Table of contents

Martensites are materials that undergo diffusionless, solid-state transitions. The martensitic transition yields properties that depend on the history of the material and may allow it to recover its previous shape after plastic deformation. This is known as the shape-memory effect (SME). We have succeeded in identifying the primary electronic mechanism responsible for the martensitic transition in the shape-memory alloy AuZn by using Fermi-surface measurements (de Haas–van Alphen oscillations) and band-structure calculations. This strongly suggests that electronic band structure is an important consideration in the design of future SME alloys.

Measuring, characterizing, and modelling the slow dynamics of glassy soft matter is a great challenge, with an impact that ranges from industrial applications to fundamental issues in modern statistical physics, such as the glass transition and the description of out-of-equilibrium systems. Although our understanding of these phenomena is still far from complete, recent simulations and novel theoretical approaches and experimental methods have shed new light on the dynamics of soft glassy materials. In this paper, we review the work of the last few years, with an emphasis on experiments in four distinct and yet related areas: the existence of two different glass states (attractive and repulsive), the dynamics of systems very far from equilibrium, the effect of an external perturbation on glassy materials, and dynamical heterogeneity.

In this review article we discuss recent work on the thermal stability, thermal fluctuations and interaction forces in lipid membrane stacks. The lipid bilayers are deposited on solid surfaces for investigation by interface sensitive x-ray and neutron scattering on length scales ranging from the molecular to the mesoscopic scale. We discuss how the structure, composition, fluctuations, and interactions in lipid and biomimetic membranes can be studied in the fluid L_α phase. Results on the interaction potentials, height–height correlation functions, elasticity parameters, fluid lipid chain ordering, and collective lipid chain motions are discussed.

Very asymmetric mixtures of hard spheres naturally arise in the modellization of colloidal dispersions. Effective potentials have emerged as a powerful tool for describing these systems and have often been employed to extract the phase diagram in both the additive and nonadditive cases. However, most theoretical investigations have been carried out by means of mean-field-like approaches, so their quantitative accuracy remains to be assessed. Here we employ previously determined effective potentials for nonadditive hard-sphere mixtures to study the fluid–fluid phase transition by the hierarchical reference theory (HRT), which is designed to take realistically into account the effects of long-range fluctuations on phase separation. Fluid–solid equilibrium is addressed by supplementing HRT with thermodynamic perturbation theory for the solid phase. We apply this approach both to a potential with adjustable nonadditivity parameter (Louis et al 2000 Phys. Rev. E 61 R1028) and to the Asakura–Oosawa (AO) potential, which represents an extreme case of nonadditivity. Our results for the phase diagram, including modified hypernetted chain (MHNC) calculations, are compared to those of other liquid-state theories and are found to agree nicely with available simulation data. Unlike commonly adopted liquid-state theories, HRT is capable both of getting arbitrarily close to the fluid–fluid critical point, and of giving nontrivial critical exponents. In particular, the fluid–fluid coexistence curve is much flatter than that obtained via perturbation theory, in agreement with a recent finite-size scaling Monte Carlo analysis of the AO model.

We study the equilibrium behaviour of a mixture of monodisperse hard sphere colloids and polydisperse non-adsorbing polymers at their θ-point, using the Asakura–Oosawa model treated within the free-volume approximation. Our focus is the experimentally relevant scenario where the distribution of polymer chain lengths across the system is fixed. Phase diagrams are calculated using the moment free energy method, and we show that the mean polymer size ξ_c at which gas–liquid phase separation first occurs decreases with increasing polymer polydispersity δ. Correspondingly, at fixed mean polymer size, polydispersity favours gas–liquid coexistence but delays the onset of fluid–solid separation. On the other hand, we find that systems with different δ values but the same mass-averaged polymer chain length have nearly polydispersity-independent phase diagrams. We conclude with a comparison to previous calculations for a semi-grand-canonical scenario, where the polymer chemical potentials are imposed; there it was found that fluid–solid coexistence was favoured over gas–liquid in some areas of the phase diagram. Our results show that this somewhat counter-intuitive result arose because the actual polymer size distribution in the system is shifted to smaller sizes relative to the polymer reservoir distribution.

