Location of superconductivity in La2-βSrβCuO4

Evidence is presented that the superconductivity of La_{2 ?β} Sr_β CuO₄ and La₂CuO4_+δ occurs via holes in the LaO planes, rather than in the CuO₂ planes.     

Electrical and thermoluminescence properties of γ-irradiated La2CuO4 crystals

Abstract

Measurements of the electrical properties of unirradiated as well as γ-irradiated La₂CuO₄ crystals were carried out at different temperatures in the frequency range of 0.1-100 kHz. Thermoluminescence (TL) studies were also performed on such crystals in the temperature range of 300-600K. The conductivity of the unirradiated La₂CuO₄ crystals were found to obey the power law frequency dependence at each measured temperature below the transition temperature (T_c = 450K). The activation energies for conduction and dielectric relaxation time have been calculated. The TL response and the dc resistance were found to increase with γ-irradiation dose up to 9-10 kGy. The results showed that the ferroelastic domain walls of La₂CuO₄ crystal as well as its TL traps are sensitive to γ-raditaion. This material can be used in radiation measurements in the range 225 Gy-10 kGy.     

Observation of magnetic order of Cu-moment aroundx=0.12 in La2−x Ba x CuO4 and La2−x sr x CuO4 by μ+SR

Zero field positive muon spin rotation method ( ⁺SR) is applied on La_2–x Sr _x CuO₄ around forx=0.12 at which the high-T _c superconductivity (SC) is suppressed. The magnetically ordered state of Cu-moments, which is not a spin glass state but an antiferromagnetic like state, appears below 15 K for 0.105x0.120. The magnetic phase boundary is very similar to the one of the La_2–x Ba _x CuO₄ in which the structural transition from the low temperature orthorhombic (LTO) phase to the low temperature tetragonal (LTT) phase is observed aroundx=0.12. The present study suggests that there is no big difference of the electronic state of the CuO₂ plane between the La-Ba system and the La-Sr system and that the magnetic ordering of Cu-moments plays an important role for the suppression of the high-T _c SC aroundx=0.12 in both of the systems, although the LTO-LTT structural transition has not been observed yet in the La-Sr system.     

Structural phase transition and structural properties in oxygenated La2CuO4+y; y ∼0.02

The crystal structure of the oxygenated La₂CuO_4+y; y ～0.02 has been studied by X-ray diffraction. It is found that the structural phase transition from tetragonal to orthorhombic symmetry occurring at about 450 K (Tc) is of second order and shows diffuse scattering above Tc. The diffuse scattering is related to static and/or dynamic short-range order of cooperative tilts of rigid-like elongated CuO₆ octahedra, which make two-dimensional networks stacked along the [001] axis. The structures at room temperature and 210K are determined using a 4-circle X-ray diffractometer and have the orthorhombic space group ¹⁰D_2h-Pccn. The structure is mainly characterized by anisotropic cooperative tilts of the CuO₄ octahedra around the [100]_p and [010]_p primitive axes in two-dimensional octahedron-networks, where suffix “p” means pseudo K₂NiF₄-type structure. The anisotropy of the tilting-angles results from pushing by excess oxygen atoms to apical oxygen atoms of the CuO₆ octahedra. It is also found by refinement of the structure that the excess oxygen atoms are randomly sited at the atomic coordinates (1/4,1/4,0.244) and/or (1/4, 1/4,0.279).     

Theory for position of muonium in α-quartz and associated hyperfine interaction

The electronic structures and hyperfine interactions of muonium and hydrogen in -quartz are investigated by the unrestricted Hartree-Fock cluster procedure. The muonium is found to be trapped near the center of the line joining two silicon atoms. On including vibrational effects, the muon hyperfine constant comes out as 1.09 times that for free muonium, this ratio being larger than unity and smaller than for protons in trapped hydrogen, both features being in agreement with experiment.Briefly reported in Abstract at the American Physical Society meeting in New Orleans, March 1988. See Bull. Am. Phys. Soc. 33 (1988) 770.     

Resistive properties of La2 ? x Sr x CuO4 low-dopped cuprates in the antiferromagnetic state at low temperatures

The nonlinear dependence of conductivity and negative differential resistance of ceramic samples of La_{2 ? x} Sr _x CuO₄ in the antiferromagnetic state is revealed. The possible influence of a spatially inhomoge-neous distribution of strontium impurities on the low-temperature behavior of conductivity is discussed.     

