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The authors have measured the damping of a vibrating wire in a saturated ³He-⁴He solution from 0.9 to 10 mK at 0.5 bar. The behavior becomes dominated by mean free path effects below 5 mK where, with decreasing temperature, the mean free path becomes first comparable to the wire radius and finally comparable to the container dimension.

Proton spin transitions were modulated while the response of the coupled electron-proton spin system was monitored for electron spin transitions. Evidence is presented showing that the response lags in time with respect to the perturbation and that it can reach an unexpectedly high degree.

The authors investigate the inelastic diffusion spectrum one may expect from fluctuating domain walls in the vicinity of the incommensurate-commensurate transition in a 3D uniaxial compound. The diffusion function exhibits a central peak, the width and height of which vary with temperature near the transition. The latter variation is due to the interwall interaction which governs the wall dynamics. They also study the structure factor for quasi-elastic scattering. They find that it contains two terms, one with a usual Debye-Waller factor and one with a Debye-Waller factor that diverges at the commensurability transition.

Develops the theory of antishielding factors of ions in solids from first principles. In addition to the Sternheimer factor, the author finds the effects arising from the overlap and covalency. He has considered the dynamics of the lattice which permits the phonon-induced charge transfer effects and thus gives rise to a zero-point contribution and a temperature-dependent contribution to the antishielding which varies as the lattice energy of the solid.

The analysis of data obtained from the interband magnetoabsorption in HgSe and in mixed crystals Hg_1-xCd_xSe and HgS_xSe_1-x with compositions close to pure HgSe led the authors to the conclusion that in these materials mod E₀ mod approximately=²/₃ Delta where the energy gap E₀=E_{Gamma 6v}-E_{Gamma 8c} and the spin-orbit splitting Delta =E_{Gamma 8v}-E_{Gamma 7v}. The results of the interband magnetoabsorption in the above mentioned materials could not be described in terms on the three-band Pidgeon-Brown model (1966), which provides a good description of the experimental data when mod E₀ mod <²/₃ Delta . In good order to interpret the magnetoabsorption data it was necessary to introduce a five-band model where the closest Gamma ₁₅ 'higher' band was explicitly taken into account. This model is used for the first time in the case of zero-gap semiconductors in a quantising magnetic field. Such a five-band model works well in Hg_1-xCd_xSe (x<0.01), oriented samples of HgSe (H//(001) and H//(111)) and HgS_xSe_1-x, x<0.05, always when mod E₀ mod approximately=²/₃ Delta (at helium temperature). Thus, a complete set of band parameters for Hg_1-xCd_xSe in the semimetallic region and HgSe are obtained. In particular, a peculiar character of the Gamma _6v level is described. Also for HgS_xSe_1-x the dependencies of fundamental band parameters on T and x were obtained by means of numerical fitting of the five-band model.

The Ward identity is derived for the electron-phonon coupling in one dimension and the spectrum of elementary excitations is calculated by assuming that the Fermi distribution is not strongly distorted by the interaction. The electron-phonon vertex is renormalised in the case of the forward scattering and Migdal's theorem (1958) is discussed. A model is proposed for the giant Kohn anomaly. The dip in the phonon spectrum is obtained and compared with the experimental data for KCP.

Co-doped LEC InP was studied using resistivity and Hall measurements, thermal and optical capacitance spectroscopy, photoluminescence (PL) and PL-excitation (PLE) techniques. The derived activation energy for n-type conduction is about 0.50 eV in semiinsulating InP:Co samples of resistivity 10⁶ to 10⁷ Omega cm. The same level at E_c-0.53 eV is found in high concentration in DLTS measurements. A hole trap at E_v+0.3 eV in similar concentrations is detected in optical DLTS and photocapacitance experiments. Relationships between optical cross sections for transitions to the valence band and to the Gamma and L regions of the conduction band are derived for the 0.53 eV level. Co is shown to substitute for In in InP from the observation of the ⁴T₂(F)₄ to ⁴A₂(F) (F) crystal field transition of Co²⁺ (d⁷) at 474.0 meV in photoluminescence. Furthermore, PLE is used to show that the Co²⁺ charge state is presented in n-type and semi-insulating InP:Co from observation of the ⁴A₂(F)-⁴T₁(F) transition of Co²⁺ at 785.8 meV. M From these results and the electrical and capacitance measurements, the level at E_c-0.53 eV is identified with the transition Co²⁺ to Co³⁺+e, whereas the level at E_v+0.3 eV is attributed to either Co⁴⁺ to Co³⁺+h or to a transition involving a Co-related complex. The trends of the ionisation energies of 2+ charge states of transition metals in III-V semiconductors are discussed and it is concluded that the value for Co²⁺ in InP (0.53 eV) is anomalously low.

