Showing papers on "Debye model published in 1990"

Elastic constants of MoSi2 and WSi2 single crystals

Abstract: Single crystals of MoSi2 and WSi2 with a body-centred-tetragonal C 1 1b structure were fabricated using a floating-zone method. The elastic wave velocity was measured for samples with various orientations using a simple pulse echo method at room temperature, and six elastic stiffness constantsc ij were calculated. The stiffness constants were a little higher for WSi2 than for MoSi2.c 11 andc 33 of these compounds were approximately equal toc 11 of tungsten and molybdenum, respectively, althoughc ij (i ≠j) was a little higher for these compounds than for molybdenum and tungsten. Young's modulus 1/s 11 was the highest in the direction, and the lowest in the direction. The shear modulus 1/s 66 was high on the {0 0 1} plane and independent of shear direction. It was generally low on the close-packed {1 1 0} plane and largely dependent on shear direction. The elastic constants for the polycrystalline materials were estimated fromc ij ands ij . Poisson's ratiov was 0.15 for MoSi2 and for WSi2, and these values were much lower than for ordinary metals and alloys. The Debye temperature θD was estimated using the elastic-wave velocity of the polycrystalline materials via the elastic constants such as Young's modulus and shear modulus: it was 759 K for MoSi2 and 625 K for WSi2.

Observation of a Bloch-Grüneisen regime in two-dimensional electron transport

Abstract: We have observed a rapid decrease in the rate by which two-dimensional electrons in very high-mobility (\ensuremath{\mu}\ensuremath{\approxeq}${10}^{7}$ ${\mathrm{cm}}^{2}$/V sec) GaAs-${\mathrm{Al}}_{\mathrm{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$As heterostructures are scattered by acoustic phonons as the temperature approaches T=0. This precipitous drop in scattering rate is caused by strong phase-space restrictions for electron-phonon scattering. The characteristic temperature for this transition is not the Debye temperature of the GaAs host material, but the temperature at which phonons of twice the Fermi wave vector cease to be thermally excited.

Electrical conductance of silver nanoparticles grown in glass-ceramic

Abstract: Conducting films consisting of silver particles of diameters ranging from 4 to 12 nm have been grown in glass-ceramic by subjecting the latter to a Li+ to or from Ag+ exchange followed by a suitable reduction treatment. The DC electrical resistance of these films has been measured over the temperature range 80-300 K. The resistivity data have been analysed in terms of the Ziman theory of electron-phonon scattering. The effective Debye temperature Theta p has been estimated by fitting the experimental data to Ziman's equation. Theta p is seen to vary from 98 to 192 K for silver particle sizes ranging from 4.3 to 11.0 nm. The silver particle aggregates in the present system have a fractal microstructure with fractal dimensions of around 1.6 and 1.9, respectively.

Thermal expansion, heat capacity, and entropy of MgO at mantle pressures

Abstract: The effect of pressure on the heat capacity, Cv, and entropy, S, of MgO was determined using vibrational spectroscopy to over 200 kbar by the measurement of several vibronic bands in the fluorescence of Cr3+ doped into the MgO lattice. The results for Cv and S demonstrate that the high pressure thermodynamic properties of MgO can be accurately estimated using a Debye model even at room temperature.

Elastic constants, debye temperatures, and electron-phonon parameters of superconducting cuprates and related oxides

Abstract: Using both measurements and modeling, we studied the cohesive and related properties of several oxides. Superconducting oxides include La1.85Sr0.15CuO4, Y1Ba2Cu3O7, and (Bi-Pb)2Sr2Ca2Cu3O10. Related nonsuperconducting oxides include SrTiO3, BaTiO3, and La2CuO4. For these materials, we give the complete quasiisotropic elastic constants corrected to the void-free state. From elastic constants and atomic volume, we calculated Debye characteristic temperatures, Θ D . Using Kresin's model, valid for all values of the electron-phonon parameter, λ, we estimated I from Tc and Θ D . For the superconducting cuprates, λ ranges from 1.3 (La-O) to 10.0 (Tl-O). Except perhaps for Tl-O, these parameters fall within a range predicted by theory. For all the above materials, we show the 295–5-K variation of Θ D . We support our elastic-constant measurements with Born-model calculations of the bulk modulus.

