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When electrons tunnel through a double-barrier structure, there exists a region of bistability (or tristability) in the current-voltage characteristics due to the dynamical charge feedback effect in the resonant well. We propose a mechanism of acoustic-phonon-assisted tunnelling to explain the experimentally observed non-monotonic behaviour of the width of the tristable region. It is found that this width has a single maximum as experimentally observed. Furthermore, contrary to the previous proposal, our result shows that electron thermal activation is not the dominant mechanism in controlling this temperature-dependent width.

Measurements are reported of the magnetic phase transition temperatures of a Cr_99.7Ru_0.3 alloy doped with V to decrease the electron concentration, and with Mn to increase it. A similarity is found between the pressure-temperature and electron concentration-temperature magnetic phase diagrams of this system by using a scaling factor of 4.4 GPa per 1% change in the electron concentration per atom.

Formation and photobleaching of optical absorption bands in the 5 eV region were examined in proton-implanted 5GeO-95SiO₂ glasses and SiO₂ glasses implanted with Si, Ge, B or P ions. A conspicuous increase in the intensity of the 5 eV band, which is attributed to oxygen vacancies, was seen in both substrates after implantation. However, a distinct difference was observed in bleaching with 5 eV light between the H-implanted SiO₂:Ge glasses and the implanted SiO₂ glasses. In the former the 5 eV band was bleached and intense absorptions above 5 eV emerged, whereas in the latter bleaching occurred in the whole UV region. This distinct bleaching nature provides implanted glasses with the possibility of modification of the refractive index by UV illumination.

Oxygen K edge and iron L_2,3 edge X-ray absorption spectra of SmFeC₃ have been measured and theoretical analysis of these data on the basis of full multiple-scattering formalism in real space has been made. Combined oxygen K edge and Fe L₃ edge XANES is shown to be a useful tool for the study of RE orthoferrite electronic structure. The full multiple-scattering approach is found to be successful in analysing experimental oxygen K edge XANES in SmFeO₃, while the single-scattering approximation is not valid in this case. Unoccupied oxygen p states interact with Fe 3d in a specific resonance manner.

The halide perovskite tetramethylammonium trichlorogermanate(II), N(CH₃)₄GeCl₃, undergoes a structural phase transition at 200 K from an orthorhombic room-temperature phase to an incommensurately modulated orthorhombic phase and at 170 K a further transition to a phase characterized by the coexistence of a monoclinic and an orthorhombic Bravais lattice. It is proposed that the superposition of the two Bravais lattices results from the low bulk modulus (9 GPa) in conjunction with the formation of ferroelastic domains. Local stresses implied by adjacent monoclinic domains allow the soft material to form orthorhombic domain boundaries. Two order parameters xi and eta governing the phase transition at 170 K can be related to a translational and a rotational part of the displacive modulation of the rigid GeCl₃ and tetramethylammonium molecules. Experimental evidence suggests that the displacive modulation in the incommensurate phase is related to short-range disorder observed in the room-temperature phase.

We study properties of multirelaxational phenomena originating due to interactions of modes in the incommensurate dielectrics of the order-disorder type described by the Ising-like model and the Bloch equations. Results found theoretically are discussed in relation to observed properties of some dielectric materials including NaNO₂.

The coefficients governing the dipole-quadrupole dispersive attractions between all the ions in solid magnesium oxide are computed ab initio using the coupled Hartree-Fock method. These confirm for oxides the previous conclusion that the Starkschall-Gordon formula provides the most reliable values for systems not amendable to ab initio computation. Combination of the ab initio results with semiempirical estimates of the much smaller contributions from electron correlation produces improved coefficients. The cohesion predicted for MgO with the fully ionic model remains essentially unchanged on replacing the Starkschall-Gordon dipole-quadrupole dispersion coefficients with the improved values. The use of improved predictions for C₈(O^2-...O^2-) in solid CaO, ThO₂ and UO₂ in place of Starkschall-Gordon values leaves essentially unchanged the cohesion predicted for CaO whilst very slightly but significantly increasing that predicted for ThO₂ and UO₂. Theory and experiment for ThO₂ then agree as excellently as for MgO and CaO, thus showing no evidence for any covalency.

Electronic structures and hyperfine interactions associated with the Cu-O environment of T, T*, and T' phases of (La_1-xGd_x)₂CuO₄ were studied, using the density functional theory in an embedded cluster approach. Mossbauer isomer shifts, electric field gradients and contact hyperfine fields for Fe-substituted species were determined for comparison with experiments performed on Fe:(La, Gd)_1.85Sr_0.15CuO₄. These data are used to describe the response of charge and spin densities around Cu sites to sixfold (T), fivefold (T*), and fourfold (T') oxygen coordination.

