Dielectric properties of thin solid films formed on silicon

We report on the measurement of the frequency-dependent complex permittivity, ()=()-i(), over the frequency range, 30 MHz to 6 GHz, of silicon wafers and of thin dielectric films formed on silicon. Measurements, as a function of temperature and time treatments, were obtained by means of an HP Network Analyzer and dielectric probe and the resulting ()and()plots for the silicon wafers are shown to have a Debye-type [1] profile, thereby indicating that the associated polarization mechanism is of the orientational variety.     

Spectral shape dependence in the very strong coupling regime of Eliashberg theory

The effect of ² F() shape dependence on several physical properties of superconductors is studied at various values of the strong coupling indexT _c/ _ln . Our results indicate that the degree of shape dependence of each property is sensitive to the value ofT _c/ _ln . Generally, for the region we examine, 0.25T _c/ _ln 1.3, the dependence on shape is found to be higher than in the conventional strong coupling regimeT _c/ _ln 0.2. However, with the exception of the mass enhancement parameter , the amount of shape dependence does not increase steadily withT _c/ _ln and there appears to be regions of maximum shape sensitivity.     

Effects of phonons and antiferromagnetic spin fluctuations on the pairing and density of states in high-T c superconductors

We solve the Eliashberg equations for a two-dimensional, tight-binding band and anisotropic interaction due to exchange of phonons and antiferromagnetic spin fluctuations. For small band fillings, a mixture of simple and extendeds-wave pairing is stable, while for band fillings closer to half-filling thed-wave pairing state becomes stable. The density of statesN() becomes highly asymmetric in for smaller band fillings, which is an effect of particle-hole asymmetry. For thed-wave stateN() is linear in for small and exhibits a logarithmic singularity at the gap amplitude. For the mixeds-wave stateN() shows the BCS singularity at the gap edge. Antiferromagnetic spin fluctuations give rise to a pseudogap inN() for the normal state.     

Theory of neutron scattering from the two-roton,two-maxon and maxon-roton states in superfluid4He

In 1970, Ruvalds and Zawadowski (RZ) outlined a microscopic theory of the two-roton pair excitation spectrum in superfluid⁴He. They pointed out that because of a Base condensate-induced coupling into the density fluctuation spectrum, these pair excitations show up in inelastic neutron data for S(Q, ) as high energy multiparticle scattering above the maxon-roton quasiparticle peak. Stimulated by recent high-resolution neutron data from ILL, we have carried out an extensive study of the single-particle ₁(Q, ) and the two-particle ₂(Q, ) spectral densities within the RZ scenario, over a wide range of wavevectors (1 Q 3 Å^–1), frequencies and temperatures. We extend the original RZ analysis (which concentrated on the two-roton spectrum) to include the multiparticle structure associated with the maxonroton and maxon-maxon spectra and present numerical results for both attractive and repulsive quasiparticle interactions. We also point out that the microscopic theory of S(Q, ) in a Bose-condensed fluid shows that it involves a weighted sum of both ₁(Q, )and ₂(Q, ). As a result, multiparticle structure exhibited by S(Q, ) is not easily related to theoretical results for ₁(Q, ) and ₂(Q, ). Previous attempts in the literature to fit neutron data for S(Q, ) to either ₁(Q, ) or ₂(Q, ) would not appear to have much quantitative basis, especially in the two-roton frequency region 2_R.     

Thermodynamic properties of the superconductorsBaPb 0.7 Bi 0.3 O 3 andBa 0.7 K 0.3 BiO 3 using Eliashberg theory

We present results of a thermodynamic analysis for the superconductors compounds BaPb_0.7Bi_0.3O₃ and Ba_0.7K_0.3BiO₃. The physical quantities are calculated making use of the Eliashberg theory and the electron-phonon spectra ²()F() as calculated by Shirai et al. For the superconductor BaPb_0.7Bi_0.3O₃, several models of the ²()F() were studied looking for a better agreement with experimental data. The best fit is achieved with a simple constant scaling (C = 1.25) of the Shirai's spectra. The functional derivative of the deviation function D(t) with respect to changes in ²()F() is also calculated.     

