Polar-center phase nuclei in He+-irradiated CuO single crystals

Irradiation of various single-crystal CuO faces [ac,bc,(110)] with 4.6-MeV He⁺ ions has been found to result in reduction of CuO to Cu₂O and Cu on the irradiated and unirradiated sides, lifting of forbiddenness from optical transitions in the [CuO₄]⁷⁻ electron center in the 0.7–0.95-eV energy range, a change in dichroism near the bands corresponding to transitions in the hole centers, [CuO₄]⁵⁻, and electron centers, [CuO₄]⁷⁻, as well as in a resonant increase of absorption at 0.95–1.30 eV with an unusual polarization dependence. The results of He⁺ irradiation of CuO single crystals are discussed in terms of a model of the nucleation of the phase of polar (electron and hole) centers in copper-oxygen systems. Fiz. Tverd. Tela (St. Petersburg) 40, 419–424 (March 1998)     

Microphase separation and magnetic Jahn-Teller polarons in LaSrAl1−xCuxO4−δ

An EPR study of the solid solutions LaSrAl_1−x Cu_xO₄, which are isostructural to La₂CuO₄, shows that microphase separation of the structure occurs already at small copper concentrations (x≈0.01). A phase enriched with Cu appears along with a phase of La₂AlO₄, which contains isolated CuO₆ centers. It is established that the nature of the states and the deformations of the CuO₆ centers is determined by internal Jahn-Teller factors. When x⩽0.1, EPR signals are detected for new dynamic centers, which are identified as CuO₆ Jahn-Teller centers with a hole delocalized among the four in-plane oxygen ions (the total spin S=1). When 0.1⩽x<1, local CuO ₆ ⁻ hole centers transform into magnetic Jahn-Teller polarons, which include five or more CuO₆ fragments, in the copper phase of the structure. Their transformations and the conditions for observing them are discussed. Fiz. Tverd. Tela (St. Petersburg) 40, 622–628 (April 1998)     

Effect of electron bombardment on a phase-inhomogeneous nanoscopic structure and the fundamental absorption spectrum of single-crystal CuO

The optical absorption spectra of single-crystal CuO bombarded with 5-MeV electrons exhibit reduced absorption in the region of the fundamental absorption edge at 17 eV, which corresponds to the b _1g →e _u transition with charge transfer in CuO ₄ ⁶⁻ . A simultaneous increase in absorption is observed in the middle infrared and in the region of high energies centered on 2.9 eV. The experimental results obtained are interpreted in terms of ideas on the phase-inhomogeneous nano-agglomorated structure in copper oxides that occurs as a result of the nucleation of polar centers (CuO ₄ ⁵⁻ , CuO ₄ ⁷⁻ ) under electron bombardment. Fiz. Tverd. Tela (St. Petersburg) 39, 2141–2146 (December 1997)     

μ− SR studies of oxide-superconductorsSR studies of oxide-superconductors

The negative muon spin rotation method (μ⁻ SR) has been applied to studies of electronic states at oxygen sites of oxide superconductors YBa₂Cu₃O₇, Nd_2−x Ce _x CuO_4−δ (x=0.15, oxygen reduced), LiTi₂O₄ and related oxide-insulators La₂CuO_4−δ, CuO, Cu₂O. The paramagnetic shifts of μ⁻ trapped at oxygen nuclei in these polycrystalline powder samples have been measured at 300 K. All the measured shifts are positive. In copper-oxides the paramagnetic shifts are of the order 10⁻³, while in LiTi₂O₄ is very small (8.4±3.34×10⁻⁵). In YBa₂Cu₃O₇, a fast μ⁻ spin relaxation timeT ₂ ^* (∼ 200 ns) has been observed; the reason for this is unknown and further studies are now in progress.     

