Table of contents

In this letter, it is pointed out that an ideal universal equation of state (EOS) of solids should have four merits. The EOS corresponding to the generalized Lennard-Jones (GLJ) potential is derived. It is pointed out that the GLJ EOS is not volume analytic, as the exponents contained in the potential function take arbitrary values. On making the exponents satisfy a relationship, the GLJ EOS becomes volume analytic with two parameters (mGLJ EOS), and has all four merits. By applying six EOSs in investigating 50 materials, it is shown that the mGLJ EOS gives the best results.

We study tagged particle diffusion at large packing fractions, for a model of particles interacting with a generalized Lennard-Jones 2n–n potential, with large n. The resulting short range potential mimics interactions in colloidal systems. In agreement with previous calculations for short range potentials, we observe a diffusivity maximum as a function of temperature at constant density. By studying the temperature dependence of the configurational entropy—which we evaluate with two different methods—we show that a configurational entropy maximum is observed at a temperature close to that of the diffusivity maximum. Our findings suggest a relation between the dynamics and number of distinct states for short range potentials.

CePt₃Si is the first heavy fermion superconductor without a centre of symmetry. Below an antiferromagnetic transition at 2.25 K superconductivity occurs at T_C = 0.73 K. The non-centrosymmetric structure is expected to have significant impact on the nature of the superconducting order parameter. This may be a reason for the unusual shape of the superconducting transition observed in the specific heat. We present specific heat data which clearly show the existence of a double anomaly in the superconducting state, signalling two consecutive phase transitions, as in the uranium based heavy fermion superconductors U_1−xTh_xBe₁₃ and UPt₃.

The spectral representation is an efficient tool to explore electrical properties of material mixtures. It separates the contributions of geometrical topology and intrinsic properties of the constituents in the system. The aim of this letter is to derive an expression for the spectral density representation, which favours dielectric relaxation phenomenon. This unfamiliar form is distinct in that the existing dielectric relaxation models and data analysis tools can be employed for extracting the spectral density function of a given system.

The real-space renormalization group approach is applied to study critical temperatures of a system consisting of interacting spin chains of spin S = 1, with an inner antiferromagnetic exchange, which form a honeycomb crystal lattice. Using the anisotropic Heisenberg model we calculate the critical temperature as a function of the anisotropic parameter and the ratio of the interchain and intrachain interactions. A comparison our results with those obtained from RSRG calculations for the same model of spin-1/2 on a square lattice is given.

Disordered two-dimensional photonic crystals with a complete photonic band-gap have been investigated. Transmission and reflection spectra have been modelled for both ballistic and scattered light. The density of states and electromagnetic field profiles of disorder-induced localized states have also been calculated, for various levels of disorder. It is found that there is a threshold-like behaviour in the amount of disorder. Below the threshold, it is seen that there is a vanishing probability of disorder-induced localized states being introduced into the centre of the photonic band-gap, but that edge-states narrow the band-gap. Above the threshold, there is a non-zero probability of disorder-induced localized states throughout the photonic band-gap, and the modification of the transmission and reflection spectra due to disorder rapidly increases with increasing disorder.

The optical properties of a dielectric waveguide coated on one side with a periodic monolayer of metallic nanospheres are studied by means of transmission and density-of-states calculations using the on-shell layer-multiple-scattering method. In particular, the strong coupling mechanism between the waveguide and collective particle–plasmon modes is analysed and its influence on the optical response of the system is elucidated.

The effects of non-locality on the focusing properties of a thin metal slab, with the real part of the permittivity equal to −1, are evaluated. Non-local effects are introduced by employing the wavenumber-dependent plasmon energy of the hydrodynamic model. Non-locality affects the dispersion of the surface plasmons, the transmissivity of the slab, and hence also its imaging ability. The results of numerical calculations for silver slabs are presented.

The present paper concerns x-ray absorption spectroscopy experiments at the As K-edge on InAs up to 80 GPa. The aim of this study is to obtain quantitative information on the local environment of As in the post-NaCl phases of InAs. These measurements confirm the absence of the CsCl phase, and the analysis of the fine structure of the x-ray absorption allows us to characterize the local structure of the high pressure phases of InAs from their onset of appearance. While the Cmcm phase is locally very similar to the NaCl structure, having a first coordination shell of six indium atoms, above 31 GPa an increasing number of As atoms are detected within a distance of 3.16 Å.

