Results of optical-absorption experiments on heavily irradiated NaCl single crystals are presented. The dose rates were between 4 and 250 Mrad/h; the doses between 1 and 7 Grad. The irradiation temperatures were in the range of 20–150 °C. Because of the intense optical bands, the thickness of the samples had to be reduced to values between 2 and 100 μm using a thinning device. With this device we are able to investigate samples containing about 1 mol % Na and ${\mathrm{Cl}}_2$. The optical spectra of the samples, irradiated at low temperatures, show intense F, M, R, and N bands. The positions of the optical bands of the electron excess centers are in agreement with the literature. With increasing irradiation temperature, the contribution of these centers to the total amount of radiation damage decreases due to coagulation to colloidal Na particles. The colloid band, however, is much wider than the literature values; in addition, it is located at longer wavelengths. These observations can be explained by assuming that we are dealing with extremely small colloids, showing appreciable quantum size effects.

We study the early stage of aggregation of F centers into colloids in pure NaCl under irradiation. The crystals have been electron irradiated with a dose rate of 2 Mrad/h up to doses of 1500 Mrad and measured by optical-absorption spectroscopy. The major bands, the F, M, and the colloid band, are analyzed qualitatively as well as quantitatively. We have observed a relationship between the concentrations of the F and the M centers, which changes from quadratic to linear. The colloid band appears to peak at two distinct wavelengths, indicating that two types of colloids are formed during the nucleation stage. The defect concentrations are determined as a function of the dose and the irradiation temperature and are discussed in terms of models which describe the kinetics of defect formation.

We report measurements on the melting behavior of colloids produced in irradiated NaCl. With differential scanning calorimetry (DSC) several latent-heat peaks near the melting temperature of pure bulk sodium metal have been detected. It appears that the total latent heat in these peaks is an accurate measure of the amount of damage in the crystal. Peak temperature and shape provide more detailed information about the properties of the colloids. The different melting temperatures can be explained by differences in the typical sizes of the colloid, based on the theory for melting of small particles. This DSC technique provides a method to evaluate the production of radiation damage in detail without changing the properties of the damaged crystal.

Raman-scattering experiments on heavily irradiated pure and doped NaCl crystals are described. The experiments have been performed at room temperature and at approximately 25 K. The crystals had been irradiated up to a maximum dose of 95 Grad by means of electrons from a Van de Graaff accelerator. The Raman spectra show the set of phonon peaks corresponding to the NaCl modes. At low frequencies the reduced Raman intensity shows a clear power-law dependence on the frequency, ${\mathit{I}}_{\mathrm{red}}$(ω)∝${\mathrm{\omega }}^{\nu }$, with an exponent ν≃1.4, indicating that the colloids in heavily irradiated samples exhibit a fractal structure. The change in the melting peak pattern reveals that the initial form of the sodium particles is strongly affected by the performance of a Raman experiment at room temperature. At low measuring temperatures two unknown phonon peaks at 330 and 560 ${\mathrm{cm}}^{\mathrm{-}1}$ are observed. These peaks are located well above the phonon cutoff frequency of NaCl at 260 ${\mathrm{cm}}^{\mathrm{-}1}$, and are associated with local modes due to the ultrafine structure of the colloids.

Radiation-enhanced diffusion (RED) in a Ni-Zr metallic glass has been studied by high-energy electron irradiation performed in situ in a transmission electron microscope. Irradiations have been carried out on thin foil cross-sectional specimens obtained from Ni-Zr bulk diffusion couples. The diffusivity under electron irradiation has been derived from the growth rate of a thin Ni-Zr amorphous film present at the Ni-Zr interface. Experimental results show that the Ni is the most mobile species in these experimental conditions and that radiation damage occurs in glassy metals at a lower electron energy relative to the corresponding crystalline compound. Moreover, the dose-rate sensitivity of RED appears to depend also on the energy of the electron beam. To explain this effect, the process of radiation displacement in metallic glasses has been modeled within the framework of the free-volume theory of the structure of metallic glasses. The results of this simple model can qualitatively explain our results as well as those relative to RED induced by high-energy ion irradiation.

We have measured the light localization length for a disordered binary multilayer system, which we have prepared by alternating layers of antimonite and cryolite. The thicknesses of the cryolite layers are all the same, while those of antimonite vary randomly around an average value. The degree of disorder is given by the average fluctuation of thicknesses. The exponential decay of transmission intensity of light with the length of the multilayer is demonstrated experimentally. The localization lengths, the characteristic length of this exponential decay, are derived from experimental data in the wavelength region 720–900 nm for a fixed degree of disorder. The localization lengths at different incident wavelengths are computed by transfer-matrix approach. The agreement between the experimental measurements and the calculations is good.

