Showing papers on "Debye model published in 1985"

GaAs, AlAs, and AlxGa1−xAs: Material parameters for use in research and device applications

Abstract: The Al x Ga1−x As/GaAs heterostructure system is potentially useful material for high‐speed digital, high‐frequency microwave, and electro‐optic device applications Even though the basic Al x Ga1−x As/GaAs heterostructure concepts are understood at this time, some practical device parameters in this system have been hampered by a lack of definite knowledge of many material parameters Recently, Blakemore has presented numerical and graphical information about many of the physical and electronic properties of GaAs [J S Blakemore, J Appl Phys 5 3, R123 (1982)] The purpose of this review is (i) to obtain and clarify all the various material parameters of Al x Ga1−x As alloy from a systematic point of view, and (ii) to present key properties of the material parameters for a variety of research works and device applications A complete set of material parameters are considered in this review for GaAs, AlAs, and Al x Ga1−x As alloys The model used is based on an interpolation scheme and, therefore, necessitates known values of the parameters for the related binaries (GaAs and AlAs) The material parameters and properties considered in the present review can be classified into sixteen groups: (1) lattice constant and crystal density, (2) melting point, (3) thermal expansion coefficient, (4) lattice dynamic properties, (5) lattice thermal properties, (6) electronic‐band structure, (7) external perturbation effects on the band‐gap energy, (8) effective mass, (9) deformation potential, (10) static and high‐frequency dielectric constants, (11) magnetic susceptibility, (12) piezoelectric constant, (13) Frohlich coupling parameter, (14) electron transport properties, (15) optical properties, and (16) photoelastic properties Of particular interest is the deviation of material parameters from linearity with respect to the AlAs mole fraction x Some material parameters, such as lattice constant, crystal density, thermal expansion coefficient, dielectric constant, and elastic constant, obey Vegard’s rule well Other parameters, eg, electronic‐band energy, lattice vibration (phonon) energy, Debye temperature, and impurity ionization energy, exhibit quadratic dependence upon the AlAs mole fraction However, some kinds of the material parameters, eg, lattice thermal conductivity, exhibit very strong nonlinearity with respect to x, which arises from the effects of alloy disorder It is found that the present model provides generally acceptable parameters in good agreement with the existing experimental data A detailed discussion is also given of the acceptability of such interpolated parameters from an aspect of solid‐state physics Key properties of the material parameters for use in research work and a variety of Al x Ga1−x As/GaAs device applications are also discussed in detail

Superheating of small solid-argon bubbles in aluminum.

Abstract: Small solid-Ar bubbles, formed by the implantation of 50-keV Ar+ ions into Al, are observed to melt at 730 K, compared with an equilibrium bulk melting temperature of 250 K. A model based on the suppression of Ar thermal vibrations at the Al interface is used to account for the superheating of 480 K. Measurements on the temperature dependence of diffracted electron intensities from these bubbles yield Debye-Waller factors consistent with an effective Debye temperature of about 140 K, compared with a bulk value of 110 K.

Melting curves of molecular hydrogen and molecular deuterium under high pressures between 20 and 373 K

Abstract: We determined the melting curve of molecular hydrogen and molecular deuterium at closely spaced intervals from 20 to 373 K by two different techniques using high-pressure diamond cells The cells were loaded with liquid at low temperature or with compressed gas at room temperature Empirical functions for the melting curves were evaluated from least-squares fits of the data Values of the compressibility and Debye temperature were computed at melting, and the results are compared with those calculated from various theoretical models The good agreement shows that the models are generally valid, although small systematic deviations may point the way toward refinements in modeling Our study also demonstrates the need to determine a one-piece intermolecular potential valid over a wide pressure range by refitting all experimental data, including the shock data recently made available

