On Debye temperature anomaly observed in Ge-Se-Ag glasses

Anomalous values of Debye temperature have been obtained from recoil free factor measurements Ge-Se-Ag glasses recently [B. Arcondo, M.A. Urena, A. Piarristeguy, A. Pradel, M. Fontana, Physica B 389 (2007) 77]. In the present paper we show that this anomaly may arise due to the presence of anharmonic potential at the high spin ferrous site. We use q Lamb Mossbauer factor and anharmonic Lamb Mossbauer factor to study this anharmonicity.     

Role of thermal conduction in single-bubble cavitation

Effect of thermal conduction on radiation from a single cavitating bubble has been studied in a hydrochemical framework including variation of heat conductivity of noble gases up to 2500 K. Results of numerical simulation show that thermal conductivity plays an important role in determining ultimate cavitation temperature. Higher thermal conductivity of lighter noble gases causes to more thermal dissipation during the bubble collapse, leading to a lower peak temperature. Moreover, at the same driving conditions, radius of light emitting region is greater for heavier noble bubbles. Therefore, sonoluminescence radiation is more intensive from heavier noble gases. Phase diagrams of single-bubble sonoluminescence have also been calculated and in comparison with available experimental data, there is a relatively good agreement between the theory and experiment for noble gases.     

Heat conduction in multi-temperature mixtures of fluids: the role of the average temperature

The theory of mixture of fluids in which each constituent has its own temperature is more realistic than the models in which all the components have a common temperature and it is necessary in several physical situations, such as plasma physics. Nevertheless, from the theoretical point of view, the main problem regards the measurement of the temperature of each constituent. In this Letter we consider the simple case of the one-dimensional steady heat conduction between two walls and we adopt the concept of average temperature recently proposed by Ruggeri and co-workers such that the whole internal energy of the mixture depends only on the average temperature. Assuming that the two walls have given temperatures, we can first verify that the temperature behavior is different from the linear one predicted in the mixture theories with a single temperature. Moreover, we can also prove that, for a mixture of n constituents, the measure of the average temperature at 2(n−1) points allows to know the temperature behavior of each constituent everywhere.     

Effect of temperature on the electron field emission from aligned carbon nanofibers and multiwalled carbon nanotubes

Effect of temperature and aspect ratio on the field emission properties of vertically aligned carbon nanofiber and multiwalled carbon nanotube thin films were studied in detail. Carbon nanofibers and multiwalled carbon nanotube have been synthesized on Si substrates via direct current plasma enhanced chemical vapor deposition technique. Surface morphologies of the films have been studied by a scanning electron microscope, transmission electron microscope and an atomic force microscope. It is found that the threshold field and the emission current density are dependent on the ambient temperature as well as on the aspect ratio of the carbon nanostructure. The threshold field for carbon nanofibers was found to decrease from 5.1 to 2.6 V/μm when the temperature was raised from 300 to 650 K, whereas for MWCNTs it was found to decrease from 4.0 to 1.4 V/μm. This dependence was due to the change in work function of the nanofibers and nanotubes with temperature. The field enhancement factor, current density and the dependence of the effective work function with temperature and with aspect ratio were calculated and we have tried to explain the emission mechanism.     

Dielectric relaxation measurements on nonlinear side-chain polymers

Dynamic dielectric relaxation measurements in the frequency range from 0.001 Hz to 5 kHz have been performed on an amorphous nonlinear optical side-chain polymer, containing the active molecule 4-dimethylamino-4-nitrostilbene as pendant group. The results yielded the relaxation times of the group in the temperature region from 20° below to 40° above the glass-transition temperature of the polymer. The temperature dependence could be described well by the Williams-Landel-Ferry (WLF) equation. It is shown that the relaxation times have a distribution, which could be fit to the form of Cole and Cole with a parameter =0.55. Consequently a substantial part of the active molecules will not be aligned when the polymer is subjected to a poling procedure.     

