Table of contents

There is considerable interest in the use of silicon devices as qubits for quantum computing. The existence of nuclear spin in a silicon isotope and the complex band structure of silicon are unfavourable for this application of silicon devices.

We study a critical behaviour of systems dominated by Coulombic interaction. For this purpose we use the method of collective variables with a reference system. Starting from the Hamiltonian of the restricted primitive model (RPM), the simplest model of ionic fluids, we obtain a functional of the grand partition function given in terms of the two types of collective variable describing fluctuations of the total number density and the charge density, respectively. As the result of integration over the charge density variables, a microscopic based effective Hamiltonian of the RPM in the vicinity of its gas–liquid critical point is constructed. Coefficients of the effective Hamiltonian describing the density fluctuations near the gas–liquid critical point are analysed. It is shown that in spite of the long-range character of the Coulombic potential the effective interactions appearing at this level of the description have a short-range character. Consequently, the effective Hamiltonian obtained for the RPM in the vicinity of the critical point is in the form of the Ginzburg–Landau–Wilson Hamiltonian of an Ising-like model in a magnetic field. This confirms the fact that the critical behaviour of the RPM near the gas–liquid critical point belongs to the universal class of a 3D Ising model.

In this work, the discovery of a large low-field-induced magnetic entropy change in single-crystal Nd_0.47Sr_0.53MnO₃ with A-type layered antiferromagnetic structure is reported. The magnetic entropy changes reach values of 11.0 and 11.5 J kg⁻¹ K⁻¹ for field changes of 20 kOe along the ab-plane andc-axis, respectively. The large magnetic entropy change occurring near T_N was attributed to a low-field-induced antiferromagnetic ferromagnetic phase transition. Our results provide a possibility for development of magnetic refrigerant substances that are operable with a permanent magnet rather than a superconducting one as the magnetic field source.

The discovery of a novel effect in the transport through a QD spin-dependently coupled to magnetic contacts is reported. For a finite range of source–drain voltages the spin projections of the current cancel exactly, resulting in a completely suppressed output current. The spin down current behaves as one normally expects whereas the spin up current becomes negative. As the source–drain voltage is increased the spin up current eventually becomes positive. Thus, tuning the source–drain voltage such that the spin up current vanishes will result in a perfect spin filter.

We review mechanisms of low-temperature electronic transport through a quantum dot weakly coupled to two conducting leads. Transport in this case is dominated by electron–electron interaction. At temperatures moderately lower than the charging energy of the dot, the linear conductance is suppressed by the Coulomb blockade. Upon further lowering of the temperature, however, the conductance may start to increase again due to the Kondo effect. We concentrate on lateral quantum dot systems and discuss the conductance in a broad temperature range, which includes the Kondo regime.

Most MEMS devices are currently based on silicon because of the available surface machining technology. However, Si has poor mechanical and tribological properties which makes it difficult to produce high performance Si based MEMS devices that could work reliably, particularly in harsh environments; diamond, as a superhard material with high mechanical strength, exceptional chemical inertness, outstanding thermal stability and superior tribological performance, could be an ideal material for MEMS. A key challenge for diamond MEMS is the integration of diamond films with other materials. Conventional CVD thin film deposition methods produce diamond films with large grains, high internal stress, poor intergranular adhesion and very rough surfaces, and are consequently ill-suited for MEMS applications. Diamond-like films offer an alternative, but are deposited using physical vapour deposition methods unsuitable for conformal deposition on high aspect ratio features, and generally they do not exhibit the outstanding mechanical properties of diamond. We describe a new ultrananocrystalline diamond (UNCD) film technology based on a microwave plasma technique using argon plasma chemistries that produce UNCD films with morphological and mechanical properties that are ideally suited for producing reliable MEMS devices. We have developed lithographic techniques for the fabrication of UNCD MEMS components, including cantilevers and multilevel devices, acting as precursors to micro-bearings and gears, making UNCD a promising material for the development of high performance MEMS devices. We also review the mechanical, tribological, electronic transport, chemical and biocompatibility properties of UNCD, which make this an ideal material for reliable, long endurance MEMS device use.