The structure refinement from x-ray diffraction data on PbTiO₃ single crystals revealed that the parameters of the mean thermal displacements for Pb ions in the high temperature paraelectric phase (SG ) showed large values. Therefore models with displacements of Pb ions in several relevant directions were tested. It is very likely that in the cubic phase there is disorder for the Pb occupancy on three equivalent positions off the position (0, 0, 0).

In the Na_xCoO₂ chemical system, a two-phase region is found at ambient temperature near x = 0.75. The manner in which that two-phase region is formed on cooling from a uniform single phase at temperatures above 340 K is reported here. The driving force that induces the phase separation appears to be a difference in Na composition of the two phases, which, though small, is accompanied by substantial differences in the unit cell parameters. Hysteretic behaviour in the transition is observed. One of the phases exhibits negative thermal expansion in the narrow temperature region where the phases are in the process of separating.

Molecular dynamics simulations have been performed on tantalum clusters using the embedded-atom-method potential. Melting simulations show that β-Ta clusters have a lower melting temperature than the same size clusters of α-Ta (bcc structure). Pure β-Ta clusters are quite stable and do not transform to the α-Ta on melting. Simulations on Ta clusters with mixed α- and β-phases reveal that inclusion of a bcc-Ta cluster within a β-Ta cluster induces the β-to-α-phase transformation at a temperature far below the melting point of a pure β-Ta cluster, depending on the cluster size and α-to-β-atom ratio. The results suggest that the observed phase transformation of β-Ta thin films is due to the presence of α-phase inclusions in the β-Ta film grains.

We present calculations of the high pressure crystal structures in selenium, including rational approximants to the recently reported incommensurate phases. We show how the incommensurate phases can be intuitively explained in terms of imaginary phonon frequencies arising from Kohn anomalies in the putative undistorted phase. We also find inconsistencies between the calculated and experimental Se II phase—the calculations show it to be a metastable metal while the experiment finds a stable semiconductor. We propose that the experimentally reported structure is probably in error.

An optical investigation of the properties of europium (II) ions introduced in Ba₂Mg₃F₁₀ single crystals is presented. The spectra and time-dependence of both the inter-configurational (f–d) and the intra-configurational (f–f) transitions are described. The emission spectrum consists of two emitting Eu²⁺ centres, each showing one broad f–d band and one ⁶P_7/2 quartet. The thermal equilibrium between the f–f and f–d emitting states is investigated and found to take place on a microsecond timescale for one of the two Eu²⁺ centres. The spectroscopic results combined with numerical modelling of the influence of the host crystal on the ⁶P_7/2 energy scheme allow the assignment of each f–f and f–d emission to its corresponding Eu²⁺ centre.

Brillouin spectra of stishovite were measured up to the amorphization temperature ( K) using large (1 mm) defect-free single crystals. At room temperature the sound velocities extracted from the BLS yield all six elastic constants. The derived bulk modulus and the polycrystalline shear modulus of stishovite at ambient conditions are B = 316 GPa, G = 222 GPa. Between room temperature and 800 K only a small decrease of the elastic constants was observed. This is an indication that the amorphization process is not due to a soft, long-wavelength, acoustic phonon. Room-temperature Raman spectra from samples heated to close to the amorphization temperature show that amorphization occurs more rapidly on the (110) than on the (001) crystallographic face. These results, together with those of x-ray measurements from the literature, allow us to conclude that amorphization in stishovite is most likely a surface phenomenon initiated by large oscillations of the Si atoms within the crystal faces whose normals are perpendicular to the c-axis.

Novel representatives REPt₃Si (RE =  La, Pr, Nd, Sm and Gd) of the CePt₃B type have been synthesized and characterized by means of Rietveld x-ray powder diffraction. Measurements of the magnetic susceptibility, isothermal magnetization, temperature dependent specific heat as well as temperature and field dependent resistivity were employed to derive basic information on the low temperature behaviour. Long range antiferromagnetic order from 2.2 K (CePt₃Si) to 15.1 K (GdPt₃Si) was observed. PrPt₃Si, however, is non-magnetic, at least down to 400 mK, as a consequence of crystalline electric field splitting of the non-Kramers ion Pr³⁺ in tetragonal symmetry. Whilst the ordering temperature of SmPt₃Si appears to be almost unaffected in external fields up to 12 T, magnetic order in GdPt₃Si, although twice as high as for the Sm homologue, is easily suppressed by external magnetic fields due to the absence of anisotropy.

