Transport properties of high-temperature superconductor + semiconductor composites with different carrier concentration

Results are presented of an experimental study of the temperature dependence of the electrical resistivity ρ(T), critical current density J _c(T), and current-voltage characteristics of polycrystalline composites based on the high-temperature superconductor Y_3/4Lu_1/4Ba₂Cu₃O₇ and copper oxide with different lithium doping levels. The experimental temperature dependence of the critical current of composites with varying volume content of the semiconductor ingredient and varying charge carrier concentration are found to be in qualitative agreement with theory which takes account of Andreev reflection of the carriers at the S-Sm and Sm-S surfaces of the S-Sm-S Josephson junction (where S is the superconductor and Sm is the semiconductor). Fiz. Tverd. Tela (St. Petersburg) 39, 829–834 (May 1997) Deceased     

Andreev reflection and experimental temperature dependences of the critical current in heterogeneous high-temperature superconductors (polycrystals and related composites)

The temperature dependences of the critical current of Y_3/4Lu_1/4Ba₂Cu₃O₇ polycrystalline high-temperature superconductors subjected to annealing for different times and those of Y_3/4Lu_1/4Ba₂Cu₃O₇ + BaPbO₃ composites were measured. The results obtained were analyzed in terms of the Gunsenheimer-Schüssler-Kümmler theory considering the Andreev reflection of carriers in a “superconductor-normal metal-superconductor” (S-N-S) junction. Good agreement between the experimental and theoretical temperature dependences of the critical current over a wide temperature range for polycrystals with both natural and artificially created boundaries (composites) made it possible to estimate the effective length of grain boundaries in the high-temperature superconductors. It was revealed that the critical current density measured at 4.2 K is an exponential function of the length of grain boundaries in samples of both types.     

Group-theoretical analysis of possible structural phase transitions in the high-temperature superconductors

A group-theoretical analysis is performed for the complete condensation of order parameters at structural phase transitions (SPT's) in the high-temperature superconductors belonging to the D ¹⁷ _4h -14/mmm space group in the high-symmetry phase. As a rule, such transformations are due to a successive softening of phonons with wave vectors k ₁ = 1/2 b ₃ and k ₂ = 1/2 (b ₁ - b ₃) belonging to the K13(X) star of the Brillouin zone of a tetragonal body-centered Bravais cell. SPT's in system La_2-x Ba _x CuO₄ are considered in detail.     

Synthesis and transport properties of La2NiO4

La₂NiO₄ compounds were prepared by a modified sol–gel auto-combustion method, which is a low-temperature combustion synthesis procedure using microwave-assisted sol–gel as precursors. The high-temperature transport properties of the samples were investigated. The band structure, total density of states (DOS), and partial density of states (PDOS) of low-temperature orthorhombic (Bmab) phase and high-temperature tetragonal (I4/mmm) phase for La₂NiO₄ were calculated in order to study the transport properties of the as-obtained samples.     

Experimental and modeling study of the low to high-temperature oxidation of the methyl isopropyl ketone in O2/N2/Ar and O2/CO2/Ar atmospheres

Few studies on the low-temperature combustion behavior of MIPK, being a promising fuel additive, have been conducted. In this work, ignition delay times (IDTs) of MIPK were measured in the temperatures ranging between 780–910 K and pressures of 20 and 25 bar using a rapid compression machine (RCM). Oxy-fuel combustion combined with biofuel could remove CO₂ from the atmosphere. The IDTs of MIPK were measured in the temperatures ranging between 1125–1600 K under the O₂/CO₂ atmosphere at the pressures of 1 and 10 bar using a shock tube. A low to high-temperature MIPK kinetic model (HUST-MIPK model) was proposed, in which the low-temperature sub-model consists of 19 low-temperature reaction classes and was constructed by analogy-based method, the high-temperature sub-model was adapted from the works of Cheng et al.. The predictions of HUST-MIPK model are in good agreement with the present low-temperature IDTs, high-temperature O₂/CO₂ atmosphere IDTs, and the literature experimental data. The negative temperature coefficient (NTC) behavior was not observed in the temperature range from 790 to 910 K in the present RCM experiments, but was observed for methyl propyl ketone (MPK) and diethyl ketone (DEK) under similar conditions. The low-temperature chemistry of three pentanone isomers (MIPK, MPK, and DEK) was compared using the flux and sensitivity analysis. The comparison of the experimental high-temperature IDTs between O₂/CO₂ and O₂/Ar atmospheres indicates the IDTs of MIPK under O₂/CO₂ atmosphere are longer than those under O₂/Ar atmosphere at 1 bar, and the effects of CO₂ are almost independent of the pressure. The physical and chemical effects of CO₂ on the ignition were studied in detail.     

