Mössbauer studies on hyperfine field measurement in titanium doped nickel-zinc ferrites

⁵⁷Fe Mössbauer effect study in the polycrystalline solid solutions of Zn_0.25Ni_{0.75+tTitFe2?2tO4} (t=0.0-0.5) has been carried out at room temperature. All the samples exhibited two pure Zeeman sextets corresponding to tetrahedral and octahedral sites. The present work has been aimed at investigating the influence of tetravalent titanium substitution on the isomer shift, quadrupole splitting, nuclear magnetic fields at ⁵⁷Fe nuclei for tetrahedral and octahedral sites and the cation distribution. The isomer shift was found to be independent of substitution level t. Nuclear magnetic field H_A decreases faster than H_B with t. From the variation of H_A and H_B with t it is concluded that for higher titanium substituted samples, titanium occupies both the B as well as A sites.     

Degradation mechanism of scandia-stabilised zirconia electrolytes: Discussion based on annealing effects on mechanical strength,ionic conductivity,and Raman spectrum

In the present study, the mechanical strength as well as the ionic conductivity of the scandia- and ytterbium oxide-codoped zirconia electrolyte is investigated during annealing. Based on the annealing effect on both properties, the degradation mechanism is discussed. The X-ray diffraction and Raman spectroscopic analyses confirmed that annealing enhanced the tetragonal phase content in the samples which initially contained the tetragonal phase. The samples with the rhombohedral phase showed no phase changes. The samples initially with the tetragonal phase showed a significant decrease in the conductivity but an increase in the strength whereas the samples with the rhombohedral phase generally showed no change and the ones with the cubic phase showed slight decreases in both properties. From above results, the conductivity degradation of the sample with the tetragonal phase was attributed to two causes: the formation of low conductive but transformable t-phase from the high conductive t′-phase and the formation of t′-phase from the t″-phase. The annealing effect was little for the samples initially with rhombohedral phase and complex for the ones with cubic phase.     

Frozen photoconductivity in PbTe films

We have studied PbTe films of thicknessd=200/10000 A made with telluride vapour deposition on glass substrate at room temperature. The estimate of the donor concentration ～10¹⁹ cm^?3 of the fresh-deposited film compared with the impurity content in the bulk raw material ～10¹⁷ cm^?3 shows that the donors were mainly film defects or nonstoichiometric Pb atoms. Electrical conductivity of the freshly deposited film increased with lowering of the temperature. After deposition the donors were compensated with an oxidation in the laboratory air. Transition to the thermally activated conductivity resulted from oxidation. At temperatures belowT≈100 K the resistance of the compensated films followed Mott’s ruleR=R ₀ exp(T ₀/T)^1/3. The square film value 1 Mohm andT ₀≈100 K ford=1000 A. At low temperatures an exposure to light resulted in sharp decrease of the film resistance. At liquid helium temperatures the resistance dropped 10³–10⁶ times and stayed at the low value for an indeterminate time. The heating of the film aboveT=100 K gave rise to an initial high resistive state. The critical temperatureT _c, when the frozen photoconductivity became negligible, varied with samples in the temperature region 90–120 K. Near the critical temperature we could measure the time dependence of the film resistance after the light exposure, which followed the equationR=A+B.lnt fort>1 sec with the empirical constantsA andB. After a time intervalτ the resistance gained the initial “dark” value and remained stationary. The value lnτ≈α.(T _c?T), where the factorα approximately wasα≈0.5 K^?1. Some results of these experiments were published earlier (Krylov and Nadgorny 1982; Krylov and Pojarkov 1984).     

