Table of contents

We propose a mechanism for the observed stacking charge order in the quarter-filled ladder compound NaV₂O₅. Via a standard mapping of the charge degrees of freedom onto Ising spins we explain the stacking order as a result of competition between couplings of the nearest and next-nearest planes with the four-fold degenerate super-antiferroelectric in-plane order.

In the present work we report x-ray powder diffraction measurements of Sr₂CaWO₆ and Sr₂MgWO₆, at different temperatures. The crystal structures at room temperature of both compounds are determined; and results showing the existence of phase transitions in them are presented. For the structure at room temperature of the Ca containing compound, the space group P 2₁/n is obtained, which is different from that previously suggested. The evolution with temperature of the structure of this compound shows the presence of two phase transitions, a discontinuous one at 1130 K and a continuous one at 1250 K, with the following phase transition sequence: . The crystal structure of the Mg containing compound at room temperature has the I4/m space group. The temperature evolution of the structure has revealed a continuous phase transition at 570 K, changing the symmetry from tetragonal (I4/m) to cubic ().

Random dense packing of binary hard discs generated under the infinitesimal gravity protocol is investigated by a Monte Carlo simulation in the entire region of the size ratio and the concentration. When the size difference of the two species is small, the packing fraction of disc assembly becomes lower than that of the monodisperse system. Defining the adjacent neighbours of a disc through the radical tessellation, we show that the packing fraction can be related to the distance of adjacent discs and that the suppressed packing fraction is caused by an increase of the adjacent neighbours which are not seen in the monodisperse system.

In this study, we investigate the adhesion of diamond coatings deposited on 0.8 µm WC–10% Co substrates using chemical vapour deposition (CVD). Polycrystalline diamond films were deposited using (i) constant methane (CH₄) flow, at 3 and 4.5 sccm, and (ii) modulated CH₄ flow at 4.5 and 3 sccm for 8 and 10 min, respectively. Constant CH₄ flow into the vacuum chamber during diamond CVD is the conventional approach to deposit diamond onto a range of substrate materials. The timed CH₄ modulations are an integral part of our recently proposed process called time-modulated CVD (TMCVD). The coating adhesion was characterized using indentation tests employing a Brinell indenter. In this study, we employ three indentation loads: 294, 490 and 612.5 N. In addition, micro-Raman spectroscopy was used to (i) characterize the deposited films for diamond-carbon phase purity and (ii) determine the biaxial stresses in the coating samples. In this paper, we correlate the adhesion strength of diamond films to biaxial stresses.

No lateral cracking occurred in diamond films deposited by the TMCVD process after a 294 N load indentation. This result coupled with the complementary stress calculations indicates a higher film–substrate adhesion in time-modulated films. Our findings suggest that at higher indentation loads the response from and the behaviour of the substrate need to be considered in determining the coating adhesion strength. The TMCVD process promotes secondary nano-sized diamond crystallites during the higher timed methane modulations. It is expected that the mechanical interlock at the film/substrate interface is higher for film coatings deposited using TMCVD, thus resulting in the improved adhesion of the time-modulated coating on WC–Co.

We present calculations of a set of UTX compounds (where T is a transition metal and X is a p element) in the framework of density functional theory applying the LSDA +U exchange–correlation energy functional. As the parameters U and J for these compounds are not known, we varied them in effort to reproduce experimental uranium magnetic moments. Using the obtained electronic structures we discuss the magnetism and the effects of hybridization. Our results are in improved agreement with experimental findings.

The low-temperature magnetic properties of the Ce atoms in the intermetallic compounds CeMn₂Ge₂ and CeMn₂Si₂ were studied. Previous neutron scattering measurements did not detect an ordered moment at Ce atoms in either compound despite the fact that they are surrounded by the Mn moments ordered ferromagnetically in the CeMn₂Ge₂ and antiferromagnetically in the CeMn₂Si₂. Contrasting with this result, a recent measurement performed with the time differential perturbed angular correlation (TDPAC) technique showed the presence of a pronounced magnetic hyperfine field (MHF) at Ce sites in the CeMn₂Ge₂ compound and no MHF in CeMn₂Si₂. The absence of the Ce magnetic moment and MHF in the silicide can be understood in terms of too weak a Ce–Ce magnetic interaction while in the germanide the TDPAC result suggests that some magnetic ordering of Ce atoms may occur. Aiming to understand the effects which result in the quenching of the Ce 4f moment in both cases, we performed first-principles band-structure calculations for both systems, using the full potential linear augmented plane wave method. It is shown that the magnetism of the Ce sublattice has fundamentally different nature in CeMn₂Si₂ and CeMn₂Ge₂. While the Ce atoms are intrinsically non-magnetic in the silicide, having a zero magnetic moment with both spin and orbital contributions identically zero, they display magnetic properties in the CeMn₂Ge₂ since their very small total moment is composed of finite spin and orbital components which almost cancel each other accidentally.

