Valence fluctuations in La2−xSrxCuO4

Ab-initio electronic band structure calculations are presented for the perovskite La₂CuO₄ and for this material doped with Sr for a supercell of composition La₃SrCu₂O₈. This material is close to the high Tc superconductor La_2−xSr_xCuO₄ discovered recently. The Sr doping gives rise to strong valence fluctuations. We discuss the effect of the valence fluctuations on the stability of the lattice, indicating a small value of U and enhancing the electron-phonon coupling λ, mainly by a mechanism of incipient peroxide formation.     

Theoretical investigations of the EPR g factors and the local structures for Ni3+ in LiAlyCo1–yO2 at various Al concentrations

The electron paramagnetic resonance (EPR) g factors and local structures for Ni³⁺ in LiAl_yCo_1–yO₂ as the cathode materials for lithium-ion batteries are theoretically investigated at various Al concentrations y (= 0.1, 0.5, 0.8 and 1.0). The [NiO₆]^9– clusters are subject to the tetragonal elongation distortions along the C₄ axis owing to the Jahn–Teller effect and the different degrees of distortions are characterised by the slightly increasing relative elongation ratio ρ (and also crystal-field strength and covalency) with y. In pure LiAlO₂ (y = 1.0), the [NiO₆]^9– cluster exhibits the largest ρ (≈1.4%) and an additional relative variation ratio τ (≈1.4%) of the planar bond lengths, responsible for the perpendicular g anisotropy δg = (g_x – g_y). The number of impurity Ni³⁺ centres increases from one at y = 0 and 1.0 to two at y = 0.1 and 0.8 and reaches the maximum four at y = 0.5, in accordance with the increasing degree of disorder of the systems. The distinct impurity centres for the same y may be accompanied by the comparable energies of the ground and near excited states, similar to ‘resonant states’ with slightly different local elongation distortions and tetragonal level splittings. The present studies will be helpful to understand the relationships between local structures and performances of LiAl_yCo_1–yO₂ type cathode materials.     

Lattice dynamics of the high-Tc superconductor La2−xMxCuO4

The phonon dispersion curves, the one-phonon density of states and the specific heat of the high-T_c superconductor La_2−xM_xCuO₄ have been calculated in the framework of a shell model. The zone center frequencies are classified according to their vibrational character and compared with the available IR and Raman data. A slight adjustment of the parameters obtained from the known interaction potentials of oxides with perovskite and rock salt structure yields reasonable agreement with the IR, Raman, specific heat, and inelastic neutron scattering data of the polycrystalline material.     

Enhanced superconductivity by reducing magnetism in strained La2−xSrxCuO4 films

I report measurement results of the temperature dependence of electrical resistivity for the compressively strained (0 0 1)-oriented films of La_2−xSr_xCuO₄, which show values of the superconducting transition temperature (T_c) higher than those for bulk materials. A comparison of the results for the films to those for bulk suggests that the number density of localized Cu spins is reduced in the films. This reduction seems to be essential for the enhancement of T_c in the films, which lead me to suggest that the magnetism in cuprates, rather than being the origin of high-temperature superconductivity (HTS), is actually an impediment to it.     

Effect of annealing conditions on high temperature superconductivity in La2−xSrxCuO4

Abstract

Besides investigating a number of oxides of the series La_2?xM_xCuO₄ (M=Ba, Pb, Sr), we have, in this investigation emphasised on the studies of the effect of annealing conditions on superconducting properties of composition La_1.8Sr_0.2CuO₄. The effect is remarkable and is di'scuss'ed in the light of X-ray diffraction results. These samples, studied after a gap of two months, showed interesting changes which has been discussed in the present paper.     

EPR study of the high-TC superconductor YBa2Cu3O7−δ

EPR spectra of superconducting YBa₂Cu₃O_7-δ and insulating Y₂BaCuO₅ have been examined. The intensity of Cu²⁺ ions in YBa₂Cu₃O_7-δ is about 1/3000 of that in Y₂BaCuO₅ at room temperature. Owing to the Meissner effect in the superconducting state, the EPR signal of Cu²⁺ ions in YBa₂Cu₃O_7-δ disappears; moreover the EPR signal of DPPH deposited on the YBa₂Cu₃O_7-δ specimen is shifted. An additional EPR line is found for YBa₂(Cu_2.94Cr_0.06)O_7-δ at temperatures below 113K; this can be related to the phenomenon of superconductivity.     

Infrared spectra of superconducting La2−xSrxCuO4 and YBa2Cu3O7−δ

Abstract

Abstract Infrared spectra of La_2?xSr_xCuO₄ and YBa₂Cu₃O_7?δ are discussed, specially with respect to the disappearance of the high-frequency (～650 cm) band of the superconducting compositions at 300K. Some of the bands persist at 300K eventhough the materials are fairly conducting. YBa₂Cu₃O_7?δ does not show evidence in the far IR spectrum for the presence of an optical gap.     

