Showing papers on "Debye model published in 1987"

Scattering of Phonons by Vacancies

Abstract: The scattering of phonons by vacancies is estimated by a perturbation technique in terms of the missing mass and the missing linkages. An argument is given why distortion effects can be disregarded. The resonance frequency of the defect is sufficiently high so that resonance effects can be disregarded for phonons in the important frequency range for thermal conduction. The theory is applied to the thermal resistance by vacancies in cases where the vacancy concentration is known: potassium chloride with divalent cations, nonstoichiometric zirconium carbide, and tin telluride.

Elastic constants and Debye temperature of polycrystalline Y1Ba2Cu3O7−x

Abstract: Using ultrasonic methods, the quasi-isotropic elastic stiffnesses of void-containing Y1Ba2Cu3O7−x were determined By a composite-material model, these were corrected to the void-free state. From these, the Debye characteristic temperature was calculated. All the elastic stiffnesses fall well below those of polycrystalline BaTiO3, an approximate crystalstructural building block of Y1Ba2Cu3O7−x. The low apparent stiffness may result from oxygen vacancies, which soften interionic forces. Also, it may result from microcracks, which reduce elastic stiffness without lowering mass density.

Elasticity and anharmonicity of potassium chloride at high temperature

Abstract: High temperature elasticity of single crystal potassium chloride has been studied by the Rectangular Parallelepiped Resonance (RPR) method up to 870 K (⋍ 3.8 times the Debye temperature, ϑ). The elastic stiffness moduli, C 11 and C 44, decrease linearly with temperature while C 12 increases slightly with temperature. The RPR method is particularly suited to measurements of elasticity at high temperatures, since no glues (which decompose at high temperature) are used to connect the transducers to a specimen. As a consequence, the measured spectrum closely approximates the theoretical spectrum of a specimen freely suspended in space with no external contact. The present elasticity data permits the investigation of the thermodynamic properties of potassium chloride far above the Debye temperature when used together with the previous zero-pressure data on thermal expansivity and heat capacity. The equation of state of potassium chloride is virtually unaffected by anharmonicity, even at T/ϑ=3.8. One result is that the thermal pressure for KCl above the Debye temperature linearly increases with temperature. There is also small dependence on volume, in contrast to NaCl where there is no dependence on volume.

New spin-crossover iron(III) complexes with large hysteresis effects and time dependence of their magnetism

Abstract: The new complexes [FeL12] ClO4(HL1= tridentate Schiff base formed by 1 : 1 condensation of o-aminophenol and pyridine-2-carbaldehyde) and [FeL22] NCS ( HL2= tridentate Schiff base formed by 1 : 1 condensation of 8-aminoquinoline and salicylaldehyde) were synthesized. The spin-crossover behaviour between S=½ and 5//2 which is dependent on temperature was observed for the solid complexes using variable-temperature 57Fe Mossbauer spectral and magnetic measurements. Both compounds show a pronounced hysteresis effect in the high spin ⇌ low spin transition. For [ FeL12] ClO4, sharp and strong Mossbauer absorptions were observed over the temperature range examined and the effective vibrating mass, Meff., was evaluated using the high-temperature approximation of the Debye model. The value for the low-spin isomer was larger than that for the high-spin isomer. The complex [FeL22]NCS when freshly recrystallized below 280 K is predominantly in a low-spin state. However, the magnetism at room temperature increases with time, spin-crossover behaviour is exhibited, and the magnetism is dependent on temperature, with a pronounced hysteresis loop of about 70 K at 286 K 10 d after preparation.