Dielectric relaxation measurements are performed with very high accuracy on a liquid crystalline compound n-octylcyanobiphenyl (8CB) in the isotropic (I), nematic (N) and smectic A (S_A) phases. The data obtained display an essential difference in the rotational diffusion process in the vicinity of the I–N phase transition in comparison to that taking place in the vicinity of the N–S_A phase transition. Thus, for the I–N transition, anomalously slow diffusion (subdiffusion), characterized by an anomalous coefficient α<1, is observed, while normal Brownian rotational diffusion with is found for the N–S_A transition. It is also shown how the fractal parameter α is temperature dependent with an extremely sharp variation at the I–N transition point in the form of a lambda-like profile.

Regeneration of thixotropic gels resulting from aggregation of positively charged magnetic nanoparticles is here investigated by mechanical spectroscopy. The frequency independence of the ratio ( and being respectively the viscous and the elastic moduli) at the gel point seen as a liquid–solid transition allows the determination of the gelation time t_g and of the power law exponent on the angular frequency for both moduli (, ). This behaviour is similar to the one observed for classical chemical and physical gels. It implies a broad relaxation time spectrum which is compatible with previous dynamic magneto-optical birefringence measurements. The value of t_g decreases exponentially with an increase of the pH that controls the interactions between particles through their surface density of charges. In contrast, the relaxation exponent is found to be nearly insensitive to the pH. A fractal dimension of 2.07 ± 0.06 is deduced from the power law exponent Δ at the gelation time.

This paper reports a new interatomic potential for lithium niobate, which has been fitted to the structure and properties of the stoichiometric ferroelectric phase of the material. The potential is based on a fully ionic description of the material, with the shell model being used for the oxygen ions and a three-body potential representing the interactions of the niobium ions with the neighbouring oxygen ions. The potential set has been tested on the paraelectric phase, whose structure it reproduced to within a few per cent. The calculation of lattice properties including elastic constants and dielectric constants, as well as powder x-ray diffraction patterns of both phases, are reported.

The crystal structure of PbSnF₄ and the nature of the anion diffusion mechanism which characterizes its high ionic conductivity have been investigated by impedance spectroscopy, powder neutron diffraction and computer simulation methods. The ionic conductivity of PbSnF₄ undergoes small, but abrupt, increases at 608(4) and 672(3) K characteristic of the and phase transitions. The ambient temperature α-PbSnF₄ phase possesses a tetragonal crystal structure (space group P4/nmm), derived from the cubic fluorite arrangement by ordering of the cations in the scheme PbPbSnSnPbPb along the [001] direction. However, the Sn²⁺–Sn²⁺ layers contain essentially no F⁻, with the displaced anions residing in the Pb²⁺–Sn²⁺ layers and showing significant disorder, particularly at temperatures close to the upper limit of stability of the α phase. Computer simulations, using interionic potentials derived from first-principles calculations and containing realistic representations of polarization effects, are in good agreement with the measured ionic conductivity and successfully reproduce the experimentally determined ionic distribution. Analysis of the simulated ionic motions demonstrate that the impressive ionic conductivity of α-PbSnF₄ at temperatures close to ambient is a consequence of anion diffusion within the Pb²⁺–Sn²⁺ layers, whilst those F⁻ within the Pb²⁺–Pb²⁺ layers are immobile. At temperatures close to the melting point of the simulated system, increased transfer of anions between the various Pb²⁺–Pb²⁺, Pb²⁺–Sn²⁺, Sn²⁺–Sn²⁺ layers is observed, as the system tends towards a more isotropic anion diffusion process.