Frequency dispersions with chain length have been experimentally determined from Raman-scattering data published previously on a series of oligomers and polymers of paraphenylene, paraphenylene vinylene, thiophene, N-protected pyrrole, pyrrole, and furan. The dispersion behavior changes noticeably in the different series of compounds. Conformational flexibility and the confinement of π electrons within each aromatic ring are the two factors considered for the explanation of such an observation. The π-electron confinement is analyzed in terms of the effective conjugation coordinate theory which is related to the amplitude mode theory. Ab initio calculations performed on model compounds are used to support the experimental evidence of the competition between π-electron confinement within the rings and delocalization along the chain.

We introduce a semimicroscopic discrete-state model appropriate to the orientational glass phase in mixed alkali halide cyanides with 〈111〉 equilibrium orientations of the ${\mathrm{CN}}^{\mathrm{-}}$ ions, such as K(CN${)}_{\mathit{x}}$${\mathrm{Br}}_{1\mathrm{-}\mathit{x}}$. The order-parameter fields are defined as symmetry adapted combinations of the occupation number operators along the cubic body diagonals, which transform according to the ${\mathit{T}}_{2\mathit{g}}$ representation of the cubic group. These interact via an infinite-range random interaction in the presence of quenched local random strains. We then use the replica formalism to derive a replica-symmetric solution for the components of the orientational-glass order parameter, the linear susceptibilities, and the elastic compliances. The high-temperature orientational-glass phase is characterized by an isotropic order-parameter matrix with only the diagonal elements ${\mathit{q}}_{\mathrm{\mu }}$ being nonzero. At high temperatures, the behavior of the order parameter q=${\mathrm{\Sigma }}_{\mathrm{\mu }}$${\mathit{q}}_{\mathrm{\mu }}$/3 is similar to that of an Ising spin glass, however, at intermediate and low temperatures the two models differ significantly. We also derive the instability line ${\mathit{T}}_{\mathit{f}}$(Δ) separating the replica-symmetric isotropic phase from the low-temperature anisotropic orientational glass phase, which is characterized by broken replica symmetry. In contrast to the random-bond–random-field model of an Ising spin glass, the instability temperature increases with random-field variance, implying that in quadrupolar glasses replica-symmetry breaking may be relevant already at relatively high temperatures. Finally, an expression for the distribution of local strains related to the NMR line shape is derived. It is also shown that the quadrupolar glass order parameter can be determined by NMR.

The central gap in the density of states and the localization of states on a Penrose lattice are investigated within a tight-binding model for the vertex problem. The results show that the gap is a consequence of frustration. There are surface states within the gap in a finite lattice. The correlation between the appearance of the gap and the localization of states is also studied.

The electronic structure and conductivity in a model approximant ${\mathrm{Al}}_{66}$${\mathrm{Cu}}_{30}$${\mathrm{Co}}_{14}$ of decagonal quasicrystals Al-Cu-Co is presented theoretically. As in most quasicrystals and approximants, the density of states exhibits a well pronounced pseudogap at the Fermi level, commonly attributed to the Hume-Rothery phenomenon. From the local density of states and band dispersion, wave functions of eigenstates at the Fermi level are analyzed. They are very flat bands, located on a specific group of atoms, and of p-d symmetry. This suggests the importance of hybridization between transition metals and aluminum in quasicrystals. The intraband electronic conductivity at T=0 K, calculated in Boltzmann theory, agrees quantitatively with experimental values. Its temperature dependence is also discussed. The strong anisotropy of decagonal phases has a crucial effect on all aspects of the electronic structure and then on transport properties and stability.

We present numerical calculations for the determination of localized modes in one-dimensional finite chains of atoms with free ends containing harmonic and quartic anharmonic interactions. By adding step by step the quartic term we can follow the formation of even and odd localized modes arising from the highest harmonic frequency mode. We have studied the role of crystal inhomogeneity by introducing a modification of the fourth-order force constant between neighboring atoms at the center of the chain, where the localized mode has its maximum displacement. For large weakening of this force constant the localized mode develops a double-peaked structure, as has been found in the continuum limit. In the case of asymmetrical local inhomogeneity the localized mode remains stable and moves toward the atom with the inhomogeneity. We also show the existence of anharmonic surface modes localized at the end of the chain.