μSR and NMR investigations on electronic and magnetic state aroundx=0.12 in La2−x Sr x CuO4 and La2−x Ba x ;CuO4

The muon spin rotation ( ⁺SR) and NMR measurements provides clear evidences of the antiferromagnetic order of Cu-moment below 35 K for La_2–x Ba _x CuO₄ and below 15 K for La_2–x Sr _x CuO₄ in the narrow range ofx where the high-T _c superconductivity (SC) is suppressed remarkably. The results suggest that the freezing of spin fluctuations of Cu-moment is relevant to the local suppression of SC under an change of the electronic state coupled with the lattice instability.     

Correlation betweenT c and elastic constants of (La−M)2CuO4

By an ultrasonic method, we measured the elastic constants of polycrystalline La_1.85M_0.15CuO₄ withM=Ca, Ba, Sr. These materials show zero-resistance normal-superconductive transition temperatures of 19, 28, and 36K. VersusT _c , all the elastic stiffnesses-bulk modulus, Young modulus, shear modulus-increase. Derived from the elastic constants and average atomic volume, the Debye temperature increases linearly withT _c . In conventional BCS superconductors,T _c decreases with increasing _D .     

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.     

Quasi-two-dimensional transport properties of the layered superconductor Nd2−xCe x CuO4+δ

The temperature dependences of resistivities ρ_ab in the ab plane and ρ_c along the c axis have been studied for single-crystal Nd_{2 ? x} Ce_xCuO_{4 + δ} (x = 0.12, 0.15, 0.17, 0.20) films with (001) and (1 $\bar 1$ 0) orientations. The superconducting transition temperature and anisotropy coefficient are shown to be maximal in optimally annealed samples (the oxygen content is close to the stoichiometric content, δ → 0). A combination of the metallic behavior of the ρ_ab(T) dependence and the nonmetallic behavior of the ρ_c(T) dependence for the optimally annealed samples is an intrinsic property of the substance and an indication of the fact that the system is quasi-two-dimensional. This layered quasi-two-dimensional system is an Anderson dielectric with a strongly anisotropic localization radius (R _loc ^ab ? R _loc ^c ). An increase in the oxygen content and, hence, in the degree of disorder in the Nd_{2 ? x} Ce_xCuO_{4 + δ} system is found to decrease the resistivity anisotropy coefficient. Thus, a disorder-induced Anderson transition takes place in this quasi-two-dimensional system.     

Fine and hyperfine structure in the X2Σ+ state of gas-phase yttrium monoxide

The A²Π_r-X²Σ⁺ visible spectrum of gas-phase yttrium monoxide has been studied at sub-Doppler resolution to obtain information on the spin-rotation and ⁸⁹Y nuclear magnetic hyperfine interactions. The observations were fitted to an effective Hamiltonian model that neglected the nuclear magnetic hyperfine interactions in the excited electronic state. An interpretation of the determined parameters in terms of possible electronic configurations is presented.     

Theoretical investigation of electronic,magnetic, transport and optical properties of the pure and doped cuprate superconductor HgBa2CuO4+δ

We report detailed DFT calculations and Monte Carlo simulations on the pure and doped cuprate superconductor HgBa₂CuO_4+δ. For the pure compound (δ = 0), we have obtained an insulating behavior with strong antiferromagnetic copper spin correlations in the CuO₂ plane. The high value of the calculated Néel temperature T_N = 333 K reflects the large in-plane exchange interaction J = -145 meV. The obtained optical properties and critical exponents demonstrate the anisotropic quasi-2D character of this type of materials. As for the doped compound the electronic structure and the transport properties have been investigated for various values of doping. Based on these data, we suggest a prediction of the value of optimum doping for HgBa₂CuO_4+δ(δ_optimal = 0.125).     

Vortex structure and anisotropy of electric resistance in Bi2Sr1.65La0.35CuO6 + δ single crystals

The vortex structure of Bi₂Sr_1.65La_0.35CuO_6+δ single crystals in tilted magnetic fields has been studied by the decoration method. From the observed pattern of vortex chains in the basal plane, the parameter of anisotropy in the superconducting state has been estimated as γ_S = 460 ± 40. The electric resistance of Bi₂Sr_1.65La_0.35CuO_6+δ single crystals has been studied in a broad range of temperatures (T _c < T < 300 K) and magnetic fields (up to 16 T). The ratio of resistivities in the direction perpendicular to the basal plane and in plane near the critical temperature T _c amounted to ρ⊥/ρ‖ = 3.2 × 10⁵. A possible relationship between the anisotropy in the normal and superconducting states is discussed.     

