasis Type="BoldItalic">x</Emphasis></Subscript> synthesized by direct and catalytic hydrogenation

Hydrofullerides with hydrogen content up to 5 wt.% were obtained by direct and catalytic reactions with H₂ gas. Hydrogen content was monitored in situ using a gravimetric system, and verified by chemical analysis ex situ. It was found that pure C₆₀ reacts rapidly when exposed to H₂ gas at 673 K and 50–100 bar. Gravimetric study of this reaction showed that hydrogenation is saturated at about 5 wt.% of hydrogen. The mass of the sample goes through a maximum and with a longer reaction time its weight starts to decrease. This proves that hydrofullerides with high hydrogen content are not stable and strong hydrogenation results in the collapse of C₆₀ molecules. XRD studies showed that samples prepared by direct hydrogenation without a catalyst retain an original fcc structure with an increase of the cell parameter a up to 15.1 Å. Catalytic hydrogenation of C₆₀ with H₂ gas results in a decrease of the reaction temperature and formation of hydrofullerides with different types of crystal structures. PACS 61.48.+c; 61.10.Nz; 81.05.Tp     

Electronic Structure and Magnetic Phase Transition in Helicoidal Fe<Subscript>1 - <Emphasis Type="Italic">x</Emphasis></Subscript>Co<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Si Ferromagnets

LSDA + U + SO calculations of the electronic structure of helicoidal Fe_{1 - x}Co _x Si ferromagnets within the virtual crystal approximation have been supplemented with the consideration of the Dzyaloshinski-Moriya interaction and ferromagnetic fluctuations of the spin density of collective d electrons with the Hubbard interactions at Fe and Co atoms randomly distributed over sites. The magnetic-state equation in the developed model describes helicoidal ferromagnetism and its disappearance accompanied by the occurrence of a maximum of uniform magnetic susceptibility at temperature T _C and chiral fluctuations of the local magnetization at T > T _C . The reasons why the magnetic contribution to the specific heat at the magnetic phase transition changes monotonically and the volume coefficient of thermal expansion (VCTE) at low temperatures is negative and has a wide minimum near T _C have been investigated. It is shown that the VCTE changes sign when passing to the paramagnetic state (at temperature T _S ).     

Optical spectroscopy of intermetallic compounds TbNi<Subscript>2</Subscript>Mn<Subscript> <Emphasis Type="Italic">x</Emphasis> </Subscript> (<Emphasis Type="Italic">x</Emphasis> = 0, 0.5, 1)

The optical properties of intermetallic compounds TbNi₂Mn_x (x = 0, 0.5, 1) have been investigated using the ellipsometric method in the spectral range from 0.22 to 16 μm. The specific features of the modification of the dispersions of spectral characteristics with a variation in the manganese concentration have been determined. The behavior of the frequency dependences of the optical conductivity in the interband absorption region has been discussed in terms of the available data on the electronic structure of these compounds. The concentration dependences of a number of electronic parameters have been calculated.     

Anomalous temperature dependence of photoluminescence in GeO<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> films and GeO<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>/SiO<Subscript>2</Subscript> nano-heterostructures

The optical properties of GeO _x film and GeO _x /SiO₂ multilayer heterostructures (with thickness of GeO _x layers down to 1 nm) were studied with the use of Raman scattering and infrared spectroscopy, ellipsometry and photoluminescence spectroscopy including temperature dependence of photoluminescence. The observed photoluminescence is related to defect (dangling bonds) in GeO _x and interface defects for the case of GeO _x /SiO₂ multilayer heterostructures. From analysis of temperature dependence of photoluminescence intensity, it was found that rate of nonradiative transitions in GeO _x film has Berthelot type, but anomalous deviations from Berthelot type temperature dependence were observed in temperature dependences of photoluminescence intensities for GeO _x /SiO₂ multilayer heterostructures.     

Physical properties,crystal and magnetic structure of layered Fe<Subscript>1.11</Subscript>Te<Subscript>1-</Subscript><Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Se<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> superconductors

The physical and structural properties of Fe_1.11Te and Fe_1.11Te_0.5Se_0.5 have been investigated by means of X-ray and neutron diffraction as well as physical property measurements. For the Fe_1.11Te compound, the structure distortion from a tetragonal to monoclinic phase takes place at 64 K accompanied with the onset of antiferromagnetic order upon cooling. The magnetic structure of the monoclinic phase was confirmed to be of antiferromagnetic configuration with a propagation vector k = (1/2, 0, 1/2) based on Rietveld refinement of neutron powder diffraction data. The structural/magnetic transitions are also clearly visible in magnetic, electronic and thermodynamic measurements. For superconducting Fe_1.11Te_0.5Se_0.5 compound, the superconducting transition with T _c = 13.4 K is observed in the resistivity and ac susceptibility measurements. The upper critical field H _c2 is obtained by measuring the resistivity under different magnetic fields. The Kim’s critical state model is adopted to analyze the temperature dependence of the ac susceptibility and the intergranular critical current density is calculated as a function of both field amplitude and temperature. Neutron diffraction results show that Fe_1.11Te_0.5Se_0.5 crystalizes in tetragonal structure at 300 K as in the parent compound Fe_1.11Te and no structural distortion is detected upon cooling to 2 K. However an anisotropic thermal expansion anomaly is observed around 100 K.     