Study on phase stability and oxide ion conductivity in the BIAGVOX system

A novel oxide ion conductor, BIAGVOX formulated as Bi₄Ag_xV_2?xO_11?(2x)?δ is obtained by substituting a fraction (x) of V⁵⁺ by Ag⁺ in Bi₄V₂O₁₁ samples of the BIAGVOX system in the composition range, 0.05≤x≤0.30 were synthesized using the standard solid-state reaction. XRPD, FT-IR, DTA and AC impedance spectroscopy were used for the investigation of the correlation between the structural phase stability and oxide ion performance of the BIAGVOX materials. It has been found that orthorhombic, β, and incommensurate tetragonal, γ′ were stabilized at room temperature for compositions with x≤0.15 and x≥0.20, respectively. The enthalpy of β→γ and γ→γ transition exhibited a general drop with increasing Ag content and a minimum transition temperature was observed for x=0.20. However, the oxide ion conductivity at lower temperatures remarkably increased in composition stability ranges of β- and γ-phase, while a maximum conductivity was reported for the high temperature γ-phase at x=0.15. However, the low temperature conductivity, σ₃₀₀°c measured for x=0.30 was found to be comparable with the maximum ionic conductivity of the BICUVOX system reported for x=0.20 at the same temperature.     

Inflation of fireballs, the gluon wind and the homogeneity of the HBT radii at RHIC

We solve analytically the ellipsoidally expanding fireball hydrodynamics with source terms in the momentum and energy equations, using the non-relativistic approximation. We find that energy transport for high-p _tjets of gluons to the medium leads to a transient, exponential inflation of the fireballs created in high energy heavy ion collisions. In this transient, inflatory period, the slopes of the single particle spectra are exponentially increasing, while the HBT radius parameters are exponentially decreasing with time. This effect is shown to be similar to the development of the homogeneity of our Universe due to an inflatory period. Independently of the initial conditions, and the exact value of freeze-out time and temperature, the measurables (single particle spectra, the correlation functions, slope parameters, elliptic flow, HBT radii and cross terms) become time-independent during the late, non-inflatory stages of the expansion, and they satisfy a new kind of scaling laws. If the expansion starts with a transient inflation caused by the gluon wind, it leads naturally to large transverse flows as well as to the simultaneous equality, and scaling behaviour of the HBT radius parameters, R _side≈R _out≈R _long≈t _f √T _f /m. With certain relativistic corrections, the scaling limit is 281-2, where m _tis the mean transverse mass of the pair.     

Anomalous low-temperature behavior of the thermal characteristics of MgB<Subscript>2</Subscript>

We have studied the behavior of the thermal expansion coefficient α(T) (in a zero magnetic field and at H≈4 T), the heat capacity C(T), and the thermal conductivity κ(T) of magnesium boride (MgB₂) in the vicinity of T_c and at lower temperatures. It was established that MgB₂, like oxide-based high-temperature superconductors, exhibits a negative thermal expansion coefficient at low temperatures. The anomaly of α(T) in MgB₂ is significantly affected by the magnetic field. It was established that, in addition to the well-known superconducting transition at T_c≈40 K, MgB₂ exhibits an anomalous behavior of both heat capacity and thermal conductivity in the region of T≈10–12 K. The anomalies of C(T) and κ(T) take place in the same temperature interval where the thermal expansion coefficient of MgB₂ becomes negative. The low-temperature anomalies are related to the presence of a second group of charge carriers in MgB₂ and to an increase in the density of the Bose condensate corresponding to these carriers at T_c2≈10–12 K.     

Quadratic divergences in gauge theories

Quadratic divergences are analysed using dimensional regularisation in gauge theories in general and the standard model in particular. We give a prediction (under dubious assumptions) thatm _t ≈115 GeV andm _H ≈180 GeV.     

A stable superconducting state at 8 K and ambient pressure in\alpha _t - (BEDT - TTF^ \star )_2 I_3 ^1

We report bulk superconductivity at 8 K and ambient pressure in crystals of α _t (BEDT-TTF)₂I₃. In contrast to the earlier observed metastable superconducting state at 8 K in crystals of β-(BEDT-TTF)₂I₃ here the superconducting state is stable and the crystals can be prepared by tempering α-(BEDT-TTF)₂I₃ above 70 °C for several days. ac-susceptibility measurements show that the observed superconducting state at 8 K is a bulk property of the crystals. Resistivity measurements indicate a sharp superconducting transition at 8 K with an onset temperature of about 9 K. The upper critical fields H_c2 at 1.3 K lie between 3 and 11 T depending on the direction of the magnetic field with respect to the crystal axes. ESR- as well as NMR-measurements indicate a total transformation of the α-phase crystals into the new superconducting α _t -crystals after tempering.     