Specific heat of the superconductor YBa2Cu4O8 from 1.5 to 330 K

Abstract: The specific heat of four samples of the 81 K-superconductor Yba 2 Cu 4 O 8 prepared under high oxygen pressure is investigated. The low-temperature specific heat is sample-dependent. For the most homogeneous sample, the coefficient of the linear term at T ⪡ T c is γ ∗ =4.9±0.2mJ/(K 2 mole) and the initial Debye temperature is θ (0)=350±10K. A small specific heat jump Δ C / T c =12 to 16 mJ/(K 2 mole) is observed at the superconducting transition; it is only weakly correlated with the field cooling Meissner fraction (14 to 55% in 20 Oe). A tentative separation of the total specific heat into electron and phonon terms is obtained by a least-squares fit of high-temperature data using a model of the phonon density of states. The electron contribution is found to be much smaller than for YBa 2 CU 3 O 7 . Thermochemical data are tabulated.

Simulations of the premelting of Al(110)

Abstract: The premelting of Al(110) is studied by molecular dynamics simulations using the potential for Al derived from effective medium theory. We find a progressive disordering of the surface region as the bulk melting point is approached. This disorder is mediated by the formation of adatom–vacancy pairs. Based on the calculated radial distribution function and the analytical expression for the energy, it is demonstrated that the thermodynamics of disorder and melting is reflected in the single particle properties by a decrease in the number of nearest neighbors. The Debye temperature for the surface layer as determined from the simulations is θD ∼250 K, compared to θD ∼320 K found for the deeper layers. The thickness of the surface region determined from the Debye temperature is 3–4 layers. From an examination of the properties of some order parameters, we find that the premelting appears anisotropic only for some choices of order parameter. This has implications for the detection of the disorder by a diffract...

On the temperature dependence of the energy gap in PbSe and PbTe

Abstract: A detailed temperature dependence of the energy gap E0 in the PbSe and PbTe was obtained from the transmittance measurements. A non-linear change of E0 in the vicinity of the Debye temperature was noticed. An attempt to find an empirical formula describing the experimental dependence E0(T) led to the conclusion that the non-linear change can be attributed to the electron interaction with the optical phonons.

Lattice site location of Te in GaAs

Abstract: The lattice site of donor Te atoms in GaAs was studied by the channeling technique in order to determine the position of the nonsubstitutional fraction. Single crystals of pure GaAs with a (100) orientation were implanted with 220 keV Te ions to a dose of 3 × 1014 cm−2. After rapid thermal annealing (820°C, 15 s) angular scans through three major axial directions were measured with 3.9 MeV He+ ions. A strong asymmetry, observed in 〈110〉 scans along a (211) plane, is discussed and quantitatively explained by Monte Carlo simulations. Analysis of the data for Te, by comparison with computer simulations, shows that practically all Te atoms are close to As sites, with an average displacement of 0.32 ± 0.04 A . A clear separation in a substitutional fraction and a nonsubstitutional fraction at a unique site is not evident. The best overall fit for the Ga + As RBS spectra was obtained for thermal vibration amplitudes corresponding to a Debye temperature of 260 ± 10 K.

On the derivation of the Debye theory of dielectric relaxation from the Langevin equation in the presence of the driving field

Abstract: It is shown how the Debye results for the behavior of an assembly of dipoles subjected to step‐on, step‐off, and ac fields which are usually obtained by calculating the linear response from the Smoluchowski equation may also be obtained from the Langevin equation by using a transformation of that equation suggested by Frood and Lal. This transformation allows one to calculate the linear response from the Langevin equation even in the presence of a driving field.

New Magnetic Material R3T System With Extremely Large Heat Capacities Used as Heat Regenerators

Abstract: We developed a new magnetic material R3T system, where R is a rare earth metal and T a transition metal, used as a regenerator matrix in a conventional two-stage Gifford-McMahon (GM) type cryocooler. These compounds, especially Er3Ni (Erbium 3 Nickel), possess extremely large heat capacities between 4 K and room temperature because of their low Debye temperatures, about 150 K, and broadened large peaks in specific heat due to complicated ferromagnetic or antiferromagnetic spin configura­tions near each transition temperature, for instance 7 K for Er3Ni.