Dynamics of the first-order magnetostructural transition in Gd<Subscript>5</Subscript>(Si<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Ge<Subscript>1-<Emphasis Type="Italic">x</Emphasis></Subscript>)<Subscript>4</Subscript>

For a given crystal structure, say body-centred-cubic, the many-body Hamiltonian H in which nuclear and electron motions are to be treated from the outset on the same footing, has parameters, for the elements, which can be classified as (i) atomic mass M, (ii) atomic number Z, characterizing the external potential in which electrons move, and (iii) bcc lattice spacing, or equivalently one can utilize atomic volume, . Since the thermodynamic quantities can be determined from H, we conclude that T _c , the superconducting transition temperature, when it is non-zero, may be formally expressed as T _c = . One piece of evidence in support is that, in an atomic number vs. atomic volume graph, the superconducting elements lie in a well defined region. Two other relevant points are that (a) T _c is related by BCS theory, though not simply, to the Debye temperature, which in turn is calculable from the elastic constants C ₁₁, C ₁₂, and C ₄₄, the atomic weight and the atomic volume, and (b) T _c for five bcc transition metals is linear in the Cauchy deviation C ^* = (C ₁₂ - C ₄₄ )/(C ₁₂ + C ₄₄ ). Finally, via elastic constants, mass density and atomic volume, a correlation between C ^* and the Debye temperature is established for the five bcc transition elements.Received: 13 May 2004, Published online: 23 July 2004PACS: 74.62.-c Transition temperature variations - 74.70.Ad Metals; alloys and binary compounds     

<Emphasis Type="Italic">Q</Emphasis><Superscript>2</Superscript> evolution of parton densities at smal-<Emphasis Type="Italic">x</Emphasis> values

In the leading twist approximation of the Wilson operator product expansion with “frozen” and analytic strong coupling constants we show that Bessel-inspired behavior of the structure function F ₂ at small x, obtained for a flat initial condition in the DGLAP evolution equations, leads to good agreement with the deep-inelastic scattering experimental data from HERA.     

Phase segregation in Na<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>CoO<Subscript>2</Subscript> for large Na contents

We have investigated a set of sodium cobaltates (Na _x CoO₂) samples with various sodium content (0.67 ≤ x ≤ 0.75) using Nuclear Quadrupole Resonance (NQR). The four different stable phases and an intermediate one have been recognized. The NQR spectra of ⁵⁹Co allowed us to clearly differentiate the pure phase samples which could be easily distinguished from multi-phase samples. Moreover, we have found that keeping samples at room temperature in contact with humid air leads to destruction of the phase purity and loss of sodium content. The high sodium content sample evolves progressively into a mixture of the detected stable phases until it reaches the x = 2/3 composition which appears to be the most stable phase in this part of phase diagram.     

Enhanced ionic conductivity in nano-composite solid polymer electrolyte: (PEG)<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> LiBr: <Emphasis Type="Italic">y</Emphasis>(SiO<Subscript>2</Subscript>)

In this paper, we report an enhancement in ionic conductivity in a new nano-composite solid polymer electrolyte namely, (PEG) _x LiBr: y(SiO₂). The samples were prepared, characterized, and investigated by XRD, IR, NMR, and impedance spectroscopy. Conductivity as a function of salt concentration shows a double peak. Five weight percent addition of silica nanoparticles increases the ionic conductivity by two orders of magnitude. Conductivity exhibits an Arrhenius type dependence on temperature. IR study has shown that the existence of nanoparticles in the vicinity of terminal O―H group results in a shift in IR absorption frequency and increase in amplitude of vibration of the terminal O―H group. This might lead to an enhancement in conductivity due to increased segmental motion of the polymer. ⁷Li NMR spectroscopic studies also seem to support this. Thus addition of nanoparticle inert fillers still seems to be a promising technique to enhance the ionic conductivity in solid polymer electrolytes.     

Diode-pumped CW Nd:YAG laser at 1356 nm based on the <Superscript>4</Superscript><Emphasis Type="Italic">F</Emphasis><Subscript>3/2</Subscript>-<Superscript>4</Superscript><Emphasis Type="Italic">I</Emphasis><Subscript>13/2</Subscript> transition

We describe the output performances of the 1356 nm ⁴ F _3/2-⁴ I _13/2 transition (generally used for a 1319 nm transition) in Nd:YAG under in-band pumping with diode laser at the 809 nm wavelength. An end-pumped Nd:YAG crystal yielded 1.02 W of continuous-wave (CW) output power for 18.2 W of incident pump power. Moreover, intracavity second-harmonic generation (SHG) has also been achieved with a power of 290 mW at 678 nm by using a LiB₃O₅ (LBO) nonlinear crystal. The red beam quality factor M ₂ was less than 1.37. The red power stability was less 3.2% in 4 h.     

