Properties of LiZn x V2−x O4 and LiMg x V2−x O4 mixed oxides

The phase composition of the mixed oxides, series LiMe _x V_2–x O₄, where Me=Zn, Mg, was studied using X-ray analysis. The minor phases present in the investigated samples together with the basic spinel phase were determined as Li₃VO₄ and Zn₂V₂O₇ or Mg₂V₂O₇ respectively. Their presence is caused by the tendency of tetravalent vanadium ions to the disproportionation. For the compositionsx=0·375 were determined the quantitative phase compositions. The lattice constant of the spinel phase is linearly changing according to Vegard's rule, for the zinc series in the range 0x0·375 and for the magnesium series in the range 0xs0·25. Further substitution in magnesium series caused tetragonal deformation of the cubic spinel lattice.The author is indebted to Dr. A.Bergstein for helpful discussions and to Mrs. E.Hrubá for technical assistance.     

Mossbauer study of ferrite systems Co x Mn1−x Fe2O4 and Ni x Mn1−x Fe2O4

Mössbauer spectra of Co _x Mn_1?x Fe₂O₄ and Ni _x Mn_1?x Fe₂O₄ ferrites withx values ranging from 0·1 to 0·8 in steps of 0·1 have been recorded at room temperature. All spectra exhibit well-defined Zeeman hyperfine patterns. It has been observed that hyperfine field at Fe³⁺ nucleus increases more rapidly by nickel substitution than by cobalt substitution. This has been explained in terms of exchange interactions and cation distribution in the spinels. Hyperfine fields, isomer shifts and quadrupole splittings have been determined.     

Photoemission investigation of the high temperature superconductors La2−x Sr x CuO4

Photoemission spectra of the La_2–x Sr _x CuO₄ system which recently has been demonstrated of being capable to produce high temperature (T _c >35 K) superconducting compounds are reported. The photoemission spectra of the basic system (La₂CuO₄) and of samples which contain up to 20% Sr are not significantly different. The density of states at the Fermi energy is small (smaller than that of metallic Cu) and the samples show a possible small shift of the Cu 2p lines towards the trivalent position with increasing tendency to superconductivity. From the Sr 3d spectra a considerable amount of oxygen defects coordinated to the Sr ions is estimated.     

On the τ↦→vx03C4;x03C4;x03C4;πω decay mode

We discuss the theoretical predictions for the τ?→vτπω decay mode and compare it with recent experimental results.     

Photoelectronic properties of the Zn x Cd1−x In2S4 system

Summary Photoelectrical investigations on the Zn _x Cd_1−x In₂S₄ system have been carried our particularly with respect to the high residual conductivity and the long-relaxation-time properties. Paper presented at the ?V International Conference on Ternary and Multinary Compounds?, held in Cagliari, September 14–16, 1982.     

Magnetic behavior of the oxide spinels: Li0.5Fe2.5−2x Al x Cr x O4

In order to study the effect of substitution of Fe³⁺ by Al³⁺ and Cr³⁺ in Li_0.5Fe_2.5O₄ on its structural and magnetic properties, the spinel system Li_0.5Al _x Cr _x Fe_2.5?2x O₄ (x=0.0, 0.2, 0.4, 0.5, 0.6 and 0.8) has been characterized by X-ray diffraction, high field magnetization, low field ac susceptibility and ⁵⁷Fe Mossbauer spectroscopy. Contrary to the earlier reports, about 50% of Al³⁺ is found to occupy the tetrahedral sites. The system exhibits canted spin structure and a central paramagnetic doublet was found superimposed on magnetic sextet in the Mössbauer spectra (x>0.5).     

Photoconductivity of Cu x Ag1−x Cd2InTe4

Summary The prominent photoelectric parameters of the semiconducting solid solutions Cu _x Ag_1−x Cd₂InTe₄ are reported. From the spectral dependence of photoconductivity at room temperature and at 77 K the energy gaps and their thermal coefficients are obtained. The detectivityD ^*, the photocurrent gainG and the carrier lifetime are measured and the carrier mobility is deduced. In order to improve theD ^* characteristics, the analysis of its parameters is also performed. Paper presented at the ?V International Conference on Ternary and Multinary Compounds?, held in Cagliari, September 14–16, 1982.     

Normal state properties of La2−x Sr x CuO4

The frequency dependence of the reflectivity spectra of doped La_2–x Sr _x CuO₄ in a high frequency region is analyzed based on a picture of correlated hole hopping within a lower Hubbard subband. It is shown that experimental trends are fairly well accounted for by this model, suggesting that the normal state of this compound is quite different from the metallic state.     

Rotational excitations of NH 4 + in (NH4) x K1−x I

We propose a gaussian model for the distribution of rotational tunneling lines which constitute the experimentally observed inelastic neutron scattering spectra in the range 0 to 0.6 meV from the mixed system (NH₄) _x K_1−x I. The intensity profiles generated by this model are in remarkable agreement with the experimentally observed ones.     

