Table of contents

We show that the entropic effect originating from the translational movement of water molecules plays critical roles in the pressure-induced denaturation of proteins. In our statistical–mechanical method, the partial molar volume governing the pressure dependence of the structural stability is expressed in terms of the two geometric measures of each protein structure, the excluded volume and the accessible surface area for water molecules, and a parameter related to the water-density profile entropically formed near its surface. An unfolded structure, which is shown to turn more stable than the native one at an elevated pressure, successfully features the experimentally observed denaturation.

Localization in the bulk is one of the most important ingredients for the theory of the quantum Hall effect and much attention has been paid to this topic for more than two decades. However, less effort has been made to model the current transport itself. Network models are frequently used in this context and an answer should be given as to whether these are also suitable for modelling the lateral distribution of experimentally excited currents and voltages in the quantum Hall effect (QHE) regime. The term 'network model' is of more general meaning and therefore the term 'circuit type simulations' should be used instead for expressing this kind of modelling. In preceding papers a Landauer–Büttiker type representation of bulk current transport has been successfully used for the numerical simulation of the magneto-transport of two-dimensional electron systems in the high magnetic field regime. This approach allows us to build up a network model, which describes correctly the effect of non-equilibrium currents injected via metallic contacts as in real experiments. In this context we suggest a network model, which serves as a circuit type representation of magneto-transport. It is demonstrated that it is in full agreement with a treatment of bulk current transport as a quantum tunnelling process between magnetic bound states, which exist in the high magnetic field regime. Additionally, we find a striking correspondence between our network representation and the bulk current picture in terms of mixed phases mapped on a chequerboard: at half filled Landau level (LL) coupled droplets of a quantum Hall (QH) liquid phase and coupled droplets of an insulator phase exist at the same time, with each of them occupying half of the bulk area. Removing a single electron from such a QH liquid droplet at half filling completes the QH plateau transition to the next higher QH plateau, while adding a single electron to such a droplet at half filling completes the QH plateau transition to the previous lower QH plateau. As a consequence, the sharpness of the QH plateau transitions on the magnetic field axis depends on the typical size of the droplets, which can be understood as a measure of the disorder in the sample. We furthermore demonstrate that our model can also be used as a theoretical concept for discretization of a long range random potential on a regular grid. We also find a correspondence between our model and the so called localization picture of the QH plateau transitions. Alternative approaches like the Chalker–Coddington (CC) network as the coherent counterpart and a resistor network as the classical counterpart are discussed in comparison with our network approach. On the one hand the CC model was originally developed to treat the localization problem; on the other hand it also assigns currents to channels, and in this context the CC model will be discussed in the context of circuit type simulations as well. Finally, we compare our simulation results to results obtained on the basis of an adapted lattice model by Ando (1998 Surf. Sci. 361/362 270) for the QHE regime and find our results in full agreement for the case of strong inelastic scattering.

The structural, transport and magnetic properties of the δ-phase bronzes Na_0.56V₂O₅ and Sr_0.5V₂O₅ with double trellis layers have been explored by means of x-ray diffraction and through measurements of electrical resistivity, thermoelectric power, magnetization and electron spin resonance. The properties of the Na compound with a uniform valence depend on the cooling rate likely due to the configuration for the order of Na: the slow cooling condition gives rise to a thermal hysteresis for the semiconducting transport and the spin-singlet state due to the large exchange coupling for the rung in the trellis layer. On the other hand, fast cooling leads to the absence of hysteresis and the metastable spin state depending on the measurement time. The Sr compound with a valence order close to V⁴⁺ and V⁵⁺ is insulating due to electron correlation, and it exhibits the singlet state originating from the alternating-exchange chain effect.

We have carried out a computational study for the nitrogen vacancy in charge states +3, +2 and +1 in AlN in the metastable zinc-blende phase. The vacancy and its four nearest-neighbour Al ions are treated as a molecular cluster, embedded in an infinite classical shell-model crystal. The following ground state properties, all of which are determinable from experiment, have been calculated: total spin, nearest-neighbour displacement, electron spin density at nearest-neighbour nuclei and breathing-mode force constant. The issue of disproportionation among the three charge states is also addressed. Most importantly, the optical excitation energies are evaluated.

Raman and polycrystalline IR spectra were obtained for Li₂TiGeO₅ and the assignment of the observed bands to the respective internal and external phonons has been proposed on the basis of lattice dynamics calculations. Temperature dependences of Raman- and IR-active phonons are also reported to probe the paraelastic–ferroelastic phase transition that takes place at 233.5 K. This study shows that the phase transition is continuous and that this phase transition leads to significant distortion of the unit cell.

