Synthesis and structural characteristics of La2 ? x Sr x NiO4 dielectric ceramics

Fine-grained single phase La_{2 ? x} Sr _x NiO_{4 + ??} powders are produced by means of self-propagating high temperature synthesis (SHS) after annealing the primary product at 670°C. Ammonium citrate was used as the reduction agent. It is shown that the reduction in unit cell volume due to lanthanum being replaced by larger strontium ions is caused by an increase in nickel oxidation.     

Luminescence of the compounds Y0.5 − x Li1.5PO4:Eu x

In this paper for the first time, the compounds Y_{0.5 ? x} Li_1.5PO₄:Eu _x (YLP:Eu) were synthesized by the simple method of solid state reaction. The technology takes much time dry low temperature and little time calcined to synthesize rare earth phosphates. Their structures were analyzed by X-ray powder diffraction experiment. The excitation and emission spectra were measured at room temperature. Y_{0.5 ? x} Li_1.5PO₄:Eu _x shows the characteristice emissions of Eu³⁺ (⁵ D ₀-⁷ F _{1, 2, 3, 4} transitions dominated by ⁵ D ₀-⁷ F ₄). Incorporation of Li⁺ ions in the phosphors promote the crystallinity, reduce the amount of Y₂O₃, and lower the cost of production, moreover, transition dominated by ⁵ D ₀-⁷ F ₄ is important for a number of practical applications.     

Admixture of the giant dipole resonance to low lying 1−-states in heavy nuclei

The mass of ¹³ Be has been measured with the reaction ¹³ C(¹⁴ C,¹⁴ O)¹³ Be at E _Lab =337 MeV. A Q-value of Q ₀=–37.02(5) MeV was obtained and the mass excess is M.E.=35.16(5) MeV. If the observed line corresponds to the ground state,¹³ Be is particle unstable with respect to the oneneutron emission by 2.01 MeV. The observed line width of 0.3(2) MeV supports an assignment ofJ =5/2⁺ or 1/2^–, but excludesJ =1/2⁺. An excited state is seen at 3.12(7) MeV; there are indications of a second excited state at 6.5(2) MeV.     

Effect of oxygen content on the magnetic state of La0.5Ca0.5MnO3−γ perovskites

The magnetization and magnetoresistance are studied in La_0.5Ca_0.5MnO_3−γ as a function of oxygen content. It is found that as the oxygen content is decreased, this compound undergoes a sequence of transitions from an antiferromagnetic to a ferromagnetic state (γ⩾0.04), from the ferromagnetic to a spin-glass state (γ ⩾0.14), and from the spin glass to an inhomogeneous ferromagnetic state (γ⩾0.25). Strongly reduced samples (γ⩾0.25) show a large magnetoresistance despite the absence of Mn³⁺–Mn⁴⁺ pairs. It is suggested that the oxygen vacancies in the strongly reduced samples (γ⩾0.25) are long-range ordered. Pis’ma Zh. éksp. Teor. Fiz. 70, No. 9, 583–587 (10 November 1999) Published in English in the original Russian journal. Edited by Steve Torstveit.     

Transient photoimpedance response of YBa2Cu3O7−0δ epitaxial films in the mixed state

This paper reports on an investigation of transient photoimpedance response to radiation, in other words, real-time variations of the impedance induced by femtosecond optical pulses in superconducting films transferred to the mixed state by an external magnetic field applied parallel to the c-axis. When the films were in a state characterized by the absence of dc resistivity, the response amplitude increased with the magnetic field faster than expected owing to the contribution of magnetic vortices to the impedance of a superconductor with s-wave pairing of electrons. It turned out that the effect is due to a growth in the effective density of quasiparticle states in the mixed state of a d-wave superconductor. In the absence of magnetic field, however, the response amplitude was higher at lower temperatures, which contradicts the predictions of both models. Possible reasons for this feature in the response as a function of temperature are discussed. Zh. éksp. Teor. Fiz. 116, 1035–1047 (September 1999)     

Structure evolution and microwave dielectric response of (Ca0.5+xSr0.5−x)[(Al0.5Nb0.5)0.5Ti0.5]O3 solid solutions

The phase assemblage, crystal structure evolution and microwave dielectric response of (Ca_0.5+xSr_0.5−x)[(Al_0.5Nb_0.5)_0.5Ti_0.5]O₃ ceramics (abbreviated as CSANT hereafter) are investigated. Single perovskite solid solution is formed in the CSANT ceramics in Sr-rich composition range of x < −0.05, however, Ca₄Ti₃O₁₀-type layered perovskite phase begins to segregate after x = −0.05. The CSANT perovskites crystallized in Fm3m cubic symmetry in the composition range of x ≤ −0.2, however, as the Ca²⁺ content in A-site increased, the oxygen octahedral began to be anti-phase tilted at x = −0.1 and the crystal structure transited to P2₁/n pseudo-orthorhombic space group thereafter. The microwave dielectric response of the CSANT ceramics is elaborately discussed in terms of their crystallographic structure and chemical composition. When sintered at 1500 °C for 4 h, a dielectric constant ɛ_r of 52.5, a Qf product of 28000 GHz and a τ_f of +25.4 ppm/°C microwave dielectric ceramic can be obtained in the CSANT ceramics at x = 0.3.     

