A second quantized temperature formalism for the correlation functions and linear response laws for detect systems representative for solid and partly ordered liquid states is developed. The partition function is expressed in terms of defect degrees of freedom for which also an effective action is derived coupled to source fields. The physical configurations of the system are expressed in terms of their geometric sources which are the defects and so are the correlation functions, as for instance the non-local Hooke tensor. Collective excitations of phonon and plasmon type are studied on a qualitative level. Most results are given in the flat approximation, where the non-trivial metric and Riemann connection generated by the geometric sources is ignored. The perturbation theory, in order to analyse explicitly the flow structures of defect systems, is not developed in this paper.

A dislocation theory of dimer melting in two dimensions is presented in which the solid state is considered as a random array of close-packed dimers on a triangular lattice. Possible application of the model to phospholipid bilayer membranes is discussed, and comparison with the melting of two-dimensional monatomic systems, as well as of layered paraffins, is made.

A theory of the melting of paraffins is developed based on dislocation theory. The transition from two- to three-dimensional melting behavior, when proceeding from the small-to large-chain limit, is qualitatively described by this theory. It also yields a tentative explanation of the tendency of large-chain polymer crystals to crystallize in a folded structure. The structural properties of the paraffin lamellae above the rotator transition and below the melting point are studied by considering vortex loop defects. The geometrical implications of the motion of vortex and dislocation loops on the lamellar structure is studied, and a roughening of the surfaces of the lamellae is predicted. The possibility of a nematic ordering of the melt is discussed.

Reduction of the photothreshold in degenerately doped EuO below the Curie temperature is shown to arise from a magnetically induced space charge layer at the surface. A good fit to the measured shift versus temperature for EuO-2% La is obtained within a simple model, yielding new information about the magnetic nature of the semiconductor surface.

The magnetoresistance of a degenarate magnetic semiconductor is calculated, using a theory in which charged impurities are screened by a spin dependent dielectric function. The screening function explicitly includes the coupling of the conduction electrons to the lattice of localized spins and thereby improves upon the work of Shapira and Kautz who consider only the effects of conduction-band splitting on the Thomas—Fermi screening length. Magnetic inhomogeneties induced by charged impurities in a magnetic semiconductor are also capable of scattering electrons. Resistivity resulting from this mechanism is also considered.

The claim that anomalous ultrasonic absorption peaks give evidence of a new spin-ordering phase in holmium is put to question. It is proposed that a mixed domain structure stabilizes at the temperature where the spiral periodicity becomes commensurate with that of the lattice. The mixed phase consists of incommensurate spiral domains and “locked-on” domains, and the absorption results from magnetoelastically-driven domain-wall resonance.

A calculation of the linear-response properties of an electron gas, coupled by the $s\ensuremath{-}f$ exchange interaction to a lattice of localized spins is presented. Coupled-charge and spin-density-response functions are determined in the random-phase approximation and used to compute a Mott-type instability of metallic Eu-rich EuO toward formation of magnetic polarons bound to oxygen vacancies. Critical carrier densities are obtained as a function of temperature and phase diagrams of the metallic and insulating states are presented and compared with existing theories and experimental results.

Ultrasonic attenuation near the ferromagnetic-spiral transition of Dy and Tb is described on the basis of magnetoelastically driven domain-wall resonance. Good agreement with the measured attenuation coefficient is obtained on the basis of a simple model, and an apparent discrepancy between zero-field observations is discussed.

The magnetic susceptibility $\ensuremath{\chi}(x,T)$ of the pseudobinary intermetallic compound $\mathrm{La}{\mathrm{Ag}}_{x}{\mathrm{In}}_{1\ensuremath{-}x}$ is given for temperatures $T$ between 20 and 250 K with $x$ between 1.0 and 0.15. For $x$ between 0.9 and 0.6, $\ensuremath{\chi}$ is very anomalous, rising strongly as the samples are cooled until a temperature ${T}_{M}(x)$ is reached where a cubic-to-tetragonal crystallographic transformation is found to occur and the susceptibility drops to a much lower value. The transformation is shown to arise from a band Jahn-Teller effect. The behavior of $\ensuremath{\chi}(x,T)$ as well as ${T}_{M}(x)$ can be described quantitatively over a broad range of $x$ assuming that the Fermi energy of LaAg lies closely below a high peak in the density of states which originates from the ${e}_{g}$ states of La. This peak becomes gradually populated as the electron concentration is increased by replacing Ag by In.

A detailed theoretical study has been made of the magnetoelastic perturbation of the spectra of elementary spin and lattice excitations in Tb and Dy metals. The theory was formulated on the basis of an interaction formed from bilinear products of local spin and strain functions. Previous ad hoc models appear in certain limits of the theory, giving a coherence to the theoretical picture of magnetoelastic coupling. It is found that uniform magnetostriction causes a smooth transition from "free-lattice" to "frozen-lattice" perturbation of the magnon spectrum depending on the wave vector of the state. The microwave absorption versus magnetic field applied along the hard planar axis of Tb and Dy is calculated. It is found that free-lattice magnons are primarily responsible for low-frequency absorption in Tb below 140 K, and for both low- and high-frequency absorption in Dy below the Curie temperature of that metal. It is shown that the transition from free- to frozen-lattice behavior of the magnon spectrum is essential to the explanation of existing data on the temperature dependence of absorption-peak positions in Tb. The dynamic interaction between spin and lattice waves is derived and used to calculate the mixed-mode splittings in regions of the Brillouin zone of Tb where phonon and magnon dispersion curves cross. The theory predicts well the splitting which occurs where the acoustical-magnon and phonon branches touch, but fails to account for the splitting between the acoustical-magnon and optical-phonon branches. A different coupling mechanism is proposed which may account for the mixing of these branches.

We show that in the long-wavelength region the magnetoelastic effect allows both the free-lattice and the frozen-lattice mode to exist in the heavy rare earths Tb and Dy. The former may be more easily detected by ferromagnetic resonance at low frequencies and low temperatures, and the latter predominates at high frequencies and high temperatures.