The quantum interference in single-walled carbon nanotubes (SWNTs) in a magnetic field parallel to their axes has been studied analytically. It is found that the quantum conductance changes with magnetic field periodically, which is consistent with the experimental observations. The rapid and slow conductance oscillation periods for both armchair and zigzag SWNTs have been derived analytically; they depend now on both the magnetic field and gate voltage in addition to the sample length. More interestingly, metallic zigzag SWNTs in an axial magnetic field can have slow conductance oscillation, which should not exist without a magnetic field being applied.

The magnetic and magnetotransport properties of the Sr₃Fe_2−xCo_xO_7−δ system (0.2≤x≤1.0) have been systematically investigated. This oxide system exhibits a giant magnetoresistance (GMR) effect at low temperatures, reaching up to 80% in 7 T at 5 K. Ac-susceptibility measurements show that there exists a strong competition between ferromagnetic (F) and spin glass states, and the balance between these two magnetic states can be controlled by varying cobalt (x) and/or oxygen contents (δ). Importantly, the MR effect is closely related to the magnetic property: the development of magnetic disordering leads to enhancement in the negative MR effect. It is suggested that the compound segregates into F clusters embedded in a non-F matrix, being a naturally occurring analogue of the artificial granular-GMR materials, as in the doped perovskite cobaltites, La_1−xSr_xCoO₃ (x< 0.18).

The ¹H and ³⁹K spin–lattice relaxation, T₁, spin–spin relaxation time, T₂ in the laboratory frame, and the spin–lattice relaxation time in the rotating frame T_1ρ in superionic K₃H(SeO₄)₂ single crystals grown by the slow evaporation method were measured in the ferroelastic and paraelastic phases. The ¹H  T₁, T_1ρ, and T₂ exhibited different trends with temperature in the ferroelastic phase, but were found to have very similar, liquid-like values in the paraelastic phase. In the paraelastic phase, the reorientational and translational motions are lost, and the proton motion can be characterized with a single correlation frequency, ω_c. The observation that the variations of ¹H  T₁, T_1ρ, and T₂ of K₃H(SeO₄)₂ crystals with temperature are very similar in the paraelastic phase indicates that the destruction and reconstruction of hydrogen bonds does indeed occur at high temperatures. In addition, the ³⁹K  T₁ and T₂ were found to be similar at high temperatures, as was also observed for the ¹H  T₁ and T₂. The crystal in the high-temperature phase is a fast ionic conductor. The motion giving rise to this liquid-like behaviour may be related to superionic motion.

Transmission electron microscopy and electron energy loss spectroscopy (EELS) were used to distinguish structural details for the cubic and uncommon monoclinic crystal phases of heavy metal rare earth oxide nanoparticles synthesized by a combustion process. Specifically, we systematically examined the EELS oxygen K edge for Ho, Tm, Er, and Yb sesquioxides in both phases. This work presents, to our knowledge, the first EELS study of the monoclinic phase in these materials and the first systematic comparative study between the monoclinic and cubic phases across a range of rare earth sesquioxides. For both phases, we observed the usual double-peak structure of the oxygen K edge typically seen for oxygen atoms tetrahedrally surrounded by metal atoms, but we found the details of the energy loss near-edge structure to differ substantially between the two phases. Our results indicate a greater peak separation for the cubic than for the monoclinic phase. Furthermore, a trend of increasing peak separation with increasing atomic number was also noted for both phases in the series of rare earth oxide nanoparticles. Our results show that the fine structure in EELS can be utilized to determine differences on the nanoscale between the common cubic structure and the less common monoclinic structure.

The dc electrical conductivity, σ_dc, of the composite system Ag₄RbI₅–(β-AgI), which is composed of a good ionic conductor and a bad one, was measured by the ac impedance method. The obtained σ_dc increases with the volume fraction, ϕ, of the good conductor, Ag₄RbI₅. This ϕ dependence of σ_dc was analysed based on the generalized effective medium (GEM) theory. The scaling law analysis was also applied. The obtained threshold value of the volume fraction, ϕ_c, was 0.162 ± 0.005. The critical exponents, t and s, were determined to be 2.0 ± 0.05 and 0.88 ± 0.005 respectively. These values are in almost complete agreement with the universal values of ϕ_c,t and s predicted by the computer simulation for the electrical conduction. So far as the present authors know, this study is the first to provide the full set of universal characteristic values of percolation, ϕ_c,t and s, for the electrical conduction from a single experimental study for the artificial composite material.

