Study of Fe−Ni−Cr−Mo−Si−B metallic glasses after neutron irradiation

Chromium containing metallic glasses are studied by transmission⁵⁷Fe Mössbauer spectroscopy after neutron irradiation. Increasing number of nonmagnetic chromium atoms causes a compositional dependence of Curie temperature. The unirradiated samples are fully paramagnetic forx≥10 at. % Cr at room temperature. Radiation induced changes in the magnetic structure imply a decrease of the Curie temperature. Ferromagnetic-to-paramagnetic transition is observed at room temperature for 8 at. % Cr after the exposure with 10¹⁹ n/cm². Using low temperature measurements, the Curie temperature for the sample containing 10 at. % Cr is estimated to be about 270 K.     

Optical temperature sensor based on up-conversion fluorescence emission in Yb3+:Er3+ co-doped ceramics glass

Yb3+:Er3+co-doped oxy-fluoride ceramics glass has been prepared.The mechanism of up-conversion emissions about Er3+was discussed,and the temperature properties of green up-conversion fluorescence between 303 and 823 K were investigated.The results show that the sensitivity of this sample reaches its maximum value,about 0.0047 K 1,when the temperature is 383 K,indicating that this kind of sample can be used as high temperature and high sensitivity optical temperature sensor.     

Diffusion and compressibility of sodium on the (110) plane of tungsten

The surface diffusion parameters and the compressibility of sodium on the (110) plane of tungsten have been measured using the field emission fluctuation method for sodium coverages from 0.2 to 3 × 10¹⁴ atoms cm^?2 and for temperatures from 170 to 500 K. Two temperature regimes can be defined. In the high temperature regime (? 300 K) the diffusion is essentially normal with an activation energy ranging from 0.28 to 0.58 eV and a preexponential coefficient D₀ from 10^?8.1 to 10^?2.7 cm² s^?1. In this regime the compressibility increases with temperature indicating an effective repulsive adatoms interaction. In the low temperature regime (? 300 K) the diffusion coefficient decreases with temperature at high coverage and slowly increases with temperature at lower coverage. The transition between both regimes appears on the compressibility versus temperature curve as an inflection point. The comparison of the present results with slow electron diffraction results furnishes strong evidence that the observed transition corresponds to a continuous short-range order-disorder transition.     

Determination of the Thermal Expansion Coefficient of Single-Wall Carbon Nanotubes by Raman Spectroscopy

We have examined the effect of high temperature on single-wall carbon nanotubes under air and nitrogen ambient by Raman spectroscopy. We observe the temperature dependence of the radial breathing mode and the G-band modes. The thermal expansion coefficient (β) of the bundled nanotubes is obtained experimentally using the estimated volume from Raman scattering. β behaves linearly with temperature from 0.33 × 10^?5 K^?1 to 0.28 × 10^?5 K^?1 in air and from 0.58 × 10^?5 K^?1 to 0.47 × 10^?5 K^?1 in nitrogen ambient, respectively. The temperature dependence of the radial breathing mode Raman frequencies is consistent with a pure temperature effect.     

X-ray determination of thermal expansion of cadmium fluoride and lead fluoride

The lattice parameters of CdF₂ andβ-PbF₂ have been determined over the temperature range 300–670 K. The coefficient of expansion at room temperature is 21·3 × 10⁻⁶ K⁻¹ and 25·4 × 10⁻⁶ K⁻¹ for CdF₂ and PbF₂ respectively and it increases linearly with temperature over the range of temperature covered. The Grüneisen parameter decreases with temperature in both the crystals.     

The temperature dependence of the Hi optical potential

Within a realistic effective nucleon-nucleon interaction, derived from the Reid soft-core potential at zero temperature, the dependence on temperature of the heavy ion optical potential is investigated for the systems ⁴⁰Ca + ⁴⁰Ca and ²⁰⁸Pb + ²⁰⁸Pb. The hot nuclear densities are generated using the Thomas-Fermi model at a finite temperature. It is found that both the real and the imaginary parts of the optical potential become more attractive and extend to large distances when the temperature increases. The barrier heights are lowered and shifted outwards.     