On the La2CuO4-Nd2CuO4 system: Influence of oxygen pressure on the phase diagram,oxygen stoichiometry and transport properties of the T/O and T′ type solid solutions

Abstract

Two types of solid solutions exist in the La₂CuO₄-Nd₂CuO₄ system, with respectively the T/O and T′ type structure of La₂CuO₄ and Nd₂CuO₄. When synthesized in air at high temperature and normal pressure, both solid solutions exhibit semi-conducting properties. A treatment under oxygen pressure causes a slight change in the unit cell parameters of the T/O type solid solution, a significant shift of its upper limit towards higher neodynium contents and a small increase of the average oxidation state of copper. All these modifications result from the insertion of some oxygen species in the T/O type-structure and induce a superconducting behaviour.     

Behavior of positive muons in highT c superconductors La2−x Sr x CuO4

An unambiguous determination of the ⁺ location in the highT _c LaSrCuO superconductor is made through zero-field spin relaxation in the paramagnetic phase and its crystal axis dependence by using single crystalline samples. The temperature dependence of the relaxation rates has a maximum between 80 and 150 K, indicating the existence of the onset of quantum diffusion at low temperature.     

On the spin and hyperfine splittings in the X2Σ+ ground states of AlO and AlS

A re-analysis of the A′²Π_i–X²Σ⁺ band system of the AlS radical has been performed. The previously published material has been refined and extended to approximately 36,000 line assignments. The line material has been fitted to a ²Σ⁺ hyperfine Hamiltonian, in the same manner as in the recently published work on the B²Σ⁺–X²Σ⁺ and A²Π_i–X²Σ⁺ systems of AlO. The present analysis shows the same trends for the ground state γ-values as in AlO, i.e. decreasing γ-values upon increasing v. It has been confirmed that the spin splittings of the X²Σ⁺ (v = 0) levels in both molecules are mainly due to nuclear hyperfine interaction and second order spin–orbit effects due to the high-lying regular ²Π state (C²Π in AlO and B²Π in AlS), while the influence of the low-lying inverted ²Π state on γ increases dramatically with increasing v, resulting in negative γ-values starting from v = 2.     

Effects of Zn on magnetic properties and pseudogap of optimally doped La2−xSrxCuO4

The effects of Zn substitution on the uniform (q = 0) magnetic susceptibility, χ(T), of optimally doped (x = 0.15) La_2?xSr_xCu_1?yZn_yO₄ sintered samples were investigated over a wide range of Zn contents (y). Non-magnetic Zn was found to enhance χ(T) systematically and depress T_c very effectively. We have extracted the characteristic pseudogap energy scale, ε_g, from the analysis of χ(T) data. Unlike T_c, ε_g was found to be fairly insensitive to the level of Zn substitution. This supports the scenario where the pseudogap phenomenon has non-superconducting origin. We have also analyzed the Zn-induced Curie-like enhancement of the χ(T) data using different models and discussed the various possible implications.     

Synthesis and optical properties of Co2+ and Ni2+ ions doped β-BaB2O4 nanopowders

Co²⁺ and Ni²⁺ ions doped β-BaB₂O₄ nanopowders have been prepared by co-precipitation method and their structural properties are studied by spectroscopic techniques. Powder XRD data reveals that the crystal structure belongs to monoclinic and the average crystallite size is calculated. Optical absorption spectra data reveal octahedral site symmetry for Co²⁺ and Ni²⁺ ions. Crystal field (Dq) and inter-electron repulsion (B and C) parameters are evaluated for Co²⁺ doped β-BaB₂O₄ nanopowders as Dq=960, B=900 and C=3850 cm^?1 and for Ni²⁺ doped β-BaB₂O₄ nanopowders, Dq=900, B=850 and C=3500 cm^?1. FT-IR spectra showed the characteristic vibrational bands related to BO₃ and BO₄ molecules. Photoluminescence spectra contain the emission bands in ultraviolet and blue regions.     

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.