Synthesis, characterization, DNA binding and cleavage studies of mixed-ligand Cu(II) complexes of 2,6-bis(benzimidazol-2-yl)pyridine

Four novel copper(II) complexes of the composition [CuLX] where L = 2,6-bis(benzimidazole-2yl)pyridine, X = dipyridophenazine (L₁), 1,10-phenanthroline (L₂), hydroxyproline (L₃) and 2,6-pyridine dicarboxylic acid (L₄) were synthesized and characterized by using elemental analysis, FT-IR, UV–vis, ESI-MS, molar conductance and magnetic susceptibility measurements. The complexes [CuLL₁](NO₃)₂ [1], [CuLL₂](NO₃)₂ [2], [CuLL₃](NO₃) [3] and [CuLL₄] (NO₃) [4] are stable at room temperature. In DMSO the complexes [1] and [2] are 1:2 electrolytes, [3] and [4] are 1:1 electrolytes. Based on elemental and spectral studies five coordinated geometry is assigned to all the four complexes. The interaction of four copper ion complexes with calf thymus DNA were carried out by UV–vis titrations, fluorescence spectroscopy, thermal melting and viscosity measurements .The binding constant (K_b) of the above four metal complexes were determined as 5.43 × 10⁴ M_,⁻¹ 2.56 × 10⁴ M⁻¹, 1.21 × 10⁴ M⁻¹ and 1.57 × 10⁴ M⁻¹ respectively. Quenching studies of the four complexes indicates that these complexes strongly bind to DNA, out of all complex 1 is binding more strongly. Viscosity measurements indicate the binding mode of complexes with CT DNA by intercalation through groove. Thermal melting studies also support intercalative binding. The nuclease activity of the above metal complexes shows that 1, 2 and 3 complexes cleave DNA through redox chemistry.     

Influence of interatomic forces on the lattice dynamics of YBa2Cu3O7 (molecular dynamics method)

The method of molecular dynamics is used to study the dynamic properties of the high-temperature superconductor YBa₂Cu₃O₇. It is shown in the system La_2−x Sr_xCuO₄ that, due to the presence of strong anharmonism and a substantial difference in the interatomic forces, local “hot” regions arise around the Ba⁴⁺ charge defects in the CuO₂ and CuO planes, where the mean kinetic energy of the vibrations of the oxygen atoms reaches ∼0.5 eV. All other types of defects: oxygen and copper vacancies, substitutional atoms, and charge defects, do not lead to such effects. Fiz. Tverd. Tela (St. Petersburg) 41, 1729–1733 (October 1999)     

Electronic superconductor Pr2CuO4−x Fx: Magnetic correlations at high temperatures (150<T<600 K) according to 19F NMR data

The longitudinal and transverse relaxation rates of ¹⁹F nuclei in Pr₂CuO_4−x F_x (x=0.20) samples are measured at high temperatures (150 K<T<600 K). A feature is found in the temperature dependence of the relaxation rates at temperature T′≅300 K. The magnetic properties of the electronic superconductor Pr₂CuO_4−x F_x as a possible system with a stripe ordering of carriers and spins are discussed. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 4, 328–332 (25 February 1997)     

Nature of low-energy excitations in La<Subscript>1.87</Subscript>Sr<Subscript>0.13</Subscript>CuO<Subscript>4</Subscript> superconducting cuprate

The spectra of the conductivity and dielectric constant of La_1.87Sr_0.13CuO₄ cuprate have been directly measured in the frequency range of 0.3 to 1.2 THz (10–40 cm⁻¹) and the temperature range of 5 to 300 K in the E | c polarization (the electric field vector of radiation is perpendicular to the copper-oxygen planes). Excitation has been observed in the superconducting phase, and its nature has been attributed to the transverse optical excitation of the condensate of Cooper pairs, which appears because Josephson junctions between CuO planes are modulated due to in-plane magnetic and charge stripes. Additional quasiparticle absorption of unknown origin has been detected at frequencies below ≈15 cm⁻¹ at liquid helium temperatures.     