Dielectric permittivity and phase transitions in the SrTiO3-KTaO3 system

The dielectric permittivity and losses of the (SrTiO₃)_0.85(KTaO₃)_0.15 solid solution have revealed an unusual behavior at temperatures of 5–300 K and frequencies ranging from 100 Hz to 1 MHz. The dielectric permittivity exhibits a broad maximum at ≈40 K, which obeys the Curie-Weiss law in the high-temperature wing, and the onset of low-temperature relaxation contributions. Besides, while for T>40 K it is a fully ergodic, uniformly ordering system, the uniform polar ordering is replaced with decreasing temperature by two low-temperature phase transitions to a glass-like state. The proposed mechanism for the dielectric relaxation and the observed phase transformations involves reorientation and ordering in the system of electron and hole polarons, which form in the course of charge compensation of the heterovalent ions Sr²⁺, K⁺ and Ti⁴⁺, Ta⁵⁺ distributed randomly in the sublattices. Fiz. Tverd. Tela (St. Petersburg) 39, 2040–2045 (November 1997)     

Low-temperature phase transition in Ba2TiGe2O8

Previous work on Ba₂TiGe₂O₈ crystals has shown an unusual low-temperature (～ 223 K on cooling, ～ 273 K on heating) phase transition. Precession x-ray photographs on Ba₂TiGe₂O₈ single crystals show an incommensurate modulation along b*, and, for the first time, also along a*. Single crystal intensity data confirm the average structure in space group Cmm2. There is positional disorder in the pyrogermanate groups, and this is the probable cause for the modulated structure. The low-temperature phase transition is proposed to be a lock-in transition, with the modulation along a* locking in at a value of 1/3. Several properties, as well as other unusual features of the low-temperature phase transition, are discussed in light of the proposed lock-in transition. Domain studies show that the ferroelastic domains are unstable in the low-temperature phase.     

Phase transformations in Pr1–x Sr x CoO3

The elastic properties and crystal structure of the Pr_1?x Sr _x CoO₃ system are studied. Two types of crystal structure transitions are found. For the composition x = 0.5, the monoclinic phase transforms to a rhombohedral one in the high-temperature transition (T ≈ 310 K), while the unit cell symmetry remains monoclinic though the unit cell parameters change drastically in the low-temperature transformation (T ≈ 110 K). It is suggested that the high-temperature transition is caused by the dimensional effect, while the low-temperature transition is associated with the presence of praseodymium ions actively involved in chemical bonding.     

Crystallographic and vibrational study of hexanitroethane

The crystal structure of hexanitroethane has been determined in the low-temperature ordered phase at 145 K (P2₁/c: a = 10.152(2) Å, b = 9.311(2) Å, c = 10.251(2) Å, β = 97.54(1)°, V = 960.6(3) ų, Z = 4 with 2 non-equivalent molecules in the unit cell).The far-infrared, infrared and Raman spectra of hexanitroethane in both low-temperature ordered and high-temperature disordered crystal phases were recorded. The internal modes are interpreted with the help of normal coordinate analysis and a conformational analysis with the MNDO method. The transition mechanism is discussed.     

Incommensurability and metastability problems in NH4HSeO4 and ND4DSeO4: A reinvestigation of phase diagrams

Abstract

X-ray and neutron diffraction results on NH₄HSeO₄ and ND₄DSeO₄ are reported. Direct evidence of an incommensurate phase sandwiched between the monoclinic high-temperature phase (space group B2) and the low-temperature ferroelectric lock-in phase k = ? (space group P1) has been obtained in NH₄HSeO₄. The phase situation is more complicated in ND₄DSeO₄, where an incommensurate phase is found sandwiched between the B2 phase and a lock-in phase k = ¼ and where a phase in which several modulations coexist has been discovered in between the two lock-in phases k = ¼ and k = ?. The non-equilibrium processes, also present in ND₄DSeO₄, have been identified. All these results have clarified the situation about the phase diagram.     