The athermal α → ω transformation in Zr-2 at% Nb alloy

The athermal transformation in Zr-2 at.% Nb alloy has been investigated by transmission electron microscopy. Analysis of the selected-area diffraction pattern has shown that the orientation relationships between the omega and the parent-phase in quenched Zr-2 at.% Nb alloy are the same as have been previously observed for the reaction in pure zirconium. Thus it was deduced that the direct transition has taken place in the alloy during cooling. The-originated -particles were visualized using the dark-field technique. The formation of the athermal omega in the-region of-stabilized Zr-Nb alloy is discussed in terms of the relative positions of the free energy equilibrium curvesT ₀ ,T ₀ ,T ₀ and the correspondingM _s ,M _s andT _s start curves. It is concluded that the omega phase can occur over a much wider range of alloy compositions than is usually recognized on the basis of transformation data.     

Phase transformations in Ti-Nb-Ta and Ti-Nb-Ta-Zr alloys

Phase transformations in solution treated and quenched Ti-(13-26) Nb-(22-38) Ta (wt.%) and Ti-(13-35.5) Nb-(5-22) Ta-(4-7.2) Zr alloys have been studied. It has been observed that phase transformations in these alloys are sensitive to both composition and cooling rate. In ternary alloys, water and oil quenching resulted in the formation of orthorhombic martensite () in a retained + _athermal matrix, whereas slower cooling showed evidence of fine and _isothermal formation within the matrix. Increase of Nb + Ta content decreases the volume percentage of martensite. Moreover, addition of Zr stabilized the phase, lowered the martensite start temperature and suppressed formation. Finally, dynamic moduli of air cooled quaternary alloys showed that the modulus was sensitive to the composition, a minima at Nb/Ta ratio of 12.0 and 5 at% Zr being observed, this minimum in dynamic modulus being consistent with phase suppression.     

Impulse approximation in solid helium

The incoherent dynamic form factorS _i(Q, ) is evaluated in solid helium for comparison with the impulse approximation (IA). The purpose is to determine theQ values for which the IA is valid for systems such as helium where the atoms interact via a potential having a steeply repulsive but not infinite hard core. For³He,S _i(Q, ) is evaluated from first principles, beginning with the pair potential. The density of statesg() is evaluated using the self-consistent phonon theory andS _i(Q, ) is expressed in terms ofg(). For solid⁴He reasonable models ofg() using observed input parameters are used to evaluateS _i(Q, ). In both casesS _i(Q, ) is found to approach the impulse approximationS _IA(Q, ) closely for wave vector transfersQ20 Å^–1. The difference betweenS _i andS _IA, which is due to final state interactions of the scattering atom with the remainder of the atoms in the solid, is also predominantly antisymmetric in (–_R), where _R is the recoil frequency. This suggests that the symmetrization procedure proposed by Sears to eliminate final state contributions should work well in solid helium.     

Transmission electron microscopic observations of a commercial Nb-Ti superconducting alloy

Microstructures in a commercial Nb-Ti alloy heat-treated or repeatedly aged and cold worked have been investigated by transmission electron microscopy. In the aged samples after solution treatment,-phase forms at ageing temperatures below 673 K and-phase precipitates at 773 K. Cold work after solution treatment or heavy cold work without prior solution treatment results in accelerated precipitation of-phase on ageing at 623 K and of-phase on ageing at 773 K, while it gives-precipitation instead of on ageing at 673 K. Repeated ageing and cold work make the microstructures very fine. The refinement is due to formation of dislocation cell structures and subgrains and precipitation of- and-phases.     

Effect of chromium content on structure and mechanical properties of Ti-7.5Mo-xCr alloys

The present work is a study of a series of Ti-7.5Mo-xCr alloys with the focus on the effect of chromium content on the structure and mechanical properties of the alloys. Experimental results show that low hardness, strength and modulus binary Ti-7.5Mo alloy is comprised primarily of fine, acicular martensitic phase. When 1 wt % Cr is added, a small amount of phase is retained. With 2 wt % or more chromium added, the entire alloy becomes equi-axed phase with bcc crystal structure. The average grain size decreases with Cr content. When the alloy contains about 2–4 wt % Cr, a metastable phase is present. In Ti-7.5Mo-2Cr alloy appears the highest intensity accompanied with high microhardness, bending strength and modulus. The -induced embrittling effect is most profound in Ti-7.5Mo-2Cr alloy that exhibits a terrace type fracture surface covered with numerous micron-sized dimples. The alloys with higher Cr contents show normal ductile type fractography with much larger deformation dimples. The present results indicate that Ti-7.5Mo-(4–6)Cr alloys seem to be potential candidates for implant application.     