We study the energetics of the self-trapped magnetic polaron (electron plus the distorted local magnetization cloud) in electron-doped manganites, e.g., Ca_1−xLa_xMnO₃, with small x and at zero temperature. A single electron moving in a cubic lattice of antiferromagnetic t_2g core spins, as appropriate for the manganites, is examined, taking into account the effects of the nearest- and the next-nearest-neighbour hoppings and the Anderson–Hasegawa double-exchange, as well as the Jahn–Teller interaction. We compute the ground-state energy and the wavefunction of the system using a set of self-consistent equations. While we show that the next-nearest-neighbour hopping significantly reduces the binding energy of the magnetic polaron, this reduction is not enough to destabilize the self-trapped state. The ground state of the polaron is found to be a seven-site ferromagnetic region, comprising the central spin and the six nearest neighbours, with a net magnetic moment of approximately 7 μ_B, in qualitative agreement with the interpretation of Neumeier and Cohn of their experimental magnetization data (Neumeier and Cohn 2000 Phys. Rev. B 61 14319). We argue that the polaron should exhibit an activated hopping as seen in the experiments, and estimate an activation energy of about 40 meV.

We consider tunnelling between a metal partially gapped by charge density waves (CDWM) and an ordinary metal (M) or a ferromagnet (FM) separated by an insulator (I) in an external magnetic field H. Zeeman paramagnetic splitting is assumed to dominate in the CDWM over orbital magnetic effects. The quasiparticle tunnel current J and relevant differential conductance G are calculated as functions of the bias voltage V. The peaks of G(V), originating from the electron density of states singularities near the charge density wave gap edges, were shown to be split in the magnetic field, each peak having a predominant spin polarization. This effect is analogous to the H-induced splitting of G(V) peaks obtained by Tedrow and Meservey for junctions between normal metals and superconductors (S). Thus, it is possible to electrically measure the polarization of current carriers in such a set-up, although the behaviours of G(V) in the two cases are substantially different. The use of M–I–CDWM junctions instead of M–I–S ones has certain advantages. The absence of the Meissner effect, which weakens the constraints upon the junction geometry and electrode materials, comprises the main benefit. The other advantage is the larger energy range of the charge density wave gaps in comparison to that for superconductors' gaps, so that larger Hs may be applied.

We have investigated the thermoelectric power (TEP) and the electrical conductance of single-walled carbon nanotube (SWNT) strands containing large numbers of aligned crystalline ropes. A linear temperature dependence of TEP and a power-law behaviour of conductance were observed at low temperatures. The TEP increases with temperature at a very large slope of below  K, peaks around 100–150 K depending on the treatment of high temperature annealing, then decreases with the further increase of temperature. The large slope at low temperatures is interpreted as a property of the Luttinger liquid in the presence of defects.

Nitrogen doped n-type amorphous carbon (n-C:N) films are deposited by a pulsed laser deposition technique at room temperature using a camphoric carbon target with different nitrogen partial pressures (NP) in the range from 0.1 to 800 mTorr. The room temperature conductivity (σ_RT) is found to increase with N incorporation perhaps due to increasing density of trihedral (sp²) hybridization of electronic states, for NP increasing from 1 to 30 mTorr, as shown by x-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR), and optical data relevant to band tails. Study of the activation energy reveals that the Fermi level of the n-C:N film moves from the valence band to near the conduction band edge through the mid-gap. The current–voltage photovoltaic characteristics of n-C:N/p-Si cells under 1 sun air-mass 1.5 (AM 1.5) illumination conditions (100 mW cm⁻², 25 °C) are improved up to 30 mTorr and deteriorate subsequently. The maximum open circuit voltage (V_oc) and short circuit current density (J_sc) for the cells are observed to be approximately 292 mV and 9.02 mA cm⁻², respectively. The highest energy conversion efficiency (η) and fill factor (FF) were found to be approximately 1.47% and 56%, respectively.