Magnetic contributions to the thermodynamic functions of pure Ni, Co, and Fe

Abstract: An empirical mathematical equation is proposed for the magnetic contribution to the specific heat of pure metals The corresponding functions for enthalpy, entropy, and Gibbs energy are of simple form Two parameters used for each element are the critical temperature,Tc, and the total magnetic entropy The parameters have been determined from a careful separation of magnetic and nonmagnetic contributions to the specific heat Debye temperatures for Ni, Co, and Fe have been determined considering data to much higher temperatures than other studies The magnetic specific heats extracted from experimental data agree very well with the proposed equation over the entire temperature range and for all three elements Comparisons with different mathematical functions found in the literature give agreement only for the case of iron The total magnetic entropy given by a classical relation is found to be high, and a quantitative correction is given Various magnetic standard states are discussed The lattice stabilities of bcc- and fcc-iron are calculated assuming that the difference of the nonmagnetic specific heats is linear from 500 K to 1810 K A simple equation is obtained in which the anomalous temperature dependence is explained by the independently determined magnetic contribution The calculated values agree very well with Orr and Chipman’s assessment The stability of bcc iron at low temperatures is quantitatively rationalized

High frequency rotational mode in liquid methyl chloride

Abstract: Complex permittivity measurements in liquid methyl chloride have been completed over the whole frequency range at temperatures and densities scanning the orthobaric curve, from 176 K up to 363 K. A report on the experimental set-ups and the accuracies obtained is given. Relaxation parameters are derived in the frame of Debye theory, and Hill's distribution of damped resonances is used to construct a model for librational absorption. A new absorption process superimposed upon the main librational and relaxational absorption-dispersion patterns is observed. Coherent rotational features show up in both complex plane representations of permittivities (Cole-Cole) and polarizabilities (Scaife). The spectral density of the new features corresponds to a resonant phenomenon rather than a secondary relaxation process as shown by its insensitivity to temperature and density variations. Its relevance to the dipolaron formalism is examined, pointing to a balance between positive clues, such as dipolar solvation, and i...

Nonperturbative theory of temperature-dependent optical dephasing in crystals. III. Comparison with experiment

Abstract: We have applied the results of the nonperturbative theory of zero‐phonon linewidths of impurities in crystals discussed by Osad’ko and us, and further developed by us in papers I and II of this series, to the analysis of several absorption, photon echo, and hole burning experiments. Two (relatively) high temperature absorption experiments on 1,3‐diazaazulene in naphthalene and dilute ruby were analyzed with a model of Debye acoustic phonons. In both cases the Debye temperature was obtained from independent experiment or theory, and a one‐parameter fit was performed on the temperature‐dependent linewidth. It was found that (especially for diazaazulene) the systems are not in the weak coupling limit. For several low temperature experiments, where the dephasing is presumably due to pseudolocal phonons, the nonperturbative theory, coupled with the results of deBree and Wiersma, provides a reasonably complete understanding of the observed dephasing rates.

A simple estimation of the bulk module of ternary chalcopyrite semiconducting compounds by means of the debye characteristic temperature

Abstract: Estimation des modules de compression B de chalcopyrites ternaires en utilisant l'equation θ B =a 1 x+a 2 (a 1 , a 2 : constantes, x: B 1/2 ρ −1 ' 6 m −1/3 , ρ: densite, m: masse) dans laquelle θ B est la temperature caracteristique de Debye, evaluee a partir de la chaleur massique ou des constantes d'elasticite a 0 K

Optical Absorption Edge Investigation of CdIn2S4 and β‐In2S3 Compounds

Abstract: CdIn2S4 and β-In2S3 crystals are grown by direct melting of stoichiometric mixtures of their components. The absorption coefficients are measured as a function of the wavelength of the incident light between 500 to 750 nm at various temperatures. For both compounds the energy gap Eg is temperature dependent and the absorption edge shifts to lower energy values with increasing temperature. For β-In2S3 it is found that the absorption edge is mainly due to direct transitions and the temperature dependence of Eg is linear. At room temperature the value Eg = 2.00 eV is measured. In the case of CdIn2S4 there appear indirect transitions for low values of the absorption coefficient while the temperature dependence of the direct energy gap is linear at high temperatures and quadratic at low temperatures. At room temperature the direct gap has the value Eg = 2.21 and the indirect one E = 2.05 eV. A rough estimation of the Debye temperature gives θD = 665 K.