The thermal expansion of ScAlO3 — A silicate perovskite analogue

Abstract: The crystal structure of ScAlO3 has been refined at temperatures up to 1100° C on the basis of x-ray powder diffraction data. The thermal expansion is adequately described by a Gruneisen-Debye model with the elastic Debye temperature and an effective Gruneisen parameter of 1.6. The volumetric thermal expansion of 3.0% between 10 and 1100° C, corresponding to a mean thermal expansion coefficient of 2.7 × 10−5 K−1, is entirely attributable to the expansion of the AlO6 octahedra. The interoctahedral angles, though not fixed by symmetry, do not vary significantly with temperature —indicating that the expansivities of the constituent AlO6 and distorted ScO8 polyhedra are well matched. Similar considerations of polyhedral expansivity suggest thermal expansion coefficients of ∼2 × 10−5K−1 for cubic CaSiO3 perovskite and a value between 2 × 10−5 K−1 and 4 × 10−5 K−1 for MgSiO3 perovskite. The lower value is consistent with the reconnaissance measurements for Mg0.9Fe0.1SiO3 (Knittle et al. 1986) below 350° C, with low-temperature measurements of single-crystal MgSiO3 (Ross and Hazen 1989), and with the results of some recent calculations. The markedly greater expansivity ∼4 × 10−5 K−1 measured at higher temperatures (350–570° C) by Knittle et al. is inconsistent with the simple Gruneisen-Debye quasiharmonic model and may reflect the marginal metastability of the orthorhombic perovskite phase. Under these circumstances, extrapolation of the measured expansivity is hazardous and may result in the under-estimation of lower mantle densities and the drawing of inappropriate inferences concerning the need for chemical stratification of the Earth's mantle.

X-ray-absorption spectroscopy on strontium titanate under high pressure.

Abstract: The pressure dependence of the interatomic distances and their mean-square deviations (Debye-Waller factors) in ${\mathrm{SrTiO}}_{3}$ are studied by x-ray-absorption spectroscopy in a diamond anvil cell up to 15 GPa. The pressure dependence of Sr-O and Sr-Ti interatomic distances have been determined and compared with x-ray-diffraction data. The comparison confirms the occurrence of a structural phase transition induced by pressure. In the low-pressure phase the Sr-O and the Sr-Ti bond compressibilities are very close to those deduced from x-ray-diffraction data, and a Debye model can describe the pressure dependence of the oxygen and titanium Debye-Waller factors. On the contrary, in the high-pressure phase, the mean Sr-O and Sr-Ti bond compressibilities become smaller, and the Debye-Waller factors increase with pressure, showing an increase of the oxygen and titanium static disorder due to various Sr-O and Sr-Ti distances.

Electrical and optical characterization of GdSi2 and ErSi2 alloy thin films

Abstract: GdSi2 and ErSi2 polycrystalline thin films were studied using electrical resistivity in the temperature range 10–900 K, Hall effect from 10–300 K and reflectivity spectra from 0.2–100 μm at room temperature. Composition and structure in these films were investigated by Rutherford backscattering spectroscopy and x‐ray diffraction techniques. These silicides are metallic with (i) a remarkable difference in their residual resistivity, (ii) a phonon contribution to the resistivity which showed a negative deviation linearity, and (iii) low energy interband transitions. Resistivity data indicated that GdSi2 and ErSi2 have a Debye temperature of 328 and 300 K respectively and a limiting resistivity value much higher than that observed in other transition metal disilicides. The charge carrier concentration was estimated to be 4×1021 cm−3 at room temperature according to Hall measurements, and the mean free path was 63 A and 320 A for GdSi2 and ErSi2, respectively, at 10 K. The parameters obtained by the optical a...