Photoproduction of massive gauge bosons in high energy electron-proton collisions

A self-consistent renormalization scheme at finite temperature and zero momentum is used together with the finite temperature renormalization group to study the temperature dependence of the mass and the coupling to one-loop order in the (φ ³)₆- and (φ ⁴)₄-models. It is found that the critical temperature is shifted relative to the naive one-loop result and the coupling constants at the critical temperature get large corrections. In the high temperature limit of the (φ ⁴)₄-model the coupling decreases.     

Kinetics of the sorption of <Superscript>3</Superscript>He by C<Subscript>60</Subscript> fullerite. The quantum diffusion of <Superscript>3</Superscript>He and <Superscript>4</Superscript>He in fullerite

The kinetics of the sorption and subsequent desorption of gaseous ³He in a C₆₀ fullerite powder has been studied in the temperature range of 2–292 K. The temperature dependences of the diffusion coefficients of ³He and ⁴He impurities in fullerite have been plotted using the measured characteristic times of filling of octahedral and tetrahedral interstices, as well as previous data. These temperature dependences of the diffusion coefficients of ³He and ⁴He impurities in fullerite are qualitatively similar. A decrease in the temperature from 292 to 79 K is accompanied by a decrease in the diffusion coefficients, which corresponds to the dominance of the thermally activated diffusion of helium isotopes in fullerite. A further decrease in the temperature to 8–10 K leads to an increase in the diffusion coefficients by more than an order of magnitude. The diffusion coefficients of ³He and ⁴He are independent of the temperature below 8 K, indicating the tunnel character of the diffusion of helium in C₆₀ fullerite. The isotope effect is manifested in the difference between the absolute values of the diffusion coefficients of ³He and ⁴He atoms at the same temperatures.     

EPR observation of phase transitions in calcium cadmium acetate hexahydrate: Using Mn2+ and Cu2+ ions as probe

Results of temperature dependence of EPR spectra of Mn²⁺ and Cu²⁺ ions doped calcium cadmium acetate hexahydrate (CaCd(CH₃COO)₄·6H₂O) have been reported. The investigation has been carried out in the temperature range between room temperature (～ 300 K) and liquid nitrogen temperature. A I-order phase transition at 146 ± 0.5 K has been confirmed. In addition a new II-order phase transition at 128 ± 1 K has been detected for the first time. There is evidence of large amplitude hindered rotations of CH₃ groups which become frozen at ～ 128 K. The incorporation of Cu²⁺ and Mn²⁺ probes at Ca²⁺ and Cd²⁺ sites respectively provide evidence that the phase transitions are caused by the molecular rearrangements of the common coordinating acetate groups between Ca²⁺ and Cd²⁺ sites. In contradiction to the previous reports of a change of symmetry from tetragonal to orthorhombic below 140 K, the symmetry of the host is concluded to remain tetragonal in all the three observed phases between room temperature and liquid nitrogen temperature.     

Temperature dependence of EPR spectrum of Mn2+ doped in ammonium iodide single crystals

EPR of Mn²⁺ doped in ammonium iodide single crystal has been studied at X-band in the temperature range 573–577 K. The observed temperature dependence of line widths and spin Hamiltonian parameter b₂⁰ below room temperature is related to the structural transformations in the crystal. The coexistence of high temperature phase (NaCl) and low temperature phase (CsCl) is attributed to the large thermal hysteresis in line widths and b₂⁰. The dissociation of ion vacancy pairs occurs near 500 K and is reflected in the reversible change of an anisotropic EPR spectrum in an isotropic sextet near this temperature. The ion vacancy pair models for NaCl and CsCl phases are discussed along with the effects of thermal processing of the samples. Heating the crystals above 500 K leads to expulsion of Mn²⁺ impurity from the crystal.     