Layered systems show anisotropic transport properties. The interlayer conductivity shows a general temperature dependence for a wide class of materials. This can be understood if conduction occurs in two different channels activated at different temperatures. We show that the characteristic temperature dependence can be explained using a polaron model for the transport. The results show an intuitive interpretation in terms of coherent and incoherent quasi-particles within the layers. Further, we extract results for the magnetoresistance, thermopower, spectral function and optical conductivity for the model and discuss application to experiments.

In order to explore the effect of the geometric phase on the spin-polarized electron tunnelling in a ferromagnet/insulator/ferromagnet (FM/I/FM) junction, in this paper, we apply a voltage drop in the insulating layer and allow it to vary adiabatically with time t. Then the wavefunction will acquire a geometric phase which will give rise to an observable effect on the physical quantities of interest. The numerical results indicate that the geometric phase certainly has an influence on the differential conductance and the tunnelling magnetoresistance (TMR) in the spin-polarized tunnelling. We also show results for the conductance and TMR obtained by changing the orientation angle and the magnitude of the molecular field in the ferromagnets. An experimental profile for observing the effect of the geometric phase on the spin-polarized electron transport in a FM/I/FM tunnel junction is suggested.

The isothermal magnetization hysteresis loops were measured for Tl₂Ba₂CuO₆ (Tl-2201) single crystals by using a SQUID magnetometer and a micro Hall sensor. In the temperature window from 15 to 30 K, the Hall sensor measurements for M(B) showed a second magnetization peak (SMP) at a peak field, B_sp, and an onset field, B_on. In this temperature region, the second peak appeared as a shoulder in the M(H) plots when measured by using SQUID. At a temperature of 20 K, time relaxation measurements for the time interval of 1–10⁴ s were carried out at different fields by using a Hall sensor. From these relaxation data, for both flux exit and entry, the activation barrier, U₀, and the creep exponent, μ, were separately calculated as a function of local field, B_z, by using the weak collective pinning theory. The variation of μ, and U₀ as a function B indicates that below the onset of the second peak field, B_on, the creep mechanism is an elastic process, but above it, a gradual transition to plastic creep takes place. At higher fields, μ and U₀ reduce sharply. This has been interpreted as a smooth transition to a 2D collective pinning state. These results are compared with that obtained in a double layer Tl compound, Tl₂Ba₂CaCu₂O₈ (Tl-2212),as well as other high-T_c materials.

The energy spectrum of cake shape normal–superconducting systems is calculated by solving the Bogoliubov–de Gennes equation. We take into account the mismatch in the effective masses and Fermi energies of the normal and superconducting regions as well as the potential barrier at the interface. In the case of a perfect interface and without mismatch, the energy levels are treated by semi-classics. Analytical expressions for the density of states and its integral, the step function, are derived and compared with that obtained from exact numerics. We find a very good agreement between the two calculations. It is shown that the spectrum possesses an energy gap and the density of states is singular at the edge of the gap. The effect of the mismatch and the potential barrier on the gap is also investigated.

Starting from a microscopic model of coupled frustrated spin-1/2 chains, we study the effect of doping with static non-magnetic impurities. The effective interaction between the moments induced by the dopants is analysed. This interaction is non-frustrated and extended in space. The picture is applied to the antiferromagnetic long-range ordering observed in spin–Peierls compounds such as CuGeO₃ doped with non-magnetic impurities. The effective diluted long-range Heisenberg spin- model is studied using the stochastic series expansion (SSE) quantum Monte Carlo algorithm. Simulations at extremely low temperature on square lattices with up to 96 × 96 sites are carried out to investigate the AF ordering down to impurity concentrations x as low as .

The crystal and magnetic structure of manganite La_0.7Sr_0.3MnO₃ has been studied in the pressure range 0–7.5 GPa and the temperature range 4–300 K. The ferromagnetic state of La_0.7Sr_0.3MnO₃ remains stable in the whole studied pressure range. The Curie temperature increases as d T_C/d P = 4.3 K GPa⁻¹. Unlike the manganites with orthorhombic crystal structure, the pressure-induced increase of T_C in La_0.7Sr_0.3MnO₃ having the rhombohedral crystal structure of symmetry may be explained by modification of structural parameters only. The difference between the properties of manganites with rhombohedral and orthorhombic structures under high pressure is discussed in terms of the symmetry of MnO₆ octahedra.

