Table of contents

Recently, a novel form of nanodiamond exhibiting unusual mechanical properties has been synthesized by high-pressure high-temperature (HPHT) treatment of C₆₀ fullerene, amorphous carbon and diamond powder. In this study, we have characterized the dominant defects in this nanodiamond by a combination of optical absorption, luminescence, Raman, electron spin resonance and elastic recoil detection techniques. Unusually high concentrations (∼0.4 at.%) of hydrogen and very low concentrations of nitrogen (∼10⁻⁵ at.%) have been detected in diamond grown from C₆₀. Although most of hydrogen is shown to originate from inclusions of foreign phases, such as water, significant concentrations (∼0.01 at.%) of hydrogen were also detected as a point defect in the nanodiamond grains. Observed structural differences between the samples made from various carbonaceous materials are attributed to different behaviour of the starting compounds during HPHT treatment.

Magnetoelectric compounds BiFe_1−xTi_xO₃ (x = 0.1, 0.2 and 0.3) have been synthesized by the conventional solid state reaction method and the effect of Ti substitution on the ferroelectric and magnetic properties studied. It is seen that Ti substitution does not affect the ferroelectric transition temperature (T_c) of the BiFe_1−xTi_xO₃ system. An anomaly in the dielectric constant and dielectric loss in the vicinity of the antiferromagnetic Néel temperature (T_N) and a small enhancement in magnetization have been observed. No systematic variation in the antiferromagnetic Néel temperatures has been observed on Ti substitution. Furthermore, it is seen that this system shows coupling between electric and magnetic dipoles exhibiting a magnetoelectric effect at room temperature and possesses a high dielectric constant.

We show that elastic interactions between adatoms and small islands on a surface exhibit a novel intermediate-ranged oscillatory behaviour. Taking small Fe islands on Cu(111) as an example, we perform atomic-scale simulations and reveal that elastic interactions strongly depend on the size of the islands. Stress variation on the atomic scale is found to be the driving force for the size effect in elastic interactions.

The effects of hydrostatic pressure on the electronic band structure of the semiconductor mineral iron pyrite FeS₂ have been investigated theoretically by an ab initio full-potential linearized-augmented plane wave (FPLAPW) method within a local approximation (LDA/GGA) to the density functional theory. The calculations predict that at a pressure of 94.1 GPa the indirect band gap of pyrite FeS₂ vanishes and the material becomes a metal. This is due to the presence of the S–S and Fe–S bonds, which provide novel energy band distortions in the process of attaining the metallic state. Analysis indicates that, under increasing high pressure, the conduction bands (3p_z of sulfur and 3d_x²−y²+3d_xy of iron) intrude downwards into the valence bands, which are predominantly 3d in nature. At normal pressure, the lattice constant, the bulk modulus, sulfur position parameter u, S–S bond length, and the indirect band gap of pyrite FeS₂ are calculated using a fully relaxed unit cell and found to be equal to 541.8 pm, 159.7 GPa, u = 0.383, 219.5 pm and 0.45 eV, respectively. Apart from the gap, which is too small (the usual 'LDA error'), these results agree well with recent experiments. The effective masses of an electron at selected points in the conduction band are reported.

We investigate theoretically the electronic structure and transport for a two-level quantum wire with Rashba spin–orbit coupling (SOC) under the irradiation of an external laser field at low temperatures. The photon-induced transitions between SOC-split subbands with the same lateral confinement quantum number and between subbands with different confinement quantum numbers are expected. Using the method of equation of motion (EOM) for Keldysh nonequilibrium Green's functions (NGF), we examine the time-averaged density of states (DOS) and the spin-polarized conductance for the system with photon polarization perpendicular to the wire direction. Through the analytical analysis and some numerical examples, the interplay effects of the external laser field and the Rashba SOC on both the DOS and the conductance of the system are demonstrated and discussed. It is found that the external laser field can adjust the spin polarization rate and the transport of the quantum wire system with some appropriate Rashba SOC strengths.