Conductivity and magnetotransport studies have been carried out in polyaniline (PANI) doped with 2-acryloamido-2-methyl-1-propane sulfonic acid (AMPSA), camphor sulfonic acid (CSA) and dodecylbenzene sulfonic acid (DBSA). Both PANI-AMPSA and PANI-CSA show a metallic positive temperature coefficient of resistivity above 150 K, and the data down to 1.6 K show that the systems are near the critical regime of metal–insulator (M–I) transition, whereas PANI-DBSA shows hopping transport. The magnetoconductance (MC) in PANI-AMPSA is positive (negative) in transverse (longitudinal) directions. The anisotropic MC in PANI-AMPSA is due to the interplay of contributions from weak localizations, electron–electron interactions and hopping transport. This suggests the presence of local molecular-scale quasi-two dimensional (q-2D) ordered regions in PANI-AMPSA, which are absent in the other two systems. The MC is weakly (strongly) negative in PANI-CSA (PANI-DBSA). The data analysis yields consistent values for various length scales (e.g., inelastic scattering length, localization length, etc) according to this model. These results indicate that the surfactant counterion plays a major role in local molecular-scale ordering and charge transport properties in doped PANI.

Dc and ac magnetic measurements have been performed on single crystals of HfFe₆Ge₆ type ErMn₆Sn_6−xGa_x compounds prepared by the flux method. At low temperature all the compounds display easy-axis magnetization and undergo a spin reorientation transition to easy-plane magnetization below room temperature. The complete ferrimagnetic order (μ_sat = 4 μ_B fu⁻¹) is reached at 10 K only in the easy direction. From low field magnetization measurements at 10 K, a first order magnetic process (FOMP) has been detected for all the compounds and ascribed to the competition between the higher order magnetocrystalline terms. An additional transition detected from the imaginary part of the ac susceptibility for the x = 0.85 and 1.20 compositions was explained in terms of displacement of Bloch walls. The anisotropy constants, evaluated, following the Sucksmith and Thompson method, for some compositions at room temperature, decrease at increasing Ga content.

The effect of isotope substitution on the electrical resistance, magnetic susceptibility and neutron diffraction patterns of nearly half-doped R_1−xSr_xMnO₃ manganites was studied for ceramic samples with R = Sm (x = 0.425–0.525), Nd/Tb (x = 0.45) and Nd/Eu (x = 0.45). In Nd/Tb and Nd/Eu manganites, the composition was chosen to yield the same mean ionic radius as in the Sm compound, but the samples differed in the cation disorder parameter σ. The oxygen isotope substitution leads to fundamental changes in the phase diagram of the manganites studied, favouring enhanced inhomogeneity, stabilizing the insulating antiferromagnetic (AFM) state and even causing a metal–insulator transition (MIT). The MIT induced by the oxygen isotope substitution was found here for the first time in manganites with A-type AFM; earlier this unusual phenomenon was observed only for compounds with the CE-type AFM structure. This suggests that such a MIT is closely related to the ferro–AFM crossover independently on the specific nature of the AFM phase and related orbital order.

The temperature dependences of the dielectric constants of the ferroelectric semiconductors TlInS₂ and TlGaSe₂ have been studied following their annealing within the incommensurate phase. Unusual memory effects accompanied by both a remarkable inflection of the temperature dependence curves in the incommensurate phase and various shifts of the incommensurate (T_i) and commensurate (T_c) phase transition temperatures have been revealed in both crystals. The observed effects are explained on the basis of a defect density wave model taking into account the interaction of modulation waves with charge carriers localized at impurity states. The thermally activated population of these states during the heating or cooling processes is responsible for the changes of the phase transition temperatures.

160 keV Ar⁺ ions were homogeneously implanted in AlGaN at room temperature for device isolation purposes. Resonance Raman spectroscopy and DC electrical measurements were used to monitor the structural and electrical changes of the non-annealed and annealed implanted AlGaN samples with a dose ranging from 3.4 × 10¹² to 1 × 10¹⁶ ions cm⁻². The annealing was carried out at 900 °C for 40 s, these conditions being necessary to achieve good Ohmic contacts. On increasing the implantation dose from 3.4 × 10¹² to 3.4 × 10¹⁴ ions cm⁻², an increase in the electrical isolation and a decrease in the photoluminescence (PL) were observed. For the highest dose, the implanted layer becomes conductive, probably due to a hopping mechanism. After annealing, the implanted samples become conductive and the PL reappears or increases as compared to the non-annealed samples using the same implantation doses. Then, it is possible to obtain good device electrical isolation by implanting ions with a 3.4 × 10¹⁴ cm⁻² dose subsequently to the annealing process necessary to achieve Ohmic contacts.