A solid-state two-qubit quantum gate was recently proposed that might be made in a silicon fabrication plant in the near future. In this class of device, entanglement between two quantum bits is controlled by a change from a largely unentangled ground electronic state to an excited state in which useful entanglement can be produced. Such gates have potential advantages, both because they exploit known solid-state behaviour and they separate the storage and manipulation of quantum information. It is important that the excitation step does not create decoherence. We analyse a type of gate proposed before, in which the excitation involves a control electron that interacts with the qubit spins in the excited state. The dynamics of an idealized (but fairly general) gate of this type show that it can be operated to produce a standard two-qubit entangling state.

The line strengths in four polarization directions of the incident radiation of the , Γ₈ and , Γ₇, Γ₈ two photon absorption (TPA) transitions of Eu²⁺ in the perovskite KMgF₃ host have been calculated using the third-order Judd–Pooler (JP) formalism. In general, the calculated relative intensities are in good agreement with experiment, except for the ratios between the linear and circular polarizations in the transitions. In particular, the scalar operator term in the third-order spin–orbital correction formula successfully explains nine transitions with different linear polarizations.

Low-frequency dielectric studies were performed on the fine-grain ceramics of BaTiO₃ doped with 2.5 mol% of non-ferroelectric perovskite La(Mg_1/2Ti_1/2)O₃ (BT:2.5% LMT). In addition to the permittivity and P–E hysteresis loop measurements as a function of temperature, local electromechanical properties of BT:2.5% LMT were analysed by piezoresponse force microscopy. It was found that the temperature evolution of the dielectric properties detected by macroscopic methods (average over many grains) is similar to that observed on the nanoscale level (in a single grain). BT:2.5% LMT was revealed to exhibit the typical features of both the ferroelectric and the relaxor. The observed dielectric behaviour is discussed in terms of heterovalent substitution at both A-(Ba²⁺/La³⁺) and B-(Ti⁴⁺/Mg²⁺) perovskite sites.

The structures and phase transitions of AgNbO₃ were investigated using neutron powder diffraction and restricted single-crystal x-ray diffraction. Both methods have revealed the high temperature M₃–O₁, O₂–T and T–C phase transitions but have not given any significant evidence of low temperature M₁–M₂ and M₂–M₃ ones. The refinements of neutron diffraction patterns allowed us to determine the symmetry, space group and crystal structure for all phases except the O₁ one. The existence of structural disorder in the T and probably O₂ phases was found. The high temperature paraelectric phase transitions can be interpreted on the basis of consecutive condensation of oxygen octahedron tilts around the main axis. The ferroelectric and antiferroelectric behaviour has been associated with Ag and Nb cations. The reason why phase transitions between low temperature ferroelectric and antiferroelectric phases are not detectable by diffraction methods is discussed. The sequence of phase transitions in AgNbO₃ can then be understood in the framework of a long range and/or local order–disorder type arrangement.

This work reports the synthesis and electrical characterization of Bi₄Ti₃O₁₂ ceramics produced successfully by a new method based on a CO₂ laser sintering process. The structural and microstructural characterizations revealed that this method is able to produce sintered samples with high quality, free from secondary phases and with density as high as 98% of the theoretical one. Additionally, ferroelectric and dielectric investigations also revealed excellent values for the polarization and dielectric constant, respectively, thus showing the viability of the proposed method.

We have investigated the elastic stiffness and electronic band structure of nano-laminate solid solutions, where M and and Cr, by means of the ab initio pseudopotential total energy method. The second-order elastic constants, bulk moduli and anisotropic Young's moduli are computed for the solid solutions, in which x is changed from 0 to 2 in steps of 0.5. The bulk moduli of is found to be approximately the average of the two end M₂AlC and phases as the substitution content x, as well as the valence electron concentration (VEC), varies in the compounds. On the other hand, the shear modulus c₄₄, which by itself represents a pure shear shape change and has a direct relationship with hardness, saturates to a maximum as VEC is in the range 8.4–8.6. It implies that solid solution hardening may be operative for alloys having VEC values in this range. Furthermore, trends in the elastic stiffness are interpreted in terms of the electronic band structure. We show that monotonically incrementing the bulk moduli is attributed to the occupying states involving transition-metal d–Al p covalent bonding and metal-to-metal dd bonding. The maximum in c₄₄, on the other hand, originates from completely filling the shear resistive transition-metal d–Al p bonding states. Most importantly, we predict a method to optimize the desired elastic stiffness by properly tuning the valence electron concentration of ceramics.

