The initiating action of continuous heating on structural remodeling in Ag3AsS3 crystals

The effect of continuous heating on the characteristics of phase transformations in proustite (Ag₃AsS₃) is established. It is shown that continuous heating, like continuous cooling, stimulates structural remodeling, in this case reducing the temperature of the known phase transitions. Fiz. Tverd. Tela (St. Petersburg) 40, 2217–2220 (December 1998)     

3D Visualization of the separated fluid flows

For the detailed investigation of the 3D unsteady incompressible viscous separated fluid flows around a sphere (for 200≤Re≤700) and a circular cylinder (for 200≤Re≤400) the direct numerical simulation and 3D visualization are used. For 3D visualization of the fluid flows around a sphere the definition of vortex core as a connected region containing two negative eigenvalues of theS ²+Ω ² tensor is used (whereS _i,j andΩ _ij are the rate of strain and the rate of rotation tensors). The formation mechanism of vortices in the sphere wake for Re=500 is described in detail. For 3D visualization of the fluid flows around a circular cylinder the 3D isosurfaces of the streamwise component of vorticity ω _x are used.     

Calorimetric study of phase transitions in the perovskite BaCeO3

Differential scanning calorimetry was used to study phase transitions (PT) in the perovskite BaCeO₃. It is shown that its phase state is determined by a second-order λ transition at T _tr=520–540 K and a first-order δ transition at T _tr=600–670 K. Differences in PT parameters between ceramic and fused BaCeO₃ have been established. Fiz. Tverd. Tela (St. Petersburg) 40, 2109–2112 (November 1998)     

Anisotropy inc-axis oriented YBa2Cu3O7−δ

Muon spin relaxation (μSR) data taken at LAMPF on ac-axis oriented fine powder sample of YBa₂Cu₃O_7−δ (YBCO) embedded in epoxy are analyzed for relaxation rate anisotropy. Clear differences beyond simple magnetic field penetration depth anisotropy are observed forB‖c andB c. The low-temperature anisotropy ratio is consistent with oriented ceramic data. Small crystallite size and anisotropic flux pinning characteristics are suggested as the fundamental cause of the additional effects.     

Weak-link bi-epitaxial Josephson junctions in a YBa2Cu3O7−δ film on BaZrO3/CeO2/SrTiO3

A bi-epitaxial (001)YBa₂Cu₃O_7−δ /(110)BaZrO₃/(001)CeO₂ three-layer heterostructure has been grown on (100)SrTiO₃ by laser ablation. The epitaxial relations between the layers making up the heterostructure were derived from x-ray diffraction data. The I _cR_n product for the bi-epitaxial Josephson junctions thus obtained was within 1–1.5 mV for 4.2 K, and 30–60 μV for 77 K. The normal resistance R _n=(2–5 Ω) was practically independent of temperature. The magnetic field dependences of I _c had typically a clearly pronounced main maximum, followed by distorted subsequent peaks. Interaction of the Josephson ac current with self-induced electromagnetic waves at the 45° grain boundary and with external microwave radiation (f=11 GHz) produced current steps in the I-V characteristics of the bi-epitaxial junctions at the corresponding voltages. Fiz. Tverd. Tela (St. Petersburg) 39, 1732–1738 (October 1997)     

Dielectric and photoelectric properties of Pb(Mg1/3Ta2/3O3 crystals

The temperature dependence of the dielectric constant ɛ and the spectral dependence of the photoconductivity of Pb(Mg_1/3Ta_2/3)O₃ crystals are investigated. The width of the band gap (3.4 eV) is determined. It is found that above the temperature of the maximum of the dielectric constant, its reciprocal 1/ɛ varies with temperature first quadratically and then according to a linear law, as is characteristic of ferroelectrics with a smeared phase transition. The smearing parameter of the transition estimated from the experimental data is close to the value calculated assuming the absence of long-range order in the arrangement of the Mg and Ta ions. Fiz. Tverd. Tela (St. Petersburg) 40, 109–110 (January 1998)     

Carrier mobility in nonstoichiometric n-Bi2Te3−x Sex solid solutions

A study has been made of the thermoelectric and galvanomagnetic properties of n-Bi₂Te_3−x Se_x solid solutions (x=0.3 and 0.36) in the temperature range 80–300 K. The lowest carrier concentrations, (0.8–1)×10¹⁸ cm⁻³, were obtained by displacing the solid solution from the stoichiometric to a Te-rich composition. At such carrier concentrations, the second subband in the conduction band of n-Bi₂Te_3−x Se_x is not filled, which results in a growth of mobility because of the absence of interband scattering, and brings about an increase of thermoelectric efficiency in the 80–120-K range. Fiz. Tverd. Tela (St. Petersburg) 39, 483–487 (March 1997)     