Electrical and optical properties of pulse plated CdS<Subscript><Emphasis Type="BoldItalic">x</Emphasis></Subscript>Te<Subscript>1-<Emphasis Type="BoldItalic">x</Emphasis></Subscript> films

Thin films of CdS_xTe_1-x were deposited by the pulse electrodeposition technique using cadmium sulfate, sodium thiosulfate, and tellurium dioxide on titanium and conducting glass substrates. Structural studies indicated the formation of polycrystalline films possessing hexagonal structure. The resistivity varies from 53 Ω cm to 8 Ω cm as the stochiometric coefficient “x” value decreases from 1 to 0. The carrier concentration increases with CdTe concentration. It is observed that as the post-heat treatment temperature increases, the photosensitivity also increases. It is observed that a post-heat treatment temperature of 550 °C results in high photosensitivity as well as low light resistance. The optical constants, refractive index (n) and extinction coefficient (k) were evaluated from the transmission spectra of the films of different composition.     

Doping induced spin state transition in Li<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>CoO<Subscript>2</Subscript> as studied by the GGA + DMFT calculations

The magnetic properties of Li _x CoO₂ for x = 0.94, 0.75, 0.66, and 0.51 are investigated within the method combining the generalized gradient approximation with dynamical mean field theory (GGA + DMFT). A delicate interplay between Hund’s exchange energy and t _2g ?e _g crystal field splitting is found to be responsible for the high-spin to low-spin state transition for Co⁴⁺ ions. The GGA + DMFT calculations show that the Co⁴⁺ ions at a small doping level adopt the high-spin state, while delithiation leads to an increase in the crystal field splitting and low-spin state becomes preferable. The Co³⁺ ions are found to stay in the low-spin configuration for any x values.     

Diode-pumped CW Nd:YAG laser at 1116 nm based on the <Superscript>4</Superscript><Emphasis Type="Italic">F</Emphasis><Subscript>3/2</Subscript>-<Superscript>4</Superscript><Emphasis Type="Italic">I</Emphasis><Subscript>11/2</Subscript> transition

We describe the output performances of the 1116 nm ⁴ F _3/2-⁴ I _11/2 transition (generally used for a 1064 nm transition) in Nd:YAG under in-band pumping with diode laser at the 809 nm wavelength. An end-pumped Nd:YAG crystal yielded 680 mW of continuous-wave (CW) output power for 18.2 W of incident pump power. Moreover, intracavity second-harmonic generation (SHG) has also been achieved with a power of 97 mW at 558 nm by using a LiB₃O₅ (LBO) nonlinear crystal. The yellow-green beam quality factor M ² was less than 1.21. The yellow-green power stability was less 2.5% in 4 h.     

Interplay between grand unification and supersymmetry in <Emphasis Type="BoldItalic">SU</Emphasis>(5) and <Emphasis Type="BoldItalic">E</Emphasis> <Subscript> <Emphasis Type="Bold">6</Emphasis> </Subscript>

Some aspects of minimal supersymmetric renormalizable grand unified theories are reviewed here. These include some constraints on the model parameters from the Higgs and light fermion masses in SU(5), and the issues of symmetry breaking, doublet–triplet splitting and fermion masses in E₆.     

Nitrogen-containing compounds <Emphasis Type="Italic">R</Emphasis>Fe<Subscript>11</Subscript>TiN<Subscript>x</Subscript> (<Emphasis Type="Italic">R</Emphasis> = Gd or Lu)

The structure and magnetic properties of RFe₁₁TiN compounds (R=Gd or Lu) containing nitrogen are investigated. Magnetic measurements are performed on a magnetometer in magnetic fields up to 100 kOe in the temperature range from 4.2 to 750 K with the use of RFe₁₁TiN single crystals, RFe₁₁TiN powders placed in a ceramic cell, and samples oriented in an external magnetic field. It is found that the nitridation leads to an increase in the Curie temperature and the saturation magnetization. The samples studied are uniaxial over the entire temperature range of magnetic ordering. The magnetic anisotropy decreases upon nitridation. It is demonstrated that, within the local anisotropy model, the decrease in the magnetic anisotropy constant K₁ can be explained by the redistribution of the electron density in the vicinity of the crystallographic positions occupied by iron atoms.     

Electronic structure and stability of complex hydrides Mg<Subscript>2</Subscript><Emphasis Type="Italic">M</Emphasis>H<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> (<Emphasis Type="Italic">M</Emphasis> = Fe,Co)

This paper reports on the results of theoretical investigations carried out for the hydrides Mg₂FeH₆ and Mg₂CoH₅ and the mixed hydride Mg₂(FeH₆)_0.5(CoH₅)_0.5 in terms of the full-potential linearized augmented plane wave (FLAPW) method. It has been shown that the partial substitution of the Co atoms for the Fe atoms leads to a slight increase in the stability of the hydride, but, at the same time, makes it impossible to increase the stability of the alloy. The high stability of the hydrides under investigation has been explained by the strong bonding between atoms of the transition metal and hydrogen.