Physica B+C最新文献

We report on polarised as well as unpolarised neutron scattering experiments on two single-crystalline samples of UPt₃ from different sources. Previously reported magnetic intensity, probably due to anti-ferromagnetic correlations with relatively high characteristic energies (of the order of 5–8 meV) along the c-axis of the hexagonal structure, was observed in both samples. However, the ratio of the magnetic intensity to the phonon intensity was found to be a factor of five less than suggested by other authors. The magnetic character of the observed scattering was unambiguously proved by means of polarised neutron scattering. Furthermore, we observed in both samples a temperature-dependent quasi-elastic response for Q-values in the neighbourhood of the magnetic satellite positions for the anti-ferromagnetically ordered compounds U_0.95Th_0.05Pt₃ and U(Pt_0.95Pd_0.05)₃. This indicates anti-ferromagnetic correlations in the basal plane along the a^∗-axis in pure UPt₃. Moreover, we observed in only one of the samples long-range anti-ferromagnetic order with an ordered moment of the order of 0.01μ_B and a Néel temperature of about 5 K.

Blank et al. [Physica 145B (1987) 222] in a recent letter reported Q-band ESR measurements on YBa₂Cu₃O_7−δ suggesting the presence of 10 to 15% of Cu³⁺. It is shown in this article that their assignment of the ESR line at g=2.08 to Cu³⁺ is incorrect. Further it is pointed out that the assignment of this line to low-spin Cu⁴⁺ cannot be ruled out.

We have considered the effect of damping generated due to streaming electrons in a relativistic electron plasma. It is observed that due to the relativistic effect the amplitude of the soliton gets smaller and it becomes more widened. But for a high-temperature electron plasma, the effect is not considerable. The KdV equation with the damping term is solved via the Bogoliubov-Krylov perturbation theory, whence the amplitude and width of the soliton are expressed in terms of the parameters of the plasma. Lastly we discuss how higher order effects can be incorporated.

Two-sublattice collinear Heisenberg-Ising (S = 1) metamagnets with uniaxial three-ion anisotropy are investigated. Thermodynamical properties of this model are examined in the approximation including Gaussian fluctuations of the molecular field. The complete phase diagrams are obtained numerically. The theoretical results fitted for the FeCl₂ metamagnet are compared with experimental ones.

Continuing the papers on alloys of Sc, Ti, V, Cr, Mn and Fe we review in the present paper, on the basis of the model developed by Miedema and co-workers, experimental and predicted enthalpies of formation of ordered binary intermetallic compounds of Co with arbitrary metal partners. The enthalpy effects are also reviewed for liquid Co alloys.

The predicted values agree quite satisfactorily with the available experimental data for binary systems of Co with a transition metal. In the binary systems where Co is alloyed with a non-transition metal larger differences are sometimes observed.

Since Co with other transition metals may often form solid solutions the enthalpy of forming solid solutions is dealt with in a concise way distinguishing between magnetic and non-magnetic matrix metals.

The mass attenuation coefficients μ/ϱ of beta particles of end point energy E₀ ranging from 0.16–2.3 MeV are determined adopting a 4π geometrical configuration, where the source is sandwiched between thin foils of aluminium as absorbers and the sandwich is kept in a 4π gas flow proportional counter for measuring transmitted beta particles. An empirical relation connecting μ/ϱ and E₀ is determined in such a geometrical configuration for the first time. The estimated values of μ/ϱ in this geometry are discussed and compared with published values, obtained by employing various other types of geometry.

Cyclotron resonance linewidths due to scattering by acoustic phonons via deformation potential and piezoelectric interaction have been calculated for a two-dimensional electron gas formed in heterojunctions. The finite extent of the wave function along the direction of quantisation and screening due to free carriers are considered. The calculated linewidth for scattering due to deformation potential acoustic phonons show a $\text{B}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ variation with magnetic field. The field dependence of the linewidth due to piezoelectric phonon scattering is however weaker.

We have studied the voltage-dependent scaling of the resistance in a 1D disordered system by using a new Landauer-type resistance formula. The scaling behaviour at high voltage is significantly different from the scaling law due to Anderson et al. [Phys. Rev. B 22 (1980) 3519]. The finite voltage difference between both ends of the wire leads to much better statistical behaviour (in ensemble sense) of the resistance due to self-averaging over energy. Numerical results obtained for a simple model system illustrate the typical behaviour that is expected to be of conceptual importance for real systems.

Quantitative measures are introduced for the indistinguishability U of two quantum states in a given measurement, and the amount of interference I observable in this measurement. It is shown that these notions obey an inequality $\text{U ≧ I}$, which can be seen as an exact formulation of Bohr's claim that one cannot distinguish between two possible paths of a particle while maintaining an interference phenomenon. This formulation is applied to a neutron interferometer experiment of Badurek et al. It is shown that the formulation is stronger than an argument based on an uncertainty relation for phase and photon number as considered by these authors.