Nanocrystalline lead fluoride was prepared by an inert gas condensation technique under ultra-high vacuum conditions. Structural studies were carried out with x-ray diffraction analysis. As-prepared and vacuum annealed samples were found to contain both orthorhombic (α) and cubic (β) phases of PbF₂. The annealed samples contain dominantly the cubic phase. Grain sizes were found to be in the range from 21 to 43 nm. Raman scattering measurements have been performed on these samples in different frequency regions. Characteristic phonon vibrational modes such as T_2g for β-PbF₂ have been observed in addition to some modes corresponding to α-PbF₂. The Raman lines were assigned to β and α-PbF₂ on comparison with the already reported, both calculated and experimental, values. Some new modes have also been observed at the higher frequency region. The modes at higher frequencies were correlated to vibrations of electronic centres whose density varied with annealing temperature. The presence of electronic centres was investigated by photoluminescence studies. The widths of the Raman spectral lines were found to decrease with the increase in grain size. Low-frequency Raman studies revealed the presence of structural defect clusters. The defect cluster size was found to increase with annealing temperature.

Collision cascades in MgAl₂O₄ are investigated using molecular dynamics simulations in order to determine the threshold displacement energies, E_d, and the damage imparted to the lattice at energies of up to 5 keV. The value of E_d is determined for MgAl₂O₄ on each of the Mg, Al and O sublattices for different orientations of the primary knock-on atom (PKA). The lowest E_d required to create permanent defects was for an O PKA along the direction with a value of 27.5 eV, while the highest was 277.5 eV along for an Mg PKA. Higher energy cascades show that a much wider variety of defects remain after the collisional phase than for similar cascades in MgO but the number of Frenkel pairs produced is smaller. The predominant defects that form are antisite defects on the cation sublattice only and O and Mg split interstitials orientated along the direction. Some Mg–Al split interstitials centred on an Mg site were also observed. However, some more extended defect complexes can also arise which have no well defined structure.

The structure of Y Fe₂D_3.5 at 290 K has been refined in a monoclinic superstructure using synchrotron radiation and neutron diffraction experiments. The evolution of the ⁵⁷Fe Mössbauer spectra versus temperature for ZrFe₂D_x and Y Fe₂D_x deuterides with large deuterium content has been studied. For x = 3.5 both ZrFe₂D_x and Y Fe₂D_x show an increase of the Fe magnetization compared to the parent intermetallic compound, related to the increase of the cell volume. For Y Fe₂D_x the Fe moment decreases to complete disappearance as x increases from 3.5 to 5 due to the stronger influence of the Fe–D bonding compared to the volume increase. The evolution of the isomer shift can be attributed to an almost pure volumetric effect in the case of the Y Fe₂ deuterides, whereas for ZrFe₂D_3.5 additional D–Fe charge transfer has to be taken into account.

The compound NpFe₄Al₈ was prepared by direct arc melting of the constituent elements, followed by annealing. It crystallizes in the ThMn₁₂-type structure (space group I4/mmm, a = 8.7480(5) Å, c = 5.0372(4) Å), with the iron atoms completely and only occupying the 8f positions. Magnetization measurements (T = 2–300 K, B = 0–7 T) show a ferromagnetic-type transition at T_C = 135(2) K and a second anomaly at 118(3) K. The low temperature magnetization cycle is characterized by a hysteresis with a step similar to that previously observed for UFe₄Al₈ single crystals. First-principles density functional theory calculations of the NpFe₄Al₈ band structure point to a magnetic structure similar to that of UFe₄Al₈, in agreement with the observed magnetization cycle. The calculations indicate that the neptunium moment is aligned along one of the a or b axes, and the iron moments form a noncollinear structure in the a–b plane, with the antiferromagnetic and ferromagnetic contributions perpendicular and antiparallel to the neptunium spin moment, respectively.