Thermal conductivity of pressure polymerized C60

60 polymerization in the temperature interval at pressures below by measurements of the time dependence of the thermal conductivity. It has been found at that the polymerization process at is slower than the reverse transformation from “polymeric” to “monomeric” phase at . The thermal conductivity of polymerized C₆₀ was measured in the temperature range and found to increase with increasing temperature, which reflects strong phonon scattering. Both the presence of non-bonded C₆₀ molecules and a high degree of structural disorder in the crystalline lattice of the polymeric phase might be responsible for the behaviour of . The results for are qualitatively similar to those reported previously for C₆₀ polymerized at higher , but an order of magnitude smaller. Received: 20 September 1996/Accepted: 11 November 1996     

Study of the magnetization and transport properties of the La(2+x)/3Sr(1−x)/3Mn1−xCrxO3 (0≤x≤0.2) system

The samples with the Mn³⁺/Mn⁴⁺ ratio fixed at 2:1 La_(2+x)/3Sr_(1−x)/3Mn_1−xCr_xO₃ (0≤x≤0.20) have been prepared. The magnetic, electrical transport, and magnetoresistance properties have been investigated. Remarkable transport and colossal magnetoresistance (CMR) effect, as well as cluster glass (CG) behaviors have been clearly observed in the samples studied. It was found that the Curie temperature T_c and insulator−metal transition temperature T_p1 are strongly affected by Cr substitution. The experiment observations are discussed by taking into account the variety of tolerance factors t; the effects of A-site radius 〈r_A〉 and the A-site mismatch effect (σ²).     

A Brillouin study of the temperature-dependence of the acoustic modes across the insulator-metal transitions in V2O3 and Cr-doped V2O3

Brillouin scattering studies have been carried out on high-quality single crystals of undoped and 0.9% Cr-doped V₂O₃. The observed modes in both the samples at ∼12 and ∼60 GHz are associated with the surface Rayleigh wave (SRW) and bulk acoustic wave (BAW), respectively. In the undoped sample, the mode frequencies of the SRW and BAW modes decrease as the temperature is lowered from room temperature to the insulator-metal transition temperature (T_IM=T_N=∼130 K). Below the transition, the modes show hardening. In the doped sample, the SRW mode shows a similar temperature-dependence as the undoped one, but the BAW mode shows hardening from room temperature down to the lowest temperature (50 K). This is the first measurement of the sound velocity below T_IM in the V₂O₃ system. The softening of the SRW frequency from 330 K to T_IM can be qualitatively understood on the basis of the temperature-dependence of C₄₄, which, in turn, is related to the orbital fluctuations in the paramagnetic metallic phase. The hardening of the mode frequencies below T_IM suggests that C₄₄ must increase in the antiferromagnetic insulating phase, possibly due to the orbital ordering.     

Electron spin resonance properties of semiconductor/granular film heterostructures with cobalt nanoparticles in millimeter waveband

Ferromagnetic resonance spectra (FMR) on heterostructures of amorphous silicon dioxide films containing cobalt nanoparticles, (SiO₂)_100−xCo_x, grown on GaAs and Si substrates have been investigated over a frequency range of 37–41 GHz at room temperature. The FMR linewidth and saturation magnetization dependencies on the cobalt concentration have been analyzed. The impact of the semiconductor type on the FMR linewidth ΔH and a sharp increase in ΔH with a decreasing concentration of cobalt nanoparticles have been noted. The effect of considerable FMR linewidth broadening has been accounted for by the spin-polarized relaxation mechanism.     

Mechanism of the growth of ZnSe nanowires with Fe catalysts

ZnSe nanowires of high optical quality have been grown with Fe catalysts at the usual temperatures for the fabrication of ZnSe-base optoelectronic devices (about 600 K). It is found that only Fe nanoparticles of 10-50 nm diameter act as catalysts in the narrow temperature range around 600 K (with a minimum temperature of 473-523 K, and a maximum temperature of 623-673 K). The growth mechanism is discussed in terms of the vapour-liquid-solid mechanism.