Mechanisms of dissipation in a Josephson medium based on a high-temperature superconductor in a magnetic field

This paper reports on the results of an investigation into the influence of magnetic fields (0–60 kOe) on the temperature dependences of the electrical resistance R(T) of the Y_3/4Lu_1/4Ba₂Cu₃O₇ + CuO composites. The structure of these composites is considered to be a network of tunnel-type Josephson junctions in which a nonsuperconducting component (CuO) forms boundaries (barriers) between high-temperature superconducting crystallites. The temperature dependence R(T) of the composites has two steps characteristic of granular superconductors: (i) an abrupt change in the electrical resistance at the critical temperature of high-temperature superconducting crystallites and (ii) a smooth transition to the superconducting state under the influence of the boundaries between the crystallites. The experimental dependences R(T) are analyzed within the Ambegaokar-Halperin model of thermal fluctuations in Josephson junctions and the flux creep model. An increase in the magnetic field leads to a crossover from the Ambegaokar-Halperin mechanism to the flux creep mechanism. The temperature dependences R(T) in the range of weak magnetic fields (from 0 to 10² Oe) are adequately described by the relationship following from the Ambegaokar-Halperin model. In the range of strong magnetic fields (from 10³ to 6 × 10⁴ Oe), the dissipation obeys the Arrhenius law R ～ exp(?U(H)/T)], which is characteristic of the flux creep model with a temperature-independent pinning energy U(H). The effective Josephson coupling energies and the pinning energies corresponding to the Ambegaokar-Halperin and flux creep mechanisms are determined.     

The effect of a magnetic field on Jahn-Teller ordering in the monoclinic crystal RbDy(WO4)2

This paper presents the results of a study of the thermal properties of monoclinic single-crystal RbDy(WO₄)₂ at temperatures of 2–15 K and in magnetic fields up to 6 T. From the results of measurements of the heat capacity and thermograms, two structural phase transitions are detected, at T _c1=4.9 K and T _c2=9.0 K. The transformation from the high-temperature phase to the low-temperature phase occurs via an intermediate phase. The field dependences of the critical temperatures are found for various magnetic-field orientations. H-T phase diagrams are constructed for H‖a and H‖c. An anomalous increase (by almost an order of magnitude) of the relaxation time of the system, associated with structural instability of the crystal lattice, is detected in the region of the structural phase transitions. A symmetry analysis is carried out, and possible crystal structures of the low-temperature phase are indicated. Fiz. Tverd. Tela (St. Petersburg) 40, 2221–2225 (December 1998)     

Effect of thermal fluctuations on the resistive properties of HTSC+CuO composites

The effect of thermally activated phase slippage (TAPS) on the resistive properties of Y_3/4Lu_1/4Ba₂Cu₃O₇+O₇ composites has been studied. The experimental temperature dependences of the electrical resistance are described in terms of the TAPS mechanism using the temperature dependences of the tunnel-junction critical current. Fiz. Tverd. Tela (St. Petersburg) 39, 1956–1957 (November 1997)     

Magnetic susceptibility and specific heat of the spin-${\frac{1}{2}}$ Heisenberg model on the kagome lattice and experimental data on ZnCu3(OH)6Cl2

We compute the magnetic susceptibility and specific heat of the spin- Heisenberg model on the kagome lattice with high-temperature expansions and exact diagonalizations. We compare the results with the experimental data on ZnCu₃(OH)₆Cl₂ obtained by Helton et al. [Phys. Rev. Lett. 98, 107204 (2007)]. Down to k_BT/J≃0.2, our calculations reproduce accurately the experimental susceptibility, with an exchange interaction J≃190 K and a contribution of 3.7% of weakly interacting impurity spins. The comparison between our calculations of the specific heat and the experiments indicate that the low-temperature entropy (below ~20 K) is smaller in ZnCu₃(OH)₆Cl₂ than in the kagome Heisenberg model, a likely signature of other interactions in the system.     

Investigation of the Josephson coupling through a magnetoactive barrier (ferrimagnet, paramagnet) in Y3/4Lu1/4Ba2Cu3O7 + Y3(Al1 − x Fe x )5O12 composites

