L. Leonyuk

Magnetic resonance in pure and diamagnetically diluted spin-Peierls CuGeO3

Magnetic resonance at frequencies 22–75 GHz is investigated in pure single-crystalline CuGeO3 in the temperature range 1.2–25 K. At temperatures below 4 K the magnetic resonance line splits into four spectral components. The line close to the ESR of the paramagnetic phase is associated with impurities or defects. The other three lines have the same intensity in different samples and are apparently characteristic for pure single crystals at low temperature. An antiferromagnetic resonance with two resonance absorption branches and a spectrum characteristic for an antiferromagnet with easy, average, and hard anisotropy axes was observed and investigated in the Néel compound Cu0.98Zn0.02GeO3 with diamagnetic dilution of the spin subsystem.

Microwave EPR spectroscopy of cobalt-doped germanium cuprate

Resonance magnetoabsorption spectra of CuGeO3 single crystals containing 2% Co impurity have been studied in the frequency range 60–360 GHz in magnetic fields of up to 16 T and in the temperature interval 2–60 K with the magnetic field B aligned parallel to the a crystallographic axis. In addition to the Cu2+ chain resonance, a new absorption line (unobserved previously in doped CuGeO3 and deriving apparently from the Co2+ ions) was detected in EPR spectra. Quantitative analysis of the spectra suggests that the spin-Peierls transition occurs in about 10% Cu2+ chains, while the spin-Peierls state in the remaining 90% chains is completely destroyed by cobalt doping. The results obtained reveal considerable deviations from the universally accepted scenario of CuGeO3 doping and are discussed within alternative theoretical models, namely, the quantum critical behavior (based on the EPR theory for quasi-one-dimensional systems) and a three-dimensional antiferromagnet with the Néel temperature lowered by disorder.

Folded modes in the infrared spectra of the spin-Peierls phase of CuGeO 3

Polarized far-infrared transmittance spectra of CuGeO_3 single crystals were measured at different temperatures (6K < T < 300K). Two spectral lines, at 284.2 cm-1 in E||c polarization and at 311.7 cm-1 in E||b polarization, appear at the temperature of the spin-Peierls transition and grow in intensity with decreasing temperature. Both of them are, most probably, folded modes of the dimerized lattice. We discuss a possible role of the spin-phonon interaction in the formation of the 311.7 cm-1 feature.

Polarisation-dependent X-ray absorption in high- and low-Tc Bi2Sr2Can-1CunO4+2n superconductors

Abstract Taking advantage of the linear polarisation of synchrotron radiation, the polarisation dependence of the O K and Cu L 3 X-ray absorption edges has been compared for in situ cleaved Bi 2 Sr 2 CaCu 2 O 8 and Bi 2 Sr 2 CuO 6 single crystals. The X-ray absorption spectra were measured by means of total electron yield detection. The spectra show for both crystals that the lowest unoccupied Cu 3d and O 2p orbitals are dominantly oriented parallel to the a−b plane, but also the presence of a small amount of unoccupied orbitals oriented perpendicular to the a−b plane can be deduced from both the O K and Cu L3 absorption spectra. A shift of the order of 0.5 eV between absorption into the in-plane and the out-of-plane Cu 3d orbitals was measured for the Bi 2 Sr 2 CaCu 2 O 8 crystal, but no such shift was found for the Bi 2 Sr 2 CuO 6 crystal.

New 80 K superconductor with structure A14Cu24O41

New 80 K superconductor with structural type A14Cu24O41 was grown and the physical properties of single crystals grown were studied.

Study of isostructural phases in 2212-type high-Tc semiconductors

Single crystals of 2212-type compounds were grown and investigated by X-ray diffraction (XRD) and electron microprobe analysis (EMPA). Several isostructural Bi-2212-type phases were indicated, which differed in chemical composition, lattice parameters and physical properties. Several sequences of solid solutions have been distinguished which differ in the Pb amount in the crystals. In each sequence the lattice constant c changes successively depending on the concentration of Y or rare earth (RE) substituting for Ca. However, in each sequence only several single-phase samples with particular compositions were found. The compounds with intermediate compositions, and hence with intermediate Y, RE concentrations, represent an epitaxial intergrowth on the stable phases or manifest themselves as inclusions at decomposition of solid solutions at melt quenching. The cation sublattice was found to be dominant in the structure formation. Determination of the sequences of solid solutions allowed us to propose a decantation method to obtain monophase single crystals. The phase intergrowth scheme was also considered.