Electrical conductivity and composition range of Ag2HgI4 between room temperature and 75°C

The composition range of α- and β-Ag₂HgI₄ near the transformation temperature is determined by X-ray diffraction. The electrical conductivity measured in the α-phase is nearly unaffected by deviation from stoichiometry. This is caused by a compensation of the composition dependences of σ₀ and of the activation energy E_A. From the experimental results the dependences of the entropy and of the Gibbs energy for migration of Ag-ions on temperature and on composition are estimated.     

Microwave absorption in high-T c superconductors at high magnetic fields

In the mixed state of superconductors (H _c1?H?H _c2) the penetration of microwaves is governed by both, complex conductivity σ(T) and driven oscillation of vortices. In this paper, we show that an effective microwave conductivity can be derived and used to fit the field dependences of the surface resistanceR _s. The fit parameter is the upper critical field. Measurements on single crystals YBa₂Cu₃O_7-δ were made in magnetic fields (H∥c) up to 2.25 T, and in the temperature range from 70–100 K. The critical temperature for mean field superconductivity appears to be 89.3 K, while the apparent onset in the curve ofR _s(T) appears at about 92 K. The magnetic dependences ofR _s clearly demonstrate that one can separate the regions of mean field superconductivity from the region of fluctuations.     

Influence of Se-deficiencies on the properties of In-doped CdCr2Se4 single crystals

Lattice constant, Curie temperature, and electrical conductivity of CdCr₂Se₄:In single crystals have been measured after heat treatments of the crystals in Se atmosphere and under streaming hydrogen. By these treatments, the concentration of the Se vacancies and of the charge carrier concentration is altered drastically. The lattice constant as well as the magnetic ordering temperature have been found not to be affected by these heat treatments.Since the Se vacancies act as doubly changed donors, the electrical conductivity is strongly dependent on the concentration of the Se vacancies. A resistivity anomaly and large magnetoresistance are observed only in crystals with considerable Se deficiency. From these results it is concluded that the magnetoresistance is caused by hopping conduction between donor sites partly emptied by compensating A-site vacancy acceptors. Large magnetoresistance is found in samples with considerable Se deficit because only in this case the conduction at lower temperatures is dominated by the impurity band.     

Phases and phase transitions of the superionic conductor Ag2HgI4 in the temperature range between 4.2 K and 370 K detected by diffuse reflectance spectrometry

Diffuse reflectance spectra of the superionic conductor Ag₂HgI₄ powder were recorded between 4.2 K and 370 K and transformed into the Kubelka-Munk function. Six parameters of the spectral band of this function, related to the absorption band of the particular phase, were considered. The graph representing the logarithm of the integrated Kubelka-Munk spectral band versus inverse temperature was most informative. The Arrhenius behaviour of this function, in a temperature interval, was related to the existence of a phase with constant lattice structure. The non-Arrhenius behaviour was tentatively attributed to a temperature sensitive phase. Thus fifteen different crystalline phases were found, only four of them having been, up to now, unquestionably admitted. In particular, evidence was given for the existence of two superionically conducting phases and two room temperature phases. All the graphs representing the chosen parameters versus inverse temperature pointed to the existence of two separate sequences of phases: one generated by cooling gradually the β-phase, stable at room temperature, to low temperatures (β sequence), the other by heating the β-phase above the σ → β (T)₊ transition point and cooling gradually the formed α'′-phase (α′ sequence). The concept of photon absorption thermal activation energy was introduced. Its largest value was of the order of magnitude of the electrical conductivity thermal activation energy of Ag₂HgI₄. Estimates of energy band gaps and their evolution with temperature were made. It is suggested that the polymorphism of the substance is the main reason for the hysteresis loop in the superionic conductivity transition region. The low temperature phases manifested luminescence caused by exciton decay.     

