Table of contents

We give upper bounds on the photoproduction in overcritical electric fields of a neutral light boson which could explain the correlated e⁺e^- production seen at GSI-Darmstadt in heavy-ion-atom collisions.

We performed extensive Monte Carlo simulations of a hard-sphere f.c.c. crystal near the melting transition in order to examine the validity of the widely used Gaussian ansatz for the density distribution in solids. Anisotropic deviations in the shape of the density distribution from the Gaussian form are found to be of the order of 10%. Popular liquid-based density functional approximations are shown to fail in predicting the magnitude and the qualitative features of the anisotropy in the crystalline density distributions.

Modification of the surface of a metastable, supercooled phase can induce the transition to the equilibrium phase. The minimal supercooling temperature of the bulk phase is determined by both the strength and spatial extent of the surface treatment. Our results show that the temperature range where supercooling is possible can indeed vanish for strong enough surface treatments, in qualitative agreement with recent experiments which probe the effect of alcohol monolayers on the freezing of water.

The bosonic Hubbard model is studied via a simple mean-field theory. At zero temperature, in addition to yielding a phase diagram that is qualitatively correct, namely a superfluid phase for non-integer fillings and a Mott transition from a superfluid to an insulating phase for integer fillings, this theory gives results that are in good agreement with Monte Carlo simulations. In particular, the superfluid fraction obtained as a function of the interaction strength U for both integer and non-integer fillings is close to the simulation results. In all phases the excitation spectra are obtained by using the random phase approximation (RPA): the spectrum has a gap in the insulating phase and is gapless (and linear at small wave vectors) in the superfluid phase. Analytic results are presented in the limits of large U and small superfluid density. Finite-temperature phase diagrams and the Mott-insulator-normal-phase crossover are also described.

Raman-scattering experiments have been performed to study the successive phase transitions of K₂ZnCl₄ over the temperature range (100 ÷ 600) K. The spectra provide the temperature dependence of the mode frequencies and linewidths for two different spatial configurations: a(bb)c and c(aa)b. Special emphasis is put upon a new phenomenon observed in the spectra around 250 K in the c*/3 superstructure. Its kinetic character is revealed by complementary DSC measurements carried out over the range (120 ÷ 300) K. This phenomenon is interpreted as a progressive structural transformation which corresponds to a slow modification of the tetrahedron distortions.

The stratification dynamics of thin circular horizontal soap films was studied experimentally. A series of metastable circular stratification domains with different thicknesses were observed. Bubble-form instabilities of Rayleigh's type always appear at the borders of the thinnest domains. It has been found that the diameter of the stratification domains with bubble instabilities grows with a constant velocity, whereas in the absence of bubbles the diameter increases proportionally to the square root of time. A phenomenological model explaining this behaviour is proposed.

We report a study of the transverse-magnetoresistance anisotropy in the spin-density wave phase of the organic conductor (TMTSF)₂NO₃ at temperatures down to 1.8 K and in fields up to 6 tesla. The transverse magnetoresistance is highly anisotropic and obeys a temperature- and angular-dependent power law behaviour: Δρ/ρ₀ ≅ Hⁿ (1 ⩽ n < 2). The indication of a possible field-induced SDW phase is discussed. We suggest that the SDW with large imperfect nesting together with a periodic anion potential might be responsible for the observed magnetic-field effects.

The relationship between the mean energy of a two-dimensional electron gas and the first moment of their radiative recombination is investigated in the regimes of the integer and fractional quantum Hall effects. It is shown that the magnetic-field dependence of the first moment provides a method to measure the cyclotron, enhanced spin and quasi-particle energy gaps at the same time. This allows for a comparative analysis of the temperature dependence of these gaps. Upon increasing temperature an abrupt thermal collapse is observed for those energy gaps which are associated with electron-electron interactions.

An effect of screening of impurity charge by electrons in leads on the resonant tunnelling is studied. We find that the one-electron description of the conductance behaviour is drastically changed because a renormalization occurs of the one-electron resonance halfwidth Γ₀: _r ∝ Γ₀(Γ₀/D)^{α/(1 - α)}. α is determined by the phases of electrons scattered by the charge impurity and D is the conduction band width (D ≫ Γ₀). In the case of a non-symmetrical junction the resonance conductance turns out to be additionally suppressed by the Coulomb interaction. Possibilities of experimental observation are discussed.

Electrons at the van Hove point of a two-dimensional band model are studied in the weak-coupling limit as the condensation of the AF mode is approached from the paramagnetic phase. The interaction with fluctuations is treated in the one-loop approximation. It is found that for underdamped fluctuations the quasi-particle peak at the Fermi level is preserved down to the transition point, i.e. the van Hove singularity is not suppressed by the pseudogap, in contrast to what was found previously in the adiabatic approximation. However, the strength of the peak vanishes quadratically with the frequency of the AF mode when the latter is small.

The time evolution of nuclear Bragg diffraction of 14.4 keV synchrotron radiation in a ⁵⁷FeBO₃ crystal was studied when the crystal was excited to magnetoelastic vibrations by a r.f. field of 2.93 MHz. The vibrations did not modify the time evolution of the diffraction. This result is due to the fact that after the excitation by the SR pulse the nuclear vibrations cause only a phase modulation of the re-emitted radiation. Energy spectra, by contrast, are extremely sensitive to vibrations. Thus the present case is an instructive example of complementarity between energy and time domain Mössbauer spectroscopy. Time domain spectroscopy, in particular, will allow to reveal phenomena, which are blurred in the energy spectra by vibrations.

I study an ensemble of bacteria colonies. Finite-size colonies always die out. Their lifetime t₁ is either size independent or exponentially increases with size. In the latter case, their lifetime mean quadratic fluctuation δt is large compared to their representative average lifetime = exp [⟨ln t₁⟩], ⟨... ⟩ denotes the ensemble average. The extinction-survival phase transition from finite to infinite is accompanied by the phase transition. These results are common for spreading-percolation phenomena.