We study the magnetic properties of the one-dimensional Hubbard model under fixed chemical potential. By using derivatives of the Lieb-Wu equation, the magnetization curve at zero temperature is obtained for various fixed band fillings and/or chemical potentials. As a well known result, in the case of the fixed band filling, there is only one second-order phase transition at magnetic saturation. On the other hand, when the chemical potential is fixed instead of the electron density, it is found that there is an additional phase transition in the magnetization curve.

The authors have studied the dynamics following a localized excitation of a molecule in a chain of stacked ring-shaped aromatic molecules. As model system they have chosen a cytosine stack, since their study is based on recent ideas on the mechanism for the creation of double-strand breaks in DNA which require energy transport through DNA. After a pi - pi *-excitation a cytosine molecule increases the size of its ring. The excitation and the accompanying geometrical changes are coupled to the electronic system of the chain in a nonlinear manner. As a mathematical model for the pi -electrons the authors use a tight-binding ansatz with parameters taken from ab initio Hartree-Fock calculations. They found that initially an excitation is localized on one molecule and that these localized excitations move in a random manner through large parts of the chain. Although this phenomenon most probably is not a solitary wave, the nonlinear quasiparticles found in the authors' system transport energy along the chain.

The 1D delta -potential with strength Omega in an external in-plane magnetic field B is considered theoretically. Energy dependences on Omega and d (with d being normalized to the magnetic radial r_B=(h(cross)/eB)^1/2 distance between the delta -potential and the vertex of the magnetic parabola) are calculated and analysed in detail in the whole ranges of d and Omega . It is shown that at d=d_jk the jth Landau level is not affected by the presence of the potential, where d_jk are the zeros of the jth-degree Hermite polynomial. For d=0, every excited even level with number j changes its energy normalized to h(cross) omega _B ( omega _B is the cyclotron frequency) from j- 1/2 at Omega =- infinity through j+ 1/2 at Omega =0 to j+3/2 ( Omega =+ infinity ). However, the ground-state energy asymptotically tends to - infinity while Omega varies from + infinity to - infinity . A simple physical explanation of the results obtained is given. Anticrossings of the levels in the dispersion relations are investigated for all values of Omega . For fixed positive (negative) Omega , every jth level (j=0, 1, ...) has, on the d axis, j minima (maxima) with absolute value j+ 1/2 , which are achieved at d_jk. It is derived that, in the anticrossing picture, gaps between levels at the points d_jk decrease on increase in mod Omega mod , and levels cross at Omega =+or- infinity . For large negative Omega a picture of the interaction of the ground state with excited levels is given. The proposed model with the simplest modification is used to investigate the combined influence of the crossed electric and magnetic fields. This problem enables comparison with earlier results for a delta -like quantum object in a uniform electric field.

The direct influence of a d.c. electric field on weak localization is investigated on the basis of a quantum diffusion model for an anisotropic two-dimensional electron gas. The anisotropy is treated both in a model of anisotropic effective masses and in a tight-binding model for parallel chains. The crossover of the delocalization edge from a power-law 1D-like dependence to an exponential decrease, characteristic for a 2D system, is investigated. The appearance of a delocalization edge gives rise to a long-time power-law current relaxation according to j(t) approximately 1/ square root t, independently of the anisotropy.

In contrast to the ground state of the Affleck-Kennedy-Lieb-Tasaki (AKLT) isotropic spin-1 Hamiltonian its elementary excitations are not known exactly in general. Based on a simple variational ansatz we construct approximate excitations in the form of dressed hidden domain walls. This allows us to give an improved upper bound for the gap of the AKLT Hamiltonian which is only 1% larger than the best currently available numerical result. We compare the oneand two-soliton dispersion curves with low-lying excitations for a chain of 16 sites as given by Fath and Solyom and find excellent agreement. Spin and string correlations are calculated and found to agree in leading order with the corresponding correlations for the bare kinks. Finally the weight of the string-ordered part of the lowest excited state is calculated and compared with the exact numerical result.

We have searched for the hidden multispin order parameters that have been proposed to explain the weak ordered moment observed in the heavy-fermion superconductor URu₂Si₂. Since the exotic spin order is predicted to cause broken-symmetry Bragg peaks that develop rapidly with applied magnetic field, we have measured their strength in fields up to 8 T, for which a large easily observable moment is predicted. The Bragg neutron scattering is found to decrease rather than increase when a field is applied along the easy tetragonal c-axis, while there is no change for a field at right angles. The results rule out the proposed symmetric multispin order parameters. The zero-field transition is found to exhibit critical softening of the spin gap and a cusp in the susceptibility at T_N, as expected for the ordering of magnetic dipoles.