Two-dimensional quantum interference contributions to the magnetoresistance of Nd2−x CexCuO4−δ single crystals

Two-dimensional (2D) weak localization effects at low temperatures T=0.2–4.2 K are investigated in a nonsuperconducting sample Nd_1.88Ce_0.12CuO_4−δ and in the normal state of a superconducting sample Nd_1.82Ce_0.18CuO_4−δ for B>B _c2≃3 T. The phase coherence time τφ (≃5×10⁻¹¹ s at 1.9 K) and the effective thickness d of a conducting CuO₂ layer (≃1.5 Å) are estimated by fitting the expressions from the 2D weak localization theory to the magnetoresistivity data for the normal-to-plane and in-plane magnetic fields. The estimated value of the parameter d ensures the condition of strong carrier confinement and justifies a model of almost decoupled 2D metallic sheets for the Nd_2−x Ce_xCuO_4−δ single crystals. Pis’ma Zh. éksp. Teor. Fiz. 70, No. 2, 93–99 (25 July 1999) Published in English in the original Russian journal. Edited by Steve Torstveit.     

Determination of Trace Mercury by Solid Substrate-Room Temperature Phosphorimetry Quenching Method Based on Catalytic Effect of Hg2+on Formation of the Ion Association Complex [Sn(XO)6]4+·[(Fin)4]

A new method for the determination of trace mercury by solid substrate-room temperature phosphorimetry (SS-RTP) quenching method has been established. In glycine-HCl buffer solution, xylenol orange (XO) can react with Sn⁴⁺ to form the complex [Sn(XO)₆]⁴⁺. [Sn(XO)₆]⁴⁺ can interact with Fin⁻ (fluorescein anion) to form the ion associate [Sn(XO)₆]⁴⁺·[(Fin)₄]⁻, which can emit strong and stable room temperature phosphorescence (RTP) on polyamide membrane (PAM). Hg²⁺ can catalyze H₂O₂ oxidizing the ion association complex [Sn(XO)₆]⁴⁺·[(Fin)₄]⁻, which causes the RTP to quench. The ΔIp value is directly proportional to the concentration of Hg²⁺ in the range of 0.016–1.6 fg spot⁻¹ (corresponding concentration: 0.040–4.0 pg ml⁻¹, 0.40 μl spot⁻¹), and the regression equation of working cure is ΔIp=10.03+83.15 m Hg²⁺ (fg spot⁻¹), (r=0.9987, n=6) and the detection limit (LD) is 3.6 ag spot⁻¹(corresponding concentration: 9.0×10^–15 g ml⁻¹, the sample volume: 0.4 μl). This simple, rapid, accurate method is of high selectivity and good repeatability, and it has been successfully applied to the determination of trace mercury in real samples. The reaction mechanism for catalyzing H₂O₂ oxidizing the ion association complex ([Sn(XO)₆]⁴⁺·[(Fin)₄]⁻) SS-RTP quenching method to determine trace mercury is also discussed.     

Quadrupolar interaction in iridium cyanide mixed-ligands complexes in alkali halide matrices by ESR spectroscopy

Quadrupolar interactions in [IR(CN)₅]³⁻, [Ir(CN)₅Cl]⁴⁻ and [Ir(CN)₄Cl₂]⁴⁻ paramagnece a complexes in alkali halide host lattices are systematically analysed by ESR spectroscopy. The measured quadrupolar interactions are shown to have no clear correlation to the measured unpaired electron populations.     

Modeling of a structural phase transition in La2−x SrxCuO4

The structural, vibrational, and elastic properties of La₂CuO₄ are calculated using a model for calculating the energy of the crystal based on interionic potentials with the multiparticle Jahn-Teller contribution included explicitly. The microscopic reasons for the structural instability of the La₂CuO₄ lattice relative to rotations of the oxygen octahedra are investigated. A structural phase transition from the orthorhombic phase (space group D _2h ¹⁸ ) into the tetragonal phase (space group D _4h ¹⁷ ) under hydrostatic compression of an La_2−x Sr_xCuO₄ crystal is modeled. The (P,x) structural phase diagram for La_2−x Sr_xCuO₄ is constructed. Fiz. Tverd. Tela (St. Petersburg) 41, 1096–1102 (June 1999)     

Anomalous millimeter-wave absorption in the superconducting phase of La2−x SrxCuO4

Direct measurements of the complex conductivity spectra of thin-film La_2−2x Sr_xCuO₄ are made at frequencies of 5–40 cm⁻¹. Narrow, intense Drude-type excitation is observed in the superconducting phase. Pis’ma Zh. éksp. Teor. Fiz. 68, No. 5, 406–409 (10 September 1998)     