About the nature of Ni3+ magnetic centres in the LaSrAl1−xNixO4 ceramics

LaSrAl_1?x Ni _x 0₄ (x≤0.11) ceramics werer studied by EPR. It was established that Ni³⁺ ions in this compound may be in two states: a low spin one with t _2g ⁶ e _g ¹ electron configuration and high spin state with t _2g ⁵ e _g ² electron configuration. It was shown that the random distribution of La³⁺ and Sr²⁺ ions in the heavy metal sublattice leads to two kinds for each type of these centres with different degree of tetragonal distortions. The total concentration of NiO₆ isolated centres in these ceramics does not exceed 5–10 % of a total number of nickel ions at a givenx. A model, explaining these peculiarities observed in an experiment by the enhanced tendency of Ni³⁺ ions to clusterization, was proposed. It appears that even at small concentrations Ni³⁺ ions form clusters and conglomerates, whose structural characteristics of the oxygen octahedra are comparable with those in LaSrNiO₄.     

Experimental evidence for fast cluster formation of chain oxygen vacancies in YBa2Cu3O7−δ as the origin of the fishtail anomaly

We report on three different and complementary measurements, namely magnetization measurements, positron annihilation and NMR spectroscopy, which give evidence that the formation of oxygen vacancy clusters is the origin of the fishtail anomaly in YBa₂Cu₃O_7−δ. In contrast to YBa₂Cu₃O_7.0, where the anomaly is intrinsically absent, it can be suppressed in the optimally doped state where vacancies are present. Thus, clusters of oxygen vacancies rather than single vacancies or point defects are responsible for this anomaly.     

Pressure Effects in the Raman Spectra of La 2−x Sr x CuO 4

Micro-Raman measurements under hydrostatic pressures up to 6 GPa have been carried out on high-quality La 2 m x Sr x CuO 4 polycrystalline compounds with Sr concentration up to x =0.45. The zz scattering polarization has been investigated, where two strong modes due to La/Sr and the apex oxygen, and (in the low Sr concentrations) the soft mode at ～100 cm m 1 are observed. The frequency of the strong modes increases almost linearly with pressure for the Sr concentrations studied. Modifications in the increment rate d y /d p and the phonon width have been detected depending on the amount of doping. For x =0.45 a considerable increase in the width of both strong phonons with pressure was found, which must reflect a separation into phases, since this concentration is close to the solubility limit. The relative intensity of the strong phonons was investigated in connection with its correlation to the transition temperature.     

Fermi surface evolution in the ensemble of spin-polarized quasiparticles in La2 − x Sr x CuO4

For the spin-fermion model, it has been shown that the concept of spin polarons makes it possible to reproduce fine details of the Fermi surface evolution in the nodal direction of La_{2 ? x} Sr _x CuO₄ occurring with changes in the doping level x. The physics here is determined by the spin-correlated hopping of charge carriers and by the doping dependence of the inverse magnetic correlation length.     

Magnetic and superconducting properties of Tl(Sr1.5R0.5)CaCu2O7−δ compounds

DC magnetic susceptibility and resistivity measurements have been performed on 14 Tl(Sr_1.5R_0.5)CaCu₂O_7−δ compounds with R=La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu. All samples except the Ce-doped compound show superconducting phase transition temperature around 80–90 K. The rare-earth atoms behave as local moments above the superconducting transition temperature of these compounds. Mechanism of filling of holes in the CuO₂ plane can be employed to interpret the suppression or enhancement of superconductivity by rare-earth ions in Tl(Sr_1.5R_0.5)CaCu₂O_7−δ.     

Dynamic spectral response of solid solutions of the bismuth-strontium ferrite Bi1 − x Sr x FeO3 − δ in the frequency range 0.3–200 THz

Ceramic samples of Bi_{1 ? x} Sr _x FeO_{3 ? δ} have been investigated using X-ray diffraction analysis, Raman spectroscopy, and infrared spectroscopy. It has been shown that the spectra of these solid solutions should be considered as a superposition of resonant phonon responses of the cubic perovskite and a relaxation response. No relaxators are observed in the pure compounds BiFeO₃ and SrFeO_{3 ? δ}. It has been found that two concentration phase transitions occurs with the symmetry changes $R3c \leftrightarrow Pm\bar 3m$ in the range 0.1 < x < 0.2 and $Pm\bar 3m \leftrightarrow P4mm$ in the range 0.8 < x < 1.0.     