Critical concentrations in Ba-doped incipient ferroelectric SrTiO3

Temperature-induced variations of light refraction and dielectric permittivity in single-crystal Sr_1−x Ba_xTiO₃ (x=0.02, 0.05, 0.07, and 0.14), Sr_1−x Ca_xTiO₃ (x=0.014), and in nominally pure strontium titanate have been studied within the 17–300 K temperature range. The spontaneous polar contribution to the refractive index has been isolated. It was used to calculate the temperature and concentration dependences of the polarization autocorrelation function 〈P _s ^2〉 in the Sr_1−x Ba_xTiO₃ system. For x⩽0.07, the polarization P _s=〈P _s ² 〉^1/2 varies proportional to (x−x _g)^1/2, where x _g=0.0027 is the new critical concentration in Sr_1−x Ba_xTiO₃, below which short-range polar order vanishes. Fiz. Tverd. Tela (St. Petersburg) 39, 704–710 (April 1997)     

Spin susceptibility of neutron-irradiation-disordered YBa2Cu3O6.9 in the normal and superconducting states

The spin-spin relaxation rate ⁶³ T ₂ ⁻¹ of ⁶³Cu nuclei in CuO₂ layers is measured in the normal and superconducting states of the compound YBa₂Cu₃O_6.9 (T _c ^onset =94 K) subjected to radiation-induced disordering by a fast-neutron flux Φ to T _c ^onset =68 K (Φ=7×10¹⁸ cm⁻²) and T _c ^onset <4 K (Φ=12×10¹⁸ cm⁻²). It is found that as the structural disorder increases, the contribution of the indirect spin-spin interaction ⁶³ T _2G ⁻¹ , which is related to the value of the spin susceptibility at the boundary of the Brillouin zone of the copper planes χ_s(q={π/a; π/a}), decreases slightly at the transition to the superconducting state for the initial sample and remains unchanged for the weakly disordered sample. This behavior of the short-wavelength contribution to the spin susceptibility attests to the stability of the x ²−y ² symmetry of the energy gap against structural disorder, in accordance with proposed theoretical models of Cooper pairing for high-T _c cuprates. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 3, 172–177 (10 February 1998)     

Ferroelectric properties of SrTiO3-PbTiO3 solid solutions

Dielectric properties of ceramic samples of Sr_1−x Pb_xTiO₃ solid solutions for x varying from 0 to 0.3 have been studied. The ferroelectric phase transition in this system has been established to persist down to x=0.005. Within the x values of 0.002 to 0.05, the ferroelectric transition temperature is shown to follow the relation T _c=A(x−x _c)^1/2, with A=440 K and x _c=0.002. Fiz. Tverd. Tela (St. Petersburg) 39, 714–717 (April 1997)     

Observation of spin-reduction anisotropy in the quasi-one-dimensional antiferromagnet CsMnI3

The NMR of ⁵⁵Mn in the quasi-one-dimensional noncollinear antiferromagnet CsMnI₃ at T=1.3 K is investigated in magnetic fields up to ∼40 kOe. Six NMR branches corresponding to six manganese spins per magnetic unit cell are observed. The NMR spectra correspond satisfactorily to the well-known magnetic structure of CsMnI₃, taking into account the dynamic frequency shift due to the interaction with the low-lying AFMR modes. The average spins 〈S _A〉=1.86 and 〈S _B〉=1.74 of the magnetically nonequivalent Mn²⁺ ions are determined from the measured values of the hyperfine fields. The results obtained agree qualitatively with the calculations of spin reduction in quasi-one-dimensional antiferromagnets [Y. Watabe, T. Suzuki, and Y. Natsume, Phys. Rev. B 52, 3400 (1995)]. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 9, 661–665 (10 May 1998)     

Spontaneous photorefractive effect in Sr1−x CaxTiO3 (x=0.014)

The temperature dependence of the principal values of the refractive index in Sr_1−x Ca_xTiO₃ (x=0.014) has been measured in the 17–275 K range under various conditions of sample illumination with 1.96 eV photons. The spontaneous photorefractive contribution δn ^ph to the temperature-induced variation of the refractive index of Sr_1−x Ca_xTiO₃, which appears after illumination of the sample in the ferrophase (transition temperature T _c=32 K) and persists in the paraphase under heating up to 150 K, has been separated. The photoinduced polarization has been estimated. Fiz. Tverd. Tela (St. Petersburg) 39, 711–713 (April 1997)     

Diffusion and surface energy barrier for labile oxygen in YBa2Cu3O7−δ

A study is made of the isotope exchange kinetics of tracer oxygen atoms between the gas phase and the CuO_1−δ plane in the YBa₂Cu₃O_7−δ crystal under isochronous and isothermal annealing. The parameters of the surface energy barrier for oxygen atoms are found. Fiz. Tverd. Tela (St. Petersburg) 39, 42–48 (January 1997)     

Direct observation of phase separation in La1−x CaxMnO3

In a study of the infrared optical absorption spectra of polycrystalline La_1−x Ca_xMnO₃ (0.1⩽x⩽0.8), a contribution from free charge carriers is observed below the Curie temperature for compositions with x⩽0.4. The existence of this contribution for samples with a high resistivity is a direct proof of the existence of “metallic” drops in an insulating matrix. The relative volume of the “metallic” phase is estimated. Pis’ma Zh. éksp. Teor. Fiz. 68, No. 1, 89–92 (10 July 1998)     