ESR of V4+ in α-RbTiOPO4 single crystals

We have recorded and investigated the ESR spectrum of vanadium-doped α-RbTiOPO₄ single crystals in the temperature interval 77–300 K. Two types of structurally distinct centers, V1 and V2, with a 4:1 ratio of the peak intensities were observed. The angular dependences of the resonance magnetic fields are described by a spin Hamiltonian corresponding to axial symmetry with the parameters g _∥1=1.9305, g _⊥1=1.9565, A _∥1=−168.2×10⁻⁴cm⁻¹, and A _⊥1=−54.3×10⁻⁴cm⁻¹ for V1 centers and g _∥2=1.9340, g _⊥2=1.9523, A _∥2=−169.0×10⁻⁴cm⁻¹, and A _⊥2=−55.2×10⁻⁴cm⁻¹ for V2 centers. A model of a paramagnetic center is proposed: The vanadium ions replace titanium ions in two structurally distinct positions Ti1 and Ti2 (V1 and V2 centers, respectively). The possibility that a VO²⁺ ion forms when α-RbTiOPO₄ crystals and crystals of the KTP group (KTiOPO₄, NaTiOPO₄, α-and β-LiTiOPO₄), studied earlier, are doped with vanadium is discussed. Fiz. Tverd. Tela (St. Petersburg) 40, 534–536 (March 1998)     

New phases in the most basic zone of the system Ba−Cu−O

Summary In the most basic part of the binary system BaO−CuO two new compounds were identified corresponding to the Ba₂CuO₃ and Ba₃CuO₄ formulae. Another more oxidized phase of composition Ba₂CuO_3+δ was also obtained, which can exist in two crystalline modifications. For all these new substances the structural type and the lattice parameters are reported.     

Change of membrane fluidity and protein conformation involved in [Ca2+]i overload in neuronal cells induced by ONOO−

Considerable evidence indicates that the formation of peroxynitrite (ONOO⁻) with superoxide anion (O₂^−⊙ot) may be involved in the neuronal toxicity of NO. Here, the effects of ONOO⁻ on intracellular free calcium concentration ([Ca²⁺]_i) in single MN9D cells was studied by the Fura-2 microfluorometric technique. The results showed that [Ca²⁺]_i was increased dose-dependently with the addition of ONOO⁻ (0–40 μmol/l) after 5 s and then decreased rapidly back to the basal level after ONOO⁻ was removed. [Ca²⁺]_i response to ONOO⁻ can be blocked by removing Ca²⁺ from the bath or adding L-type calcium channel antagonist nifedipine (10 μmol/l) to the bath. [Ca²⁺]_i transients by ONOO⁻ were substantially inhibited by dithiothreitol (DTT), which indicated ONOO⁻ may alter the L-type calcium channel on neuronal cell by thiol oxidation. To elucidate the mechanism of ONOO⁻ on [Ca²⁺]_i, the electron spin resonance spin-labeling technique was used to study the effects of ONOO⁻ on the membrane fluidity and the membrane protein conformation on freshly dissociated neurons. The results indicate that ONOO⁻ decreases membrane fluidity both near the surface and deep in the membrane and affects protein conformation. The fact that DTT effectively inhibits the deterioration supports the conclusion that the change of membrane fluidity and protein conformation is involved in [Ca²⁺]_i overload in neuronal cells induced by ONOO⁻.     

Magnetism in electron-doped copper oxides

We report muon spin relaxation/rotation measurements on sintered powder samples of Nd_2−x Ce _x CuO_4−y and a large single crystal of Nd₂CuO_4−y . We find an electronic phase diagram which is quite similar to that of hole-doped superconductors such as La_2−x Sr _x CuO_4−y , although the doping of electrons into the system is less efficient in destroying the static moments on the copper spins. Static magnetic order in Nd₂CuO_4−y appears below about 250 K, and two spin reorientations are seen atT=75 K andT=35 K. Measurements of the magnetic field penetration depth have been unsuccessful due to the rare-earth paramagnetism of these materials.     