Singly charged tungsten and tantalum ions observed during high-temperature field evaporation

High-temperature field evaporation of tungsten and tantalum emitters in the temperature range from room temperature to 2500 K is studied using a static magnetic mass spectrometer equipped with a field source of ions. At room temperature, triply charged W³⁺ and Ta³⁺ ions alone are observed in the mass spectra. However, as the emitter temperature grows, the charge of the ions decreases. At T ≈ 1000 K, doubly charged W²⁺ and Ta²⁺ ions dominate in the spectra, and singly charged W⁺ and Ta⁺ ions appear in the temperature range 1900 < T < 2500 K. The evaporation rate of the singly charged ions is one to two orders of magnitude lower than the evaporation rate of the doubly charged particles. The energy parameters of field evaporation for differently charged tungsten ions are found.     

The temperature dependence of the57Fe hyperfine magnetic field distribution in Fe-Ni

The temperature dependence of the⁵⁷Fe hyperfine magnetic field (hmf) in Fe-Ni is stronger than the temperature dependence of the⁵⁷Fe hmf in pure Fe. By analyzing the shape of the⁵⁷Fe hmf distribution, and with the help of experiments with Si in Fe-Ni, we deduce that this anomalous temperature dependence originates from a large thermal sensitivity of the magnetic moments at those Fe atoms with more Ni nearest neighbors. A strong temperature dependence of the recoilfree fraction was also observed in Fe-Ni alloys. We suggest that a large mean square thermal displacement of Fe atoms in Fe-Ni is the cause of the anomalous temperature dependence.     

Luminescence and energy transfer in TbAl3B4O12

An investigation of the luminescence properties of TbAl₃B₄O₁₂ in the temperature region 1.4–300 K is reported. Laser site-selection and time-resolution techniques were used.The results show that energy migration among Tb³⁺ ions on the regular crystallographic sites occurs. The transfer characteristics at room temperature are consistent with a diffusion-limited transfer process to Mo³⁺ quenching centres. The diffusion constant and the critical transfer distance for Tb³⁺ → Mo³⁺ transfer are derived. The rate of diffusion increases for decreasing temperature. At about 60 K a transition from diffusion-limited transfer to trapping-limited transfer occurs. This behaviour is due to the variation in the diffusion constant with temperature. In the temperature region below 60 K transfer to Tb³⁺ traps is observed. The intensity of the emission from the traps increases exponentially with decreasing temperature. However, the overall transfer rate to Mo³⁺ and traps remains roughly constant. A simple model including time-independent transfer rates and back transfer from traps to intrinsic Tb³⁺ ions is proposed to explain the results. The rates of transfer to Mo³⁺ and traps are obtained.     

非晶态Fe87-xSixB13合金的磁化强度、电阻与温度的关系

本文研究了非晶态Fe_87-xSi_xB₁₃(x=0,9.6,14.5)合金的饱和磁化强度、电阻率与温度的关系。得到样品的居里温度T_C和晶化温度T_cr随Si含量的增加而明显提高。低温下的热磁关系符合布洛赫的T^3/2定律,计算出自旋波劲度系数D从x=0时的62meV·A²增加到x=14.5时的111meV·A²。从D值和Handrich理 关键词：     

Low frequency dielectric permittivity of quasi-one-dimensional conductor (TMTTF)2Br

Conductivity and permittivity of the organic transfer salt (TMTTF)₂Br have been measured at low frequencies (10²-10⁷ Hz) between room temperature down to 4 K. The real part of the permittivity, , is shown to grow below the temperature at which the conductivity is maximum due to charge localization of Mott-Hubbard type. reaches a maximum of 10⁵-10⁶ at 35 K-50 K depending on the samples. Decreasing temperature below , sharply decreases down to helium temperature through the antiferromagnetic phase transition at T _N = 15 K. We explain the magnitude, the temperature and frequency dependence of as resulting from short range charge density wave states in the temperature range where charge localization occurs. This interpretation is supported by recent X-ray scattering measurements. Received: 10 October 1997 / Revised: 28 February 1998 / Accepted: 3 March 1998     