Colossal magnetoresistance of the Sm1−x SrxMnO3 system

The results of investigating the temperature dependence of the resistivity, the differential magnetic susceptibility, and the magnetoresistance of a partially substituted perovskite Sm_1−x Sr_xMnO₃ (x=0.16–0.4) are presented. Colossal intrinsic magnetoresistance, reaching 90% in an external magnetic field of 30 kOe, is discovered in the compound with x=0.30 at 77 K. Fiz. Tverd. Tela (St. Petersburg) 39, 1831–1832 (October 1997)     

Strontium diffusion in Sr3−3x La2x □x(VO4)2 solid solutions

Layer-by-layer radiometric analysis in the temperature range 800–950 °C was used to measure the diffusion coefficients of ⁹⁰Sr, ⁴⁵Ca, ¹³³Ba radionuclides in Sr_3−3x La_2x □_x(VO₄)₂ (x=0,0.1,0.15,0.2,0.25) solid solutions. It was established that D _Ca ^* >D _Sr ^* >D _Ba ^* . An increase in the concentrations of vacancies (□) in the strontium sublattice leads to increased coefficients of diffusion of rare-earth metal cations. The activation energies for radionuclide diffusion were determined. It was shown that M²⁺ cations migrate via Sr(2) positions. Fiz. Tverd. Tela (St. Petersburg) 40, 223–226 (February 1998)     

Magnetic and structural transitions in La1−x Sr x MnO3: T-x phase diagram

The electrical conductivity, magnetic susceptibility, magnetization, and submillimeter (v=5∓20 cm⁻¹) permittivity and dynamic conductivity of La_1−x Sr _x MnO₃ (0≤x≤ 0.45) single crystals are investigated. The anomalies in the temperature dependences of these quantities are identified with diverse magnetic and structural phase transformations, including antiferromagnetic and ferromagnetic ordering, structural transitions between strongly distorted (Jahn-Teller) and weakly distorted (pseudocubic) orthorhombic phases, structural transitions to a rhombohedral phase and unusual transitions to a polaron-ordering state. As a result, the complete T-x phase diagram of the system La_1−2x Sr _x MnO₃ is constructed in a wide interval of temperatures T=4.2∓1050 K and concentrations x=0−0.45. Pis’ma Zh. éksp. Teor. Fiz. 68, No. 4, 331–336 (25 August 1998)     

Nuclear magnetic resonance of 55Mn in the antiferromagnet CsMnBr3 in a variable longitudinal magnetic field

The spectrum and intensities of NMR lines are investigated experimentally and theoretically for excitation by an alternating magnetic field h‖ parallel to a static field H in the quasi-one-dimensional, six-sublattice antiferromagnet CsMnBr₃. According to theory, two new NMR lines, which are not excited by a transverse magnetic field h _⊥, are observed near the phase transition from triangular to collinear structure (H=H _c ) [JETP 86, 197 (1998)]. Zh. éksp. Teor. Fiz. 115, 2228–2241 (June 1999)     

Superhyperfine structure of the EPR spectra of Nd<Superscript>3+</Superscript> and U<Superscript>3+</Superscript> ions in Li<Emphasis Type="Italic">R</Emphasis>F<Subscript>4</Subscript> (<Emphasis Type="Italic">R</Emphasis> = Y,Lu, Tm) double fluorides

The superhyperfine structure of the EPR spectra of impurity Nd³⁺ and U³⁺ ions in LiYF₄, LiLuF₄, and LiTmF₄ double-fluoride single crystals has been observed and discussed. In LiYF₄: Nd (g _| = 1.987, g _⊥ = 2.554) and LiTmF₄: Nd, the superhyperfine structure is observed at the orientation of the external magnetic field B in parallel to the c axis of the crystals and consists of nine components with a splitting of ∼15.4 MHz. In LiYF₄: U (g _| = 1.149, g _⊥ = 2.508) and LiLuF₄: U, the superhyperfine structure is observed at both B | c and B ⊥ c and consists of nine and eleven components, respectively, with a splitting of ∼21.5MHz. It should be noted that the resolution of the superhyperfine structure of the EPR spectrum of LiLuF₄: U³⁺ becomes significantly higher with a deviation from the orientation B ⊥ c.     

EPR and dielectric relaxation of Fe3+ in KTaO3

EPR and the method of dielectric losses have been used to investigate Fe³⁺ centers of axial and orthorhombic symmetry in KTaO₃ single crystals. The EPR spectrum of orthorhombic-symmetry Fe³⁺ obtained in the 8-mm wavelength range at T=77 K is described by the spin-Hamiltonian with parameters g _x=1.98, g _y=2.01, g _z=2.00, D=0.43 cm⁻¹, and E=5.87×10^t-2 cm⁻¹. From the dielectric measurement data we have obtained the following parameters of the relaxation of the orthorhombic Fe³⁺ centers in KTaO₃: characteristic relaxation frequency τ ₀ ⁻¹ =2.33×10¹² Hz and activation energy E _a=0.044 eV. A model of the orthorhombic Fe³⁺ center in KTaO₃ is discussed within the framework of the kinetic parameters obtained. Fiz. Tverd. Tela (St. Petersburg) 39, 861–864 (May 1997)