Two-dimensional weak localization effects in high temperature superconductor Nd2−x CexCuO4−δ

A systematic study of the resistivity and Hall effect in single-crystal films Nd_2−x Ce_xCuO_4−δ (0.12⩽x⩽0.20) is presented, with special emphasis on the low-temperature dependence of the normal state conductance. Two-dimensional weak localization effects are found both in a normally conducting underdoped sample (x=0.12) and in situ superconducting optimally doped (x=0.15) or overdoped (x=0.18) samples in a high magnetic field B>B _c2. The phase coherence time τ _ϒ (5.4 · 10⁻¹¹ s at 2 K) and the effective thickness of a CuO₂ conducting layer d (≃1.5Å) have been estimated by fitting 2D weak localization theory expressions to magnetoresistivity data for magnetic fields perpendicular to the ab plane and in plane. Estimates of the parameter d ensure strong carrier confinement and justify a model consisting of almost decoupled 2D metallic sheets for the Nd_2−x Ce_xCuO_4−δ single crystal. Zh. éksp. Teor. Fiz. 116, 1723–1734 (November 1999) Published in English in the original Russian journal. Reproduced here with stylistic changes by the Translation Editor.     

Optical,electrochemical, and <Superscript>1</Superscript>H NMR characteristics of Mono-, Bi-, and tetranuclear cyclopalladated complexes with 4,4′-bipyridyl

The [Pd(C∧N)(4,4′-bpy)Cl], [Pd(C∧N)NO₃]₂(μ-4,4′-bpy) and [Pd(C∧N)(μ-4,4′-bpy)]₄(NO₃)₄ complexes (C∧N⁻ are deprotonated forms of 2-phenylpyridine (ppy), 2-(2′’-thienyl)pyridine (tpy), and 2-phenylbenzothiazole (bt); bpy is 4,4′ bipyridyl) are synthesized and characterized by ¹H NMR spectroscopy, electronic absorption and emission spectroscopy, and cyclic voltammetry. The upfield shifts of the chemical shift of a proton in the ortho-position to the donor carbon atom of the cyclopalladated ligand of complexes (Δδ = −(1.1–1.5) ppm) is assigned to the anisotropic effect of the circular current of the pyridine rings of 4,4′-bipyridyl orthogonal to the coordination plane. The characteristic long-wavelength absorption and phosphorescence bands of the complexes are assigned to the chromophore metal-complex fragment {M(C∧N)}. The quasi-reversible reduction waves of complexes are assigned to the ligand-centered processes of successive electron transfer to the π*-orbitals localized mainly on the coordinated pyridine components of 4,4′-bipyridyl.     

Optical and IR study of Li2O–CuO–P2O5 glasses

Infrared (IR) and UV spectra of ternary Li₂O–CuO–P₂O₅ glasses in two series Li₂O(65−X)%–CuO(X%)–P₂O₅(35%), X = 20, 30, 40 and Li₂O(55−X)%–CuO(X%)–P₂O₅(45%), X = (10, 20, 30) were studied. Infrared (IR) investigations showed the metaphosphate and pyrophosphate structures and with increase of CuO content in metaphosphate glass, the skeleton of metaphosphate chains is gradually broken into short phosphate groups such as pyrophosphate. IR spectra showed one band at about 1,220 and 1,260 cm⁻¹ for P₂O₅(35%) and P₂O₅(45%) series, respectively, assigned to P=O bonds. For CuO additions ≤20 mol%, the glasses exhibit two bands in the frequency range 780–720 cm⁻¹ which are attributed to the presence of two P–O–P bridges in metaphosphate chain. But for CuO addition ≥30 mol%, the glasses exhibit only a single band at 760 cm⁻¹ which is assigned to the P–O–P linkage in pyrophosphate group. In optical investigations, absorption coefficient versus photon energy showed three regions: low energy side, Urbach absorption, and high energy side. In Urbach’s region, absorption coefficient depends exponentially on the photon energy. At high energy region, optical gap was calculated and investigations showed indirect transition in compounds and decreases in optical gap with increases of copper oxides contents that is because of electronic transitions and increasing of nonbridging oxygen content.