Table of contents

We propose a model for the calculation of the second order susceptibilities of groups II–VI and III–V compound semiconductors. The model is based on the assumption that the dominant effect that contributes to the second harmonic generation is the variation of the polar charge of the bond under the action of the laser radiation field. We show that a simple relation for the second order susceptibility in terms of the basic parameters of the bond follows from the dielectric theory of Phillips. This relation is checked for many experimental values for nonlinear susceptibilities of polar semiconductor compounds taken from the literature.

Using ferrocene as the catalyst and xylene as the precursor, films of multi-walled carbon nanotubes (CNTs) that are well aligned can be synthesized on flat substrates by chemical vapour deposition. Here we report that, by controlling the growth speed of CNTs at specific catalyst/carbon ratios, vertically aligned pillars of densely packed nanotubes with uniform diameters (within 10–30 µm) and lengths of up to 0.3–0.4 mm can be fabricated on a large scale on planar SiO₂ substrates, forming a continuous film. The growth mechanism of the pillars was also investigated by observing the growth of CNTs at different growth stages.

The absolute thermopower (S) of superconducting is p-type. It increases superlinearly with temperature up to  K; at higher temperatures its rate of change decreases and saturation at is observed above 150 K. S(T) for resembles that of anhydrous Na_xCoO₂ (), but is lower by a factor of and its saturation is more pronounced. These results are consistent with electronic transport by strongly correlated electrons in a very narrow band.

We investigate spin diffusion in geometrically frustrated Heisenberg antiferromagnets. We find that the diffusion constant gradually increases from its high temperature limit as the temperature approaches the peak temperature of the susceptibility. Below the peak, the exponential decrease in the susceptibility leads to a rapid increase in the rate of spin diffusion. Detailed results are presented for the pyrochlore lattice with nearest-neighbour interactions.

We report the parameter-free, density-functional theory calculations of the electronic structure, interatomic exchange interaction and magnetic critical temperature of Zn_0.75Cr_0.25Te. This system has recently gained exceptional importance as the first diluted magnetic semiconductor (DMS), where the intrinsic ferromagnetism with Curie temperature higher than room temperature is confirmed by magnetic circular dichroism measurements. We obtain a value for the Curie temperature that is in good agreement with experiment. The role of the holes in the valence band in mediating the ferromagnetic exchange interactions is demonstrated. The application of the same calculational scheme to Zn_0.75Mn_0.25Te shows that, in good correlation with experimental data, this system does not possess charge carriers and is characterized by strong antiferromagnetic exchange interactions. Comparing (ZnCr)Te with III–V DMS, we note the role of a large semiconducting gap for preserving high spin polarization of the states at the Fermi level at finite temperatures.

Optical absorption measurements on different growth sectors of a polished slice of electron-irradiated type Ib synthetic diamond show that the vacancy concentration is almost four times higher in the high-nitrogen {111} sectors than in the low-nitrogen {115} sectors. Evidence is presented to show that the self-interstitials are trapped by nitrogen, thereby reducing the amount of correlated recombination with vacancies which occurs in the absence of nitrogen.

Zircon has been proposed as a nuclear waste form to safely encapsulate weapons-grade plutonium. In order to study the impact of self-irradiation damage in zircon on its aqueous durability, we performed a hydrothermal experiment (2 M CaCl₂ solution, 600 °C, 100 MPa) with several variably radiation-damaged, i.e. amorphized, zircon samples. We found an anomalous increase in the alteration rate at two critical concentrations of amorphous domains. The first dramatic increase sets in when the amorphous domains form interconnected clusters in the structure. The second increase is related to the percolation of fast diffusion pathways consisting of nano-sized regions of depleted matter that are formed during strongly overlapping α-recoil events, as seen by molecular-dynamics simulations and small-angle x-ray scattering measurements. The two percolation thresholds provide model benchmarks for the safety performance of a zircon waste form.

Oxide-diluted magnetic semiconductors (O-DMS) have attracted a great deal of interest in recent years due to the possibility of inducing room temperature ferromagnetism. These materials are of particular interest for spintronic devices such as spin valves. This review describes the experimental status of the O-DMS including the recent results on ZnO- and TiO₂-based systems.

Based on a detailed discussion of the energy band structure and optical transitions in very long wavelength () GaAs/AlGaAs quantum well infrared photodetectors (QWIPs), we have built a practical QWIP model. We have studied various key factors that determine the photogenerated carriers and detection wavelength of very long wavelength QWIPs. Consequently, we have found and confirmed a distinctive difference between the photocurrent of QWIPs with only one confined state in the quantum well (QW) and those binding two confined states, which resulted in a different dependence of the detection wavelength on the QW width. Also, we have discussed the dependence of the response wavelength on several other parameters for very long wavelength QWIPs, such as barrier width and Al mole fraction. Afterwards, a series of very long wavelength QWIP devices have been fabricated and measured, whose photocurrents are well reproduced by our calculation. Based on the theoretical model and experimental data, a simple and convenient polynomial equation has been deduced, using structure parameters of very long wavelength QWIPs to calculate the detection wavelength and offering convenience in device designing.

