Table of contents

Co/Ag multilayers were prepared by an alternate deposition of Co and Ag metals using electron beam evaporation. These multilayers were irradiated with 130 keV argon ions at room temperature. Upon irradiation to a fluence of 7 × 10¹⁶ ions cm⁻², a new metastable phase (DO₁₉) with hcp structure having nanosized grains embedded in a matrix having an amorphous structure was observed.

In an extension of our previous study on the electron polarons and excitons in KNbO₃, KTaO₃ and BaTiO₃ (Kotomin E A, Eglitis R I and Borstel G 2000 J. Phys. Condens. Matter12 L557; Eglitis R I, Kotomin E A and Borstel G 2002 J. Phys. Condens. Matter14 3735) by the semiempirical Hartree–Fock method we present here results for free electron polarons in the PbTiO₃ perovskite crystal. We discuss the origin of the intrinsic visible band emission of PbTiO₃ perovskite oxides (so-called 'green luminescence') which has remained a topic of high interest during the last quarter of a century. We present a theoretical calculation modelling this emission in the framework of a concept of charge transfer vibronic excitons, i.e. as a result of radiative recombination of correlated (bound) self-trapped electron and hole polarons in the highly polarizable PbTiO₃-type matrix. The intermediate neglect of differential overlap method combined with the large unit cell periodic defect model was used for quantum chemical calculations and theoretical simulation of the green emission for a PbTiO₃ perovskite. The calculated 'green' luminescence energy for PbTiO₃ perovskite-type crystals agrees well with experimental measurements presented in this letter.

It is shown that a new type of superconductivity is possible in one-dimensional (1D) and quasi-one-dimensional (Q1D) metals where the pairing interaction is mediated by both spin and charge fluctuations. The gap function inevitably changes sign in the radial direction near the Fermi surface; it vanishes at the Fermi points for 1D, and for Q1D it has line nodes on the Fermi surface as in usual anisotropic pairings. The transition temperature T_c is generally higher for the singlet pairing than for the triplet, but only by a few times. This implies that T_c for the triplet pairing can exceed that of the singlet under a moderately large Zeeman magnetic field, i.e., field-induced triplet pairing. This feature seems to explain the anomalous behaviours of H_c2 and the Knight shift observed in (TMTSF)₂PF₆.

Studies on a large number of compositions among the ferromagnetic self-doped manganites, La_1−xMnO₃, show that the ferromagnetic Curie temperature increases with x and reaches a maximum value for x = 1/7. T_c remains independent of x for x ≥ 1/7 and the maximum value of the magnetization is obtained for x = 1/8. The presence of a second phase, due to Mn₃O₄, whose contribution increases with x, is observed in the powder x-ray diffraction patterns for x > 1/8 and the lattice parameters remain independent of x for x > 1/8. The results indicate that the limiting value of x or the extent of self-doping possible in La_1−xMnO₃ is x = 1/8.

Single-electron devices (SEDs) are attracting a lot of attention because of their capability of manipulating just one electron. For their operation, they utilize the Coulomb blockade (CB), which occurs in tiny structures made from conductive material due to the electrostatic interactions of confined electrons. Metals or III–V compound semiconductors have so far been used to investigate the CB and related phenomena from the physical point of view. However, silicon is preferable from the viewpoint of applications to integrated circuits because, on a silicon substrate, SEDs can be used in combination with conventional complementary metal-oxide-semiconductor (CMOS) circuits. In addition, the well established fabrication technologies for CMOS large-scale integrated circuits (LSIs) can be applied to making such small structures. LSI applications of the silicon SEDs can be categorized into two fields: memory and logic. Many kinds of device structure and fabrication process have been proposed and tested for these purposes. This paper introduces the current status of silicon-based SED studies for LSI applications.

In this overview, we discuss theoretical and experimental aspects of nonlinear two-dimensional infrared (2D-IR) spectroscopy. With this technique both peptide conformation and conformational flexibility can be probed. The quantitative relation between the experimental 2D-IR spectrum and the peptide conformation is discussed, and examples of how the conformation of a peptide and the timescale of its fluctuations are derived from its (time-resolved) 2D spectrum are presented.