The effects of thermal activation on the complex conductivity of Ag2HgI4 in the microwave region

The complex conductivity of polycrystalline Ag₂HgI₄, a superionic conductor, has been measured as a function of temperature at 10, 24 and 70 GHz. Both conductivity and permittivity exhibited sharp changes at the β?α phase transition. The microwave conductivity of the β-phase was found to be insensitive to temperature changes and that of the α-phase has thermal activation energies lower than that of dc. The observed monotonic increasing conductivity, decreasing permittivity, together with thermal activation effects are indicative of hopping ionic transport.     

Electron localization in conducting langmuir-blodgett films

The temperature dependence of the intracrystallite conductivity of Langmuir-Blodgett films of the (C₁₆H₃₃-TCNQ)_0.4(C₁₇H₃₅-DMTTF)_0.6 charge-transfer complex (CTC) is studied by measuring the surface acoustic wave attenuation in a piezoelectric delay line coated with such a film. (C₁₆H₃₃-TCNQ)_0.4(C₁₇H₃₅-DMTTF)_0.6 is a surface-active CTC made from a 1.5: 1 mixture of heptadecyl-dimethyltetrathiafulvalene (C₁₇H₃₅-DMTTF) and hexadecyl-tetracyanoquinodimethane (C₁₆H₃₃-TCNQ). The temperature dependence of the intracrystallite conductivity is found to have a maximum at T_MD=193.5 K. Above T_MD, the conductivity of the films is metallic (?σ/?T<0), while below this temperature it obeys a law that is close to the one-dimensional Mott law. The decrease in the conductivity with decreasing temperature at T<T_MD is shown to be related to the localization of electron states in the quasi-one-dimensional system under study and to be caused by the presence of impurities and defects in the TCNQ chains, along which a charge is transferred. The detected variation in the conductivity with temperature below T_MD is found to qualitatively and quantitatively agree with the model of localization in a weakly disordered quasi-one-dimensional system proposed earlier by Nakhmedov, Prigodin, and Samukhin. Fitting the experimental results to the theoretical dependences obtained in the framework of this model allows us to find the electron-phonon and electron-impurity scattering times. The structural parameters of the conducting layer are used to estimate the density of states at the Fermi level and the Fermi velocity in the films. With these values, the mean free path and the localization length in the films under study are determined.     

Conductivity studies on PEO:NaClO3 electrolyte system with different plasticizers

A new ion conducting solid polymer electrolyte thin film based on Polyethylene oxide (PEO) with NaClO₃ salt is prepared by solution-casting method. The solvation of salt with PEO has been confirmed by X-ray diffraction and IR spectral studies. Plasticizer effects were studied in PEO:NaClO₃ system by using low molecular weight polyethylene glycol (PEG), dimethyl formamide (DMF) and propylene carbonate(PC). AC conductivity in the temperature range (308–378 K) was measured to evaluate the conductivity of the polymer electrolytes. From the conductivity data, it was found that the conductivity value of pure PEO increases 10²–10⁴ order of magnitude with the addition of salts as well as plasticizers. From the transference number experiments, it was confirmed that the charge transport in these electrolyte is mainly due to the ions (t_ion≈0.94). Finally, the conductivity value of all PEO: NaClO₃ systems were compared.     

Thermoelectric properties of SnzCo8Sb24 skutterudites

Sn-filled CoSb₃ skutterudite compounds were synthesized by the induction melting process. Formation of a single δ-phase of the synthesized materials was confirmed by X-ray diffraction analysis. The temperature dependences of the Seebeck coefficient, electrical resistivity and thermal conductivity were examined in the temperature range of 300-700 K. Positive Seebeck and Hall coefficients confirmed p-type conductivity. Electrical resistivity increased with increasing temperature, which shows that the Sn-filled CoSb₃ skutterudite is a degenerate semiconductor. The thermal conductivity was reduced by Sn-filling because the filler atoms acted as phonon scattering centers in the skutterudite lattice. The lowest thermal conductivity was achieved in the composition of Sn_0.25Co₈Sb₂₄.     