The doping dependence of the optical properties of La_1−yCa_yTiO_{3.4 ± δ} crystals was studied by polarization-dependent infrared reflectivity measurements at room temperature. For y<0.2 the optical conductivity spectra reveal a quasi-one-dimensional conducting character, and the phonon spectra suggest small changes in the crystal structure and effective charge induced by the substitution of trivalent lanthanum with divalent calcium. The pronounced mid-infrared absorption band observed along the conducting direction shifts to lower frequencies with increasing electron concentration. The possible origin of the band is discussed in terms of polaronic excitations, Mott–Hubbard excitations, phonon-assisted Ti t_2g d–d transitions, and charge-transfer (2p–3d) transitions. Among these interpretations the polaron model is most likely. La_1−yCa_yTiO_{3.4 ± δ} with y = 0.2 and δ = 0 exhibits an insulating character and a less anisotropic optical response. The observed differences in the electronic and vibrational excitations in La_0.8Ca_0.2TiO_3.40 compared to the other studied titanates could be due to local changes of the crystal structure.

We have studied the effects of dimerization on the energy levels of a one-dimensional molecular chain attached between two electrodes. Analytic expressions for the change in energies in the presence of a small perturbing external potential have been obtained for the three limiting cases: (a) uniform, (b) partially dimerized and (c) completely dimerized chains. We find that the presence of dimerization enhances the mixing between low-lying energies in the system resulting in a situation conducive to showing negative differential resistance (NDR) in the current–voltage characteristics. The effect of spin-polarized molecule–electrode couplings on a dimerized chain has also been studied, where both spin-parallel and spin-antiparallel current show NDR behaviour. Strong dimerization however is found to destroy the spin-valve effects that are most essential for spintronic devices.

Magnetic properties of polycrystalline Sm_0.1Ca_0.9−ySr_yMnO₃ (y = 0–0.3) samples have been investigated in the temperature range 5–250 K, magnetic fields up to 16 kOe and under hydrostatic pressures up to 11 kbar. The studies involved sequential measurements of zero field cooled (ZFC) magnetization and measurements of magnetization upon cooling in the same magnetic field (FCC). The low-doped group (y = 0–0.1) exhibits magnetic phase separation below T_N≈T_C≈100–110 K, consisting of ferromagnetic (FM) clusters embedded in an antiferromagnetic (AFM) G-type matrix. Magnetization and ac-susceptibility measurements of this group of materials indicate features reminiscent of a cluster glass-like state, below T_C. It was found that the volume fraction of the FM phase at 10 K decreases with increasing y from 28% for Sm_0.1Ca_0.9MnO₃ to 18% for Sm_0.1Ca_0.8Sr_0.1MnO₃. It was found that an applied pressure enhances T_C with a pressure coefficient of dT_C/dP≈0.4–0.5 K kbar⁻¹. The low-temperature magnetization at this doping range (y = 0–0.1) depends on pressure only slightly, except for the case of Sm_0.1Ca_0.8Sr_0.1MnO₃, where an applied pressure enhances the FM phase volume considerably. Samples with y = 0.2 and 0.3 exhibit a heterogeneous spin configuration in their ground state, consisting of a C-AFM phase and a G-AFM phase with a very weak FM moment. The temperature of transition from the paramagnetic to the C-AFM state is almost insensitive to applied pressure, whereas the lower transition temperature to the G-AFM state increases slightly under pressure. It was found also that an applied pressure considerably reduces the FM correlations in the paramagnetic phase as well as the FM component of the G-AFM state.

We have investigated Sr_1−xLa_x/2K_x/2RuO₃ and Sr_1−xPb_xRuO₃, which have a larger average size of the A-site cations. They manifest a gradual loss of ferromagnetism in a similar way as their counterparts with smaller A-site cations. There is also evidence for a magnetism-suppressing disorder effect similar to that observed in Sr_1−xLa_x/2Na_x/2RuO₃. Therefore, the Stoner ferromagnetism in SrRuO₃ is rather unique and cannot be easily tuned by lattice distortion to yield a higher Curie temperature.

Soft x-ray resonant photoemission spectroscopy has been used as a tool to investigate the cerium 4f electronic levels of a single-crystalline sample of CeCo₂Ge₂. Temperature-dependent angle-integrated and angle-resolved results are discussed. For the lowest investigated temperatures two distinct 4f spectral features are clearly observed close to the Fermi level: the Kondo resonance and its lower lying spin–orbit partner. As the temperature is increased, the intensity of such features evolves consistently with the theoretical predictions of the single-impurity Anderson model. However, the angle-resolved results show momentum-dependent effects that require a modification of the single-impurity theory.

