Showing papers on "Debye model published in 1976"

Transport properties of dense matter

Abstract: Transport properties of stellar matter are important ingredients in understanding stellar evolution and stellar structure. Using theoretical techniques that have proven useful in solid state physics, Fermi liquid theory, and the theory of liquid metals, we have calculated the electron contribution to the electrical conductivity, the thermal conductivity, and the viscosity of neutron star matter in the absence of magnetic fields for densities less than 2 x 10/sup 14/ g cm/sup -3/ (regions where there is solid matter). The results are also applicable to high-density white dwarf matter. Variational solutions of the transport equations are used except where exact solutions exist. Transport by electrons of any energy both in the presence of a lattice and without the lattice present are considered. At temperatures very much below the Debye temperature of the lattice we have been able to do only an approximate treatment of the electron-phonon interaction, and for temperatures above the melting temperatures of the lattice ionic correlations are included. Relativistic electron-electron scattering is considered, as is electron-impurity and electron-neutron scattering. This work complements previous investigations of transport properties and extends the density regions over which we know the transport properties of dense matter. (AIP)

Elastic properties of UO2 at high pressure

Abstract: The elastic constants of UO2 have been measured as a function of pressure up to 20 kbar (2.0 GPa) at 23 °C. To within experimental uncertainty, the constants are linear functions of pressure, and no pressure‐induced phase transitions occur in this 20‐kbar pressure range. The elastic constant data are used to estimate the various polycrystalline elastic moduli and their pressure dependences, and the zero‐pressure moduli are compared with data from polycrystalline material. Isothermal moduli are calculated from the adiabatic values. The elastic‐constant data are also used to discuss the separation of the ’’pure’’ volume and temperature contributions to the (isobaric) temperature dependences of the elastic constants as well as to estimate the Gruneisen constant, Debye temperature, and equation of state of this material.

Specific Heat Study of Magnetic Ordering and Band Structure of 3d Transition Metal Disulfides Having the Pyrite Structure

Abstract: Specific heats of FeS 2 -CoS 2 -NiS 2 Solid solutions and of Ni 0.95 Cu 0.05 S 2 have been measured from 10 K to 350 K. Debye temperatures, coefficients of electronic contribution γ, magnetic ordering temperatures, and magnetic entropies have been determined. The magnetic entropy of ferromagnetic Fe 1- x Co x S 2 is proportional to cobalt concentration and is 40% of that expected for a localized spin 1/2 per Co atom, suggesting an itinerant character of e g electrons. The magnetic entropy of NiS 2 is smaller than that of a localized spin 1 and is in good agreement with that expected for the magnetic moment observed by neutron diffraction. The magnetic entropy of antiferromagnetic Co 1- x Ni x S 2 Suggests that only Ni atoms have magnetic moment in Ni-rich phase. The density-of-states curve of e g band for NiS 2 has been constructed from γ values assuming a rigid band. The band width of lower half of e g band is 0.6 eV. The Debye temperature decreases monotonously from 605 K of FeS 2 to 455 K of NiS 2 .

Analysis of heat transfer between solids at low temperatures

Abstract: A detailed analysis is given of the acoustic mismatch formulation first given by Little for the thermal contact resistance between solids for the case of phonon transport in a Debye model. Extrema ...

Molecular motions in liquid. I. Rotation of water and small alcohols studied by deuteron relaxation

Abstract: The deuteron spin lattice relaxation time T1 of CH3OD, C2H5OD, (CH3)2CHOD, and (CH3)3CHOD is reported as a function of temperature and frequency. The deuteron T1 of the alcohols in dilute benzene solutions was also measured at 25°C. A detailed analysis of the experimental results and other magnetic and dielectric relaxation data from the literature was made. Based upon the analysis, it is suggested that the success of the Debye model in explaining the dielectric and magnetic relation behavior of water and small alcohols may be purely coincidental. It seems that the error in applying Stokes law for the rotation of a macroscopic sphere to small molecules in liquid and the error in assuming that water and alcohols rotate as monomers largely cancel each other, yielding much better apparent agreement between calculated and experimental data than those for nonassociating liquids. The deuteron T1’s for the alcohols are further analyzed in terms of anisotropic rotation.

Theory and analyses of the ac characteristics of defect thin‐film insulators

Abstract: In the past, ac properties obtained from thin‐film metal‐insulator‐metal (MIM) samples have often been analyzed qualitatively in terms of the Debye relaxation process. Here we point out several anomalies and discrepancies associated with these analyses. We then go on to develop the theory of ac electrical properties in MIM systems in which Schottky barriers are assumed to exist at the metal‐insulator interface. The resulting capacitance and conductance vs frequency, temperature, and voltage bias are shown to exhibit all the salient features of the observed data, suggesting that such a model is more applicable than the Debye model, at least in the case of the materials reviewed.

