Nuclear magnetic resonance experiments are reported on multilayer ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ films adsorbed on the surface of graphite. Initially the second layer exhibits antiferromagnetic exchange, which has been measured as a function of coverage. Measurements of the nuclear magnetic susceptibility and resonant frequency for coverages around the third layer promotion precisely identify the onset of ferromagnetic exchange in the film. The evolution of the structure of the film is discussed in the light of these results and previous heat-capacity and NMR data.

The zero-field-cooled magnetization M(H,T) of a single crystal of ${\mathrm{Tl}}_2$${\mathrm{Ba}}_2$${\mathrm{CuO}}_6$ (${\mathit{T}}_{\mathit{c}}$=82.5 K) has been measured with the field applied parallel to the ${\mathrm{CuO}}_2$ planes. For field sweeps carried out below T=50 K and temperature sweeps carried out in applied fields H≥25 Oe, a large jump in magnetization is observed at H=${\mathit{H}}_{\mathit{d}}$, close to the field of first flux penetration. The data provide strong evidence that ${\mathrm{Tl}}_2$${\mathrm{Ba}}_2$${\mathrm{CuO}}_6$ behaves as a weak-Josephson-coupled layered superconductor, the discontinuity arising from the destruction of Josephson screening currents between the planes at a field close to the lower critical field ${\mathit{H}}_{\mathit{c}1}$. The value of ${\mathit{H}}_{\mathit{d}}$ is an order of magnitude lower than the predicted surface-barrier penetration field.

The hypersonic sound velocity and refractive index of ${\mathrm{SiO}}_2$ glass has been measured to 57.5 GPa at room temperature by Brillouin scattering in diamond cells. On compression, both longitudinal and tranverse modes exhibit an anomalous change in slope in the low-pressure region. Between 12 and 23 GPa, the sound velocities increase rapidly. At higher pressures, the bulk velocity follows a trend similar to that expected for coesite. At 57.5 GPa, the longitudinal velocity of ${\mathrm{SiO}}_2$ glass is 11.85 (±0.51) km/s and the transverse velocity is 6.12 (±0.06) km/s. The refractive index increases monotonically with pressure and reaches a value of 1.924 (±0.081) at 57.5 GPa. It decreases reversibly on decompression to 26 GPa, but displays an irreversible change when decompressed from 16 GPa to ambient pressure. The pressure-density relation calculated from the measured sound velocities within the elastic compression region is in good agreement with several previous determinations.

Elastic constants of the high-temperature phases of the shape-memory alloys NiTi and PdTi are determined by means of full-potential linearized augmented-plane-wave total-energy calculations. Thereby, insight into the driving mechanism of the martensitic transformation of these alloys can be obtained. In the case of NiTi, the c’ and ${\mathit{c}}_{44}$ moduli are found to be low and to decrease with rising pressure. We find that the (parent) PdTi phase is metastable with respect to tetragonal deformations. From these data, we can explain the different transformation behavior of the two alloys. In the case of PdTi, the Ti-Ti bonding seems to be responsible for the formation of the martensitic phases. Experimental data, if available, are found to be in good agreement with the results of our calculations.

A method of constructing a susceptibility scaling function and extracting critical exponents from experimental susceptibility data for physical systems with discontinuous phase transitions and/or unsymmetrical critical points induced by a nonzero field is proposed. The coordinates of these points do not appear in the derived form of the scaling function. The transformation of the field E conjugate to the order parameter to a new field h=E-${\mathit{E}}_0$(T) where ${\mathit{E}}_0$(T) represents the critical isochore and/or two-phase equilibrium line, is shown to reduce the scaling function for unsymmetrical systems to that for symmetrical ones. Transforming original isothermal experimental data for deuterated triglycine selenate with a deuterium content of x≃100%, the asymmetrical scaling function and values of the nonzero-field critical invariants δ, a=Δ/γ, and Q (equal to 1.5, 3, and 1.2, respectively) were obtained.

