Physica最新文献

Some simple but crude models accounting for ionic polarization in alkali halides have been examined by means of Monte-Carlo constant-pressure computations; for comparisons to be made in a way as unambiguous as possible, the models have been applied to simulate solid and molten potassium chloride, with the same two-body potential and at the same temperatures and pressure as in a previous paper. One of those models was found to lead to a considerably improved agreement with experimental data. The polarization contribution was found to be around 1% of the total energy.

Energy distributions of neutrons scattered incoherently from nematic MBBA have been recorded. Measurements for various scattering angles and for two geometries (neutron momentum transfer parallel and perpendicular to the nematic director) have been made. The results have been interpreted as giving evidence of rotational proton jumps, whose correlation times are of the order of 10^-12 s.

An oscillating disc viscometer is described with which measurements have been made on the viscosities of the liquids ²⁰Ne and ²²Ne along the boiling line between 25.4 K and 32.8 K. The viscosity of ²²Ne was found to be 9 ±2% greater than that of ²⁰Ne at 25.4 K, the difference dropping to 5 ± 2% above 30 K. Classically a 5% difference is predicted at all temperatures. An estimate is made of the first quantum correction to the viscosity which compares well with this departure from the classical prediction.

Scaling relations between the critical indices are derived for two similar systems exhibiting λ lines: binary liquid systems and ferromagnets under pressure. In addition to the usual scaling relations, this procedure gives information about other weakly divergent quantities like isothermal compressibility and thermal expansion. Suggestions for more detailed investigations are made.

In a series of vanadium based A 15 intermetallic compounds, the nuclear spin-echo patterns of vanadium $\text{(I=}\text{7}\text{2}\text{)}$ were recorded and analysed. The spin system was inhomogeneously broadened by magnetic as well as quadrupolar interactions, which resulted in the observation of “allowed” (Δm′ = 1) and “forbidden” (Δm′ > 1) echos. A comparison was made between the c.w. line shapes and the spin-echo patterns. Higher-order echoes at t = 4τ (Δm′ = 3) and t = 6τ (Δm′ = 5) were observed in V₃Au and V₃Co. An analysis of higher-order quadrupolar spin echoes is given, based on the theory in which the quadrupolar interaction during the pulses is taken into account.

A self-consistent iteration scheme has been used to calculate velocity autocorrelation functions in classical fluids. For this calculation it is not necessary to introduce any adjustable parameters. The calculation also bypasses the necessity for a detailed knowledge about the many-body hamiltonian. Results are in good with computer experiments on liquid argon.

Further exact and approximate investigations are made into the potential-barrier problem. A generalized barrier is considered, bounded at one end by a transition point of odd order greater than unity, namely by a point of inflexion in the barrier profile. A hierarchy of such barriers is investigated analytically based on certain functions without finite singularities expressed in terms of the Whittaker function. The phase-integral method is then developed in an acceptable manner, avoiding the unrigorous speculations of many authors in the past, in order to show the power of the method to produce results corresponding approximately to the exact solution.

It is shown that, within Kadanoff's scaling theory for the Ising model, the amplitudes of the singularities depend upon an analytic contribution to the free energy of the scaled model, which is the self-energy of the cells. In particular a logarithmic singularity of the specific heat occurs in this theory only if the self-energy is nonzero. In Wilson's renormalisation group theory amplitudes furthermore depend upon the second-order contribution in ε to the renormalised temperature.

A new formula which gives the many-body effects on the De Haas-van Alphen oscillations is presented. The oscillating susceptibility consists of sine and cosine terms. The amplitude of the sine oscillations is decreased by the exchange interaction and by the collective interaction, the latter causing a dependence on field strength. The cosine oscillations are enhanced but are smaller than the sine oscillations. The difference in the field dependences of these two types of oscillations will introduce a field-dependent amplitude when the two are combined. The frequency of the oscillations is not affected by the interaction. In addition to the oscillating susceptibility, the internal energy is also determined.