Electron paramagnetic resonance in nickel fluosilicate

Abstract: EPR data for NiSiF6⋅6H2O at Ku band and Ka band between 77 and 298 °K were fitted to an isotropic g value of approximately 2.25 with ‖D‖ varying from (0.166±0.005) cm−1 at 77 °K to (0.51±0.01) cm−1 at 298 °K. The values g∥=2.26±0.01 and ‖D‖= (0.125±0.005) cm−1 were obtained from a Ku band measurement at 4.2 °K. The magnitude of the temperature gradient ∂D/∂T had a broad maximum of about 1.8×10−3 cm−1/ °K near 200 °K, decreasing to about 1.5×10−3 cm−1/ °K near room temperature. The spin–lattice relaxation rate was determined from linewidth measurements to vary as T2 above 200 °K. From an expression for the Raman process in the high temperature limit, the Debye Temperature ϑD was deduced to be approximately 107 °K.

Temperature Dependence of Electrical Conductivity in Films of Fine Particles

Abstract: The dc electrical resistance of films of Ag and Al soot which consists of fine particles has been measured between 1.5 and 300 K. The specimens are prepared by the gas evaporation method. The resistance is constant in the low temperature region, while it increases with temperature above a certain point. By analyzing the temperature dependence of the resistance, the Debye temperature for fine particles \(\varTheta_{P}\) was estimated. It was about 1/2∼2/3 of that of bulk metal for Al of the particles of size 150∼200A and 3/4∼5/6 for Ag of 100∼300A.

Theoretical study of gas-solid energy transfer: An isotropic Debye model

Abstract: We present a model calculation for gas–solid energy transfer in which the solid is approximated as an isotropic Debye solid. For a given initial state of the solid (described by phonon occupation numbers), only states differing from this arbitrary initial state by one phonon (created or destroyed in an arbitrary state) are included in the basis set describing the solid. Using this basis set, a close‐coupling calculation is carried out describing energy transfer during a gas–solid collision. Since the scattering calculation does not employ perturbation, all states in the basis set are coupled. Averages over phonon momenta and directions are carried out before the calculation, which results in considerable simplification. Comparisons are made with He+Ag(111) experimental results. Qualitative agreement is good, and quantitative agreement is within a factor of 2. Our model tends to underestimate inelastic effects. The importance of kinematic and geometric considerations is stressed.

Self-Consistent Einstein Model and Surface Debye Temperature

Abstract: On the basis of a self-consistent Einstein model, which has been applied to a theory of melting of metallic fine particles in a previous publication, we discuss the problem of the surface Debye temperature. A semi-infinite metal crystal with a plane surface is considered and the vibrational frequency of each atom is self-consistly determined as function of atomic position. By making use of a simplified interatomic potential, an expression for the difference between square of the bulk and surface Debye temperatures is derived.

Helium diffraction from tungsten (112)

Abstract: The diffractive scattering of helium from W(112) in the [110] azimuth is presented for incidence angles from 20° to 70°, surface temperatures from 1000 to 1600 K, and beam temperatures from 300 to 650 K. The first order beams are the only nonspecular features observed and their angular positions and relative intensities are reproduced well by semiclassical scattering calculations. Debye–Waller analysis of thermal attenuation gives a surface Debye temperature lower than the bulk, but larger than that derived from electron scattering.

The low-temperature heat capacities of Tb, Lu and Y

Abstract: The heat capacities of Tb, Lu and Y, refined by solid state electro-transport processing have been measured between 1.5 and 16K. Below 4K the results were fitted to the expression C= gamma T+AT3 where for Tb the nuclear and magnetic contributions were first calculated and subtracted from the total heat capacity. The resultant values of gamma (mJ mol-1K-2) and limiting Debye temperatures theta D(T to 0) were as follows. Tb: gamma =4.4+or-0.1, theta D=178+or-2K; Lu: gamma =6.8+or-0.1, theta D=205+or-3K; Y: gamma =8.2+or-0.1, theta D=248+or-3K. The Debye temperature was found in all instances to decrease by about 10% between 4 and 16K.

Elastic Constants of ReO3

Abstract: Elastic constants of ReO 3 were determined between –195°C and 30°C by a rectangular parallelepiped resonance technique. It was found that the elasticity of ReO 3 is highly anisotropic. Discussions were made on the Debye temperature, intermolecular force constants, and the effective charges of the ions.

Thermoelastic properties of ULE® titanium silicate glass

Abstract: The elastic moduli, and their temperature and pressure derivatives, for ULE titanium silicate glass, have been measured by the ultrasonic pulse superposition technique. From the extrapolated 0 K values of the elastic moduli and their pressure derivatives, the elastic 0 K Debye temperature and Gruneisen constant have been evaluated, and compared with the thermally measured values. The same anomalies in the dynamic propeties as found in other glasses, i.e. positive temperature and negative pressure derivatives, are also found for ULE glass. The correlation between the changes in the bulk modulus and additive in silica-rich glasses is discussed.