A time-domain THz transmission experiment was performed on a SrBi₂Ta₂O₉ film deposited on a sapphire substrate. Temperatures between 300 and 923 K were investigated and complex permittivity spectra of the film were determined. The lowest-frequency optical phonon near 28 cm⁻¹ reveals a slow monotonic decrease in frequency on heating with no significant anomaly near the phase transitions. We show that the dielectric anomaly near the ferroelectric phase transition can be explained by the slowing down of a relaxational mode, observed in the THz spectra. A second-harmonic generation signal observed for a single crystal confirms the loss of a centre of symmetry in the ferroelectric phase and the presence of polar clusters in the intermediate ferroelastic phase.

The composition and orientation dependence of the room temperature piezoelectric constant d₃₃ and the dielectric constant tunability was investigated in poled single crystals of (1− x)Pb(Mg_1/3Nb_2/3)O₃–xPbTiO₃ (PMN–PT) with x varying from 0.24 to 0.38. The temperature, frequency, orientation and composition dependence of the dielectric constant in the poled crystals shows that domain configurations and phase transitions in the poled crystals strongly depend on both composition and orientation. Poled polydomain crystals near the morphotropic phase boundary, especially for -poled 0.70PMN–0.30PT crystals with a critical composition for the relaxor state, possess a large piezoelectric constant d₃₃ = 2210 pC N⁻¹ and a high dielectric constant tunability of 40.6% (at a dc bias of 1 kV mm⁻¹). The large tunability of ε_r in PMN–PT crystals is expected to be the most promising candidate for potential applications in continuously adjustable capacitors and dielectric amplifiers.

We have performed calculations of the size effect in the temperature dependence of the BaTiO₃ nanograin ceramic specific heat and dielectric permittivity. We took into account the distribution of grain sizes that exists in any real nanomaterial. This distribution led to a distribution of the temperatures of the size driven transition from the ferroelectric to the paraelectric phase because of the relation between the temperature and the size. We calculated the transition temperature distribution function on the basis of the size distribution function. This function allows us to calculate the temperature dependence of any physical quantity for a nanomaterial. As examples, we calculated the specific heat and dielectric permittivity for nanograin ferroelectric ceramics. The results demonstrate the strong influence of the size distribution on the observed properties and especially on the values of the critical size and temperature extracted from experiment. We carried out a comparison of the theory with the measured specific heat and dielectric permittivity for BaTiO₃ nanomaterial. The theory developed described the experimental data fairly well. The possibility of extracting size distribution function parameters as well as real values of critical parameters from experimental data is discussed.

Recent experiments in type II superconductors on the dynamics of vortices driven by external currents have revealed strong off-equilibrium phenomena, such as 'memory' and history dependent effects in I–V characteristics, which set in beyond the regime of validity of current theories on vortex stationary elastic flow. Here we discuss these phenomena in the framework of a simple model of a Monte Carlo interacting lattice gas moving in a pinning background under an external drive. Via computer simulations we give a comprehensive picture of time dependent phenomena in transport properties, response functions and I–V characteristics of the model and outline its quantitative connections to experimental findings.

In a review of supercooled and glassy water Debenedetti (2003 J. Phys.: Condens. Matter15 R1669) proposes that the boiling line for supercooled water (a binodal) can meet a conjectured low temperature limit of stability for supercooled water (a spinodal) only at a critical point. This comment offers a counter-example to show that the meeting of a binodal and a spinodal need not be a critical point.

In a recent review on supercooled and glassy water (Debenedetti 2003 J. Phys.: Condens. Matter 15 R1669) I used a thermodynamic consistency argument to rule out the possibility, proposed originally by Speedy (1982 J. Phys. Chem. 86 982), that the spinodal curve for superheated liquid water can retrace to positive pressure. The argument is based on the impossibility that a thermodynamic state other than a critical point be a limit of stability (i.e., a point along a spinodal line) and simultaneously coexist with another phase. Speedy's comment offers a counter-example involving the freezing transition to argue that a thermodynamic state can in fact be a limit of stability (spinodal) and simultaneously coexist with a different phase (i.e., a non-critical point along the phase coexistence locus). Here I argue that instability to infinitesimal density fluctuations and coexistence with a different phase are mutually exclusive conditions.

The full text of this erratum is given in the PDF.