Pressure and temperature induced modification of luminescence from tetracene single crystals

Fluorescence spectra of crystalline tetracene have been recorded in the temperature range 120 to 300 K under hydrostatic pressure up to 600 MPa. From discontinuities in both emission spectra and spectral intensities it is concluded that two phase transitions occur. The room temperature phase is transformed to a low temperature phase/high pressure phase I at T^I_t (p = 0) = 182 K, the temperature coefficient being dT^I_t/dp = 0.395 K/MPa. The phase transition is induced by a decrease of the specific volume under pressure and/or upon cooling. Lack of a significant shift of the origin of the fluorescence band near T^I_t at constant pressure is an artifact resulting from the neglect of reabsorption effects. The Stokes shift is 260 cm^-1, independent of temperature and crystal modification. In accord with previous Raman data a second phase transition occurs at T^II_t (p) = 143 K, the pressure shift being dT^II_t/dp = 0.088 K/MPa.In addition, the shift of the triplet energy as a function of pressure as well as the pressure-dependence of the rate constants governing fission of a singlet exciton into a pair of triplets is discussed utilizing their magnetic field dependences.     

Phase transformations in equiatomic alloy TiZr at pressure up to 70 kbar

The effect of pressure on the α ? β and ω ? β transformations in the equiatomic alloy TiZr is studied by the differential thermal analysis (DTA) and calorimetric technique. The α-β equilibrium at atmospheric pressure occurs at a temperature of 579°C, and the heat of transition ΔH is 40.9±2.0 J/g. As the pressure increases up to 28 kbar, the temperature of the α-β equilibrium linearly decreases, dT/dP=?2.2±0.3 K/kbar. In the pressure range 28–48 kbar, the β-phase undergoes a transition to the two-phase (α + ω) state upon cooling to room temperature. At pressures above the triple point with the coordinates P=49±3 kbar and T=460±30°C, the cooling of the β-phase gives rise to only the hexagonal ω-phase with the unit cell parameters a=4.843 Å, c=2.988 Å, and c/a=0.617 under normal conditions. The slope of the ω-β equilibrium boundary is positive at pressures up to 70 kbar, dT/dP≈0.46 K/kbar. The ω → α transformation at atmospheric pressure proceeds in the temperature range T=425–470°C with the enthalpy of transition ΔH=2.8 J/g.     

Low-temperature anomalies in the specific heat and thermal conductivity of MgB<Subscript>2</Subscript>

The temperature dependences of the specific heat C(T) and thermal conductivity K(T) of MgB₂ were measured at low temperatures and in the neighborhood of T _c . In addition to the well-known superconducting transition at T _c ≈40 K, this compound was found to exhibit anomalous behavior of both the specific heat and thermal conductivity at lower temperatures, T≈10–12 K. Note that the anomalous behavior of C(T) and K(T) is observed in the same temperature region where MgB₂ was found to undergo negative thermal expansion. All the observed low-temperature anomalies are assigned to the existence in MgB₂ of a second group of carriers and its transition to the superconducting state at T_c2≈10?12 K.     

Comparison of superconducting parameters of A 15- and σ-phases of Nb−Pt

The superconducting properties of A 15- and σ-phases of the system Nb−Pt were investigated. Alloys of compositions Nb₃Pt, Nb₂Pt, and Nb₆₂Pt₃₈ were prepared by arc melting and subjected to optical microscopic and x-ray analysis for structure determination. The transition temperatures were measured inductively as well as resistively. The temperature dependences of critical fields were measured up to 35 kG. The A 15-phase has aT _c value of 8.97K and a zero temperature upper critical fieldH _c2(0) of 100 kG as compared to aT _c value of 2.14K and anH _c2(0) value of 14kG for the σ-phase. A comparison with theoretical upper critical field values seems to indicate that a spin-orbit coupling mechanism is responsible for the higher value ofH _c2(0) in the case of the A 15-phase alloy. The temperature dependent resistance up to 300 K is also measured and discussed for both the phases.