Specific heat and thermal expansion in GdxY1-xCu2

Abstract: Specific heat and thermal expansion experiments have been performed on GdxY1-xCu2 compounds between 1.2K and 50K and from 1.4K up to 77K, respectively. The specific heat data are analysed in an electronic, lattice and magnetic part resulting in a gradual variation of the Debye temperature with composition and in a magnetic entropy proportional to the number of Gd atoms. From the thermal expansion data below TN a magnetic contribution to the volume at zero temperature of negative sign is deduced. The magnetic volume is roughly proportional to x2 and is attributed to the loss of short-range polarisation at the antiferromagnetic spin configuration. By comparing the specific heat and thermal expansion data, the electronic, lattice and magnetic Gruneisen parameters have been determined.

Phonon anomalies and local atomic displacements in the exchange-enhanced superconductor U6Fe.

Abstract: Moessbauer effect and neutron-diffraction measurements have been made as a function of temperature on the exchange-enhanced superconductor U/sub 6/Fe. Non-Debye behavior is observed in the thermal mean-squared displacement of Fe with the Debye temperature changing from THETA/sub f/approx. =320 K above 100 K to THETA/sub f/approx. =400 K below 80 K. Changes in the Moessbauer shift below 100 K are attributed principally to electronic-structure changes of the Fe atom which lead to a small increase in the number of d electrons and a concomitant reduction in the number of s electrons. The neutron-diffraction results indicate a pronounced anisotropy in the unit-cell thermal expansion and below 100 K an anomalous temperature dependence for the c-axis component of the total Debye-Waller factor of Fe. These observations taken together imply that along the c axis, static displacements of the Fe atoms begin around 100 K and increase with decreasing temperature.

Low-temperature specific heat and electrical resistivity measurements of hydrogen-absorbed Pd-Zr and Ni-Zr metallic glasses

Abstract: The low-temperature specific heats (1.5–6 K) and the electrical resistivities (2–300 K) have been measured on liquid quenched Pd 0.33 Zr 0.67 H x ,Pd 0.35 Zr 0.65 H x and Ni 0.35 Zr 0.65 H x (0≤ x ≤1.2) ternary metallic glasses. Low-temperature specific heat data showed a substantial reduction in both the superconducting transition temperature and the density of states at the Fermi level but an increase in the Debye temperature upon hydrogenation. The results can be interpreted on the basis of the fact that hydrogen atoms tend to be trapped in tetrahedral sites surrounded by Zr-atoms. The electrical resistivities in the range 30–300 K can be well fitted to an empirical equation ρ/ρ 0 = A + B exp (- T / Δ ) for all alloys studied. Attention is focused on the Mooij correlation in the resistivity range 200–300 µΩ·cm and also on the presence of a correlation between the characteristic temperature Δ and the Debye temperature \(\varTheta_{\text{D}}\).

Thermal disorder in Pt and Ir metal: Comparison of model and experimental extended x-ray-absorption fine structure

Abstract: Extended x-ray-absorption fine-structure (EXAFS) spectra were measured for Pt (bulk foil) and Ir (sintered supported catalyst) over the temperature range from 10 to 822 K and evaluated for the effects of thermal disorder on amplitude and phase. A procedure based on parallel analysis of experimental EXAFS spectra and model EXAFS spectra derived from an adjustable pair interaction potential is used to deduce structural and disorder parameters corrected for observed disorder-related effects present in conventional analysis methods. Comparison of results with a correlated Debye theory of the effect of disorder on EXAFS spectra shows close agreement.

Theory of lattice-dynamical properties of compressed solids

Abstract: The Thomas-Fermi-Dirac statistical model is generalized to treat the mechanical and thermal properties of compressed solids where the effect of the periodic lattice is taken into account in the muffin-tin approximation. A general lattice-dynamical theory is presented in the same approximation, where the core effect on the dynamical matrix is treated consistently with the statistical model. Calculated results are given for the elastic constants, the Debye temperature, the Gr\"uneisen constant, and the melting temperature for Al and partly for Fe covering a wide range of density. The present theory has been examined particularly for phonon spectra at the normal density, and is in good agreement with the experiments for Al.