When strontium titanate (SrTiO₃) single crystal is irradiated at room temperature with a 325 nm laser light in an evacuated specimen chamber, the luminescence intensity increases, creating a broad visible luminescence centred at about 2.4 eV. Then, introducing oxygen gas into the specimen chamber, the photoluminescence spectrum returns reversibly to the original weak luminescence under the same laser light irradiation. After removing the laser light irradiation, each photoluminescent state is stored for a long time at room temperature under room light, regardless of any changes of atmosphere. Such photo-induced spectral change has been observed also at different temperatures from 13 K to room temperature. The observed phenomenon is explained by means of the photo-induced oxygen defect formation at the surfaces of SrTiO₃ crystal. For the same SrTiO₃ single crystal, we have studied the photoluminescence properties. Besides the 2.4 eV luminescence band, we have observed new two luminescence bands centred at about 3.2 eV and about 2.9 eV. The energy, 3.2 eV, is close to both the photoluminescence excitation edge energy and the reported band edge energy of SrTiO₃ crystal. Both the 3.2 eV luminescence and the 2.9 eV luminescence decay rapidly after a pulsed photoexcitation, while the 2.4 eV luminescence lasts for several seconds at 13 K. The excitation light intensity dependence of these luminescence bands has been also measured at 13 K. The 2.4 eV luminescence increases in intensity with increasing excitation intensity up to 4 mJ cm⁻², and then it becomes decreased with further increase in the excitation intensity. On the other hand, both the 3.2 eV luminescence and the 2.9 eV luminescence increase in intensity with increasing excitation intensity, without any saturation. Although the 2.4 eV luminescence had been assigned to the radiative decay of intrinsic self-trapped excitons in a superparaelectric state by several workers, the present studies have clarified that the luminescence originates mainly from crystal defects (oxygen defects and chemical heterogeneity in the surface region). Both the 3.2 eV luminescence and the 2.9 eV luminescence are discussed qualitatively.

We have studied the electronic structure of vertically assembled quantum discs in a magnetic field with varying orientation using the effective mass approximation. We calculate the four energy levels of single-electron quantum discs and the two lowest energy levels of two-electron quantum discs in a magnetic field with varying orientation. The change of the magnetic field as an effective potential strongly modifies the electronic structure, leading to splittings of the levels and anticrossings between the levels. The calculated results also demonstrate the switching between the ground states with the total spin S = 0 and 1. The switching induces a qubit controlled by varying the orientation of the magnetic field.

On the basis of the multichannel Landauer formula, we investigated the effects of incommensurability on the electron transport properties of clean incommensurate double-walled carbon nanotubes (DWNTs). The results obtained clearly show that electron transport in incommensurate DWNTs can be either ballistic or non-ballistic depending on the energy region. In the lower energy region, the conductances remain at 2G₀ (where G₀ = 2e²/h is the conductance quantum) except for at a very few positions, at least up to the system length of about 1.4 µm. And with increasing length, the conductances tend to change from 2G₀ to 1G₀ due to the antiresonances. This offers a possible explanation for the measurement results reported by Frank et al (1998 Science280 1744) and by Urbina et al (2003 Phys. Rev. Lett. 90 106603). In the other energy regions, electron transport is non-ballistic and the conductances show a power law decay.

Based on the experimentally-found existence of two superconducting gaps in MgB₂ (one gap associated to the boron σ-states and the other to the boron π-states), the different contributions to the transport properties, electrical conductivity and Hall coefficient were studied using the full potential-linearized augmented plane wave method and the generalized gradient approximation. Four different relaxation times were needed to adjust the electrical conductivity and Hall coefficient to experimental values. MgB₂ doping was analysed in the rigid band approximation; this permitted a detailed study of the partial substitution of magnesium for aluminium (Mg_1−xAl_xB₂). Other substitutions such as AB₂ (A = Be, Sc, Zr, Nb and Ta) are also discussed. The MgB₂ σ-bands (boron σ-states), which are associated to the large gap, are very anisotropic at E_F, while the π bands have very little anisotropic character. In Mg_1−xAl_xB₂, T_c diminishes with Al content; the other compounds are not superconductors or have a low T_c. In this work it was found that with electron doping, such as Al substitution, the σ-band conductivity decreases and the corresponding bands become less anisotropic. The σ-band contribution for BeB₂ and ScB₂ at E_F is very small and the anisotropy is much lower. For Zr, Nb and Ta there are no σ-bands at E_F. These results give a clear connection between superconductivity and the character of the σ-band, band conductivity, and band anisotropy. This gives a plausible explanation for the diminution of T_c with different doping of MgB₂.