We calculate the pair distribution function, g(r), in a two-dimensional electron gas and derive a simple analytical expression for its value at the origin as a function of r_s. Our approach is based on solving the Schrödinger equation for the two-electron wavefunction in an appropriate effective potential, leading to results that are in good agreement with quantum Monte Carlo data and with the most recent numerical calculations of g(0) (Bulutay and Tanatar 2002 Phys. Rev. B 65 195116). We also show that the spin-up spin-down correlation function at the origin, , is mainly independent of the degree of spin polarization of the electronic system.

Those nanocrystalline materials which consist of ferromagnetic nanograins embedded in a ferromagnetic matrix were modelled as a cubic lattice composed of a central sphere with strongly coupled spins surrounded by weakly coupled spins. The magnetic behaviour was studied by Monte Carlo simulation, especially the low temperature spin ordering and features exhibited in the temperature range between the Curie temperatures of the two phases. The magnetization and magnetic susceptibility are calculated as a function of temperature for different values of the interfacial exchange interactions. It is shown that the exchange coupling between matrix and nanograin influences the magnetic properties of the nanograin, the matrix and the interfacial regions differently. The magnetic behaviour of different regions has been explained in terms of a polarization mechanism acting on the surface and in the core, leading to magnetic correlation between the spins. Such features are quite consistent with the experimental results obtained on nanocrystalline alloys.

A slab of negatively refracting material, thickness d, can focus an image at a distance 2d from the object. The negative slab cancels an equal thickness of positive space. This result is a special case of a much wider class of focusing: any medium can be optically cancelled by an equal thickness of material constructed to be an inverted mirror image of the medium, with , μ reversed in sign. We introduce the powerful technique of coordinate transformation, mapping a known system into an equivalent system, to extend the result to a much wider class of structures including cylinders, spheres and intersecting planes, and hence show how to produce magnified images. All the images are 'perfect' in the sense that both the near and far fields are brought to a focus and hence reveal sub-wavelength details.

As-quenched icosahedral Al₅₅Si₇Cu_25.5Fe_12.5, its 1/1 approximant of the same composition, and icosahedral Al_62.5Cu_24.5Fe₁₃ alloys have been studied using x-ray diffraction, scanning transmission electron microscopy and the high-angle annular dark field technique, zero-field and in-field ⁵⁷Fe Mössbauer spectroscopy, and electrical resistivity. The crystal structure of the 1/1 approximant Al₅₅Si₇Cu_25.5Fe_12.5 has been refined with the Rietveld method and shown to be compatible with the measured high-angle annular dark field images. The distribution of the principal component of the electric field gradient tensor has a bimodal character with a dominant negative sign in the icosahedral Al–Cu–Fe system. The local order of the Fe structural environment is compared in icosahedral Al₅₅Si₇Cu_25.5Fe_12.5, its 1/1 approximant, and icosahedral Al_62.5Cu_24.5Fe₁₃. The average quadrupole splitting decreases with temperature as T^3/2 for all alloys studied, and its value is significantly larger for the icosahedral alloys. The vibrations of the Fe atoms in the alloys studied are well described by a Debye model, with characteristic Mössbauer temperatures of 468(25), 487(19), and 455(6) K for icosahedral Al₅₅Si₇Cu_25.5Fe_12.5, its 1/1 approximant, and icosahedral Al_62.5Cu_24.5Fe₁₃, respectively. The electrical resistivity is discussed in terms of quantum interference effects and structural disorder.

Nanometric Sn₂P₂S₆ crystalline powders and ceramics are studied by x-ray diffraction and Raman spectroscopy. The average crystallite size is estimated from TEM and x-ray data. The observed features in the Raman spectra are discussed in view of confinement-related selection rules relaxation, surface phonon modes and possible partial substitution of sulfur by oxygen at the nanometric powder surface.

Magnetic relaxation behaviour, critical current density J_c and lower critical field H_c1 have been investigated in MgB₂/Ta/Cu wires. It is found that J_c and H_c1 decrease linearly with temperature in the whole temperature region below T_c. The relaxation rate is very small and has a weak temperature dependence compared to high-T_c superconductors. Also, the pinning potential is much larger and the temperature and field dependences of the pinning potential are briefly discussed.