Photoconductive characterization of ZnxCd1−xTe (0≤x≤0.25) single crystal alloys

Abstract: Large grain polycrystalline and single crystals of ZnxCd1−xTe grown by a modified Bridgman method were studied using the photoluminescence and photoconductivity techniques. The temperature dependence of the band gap, as determined by photoluminescence, follows the Varshni equation for measuring temperature in the range of 15–290 K. One of the fitted parameters, the Debye temperature, monotonically decreases with the increase of the atomic zinc concentrations. A a close correlation between the photoluminescence and photoconductivity measurements is also found. Samples in which the photoluminescence spectra exhibit emission bands associated to cadmium vacancies and other structural defects, show a photoresponse curve which includes, in addition to the intrinsic band, another broad band at lower energies. Using the ionization energies of the defect related bands in the photoluminescence spectra we have identified the second band in the photoresponse curve due to the photoexcitation of trapped carriers at lev...

Pressure-dependent phonon properties of III-V compound semiconductors.

Abstract: Using a phenomenological lattice-dynamical theory in the quasiharmonic approximation, we present a comprehensive study to understand the effects of pressure on the vibrational properties of Ga-In pnictides that exhibit a sphalerite crystal structure. The existing pressure-induced Raman scattering data for phonon frequencies, the ultrasonic measurements of elastic and lattice constants, are used as constraints to stringently test the reliability of our rigid-ion model. The effects of high pressure on phonon dispersion curves are shown to lead to a softening in the transverse acoustic modes. At low temperatures this provides a possible driving mechanism for the decrease in the Debye temperature and the occurrence of a negative Gr\"uneisen constant and thermal-expansion coefficient in semiconductors. With a few exceptions (InP, GaAs, and GaSb), our calculated values for several elemental and compound semiconductors have qualitatively satisfied the empirical linear relationship between the Gr\"uneisen parameter ${\ensuremath{\gamma}}_{\mathrm{T}\mathrm{A}(\mathit{X})}$ and the transition pressure ${\mathit{P}}_{\mathit{t}}$. Numerical results for the lattice dynamics, one-phonon and two-phonon density of states, Debye temperature, Gr\"uneisen constant, and linear thermal-expansion coefficient are all shown to be in reasonably good agreement with the existing experimental data.

Low-temperature specific heat of the diluted magnetic semiconductor mercury cadmium iron selenide

Abstract: The specific heat of the diluted magnetic semiconductors Hg1-x-yCdyFexSe has been measured for temperatures below 25K. The excess specific heat (Cm) has been extracted by scaling of nonmagnetic lattice contributions. The resulting Cm of these systems reveals a broad maximum at T?10 K, whereas for low temperatures a Schottky-type exponential decay of Cm can be observed. These phenomena can be fairly well understood on the basis of a simple crystal-field model and a random distribution of Fe2+. On the other hand, no drastic effect of the energy gap is clearly reflected by our data and therefore we suggest that only Fe2+ states involving the valence bands dominate the antiferromagnetic d-d interactions, analogously to the Mn2+ case.

Surface structure of VN0.89(100) determined by low-energy electron diffraction.

Abstract: The structure of the (100) surface of substoichiometric vanadium nitride was studied by low-energy electron diffraction on a ${\mathrm{VN}}_{0.89}$(100) sample. A simple 1\ifmmode\times\else\texttimes\fi{}1 (100) diffractogram was observed. To describe the electron scattering in substoichiometric VN we apply the averaged t-matrix approximation to the nitrogen atoms. We find that the best structural model is one having no nitrogen vacancies in the surface region. It turns out that the first layer is rippled with the N atoms displaced 0.17 \AA{} above the subplane of V atoms, that the spacing between this subplane and the second layer is 1.92 \AA{}, and that the spacing between the second and the third layer is 2.08 \AA{}. In relation to the (100) spacing of the bulk, 2.06 \AA{}, these spacings are 6.8% contracted and 1% expanded, respectively. The Debye temperature of VN is found to be 660 K in good agreement with a prediction from entropy data and from neutron diffraction and helium-ion channeling experiments.

Atomic vibrations in a self-consistent-field atom-in-jellium model of condensed matter.