A semiempirical model is used to analyse the results of published experiments reporting on the solid-state amorphization reactions in bilayers and multilayers formed by Gd and Co. The role of the interfacial effects in raising the free energy of the initial arrangement in a multilayered configuration, and in promoting the amorphization reaction, is studied in detail. The model explains the observation of amorphous alloys over a broad composition range in the bilayer experiments. The preferred composition obtained in the multilayer experiments is discussed critically and the model prediction of a preferred composition Gd_0.46Co_0.54 is in good agreement with the compositions observed in recent experiments.

The Raman spectra of tetragonal CaMoO₄ (scheelite structure, C_4h⁶ space group) have been measured in the temperature range 12–1300 K. At high temperatures, the frequency and linewidth of all Raman-active phonons vary almost linearly with temperature, indicating that the three-phonon decay processes are dominant over the four-phonon ones. All Raman phonons display normal negative (∂ω/∂T) slopes throughout the temperature range, except for the lowest-frequency B_g phonon at 111.5 cm⁻¹ which is almost temperature independent in the region 12–400 K, but then for T > 400 K it shows a slight (normal) softening with temperature. The reduced (∂ω/∂T)/ω slopes of the internal modes (vibrations of atoms within the tetrahedral MoO₄²⁻ units) are an order of magnitude smaller than the respective slopes of the external ones (pure lattice modes). There are no discontinuities or sharp changes of slope in the ω(T) plots, implying that CaMoO₄ remains stable over the entire temperature range. Combining the temperature-dependent Raman data of this work and the previously reported pressure-dependent Raman data on this crystal (Christofilos D, Kourouklis G A and Ves S 1995 J. Phys. Chem. Solids56 1125), as well as the thermal expansion coefficient β(T) and compressibility κ(T) data, it has been possible to separate and evaluate quantitatively the volume (expansion) Δω_vol and the pure temperature (anharmonic) Δω_anh contributions to the total Δω_tot shift of phonons with temperature. It has been found that for most phonons at high temperatures, the volume effect is greater than the pure temperature one, thus indicating that most of the bonds in CaMoO₄ are predominantly ionic in character.

The growth modes of Ge islands on SiN_x-covered Si with and without a surfactant Sb layer are studied by scanning tunnelling microscopy. It is observed that on SiN_x/Si(111), Sb cannot enhance the coverage of (111) facets of Ge islands, which are the dominant features in the late stage of Ge overlayer growth when Sb is not used. However, on SiN_x/Si(001), Sb favours the growth of Ge(001) facets, which will shrink during the islanding growth if Sb is not used. The different behaviours with the (111) and (001) substrates suggest that the surfactant effect of Sb on the islanding growth of Ge on SiN_x/Si is strongly orientation dependent.

We report on investigations of the crystallographic structure and the magnetic anisotropies of epitaxial Fe films deposited on vicinal Au(001) surfaces for two miscut directions and various miscut angles. The stepped surface generates a uniaxial in-plane anisotropy contribution in addition to the fourfold anisotropy characteristic for Fe(001). With the step edges perpendicular to [100]_Fe the uniaxial anisotropy constant is a non-monotonic function of the vicinal angle. At low vicinal angles the easy magnetization axis is parallel to the step edges. The effective uniaxial anisotropy constant first increases and then decreases for increasing vicinal angle, crossing zero at about 1.3°± 0.2°. For higher angles the easy axis is perpendicular to the step edges. For the step edges perpendicular to [110]_Fe the step-induced in-plane uniaxial anisotropy increases approximately linearly with the vicinal angle.