Scattering in a Spin-Charge Separated Non-Fermi Liquid

The electron self-energy due to the scattering on nonmagnetic impurities in a spin-charge separated non-Fermi liquid is calculated. The frequency dependence of the inverse scattering time is proportional to , for the case when the anomalous parameter = 0, and to ¹⁺ for 0. The scattering time is cutoff dependent when the anomalous parameter is nonzero.     

The significance of many-body interactions in physical adsorption

The significance of many-body interactions in the physical adsorption of rare gas atoms on solid surfaces has been investigated. A simple model of the system was considered in which the atoms of the solid and the adsorbed atom were represented by interacting isotropic point dipoles. The fullN-body interaction energy between the adsorbed atom and the solid was calculated exactly to lowest order in the expansion parameter [₀(₀/)²]/4d ³ (, and ₀, ₀ are the solid atom's and adsorbed atom's characteristic frequency and polarizability,d is the distance from the surface of the adsorbed atom, and is the number of atoms per unit volume in the solid). The interaction was then evaluated for the adsorption of various rare gas atoms on rare gas solids. It was found that the fullN-body interaction deviated at most by only2 1/2% from the interaction obtained by considering only two-body interactions. In addition, it was found that a finite expansion in two-, three-, ... body expressions may lead to erroneous results for the interaction energy since extensive cancellations occur between successive terms of the series.     

Transport results from untwinned YBCO: Washboard frequency of the vortex lattice,and the quasiparticle mean free path

The washboard frequency of the moving vortex lattice in untwinned YBa₂ Cu₃ O_6.93 may be observed through mode-locking to an externally applied ac current of frequency _ext. The interference between and _ext results in jumps in the dc current-voltage characteristics when and _ext are harmonically related¹. The interference effect disappears in the vortex liquid state. The Hall conductivity _xy below T_c in YBCO contains contributions² from a positive quasiparticle (qp) term (H) and a negative vortex term (1/H). The qp term is surprisingly large well below T_c and implies a large gap anisotropy and a long qp mean free path (mfp). The thermal Hall effect³ _xy is closely related to the qp _xy; _xy is produced by asymmetric scattering of qp by pinned vortices. The qp mfp at H = 0, extracted from _xy and extended to low T by _xy, increases remarkably from 90 Å at T_c to more than 0.5m at 22 K.     

Pseudogap and Superconducting Gap in YBa2Cu3o6+x: A Raman Study

We present results of electronic Raman-scattering experiments in differently doped YBa ₂ Cu ₃ O _6+x . In B _2g symmetry, an analysis of the data in terms of a memory function approach is presented and dynamical relaxation rates (, T) and mass-enhancement factors 1 + (, T) for the carriers are obtained. Starting from temperatures T > 180K, (, T) and 1 + (,T) are extrapolated to lower temperatures and used to re-calculate Raman spectra. By comparison with our data, we find a loss of spectral weight between T _c < T < T* at all doping levels x. T* is comparable to the pseudogap temperature found in other experiments. Below T _c , the superconducting gap is observed. It depends on x and scales with T _c whereas the energy scale of the pseudogap remains the same.     

Wide-band phase-metering frequency transducers with fixed frequency heterodynes

Conclusions It has already been pointed out [3] that the PMFTs with a repeated sequential transformation of frequency in ISSs and restoration of the heterodyne frequency contain a minimum number of transducers (two WBFSs) and provide, only for the condition that <, an adequately small error due to an incomplete suppression of the image frequencies (₁₂) in the WBFSs.Over a wider frequency range (), i.e., at any frequency with the exception of=, the PMFTs operate with the same small errors due to an incomplete suppression of the WBFS image frequencies (₁₂), provided that the ISS contains two WBFSs and an SM, or three WBFSs, and that the reference-signal shaping channel consists of a frequency doubler and k₁=k₂=1.Certain PMFTs which contain an FST at one of the heterodyne outputs [5] (k₂>k₁=1) have even a. wider band. Thus, the PMFTs corresponding to combinations of their component transducers shown in the lines 2, 5, 8, and 11 of Table 1 operate, for any relationship of the frequencies ₁ and k₂, with adequately small errors due to an incomplete supression of the image frequency (₁₂) in the WBFSs.The above error, with the remaining conditions being the same, is minimal (¹²³) in the PMFTs which consist of three WBFSs and have transducer combinations shown in the lines 1 and 3 of Table 1.Bearing in mind that WBFSs can now be made with an image frequency suppression factor of the order of 5·10^–3, the error under consideration of such PMFTs can be of the order of 10^–7 rad or 5·10^–5 degrees, i.e., it can be small as compared with other modern phase-metering devices' error components (due to coupling between channels, presence of higher signal harmonic components, etc).Translated from Izmeritel'naya Tekhnika, No. 1, pp. 21–25, January, 1974.     

