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Here are reminiscences of some of my interactions with Marshall Stoneham and my career in industry, and particularly of his timely support for my work; and with some illustration of the importance of keeping a firm grasp on basic science to help see the wood from the trees in evaluating new technologies. It is interesting to see that fundamental theory established several decades ago needs to be further developed with some quite radical change of viewpoint when it is applied to new technology; and it is ironic that the impetus for such development of fundamental theory can be technological and commercial, rather than purely academic.

We present a quantum theory of friction in which interactions with the surrounding medium are described by generalized measurements of the particle's position and momentum. The theory predicts intrinsically quantum contributions to the particle's steady-state energy and to the associated diffusion in position. We discuss the physical significance of these and demonstrate their significance in ensuring a well behaved theory.

It is well established that nucleation of metal clusters on oxide and halide surfaces is typically dominated by defect sites. Rate equation models of defect nucleation have been developed and applied to these systems. By comparing the models with nucleation density experiments, energies for defect trapping, adsorption, surface diffusion and pair binding have been deduced in favourable cases, notably for Pd deposited on Ar-cleaved MgO(001). However, the defects responsible remain largely unknown. More recently, several types of ab initio calculation have been presented of these energies for Pd and related metals on MgO(001) containing several types of surface defect; these calculated values are surveyed, and some are widely divergent. New rate equation nucleation density predictions are presented using the calculated values. Some calculations, for some defect types, are much closer to experiment than others; the singly charged F_s⁺ centre and the neutral divacancy emerge as candidate defects. In these two cases, the Pd/MgO(001) nucleation density predictions agree well with experiment, and the corresponding surface defects deserve to be taken seriously. Energy and entropy values are discussed in the light of differences in calculated charge redistribution between the metal atoms, clusters and (charged) surface defects, and (assumed or calculated) cluster geometries.

In order to understand how to enhance the performance of polymer light-emitting diodes (PLEDs), we used a mesoscopic hopping model, taking into account molecular properties and polymer morphology, to investigate the impact of a number of conjugated polymers and molecular arrangements on the functioning of single-layer devices. The model is applied to devices with the active polymer consisting of poly(p-phenylene vinylene) (PPV) and PPV derivatives with stiff conjugated segments having their long axis oriented parallel and perpendicular to the electrode surfaces as well as randomly oriented, which are three of the molecular arrangements that can be obtained experimentally at microscopic scale in solution-processed conjugated polymer thin films.

The model provides insight into current efficiency, charge distribution, internal electric field and consequently recombination throughout the polymer layer. We found that the details of molecular arrangement crucially affect the distribution of recombination events far from the electrodes and its field dependence, which has implications for the efficiency of PLEDs. In particular, we found a variation of recombination efficiency in the bulk of uniform ordered polymer films depending on the molecular alignment relative to the electrode surfaces. It turns out that molecular orientation perpendicular to the electrodes increases recombination in the centre of the polymer film as compared to the case of parallel orientation. We conclude that the orientational alignment perpendicular to the electrode surfaces might be a viable strategy towards efficient polymer-based LEDs.

Recent experiments that determine mass diffusion D_s on the close packed surfaces of vacuum compatible metals are reviewed. The results turn out to be approximately universal when scaled to homologous temperatures T/T_m, with T_m the melting temperature. Similar behaviour for vacancy-driven diffusion in bulk metals has been recognized for decades. Remarkably, the uncertainty with which this scaling occurs is only ∼10%. Possible origins of the universality are discussed.

The significance of the part played by the angular-dependent components of forces associated with d–d bonding between atoms in a transition metal has long remained a subject of debate. While almost all the large-scale molecular dynamics simulations of collision cascades and radiation damage in transition metals and alloys are currently performed using spherically symmetric many-body potentials, density functional calculations exhibit a highly anisotropic pattern of charge density deformation in and around the core of interstitial atom defects. This paper describes a fast second-order matrix recursion-based algorithm for including effects of angular anisotropy of d–d bonds in a large-scale molecular dynamics simulation.

