Table of contents

Subpicosecond transient Raman spectroscopy has been used to study electron transport properties of an In_0.53Ga_0.47As based p–i–n nanostructure. Both the electron distribution function and the electron drift velocity have been directly measured as a function of the photoexcited electron–hole pair density. We have found that, at low electron–hole pair densities such as n = 5 × 10¹⁶ cm⁻³, the electron distribution function has an extremely non-Maxwellian shape. However, as the photoexcited electron pair density gradually increases, the non-Maxwellian distribution gradually evolves into a shifted Maxwellian distribution. We attribute these findings to the direct effects of the role of electron–electron scattering in momentum randomization.

We propose a controlled Josephson spin current across the junction of two non-centrosymmetric superconductors like CePt₃Si. The Josephson spin current arises due to the direction-dependent tunnelling matrix element and different momentum-dependent phases of the triplet components of the gap function. Its modulation with the angle ξ between the non-centrosymmetric axes of two superconductors is proportional to sinξ. This particular dependence on ξ may find application of the proposed set-up in making a Josephson spin switch.

Photoluminescence of Y_2.3−xTb_xCe_0.05Gd_0.65Al₅O₁₂ (x = 0.575, 1.15, 1.725 and 2.3) has been measured at room temperature at high hydrostatic pressure. Under excitation at 457 nm, the broad band emission related to interconfigurational transition from the lowest state of excited electronic configurations 5d¹ to the ground state split by spin–orbit interaction into ²F_5/2 and ²F_7/2 components of Ce³⁺ ions, peaked at 550 nm, is seen. Under excitation with 325 nm one observes mainly the sharp luminescence lines related to , , and transitions in Tb³⁺ ions. One observes a strong pressure induced red-shift of the Ce³⁺ emission approximately equal to −20 cm⁻¹ kbar⁻¹ and much smaller shifts (approximately −1 cm⁻¹ kbar⁻¹) of the energies of the sharp lines related to Tb³⁺ luminescence. It has been found that the efficiency of energy transfer depends on Tb content and excitation energy; specifically, it is less effective when Tb³⁺ is excited to the lowest state of the excited 5d¹4f⁷ electronic configuration. This effect has been tentatively attributed to the nonradiative de-excitation of the Tb³⁺ ion to the metastable ⁵D₄ state. A configurational model has been developed, which explains peculiarities of relaxation in the excited states of Tb³⁺ and energy transfer.

An Fe₃O₄ nanotube array was successfully prepared in pores of an anodic aluminium oxide (AAO) template. Fe₃O₄ nanotubes and the nanotube array were characterized by transmission and scanning electron microscopy. The average diameter of the nanotubes was about 200 nm, and the length was more than 10 µm. The static distribution of the magnetic moments was investigated by means of magnetostatic energy analysis and Mössbauer spectrum measurement. The resulting Mössbauer spectrum shows that the distribution of the magnetic moments in the Fe₃O₄ nanotube array is spatially isotropic. However, macroscopic magnetic measurement shows the Fe₃O₄ nanotube array to have obvious anisotropy, and the easy axis is parallel to the nanotube axis. These magnetic behaviours are discussed on the basis of analysis of the magnetostatic energy.

On the basis of the Rashba spin–orbit interaction, we propose and investigate a mesoscopic spin interferometer for generating spin polarized current. We find that the output current is in general spin polarized if the interferometer is a multi-terminal device with two non-equal-length arms. We study the dependence of the spin polarization and output probability on the system parameters, and find that both of them can be quite large, simultaneously. The suggested device does not contain any magnetic material or magnetic field; moreover, the spin–orbit interaction can be either uniform or non-uniform. This spin device can be realized with today's technology.

The distribution of carbon and nitrogen atoms on the octahedral interstitial sites of the face-centred-cubic austenite phase in Fe–C and Fe–N alloys, especially in austenitic stainless steel, is still causing controversy. In this work, results of Mössbauer experiments are presented in order to advance the understanding of this interstitial occupation. Therefore, laser carburized and laser nitrided austenitic stainless steel was investigated by means of x-ray diffraction and Mössbauer spectroscopy. Three subspectra in terms of different iron sites were resolved in the Mössbauer spectra for these iron–carbon and iron–nitrogen austenites. The isomer shifts, the quadrupole splittings and in particular the subspectra fractions depend on the type of the introduced atom and undergo changes when increasing the carbon or nitrogen content. This is discussed in connection with the existing ordering models for interstitial atoms. No clear evidence could be found for a perfect random occupation, nor for a perfect ordered occupation of the interstitials. Nevertheless, there seems to a tendency for a weak attractive interaction for nitrogen interstitials, and for a stronger repulsive force for the carbon interstitials in laser nitrided/carburized austenitic stainless steel.