Strength enhancement in a metastable β-titanium alloy: Ti-15Mo

The tensile properties of a metastable alloy, Ti-15Mo*, in a number of different aged conditions have been correlated with the microstructures observed by TEM. Ageing at a temperature, which is approximately the upper limit of phase stability, allowed the and phases to coexist, the a phase apparently having been nucleated on the particles. An extremely fine distribution of the phase was achieved in this way, resulting in increased strength, limited ductility and good toughness. This method of precipitate refinement is then compared with the other treatments which have been applied to metastable -titanium alloys. Changes in strength due to variations in heating rate to the ageing temperature, to simulate the effect of section size, are also reported and discussed in terms of the refinement in a precipitate size. It is shown that these refinements in precipitate size are reflected in higher values of the ratio 0.2% proof stress/Young's modulus.     

Superconductivity in amorphous Bi and Ga

The electron-phonon ²()F() has been measured by Chenet al. in films of amorphous Bi and Ga. This was accomplished by inversion of low-temperature quasiparticle tunneling data. From ²()F(), the phonon renormalization of the electronic effective mass was calculated and was found to be considerably greater than it is in the case of the well-known strong-coupling superconductors Pb and Hg. These amorphous materials should, therefore, be ideal for a study of strong-coupling effects, i.e., deviations from BCS laws. In this paper, we present results of a theoretical study of select thermodynamic properties of these materials based on numerical solutions, at finite temperatures, of the Eliashberg equations.Work performed under the auspices of the U.S. Atomic Energy Commission, and supported in part by the National Research Council of Canada.     

The Infrared Hall Effect in YBCO: Temperature and Frequency Dependence of Hall Scattering

We measure the Hall angle, _H , in YBCO films in the far- and mid-infrared to determine the temperature and frequency dependence of the Hall scattering. Using novel modulation techniques we measure both the Faraday rotation and ellipticity induced by these films in high magnetic fields to deduce the complex conductivity tensor. We observe a strong temperature dependence of the mid-infrared Hall conductivity in sharp contrast to the weak dependence of the longitudinal conductivity. By fitting the frequency dependent normal state Hall angle to a Lorentzian _H () = _H /( _H – i) we find the Hall frequency, _H , is nearly independent of temperature. The Hall scattering rate, _H , is consistent with _H T ² up to 200 K and is remarkably independent of IR frequency suggesting non-Fermi liquid behavior.     

Deformation behaviour of retained β phase inβ-eutectoid Ti-Cr alloys

Plastic deformation mode and its relation to tensile properties were investigated in retained phase of-eutectoid type Ti-Cr alloys. A plate-like single variant phase is induced during deformation of the most unstable phase having a minimum chromium content required to suppress martensitic transformation. A selected area electron diffraction pattern taken from a boundary region of the stress-induced phase plate can be explained by the idea that a single variant of phase is induced in a {3 3 2} 1 1 3 twin produced during deformation. Anisotropy in population of four phase variants decreases with increasing chromium content. On further increasing chromium content, deformation occurs by slip. Enhanced ductility is obtained in as-quenched Ti-Cr alloys accompanied by phase transformation or {332} 1 1 3 twinning during deformation, phase of as-quenched Ti-Cr alloys changes continuously from commensurate structure with sharp reflections to incommensurate structure with diffuse reflections with increasing chromium content. The obtained results in-eutectoid type Ti-Cr alloys are quite similar to those in-isomorphous type Ti-V alloys.     

Energy Scales in the High-Tc Superconductor YBa2Cu3O6+x

The optical conductivity sum rule is used to examine the evolution of the spectral weight N() in both the normal and superconducting states of optimally and underdoped YBa₂Cu₃O_6+x along the a axis. Differences in N() above and below T _c allow the strength of the superconducting condensate _s to be determined. In the optimally-doped material, _s is fully formed at energies comparable to the full superconducting gap maximum (0.1 eV), while in the underdoped material the energy scale for convergence is considerably higher (0.6 eV). This difference is discussed in terms of normal-state properties.