Study of the crystalline structure of the superconducting phase of the YBaCuO ceramic compound

We present the results of a study of the crystalline structure of the high-temperature superconducting (above 110K) phase in the single-phase ceramic compounds YBaCuO. We have concluded that the crystal symmetry of this HTS phase with chemical composition Y_1.2Ba_0.8CuO_4−δ is orthorhombic type with crystallographic lattice constants a=0.3815 nm, b=0.3457 nm, c=2.650 nm.     

On the hall resistivity of nHgCdTe

We discuss the measurements of the Hall resistivity of nHgCdTe and InSb which have demonstrated the similarity of the electronic properties of these two materials. Gebhardt and Nimitz [1] dispute the validity of our results. We cite the evidence that confirms the correctness of our original observations.     

Molecular structure of cyclopropenylidene,HCCCH from the millimeter wave spectra of its isotopomers

The rotational spectra of the two ¹³C and of the D monoisotopic substitutions of cyclopropenylidene C₃H₂ have been measured up to 300 GHz. The determination of the rotational and centrifugal distortion constants of four isotopomers has led to a complete substitution structure. The values of the two CC bond lengths which define the three-membered ring have been improved by using Watson's “mass-dependence” (r_m) method.     

High Tc superconductivity of a TlBaCaCuO compound

A TlBaCaCuO_4.5+x compound has been investigated from the point of view of superconductivity. Depending on the heat treatment, one part of the samples exhibits superconductivity with an onset of 121 K and a zero resistivity of 106 K, while the other part of them shows only a sharp drop in resistivity at 130 K which hints at the existence of superconducting domains. This picture was confirmed by magnetic and ESR measurements giving possibility for estimation of the critical magnetic fields.     

Anisotropy of the superconducting critical magnetic field HC2 of LaMCuO system(M = Sr and Ba)

Preliminary measurement of the superconducting critical magnetic field H_c2 of LaMCuO system (M = Sr and Ba) with layered perovskite structure has been carried out. The results show a quite large anisotropy of H_c2 indicating that the present system has a two dimensional character of the electron transport. The measured conductivities seem to be consistent with the values estimated from the observed H_c2 by use of an ideal two dimensional band with free electron mass.     

On the homogeneity range of β″-alumina in the Na2OMgOAl2O3 system

The Al₂O₃ rich region of the Na₂O:MgOAl₂O₃ system has been investigated by means of X-ray phase analytical methods. The main features of the isothermal sections at 1870 K and 1950 K have been determined. The two sections exhibit the same phase equilibria except in the Na₂Al₂O₄-β″ region where a liquids phase appears at 1950 K. The magnesium stabilized β″-alumina phase with the nominal formula Na_1xxAl₁₁−xO₁₇ exhibits an extended homogeneity range with the magnesium content ranging over x=0.59-0.72. The homogeneity ranges are accompanied by variations in the unit cell dimentions. The composition of β″-alumina appears to correspond to a far lower relative sodium cntent than that indicated by the ideal formula at large magnesium + aluminium contents.     

Energy dependence of the 16O208Pb effective interaction

The Coulomb-nuclear interference in the excitation probability of the 2.615 MeV (3⁻) state of ²⁰⁸Pb by ¹⁶O at θ_CM=172° for bombarding energies 57 MeV ⩽E_lab⩽79 MeV has been studied. The data are described by an effective interaction which has a behaviour similar to that expected from the dispersion relation connecting the real and imaginary parts of the generalized optical potential.     

The origin of graduated demagnetization curves of NdFeB magnets

We present an analysis of the magnetization process of NdFeB-type magnets which exhibit a constriction or a shoulder of the demagnetization curve near the remanence point B_r. This constriction which drastically reduces the maximum energy product of the permanent magnet is discussed in terms of i) a siperposition of a soft and a hard magnetic phase, ii) an oxidation process of surface layers, iii) and the spin reorientation which changes the magnetic structure from easy c-axis to easy cone below 135 K. We demonstrate that the effect of surface oxidation and of a soft magnetic phase are of considerable influence over the entire temperature range while the influence of the spin reorientation is dominant below 150 K. The appearance of the cone structure prevents the application of NdFeB magnets in a magnetic circuit below 150 K.     