Mo/Si multilayer (ML) mirrors play a decisive role in an extreme-ultraviolet (EUV) lithography process. In this study, the surface and interfacial roughness, as well as the lateral and vertical correlation lengths, of a series of Mo/Si MLs deposited by RF-magnetron sputtering (RF-MS) have been characterized using diffuse x-ray scattering and atomic force microscopy. We have investigated the influence of the substrate quality and material (silicon, ule and zerodur) on the propagation and the value of ML roughness. We show that, whatever the substrate is, the film deposited by RF-MS presents a reduced roughness compared with that of the substrate. Moreover, rocking-curve analyses show that, for Si and ule substrates, the ML average roughness is very low (< 1.5 Å), associated with high spatial frequency oscillations, while in the case of zerodur substrates, the roughness is significantly increased (> 2 Å), and the high spatial frequency oscillations are reduced. Finally, the combination of specular and non-specular small-angle x-ray results allows us to evaluate another key parameter, namely, the uncorrelated roughness which is an intrinsic characteristic related to the choice of both the deposition technique and the materials. This intrinsic roughness is found to be very low (2 Å) and constitutes a good argument in favour of the use of the RF-MS technique for EUV mirror deposition.

Magnetization reversal of a Fe/Si/Fe trilayer having a ferromagnetic bilinear and biquadratic exchange is investigated at 10 and 300 K. The experiments were performed using the magneto-optical Kerr effect technique and a SQUID magnetometer. The type of stable magnetic configuration is determined for both temperatures of the study. The hysteresis loops contain jumps in magnetization, which could be associated with the overcoming of the energy barriers related to the uniaxial anisotropy hard axes in the two iron layers. The analysis of the experimental results was performed in the framework of a model in which the ferromagnetic bilinear and biquadratic exchange between two magnetic layers separated by a nonmagnetic spacer and the cubic and uniaxial anisotropy were taken into account. Good qualitative agreement is obtained between the experimental and calculated field dependences of the magnetization.

Li was deposited at low temperature (80 K) onto cleaved van der Waals surfaces of the layered compounds MSe₂ (M ≡ Ti, Zr and Hf). The adsorption systems were investigated by means of low-energy electron diffraction, work-function measurements and soft-x-ray photoelectron spectroscopy using a synchrotron radiation source. The results suggest that at low coverages, Li is uniformly distributed near the surface, leading via a decomposition reaction to the formation of Li₂Se and M⁰. At high adsorbate concentration, some of the Li intercalates into the substrate in the interlayer region. The intercalation process seems to depend on the temperature and the lattice parameter of the substrate.

A geometrical model is proposed for the recently observed transformation under the action of highly localized stresses during the surface scratch test of the icosahedral phase into a body-centred cubic (BCC) phase, the disordered version of the B2 phase with the CsCl structure, which occupies a large portion of the Al–Cu–Fe phase diagram around the Al₅₀(Cu, Fe)₅₀ concentration. The model is founded on a polytope concept and the concept of the eight-dimensional root lattice E₈. In accordance with these concepts many crystalline or quasicrystalline structures can be derived from the polytope (the four-dimensional polyhedron) by fulfilling operations of lowering its local curvature with subsequent mapping of decurved polytope fragments onto Euclidian three-dimensional space. The properties of the E₈ lattices give foundation to the possibility of mapping a quasicrystalline structure on a crystalline structure. The structural transformation is effected through intermediate atomic configurations coinciding with both structures, which are determined by a four-dimensional icosahedron (the {3, 3, 5} polytope). For the transformation of the icosahedron of the icosahedral phase into a rhombic dodecahedron of the cubic B2 phase, the cubic A15 structure plays the role of an intermediate configuration since it can be represented as a three-dimensional packing of linearly interlaced chains of Frank–Kasper polyhedra with coordination numbers Z = 12 (icosahedron) and Z = 14. The transition between the rhombic dodecahedron of a B2 structure and the Frank–Kasper polyhedron with Z = 14 requires insertion of disclination quadruplets into some facets of the rhombic dodecahedron. The proposed geometrical model can be applied also to the polymorphic BCC–FCC transformation since the Miller indices of the Frank–Kasper polyhedron with Z = 14 coincide with the observed indices of habit planes of iron martensites.