Absence of the long-range antiferromagnetic order in doped La2−x Sr x CuO4

Based on a simple argument, it is shown that a small concentration of holes in a single band Hubbard Hamiltonian on a quadratic lattice with moderateU/t ratio is enough to destroy the long range antiferromagnetic order in the ground state. The particular emphasis is made on the role of large amplitude zero-point quantum spin fluctuations inherent in low-dimensional spin system with the small magnitude of spinS=1/2. The relevance to the highT _c superconductivity materials La_2–x Sr _x CuO₄ is discussed.     

Orbital-selective Mott-insulator transition in Ca 2 - x Sr x RuO 4

The electronic structures of the metallic and insulating phases of the alloy series Ca_2-xSr_xRuO₄ ( 0 ? x ? 2) are calculated using LDA, LDA+U and Dynamical Mean-Field Approximation methods. In the end members the groundstate respectively is an orbitally non-degenerate antiferromagnetic insulator (x = 0) and a good metal (x = 2). For x > 0.5 the observed Curie-Weiss paramagnetic metallic state which possesses a local moment with the unexpected spin S = 1/2, is explained by the coexistence of localized and itinerant Ru-4d-orbitals. For 0.2 < x < 0.5 we propose a state with partial orbital and spin ordering. An effective model for the localized orbital and spin degrees of freedom is discussed. The metal-insulator transition at x = 0.2 is attributed to a switch in the orbital occupation associated with a structural change of the crystal. Received 27 July 2001     

The role of oxygen in the electron-doped superconducting system Nd2−x Ce x CuO4

Mössbauer effect studies suggest the presence of extraneous oxygen in the whole family of title compounds. At certain concentrations of Ce, oxygen is rather tenaciously bound. Such oxygen causes superparamagnetism in the magnetically ordered material, and also brings about the formation of spin clusters in the metallic material.     

Mössbauer effect study of ZnAl x Fe2−x O4

⁵⁷Fe Mössbauer effect study in the polycrystalline solid solutions of ZnAl _x Fe_2?x O₄ (x=0.0 – 1.5) has been carried out at 78 and 300K. All the samples exhibited well resolved quadrupole doublets corresponding to Fe³⁺ ions at octahedral sites. A monotonic increase in the value of quadrupole splitting with increasing aluminium content is observed. This is attributed to the change in the oxygen parameter of the spinel structure with the addition of aluminium. Quadrupole splitting does not show any detectable change with temperature. The value of isomer shift has been found to be independent of substitution levelx which suggests that the s-electron charge distribution of Fe³⁺ ions is not influenced by Al³⁺ substitution.     

A Mössbauer study of hyperfine interaction in the systems Co x Mn3−x−y Fe y O4 and Ni x Mn3−x−y Fe y O4

A Mössbauer study of systems Co _x Mn_3?x?y Fe _y O₄ and Ni _x Mn_3?x?y Fe _y O₄ for values ofx=0·1, 0·5, 1·0 andy ranging from 0·1 to 2·0 in steps of 0·2 have been made. At room temperature samples fory values ranging in between 0·1 to 0·5 exhibit paramagnetic behaviour while all spectra for values ofy between 0·6 to 0·8 show relaxation effects. Well-defined hyperfine Zeeman spectra are observed for all the samples withy>0·8 and resolved in two sextets corresponding to octahedral and tetrahedral site symmetries and a central doublet probably due to the presence of super-paramagnetic particles in the system. The hyperfine field at⁵⁷Fe nucleus reduces with decreasing iron cobalt and nickel concentration. These observations have been explained in terms of site preference of cations and exchange interactions.     

Submicrosecond rearrangeable nonblocking silicon-on-insulator thermo-optic 4x4 switch matrix

A rearrangeable nonblocking silicon-on-insulator-based thermo-optic 4x4 switch matrix is designed and fabricated. A spot-size converter is integrated to reduce the insertion loss, and a new driving circuit is designed to improve the response speed. The insertion loss is less than 10 dB, and the response time is 950 ns.     

A study of the x4 model in the Tsallis nonextensive statistics

The author studied the effects of the environment described by Tsallis statistics in quantum mechanics, when the deviation from Boltzmann–Gibbs (BG) statistics is small. The $x^4$ model was used and the squeeze angle caused by the difference between Tsallis and BG statistics was calculated perturbatively in the mean field approximation as a function of the dimensionless parameters: the inverse temperature ${\beta }_p$ and the coupling strength ${\lambda }_p$. The author found that the effect of the deviation from BG statistics is relatively large at high temperature. The squeeze angle as a function of ${\beta }_p$ has a dip structure, and the dip is deeper with the increase of ${\lambda }_p$. The sign of the angle as a function of ${\beta }_p$ changes. These facts indicate that the frequency is modulated by the difference between these statistics.     

Comparative Studies of Frustrated Cobaltites YBaCo4O7 +x and DyBaCo4O7 +x with Small Oxygen Nonstoichiometry (x ≤ 0.2)

Journal of Experimental and Theoretical Physics - Comparative experimental studies of the structural and elastic characteristics of the YBaCo4O7 + x and DyBaCo4O7 + x (x = 0, 0.1, 0.2) cobaltite...