Recently there has been considerable interest in the displacive ferroelectric phase transition near T = 28 K in O-18 isotopic strontium titanate. Special efforts have been made to combine the quantum criticality exponents α = −2 (2D) or −3 (3D), δ = 3, and γ = 2 with the thermodynamic inequalities of Rushbrooke, Griffiths, Widom et al, which become exact equalities under the hypothesis of scaling. In particular, these have led others to the inference that γ = 2.0 and β = 1.2 in SrTiO₃. First we show that this is mathematically incorrect and explain why (quantum criticality is exact only at T = 0, whereas the thermodynamic (in)equalities are valid everywhere except T = 0). Second, we show that the inferred values strongly violate a new equality, γ−2β = ν(4−d−2η)>0, we derive from hyperscaling. Third, we show that the existing soft mode frequency data ω(T) from Takesada et al (2006 Phys. Rev. Lett. at press) yield above T_c (from the Lyddane–Sachs–Teller relationship) γ = 1.0. Fourth, we remeasure β from the polarization P(T) and find β = 0.50 ± 0.02. Fifth, we remeasure the electric susceptibility and find that it perfectly satisfies the Salje–Wruck–Thomas equation, which requires γ = 1.0. The important conclusions are: (a) O-18 SrTiO₃ near T_c is mean-field; (b) the thermodynamic scaling equalities of Rushbrooke, Griffiths et al are mathematically incompatible with quantum criticality theory; (c) a new hyperscaling relationship makes β = 1.2 and β>γ/2 impossible.

Femtosecond real-time spectroscopy is an emerging new tool for studying low energy electronic structure in correlated electron systems. Motivated by recent advances in understanding the nature of relaxation phenomena in various correlated electron systems (superconductors, density wave systems) the technique has been applied to heavy electron compounds in comparison with their non-magnetic counterparts. While the dynamics in their non-magnetic analogues are similar to the dynamics observed in noble metals (only weak temperature dependences are observed) and can be treated with a simple two-temperature model, the photoexcited carrier dynamics in heavy electron systems show dramatic changes as a function of temperature and excitation level. In particular, below some characteristic temperature the relaxation rate starts to decrease, dropping by more than two orders of magnitude upon cooling down to liquid He temperatures. This behaviour has been consistently observed in various heavy fermion metals as well as Kondo insulators, and is believed to be quite general. In order to account for the experimental observations, two theoretical models have been proposed. The first treats the heavy electron systems as simple metals with very flat electron dispersion near the Fermi level. An electron–phonon thermalization scenario can account for the observed slowing down of the relaxation provided that there exists a mechanism for suppression of electron–phonon scattering when both the initial and final electronic states lie in the region of flat dispersion. An alternative scenario argues that the relaxation dynamics in heavy electron systems are governed by the Rothwarf–Taylor bottleneck, where the dynamics are governed by the presence of a narrow gap in the density of states near the Fermi level. The so-called hybridization gap results from hybridization between localized moments and the conduction electron background. Remarkable agreement with the model suggests that carrier relaxation in a broad class of heavy electron systems (both metals and insulators) is governed by the presence of a (weakly temperature dependent) indirect hybridization gap.

Here we review the experimental results on a variety of heavy electron compounds, point out the common features as well as the peculiarities observed in some compounds, and compare the data with existing theoretical models.

An investigation into the high-pressure behaviour of YAlO₃ perovskite was conducted using first-principles calculations based on density functional theory within the generalized gradient approximation. Five candidate phases were considered, Pbnm, Cmcm, I4/mcm, P4/mbm and Pm3m respectively. Our results demonstrate a phase transition of YAlO₃ from Pbnm to I4/mcm at 80 GPa and 0 K, and no tendency to the cubic phase or the post-perovskite phase in our pressure range. This high-pressure behaviour of YAlO₃ is similar to that of CaSiO₃. The pressure dependence of the distortion of AlO₆ octahedra is described by the octahedral tilting and rotation angles, the tolerance factor, the polyhedral volume ratio and the valence charge density. We also summarize the relations between tolerance factor, bulk modulus and radius of the A-site in Pbnm-AAlO₃ systems.