The magnetic structure and hyperfine interactions in the polycrystalline pseudo-binary system (Zr_1−xTi_x)Fe₂ for and were studied using the techniques of neutron diffraction and the Mössbauer effect. The results of our measurements reveal the cubic C15 and the hexagonal C14 crystal structure for low and high Ti atom concentration x, respectively. For the ferrimagnetic region of the C15 structure the reduction from 1.58(5) to 1.30(6) μ_B of Fe magnetic moments with increasing x at room temperature was found. The presence of two hyperfine magnetic fields in the antiferromagnetic C14 phases suggests the existence of small magnetic moments on Fe atoms at 2a in addition to those present at the 6h sites. The magnetic structure refinement for the (Zr_0.3Ti_0.7)Fe₂ alloy yielded magnetic moments perpendicular to the c axis of 0.19(10) μ_B at Fe 2a sites at 10 K. The results confirm the spin-canted antiferromagnetic structure with a canting angle of 8° at 10 K in the pseudo-binary phase.

The dynamics of mixed single crystals of Ag_xNa_1−xCl has been investigated by inelastic neutron scattering before and after chemical demixing. In the homogeneous phase, the concentration dependence of acoustic phonons reveals that doping of NaCl with silver chloride leads to a considerable softening of the lattice, while the elastic properties of AgCl are almost independent of sodium chloride additives. After quenching into the miscibility gap, the phase separation is associated with a well-defined splitting of acoustic phonons which provide the most direct information about the underlying mechanism. In contrast to the local dynamical properties, the lattice structure is essentially determined by coherency strains which hinder the relaxation of lattice parameters. Thus diffraction and inelastic scattering yield independent and complementary information about demixing processes in ionic solids.

The Langevin diffusion equation approach is proposed for studying the metal–nonmetal (M–NM) transitions in expanded fluid mercury from a mesoscopic and dynamical viewpoint. In this theory, time evolutions of coarse-grained atomic densities are calculated in accordance with the free energy functionals that incorporate changes in the electronic states across the M–NM transitions. Because of the intrinsically discontinuous nature of the M–NM transitions, irregular mixing of high-density metallic domains and low-density nonmetallic domains is predicted in the M–NM transition range. Thermal fluctuations cause local transformation between metallic and nonmetallic domains. The timescale of structural relaxation involving such a local M–NM transition is remarkably slow, which is reflected in the strikingly slow time decay of the dynamical density correlation functions in the M–NM transition range. This result strongly supports the experimental evidence of slow structural relaxation found through anomalous sound wave attenuation. An increase in isothermal compressibilities and a modest enhancement of long-wavelength static structure factors are predicted across the transition. Discontinuous density jumps are smeared out by structural disorder, leading to a continuous M–NM transition in agreement with observations.

A recent model for the large radiation-induced swelling exhibited by irradiated zircon (ZrSiO₄) is partially based on results of molecular dynamics (MD) simulations of the partial overlap of two collision cascades that predict a densified boundary of polymerized silica and the scattering of the second cascade away from the densified boundary (Trachenko et al 2003 J. Phys.: Condens. Matter15 L1). These MD simulations are based on an atomic interaction potential for zircon (Trachenko et al 2001 J. Phys.: Condens. Matter13 1947), which, according to our analysis, only reproduces some of the crystallographic properties at equilibrium and does not adequately describe the atomic scattering physics for zircon, and on simulation methodologies for energetic events that are ill defined. In fact, the interatomic potential model used by Trachenko et al yields a significantly more rigid structure, with very high Frenkel defect formation energies and extremely low entropy and specific heat capacity. Consequently, the reported results of the cascade simulations, which are events far from equilibrium, may be artifacts of both the potential model and simulation methodologies employed. Thus, the structural changes predicted by the simulations must be viewed cautiously, as these simulation results cannot be taken as confirmation of a new scattering physics process that is the basis for the proposed swelling model. In this comment, the deficiencies in the atomic interaction potential and methodologies employed by these authors are critically reviewed, and the validity of the cascade overlap simulations and proposed physics is discussed.

The authors of the comment (Corrales et al 2003 J. Phys.: Condens. Matter15 6447) on our paper (Trachenko et al 2003 J. Phys.: Condens. Matter15 L1) suggested that different simulation conditions could result in different results, including a wider spread of the damage. To show that this is not the case, we have repeated the simulations, exactly as proposed by the authors of the comment, using new potentials with ZBL short-range terms and with potentials proposed by the authors of the comment (Park et al 2001 Phys. Rev. B 64 174108). We find that, contrary to the suggestions of the authors of the comment, the damage is well localized in the simulation box and is generally similar to that found in the original paper. We find that, similar to our previous results, the damage has a depleted region in the centre and is more dense at the boundaries. We show that the suggestions of the authors of the comment, that the damage should be more spread in our simulations, probably originate from unphysical results derived in their previous simulation work.