Luminescent properties of Sr2.5−3x/2Ba0.5SmxAlO4F oxyfluorides

Effective orange Sm³⁺-doped Sr_2.5Ba_0.5AlO₄F phosphors excited at 254 and 408 nm excitation were prepared by the solid-state method. The excitation and emission spectra of Sr_2.5?3x/2Ba_0.5Sm_xAlO₄F and Sr_2.5?3x/2Ba_0.5Sm_xAlO_4?αF_1?δ (x=0.001～0.1) based on photoluminescence spectroscopy are investigated. The defects in anion-deficient Sr_2.5?3x/2Ba_0.5Sm_xAlO_4?αF_1?δ (x=0.001, 0.01) are monitored by broad-band photoluminescence emission centered near 480 nm along with the orange emission transitions of Sm³⁺. CIE values and relative luminescent intensities of Sr_2.5?3x/2Ba_0.5Sm_xAlO₄F and Sr_2.5?3x/2Ba_0.5Sm_xAlO_4?αF_1?δ by changing the Sm³⁺ content (x=0.001～0.1) are discussed.     

Gamow-Teller β transition from the 2− ground state of88Rb to the 3− excited state of88Sr

The Gamow-Tellerβ-transition from the ground state 2^? of⁸⁸Rb to the 3^? level at 2.734 MeV of⁸⁸Sr is studied. The nuclear matrix element 〈σ〉 and the lofft value are calculated using complete nuclear wave functions for the initial and final states. It is shown that, contrary to the normal assumption, the component ¦π2p ₃ ^2/?1 π1g _9/2〉 of the final state does give a very important contribution to 〈σ〉 due to the presence of strong cancellation effects. Although our calculations favour a wave function for the 3^? level⁸⁸Sr where neutron 1h—1p configurations are not included, there are still some facts which make that our results cannot be taken as conclusive. Nuclear Structure⁸⁸Rb,⁸⁸Sr, calculated the Gamow-Teller nuclear matrix element 〈σ〉 and the logft value using a shell model.     

Microstructure and electrochemical properties of porous La2NiO4+δ electrodes spin-coated on Ce0.8Sm0.2O1.9 electrolyte

Porous La₂NiO_4+?? electrodes were prepared from superfine starting powder on dense substrates of Ce_0.8Sm_0.2O_1.9 electrolyte by a spin coating technique. The microstructure and electrochemical properties of the electrodes were investigated within the sintering temperature range of 1,000?C1,100?°C. An obvious effect of sintering temperature on the microstructure and electrochemical properties was detected. The variation of the electrochemical properties with sintering temperature was explained in relation to the microstructural evolution of the porous electrodes. It was detected that the electrode processes greatly depended on the microstructure of the electrodes. The polarization of surface oxygen exchange process was found to be the major contribution to the total electrode polarization. The electrode sintered at 1,050?°C showed the optimum electrocatalytic activity among the investigated electrodes. At 800?°C, the electrode exhibited a polarization resistance of 0.42????cm², an overpotential of 48?mV at a current density of 200?mA?cm^?2 and an exchange current density of 121?mA?cm^?2.     

Modulation response of dynamic single-mode quantum-well heterolasers in the 1.5 μm range

Numerical modelling is used to study the effect of tuning the laser output over the gain bandwidth on the modulation response of GaInAs–GaInAsP quantum-well heterolasers for different modulation frequencies of the pump current. It is found that the maximum frequency bandwidth of the response band and the greatest feasibility of high speed modulation for transmission of signals in information systems are achieved in the center of the gain band. Raising the dc component of the pump current increases the response bandwidth. For typical parameters of this system (near 1.5 μm) the maximum response bandwidth can approach ≈40 GHz. For certain parameters, the amplitude-frequency characteristics of the heterolasers have two local maxima: one at low frequency corresponding usually to a resonance for the 1/2 subharmonic and one at high frequency, for the fundamental resonance.     

Nonlinear dielectric response of antiferroelectric liquid crystals in the smectic Cα * phase

Nonlinear dielectric response of antiferroelectric liquid crystals has been studied in the smectic C_α* (SmC_α*) phase. The linear dielectric spectrum shows a single relaxation of Debye type and its relaxation frequency is as high as one hundred kHz. The profile of the third-order nonlinear dielectric spectrum varies in complex fashion with temperature. In the vicinity of the SmA-SmC_α* phase transition, experimentally obtained nonlinear spectra are well described by those calculated with phenomenological theory of Landau type. The soft mode of the SmC_α* phase shows critical slowing-down near the transition temperature. In the lower-temperature region of the SmC_α* phase, the contribution from the soft mode of the SmC_α* phase reduces and the other relaxation mode with Debye-type spectrum appears at several hundred Hz in the nonlinear spectrum. The appearance of this low-frequency mode suggests that the cooperative fluctuation of directors over long range exists in the SmC_α* phase. Received 19 April 2002 and Received in final form 23 July 2002 RID="a" ID="a"e-mail: kimura@exp.t.u-tokyo.ac.jp