EPR study of the Jahn-Teller effect of Cu2+ ions in ZnGa2O4

The Jahn-Teller effect in the ZnGa₂O₄ spinel single crystal has been investigated using electron paramagnetic resonance of Cu²⁺ ions in the temperature range 110–560 K. It has been shown that copper ions occupy octahedral sites 16d in the ZnGa₂O₄ crystal with cubic symmetry O _h ⁷ (Fd-3m). At T < 560 K, the octahedra undergo tetragonal distortions (predominantly tension) and rotation around the fourfold axes by the angle θ ≈ 2.6°. The parameters of the spin Hamiltonian, which characterize the prolate (g _‖ = 2.355, g _⊥ = 2.077, A _‖ = 116 Oe, A _tp = 12 Oe) and oblate (g _‖ = 2.018, g _⊥ = 2.246, A _‖ = 75 Oe, A _⊥ = 44 Oe) octahedra, have been determined. At temperatures above 560 K, the static Jahn-Teller effect transforms into the dynamic effect and the spectrum of the magnetic resonance becomes isotropic with g = 2.116 (the experimental frequency corresponds to the X band).     

Anomalous magnetism of La2CuO4+x single crystals

Measurements of the differential magnetic susceptibility are performed to study the changes in the magnetic properties of La₂CuO_4+x due to oxygen doping in the concentration interval 0<x<0.011. For crystals with 0.005<x<0.011, a ferromagnetic-type phase transition is observed at T _c >T _N , and it occurs even in crystals with T _N ～0. The concentration dependence of the transition temperature T _c (x) is obtained.     

Weak ferromagnetism and oxygen ordering in La2CuO4+x single crystals

The changes produced in the magnetic properties and structure of La₂CuO_4+x (0< x<0.015) single crystals by doping with oxygen are investigated by differential magnetic susceptibility and x-ray diffraction methods. It is found that the appearance of a weak ferromagnetism in weak fields H<50 Oe is accompanied by a lowering of lattice symmetry as a result of the oxygen ordering. Pis’ma Zh. éksp. Teor. Fiz. 66, No. 3, 159–162 (10 August 1997)     

Magnetic properties of La2CuO4 single crystals

Magnetic susceptibility x of La₂CuO₄ crystals have been measured with external magnetic field perpendicular to and within the c plane. A kind of ordering or condensation of the spin system with the spin direction parallel to the c axis has been found although it is difficult to consistently explain all of the observed behaviors. Even above the ordering temperature (⋍ 250K), uniaxial anisotropy of x has also been observed.     

Neutron diffraction investigation of phase separation in La2CuO4+y single crystals

La₂CuO_4+y single crystals with y=0.03 and 0.04 are investigated with a high-resolution neutron diffractometer in the temperature range 10<T<293 K. Althouogh the excess oxygen concentration y falls in the range of the dissolution gap, the crystal with y=0.03 does not undergo phase separation (PS)—the oxygen distribution in this crystal remains uniform over the entire experimental temperature interval. Macroscopic PS is observed in the crystal with y=0.04. An analysis of the dependence of the widths of the reflection orders on the interplanar separation gives the sizes of the coherent regions of the two coexisting phases: (970±30) Å in the direction of the tetragonal axis c and (1460 645) Å in the basal plane.     

Percolative phase separation in La2CuO4+δ and La2−x Sr x CuO4

We report on the formation of conducting phases in slightly doped La₂CuO₄ samples by the existence of a percolative phase separation. Phase separation can be quenched by rapid cooling and can be restored by the application of a 3 T magnetic field. Magnetically polarizable quasiparticles are shown to be formed by hole doping which fuse to form percolative conducting and below 37 K superconducting phases.     

Semiempirical model for calculating the lattice dynamics of the Jahn-Teller oxide La2CuO4

Calculations of the properties of NiO, CaO, SrO, and La₂NiO₄ crystals are used as a basis for determining empirical pairwise potentials which are employed in calculating the structure, lattice dynamics, and certain properties of La₂CuO₄. The model developed here differs from those proposed before. First, the many-particle Jahn-Teller contributions to the energy and dynamic matrix of the crystal are taken into account. Second, the analytic expressions for the pairwise potentials contain an additional short-range electrostatic screening term. The results are in satisfactory agreement with experimental data. Fiz. Tverd. Tela (St. Petersburg) 39, 1094–1100 (June 1997)     

Second harmonic of magnetization and Cu magnetic system behavior at phase separation in moderately oxygenated single crystals of La2CuO4+x

The results on the nonlinear longitudinal response (second order uniform dynamic susceptibility χ₂(ω)) of two moderately doped La₂CuO_4+x single crystals in the temperature region of the phase separation of extra oxygen are reported. Large 2D-fragments (I≈7·10³ Å) of stoichiometric La₂CuO₄ are shown to be responsible for the signal in sample withT _ps>-T _N≈243 K. The signal of the crystal withT _ps<T _N≈272 K is due to oxygen-rich phase. Its peculiarities are related either to change and destruction of magnetic ordering in the course of the phase separation or to the ordering of the extra oxygen.     