The electrical resistivity and low-temperature specific heat of exchange-enhanced Pauli paramagnetic Laves phase quasi-binary compounds Lu(Co_1−xM_x)₂ (M =  Al, Ga and Si) have been investigated in order to discuss the spin fluctuation characteristics and the Kadowaki–Woods plot. In these three kinds of quasi-binary system, both the electrical resistivity coefficient A and the specific heat coefficient γ_tot at low temperatures decrease at first and then significantly increase with increasing x. The concentration dependence of magnetic susceptibility at 0 K, χ(0), is also similar to that of A and γ_tot. The Wilson ratio becomes very large because of the enhancement of magnetic susceptibility. The Kadowaki–Woods plot is fitted to the same universal line of heavy-fermion compounds, A15-type Nb₃Sn and V₃Si compounds because of significant spin fluctuations in the present quasi-binary compound systems.

Electrical and magnetic transitions in Pr_1−xSr_xMnO₃ and Pr_1−xSr_xMn_0.95Fe_0.05O₃ ceramic samples with x = 1/4, 3/8, and 1/2 have been investigated. It is proved that 5% Fe doping in the system suppresses electrical conduction and ferromagnetism severely, but favours antiferromagnetism. The existence of spin clusters at temperatures above the ferromagnetic transition T_C is demonstrated in the Fe-doped x = 1/2 sample, which is caused by the Fe-doping-induced magnetic inhomogeneity. The transport behaviour above T_C in this sample is governed by variable-range hopping of the magnetic polarons. For the Fe-doped x = 1/4 sample, the insulator-to-metal (I–M) transition drops behind T_C by nearly 80 K; accordingly, two magnetoresistance peaks appear. One is around T_C, and the other lies just at the I–M transition temperature. The possible underlying mechanism is discussed qualitatively.

The crystal structures and magnetic properties of quaternary oxides Ba₃LnIr₂O₉ (Ln =  Y, lanthanides) are reported. Rietveld analyses of their x-ray diffraction data indicate that they adopt the 6H-perovskite-type structure with space group P6₃/mmc or, in the case of Ln =  La and Nd only, a monoclinically distorted structure with space group C 2/c. They have the valence state of Ba₃Ln⁴⁺Ir₂⁴⁺O₉ (for Ln =  Ce, Pr, and Tb) or Ba₃Ln³⁺Ir₂^4.5+O₉ (for Ln =  Y and other lanthanides). Measurements of the magnetic susceptibility and specific heat were carried out for Ln =  Y, Ce, Pr, Tb, and Lu. It was found that the effective magnetic moment of Ir ions is significantly small, which indicates the existence of the strong antiferromagnetic interaction between Ir ions in the Ir₂O₉ bioctahedra. The Y and Lu compounds with an unpaired magnetic moment remaining in the Ir₂^4.5+O₉ show an antiferromagnetic transition at 4.0 and 5.1 K, respectively. For Ba₃TbIr₂O₉, a long-range magnetic ordering of Tb⁴⁺ ions was found at 2.0 K.

The effect of low substitution levels of Ga for Mn and of Ba for Ca upon the magnetic properties of orbital–charge ordered manganites has been studied for in Pr_1−xCa_xMnO₃. It is shown that, for very small Ga or Ba contents (), a ferromagnetic ground state can be created. More importantly, the ferromagnetic fraction, very small for and x = 0.50, goes abruptly through a maximum for the compound, which can be considered as a full ferromagnet at 2.5 K, with ferromagnetic fractions of 95–100% for 2–3% Ba. The role of the local 'counter-distortion' introduced by Ga or Ba in the appearance of ferromagnetism is discussed in the context of orbital–charge ordering and phase separation.

We present and compare the results of temperature-dependent electron paramagnetic resonance (EPR) studies on Pr_1−xCa_xMnO₃ for x = 0.64, which is electron-doped, with results of studies on the hole-doped, x = 0.36 composition. The temperature dependence of the various parameters obtained from the powder and single crystal spectra show significant differences between the two manganites. At room temperature the 'g' parameter for the electron-doped system is less than the free electron 'g' value 'g_e ', whereas for the hole-doped system it is more than 'g_e '. Further, the linewidth obtained from the powder spectra as well as the single crystal spectra show different functional dependences on temperature in the two systems. Quite strikingly, the peak observed at T_co in the temperature dependence of the asymmetry parameter, α, of the single crystal spectra in the hole-doped system, is absent in the electron-doped system. We understand this contrasting behaviour of the EPR parameters in the two systems in terms of the very different nature of microscopic interactions in them.