We present resonant ultrasound spectroscopy measurements of the complete elastic moduli of ${\mathrm{La}}_{2\mathrm{-}\mathit{x}}$${\mathrm{Sr}}_{\mathit{x}}$${\mathrm{CuO}}_4$ (x=0.00 and 0.14). The x dependence of both the absolute values of the moduli in the tetragonal phase and the temperature dependences of the moduli near the structural phase transition is weak. In both materials ${\mathit{c}}_{44}$ and 1/2 (${\mathit{c}}_{11}$-${\mathit{c}}_{12}$) are unaffected by the transition, ${\mathit{c}}_{33}$ shows a steplike drop broadened by fluctuations, and ${\mathit{c}}_{11}$ and ${\mathit{c}}_{66}$ decrease as 1/(T-${\mathit{T}}_{\mathrm{SPT}}$) for T>${\mathit{T}}_{\mathrm{SPT}}$ (where ${\mathit{T}}_{\mathrm{SPT}}$ is the structural phase transition temperature). We also present measurements of the anisotropic thermal expansion coefficients in ${\mathrm{La}}_{1.86}$${\mathrm{Sr}}_{0.14}$${\mathrm{CuO}}_4$ using a high-resolution capacitance dilatometer. These data when taken together raise serious questions about the standard interpretation of the structural phase transition in ${\mathrm{La}}_{2\mathrm{-}\mathit{x}}$${\mathrm{Sr}}_{\mathit{x}}$${\mathrm{CuO}}_4$.

The hydrostatic-pressure-induced kinetics of paraelectric-ferroelectric phase transitions is considered on the basis of the time-dependent Ginzburg-Landau theory. The exact solution describing the interphase boundary at first-order ferroelectric phase transitions is used for analyzing the movement of the interphase boundary under the influence of hydrostatic pressure. Temperature and pressure dependences of the interphase boundary width and velocity are calculated by using the experimental data in ${\mathrm{BaTiO}}_3$. Power laws of the temperature and pressure behavior of the interface velocity v=v(‖T-${\mathit{T}}_{\mathit{c}}$‖) and v=v(‖p-${\mathrm{\rho }}_{\mathit{c}}$‖) are found. The pressure hysteresis is calculated.

Electric-field-modulated two-pulse photon-echo experiments were performed in heavily doped ruby crystals containing photoinduced domains possessing internal electric fields. The photon-echo modulation patterns are sensitive to the photoelectric domain structure in the crystals. Studies as a function of the relative orientation of the optical excitation beam and the crystal axes make it possible to obtain information about the size and shape of the domains. The domains occur in sheets oriented perpendicular to the c axis and are about 20 μm in thickness.

A possible mechanism of the laser-field-induced superionic transition in the near-surface region is proposed theoretically in ionic-electronic materials, i.e., mixed conductors. The laser field does not expose ions immediately but via intermediate interaction with conduction electrons and induces many nonequilibrium defects in the Ag sublattice. These processes cause the decreasing of activation energy of stable Frenkel defects and, consequently, the decreasing of the superionic transition temperature.

White-beam synchrotron-radiation topography is applied to an investigation of the second-order ferroelastic-paraelastic phase transition in ${\mathrm{NdP}}_5$${\mathrm{O}}_{14}$ single crystals. The displacements of images of adjacent domains in the 200 reflection topographs and their variation during the transition are directly observed with a television recording system. The spontaneous deformation, a monoclinic shear strain ${\mathit{e}}_5$, is calculated from the image displacements at different temperatures. The temperature dependence of ${\mathit{e}}_5$ and the evolution of domains are well consistent with the Landau theory of second-order phase transitions. The transition process is also observed under a polarizing microscope.

The phase-transition sequence undergone by [N(${\mathrm{CH}}_3$${)}_4$${]}_2$${\mathrm{ZnI}}_4$ has been studied by adiabatic calorimetry. The specific-heat results show the P${112}_1$/a-P$2_1$ca phase transformation around 215 K to take place in two independent steps. The corresponding specific-heat anomalies differ by only ≊0.4 °C on heating, and ≊3 °C on cooling. An unusual thermal hysteresis effect has been found to be present, which leads to the reversal in the order of appearance of the two specific-heat peaks. This fact has been unambiguously confirmed by means of an exhaustive experimental specific-heat study and by the use of the thermodynamic-functions values associated with this anomalous phase transformation.