Spin dynamics in Er₂Ti₂O₇ and Er₂Sn₂O₇ have been probed by means of muon-spin relaxation (μ⁺SR) in the temperature range  K. Both compounds are thought to constitute experimental realizations of the highly frustrated XY antiferromagnet on the pyrochlore lattice, for which theory predicts fluctuation-induced magnetic order. Our results for Er₂Ti₂O₇ are consistent with a transition into an ordered state at  K, in agreement with previous neutron measurements. Below this temperature, the muon relaxation rate λ(T) remains large ( MHz) and temperature independent, in contrast to the behaviour in conventional magnets. The thermal evolution of λ(T) for Er₂Sn₂O₇ is somewhat similar to that of the Ti material. However, the depolarization curves remain exponential over the entire temperature range, suggesting a dense distribution of rapidly fluctuating magnetic moments, and, thus, are compatible with the absence of long-range order at least for T>0.02 K.

We have performed transport, magnetization and specific heat measurements on the ABO₃-type half-doped (La_0.3Eu_0.2)(Ca_0.3Sr_0.2)MnO₃ (LECSMO) manganite. At T = 2 K, a sharp step-like metamagnetic transition in the magnetization with a critical field of 3.3 T is observed. The transition is much sharper than that seen at higher temperatures ( K). Such step-like transitions have previously been reported only in Pr-based manganites, but the present data show that the step-like metamagnetism can also be found in other manganite compounds.

We report the parameter-free, density functional theory calculations of interatomic exchange interactions and Curie temperatures of half-metallic ferrimagnetic full Heusler alloys Mn₂V Z (Z = Al, Ge). To calculate the interatomic exchange interactions we employ the frozen-magnon approach. The Curie temperatures are calculated within the mean-field approximation to the classical Heisenberg Hamiltonian by solving a matrix equation for a multi-sublattice system. Our calculations show that, although a large magnetic moment is carried by Mn atoms, competing ferromagnetic (inter-sublattice) and antiferromagnetic (intra-sublattice) Mn–Mn interactions in Mn₂V Al almost cancel each other in the mean-field experienced by the Mn atoms. In Mn₂V Ge the leading Mn–Mn exchange interaction is antiferromagnetic. In both compounds the ferromagnetism of the Mn subsystem is favoured by strong antiferromagnetic Mn–V interactions. The obtained value of the Curie temperature of Mn₂V Al is in good agreement with experiment. For Mn₂V Ge there is no experimental information available and our calculation is a prediction.

Solid solutions of nominal composition [ZnGa₂O₄]_1−x[Fe₃O₄]_x, of the semiconducting spinel ZnGa₂O₄ with the ferrimagnetic spinel Fe₃O₄, have been prepared with x = 0.05, 0.10, and 0.15. All samples show evidence for long range magnetic ordering with ferromagnetic hysteresis at low temperatures. The magnetization as a function of field for the x = 0.15 sample is S-shaped at temperatures as high as 200 K. Mössbauer spectroscopy of the x = 0.15 sample confirms the presence of Fe³⁺, and spontaneous magnetization at 4.2 K. The magnetic behaviour is obtained without greatly affecting the semiconducting properties of the host; diffuse reflectance optical spectroscopy indicates that Fe substitution up to x = 0.15 does not affect the position of the band edge absorption. These promising results motivate us to suggest the possibility of dilute ferrimagnetic semiconductors which do not require carrier mediation of the magnetic moment.

Ferroelectric poly(vinylidene fluoride-trifluoroethylene) [P(VDF-TrFE)] copolymer (50% of VDF) was transformed into the relaxor ferroelectric state by high-energy electron irradiation. The field-induced piezoelectric response in the relaxor P(VDF-TrFE) copolymer was obtained by measuring the electrostrain induced by a small ac voltage superimposed on a dc bias voltage. It was shown that the effective piezoelectric coefficient in the relaxor ferroelectric copolymer, which was enhanced as compared to the unirradiated ferroelectric copolymer, could be tuned by dc biasing field. The effective piezoelectric coefficient d₃₁ initially increased with the dc biasing field and reached a value of 84 pm V⁻¹ at 30 MV m⁻¹; afterwards, d₃₁ began to decrease with the biasing field and had a value of 52 pm V⁻¹ at 50 MV m⁻¹. The mechanism for the tunable field-induced piezoelectricity is discussed based on the high-field electromechanical coupling behaviour in the relaxor ferroelectric polymer.