Specific Heat of La2-xSrxCuO4and YBa2Cu3O7Superconductors from 1 K to Room Temperature

Abstract: The specific heat of the superconductors La1.85Sr0.15CuO4 and YBa2Cu3O7 was measured from 1 to 350 K. A maximum in C/T3 near 22 K indicates enhanced phonon density of states at energies ω~11 meV. A lattice specific heat still below 3R at room temperature implies the existence of high energy phonon states at ~50 meV or beyond. The initial Debye temperature lies in the 380–400 K range. At low temperatures, abnormal electronic contributions are seen, mainly in YBa2Cu3O7. The specific heat jumps are resolved in six samples: ΔC/Tc14 mJK-2mole–Cu-1 in La1.85Sr0.15CuO4 and ΔC/Tc13 mJK-2mole–Cu-1 in YBa2Cu3O7.

Thermal expansion of the clathrate hydrates of ethylene oxide and tetrahydrofuran

Abstract: The unit cell parameters of clathrate hydrates of ethylene oxide and tetrahydrofuran have been measured from 20 to 250 K by X-ray powder diffraction. The coefficients of linear expansion for the two clathrate hydrates are of comparable magnitude and larger than that for hexagonal ice. The larger thermal expansivity in the hydrates indicates a greater anharmonicity in the crystal potential. A positive deviation of the Gruneisen parameter at low temperature was observed. The characteristic Debye temperature and the moments of the lattice vibrational density of states of the hydrate have been evaluated from quasi-harmonic theory.

Breakdown of the Debye theory of bulk ion recombination

Abstract: We have investigated how the rate of diffusion‐controlled bulk recombination between electrons and positive ions is influenced by the mean free path of electrons. By use of the Monte Carlo method previously developed we have calculated the rate constant as a function of the mean free path. For mean free paths negligibly short compared with the Onsager length, the rate constant increases linearly with increasing mean free path, in agreement with the Debye theory of bulk ion recombination. However, as the mean free path further increases, the rate constant goes through a maximum and then decreases. The present result quantitatively explains recent experimental data on electron–ion recombination rate constants in liquid methane in which the mean free path of electrons is comparable with the Onsager length.

Magnetic properties of fine iron particles

Abstract: Iron particles were prepared by a gas evaporation method in a nitrogen atmosphere. The particle size, shape, and magnetic properties were influenced by the nitrogen pressure. Particles prepared by this method were relatively stable in air and were not seriously oxidized. The recoilless fraction is low for the oxide layer, so that it cannot be detected at room temperature by Mossbauer spectrum. A superparamagnetic peak associated with the oxide microcrystal layer did not occur in the Mossbauer spectra due to the action of the magnetic field from the iron core and the interaction between iron particles and the low Debye temperature. From the electron micrography, the magnetic reversal process seems to be determined by a chain‐of‐spheres mechanism but Hc∼T curve cannot coincide with the Ms∼T curve at low temperatures. An increase of Hc may be associated with magnetic anisotropy. It is found that the magnetization versus temperature shows an anomalous increase in M(T) at higher T. We think that this means tha...

Linear temperature behavior of the resistivity in the new high-Tc superconductors.

Abstract: We propose that the linear temperature behavior of the resistivity observed in the new high-T/sub c/ materials might be linked to their two-dimensional character for electron transport. For three-dimensional electron transport a linear temperature law for the resistivity is obtained for temperatures above the Debye temperature FTHETA/sub D/. For two-dimensional transport, however, such a linear law holds above a characteristic temperature T/sup */ = 2h-dash-barsp/sub F/ which can be well below FTHETA/sub D/ (s denoting the sound velocity and p/sub F/ the Fermi momentum) provided that p/sub F/<<..pi../a which is the case for these high-T/sub c/ materials.

Superconducting EuBa2Cu3O7. A Mössbauer study

Abstract: EuBa2Cu3O7 shows a sharp transition to zero resistance atT sc =85(1) K. Europium is entirely trivalent. The151Eu Mossbauer recoilless fraction in the range 4–300 K is well-fitted by a Debye model withT sc = 85(1) K. There is no anomaly around 240 K, nor atT sc , and therefore no significant change in the vibrational amplitude of the rare-earth ion. Another sample doped with 1%57Fe, shows two ferric quadrupole doublets associated with Fe3+ on Cu1 and Cu2 sites. There is no sign of any magnetic order down to 4.2 K.