In this paper we present a plausible explanation for the exponential relaxation in TbFeO₃ single crystals observed below 3 K. Our interpretation is based on the formation and consequent propagation of the nucleated domain wall at the (010) face. The analytical calculations along the lines of current theories of macroscopic tunnelling of magnetization are in agreement with experimental results.

The non-polarized Raman scattering spectra of indium-doped PbTe single crystals were measured in the temperature range between 10 and 300 K. Well resolved peaks at about 68, 126 and 143 cm^-1 were observed for all temperatures. An additional mode appears at about 115 cm^-1 for temperatures below 100 K. The intensity of this mode increases sharply when the temperature is lowered below 25 K, the temperature where a persistent photoconductivity effect in PbTe(In) appears. This mode is assigned as a local In impurity mode and represents a population of metastable states due to the transfer of electrons from two-electron to one-electron metastable impurity states.

There are two non-equivalent sites of yttrium in the lattice of Y₂SiO₅. Both of them may be occupied substitutionally by Er³⁺, resulting in a laser active material. We measured absorption spectra of Y₂SiO₅:Er³⁺ at several low temperatures and we studied its emission spectrum at 4.2 K following selective excitation of individual Stark components of the ⁴I_13/2 to ⁴ I_15/2 transition by the light of a tunable colour centre laser. The results of these two experiments allow classification of the absorption and emission lines into two independent systems, thus yielding two energy level schemes 1 and 2. Moreover, simple crystal field calculations strongly suggest from which Y₂SiO₅ crystallographic site spectrum 1 arises, and from which one spectrum 2.

The optical absorption of the Tm³⁺ ion in the gadolinium oxychloride (GdOCl) matrix in the UV, visible, and NIR range was studied at temperatures between 9 and 300 K. The visible luminescence of GdOCl:Tm³⁺ under Ar⁺ ion laser and mercury lamp excitation was recorded at 9, 77, and 300 K, too. The crystal field (CF) splitting of the ³H_4-6, ³F_2-4, ¹G₄, ¹D₂, and ¹I₆ levels of the Tm³⁺ ion deduced from the spectra was analysed according to the C_4v point symmetry of the RE³⁺ site. The resulting energy level scheme, consisting of 39 levels (i.e. 55 Stark components) out of the total of 70 (91) for the whole 4f¹² configuration, was simulated with the aid of a phenomenological theory taking simultaneously into account both the free-ion and CF effects. The model included 13 adjustable parameters describing the electrostatic (the Racah parameters E_0-3) and the configuration interaction (the Trees parameters alpha , beta , and gamma ) as well as the spin-orbit coupling (the coupling constant zeta _4f). The CF effect was accounted for by the five non-zero B_q⁴ parameters B₀², B₀⁴, B₄⁴, B₀⁶, and B₄⁶. Good simulation of the experimental energy level scheme was achieved with a root mean square deviation equal to 21 cm^-1. A comparison to the energy level parametrization of the Pr³⁺ (4f²), Nd³⁺ (4f³), Eu³⁺ (4f⁶), and Tb³⁺ ions (4f^B electron configuration) in other REOCl matrices shows the consistency of the present results. The systematic trends in the CF effect on the energy level schemes of the RE³⁺ ions are discussed, too.

Annihilation characteristics of positrons in vitreous silica (v-SiO₂) were studied in the temperature range between 20 K and room temperature (RT). In the measured temperature range, almost all positrons are trapped by open spaces and annihilate mainly from positronium (Ps) states. Lifetimes of ortho-Ps increase with increasing temperature, where the distribution of the lifetimes at 20 K is shallower than that at RT. This fact was attributed to the thermal expansion of the open spaces and the broadening of their size distribution. In the temperature range between 170 K and 200 K, a certain structural transition of the open spaces was found. The transition temperature is decreased by electron irradiation or by the presence of hydroxyl groups. These results were attributed to the fact that an activation energy of the transition was lowered by the introduction of defects.

Calculations of the permittivity tensor (PT) of a hexagonal BN (h-BN) crystal carried out near the B K edge by the Green function method show that the anisotropy of the PT can be considerable in the X-ray region of the spectra. Such anisotropy causes pronounced crystal optic effects in X-ray absorption spectroscopy such as the existence of X-ray polaroid and quarter-wave plate effects. It is shown that absorption spectra of the left and right circularly polarized radiation can differ from each other in biaxial crystals. Anisotropy of PT leads to violation of the exponential decrease of X-ray intensity.