X-ray diffraction and Raman scattering studies on the scheelite structured barium molybdate show that, at ∼5.8 GPa, it undergoes a first order phase transition to the fergusonite structure (I 2/a,Z = 4)—as also observed in iso-structural barium tungstate. At still higher pressures, barium molybdate transforms to another phase between ∼7.2 and 9.5 GPa. On release of pressure from 15.8 GPa, the initial phase is recovered, implying that the observed structural modifications are reversible.

The temperature dependence of the magnetic susceptibility shows a broad maximum at ∼550 and 630 K for LuMn₄Al₈ and ScMn₄Al₈, respectively, which can be interpreted as due to the presence of a pseudogap in the effective bands as in LaMn₄Al₈ and YMn₄Al₈. The anisotropic thermal expansion observed for RMn₄Al₈ (R = La, Y, Lu and Sc) and the sensitive volume dependence of the gap width throughout the RMn₄Al₈ system suggest dominant magnetic coupling in Mn spin chains along the c axis.

The magnetic and crystallographic structures in the Er₅(Si_xGe_1−x)₄ system have been investigated for x = 1,0.875 and 0 using macroscopic magnetic measurements and neutron powder diffraction. The existence of a first-order structural transformation from a high-temperature orthorhombic O (I) structure to a low-temperature monoclinic M polymorph in the temperature range 160–220 K has been confirmed for Er₅Si₄. In contrast, compounds with x = 0.875 and 0 retain their room-temperature crystallographic phases—M and O (II), respectively—on cooling down to 2 K. Both Si-containing alloys adopt canted ferromagnetic structures below  K with an easy magnetization direction along the b-axis and a weak antiferromagnetic coupling in the (010) plane. The Er₅Ge₄ compound orders antiferromagnetically at T_N = 14 K in a complex incommensurate structure.

The exciton ground state and excited state energies are calculated for a model system of an infinitely long cylindrical wire. The effective Coulomb potential between the electron and the hole is studied as function of the wire radius. Within the adiabatic approximation, we obtain 'exact' numerical results for the effective exciton potential and the lowest exciton energy levels which are fitted to simple analytical expressions. Furthermore, we investigated the influence of a magnetic field parallel to the nanowire on the effective potential and the exciton energy.

We report on time-resolved studies of non-radiative relaxation of V³⁺ ions in LiAlO₂ by means of a two-beam, pump–probe saturation experiment performed with the 150 fs pulsed output of a Ti–sapphire laser. Exciting into the vibronically broadened transition at 800 nm, a ³T₁ relaxation time of 199 ps has been measured at 4 K. This value decreases to 82 ps at room temperature, representing a reduction in the lifetime of a factor of 2.5 due to internal-conversion processes. The relative probabilities for non-radiative, phonon-assisted barrier hopping and quantum mechanical tunnelling through the potential barrier to the ³A₂ ground state have been obtained using Mott's expression, yielding best-fit parameters of W₀ = (5.2 ± 1) × 10⁹ Hz and W₁ = (7.5 ± 1) × 10¹⁰ Hz for a potential barrier of E_nr = 530 ± 50 cm⁻¹.

Absorption and emission properties and fluorescence lifetimes for the transition of Nd³⁺ ions embedded in P₂O₅–K₂O–MgO–Al₂O₃ (PKMA)-based glasses modified with AlF₃ and BaF₂ are reported at room temperature. The observed energy levels of Nd³⁺ ions in these glasses have been analysed through a semi-empirical free-ion Hamiltonian model. The spin–orbit interaction and net electrostatic interaction experienced by the Nd³⁺ ions follow the trend as PKMA>PKMA+AlF₃> PKMA+BaF₂ glasses. Judd–Ofelt analysis has been carried out on the absorption spectra of 1.0 mol% Nd³⁺-doped glasses to predict the radiative properties for the fluorescent levels of the Nd³⁺ ion. Branching ratios and stimulated emission cross-sections show that the transition of the glasses under investigation has the potential for laser applications. The Inokuti–Hirayama model has been applied to investigate the non-radiative relaxation of the Nd³⁺ ion emitting state, ⁴F_3/2. Based on the decay curve analysis, concentration quenching of the ⁴F_3/2 emission has been attributed to a cross-relaxation process between the Nd³⁺ ions.