The study of the substitution of Ba for Ca in the orbital-charge ordered manganites Nd_1−xCa_xMnO₃, with , for low Ba levels (smaller than 6%) shows that, similarly to the Pr_1−xCa_xMnO₃ system (Zhu et al 2004 J. Phys.: Condens. Matter16 2861), a ferromagnetic ground state is obtained. The evolution of the ferromagnetic fraction in zero field is quite similar: it goes through a maximum at x = 0.43, which, for 4% Ba, is like for Pr a full ferromagnet at 2.5 K, with a ferromagnetic fraction of 90%. The higher Ba level necessary to reach the FM state in Nd phases compared to Pr is explained in terms of the antagonist effects of charge-orbital stability and local counter-distortion due to Ba. The different behaviour in a magnetic field, and especially the much larger magnetizations reached by Nd phases with respect to the Pr ones, , is explained in terms of Nd–Mn magnetic interactions, which destabilize the CE-type AFM structure.

Molecular dynamics (MD) simulations of the pressure-induced phase transition of potassium chloride (KCl) were performed with 2744 and 9000 atoms to study crystalline states, the process and the effects of defects under high hydrostatic pressure. In this study, we adopted the Born–Mayer–Huggins-type potential function to describe the interatomic interaction. The potential parameters used in this study were empirically optimized on the basis of Hugoniot equation of state data. The simulation results for perfect crystals (1372 K⁺ and 1372 Cl⁻) showed that the B1–B2 phase transition occurred with large hysteresis, and the thermodynamic transition pressure was calculated to be 3.5 GPa. The simulation results indicated that the phase transition proceeded through displacements of atomic lines parallel to the axis direction of the B1-type structure, and that these lines corresponded to the atomic lines parallel to the axis direction of the B2-type structure after the phase transition. In the case of the larger cells containing 9000 atoms with weak or strong van der Waals interactions, some clusters or dislocations, respectively, were generated in the resultant B2 phase. As regards dislocations, the phase transitions started around dislocations and the phase transition pressure decreased.

New neutron diffraction and ac magnetic susceptibility data concerning phase transitions and magnetic structures of PrCu₂Ge₂ and NdFe₂Ge₂ are presented here. In the case of PrCu₂Ge₂ at low temperatures collinear antiferromagnetic ordering of the AFI type is observed. In the temperature range between T_t = 5.5 K and T_N = 15.0 K the collinear and a sine wave modulated structure coexist. The latter phase is a long period modulation along the a-axis described by the propagation vector k = (0.0336(5),0,0). In the case of NdFe₂Ge₂ the situation is similar: a collinear AFII-type low temperature phase (k = (0,0,1/2)) changes into a non-collinear one (k = (0,0,0.46(1))) at about 12 K. In both compounds in all phases magnetic moments localized on rare earth ions are parallel to the c-axes.

Crystal structures and their stabilities for molybdenum under increasing hydrostatic pressures are investigated by first-principles calculations of the Gibbs free energy. Three structures are considered: body-centred cubic (bcc, the ground state at zero pressure), hexagonal close-packed (hcp) and face-centred cubic (fcc). For each structure and each pressure (up to 8 Mbar) the equilibrium states are found from minima of the Gibbs free energy at zero temperature. The stability is tested by calculating the elastic constants and checking whether they satisfy the appropriate stability conditions. The bcc structure is confirmed to be stable at zero pressure and at 6 Mbar. At and above 6.2 Mbar the ground-state structure changes to hcp, which is found to be stable at 7 Mbar. At 7.7 Mbar another transition occurs, and the ground-state structure changes from hcp to fcc. The fcc structure, which is unstable at zero pressure, becomes metastable over the range from 3 to 7.7 Mbar and becomes the ground state at higher pressures (at least up to 8 Mbar). Direct confirmation of these calculated transition pressures with experiment is not now possible, as the maximum static pressure currently reached experimentally is 5.6 Mbar, where Mo is found to be still in the bcc phase.