We perform a numerical study of the dynamical properties of solitons in a one-dimensional deterministic aperiodic nonlinear lattice. The lattice is described by a discrete nonlinear Schrödinger equation, where the aperiodicity enters through an on-site potential chosen according to the substitutionally generated Thue-Morse sequence. Rapidly moving solitons are shown to propagate with small dispersion and radiation if they have a phase-modulation wavelength corresponding to regions of small amplitudes in the Fourier spectrum of the on-site potential. Such solitons also remain essentially unchanged after collision. Furthermore, we show that, for slowly moving solitons, an effective particle model is useful in order to understand some of the dynamic behavior, e.g., trapping of the solitons around points where they experience a local extremum in energy. The relation between the width and energy of the solitons is also discussed.

Comprehensive computer simulations of a tracer particle hopping in a lattice where the sites could be either empty or occupied by one or two background particles are performed using a standard Monte Carlo technique. The results for the diffusion coefficient are compared with those of a mean-field theory derived from a random-walk approach. The blocking effects due to dynamical background particles are obtained exactly. The correlation effects due to many-particle interactions are analyzed qualitatively. This kind of double-occupancy model can be applied to the study of ionic conduction in glasses.

Lattice dynamics of tin monoxide was investigated by Mössbauer spectroscopy in the range 16–294 K. From the temperature dependence of the absolute recoil-free fraction, f(T), the second moment of the phonon spectrum and the anharmonic constant ɛ(-2) were determined. These results suggest that the dominant anharmonic contribution arises from the cubic term of the vibrational phonon-phonon coupling Hamiltonian. The absolute recoilless fraction was determined at room temperature by the linewidth method and found to be f(295 K)=0.${12}_1$. No phase transitions were observed in this temperature range. The Mössbauer-line intensity-ratio-data temperature dependence displays a likely vibrational anisotropy.

Results of a reinvestigation of paramagnetic scattering from ${\mathrm{YbAl}}_3$ are presented. They confirm earlier observations of the gaplike spectral response at low temperatures (5 K), viz., a low level of magnetic scattering up to ∼30 meV followed by a sharp rise peaking on ∼32 meV forming a ridge to the bulk of the spectral response lying beyond. The gap in the magnetic response smears out rapidly with increasing temperature such that for temperatures close to and above the maximum in the bulk susceptibility (∼130 K), the observed scattering is indistinguishable from a quasielastic distribution, viz., a broad Lorentzian of half-width ∼35 meV centered on zero energy. Measurements performed using low-energy neutrons under closely similar experimental conditions as in another reported neutron study of ${\mathrm{YbAl}}_3$ demonstrate the strong influence of phonon scattering, misinterpreted by the authors of that investigation as representing a transition to a Kondo resonance around ∼10 to 12 meV at temperatures between 120 and 50 K.

The magnetic and structural phase diagrams of the C15 Laves phase system ${\mathrm{Dy}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Y}}_{\mathit{x}}$${\mathrm{Mn}}_2$ (0<x<1) have been mapped in detail using neutron powder diffraction. At concentrations x<0.35 and temperatures less than 45 K a complex antiferromagnetic structure, related to that of ${\mathrm{DyMn}}_2$, is observed. In this structure only one in four of the chemically equivalent Mn atoms carries a magnetic moment, of magnitude 1.4${\mathrm{\mu }}_{\mathit{B}}$. This moment is induced by a strong molecular field at this Mn site, which results from the canted magnetic structure of the Dy sublattice. At concentrations greater than x=0.75 the magnetic order is similar to that of ${\mathrm{YMn}}_2$: the transition to antiferromagnetism at T<100 K is of first order and is accompanied by a 5% expansion of the volume of the unit cell. This is a consequence of the condensation of local Mn moments, of magnitude 2.8${\mathrm{\mu }}_{\mathit{B}}$, from the spin-fluctuating state which characterizes the paramagnetic phase. For a wide range of intermediate compositions, 0.35<x<0.75, all long-range magnetic order is lost. However, a small (1%) cell-volume expansion persists across this magnetically disordered phase, implying the presence of an intrinsic Mn moment in this region. It is suggested that the loss of long-range magnetic order at intermediate compositions is due not simply to Dy-Y disorder, but also to competition between the magnetic structures associated with the induced and intrinsic moments.

We discuss magnetostatic modes in semi-infinite magnetic-nonmagnetic superlattices in an arbitrary-angle magnetization geometry. In this case, some very interesting nonlinearity features appear. We also discuss the consistency condition for the existence of surface waves.

