Physica最新文献

The Luttinger-Tisza method and generalizations of this method for determining the minimum energy spin arrangement in a crystal subject to certain strong conditions are reviewed. It is shown that one can always replace the strong conditions by a set of additional conditions which in the simplest cases is identical with the single weak condition introduced in the Luttinger-Tisza method. A general method of calculating the minimum energy spin arrangement, based on the space group symmetry of the magnetic atom arrangements, is given, and the method of Niemeyer, based on permutation groups, is shown to be equivalent to this method. It is also known that the Luttinger-Tisza conjecture concerning the periodicity of the minimum energy spin arrangement is equivalent to a sufficient, but not necessary, condition for a minimum of the energy.

A theory for combined Doppler and pressure broadening has recently been proposed by Nienhuis [Physica 66 (1973) 245]. We show that his dynamical classical limit can obtained as a result of binary collisions in which the radiator returns to its initial translational state after every collision, and thus the Doppler effect is not fully incorporated.

It is shown how persistent currents in the saturated helium-II film can be created by rotation at velocities greater than the critical one.

1/f noise has been observed in the open circuit thermo e.m.f. of both intrinsic germanium and extrinsic germanium and silicon. The experimental results can be explained only by assuming that the 1/f noise is due to fluctuations in the mobility of free charge carriers. The 1/f noise behaviour of the open circuit thermo e.m.f. of intrinsic germanium is similar to the noise behaviour of the open circuit e.m.f. of concentration cells of ionic aqueous solutions.

The Anderson-Grüneisen parameter δ has been evaluated for NaF, BaF₂ and CaF₂ from the measured third-order elastic constants of these crystals. Using these δ values, the temperature dependence of the bulk modulus of these three fluorides is calculated from Anderson's theory in the temperature range up to their melting points. The bulk moduli of the alkaline earth fluorides CaF₂ and BaF₂ have similar variation with temperature. The temperature variation of the bulk modulus of the alkali halide NaF deviates from those of CaF₂ and BaF₂ and this may be attributed to the difference in the crystal structures.

We generalize the Fokker-Planck equation for the laser radiation-field distribution. Our kinetic equation is valid for a monomode solid-state laser in the case of arbitrary coupling strength but low concentration in active material; hence it has a larger domain of validity in the region of high intensity than previous theories.

The near-infrared region of the lanthanum spectra was recorded, between 3500 and 14 500 cm^-1, using a Fourier transform spectrometer. We report here the results concerning the classification of the even configurations (5d + 6s)³ of the first spectrum. 35 out of the 37 existing levels are now known. We performed the theoretical interpretation of the energies and Landé factors and we used the intermediate- coupling eigenfunctions to compute the angular part of the hyperfine interactions. From our measurements and earlier experimental results, the magnetic hyperfine constants of 29 even levels are known; we discuss the values of the radial hfs parameters that we deduced.

In a previous paper we reported the measurements of both C/T and đQ/dH of high-purity niobium as functions of the applied field at a number of chosen temperatures. In the present paper we analyze the slopes of these “magnetocaloric” quantities in the reversible region near H_c2. These slopes are related to the nonlinearity of the magnetization curve by means of a third-order field term in the Gibbs free energy. At the chosen temperatures the small values of the coefficient of this field term are calculated and found to be in agreement with the model of the reversible magnetization curve proposed by Kes.

The influence of various physical parameters on the lowest-order second-harmonic process in a model solid is investigated. The medium is assumed to consist of two-level atoms obeying a simple harmonic-oscillator velocity distribution. It is seen that a definite phase relationship between the incident and the emergent fields is necessary for efficient second-harmonic conversion. In addition, the second-harmonic field and the quality factor are seen to depend on parameters like thickness of medium, relaxation time, level population and motion of the atoms. A comparison is made between this process and parametric oscillations.

In low-frequency relaxation experiments on paramagnetic crystals, placed in liquid helium, often dispersion- and absorption curves are found, strongly deviating from those following from the thermodynamic theory of Casimir and Du Pré. For the relaxation time τ_abs, related to the maximum in the absorption curve, a temperature dependence $\text{τ}{}_{\mathrm{abs}}{}^{\mathrm{\alpha }}\text{T}{}^{\mathrm{-\alpha }}$ with 1 < α < 6 has been found frequently, accompanied sometimes with a jump at the helium-transition temperature T_λ. We computed, starting from a model including the heat conduction of lattice and helium bath and a heat resistance from crystal to bath, dispersion- and absorption curves and found that, assuming the direct process for the spin—lattice relaxation, τ_abs(T) (related to the maximum of the calculated absorption curve) can be described roughly as mentioned above with a jump at T_γ.