Relationship between stoichiometry and physical properties of R(123)-type (R=Y, RE) single crystals

A relationship between the deviation of the cation sublattice from stoichiometry, on the one hand, and the crystal symmetry, superconducting properties and optical spectra, on the other hand, has been analysed in R(123)-type single crystals, where R denotes Y or rare earth (RE) elements. It has been shown that this relationship can be quantitatively characterized by the deficiency of Cu sites dCu. Two regions of dCu values have been distinguished, in which superconducting single crystals have been synthesized. The regularities of the redistribution of the elements in the cation sublattice of RE-doped single crystals of the (123)-type structure have been revealed.

The influence of oxygen vacancies on flux pinning in (RE)BA2CU3O7−δ

Abstract Measurements of the irreversible magnetisation in single crystal and melt-textured (RE)Ba 2 Cu 3 O 7−δ , with 0.05 weaken the pinning, contradicting earlier suggestions that vacancies are significant pinning centres in these materials. However, there is almost no effect on the zero field remanent magnetisation.

Raman scattering in Bi2Sr2Ca1−x(Y,Er,Tm)xCu2O8+δ single crystals

Abstract Bi 2 Sr 2 Ca 1−x (Y,Er,Tm) x Cu 2 O 8+δ single crystals have been investigated by Raman spectroscopy as a function of doping (0 ≤ x ≤ 0.85), incident laser wavelength, temperature and polarization directions of the incident and scattered light. Compared to undoped Bi 2 Sr 2 CaCu 2 O 8+δ , additional phonons are found at 136 cm −1 , 187 cm −1 , 320 cm −1 and 434 cm −1 . In contrast to superconducting samples, the B 1g -continuum of electronic scattering is not observed in semiconducting Bi 2 Sr 2 Ca 1−x Y x Cu 2 O 8+δ (x ≥ 0.5), corroborating the identification of narrow (≈ 60 me V) band-like states near the Fermi energy in superconducting samples. Two-magnon scattering in non-superconducting Bi 2 Sr 2 Ca 1−x (Y,Er,Tm) x Cu 2 O 8+δ (x ≥ 0.5) gives rise to a broad peak with its maximum at ≈ 2900 cm −1 , becoming more intense with increasing x.

ESR millimeter-band spectroscopy of magnetic ordering in the low-dimensional magnet CuGeO3

Resonant absorption of microwaves in CuGeO3 single crystals in a frequency band of 40 to 120 GHz, in magnetic field B⩽15 T, at temperatures ranging between 0.5 and 300 K, and in the configuration B∥a has been investigated. Several absorption lines (S0, Sa, and Sb) whose parameters strongly depend on temperature have been detected close to ESR. The temperature dependence of the total absorption in the main line S0 with the Landé g-factor g0=2.154 at temperatures above the spin-Peierls transition temperature is in good agreement with Bonner and Fisher’s theoretical prediction for a one-dimensional Heisenberg spin chain. In addition to the main resonance, a resonance of smaller amplitude, Sa, with the g-factor ga=2.72 has been detected at temperatures ranging down to a characteristic temperature T≃1 K, below which the amplitude of this feature drops to zero. A radical restructuring of the magnetoabsorption spectrum occurs at the temperature of the spin-Peierls transition TSP≈14 K. At T<12 K new features emerge in the spectrum, namely, a broad absorption line overlapping with the narrow lines S0 and Sa, and a line Sb with gb=1.83, which is not detected at temperatures above TSP. An analysis of amplitudes and total absorption of ESR lines as functions of temperature has shown that the temperature range below 1 K is anomalous, which may be caused by an additional ordering in the CuGeO3 magnetic subsystem at low temperatures.