Electron transport studies of Ni33Zr67-based metallic glasses containing H, B, Al and Si

Abstract: The electrical resistivity has been measured on Ni33Zr67Hx (0

Studies of negative TCR and electronic structure of nonmagnetic metallic glasses based on Y and La

Abstract: The temperature dependence of the electrical resistivity has been measured over a wide temperature range, 2-300 K, for a number of Y- and L-based metallic glasses. Low-temperature specific heats have also been measured to determine the density of states at the Fermi level and the Debye temperature. All of the resistivity data in the range from about 40 to 300 K can be well described in terms of the equation rho / rho 0=A+B exp(-T/ Delta ). According to Mizutani's classification of metallic glasses, the present alloy systems are to be classified in group (4), together with systems such as Cu-Zr and Cu-Ti. The authors revealed that: (i) the above equation is valid for all group (4) metallic glasses studied so far; (ii) the characteristic temperature Delta increases with increasing Debye temperature theta D; and (iii) the resistivity increases with decreasing density of d states at the Fermi level. Using the generalised Faber-Ziman theory; it is difficult to interpret all these findings in a consistent manner. It is suggested that the d electrons at the Fermi level play a key role in the electron transport in group (4) metallic glasses.

The heat capacity of high-purity gadolinium from 0.5 to 4 K and the effects of interstitial impurities

Abstract: Two sets of experiments are presented. In the first, the effects of interstitial impurities on the heat capacity are explored in a gadolinium specimen of moderate purity. The second set consists of measurements on a number of samples of the highest available purity prepared by the solid-state electrotransport process. The heat capacities of the two highest-purity specimens are found to agree to 1% or better, and that of the most pure specimen can be represented by the expression Cp=4.48T+1.37T1.5+0.404T3 mJ K-1 mol-1 implying that the electronic specific heat coefficient gamma =4.48+or-0.07 mJ K-2 mol-1 and the Debye temperature theta 0=169+or-1 K. The quoted errors are standard deviations derived from the scatter in the experimental points, and do not include systematic errors; these are discussed in the text.

High resolution Mössbauer spectroscopy with67Zn in metallic systems

Abstract: The high energy resolution of the 93.3 keV Mossbauer transition in67Zn is used to investigate changes of s-electron density at the Zn nucleus in Cu-Zn alloys with special emphasis on the α-phase and on Zn-metal itself. We observe the presence of short-range order in α-brass with only four different Cu-Zn configurations, instead of the expected binomial distribution. In Zn metal, no change of center shift was observed when crossing the superconducting phase transition. The tremendous anisotropy of the Lamb-Mossbauer factor, as well as the temperature shift found for hexagonal Zn metal, demonstrate that the 93.3 keV resonance is also extremely sensitive to lattice dynamical effects. Our results, as well as specific heat data, are quantitatively described by an extended Debye model characterized by two Debye temperatures, θ⊥ and θ‖, which correspond to the lattice vibrations perpendicular and parallel to thec axis, respectively.

Electrical resistivity of amorphous Fe 82 B 18-x Ge x alloys: Coherent electron-magnon scattering contribution

Abstract: Detailed analysis of the electrical resistivity (rho) data taken on amorphous Fe/sub 82/B/sub 18-//sub x/Ge/sub x/ alloys in the temperature range 4.2 to 320 K reveals that physically meaningful values of the Debye temperature, structure factor, and Fermi wave vector can be deduced from these data only when the existence of an additional contribution to rho, besides the dominant structural contribution (rho/sub str/) arising from the coherent scattering of conduction electrons from the long-wavelength magnons (rho/sub mag/), is duly recognized. Moreover, rho/sub mag/ is found to possess values comparable in magnitude to those previously reported for crystalline ferromagnets.