Observation of a soft phonon in linear chain compound (NbSe4)3I by Raman scattering

The phase transition of the linear chain compound (NbSe₄)₃I was studied by Raman scattering. At 78 K three new peaks were observed at 73 cm^?1, 205 cm^?1 and 261 cm^?1. The totally symmetric Raman peak at 73 cm^?1 shows anomalous temperature dependence. The frequency decreases with increasing temperature, and at high temperatures an anticrossing occurs with another peak observed at about 58 cm^?1. The Raman intensity decreases and the linewidth broadens remarkably as the temperature increases. These properties allow us to assign this peak to a soft phonon. This fact indicates clearly the existence of a structural phase transition of a displacive type below room temperature.     

Investigation of the dynamical behaviour of the FH(CN−) centre in KCl with temperature dependent endor spectroscopy

Abstract

The temperature dependence of the electron nuclear double resonance (ENDOR) spectra of F_H(CN^?) centres in KCl was investigated in the temperature range between 10–220 K. At the lowest temperature of 10 K only one CN^? orientation with respect to the F centre electron is present, in which the nitrogen is thought to be nearer to the F-electron than the carbon. With the very small thermal activation energy of 2.9 meV the opposite orientation is occupied. The superhyperfine interactions of those first shell K nuclei nearest to CN^? and of the ¹³C interaction of the CN^? molecule are strongly temperature dependent between 10 and 60 K, following an exponential law with a thermal activation energy of 4.2 meV. It is assumed that a soft local mode involving those two nearest K nuclei and the CN^? is causing the strong temperature dependence. The shf interactions of ¹⁴N nuclei have not been seen, probably because of the dynamical effects.     

The electric field gradient and its temperature dependence for ruthenium in scandium and yttrium

The electric field gradients (EFG) for ruthenium in scandium and yttrium metal were determined by TDPAC measurements to be 19(4) × 10¹⁷ V/cm² and 5.5(12) × 10¹⁷ V/cm² respectively at room temperature. The EFG for Ru in Sc was found to vary considerably in the temperature range from 14 to 700 K, whereas for Ru in Y only an extremely small temperature dependence of the EFG was observed.     

Application of the thermal spike model for explanation of variations of surface structure of highly oriented pyrolytic graphite under bombardment by 86Kr and 209Bi fast ions with high ionization energy loss

Temperature effects in the highly oriented pyrolytic graphite (HOPG) under bombardment by ⁸⁶Kr (253 MeV) and ²⁰⁹Bi (710 MeV) heavy ions are studied in the framework of a three-dimensional thermal spike model. It is shown that the surface temperature of an HOPG target under bombardment by bismuth ions can exceed the sublimation temperature at particular values of the electron-phonon interaction coefficient. At the same time, the temperature at the target surface during bombardment of HOPG by krypton ions does not exceed the sublimation temperature over a wide range of variations in the electron-phonon interaction belongs. The calculations allow the explanation of the observed changes in the surface structure of HOPG single crystal under bombardment by ²⁰⁹Bi and ⁸⁶Kr ions.     

Ferromagnetism, variable range hopping and anomalous Hall effect in epitaxial Co:ZnO thin film

A series of high quality single crystalline epitaxial Zn_0.95Co_0.05O thin films is prepared by molecular beam epitaxy. Superparamagnetism and ferromagnetism are observed when the donor density is manipulated in a range of 10¹⁸ cm^-3-10²⁰ cm^-3 by changing the oxygen partial pressure during film growth. The conduction shows variable range hopping at low temperature and thermal activation conduction at high temperature. The ferromagnetism can be maintained up to room temperature. However, the anomalous Hall effect is observed only at low temperature and disappears above 160 K. This phenomenon can be attributed to the local ferromagnetism and the decreased optimal hopping distance at high temperatures.