The temperature dependence of the conductance and the nonlinear electrical response of a Pt-film percolation system, deposited on fracture surfaces of α-${\mathrm{Al}}_2$${\mathrm{O}}_3$ ceramics, have been measured over three decades of sheet resistance. We find that in the temperature interval of T=77–300 K, the resistance temperature coefficient β=(1/R)dR/dT is not a constant, which is different from that for flat films. A dc I-V characteristic which strongly depends on the thickness of the film is found and it can be interpreted as a competition among the local Joule heating, hopping, and tunneling effects. The third-harmonic measurement suggests that the critical exponent comes from 1/f noise, which obeys the power-law dependences ${\mathit{S}}_{\mathit{R}}$∝(p-${\mathit{p}}_{\mathit{c}}$${)}^{\mathrm{-}\mathrm{\kappa }}$, R∝(p-${\mathit{p}}_{\mathit{c}}$${)}^{\mathrm{-}\mathit{t}}$, and then ${\mathit{S}}_{\mathit{R}}$∝${\mathit{R}}^{\mathit{w}}$ with w=κ/t, where ${\mathit{S}}_{\mathit{R}}$ is the mean square of resistance fluctuations, p the surface coverage fraction, and ${\mathit{p}}_{\mathit{c}}$ its percolation critical value. We find that w=0.45±0.06, which is lower than the flat-film exponent. This result indicates that the tunneling and hopping effects in the fractal samples are much stronger than that of flat films.

We have measured quantitatively the temperature dependence of the local distortions of ${\mathrm{PbTiO}}_3$ crystals below and above the structural ferroelectric phase transition, using x-ray absorption fine-structure (XAFS) measurements. Two probe atoms, Pb and Ti, were used. The results were analyzed by fitting parametrized theoretical XAFS spectra to the experimental results. These measurements provide quantitative ‘‘distortion parameters’’ defined as the difference between the distance of the probe to its nearest neighbor measured in the actual structure and that in a centrosymmetric structure with the same unit cell dimensions. At low temperatures the Pb-edge spectra were fit using four shells and included single, double, and triple scattering configurations. The high-temperature fits included only two shells. The Ti-edge spectra were fitted with the first shell only. The Pb and Ti distortions vary relatively little with temperature below the transition and decrease faster near the transition temperature. Above the transition temperature, the Pb and Ti distortions at the peaks of their distortion distribution function (DDF’s) are both more than 70% of the corresponding low-temperature values. These results show that an essential element of order-disorder is present even in this nominally pure ferroelectric crystal which displays a soft mode and a dielectric constant typical of displacive-type ferroelectrics. The presence of the local distortions suggests that the displacements should have at least two correlation length scales, one associated with the local distortions, the other with the order parameter.

The crystal structure and phase stability of selenium and tellurium have been investigated by total-energy calculations within the local-density approximation (LDA), without and including generalized-gradient corrections (GGC). We find that the LDA underestimates the equilibrium volume by as much as 15% and predicts a crystal structure that is much more isotropic than observed: the nearest-neighbor distances in the helical chains are increased, but the interchain distances are reduced. Adding generalized-gradient corrections to the LDA results in a crystal structure, equilibrium volume, and binding energy in better agreement with experiment. We show that the effect of the GGC’s is to add a uniform internal pressure to the system, the local bonding properties at constant volume, however, are unchanged.