The transport properties of two-phase composites consisting of a high-temperature superconductor and a nonsuperconducting component with magnetic ordering are analyzed. These composites are considered as a network of “superconductor-magnetoactive insulator-superconductor” weak links of the Josephson type. Substituted garnets Y₃(Al_{1 ? x} Fe _x )₅O₁₂ (x = 0, ..., 1.0) are used as a magnetoactive component. The composites under investigation contain 92.5 vol % Y_3/4Lu_1/4Ba₂Cu₃O₇ (the high-temperature superconductor) and 7.5 vol % Y₃(Al_{1 ? x} Fe _x )₅O₁₂ (x = 0, ..., 1.0). It is shown that an increase in the iron content in the Y₃(Al_{1 ? x} Fe _x )₅O₁₂ garnet leads to a reduction of the Josephson coupling strength: the temperature range in which the electrical resistance of the composites is equal to zero is reduced, and the critical current density at a temperature of 4.2 K decreases exponentially. For composites in which the iron content in the Y₃(Al_{1 ? x} Fe _x )₅O₁₂ garnet is higher than 0.1, the temperature dependence of the electrical resistance R(T) at temperatures below the transition point T _C of high-temperature superconductor crystallites has a portion in the range T _m -T _C where the resistance R(T) is independent of the transport current and the magnetic field strength. Below the temperature T _m , the dependences of the electrical resistance R(T) of the composites are nonlinear functions of the current and involve a considerable contribution from magnetoresistance. This behavior is characteristic of a network of Josephson junctions. The temperature T _m decreases with an increase in the iron content in the Y₃(Al_{1 ? x} Fe _x )₅O₁₂ garnet. The appearance of the above feature in the temperature dependences of the electrical resistance R(T) is interpreted as complete suppression of the Josephson coupling in the temperature range above T _m due to the interaction of supercurrent carrier pairs with magnetic moments of iron atoms in the dielectric barriers separating high-temperature superconductor grains.     

Influence of high-temperature treatment on the structure and emission properties of opal doped with erbium

The influence of high-temperature treatment of opal—erbium composites on their spectral properties in the wavelength region of 1.5 μm is investigated, the dependence of the luminescence intensity on the erbium concentration is measured, and the structure of the composite after high-temperature treatment is examined using transmission electron microscopy. The regular structure of the composites undergoes considerable transformations up to the complete loss of periodicity upon annealing at a temperature of 1500 K for 10 h and more. The composites in this case are sintered into a homogeneous substance. Pores disappear, and erbium oxide inclusions in the form of 30- to 70-nm regular solid spheres are observed instead. Measurements of the luminescence spectra in the wavelength region of 1.5 μm for the opal-erbium composites with erbium oxide concentrations in the range 0.25–16.00 wt % have shown that the luminescence yield reaches the maximum at an Er₂O₃ content of 1 wt % in the composite. Analysis of the influence of the temperature and time of heat treatment on the luminescence intensity of the samples has revealed a nonmonotonic character of the dependence on the annealing time for all the temperatures under investigation. The factors responsible for this behavior are discussed. The reflection spectra of the opal-Er₂O₃(1 wt %) composites after heat treatment have demonstrated that the photonic band gap of the samples degrades at annealing temperatures of 1300 K and higher. A method proposed for retaining the periodic structure upon heat treatment consists in introducing the stabilizing phase into the opal.     

Importance of annealing on the order-disorder phase transition in alkali cyanides

Abstract

Contrary to monocrystals powdered KCN displays, after grinding and without annealing, an intermediate monoclinic phase between the high-temperature pseudocubic and the low-temperature orthorhombic phases. The consequence of annealing is the total disappearance of this intermediate phase. A further effect of the annealing process is the reduction of the cubic distortion at the transition. In this paper, we analyse this effect on the intermediate rhombohedral phase observed in the (KCN) _x (KBr)_1?x mixed system.     

Superionic phase transition in (NH4)2SeO4·2NH4HSeO4 single crystal at 378 K

Abstract

DTA, structural and electric conductivity investigations were made for (NH₄)₄H₂(SeO₄)₃ single crystals. A high-temperature phase transition at 378 K to a superionic phase was found. The phase is characterized by a high electrical conductivity (～4.10^?3 Ω^?1 cm^?1) and a low activation energy (0.11 eV).     

A study of the new compound YBa4Fe3O11−y

The compound YBa₄Fe₃O _y was synthesized where the Fe atoms completely substituted for the Cu atoms in YBa₄Fe₃O _y . The X-ray phase analysis of the synthesized material showed that the diffraction pattern corresponds to the data for the compound YBa₄Fe₃O _y .⁵⁷Fe Mössbauer measurements of the compound were performed at room temperature after different heat treatments: high-temperature synthesis in an oxygen atmosphere and in air, and low-temperature annealing in oxygen and in vacuum. No magnetic components are observed after any heat treatments. The Fe atoms in the compound have two valence states, Fe³⁺ and Fe⁴⁺. The idealized model of the 1-4-3 Fe structure is discussed.