The requirement from the industry of precise lifetime estimation of Bragg gratings in optical fibres and other optical devices motivated us to perform calculations in a region of a master curve (describing the time evolution of a system at any temperature) where an approximation, called demarcation energy, is no longer valid. We describe a correction procedure that leads to reintroducing afterwards a temperature dependence lost in this approximation while preserving most of the advantages of the demarcation energy approach. With our procedure, a precise determination of burning-in parameter for achieving lifetime specification is possible.

A Bragg grating written in H₂ loaded Ge doped silica core optical fibres is used as application example.

We report molecular dynamics simulations of the production of radiation cascades in pyrochlores. We consider the apparently similar systems Gd₂Ti₂O₇, Gd₂Zr₂O₇ and Gd₂Pb₂O₇, the first two of which have been put forward as potential materials for high-level radioactive waste storage. The effects of changing the mass of the 'primary knock-on' atom are also examined and we investigate whether the change in behaviour from Ti to Zr to Pb is largely due to the mass or the size difference between the elements. Problems associated with analysing the cascades and the damage created are discussed. There are clear differences between the three compounds. The simulations see no direct amorphization but rather a transition to the fluorite structure which is more pronounced for the Zr and Pb compounds than the Ti system. Underlying chemical trends are examined.

Pure single-phase iron nanostructured particles with pseudo-cubic shape crystalline grains and linear dimensions of around 11 nm can be produced by the low energy ball milling of microcrystalline Fe under argon atmosphere. The long range ferromagnetic correlation of exchange coupled crystallites extending across grain boundaries leads to a reduction of the effective anisotropy, as expected from the generalized random anisotropy model. This ferromagnetic network of correlated grains is preserved at low temperatures. No spin-glass freezing process is detected. Slight oxidation of the particles with formation of an FeO phase is achieved with deliberately prolonged milling. This FeO phase leads to non-collinear spin structure at the interfaces that suppresses the intergrain correlations and enhances the role of long range dipolar interactions. The interface spin disorder and the complex state of the intergrain interactions are the sources of the spin-glass-like behaviour found in these Fe–FeO nanocomposites.

Magnetization and high resolution neutron powder diffraction measurements on the magnetic shape memory compound Ni₂Mn_1.44Sn_0.56 have confirmed that it is ferromagnetic below 319 K and undergoes a structural phase transition which takes place at T_M = 221 K on cooling and 239 K on warming. The high temperature phase has the cubic L 2₁ structure, a = 5.973 Å, with the excess manganese atoms occupying the 4(b) tin sites. In the cubic phase at 245 K the manganese moments at both sites were found to be ferromagnetically aligned. The magnetic moment at the 4(a) sites was 1.88(10) μ_B but it was only 0.53(18) μ_B/Mn at the 4(b) sites. The low temperature phase stable below T_M has an orthorhombic structure with space group Pmma related to the cubic phase through a Bain transformation a_ortho = (a_cub+b_cub)/2; b_ortho = c_cub; c_ortho = (a_cub− b_cub). The change in cell volume in the transition is only ≈0.5%, suggesting that the atomic moments are unchanged although the spontaneous magnetization drops significantly.

Pb₂MnWO₆ has been synthesized and its structure determined by powder x-ray diffraction. This sample undergoes a first order structural phase transition at ∼445 K. This transition is coupled to a change in the electrical properties manifested by a sharp discontinuity of the dielectric permittivity at the phase transition temperature. Pb₂MnWO₆ is cubic paraelectric above 445 K and orthorhombic with electric ordering below such a temperature. The structural refinement reveals Pb displacements in the low temperature phase in agreement with an antiferroelectric ordering.

Diferential scanning calorimetry and magnetic susceptibility measurements are also reported. The large entropy content of the transition evidences the presence of an ordering process. Pb₂MnWO₆ is paramagnetic in a broad temperature range in agreement with the presence of high-spin Mn²⁺ ions. A magnetic anomaly at 45 K may be associated with antiferromagnetic ordering of these ions that would coexist with the antiferroelectric state at very low temperature.