A method for computer simulation of time-resolved x-ray diffraction (TRXD) in asymmetric Laue (transmission) geometry with an arbitrary propagating strain perpendicular to the crystal surface is presented. We present two case studies for possible strain generation by short-pulse laser irradiation: (i) a thermoelastic-like analytical model; (ii) a numerical model including the effects of electron–hole diffusion, Auger recombination, deformation potential and thermal diffusion. A comparison with recent experimental results is also presented.

We have undertaken a study of the diluted magnetic semiconductors Ga_1−xMn_xN and Ga_1−xCr_xN with x = 0.0625,0.125, using the all-electron linearized augmented plane wave method (LAPW) for different configurations of Mn as well as Cr. We study four possible geometries of the impurity atoms in the wurtzite GaN structure to predict the energetically most favourable structure within a 32-atom supercell and conclude that the near-neighbour geometry has the lowest energy. The on-site magnetic moment at the transition metal site shows insignificant variation with distance. We have also analysed two configurations in which the local moments on the impurity atoms are parallel and anti-parallel. The density of states as well as band structure indicate a half-metallic state for all the systems. T_c has also been estimated for these systems and shows that for near-neighbour substitution of dopants T_c is above room temperature.

Thermoluminescence signals and morphological studies are reported for a new phosphor of Gd₂O₃. The phosphor particle material was synthesized by an aerosol technique and it is heavily doped with Eu. Annealing at 1200 °C increases the luminescence efficiency by ∼1000-fold relative to the as-prepared material. The emission spectra are primarily from the red transitions of the Eu, but weaker short wavelength emissions can also be detected during low temperature thermoluminescence. There are numerous wavelength dependent differences between the low temperature thermoluminescence curves. These variations are interpreted to show that there are a range of defect and luminescence sites in the Gd₂O₃:Eu phosphors. These are associated with the symmetry of the crystal field around the europium ion, which is a function of the phase content, i.e. resulting from sample preparation and annealing. The sites are almost independent in terms of their emission spectra, but the stability of some sites is enhanced by the presence of Eu so that the TL peaks appear at higher temperatures than the intrinsic defect TL of the host material. The synthesis of uniform and submicron sized spherical particles with nano-clustered inner structure is demonstrated with various analysis techniques (XRPD, FE-SEM and HR-TEM). The effect of processing parameters and post-annealing treatment is discussed from the viewpoint of different phase formation.

Within the Ginzburg–Landau theory applied to a planar array of chains, characterized by a real order parameter, the inter-chain fluctuations are taken into account exactly. So, the 'linear chain approximation' is replaced by a rigorous treatment. The single-chain problem is addressed taking advantage of a precise and simple non-perturbative solution of the Schrödinger equation for the anharmonic oscillator.

The effects of the spin–orbit (SO) interaction strength of the ground and excited configurations of rare-earth ions on the paramagnetic and diamagnetic Faraday rotation (FR) spectra in Ce-substituted yttrium iron garnets (YIG) are studied on the basis of quantum theory. It is found that the effect of the SO interaction strength of the ground configuration is stronger than the corresponding effect of the excited configuration. The reasons for this difference have been analysed in connection with the analysis of the large FR effect in Bi-substituted YIG.

We have studied the ground-state phase diagram of a spin ladder with alternating rung exchange in a magnetic filed, in the limit where the rung coupling is dominant. In this limit the model is mapped onto an XXZ Heisenberg chain in uniform and staggered longitudinal magnetic fields, where the amplitude of the staggered field is ∼δ. We have shown that the magnetization curve of the system exhibits a plateau at magnetization equal to half of the saturation value. The width of a plateau scales as δ^ν, where ν = 4/5 in the case of a ladder with isotropic antiferromagnetic legs and ν = 2 in the case of a ladder with isotropic ferromagnetic legs. We have calculated four critical fields (H_c1 ^± and H_c2 ^± ) corresponding to transitions between different magnetic phases of the system. We have shown that these transitions belong to the universality class of the commensurate–incommensurate transition. The possibility for the realization of a new type of spin-Peierls instability, characterized by the spontaneous appearance of an alternating rung exchange in a uniform magnetic field and at a system magnetization equal to the half of its saturation value, is briefly discussed.

The unit on the y axis of figure 5 should be (sec^-1).