The ferroelectric phase transition of crystalline sodium trihydrogen selenite has been characterized by domain observations and measurements of electric permittivity, pyroeffect and spontaneous polarization. The first-order character of the phase transition is clearly demonstrated by the phase coexistence and temperature autostabilization. The considerable heating effect at 50 Hz ac field is described. The electric field effect on the temperature variation of the electric permittivity, in the phase transition region, shows a considerable domain structure contribution to the permittivity value. It is demonstrated that the dielectric properties of x- and y-samples can be described by classical dielectric state equations: the set of coefficients has been determined. It is concluded that the x-component of spontaneous polarization plays a predominant role in the phase transition.

We have studied by electron paramagnetic resonance the mechanism of defect production by electron irradiation in barium hollandite, a material used for immobilization of radioactive caesium. The irradiation conditions were the closest possible to those occurring in Cs storage waste forms. Three paramagnetic defects were observed, independently of the irradiation conditions. A hole centre (H centre) is attributed to a superoxide ion O₂⁻ originating from hole trapping by interstitial oxygen produced by electron irradiation. An electron centre (E₁ centre) is attributed to a Ti³⁺ ion adjacent to the resulting oxygen vacancy. Another electron centre (E₂ centre) is attributed to a Ti³⁺ ion in a cation site adjacent to an extra Ba²⁺ ion in a neighbouring tunnel, originating from barium displacement by elastic collisions. Comparison of the effects of external irradiations by electrons with the β-decay of Cs in storage waste forms is discussed. It is concluded that the latter would be dominated by E₁ and H centres rather than E₂ centres.

In this work we study the microwave photoconductivity of a two-dimensional electron system (2DES) in the presence of a magnetic field and a two-dimensional modulation (2D). The model includes the microwave and Landau contributions in a non-perturbative exact way; the periodic potential is treated perturbatively. The Landau–Floquet states provide a convenient base with respect to which the lattice potential becomes time dependent, inducing transitions between the Landau–Floquet levels. Based on this formalism, we provide a Kubo-like formula that takes into account the oscillatory Floquet structure of the problem. The total longitudinal conductivity and resistivity exhibit strong oscillations, determined by = ω/ω_c, with ω the radiation frequency and ω_c the cyclotron frequency. The oscillations follow a pattern with minima centred at , and maxima centred at , where j = 1,2,3..., δ∼1/5 is a constant shift and l is the dominant multipole contribution. Negative resistance states (NRSs) develop as the electron mobility and the intensity of the microwave power are increased. These NRSs appear in a narrow window region of values of the lattice parameter (a), around a∼l_B, where l_B is the magnetic length. It is proposed that these phenomena may be observed in artificially fabricated arrays of periodic scatterers at the interface of ultraclean GaAs /Al_xGa_1−xAs heterostructures.

The temperature dependence of acoustic properties of the Sn₂P₂(Se_xS_1−x)₆ uniaxial ferroelectric in the vicinity of the Lifshitz point (LP) was investigated by Brillouin spectroscopy and analysed in the Landau–Khalatnikov approximation. An anomalous decrease of the longitudinal hypersound velocity in the paraelectric phase caused by fluctuation effects and crystal structure defects has been found near the LP. Besides this, a small softening of the transverse acoustic phonons is observed, which is due to their linear interaction with the soft optic mode found for incommensurate phase transitions in proper ferroelectrics. The lattice instability analysis of Sn₂P₂S₆ and Sn₂P₂Se₆ crystals and their solid solutions in a polarizable ion model shows that a possible reason for the absence of a soft acoustic mode is nonorthogonality of the spontaneous polarization vector and modulation wavevector which both lie in the monoclinic symmetry plane.

We report our investigations of the electrodynamic response of the S = 1 quantum spin systems Ni₅Te₄O₁₂X₂ (X = Cl and Br), which develop a magnetic ordered state below 30 K. We measure the optical reflectivity over a broad spectral range extending from the far infrared up to the ultraviolet. Besides identifying the electronic interband transitions, we primarily focus our attention on the lattice dynamics, emphasizing the phonon modes spectrum and its temperature dependence. Our findings do not reveal any direct link between possible structural anomalies and the transition into the magnetically ordered state at low temperatures.

