A.A. Gippius

The new family of complex copper oxides with possible superconducting properties: Pb2Sr2R1.33Ce0.67Cu3O10+δ

Abstract The new family of complex copper oxides Pb 2 Sr 2 R 1.33 Ce 0.67 Cu 3 O 10+δ ( R = La - Tm ) has been prepared. These compounds have two crystal structure types with different values of δ, which depend on the synthesizing conditions (partial oxygen pressure). The crystal structure models for the Sm-containing phases with δ =-0.2 and 1.7 have been determined on the basis of X-ray powder diffraction. The structures of these compounds are closely related to the structure of the known high- T c oxides Pb 2 Sr 2 R 0.5 Ca 0.5 Cu 3 O 8+δ but differ from them by the existence of flourite-like (R 1.33 Ce 0.67 O 2 ) blocks instead of the (Y 0.5 Ca 0.5 □) sheets which divide the (CuO 2 ) sheets. All these compounds are semiconductors on the basis of magnetization and resistivity measurements, but Eu-containing samples which were prepared in a nitrogen atmosphere exhibited a superconducting transition at 24 K with a relatively small volume of the superconducting phase (

Quadrupole and magnetic mechanisms of 209Bi spin-lattice relaxation in Bi4Ge3O12

The nuclear spin-lattice relaxation in the Bi4Ge3O12 single crystal was studied by the 209Bi NQR spin-echo technique. The recovery curves were measured for all 209Bi NQR transitions of nuclear spin I = 9/2 in a large temperature range 10-230 K. The experimental relaxation curves were described in terms of the single effective spin-lattice relaxation time T1*. The temperature dependence of 1/T1* was close to a Tn-law with n = 2.4-2.7. A theoretical treatment was given for the nuclear spin-lattice relaxation in the multi-level system for the case of a single-axial crystalline electric field. Both quadrupole and magnetic mechanisms of relaxation were taken into account. In separating the quadrupole and magnetic contributions, an idea of Rega (Regaxa0T 1991 J.xa0Phys.:xa0Condens.xa0Matter 3 1871) was used. To extract the temperature dependences of the parameters W1, W2 and WM the fitting procedure for calculated and experimental relaxation times was used. High-accuracy measurements of the recovery curves enable us to conclude that the magnetic mechanism contributes noticeably to the spin-lattice relaxation in Bi4Ge3O12 at T≤50xa0K.

Application of199Hg solid-state NMR to Hg based HTSC

It is proposed to study Hg-oxide ceramics, which are high-temperature superconductors, with 199Hg NMR spectroscopy. As an exemplary application of this method first room-temperature 199Hg spectra and spin-lattice relaxation times of samples with different macrostructure are presented. A spin-lattice relaxation time of T~ = 35 ms is found. All samples exhibit similar characteristic powder spectra caused by an axially symmetric 199Hg spin interaction. The isotropic value and the anisotropy of the tensor relative to solid HgCI2 as a standard substance is estimated.

Temperature dependences of lower critical fields in TmBa2 Cu3O7−x single-crystals

Abstract Temperature dependences of lower critical fields Hc1 of the monodomain single crystals TmBa2 Cu3 O7−x were measured for different magnetic fields orientations with respect to the crystallographic axes. The linear Hc1 (T) behaviour for T> 10 K and a saturation for lower temperatures were found. The Meissner magnetic susceptibilitys and Hc1s angular dependences in the (bc)-plane were studied. The expression derived in the ellipsoid approximation shows a good agreement with these dependencis.

Synthesis and investigation of the complex copper oxides Bi2Sr2R2−xCexCu2O10+δ (R=rare earth)

Abstract Bi-2222 phases were prepared by solid state reactions and were studied using X-ray diffraction, chemical and EDS-analysis. Compounds with composition Bi2Sr2R1.33Ce0.67Cu2O10+δ were obtained for R=Pr, Nd, Sm-Er. The phases Bi2Sr2Cu2O10+δ with similar structure but without Ce were prepared for R = Pr, Sm-Dy. For Bi2Sr2Eu2−xCexCu2O10+δ the solubility limit was determined and the effect of treatment under different oxygen pressures was investigated. Weak diamagnetic signals (

The Ln2Cu2O5 ESR study (Ln= Y, TbLu)

Abstract The method for the growth of large Ln2Cu2O5 single crystals is developed. No ESR signals is found in all studied samples except Lu2Cu2O5. Temperature dependences of Cu2+ ESR spectra of Lu2Cu2O5 compounds were studied for both policrystalline and single crystal samples. The significant anisotropy of ESR spectra of Ln2Cu2O5 single crystal was found. The g-factor main values were determined. The sharp broadening and disappearance of the ESR signals at temperatures below 20K seems to be caused by the enhancement of spin fluctuations in the vicinity of the antiferromagnetic transition in these compounds. The effective Cu2+ interaction with rare earth ions magnetic moments in Ln2Cu2O5 compounds is supposed.

Time relaxation of the untwinned TmBa2Cu3O7−δ single crystal remanent magnetization

Abstract Measurements of the remanent magnetization relaxation M(t) in the TmBa 2 Cu 3 O 7−δ single crystals with the small twins density are reported. The relaxation rate r(T)=dM(T.t)/d(lnt) has a sharp maximum at temperature T=T max . The T max value shifts to lower temperatures as H is increased. The r(T) maximum is reproduced by the calculation taking into account the thermal activated flux creep and the Abrikosov vortices distribution according to the Beans model.

Quadrupole effects and electron-phonon interaction in the non-equilibrium superconductors Al1-xSix

27Al NMR (nuclear magnetic resonance) spectra and the spin-lattice relaxation were measured for the non-equilibrium Al-based superconductors Al1-xSix (x = 0, 0.02, 0.05, 0.06 and 0.08) in the temperature range 2-300 K. The NMR data confirm a substantial enhancement of the electron-phonon interaction in the vicinity of the FCC (face-centred cubic) lattice instability in these model superconductors. The observed quadrupole effects can be interpreted in terms of a substantial distortion and disordering of the Al-based FCC lattice.

199Hg- solid state NMR and susceptibility studies of mercury - based HTSC

Abstract 199 Hg- NMR Spectroscopy was applied to study Hg- oxide ceramic high temperature superconductors at room temperature. Spectra of samples with different macro- structure and transition- temperatures have been measured. A spin- spin relaxation time of T 2 = 1.6 msec and a spin- lattice relaxation time of T 1 = 35 msec was found. All spectra exhibit a characteristic powder line shape caused by an axially symmetric 199 Hg spin interaction.

TEMPERATURE DEPENDENCES OF Y2Cu2O5 ESR SPECTRA

Temperature dependences of the ESR spectrum in Y2Cu2O5 compound are studied for both polycrystalline and single crystal samples. The g-factors along the main axes are found to be ga=2.099, gb=2.048, and gc=2.21 at room temperature, T=293 K. As temperature decreases, the linewidth ΔH increases in agreement with the Huber’s formula ΔH=A(TN/(T−TN))β+B(θ/T+1) with the following parameters: A=1513±10 Oe, B=509±10 Oe, TN=7±1 K, θ=18±2 K, β=1.3±0.3. In the vicinity of the Neel Temperature, TN, the ESR spectrum has a very complicated form indicating the existence of clusters with different internal magnetic fields. Below TN, a single line symmetrical ESR spectrum is recovered.