Ground-state recovery measurements on the F center in NaI were performed with time resolutions of both 7 ps and 200 fs. The measurements confirm the expectation of an electronic relaxation with a decay time of the order of tens of nanoseconds, which can be associated with a nonradiative transition from the relaxed excited state (RES) at low temperatures. Different from the case of NaBr, the decay channel related to the recapture of conduction electrons cannot clearly be distinguished. This is possibly related to the fact that the available time delay is considerably shorter than the ground-state recovery in the temperature range in which the dominant contribution changes from the component related to the RES lifetime to the retrapping component. Measurements of the ground-state recovery with a 200-fs time resolution show that a very fast relaxation component is present for the F center in NaI. It possesses a time constant of 9 ps at low temperature and its relative contribution with respect to the total induced transparency signal is roughly 33%. The temperature dependence of its time constant follows closely the vibrational lifetime derived from linewidth measurements of the resonant Raman spectrum. This feature is expected for the contribution to the induced transparency arising from an electronic transition during vibrational relaxation, the so-called ‘‘crossover process.’’

The mechanism of redistribution of vacancies in the depleted zone of a displacement cascade in copper has been investigated by molecular-dynamics simulations. A simplified model of the thermal spike was used, for which the calculation cell was cooled down along one axis after vacancies and kinetic energy had been introduced in the center of it. The influence of electron-phonon coupling on the sweeping of vacancies in this highly disordered hot core as it cools was then investigated. The characteristic cooling time, τ, for transfer of heat to the electron system by ion-electron interaction was a variable parameter and was varied from 1 to 10 ps. According to Finnis et al. [Phys. Rev. B 44, 291 (1991)] the values of τ≤1 and τ≥5 ps are typical of metals such as Ni and Cu, respectively. In the conditions of the nonuniform distribution of the thermal spike, the maximum size of the real melt becomes a function of τ. The influence of the strong coupling on the vacancy redistribution is more strongly pronounced for thermal spikes in which the average atomic kinetic energy in the melt is close to the value (3${\mathit{k}}_{\mathit{B}}$${\mathit{T}}_{\mathit{m}}$+L), where ${\mathit{T}}_{\mathit{m}}$ is the melting temperature and L is the latent heat of fusion. In this case the time to form the liquid structure by melting is comparable with the time to cool the zone to below the temperature of crystallization. It has been demonstrated that the coupling also influences the number of vacancies trapped by the melt during the advance of the solid/liquid interface. It is shown that in metals with strong electron-phonon coupling, the highly disordered structure of the melted region can be frozen in when a depleted zone with a high concentration of vacancies (≥2–5 at. %) solidifies.

Molecular-dynamics computer simulations of collision cascades in intermetallic ${\mathrm{Cu}}_3$Au, ${\mathrm{Ni}}_3$Al, and NiAl have been performed to study the nature of the disordering processes in the collision cascade. The choice of these systems was suggested by the quite accurate description of the thermodynamic properties obtained using embedded-atom-type potentials. Since melting occurs in the core of the cascades, interesting effects appear as a result of the superposition of the loss (and subsequent recovery) of the crystalline order and the evolution of the chemical order, both processes being developed on different time scales. In our previous simulations on ${\mathrm{Ni}}_3$Al and ${\mathrm{Cu}}_3$Au [T. Diaz de la Rubia, A. Caro, and M. Spaczér, Phys. Rev. B 47, 11 483 (1993)] we found a significant difference between the time evolution of the chemical short-range order (SRO) and the crystalline order in the cascade core for both alloys, namely the complete loss of the crystalline structure but only partial chemical disordering. Recent computer simulations in NiAl show the same phenomena. To understand these features we study the liquid phase of these three alloys and present simulation results concerning the dynamical melting of small samples, examining the atomic mobility, the relaxation time, and the saturation value of the chemical short-range order. An analytic model for the time evolution of the SRO is given.

We have studied by incoherent quasielastic neutron scattering the molecular dynamics of the ferroelectric copolymers of vinylidene fluoride and trifluoroethylene with two different molar ratios. Thermal hysteresis effects accompanying the structural changes observed in these materials are found. In the temperature range investigated three motions were identified: (i) a vibrational motion of Debye-Waller type which shows increasing amplitude until the vicinity of the Curie temperature and then it follows a ${\mathit{Q}}^4$ law, (ii) a slow jump diffusive motion of each chain in the restricted volume defined by other chains, thermally activated with an activation barrier of ≊8 kcal/mol, and (iii) a fast local diffusive motion with only some fraction of the chains concerned.