We have studied the electronic structure and elastic properties of Y_n+1Co_3n+5B_2n (space group P6/mmm), where n = 1, 2, 3 and , using ab initio calculations. These ternary borides exhibit a bulk-modulus-to-C₄₄ ratio from 1.6 to 1.9, which is rather unusual for ceramics. This may be understood on the basis of the electronic structure: predominantly covalent–ionic YCo₃B₂ layers are interleaved with predominantly metallic YCo₅ layers. Covalent–ionic bonding between B and Co may give rise to a large bulk modulus, while weak coupling between the YCo₃B₂ and YCo₅ layers may be responsible for the low C₄₄ value. On the basis of the similarity in electronic structure and elasticity data, it is reasonable to assume that the Y_n+1Co_3n+5B_2n compounds investigated here may exhibit similar properties to the so-called MAX phases (Barsoum 2000 Prog. Solid State Chem.28 201).

An efficient and reasonable procedure is proposed for calculating the electronic structure of carbon nanotori in terms of rotational symmetry, within the tight binding formalism. It is shown that the curvature induced σ^*–π^* hybridization effects play an important role in determining the electronic structure of this novel nanostructure. The energy gap of the carbon nanotorus exhibits a well defined oscillation feature with increasing size of the torus, converging to that of the corresponding infinite nanotube, while its density of states spectrum evolves from the characteristic of a zero dimensional system to that of a quasi-one dimensional system. Effects of disorder on the electronic properties are also discussed.

We present angular-dependent current–voltage (I–V) measurements in borocarbide YNi₂B₂C single crystals near the vortex-glass irreversible line. External magnetic fields are applied along the angle θ with respect to the c-axis. The nonlinear I–V curves reveal scaling behaviour near the transition. Using the scaling analysis, the relevant critical exponents and vortex transition temperatures are determined for all orientations. The data agrees well with the vortex-glass (VG) model. No evidence was found that supports the existence of a Bose-glass (BG) type of transition.

Ferroelectric materials such as lithium niobate and lithium tantalate show a non-linear hysteresis behaviour, which may be explained by dynamical system analysis. The behaviour of these ferroelectrics is usually explained by domains and domain wall movements. So, the spatial variation of the domain wall was studied previously in order to see its effect on the domain wall width in the context of the Landau–Ginzburg functional. In the present work, both temporal and spatial variations of polarization are considered, and by using the Euler–Lagrange dynamical equation of motion, a Klein–Gordon equation is derived by taking the ferroelectrics as a Hamiltonian system. An interaction has been considered between the nearest neighbour domains, which are stacked sideways in a parallel array with uniform polarization. This interaction term is associated with the spatial term and when this interaction is assumed to be zero, the spatial term vanishes, giving rise to a Duffing oscillator differential equation, which can be also studied by a dynamic system analysis.

The nonequilibrium spin accumulation on a one-dimensional (1D) mesoscopic Rashba ring is investigated with unpolarized current injected through ideal leads. Due to the Rashba spin–orbit (SO) coupling and back-scattering at the interfaces between the leads and the ring, a beating pattern is formed in the fast oscillation of spin accumulation. If every beating period is complete, a plateau is formed, where the variation of spin accumulation with the external voltage is slow, but if new incomplete periods emerge in the envelope function, a transitional region appears. This plateau structure and the beating pattern are related to the tunnelling through spin-dependent resonant states. Because of the Aharonov–Casher (AC) effect, the average spin accumulation oscillates quasi-periodically with the Rashba SO coupling and has a series of zeros. In some situations, the direction of the average spin accumulation can be reversed by the external voltage in this 1D Rashba ring.

The expression for the effective diffusion of an inertial, periodically driven Brownian particle in an asymmetric, periodic potential is compared with the step number diffusion which is extracted from the corresponding coarse grained hopping process specifying the number of covered spatial periods within each temporal period. The two expressions are typically different and involve the correlations between the number of hops.