We have performed atomistic simulations for helical multi-shell (HMS) Cu nanowires and nanotubes. Our investigation on HMS Cu nanowires and nanotubes has revealed some physical properties that were not dealt with in previous works that considered metal nanowires. As the diameter of HMS nanowires increases, their cohesive energy per atom decreases but their optimal lattice constants are almost constant. Shell–shell or core–shell interactions mainly affect the lattice constant and the diameter of HMS nanowires or nanotubes. Our molecular dynamics simulations show that parts of ultrathin Cu nanotubes collapse into the empty core at low temperature and this is the main mechanism for the transformation from ultrathin nanotube to nanowire structures. This study shows that HMS Cu nanotubes can retain their own structures when the internal stresses on HMS nanotubes are zero or towards the outside.

The TbBaCo_1.92Fe_0.08O_5+δ layered perovskite, exhibiting a metal–insulator phase transition at T_MI ∼ 340 K, has been investigated by the Mössbauer spectroscopy method. It was found that the Fe³⁺ ions have two different positions in the low-temperature insulator phase (at T < T_MI) and predominantly one position in the high-temperature metallic region above T_MI. X-ray diffraction experiments have revealed an orthorhombic-to-tetragonal crystal structure transformation taking place at T_MI. These results have been interpreted as favouring an oxygen vacancy order–disorder transition that occurs simultaneously with the metal–insulator one.

We investigated the damping of phonon polaritons in congruent lithium tantalate (LiTaO₃) single crystals by measuring spontaneous Raman spectra of the low-frequency wing of the A₁ phonon line, the dependence of the linewidth of the stimulated Raman scattering gain curves on polariton frequency and the spatial damping of propagating phonon-polariton pulses by nonlocal time-delayed coherent anti-Stokes Raman scattering. The damping and the dispersion curve of the lowest-frequency A₁ phonon polaritons were determined from 10 to 200 cm⁻¹ at room temperature and at 77 K. The observed frequency dependence of the damping is discussed in the context of low-frequency excitations and a Debye relaxational mode.

A theoretical model describing quasicritical behaviour of the nuclear magnetic resonance (NMR) relaxation rates 1/T₁, 1/T₂ and muon relaxation rate λ for cubic ferromagnets and antiferromagnets with spin fluctuations is presented. The model extends and generalizes the model of Moriya and explains why the observed experimental values of the critical exponents are different from the values predicted by the theory of critical phenomena. The formulae for the relaxation rates obtained from this model are used for fitting to the experimental NMR data for YMn₂D_x, TbMn₂D₂ and the muon spin-rotation (μ SR) data for YMn₂, YMn₂D_x, GdMn₂ and Y(Co_1−xAl_x)₂ (ferromagnetic). It is shown that the dependence of the exchange coupling constant on the wavevector k is important for understanding the observed quasicritical effects.

The effects of substitution of chromium and ruthenium for manganese on the magnetic and transport properties of electron-doped manganite Ca_0.9Ce_0.1MnO₃, with d_z² orbital-ordered and the associated antiferromagnetic (AFM) characters, have been studied. As expected, due to the participation of the Cr³⁺ (t_2g³ e _g⁰) and Ru^4+/5+ [t_2g⁴ e _g⁰/t _2g³ e _g⁰)] ions in the e_g band broadening for the Mn³⁺ (t_2g³ e _g¹) and Mn⁴⁺ (t_2g³ e _g⁰) species, the suppression of the AFM transition by Ru and Cr doping is accompanied by an increase in the ferromagnetic (FM) character. The coexistence of FM and AFM phases in these Ru-and Cr-substituted compounds leads to the observations of cluster glass behaviour, the giant magneto-resistive memory (GMM) effect and other magnetic-history-dependent magnetic and transport properties. Our observation of the GMM effect in these Cr-and Ru-doped compounds is the first of its kind in the magnetically phase-separated bulk polycrystalline compounds.

In this paper we will study the properties of several unusual Raman scattering peaks in heavily irradiated NaCl with vast amounts of colloidal sodium and chlorine precipitates. It appears that the laser excitation light interacts with both the electronic and vibration systems of the Na colloids, which gives rise to new Raman scattering peaks, which can be associated with electronic and vibrational excitations confined in extremely thin (about 6 nm) quantum wires.