Theoretical studies of the dependence of EPR g-factors on local structure for the trigonal Er3+–VK centres in KMgF3 and KZnF3

The dependence of the EPR g-factors on the local structural parameter for a 4f¹¹ configuration ion Er³⁺ in a trigonal crystal-field has been studied by diagonalizing the 364×364 complete energy matrices. Our studies indicate that the EPR spectra of the trigonal Er³⁺–V_K centers in KMgF₃ and KZnF₃ may be attributed to the translation of the cubic Kramers doublet Γ₇. Furthermore, the EPR g-factors of the trigonal Er³⁺–V_K centers may be interpreted reasonably by the shifts ΔZ≈0.340 Å and ΔZ≈0.303 Å of the Er³⁺ ions toward the charge compensator V_K along the C₃ axis for the KMgF₃:Er³⁺ and the KZnF₃:Er³⁺ systems respectively.     

Research on the defect structure and g factors for the tetragonal (CrO4)3− impurity center in ZrSiO4: Cr5+ crystal

ABSTRACT

The EPR g factors g// and g⊥ for the tetragonal (CrO₄)^3? impurity center in ZrSiO4: Cr⁵⁺ crystal are studied from the high-order perturbation formulas based on the two-mechanism (the crystal field and charge-transfer mechanisms) model. The studies indicate that differing from the tetragonally-elongated host (SiO4)^4- tetrahedron, the dominant defect structure of the substitutional (CrO₄)^3? tetrahedron is tetragonally- compressed with the ground state |d_z² due to the Jahn-Teller distortion. Furthermore, the agreement of g factors between calculation and experiment requires a small admixture of the first excited state |d_x²_?y² to the ground state |d_z² due to the vibrational motion of ligands, which leads a compressed (CrO₄)^3? tetrahedron to become a twinkling elongated one, These results are discussed.     

Unconventional magnetic phase diagram of cuprate superconductor La2−xSrxCuO4 at quantum critical point x=1/9

We propose a new magnetic phase diagram of La_2?xSr_xCuO₄ around a quantum critical point x=1/9 based on field-cooled magnetization measurements and critical fittings. A new phase boundary T_m2(H) is discovered which buries deeply below the first order vortex melting line in the vortex solid phase. The coupling between superconductivity and antiferromagnetism is found to be attractive below T_m2(H) while repulsive above. The attractive coupling between superconducting order and static antiferromagnetic order provides compelling experimental evidence that the antiferromagnetism microscopically coexists and collaborates with the high temperature superconductivity in cuprates.     

Introduction of bismuth into the highT c superconductor La2−x Sr x CuO4−y

Novel high-T _c superconductors La_2–x Bi _x Sr _x–x CuO_4–y have been isolated and studied. The influence of the experimental conditions of synthesis on the superconductivity has been especially studied for the composition La_1.7Bi_0.1Sr_0.2CuO_4–y ; it is shown thatT _c decreases as oxygen is introduced into the matrix, likely due to the partial oxidation of Bi(III) to Bi(V). The highT _c value observed for one sample (T _c 42 K), as compared withT _c 38 K of La_1.8Sr_0.2CuO_4–y suggests that bismuth is also a potential candidate for improving the superconducting properties in these oxides.     

Ultrasonic studies of the high Tc superconductor (La1−xSrx)2CuO4−δ

We synthesized (La_1−xSr_x)₂CuO_4−δ crystals with 0 ≦ x ≦ 0.4 by the sintering method, and obtained several samples with good homogeneity, high onset superconducting transition temperature T_c ≲ 38 K and narrow transition width ΔT_c. For the sample with the maximum T_c among them, ultrasonic measurements were performed. In the curve of the sound velocity change ΔV_s(T) versus temperature T, there was observed an anomalously large decrease of sound velocity as T decreases in the range 150 K < T < 240 K. Furthermore, in the plots of the ultrasonic attenuation coefficient α(T) as a function of T, there were observed a broad and large peak located at T ∼ 100 K and a small peak located at T ∼ 200 K. These results show the existence of an optical mode of the energy ℏω_op/k_B ∼ 100 K and, probably, also of that of ∼ 200 K. On the basis of these experimental results of elastic properties, we suggest the origin of high T_c of this material.     

Jahn–Teller effect in the 4T2g excited state of Cr3+ ion in Cs2NaYF6 crystal

Calculations of the fine structure of Cr³⁺ energy levels in Cs₂NaYF₆ accompanied by estimations of the Jahn–Teller (JT) stabilization energy in the first excited ⁴T_2g state of Cr³⁺ ion are presented. Two independent approaches—effective second-order spin–orbit Hamiltonian and analysis of the potential energy surfaces—are used. The JT energy was estimated to be 216 and 257 cm^–1 in the first and the second models, respectively. It is shown that the octahedral [CrF₆]^3? complex undergoes an equatorial expansion by 0.09 Å and an axial elongation by 0.02 Å due to the combined effect of the a_1g and e_g normal modes.     

Microwave Absorption by Charge Density Waves in La2–xSrxCuO4

The results of studying the La_2–xSr_xCuO₄ (x = 0.077−0.195) crystals with the microwave absorption, direct current resistance and magnetic susceptibility measurements are presented. The doping-temperature phase diagram with the boundaries of the areas with charge density waves, superconducting fluctuation and the bulk superconductivity state is plotted. The doping range of the charge density wave existence is found to be broader than that obtained from the X-Ray Diffraction study. It spreads at least down to x = 0.077.