We study the behavior of a model of two XXZ spin chains coupled by an isotropic exchange interaction ${\mathit{J}}_{\mathit{K}}$ as the anisotropies and spin velocities in the individual chains are varied. In the limit of large antiferromagnetic ${\mathit{J}}_{\mathit{K}}$ we map the model’s low-energy sector onto an Ising model in a transverse field; in the limit of large ferromagnetic ${\mathit{J}}_{\mathit{K}}$ we map the model’s low-energy sector onto that of a single spin-one chain. For small ${\mathit{J}}_{\mathit{K}}$, we study the model perturbatively via Abelian and non-Abelian bosonization. We determine the possible phases, approximate locations of the phase boundaries, universality classes of the transitions, and asymptotic behavior of correlation functions. We believe our model captures the essential physics of the interplay between the compensation of local moments and magnetic ordering, which may be important for the heavy-fermion materials.

We have studied the magnetic properties of ${\mathrm{Y}}_2$${\mathrm{CuO}}_4$, synthesized at high pressures. This cuprate crystallizes in the ${\mathrm{Nd}}_2$${\mathrm{CuO}}_4$-type structure (T’), characteristic of the electron-doped superconductors. Since the Y ions are nonmagnetic, this compound is very suitable to study the magnetism of the Cu lattice, without any interference from the rare earths. We have measured dc magnetization vs temperature in several magnetic fields up to 50 kOe, both after field cooling (FC) and zero-field cooling (ZFC) the samples, and we have made isothermal magnetization measurements vs field, from 5 to 340 K. We have also performed a detailed study of the ac-susceptibility dependence on the dc field, the ac field, and the frequency. All the measurements indicate a three-dimensional antiferromagnetic (AF) ordering of the Cu lattice below ${\mathit{T}}_{\mathit{N}}$=257(1) K, with the Cu spins slightly canted away from perfect AF alignment. This canting produces a weak ferromagnetic (WF) component for each ${\mathrm{CuO}}_2$ plane. However, at zero field, the AF coupling between different planes makes that the WF components remain almost compensated, and only after a metamagnetic transition can the weak ferromagnetic behavior be evidenced. Above ${\mathit{T}}_{\mathit{N}}$, the WF component is still induced by the field in a large temperature interval. Below ${\mathit{T}}_{\mathit{N}}$, an irreversibility line has been determined from hysteresis loops and magnetization vs temperature measurements in FC and ZFC conditions. It follows a de Almeida–Thouless law ${\mathit{H}}_{\mathrm{irr}}$∝(${\mathit{T}}_{\mathit{N}}$-${\mathit{T}}_{\mathrm{irr}}$${)}^{3/2}$. Below the irreversibility line, typical logarithmic relaxation processes of the ZFC magnetization are detected. Finally, an activated dynamic scaling describes the frequency dependence of the ac-susceptibility peak found below ${\mathit{T}}_{\mathit{N}}$ and associated with magnetic freezing processes. All these features are characteristic of the existence of finite-size weak ferromagnetic clusters.

Earlier magnetization and susceptibility measurements on the mixed magnet ${\mathrm{Co}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Mn}}_{\mathit{x}}$${\mathrm{Cl}}_2$⋅${2\mathrm{H}}_2$O, which has random competing short-range antiferromagnetic and ferromagnetic exchange interactions, revealed a spin-glass transition near 2.45 K over a wide composition range. The time dependence of the thermoremanent magnetization (TRM) below ${\mathit{T}}_{\mathit{g}}$ for an x=0.452 sample was found previously to conform approximately to decay of stretched exponential type. Small systematic deviations of data from fitted curves were apparent, however. Recently a percolation model for relaxation in random systems was proposed by Chamberlin and Haines, and shown to fit well the TRM decay in a Au:Fe spin glass and to account plausibly for relaxation in certain glasses. The model assumes dispersive excitations within fixed finite domains, and includes among its parameters the fastest and slowest relaxation rates characterizing the distribution of domains. We find that this model also leads to much better fits to the TRM decay in ${\mathrm{Co}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Mn}}_{\mathit{x}}$${\mathrm{Cl}}_2$⋅${2\mathrm{H}}_2$O, with systematic deviations either eliminated or much reduced. The variation of the fitted parameters with cooling field and temperature is explored. The prefactor ${\mathit{M}}_{\mathit{i}}$ displays a field dependence similar to that of the TRM at arbitrary time, and a temperature dependence consistent with the independently determined ${\mathit{T}}_{\mathit{g}}$ value. The correlation coefficient C decreases with increasing temperature somewhat faster than 1/T. The slowest relaxation rate for the largest antialigned domain, ${\mathrm{\omega }}_{+}$, increases with both field and temperature, while ${\mathrm{\omega }}_{\mathrm{-}}$, the fastest relaxation rate for the largest aligned domain, decreases with increasing temperature.