The Mössbauer Effect

Abstract: The study of recoilless nuclear resonant absorption or fluorescence is more commonly known as Mossbauer spectroscopy. From its first origins in 1957, it has grown rapidly to become one of the most important research methods in solid-state physics and chemistry.

Debye temperature of MgO powder by elastic neutron scattering

Abstract: The Debye-Waller exponent B and the Debye temperature ΘD for a powdered sample of MgO has been determined at room temperature by elastic neutron scattering using a triple-axis spectrometer. Detailed calculations were performed to estimate the thermal diffuse scattering (TDS) under the Bragg peaks. The TDS contribution proved to be negligible. The value obtained for B is 0.354 ± 0.008 A2 which corresponds to a ΘD of 743 ± 8 K. The ΘD value obtained here for a powdered sample is about two percent lower than the corresponding value obtained from the inelastic neutron scattering for a single-crystal sample.

Theoretical study of energy transfer in molecule–solid collisions: H2, D2+Ag(111)

Abstract: A quantum mechanical calculation of the angular distribution of H2 and D2 scattered from a Ag(111) surface is presented Energy transfer between the gas and the solid is included using an isotropic Debye model of the solid The coupling of the rotational levels of H2, D2 with the phonon modes of the solid is investigated treating the molecules as rigid rotors The results indicate that the mass difference between H2 and D2 is more important than the different rotational energy spacing in determining the angular scattering pattern Differences between the present results and classical ’’cubes’’ models are observed and compared with experimental results

Critical-magnetic-field curve of lanthanum sesquicarbide

Abstract: The critical-magnetic-field curve of lanthanum sesquicarbide has been measured. The initial slope was extracted from these data and used, along with measured values of the upper and lower critical fields, to obtain the electronic coefficient of specific heat and the Ginzburg-Landau $\ensuremath{\kappa}$ value. Experimental evidence indicates the absence of paramagnetic limitation. Values for the electron-phonon coupling constant and the band-structure density of states were also obtained after estimating the Debye temperature for this material.

Low-temperature specific heat and superconductivity of α -phase A u − Ga alloys

Abstract: Low-temperature specific-heat measurements are reported on ..cap alpha..-phase Au--Ga alloys. The electronic specific-heat coefficient ..gamma.. is observed to increase upon alloying at the initial rate dln..gamma../dz = 0.95. The Debye temperature is observed to decrease in the alloys at the rate -1.63 K/at.%. The increase in ..gamma.. is compared with increasing values of lambda, the electron--phonon mass enhancement as evaluated from measurements of the superconducting transition temperature. Comparison is also made between the values of ..gamma.. and lambda for Ag--Ga and Cu--Ga ..cap alpha..-phase alloys. We find that the band-structure density of states at the Fermi level, N/sub bs/, for Au--Ga and Ag--Ga alloys is a weakly increasing function of composition, while for Cu-Ga alloys it is slightly decreasing and in good agreement with the calculations of Lasseter and Soven. (AIP)

Low-temperature heat capacity, thermal expansion and Gruneisen parameters for SnTe

Abstract: Measurement of the heat capacity from 1.8 to 30K and the thermal expansion from 1.5 to 35K, from 65 to 90K and at room temperature have been made on a sample of Sn/50.4 at.% Te. Values of the Debye temperature and the lattice Gruneisen parameter derived from these measurements exhibit strong temperature dependences below 20K which are attributed to thermal excitation of the low-lying transverse optic mode.

The molar heat capacity of superconducting PdDx in the temperature range from 2 to 12 K

Abstract: The molar heat capacity of superconducting PdDx (3.8 K ≦ Tc ≦ 5.4 K) is measured in the temperature range from 2 to 12 K. The γ-coefficients and Debye temperatures of samples with different atomic ratio x = D/Pd (0.87 ≦ x ≦ 0.91) are determined. In dependence on Tc a remarkable decrease of the Debye temperature and a small decrease of the γ-coefficient is observed. Therefore, from the calorimetric results it seems probable that the increase of Tc with increasing x-value and the isotope effect are a consequence of the change of the phonon spectrum and the different electron-magnon interaction. Die molare Warmekapazitat von supraleitendem PdDx (3,8 K ≦ Tc ≦ 5,4 K) wird im Temperaturgebiet von 2 bis 12 K gemessen. Die γ-Koeffizienten und Debye-Temperaturen von Proben mit verschiedenen Atomzahlenverhaltnissen x = D/Pd (0,87 ≦ x ≦ 0,91) werden bestimmt. In Abhangigkeit von Tc wird eine bemerkenswerte Abnahme der Debye-Temperatur und eine kleine Abnahme des γ-Koeffizienten beobachtet. Es scheint deshalb, ausgehend von den kalorimetrischen Ergebnissen, wahrscheinlich, das die Erhohung von Tc mit steigendem x-Wert und der Isotopeneffekt eine Folge des veranderten Phonon-spektrums und der unterschiedlichen Elektron-Magnon-Wechselwirkung sind.