More experimental insight on Eu2+ ground state splitting

Experimental results obtained on Eu²⁺ incorporated into MeFX (Me = Ca, Sr, Ba and X = Cl, Br, I) single crystals are discussed. A possible origin of the observed ground state splitting is traced back to the physico-chemical features of the host crystals. From X-ray diffraction measurements performed to determine the lattice cell structure at different temperatures one concludes that implicit effects are more likely to explain the ground state splitting temperature dependence.     

The effect of off-center Cu2+ ions on the phase transition in lead germanate crystals

The properties of Pb₅Ge₃O₁₁: Cu²⁺ crystals near the temperature of the ferroelectric phase transition are discussed in terms of a phenomenological approach. Assuming a quadratic interaction with the order parameter, the effect of Cu²⁺ is considered a result of static distortions, which result from the off-center position of the copper impurity ions in the lead germanate structure. In this approximation, Cu²⁺ jumps between off-center positions do not affect the dynamic properties of the crystal matrix near T _C.     

Dc and Ac conductivity of La2CuO4+δ

The complex conductivity of La₂CuO_4+δ has been investigated for frequencies 20 Hz≤ν≤4 GHz and temperatures 1.5K≤T≤450 K. Two single crystals with δ≈0 and δ≈0.02 were investigated, using dc (four-probe), reflectometric and contact-free techniques. At high temperatures the dc conductivity is thermally activated with low values of the activation energy. For low temperatures Mott's variable range hopping dominates. The real and imaginary parts of the ac conductivity follow a power-law dependence σ∼v ^s, typical for charge transport by hopping processes. A careful analysis of the temperature dependence of the ac conductivity and of the frequency exponents has been performed. It is not possible to explain all aspects of the ac conductivity in La₂CuO_4+δ by standart hopping models. However, the observed minimum in the temperature dependence of the frequency exponents strongly suggests tunneling of large polarons as dominant transport process.     

Dc and Ac conductivity of La2CuO4+δ

The complex conductivity of La₂CuO_4+δ has been investigated for frequencies 20 Hz≤ν≤4 GHz and temperatures 1.5K≤T≤450 K. Two single crystals with δ≈0 and δ≈0.02 were investigated, using dc (four-probe), reflectometric and contact-free techniques. At high temperatures the dc conductivity is thermally activated with low values of the activation energy. For low temperatures Mott's variable range hopping dominates. The real and imaginary parts of the ac conductivity follow a power-law dependence σ～v ^s, typical for charge transport by hopping processes. A careful analysis of the temperature dependence of the ac conductivity and of the frequency exponents has been performed. It is not possible to explain all aspects of the ac conductivity in La₂CuO_4+δ by standart hopping models. However, the observed minimum in the temperature dependence of the frequency exponents strongly suggests tunneling of large polarons as dominant transport process.     

Magnetic studies of phase separation in La2CuO4+° single crystals

The phase seperation effect in La₂CuO_4.015 single crystal has been studied by magnetic susceptibility and EPR experiments. The experiments confirm the percolative phase separation model based on the concept of magnetic polarons formed by hole doping. It has been shown that the superconducting subphase has specific spatial structure which is responsible for unconventional low temperature behavior of susceptibility. The EPR signal as well as the paramagnetic contribution to the lowtemperature susceptibility have been shown to arise from isolated Cu²⁺ ions located inside the phase domain walls.     

Intercalation and staging behavior in super-oxygenated La2CuO4 + δ

A high temperature electrochemical oxidation process has been used to produce large single crystals of La₂CuO_{4 + δ} suitable for neutron scattering experiments. Below room temperature the oxygen-rich phases have structural superlattice scattering peaks which indicate new periodicities ranging from 2 to 6.6 layers perpendicular to the copper oxide planes. A model structure originally proposed for La₂NiO_{4 + δ} can account for the superlattice peaks as a result of anti-phase domain boundaries between different tilt directions of the CuO₆ octahedra. Within this model, the changes in CuO₆ tilt directions are induced by segregated layers of interstitial oxygen which order in a manner similar to intercalants in graphite. This structural model thus clarifies previous work and establishes La₂CuO_{4 + δ} as a unique lamellar superconducting system with annealed disorder.