Raman spectroscopy of the high Tc superconductor YBa2Cu3O7 and the semiconductor YBa2Cu3O6

Abstract: The recent discoveries of superconducting materials with extraordinarily high-transition temperatures immediately raise the question of whether an electron-phonon mechanism as in the Bardeen-Cooper-Schrieffer (BCS) theory is operative or if some entirely new mechanism must be invoked. The common structural features of these materials are extended Cu-O chains and sheets. Since the standard BCS theory predicts that T/sub c/ is proportional to the Debye temperature, it has been suggested that the comparatively high frequencies of Cu-O breathing modes could be responsible for these very high-transition temperatures. Motivated by the generally accepted empirical correlation between the existence of phonon anomalies, lattice instabilities, and high T/sub c/'s in other materials, the authors have begun to investigate the role of lattice phonons in high T/sub c/ superconductors. They report here the Raman spectra of the superconducting YBa/sub 2/Cu/sub 3/O/sub 7/ phase and the semiconducting YBa/sub 2/Cu/sub 3/O/sub 6/ phase. A high-frequency mode was identified in the Raman spectrum of the high temperature superconductor YBa/sub 2/Cu/sub 3/O/sub 7/ which is shifted to much lower frequencies in the oxygen deficient nonsuperconducting compound YBa/sub 2/Cu/sub 3/O/sub 6/. This mode is assigned to the symmetric stretch of the terminal Cu-O bonds in the linear chain of corner-shared CuO/sub more » 3/ units; this assignment is consistent with the structural models derived from diffraction experiments. While these experiments do not definitively establish a causal link between the frequency shift of this mode and susperconductivity, the correlation is nevertheless intriguing. « less

Effective and Debye temperatures of Ti in TiC, TiO 2 , and TiH 2

Abstract: The relative intensities of resonantly scattered gamma rays by $^{48}\mathrm{Ti}$, at room and liquid-nitrogen temperatures, were measured in TiC, ${\mathrm{TiO}}_{2}$, and ${\mathrm{TiH}}_{2}$ with use of the nuclear-resonance photon-scattering technique. The results enable determination of the effective temperature ${T}_{e}$ and the Debye temperature ${\ensuremath{\Theta}}_{\mathrm{D}}$ of Ti in the various compounds with assumption of the Debye model. A significant increase in ${T}_{e}$ and ${\ensuremath{\Theta}}_{\mathrm{D}}$ of the Ti atoms was observed in TiC and ${\mathrm{TiO}}_{2}$, while no detectable change in ${T}_{e}$ and ${\ensuremath{\Theta}}_{\mathrm{D}}$ was observed in pure Ti upon hydrogenation. The present results are discussed and compared with data obtained by other techniques.

Temperature dependence of planar channeling radiation.

Abstract: Peak energies and linewidths of 54-MeV electron planar channeling radiation from a silicon crystal have been measured as a function of temperature. Our measured peak energies are compared with our theoretical calculations to obtain a Debye temperature for silicon of 495\ifmmode\pm\else\textpm\fi{}10 K. This value is appreciably lower than the value of 543 K obtained from an x-ray diffraction measurement, but is in excellent agreement with the value of 500 K obtained recently from a measurement of axial channeling radiation.

Reduction of lattice thermal conductivity by point defects at intermediate temperatures

Abstract: The lattice thermal conductivity is reduced by point defects because they scatter phonons. An analytic expression can be derived only in the limit of high temperatures; at lower temperatures one must have recourse to numerical calculations. Because the conductivity is due mainly to phonons of low frequencies when point-defect scattering is strong, the high-temperature approximation can be used at temperatures above half the Debye temperature. Numerical calculations, using the Ge-Si system as an example, show that the error incurred by using the high-temperature approximation is less than 10%.

Transport Properties and Specific Heat of (La1-xBax)2CuO4-y

Abstract: X-ray analysis, covering a wide temperature range between 10 and 300 K, and the measurements of the thermoelectric power and specific heat are performed on a recently discovered high-Tc superconductor, (La1-xBax)2CuO4-y with x=0075 A large positive thermoelectric power is observed above Tc The Debye temperature is 385 K and no specificheat jump is detected across Tc