In order to investigate the various interactions in a system consisting of both a 4d transition metal (Ru) and a 4f rare earth (Gd), a combined nuclear magnetic resonance (NMR) and magnetization study has been carried out on the ordered double-perovskite Sr₂RuGdO₆. As the temperature is reduced, an antiferromagnetic ordering (type-I) of the Ru sublattice occurs at T₁ = 33 K, which is a consequence of the Ru–O–O–Ru superexchange interactions. A second magnetic transition occurs between 30 and 10 K, which is characterized by a maximum in magnetization at T₂ = 17 K, along with hysteretic behaviour, and reflects the importance of the Ru–O–Gd interactions over this temperature range. For T₃<2 K, the Gd sublattice orders antiferromagnetically, which is a consequence of the emergence of strong Gd–O–O–Gd interactions. In an external magnetic field, a temperature-dependent spin reorientation is observed. The NMR spectrum for Sr₂RuGdO₆ at 1.3 K consists of two peaks at 119 and 133 MHz corresponding to the ⁹⁹Ru and ¹⁰¹Ru isotopes, respectively, and a hyperfine field of 605 kOe. The evolution of the ¹⁰¹Ru NMR peak in an applied magnetic field reveals the existence of a ferromagnetic phase in the antiferromagnetic matrix at low temperature, indicating a complex competition and coexistence of different magnetic interactions in Sr₂RuGdO₆.

Single-ion anisotropy is of importance for the magnetic ordering of the frustrated pyrochlore antiferromagnets Gd₂Ti₂O₇ and Gd₂Sn₂O₇. The anisotropy parameters for Gd₂Sn₂O₇ were measured using the electron spin resonance technique. The anisotropy was found to be of the easy plane type, with the main constant D = 140 mK. This value is 35% smaller than the value of the corresponding anisotropy constant of the related compound Gd₂Ti₂O₇.

Nonequilibrium electronic transport through a quantum dot coupled to ferromagnetic leads (electrodes) is studied theoretically by the nonequilibrium Green function technique. The system is described by the Anderson model with arbitrary correlation parameter U. Exchange interaction between the dot and ferromagnetic electrodes is taken into account via an effective molecular field. The following situations are analysed numerically: (i) the dot is symmetrically coupled to two ferromagnetic leads, (ii) one of the two ferromagnetic leads is half-metallic with almost total spin polarization of electron states at the Fermi level, and (iii) one of the two electrodes is nonmagnetic whereas the other one is ferromagnetic. Generally, the Kondo peak in the density of states (DOS) becomes spin-split when the total exchange field acting on the dot is nonzero. The spin-splitting of the Kondo peak in DOS leads to splitting and suppression of the corresponding zero-bias anomaly in the differential conductance.

We investigate an alternative approach to the connection between plasmons and the ground-state wavefunction of many-electron systems, starting with a classical plasmon Hamiltonian. We obtain a prescription for the part of the wavefunction which determines the long-ranged electron–electron correlations; we then show how this prescription can be used to construct improved trial wavefunctions for use in quantum Monte Carlo simulations.

Using the quasi-2D electron gas as a test system, we construct a trial wavefunction of the Slater–Jastrow form, combining mean-field single-electron orbitals with short-ranged and plasmon-derived long-ranged correlations. Variational quantum Monte Carlo calculations show that the new wavefunction reduces the expectation value of the energy, but the short-ranged correlations are more important than the plasmon-derived ones. Finally, we present a computationally efficient Jastrow factor, which we recommend for future simulations.

We describe a further development of the stochastic state selection method, a new Monte Carlo method we have proposed recently to make numerical calculations in large quantum spin systems. Making recursive use of the stochastic state selection technique in the Lanczos approach, we estimate the ground state energy of the spin-1/2 quantum Heisenberg antiferromagnet on a 48-site triangular lattice. Our result for the upper bound of the ground state energy is −0.1833 ± 0.0003 per bond. This value, being compatible with values from other work, indicates that our method is efficient in calculating energy eigenvalues of frustrated quantum spin systems on large lattices.

It has previously been noted that the implantation of argon atoms into amorphous SiBCN materials (prepared by magnetron sputtering in various N₂+Ar mixtures) leads to variations in a number of material properties; for example an increase in compressive stress. Little is known about the mechanism by which Ar incorporation affects structural and mechanical properties of these materials. Here, we report ab initio molecular dynamics simulations of amorphous SiBCN materials. Using liquid-quench simulations, we investigate how the presence of Ar atoms in the sample affects the liquid quench and the final structure of the material. In the absence of Ar, we find that the final structures are homogenous. The presence of Ar, however, leads to the formation of Si-enriched regions in the vicinity of implanted Ar atoms. This result provides new insight into the role of implanted Ar in the formation of structures in amorphous SiBCN materials. It can also explain the ability of Si to relieve stress generated by these implanted Ar atoms.