Physica B+C最新文献

In an analysis of the influence of the Jahn-Teller (JT) distortion in KCuF₃ on the exchange-coupling constants (ecc) along (J_S ≈ -190K), and perpendicular to (J_AS ≈ 0) the c-axis, two models are considered. First, a three-center, four-electron model is adopted with d(e_g)- and p_σ-orbitals at the cations and anion, respectively. Experimental values of ionization potentials and electron affinities are employed. The model accounts for the JT-distortion by using the induced admixtures of the e_g-orbitals on the copper cations. The computations involve CI. Secondly, taking s-orbitals at all centers, we perform three-center, four-electron model calculations supplemented with two-anion non-additive exchange contributions obtained from four-center, six-electron model calculations. The latter contributions, strongly geometry dependent, account for the effect of JT-distortion on the ecc's. First-order exchange-perturbation theory is used. In the applications to KCuF₃ we adopt in the two models the same orbital parameterization, based on atomic properties, with which the ecc's in the regular perovskites KMF₃ : M = Mn, Co and Ni, are reproduced. The resulting values for J_S and J_AS agree with experiment. The vanishing value of J_AS is structural in the first (pd-) model and accidental in the second (s-) model. This discrepancy is discussed.

We have carried out a detailed investigation of squeezing and anti-bunching in the Jaynes-Cummings model, thereby extending the earlier work of Meystre and Zubairy [Phys. Lett. 89 A (1982) 390]. Analytic criteria are formulated relating the initial states of atom and field to the tendency of the field to undergo squeezing or anti-bunching when the atom enters the cavity. Some interesting connections are established between atomic squeezing and field squeezing/anti-bunching. Using the initial conditions suggested by our criteria, we explore the generation of non-classical states of the cavity field. The relationship of our work to other recent work on squeezing and anti-bunching in the Jaynes-Cummings model is discussed.

Measurements of loss factor and capacitance made on single crystals of P-doped n-type silicon are reported. Samples are irradiated with (1.00 ± 0.20) MeV electrons at temperatures <12K. The production of divacancies in the irradiated samples is monitored using hopping conductivity changes at 4.2 K. Higher divacancy production is observed in Czochralski-grown samples and samples containing appreciable concentrations of group III impurities. This suggests that oxygen and group III impurities trap the self interstitials, mobile under irradiation conditions, thus, inhibiting divacancy-self-interstitials annihilation. Effects of irradiation on the free carriers conductivity of the samples are also investigated.

The Slater-Koster interpolation method is used in combination with the standard KKR method to get a set of quickly applicable energy bands for a mixed valence intermetallic compound CeAg.

The standard molar enthalpies of formation of AlGd₂, Al₂Gd₃, AlGd, Al₂Gd and Al₃Gd compounds were determined by dissolution calorimetry, using a calorimeter based on liquid aluminium. Experimental results are compared with model predictions. More particularly the alloy cluster Bethe lattice method has been used to analyse the chemical trend in the bonding properties of these compounds.

Electron impact excitation followed by fluorescence induced by N₂-laser absorption was used to study the lifetime of the lowest vibrational level of the B ³Π_g electronic state of N₂. The experimental result of this work is 13 ± 1 μs and comparison is made with recent experimental and theoretical results.

In the presence of an applied magnetic field, an energy gap exists in the spin-wave dispersion relation. As a consequence, the temperature dependence of the magnetization for ferromagnets must differ from the simple Bloch T^{$\text{3}\text{2}$} law. For the first time, the modification factors have been calculated systematically.

The lattice dynamics of the A15 superconducting compounds have been investigated using a model in which the interatomic forces include, in addition to central forces, the angular forces of the type employed by de Launey. This model has been applied to Mo₃Si, Cr₃Si and V₃Si to investigate the phonon dispersion curves in the three symmetry directions of the Brillouin Zone. The force constants have been evaluated by fitting them to the measured values of the phonons at the zone centre. The calculated results are found to be in good agreement with the available experimental results. It has been inferred that the angular forces are not important for these compounds. The physical aspects for the same are discussed.

Using a new type of NMR dispersion spectrometer we have made an extensive study of the tunnelling induced nuclear dipolar polarization effect or Haupt effect in lithium acetate single crystals. Special attention is paid to the temperature dependence and angular dependence of the Haupt effect.

The dielectric response function χ of a uniform electron gas is investigated up to the second order of the Coulomb interaction with different methods. When examining all polarisation diagrams with two interaction lines, it is confirmed that previous work in the Green's function formalism does not contain all second order processes and the importance of the corrections is pointed out. It is further shown, how the evaluation of χ with Green's function can be greatly simplified when taking into account the symmetry of the expressions.