When computing the Peierls stress of screw dislocations in tantalum by means of classical molecular dynamics (CMD), we have observed that a suitable potential should be able to reproduce the complicated angular dependence of forces arising in the core structure of dislocations. This prevents the use of simple embedded atom potentials, but we notice that more generally, effective interatomic potentials tend to overestimate the Peierls stress. Since the Peierls–Nabarro model of dislocation requires an accurate description of the generalized stacking fault (GSF) energy surface, we have decided to use the modified embedded atom method (MEAM) potential, which explicitly incorporates angular terms, whose coefficients are constrained to reproduce ab initio GSF data.

Calculations with our new tantalum empirical model show the importance of hard core screw dislocations and underline the problem of kinetics in Peierls stress calculations.

The dynamics of magnetic relaxation in a system of isolated ferrimagnetic nanoparticles depends on the ratio between the magnetic relaxation time (τ) and the measurement time (t_m), which is usually considered to be equal to the period (T_H) of the external alternating magnetic field (t_m = T_H). When t_m approaches τ (τ<t_m), the magnetic moments cannot relax completely, thus leading to a deviation from the superparamagnetic behaviour (SPM), and a magnetic remanence of the system when the deviation is large. An external magnetic field (H) can significantly change the dynamics of the relaxation, especially when its amplitude (H_m) is high. This paper shows that there is a limit field (threshold field (H_p)) that depends on the anisotropy field of the nanoparticle, its magnetic volume and on the temperature; beyond this field, the magnetic moments cannot pass the potential barrier and they remain blocked. It will be shown that under these conditions the measurement time can no longer be considered to be t_m = T_H, but is a measurement time t_mH< T_H that in addition to T_H will also depend on H_p and H_m. When the amplitude of the alternating magnetic field is lower than the value of the threshold field (H_m< H_p), the measurement time is reduced to the period of the magnetic field. The theory proposed for a system of aligned nanoparticles has been verified experimentally in the case of a ferrofluid-type system. The result obtained brings in important corrections for determining the magnetic volume of the nanoparticles or the magnetic anisotropy constant if the condition t_m = t_mH< T_H is used when H_m is high (H_m> H_p), instead of t_m = T_H.

Electronic structures of double perovskites Sr₂(Mn_1−xFe_x)MoO₆ (x = 0.0, 0.5, 1.0) have been studied by using the LSDA +U method, where the on-site Coulomb interaction U of localized d electrons has been considered in the local-spin density approximation. The band structure of Sr₂MnMoO₆ with antiferromagnetic structure is found to be semiconducting with a gap of 0.5 eV, which is in good agreement with recent optical absorption results. The optical absorption corresponding to this optical threshold is found to be a non-vertical transition, which can occur only if a non-zero momentum such as from phonons participates in the transition. With the increase of x, the band becomes half-metallic and ferrimagnetic. These semiconductor–metal and magnetic structure transitions can be rationalized by different characteristics of charge transfers between Fe–Mo and Mn–Mo via intermediate O 2p orbitals. The down-spin Mo t_2g states cross the Fermi energy (E_F) in the presence of Fe ions. With the increase of x, more states cross over E_F, which explains the high conductivity in the compound with large x observed in experiment.

A concise and general formula is introduced to obtain ab initio pair potentials between atoms across a metal–ceramic interface by inversion of the adhesive energies of the interface. Derivation of interfacial potentials Φ_Ag−Mg and Φ_Ag−O from ab initio adhesive energies is performed by applying the formula to the Ag/MgO(001) interface. Transferability of these potentials at Ag/MgO(100), Ag/MgO(110) and Ag/MgO(111) interfaces is discussed.

The line width of 5.5 μeV fitted to the data in figure 2 panel l is not the FWHM as shown but the HWHM. As a consequence the resulting FWHM line width of 11 μeV does not change under shear for the direction of the flow but the diffusivity is slowed down in the direction of the shear gradient.