Abstract: The electronic structure of many of the elemental solids is fairly well represented by a single atom embedded in a degenerate electron gas. This is particularly true for bulk properties such as the equation of state of highly compressed matter. Because the atom-in-jellium model is spherically symmetric, it is simpler than band-structure models. We study what happens when the nucleus is moved off center in the atomic cell to form a nonspherical system. This forms the basis of an Einstein model of atomic vibrations. The model is used to calculate Einstein temperatures and Grueneisen constants of simple solids by self-consistent-field electronic-structure methods.

Specific heat of the superconductor Yba2Cu4O8 from 1.5 to 300K

Abstract: The specific heat of selected ceramic samples of the 81k-superconductor YBa 2 Cu 4 O 8 prepared under an oxygen pressure of 85 bar is investigated. For the most homogeneous sample, the coefficient of the “linear” term at T≪T c is γ * =4.9±0.2 mJ/(K 2 mole), and the initial Debye temperature is Θ(0)=350±10K. The specific heat jump at T c2 , ΔC/T c −16 mJ/(K 2 mole), and the Sommerfeld constant evaluated at high temperatures, γ−12 mJ/(K mole), are much smaller than those of YBa 2 Cu 3 O 7 .

Specific heat of the high-Tc superconductor (Bi, Pb)2Sr2Ca2Cu3O10, and related phases Ca2CuO3 and (Ca0.86Sr0.14)CuO2 from 0.4 to 20 K

Abstract: Specific heat, Cp, data from 0.4 to 20 K for four polycrystalline specimens of (Bi, Pb)2Sr2Ca2Cu3O10 are reported, and compared with similar measurements made on Bi2Sr2CaCu2O8. Both the upturn in Cp/T and the coefficient of the linear term, γ0, are sensitive to the phase composition of the specimen. γ0 decreases from 10.8 mJ/(mol K2) in the specimen that has roughly equal proportions of the low-Tc (2:2:1:2) and high-Tc (2:2:2:3) phases to zero in the specimen that is close to 100% 2:2:2:3 phase. The variability of γ0 with phase purity suggests that it is extrinsic to the superconducting state of (Bi, Pb)2Sr2Ca2Cu3O10. Above 4 K, Cp of (Bi, Pb)2Sr2Ca2Cu3O10 shows a rapid departure from simple T3 (Debye) behavior due to dispersion and the excitation of low-lying optic modes. Values obtained for the Debye temperature, θD0, are in the range 276–292 K for (Bi, Pb)2Sr2Ca2 Cu3O10 (cf. 253 ± 7 K for Bi2Sr2CaCu2O8). To better understand how other copper-oxide compounds, which may appear as impurity phases in this system, contribute to Cp of (Bi, Pb)2Sr2Ca2Cu3O10 we have carried out Cp measurements between 0.4 and 20 K on Ca2CuO3 and on the structural parent phase (Ca0.86Sr0.14)CuO2. For Ca2CuO3, no upturn in Cp/T is observed and values of 1.63 mJ/(mol K2) and 516 K are obtained for γ0 and θD0, respectively. (Ca0.86Sr0.14)CuO2 behaves somewhat differently with an upturn in Cp/T, γ0=0 mJ/(mol K2) and θD0=496 K.

Physical properties of amorphous Al90Fe5Ce5

Abstract: Amorphous Al90Fe5Ce5, prepared by rapid solidification from the melt, shows unusual tensile strength. The authors have investigated various physical properties of this alloy. X-ray diffraction measurements indicate a mean interatomic spacing comparable to that in FCC aluminium. Thermal analysis studies show an onset of crystallisation at 543 K, corresponding to the precipitation of FCC Al in an amorphous matrix. A second exothermic reaction at 593 K corresponds to the crystallisation of the remaining amorphous component into FCC Al and an intermetallic phase, possible Al(Ce, Fe)3. The electrical resistivity is well described by the Ziman-Faber theory with a Debye temperature of approximately 200 K. Magnetic susceptibility shows the alloy to be paramagnetic and to exhibit combined Curie and Pauli behaviour. 57Fe Mossbauer effect measurements show the short range ordering of this alloy to be similar to that found in transition metal-metalloid glasses.