Anion Ordering and Atomic Vibrations in (TMTSF)2ClO4

Abstract: We report the first observation of the specific heat anomaly associated with anion ordering in (TMTSF)2ClO4. The phase transition analyse reveals a configuration entropy R Log (2) which can be reduced by 20% on fast cooled sample. The low temperature lattice specific-heat deviate strongly from the prediction of a 3D debye model.

Specific heat of GdRh

Abstract: GdRh, a ferromagnetic intermetallic, is of interest as a working material for magnetic refrigeration. The requirements for such materials have been discussed by Barclay et al. and it is in this context that we analyze our data. Magnetic measurements of Buschow et al. have indicated TC =24 K while specific heat measurements of Olijhoek et al. have shown a maximum at 20 K and have yielded a magnetic entropy of 16.7 J/mole K. No Debye temperature has been reported previously. We have measured the specific heat from 1.8 to 77 K and our data show a peak at 19.93 K that is 17% higher than in the previous work. Fitting of the data from 40 to 77 K to C=A/T2+D(T/θ{T}), where A and θ{T} are adjustable parameters and D is the Debye function, yields a Debye temperature of 198±2 K. For the magnetic entropy we obtain 17.4 J/K, very close to R ln 8.

Thermodynamic Assessment of the System CoO‐MnO

Abstract: The thermodynamic properties of pure MnO and NiO were analyzed in terms of the Debye model and a model for the magnetic transitions. The classical formula R In (β+ 1) for the magnetic entropy was found to overestimate the effect of magnetic ordering in these systems. A previous interpretation of the data for NiO was corrected. Thermodynamic functions were derived for the solid and liquid states and are given as analytic expressions. A previous assessment of the Gibbs energy of the solid solution (Mn, Ni)O from activity data was modified on statistical grounds. The results indicate that there should be a miscibility gap below 41O°C. By estimating the Gibbs energy of the liquid phase, it was possible to calculate the complete phase diagram.

X-ray determination of mean Debye-Waller factors and Debye temperatures of the KxRb(1−x)Br mixed crystal system

Abstract: The integrated intensities of Bragg reflections have been measured for mixed crystals in the KxRb(1−x)Br system with an x-ray powder diffractometer. From the intensities, the mean Debye-Waller factors are determined. The Debye-Waller factors are corrected for static contribution and Debye temperature values are determined for the entire composition range. The x-ray Debye temperatures follow the Kopp-Neumann equation closely.

Normal vibrations in nickel, palladium, and platinum

Abstract: Calculations of the phonon frequencies, temperature variation of Debye temperature, and the Debye–Waller factor for nickel, palladium, and platinum have been made by employing a four parameter model. Expressions for electron pressure and equilibrium condition which preserves the crystal stability are derived using the Coulombian potential. The model satisfies the symmetry requirements of the lattice and the lattice is in equilibrium without recourse to external forces. A comparison between the theoretical results and the available experimental data exhibits good agreement.

Normal and superconducting state properties of U6Fe at low temperatures and high magnetic fields

Abstract: High purity U 6 Fe polycrystals with a resistance ratio rrr =9 at T =4 K are found to exhibit a record onset temperature (T 0 > 4.0 K) to superconductivity for U materials. Our measured values of the electronic coefficient of heat capacity γ ∗ =(150±3) mJ/mol · K 2 , the mean Debye temperature Θ ( T →0)=166 K and the normalized jump in heat capacity at T c , Δ / γT c =2.3±0.1, are all in good agreement with previous work † , and suggest that U 6 Fe is a strong-coupled superconductor. Resistive upper critical field measurements on high purity samples revealed a pronounced S-shaped curvature of H c2 ( T ) and evidence for anisotropy, similar to effects seen in UPt 3 ‡ , but not observed in lower purity U 6 Fe samples † . We find H c2 (0) > 10T and transition widths ΔT c ∝H 1 2 at moderate fields, followed by an abrupt increase in Δ T c for H>8T. Our H c2 data cannot be explained by existing theoretical models. Preliminary heat capacity measurements in magnetic fields 0⩽H⩽10T suggest that γ ∗ may decrease by ≈10% at H =10 T . These results and electrical resistivity data will be discussed in terms of a potential charge density wave instability in U 6 Fe at T⪅110 K.