The cubic to tetragonal phase transition in the solid solution K(Mn,Ca)F₃ has been investigated by conduction calorimetry and x-ray diffraction. The behaviour of the excess specific heat, latent heat and spontaneous strain has been explained in terms of a 2–4–6 Landau potential. The coefficients A and C, prefactors of Q² and Q⁶ in the free energy expansion, are practically constant with composition, but B (the prefactor of Q⁴) and T_C are functions of composition. The tricritical point occurs when the sign of B changes from negative (in pure KMnF₃) to positive (in Ca rich samples). However, the variation of the parameters B and T_C with dopant concentration x is non-linear. The dependence of T_C with composition is explained in terms of an internal stress due to the doping ion of Ca and is compared with the effect of external uniaxial stress on pure KMnF₃.

We report on a density functional theory calculation of the electronic structure and optical properties of γ-Al₂O₃. We have made a comparison between the optical and electronic properties of the α and γ phases of alumina. The calculated bulk modulus of the γ phase is slightly lower than that of the α phase. The calculated static dielectric function and the optical constant of the γ phase are very close to those of the α phase.

We report the effect of femtosecond laser irradiation conditions on precipitation of silver nanoparticles in silicate glasses. Absorption spectra show that the intensity of the absorption peak due to the surface plasmon resonance of silver nanoparticles increases with increase of the light intensity of the laser beam, beam diameter in the focal plane, Rayleigh length of the focusing lens and shot number of the laser pulse. The position of the surface plasmon resonance peak remains constant regardless of the variation of the irradiation conditions. The influences of laser irradiation conditions on the size and spatial distribution of silver nanoparticles are discussed.

A mechanism of attraction between like-charged macroions due to charge fluctuation has been revisited for macroions of different geometries. For higher charge asymmetry, macroion geometry plays an important role in charge fluctuation, while for comparatively lower charge asymmetry the fluctuation is almost independent of the geometry. At very low temperatures and in a high Coulomb coupling regime, stable ionized states due to auto-ionization are able to exist, which leads to long range Coulomb attraction. Overcharging of a single macroion and its counterion distribution minimizing the total energy has been studied briefly to explain the attraction properties between two like-charged macroions. The investigations have been carried out using a technique based on total energy minimization which has been verified successfully for some special cases with molecular dynamics simulation results previously published. A theoretical model, derived by modifying the Scatchard approach, is proposed to explain the overcharging curves obtained from the simulation. It has been found that the model is equally efficient for both spherical and non-spherical macroion geometries. Wigner crystal theory has also been employed for spherical cases, and excellent agreement has been found between the two theoretical approaches.

The atomic structures of calcium sialons Ca_xSi_12−mAl_mN₁₆ (m = 0.5, 1, 2, 4) with the α- or β-Si₃N₄-type structure were calculated, using the ab initio DFT approach, including relaxations of both lattice parameters and positions of the atoms. For all m values the α-Si₃N₄-type structure has a lower energy than the β-type. For m = 1, 2 and 4 the α-type structure is also stable against decomposition into the metal nitrides, in agreement with experimental data. For m = 0.5 stability becomes uncertain. Al ions prefer positions close to Ca. Rather strong structural relaxations occur around the Ca ion.

The problem of quantum lenses has been addressed recently by two papers (Rodriguez et al 2003 J. Phys.: Condens. Matter 15 8465, Even and Loualiche 2003 J. Phys. A: Math. Gen. 36 11677). The aim of this comment is to compare the results of both mathematical methods.

The suitability of conformal transformation (CT) analysis, and the eigenvalue moment method (EMM), for determining the eigenenergies and eigenfunctions of a quantum particle confined within a lens geometry, is reviewed and compared to the recent results by Even and Loualiche (2003 J. Phys.: Condens. Matter 15 8465). It is shown that CT and EMM define two accurate and versatile analytical/computational methods relevant to lens shaped regions of varying geometrical aspect ratios.