Properties of 57Fe hot-implanted into diamond crystals studied by Mössbauer emission spectroscopy between 4 and 300 K

Abstract: We investigated the hyperfine interactions of 57 Fe ions implanted into diamond crystals that were heated to 830 K during implantation. The implantation of radioactive parent 57 Co ions was performed to a fluence of 2 × 10 13 ions/cm 2 , using the 50 kV isotope separator. The Mossbauer emission spectra were measured in the temperature range between 4 and 300 K. using a helium flow cryostat. About 20% of the implanted ions end up in high-symmetry (HS, presumably substitutional) sites, with the remaining 80% of ions appearing in low-symmetry (LS, presumably hexagonal interstitial) sites. The HS sites are characterized by very high Debye temperature θ D ~ 1300 K and by high s-electron density characteristic for Fe 4+ (3d 4 sp 3 ) electron configuration. Impurities at LS sites exhibit a Debye temperature of 550 ± 50 K and a very large electric field gradient, V zz = 1.2 × 10 18 V cm 2 . An anomalous rise of the recoil-free factor for LS sites below 100 K was observed. This effect can be an evidence of localized jumps of interstitial impurity atoms between adjacent tetrahedral and hexagonal positions in the host lattice. The values of the s-electron densities and electric field gradients at Fe nuclei implanted in diamond are compared with the corresponding values in other IV-group matrices and are found to be correlated with the atomic volumes of the corresponding host elements.

Correlation between T c and Eu Lattice Vibration in T'-Phase Superconductor (La 1-x Eu x ) 2-y Ce y CuO 4-δ Observed by 151 Eu Mössbauer Effect

Abstract: The electronic state and lattice vibration of Eu in T'-phase (La1-xEux)2-yCeyO4-δ was studied by 151Eu Mossbauer effect. The isomer shift and the Debye temperature of Eu, \varThetaD, showed that Eu is trivalent, and the Eu-O distance is larger than those in the hole-doped superconductors. With increasing Eu concentration, in spite of the shortening of the lattice constants, \varThetaD in T'-phase decreased. This is probably caused by the increase in the Eu-O distance with the change of Eu-O-La bonding into Eu-O-Eu bonding. A tight correlation between Tc and \varThetaD was found and the relation, Tc=0.772\varThetaD exp (-\varThetaD2/13000), was derived.

On the Possibility of Describing Lattice Properties of Iridium in Terms of Pseudopotential Theory

Abstract: The possibility is discussed of describing lattice properties of iridium in terms of pseudopotential perturbation theory with allowance for second and third order. Values of pseudopotential parameters are proposed which provide a good description of the lattice properties of iridium under zero pressure, the contributions of the three-ion forces being small. The total energy, pressure, elastic moduli, Debye temperature, low-temperature Gruneisen parameter, pairwise potentials, and also effective solid sphere packing parameters are calculated. In addition, by invoking molecular dynamic methods, the vacancy formation and migration energies and the stacking fault energies are computed. The electron energy spectrum and group velocity calculations suggest that the electron density of states curve of Ir exhibits no van Hove singularities near the Fermi level, a fact which apparently accounts for the successful description of lattice properties in terms of the pseudopotential perturbation theory. [Russian Text Ignored].

Characterization of the Surface Oxide Layer on Iron Particles for Magnetic Recording by Mössbauer Spectrometry

Abstract: The average valency of iron in the oxide layer on iron particles for magnetic recording and the saturation magnetization of the oxide layer were estimated to be 28 and 39 emu g−1, respectively, by examining the Debye temperature and the recoilless fraction of oxide layer which were calculated with the Mossbauer parameters

Core-Hole Lifetime and Screening are Different in the Surface of W(110)

Abstract: High resolution 4f photoemission spectra from clean W(110) show that the natural lifetime width and the singularity index, which characterizes the conduction electron screening, are both larger in the first atomic layer than in the bulk. The phonon broadening of the surface and bulk components are smaller than earlier theoretical estimates. The excess broadening at the surface is compatible with a simple Debye model. These findings are very different from the interpretation previously given surface-atom core-level line shapes and have implications extending to other systems.