Indium-doped silicon, well compensated with phosphorus, was illuminated in order to neutralise the indium acceptors. The photoluminescence spectra and their decays were measured for temperatures between 4K and 135K. Below about 60K the spectra showed a phosphorus-indium pair recombination which was fast and had a recombination rate constant W_max approximately=10⁴ s^-1. Above 80K the conduction-band-indium transition dominated and gave a value for the electron capture coefficient B_ca=2*10^-15 cm³ s^-1. Approximate quantum mechanical calculations of the radiative part of B_ca were performed both for the phononless and the phonon-assisted transitions. The results of these calculations agreed within the expected accuracy with the experimental results and suggested that the electron capture of indium is a radiative process.

Electron paramagnetic resonance measurements of the cubic crystalline field splitting parameter of Fe³⁺ at the Ta site in KTaO₃ are reported as a function of temperature, pressure, and Nb doping concentration. The relative dependence of the splitting parameter a on the unit-cell volume was found to be similar in the two incipient ferroelectrics KTaO₃ and SrTiO₃. The explicit temperature effect, i.e. the derivative delta a/ delta T at constant volume and concentration is negative in KTaO₃ (and SrTiO₃) whereas it is positive in MgO, a typical inert dielectric oxide. This distinction appears to be related to the driving mechanism of ferroelectricity.

The authors have observed a peculiar behaviour in the NQR relaxation of both chlorine isotopes in (NH₄)₂SnCl₆ below 80K. An unusual ratio T₁(³⁷Cl)/T₁(³⁵Cl) approximately=2.4 at low temperatures can be explained by the dipolar coupling to the tunnelling ammonium protons causing relaxation by a mechanism similar to that for the proton NMR relaxation, but at slower rates. A very large maximum value around 55K is ascribed to relaxation of the tunnel system towards the lattice, after resonant transfer of energy from the NQR system. It is argued that this relaxation time should be of the same order of magnitude as the proton T₁ at similar frequencies and temperatures.

Mossbauer spectroscopic measurements of hyperfine fields have been used to obtain information on the magnetisation curves of single crystals of K₂FeF₅ in the presence of applied magnetic fields in the antiferromagnetic region below the Neel temperature. In zero applied field the magnetisation curve is of the expected form but with an anomalously low value of the saturation hyperfine field. With fields applied perpendicular to the magnetic easy axis the magnetisation curve is raised with respect to that in zero field but remains essentially parallel to it. For fields applied parallel to the magnetic easy axis the magnetisation curve is lowered compared with that in zero field with the effect decreasing with decreasing temperature so that the curves converge as absolute zero is approached.

Dielectric measurements on the ferroelectric CsH₂PO₄ with chain-like hydrogen bonds have been performed in the frequency region below 1 GHz. The temperature dependence of the dielectric constant and loss were explained well by the Debye equation with single relaxation time. The relaxation time obtained near the ferroelectric phase transition point exhibits a pronounced critical slowing down. This characteristic divergence of relaxation time has been explained very well using the results of the pseudo-one-dimensional kinetic Ising model. The parameters obtained in this experiment have considerable anisotropy of interactions: J/sub ////k=234K of intrachain and J_{perpendicular to} /k=6.78K of interchain. The marked long relaxation time compared with that of KH₂PO₄ is well understood with the critical dynamics of the hydrogen motion in the weakly-coupled one-dimensional chains.

The cathodoluminescence near-band-edge emissions of as-grown and zinc-treated zinc selenide single crystals have been studied and compared. Although the blue emission observed at room temperature occurs at an energy that is less than expected from observation of the free exciton at low temperature, it is shown to have many of the characteristics of the free exciton. These include (i) a superlinear dependence on excitation intensity (also shown by bound excitons), (ii) for the spectral intensity on the high-energy side of the emission, a temperature dependence which follows that of the free exciton and (iii) the same temperature dependence of the emission half-width as that followed by the free exciton.

Much work has been done on threshold effects in LEED assuming their resonance origin. The authors use here a perturbation expansion to separate the contributions of interference and resonance effects in LEED profiles. They show, on five typical examples, that oscillations near thresholds have a clear interference origin. It appears that it is difficult to find situations for which resonance conditions are actually satisfied. This fact has important consequences on the information that one can extract from analysis of threshold effects.

Photoconductivity in evaporated films of Ge_xSe_1-x glasses (x=0.33 and 0.4) has been investigated as functions of wavelength, chopping frequency and temperature. Contribution of electrons to the photoconductivity may dominate that of holes. When illuminated with a light at the Urbach tail, the photoconductivity may be produced by means of electron excitation assisted by thermal phonons from the tail states to the conduction band. The temperature dependence of the photoconductivity has quite similar features to other chalcogenide glasses.