Measuring the anelasticity of CuBe film under tensile stress using the X-pendulum

We measured the anelasticity of CuBe foil at very low frequencies by incorporating it in an X-pendulum. The imaginary part of the spring constant was constant from 0.03 Hz to 0.5 Hz under tensile stresses of 12–20 MPa. The X-pendulum technique proved convenient for low frequency use.     

Superconductivity of TiZrNi alloys containing quasicrystals

Superconductivity of rapidly quenched ribbons Ti₅₃Zr₂₇Ni₂₀ at 1.94 K and the beginning of the transition to the superconductivity in ribbons Ti₄₅Zr₃₈Ni₁₇ below 1.5 K were found. X-rays diffractometry measurements showed that the icosahedral quasicrystal phase is dominant in all samples.     

Electrical properties of LiBSe ternary system

Amorphous products were obtained in the LiBSe ternary system by quenching melts of Li₂Se, B and Se mixture prepared at 100°C in sealed silica tubes. The vitreous region was slightly lower selenium composition than that of the Li₂SeB₂Se₃ tie-line. The amorphous products were lithium ionic conductors and most of them showed contributions to their total conductivity. The amorphous product of composition Li₂₅B₃₆Se₃₉ has the least electronic contribution to its total conductivity of 6.0 × 10⁻⁶ S/cm at room temperature. A new crystalline compound and crystalline LiBH₄ were also obtained in LiBSe ternary. Both of them were lithium ionic conductors having conductivities of about 1 × 10⁻⁶ S/cm at room temperature.     

Texture in NdFeB magnets analysed on the basis of the determination of Nd2Fe14B single crystals easy growth axis

We have shown by X-ray analysis and magnetic measurements, that the easy growth axis of Nd₂Fe₁₄B crystals corresponds to the “a” axis of the tetragonal structure while the easy magnetization axis is the “c” axis at temperatures above 135K. This correlation allows to understand some interesting features in NdFeB magnets:

• •-anisotropic contraction during sintering in magnets obtained by classical powder metallurgy,
• •-orientation mechanism during hot pressing (“die upset”) of magnets based on melt spun ribons.

     

Microscopic multicluster description of the 7Li7Be, 8Li8B and 9Li9C mirror nuclei

The ⁹Li and ⁹C mirror systems are investigated in a microscopic α+³H+n+n and α+³He+p+p multicluster framework using the stochastic variational method. Possibility of existence of neutron (proton) halo structure is studied. The quadrupole moment of ⁹C is predicted to be −5.04 e fm².     

The superconductivity,structure and microstructure of BaYScCu oxides

We added small amount of Sc (0.05–0.35at.%) to BaYCu oxide and found that the superconductivity deteriorated drastically when the Sc content reached 0.325at.%. X-ray and electron diffraction showed that the orthorhombic perovskite structure changed to multi-phase structure with unidentified phases. The microstructures observed by TEM are of needle like morphology. Their phases are not yet identified.     

Electron diffraction and electron microscopic study of BaYCuO superconducting materials

Electron microscopy and electron diffraction have been applied to show that the orthorhombic phase in the compound Ba₂YCu₃O_7−δ is responsible for the high superconducting transition temperature. A positive correlation is found between the volume fration of the orthorhombic phase and the superconducting transition temperature. By means of an “in-situ” heating experiment it is found that the orthorhombic phase is formed on cooling from a high temperature tetragonal phase with disordered vacancies. It is suggested that the low temperature tetragonal phase that occurs in the same specimens as the orthorhombic phase also contains an ordered arrangement of vacancies different from that present in the orthorhombic phase. The order-disorder transition associated with the structural vacancies is shown to be reversible, provided there has been no oxygen loss.     

Mössbauer studies of the charge-density-wave state of 57FeK0.30MoO3

Mössbauer studies have been performed between 6 K and 300 K on single crystals of ⁵⁷Fe-doped K_0.30MoO₃, which show a Peierls transition at 180 K towards on incommensurate charge density wave state. The spectra show three doublets. A strong line broadening below 120 K indicates that the Fe impurities are strong pinning centers for the charge density wave above ∼ 120 K and become progressively weak pinning ones below, in agreement with theoretical models.