We report the ultrafast dynamics of simultaneously excited coherent A_1g and E_g phonons in semimetals Bi and Sb. At a low level of excitation, the doubly degenerate E_g phonons in pump–probe data occur at low temperature only, and they nearly vanish at room temperature. Their initial phase is shifted by π/2 with respect to that of the fully symmetric A_1g phonons indicating that the E_g and A_1g phonons are excited predominantly impulsively and displacively, respectively. At a high level of excitation, both phonons display an asymmetric (varying in time) line shape with more spectral weight at low frequencies testifying to quantum interference between the one-phonon state and a continuum of states. Furthermore, above a threshold fluence, these phonons of different symmetry exhibit collapse and revival, albeit with different characteristic times.

The discovery of new materials with peculiar optical properties as well as the prediction of their behaviour given the microstructure is a matter of remarkable interest in the community of material scientists. A complete theory allowing such a prediction is not yet available. We have formulated a theory able to analytically predict the effective second- and third-order nonlinear electrical behaviour of a dilute dispersion of randomly oriented anisotropic nonlinear spheres in a linear host. The inclusion medium has non-vanishing second- and third-order nonlinear hypersusceptibilities. As a result, the overall composite material is nonlinear but isotropic because of the random orientation of the inclusions. We derive the expressions for the equivalent permittivity and for the Kerr equivalent hypersusceptibility in terms of the characteristic electric tensors describing the electrical behaviour of the spheres. The complete averaging over inclusion positions and orientations led to general results in the dilute limit. We show that these results are consistent with earlier theories and that they provide null second-order hypersusceptibility as expected in a macroscopically isotropic medium. This theory generalizes the well-known Maxwell-Garnett formula and it can be easily specialized to any of the 32 crystallographic symmetry classes. Despite this study assuming static conditions, it can be generalized to the sinusoidal regime, pointing at an interesting way to engineer optically active materials with desired behaviour.

Optical absorption, excitation and emission spectra have been measured for double indium tungstate crystals with the formula M^ICr_xIn_1−x(WO₄)₂ (x = 0.5–2%, M^I = Li, Na, K and Rb) at temperatures ranging from 5 to 300 K. The broadband ⁴T₂–⁴A₂ luminescence from low-crystal-field Cr³⁺ sites was observed in all the crystals that were studied. The local structure of the Cr³⁺ surroundings is discussed in terms of the spectroscopic results, and the crystal field parameters, Dq, B and C of the Cr³⁺ ion are derived for all the materials. The Huang–Rhys parameters, S, and breathing phonons, , are reported. The reported data indicate a possible phase transition at about 150 K for the trigonal RbIn(WO₄)₂ chromium (III) doped crystals. The possible application of these crystals as laser systems is discussed.

Room temperature Co K edge extended x-ray absorption fine structure (EXAFS), x-ray absorption near edge structure (XANES), including the pre-edge structure, and x-ray diffraction (XRD) studies are carried out on LaCoO₃ and PrCoO₃. The Co–O, Co–La/Pr and Co–Co bond lengths are obtained from EXAFS analysis and compared with those obtained from XRD study. The EXAFS analysis of the data indicates that the CoO₆ octahedron is distorted in both LaCoO₃ and PrCoO₃. There are two Co–O bonds with bond length 1.863 (1.886) Å and four Co–O bonds with bond length 1.928 (1.942) Å for LaCoO₃ (PrCoO₃). Such distortion is expected in orthorhombic PrCoO₃ but not in rhombohedral LaCoO₃. This distortion in the CoO₆ octahedron is attributed to Jahn–Teller active Co³⁺ ions in an intermediate spin state in these compounds. Higher shell studies reveal that Debye–Waller (DW) factors of Co–Pr and Co–Co bonds in PrCoO₃ are greater in comparison with those of Co–La and Co–Co bonds in LaCoO₃, indicating that these bonds are structurally more disordered in PrCoO₃. The comparison of Co–Co bond lengths and corresponding DW factors indicates that the structural disorder plays an important role in deciding the insulating properties of these compounds. XANES studies have shown changes in the intensities and positions of different near edge features. The comparison of experimental spectra with the calculated ones—using the Co 4p density of states obtained from local density approximation calculations and matrix elements calculated using an atomic like core state as the initial state and a confluent hypergeometric function as the final state—indicates that for orthorhombic structure, the intensities of different features are lower as compared to those for the cubic structure. The pre-edge peaks attributed to and transitions show the effects of hybridization of the e_g orbitals with O 2p orbitals, and their relative intensities in PrCoO₃ and LaCoO₃, can be explained by using the average Co–O bond length obtained from the EXAFS.

The plane elasticity equations of two-dimensional quasicrystals of point group 10 are reduced to a single partial differential equation with eighth order by introducing a stress potential function. Further, we develop the complex variable function method for classical elasticity theory to that of the quasicrystals. The complex representations of stress and displacement components of phonon and phason fields in the quasicrystals are given. With the help of conformal transformation, an exact solution for the elliptic notch of the quasicrystals is presented. The solution of the Griffith crack problem as a special case of the results is also observed. This work shows that the stress potential and complex variable function methods are powerful for solving the complicated boundary value problems of higher order partial differential equations originating from quasicrystal elasticity.

The composition and temperature dependences of the thermal and electrical conductivities of three different Cd–Zn alloys have been investigated in the temperature range of 300–650 K. Thermal conductivities of the Cd–Zn alloys have been determined by using the radial heat flow method. It has been found that the thermal conductivity decreases slightly with increasing temperature and the data of thermal conductivity are shifting together to the higher values with increasing Cd composition. In addition, the electrical measurements were determined by using a standard DC four-point probe technique. The resistivity increases linearly and the electrical conductivity decreases exponentially with increasing temperature. The resistivity and electrical conductivity are independent of composition of Cd and Zn. Also, the temperature coefficient of Cd–Zn alloys has been determined, which is independent of composition of Cd and Zn. Finally, Lorenz number has been calculated using the thermal and electrical conductivity values at 373 and 533 K. The results satisfy the Wiedemann–Franz (WF) relation at T<373 K, which suggests the dominant carriers of thermal conduction are mainly electrons. Above this temperature (T>373 K), the WF relation could not hold and the phonon component contribution of thermal conductivity dominates the thermal conduction.

The excitons in boron carbide recently independently proved to be present by luminescence measurements (by Schmechel and co-workers) and by x-ray Raman scattering investigations in connection with ab initio calculations (by Feng and co-workers) are discussed, taking the actual structure of boron carbide within the homogeneity range (B_4.3C–B_∼11C) into account. The excitonic levels at 1.560 and 1.5695 eV obtained from the luminescence spectrum are attributed to the central B atom in the C–B–C and the C–B–B chains respectively. Considering the transition energies related to the different gap states obtained by optical and electrical measurements for energies below the interband transition of 2.09 eV, an actualized energy band scheme of boron carbide is presented.

Si₃N₄ and Ge₃N₄ are important structural ceramics with many applications because of their outstanding high-temperature and oxidation-resistant properties. Two stable phases of them, α and β, have thus far been synthesized. The high-pressure and high-temperature spinel phases of these two materials were noticed to have wide, direct electronic band gaps that are comparable to those of the promising newly developed solid-state optoelectronic materials such as lasers and LEDs. Another high-pressure and high-temperature phase, the olivine phase, has also attracted attention recently. In the present work, the structural and electronic properties of the new olivine nitrides Si₃N₄ and Ge₃N₄ are studied by the FLAPW method with PBE-GGA exchange and correlation potential. The stability of the two materials and the transition pressure are investigated. It is found that olivine-Ge₃N₄ is not stable and is difficult to be observed, while olivine-Si₃N₄ can be synthesized under appropriate conditions. The atomic sites have been optimized and the ground-state properties such as equilibrium lattice constant, bulk modulus, band structure and density of states have been obtained. Furthermore, the dielectric function has been calculated based on the random phase approximation.

The decoherence and fidelity of spin states in a localized single-electron quantum dot in the presence of a dc magnetic field, arising either from the nuclear hyperfine interaction within the dot or due to its coupling with another localized quantum dot, are examined in detail. A general framework for determining the time evolution of the reduced density matrix ρ for a single dot is presented, which is exact up to the second order in interaction with any reservoir. In particular, it is applied to the problem of nuclear hyperfine coupling, and approximate estimates of coherence decay time are made when the nuclear spins are either polarized or unpolarized and the internal dynamics of nuclear spins is determined mainly by the nuclear magnetic dipole–dipole interaction. The fidelity of a pure unperturbed electronic one-qubit spin state is obtained as a function of time, which is exact even on a very short timescale of logic gate operations. The time variation of the fidelity of the same one-qubit state on the localized dot as a part of the direct product with another one-qubit state on another localized dot arising because of coupling between these quantum dots is also calculated in this paper. In this case, we include both the single-particle tunnelling between the dots as well as the direct and exchange Coulomb interactions, including on-site Coulomb repulsion. This allows for the double occupation of a single dot. It is found that the loss of fidelity of such two-qubit states due to double occupancy and additional phase errors in the presence of appreciable dot–dot coupling can become a more severe limiting factor than that due to the hyperfine interaction in individual dots.

Electronic and transport properties of finite carbon nanotubes subject to the influences of a transverse electric field and a magnetic field with varying polar angles are studied by the tight-binding model. The external fields will modify the state energies, destroy the state degeneracy, and modulate the energy gap. Both the state energy and the energy gap exhibit rich dependence on the field strength, the magnetic field direction, and the types of carbon nanotubes. The semiconductor–metal transition would be allowed for certain field strengths and magnetic field directions. The variations of state energies with the external fields will also be reflected in the electrical and thermal conductance. The number, the heights, and the positions of the conductance peaks are strongly dependent on the external fields. The heights of the electrical and thermal conductance peaks display a quantized behaviour, while that of the Peltier coefficient does not. Finally, it is found that the validity of the Wiedemann–Franz law depends upon the temperature, the field strength, the electronic structure, and the chemical potential.

In figure 4(c) the diffraction pattern of the Si substrate was inadvertently included instead of the diffraction pattern of Ge nanocrystals, and should be replaced by the new figure given (see pdf for details).