Elastic properties of mono- and polycrystalline ( and Lu) under pressure effect

The structural and elastic properties of perovskite-type RCRh₃, with R=Sc, Y, La and Lu, under pressure effects have been investigated using the pseudo-potential plane-wave method based on the density functional theory within the generalized gradient approximation. For monocrystalline RCRh₃, the optimized lattice constants, elastic constants and directional elastic wave velocities are calculated and analyzed in comparison with the available experimental and theoretical data. An increase in the lattice constant has been found with increasing atomic size of the R element and a corresponding decrease in the hardness. The anisotropic elastic constants and directional elastic wave velocities increase linearly with increasing pressure. A set of elastic parameters and related properties, namely bulk and shear moduli, Young’s modulus, Poisson’s ratio, Lamé’s coefficients, average sound velocity and Debye temperature are predicted in the framework of the Voigt–Reuss–Hill approximation for polycrystalline RCRh₃. We have found that the toughness of RCRh₃ compounds can be improved at high pressure.     

Lattice, magnetic and transport properties in antiperovskite compounds

The temperature-dependent magnetization, lattice, and transport properties of Mn₃Sn_1−xGe_xC (0≤x≤0.5) compounds are systematically investigated. The Mn–Mn atomic distance decreases as Ge content is increased, and the transition temperature from ferromagnetic (or ferrimagnetic) to paramagnetic state decreases too. Mn₃SnC has a large magnetovolume effect (MVE). However, Ge-doping in Mn₃SnC gradually reduces the MVE, till the MVE disappears. Whether there is an abnormal lattice change or not, there always exists an anomalous increase in resistivity near the magnetic phase transition point with decreasing temperature.     

First-principles study of pressure effects on and

We have performed first-principles calculations to investigate the pressure effects on CaFe₂As₂ and BaFe₂As₂. Our calculations show that in CaFe₂As₂, the orthorhombic structure transforms to a collapsed tetragonal structure at 0.4 GPa with a volume collapse of 9.5%, which is in agreement with experiments. Together with the structural phase transition, CaFe₂As₂ undergoes a magnetic transition from the stripe antiferromagnetic ordering to the nonmagnetic state. For BaFe₂As₂, we predict that the orthorhombic structure transforms to the tetragonal structure at 9.4 GPa. Unlike CaFe₂As₂, the magnetic moments of Fe ions in BaFe₂As₂ are not zero and the stripe antiferromagnetic ordering transforms to the checkerboard antiferromagnetic ordering together with the structural phase transition. The stability of the orthorhombic structure up to 9.4 GPa suggests that superconductivity and magnetism coexist in BaFe₂As₂.     

On the low-temperature properties of

Magnetic, electrical transport and heat capacity measurements have been performed on polycrystalline sample of the compound TmRu₂Si₂, which crystallizes with the tetragonal ThCr₂Si₂-type crystal structure. The results show paramagnetic behaviour down to 0.3 K, at variance with the literature data. On the basis of the collected data, possible origins of non-magnetic behaviour are discussed. Mixed valence of Tm ions or hybridisation between Tm 4f and Ru 4d states are the most probable reasons for paramagnetic behaviour of the TmRu₂Si₂.     

Magnetic transitions and magnetoresistance of -type () compounds

Magnetic transitions and magnetotransport properties of polycrystalline Tb_1−xGd_xMn₆Ge₆ (x=0.2–1.0) compounds have been investigated by magnetic property and resistivity measurements in an applied magnetic field up to 50 kOe. The cell parameter a,c and cell volume V of compounds (x=0.2–1.0) increase with an increasing Gd content. The compounds (x=0.2–1.0) show a rich variety of magnetic behavior, such as antiferromagnetic, ferrimagnetic and paramagnetic state with increasing temperature. Their Curie temperatures increase almost linearly with an increasing Gd content from 460 K for x=0.2 to 484 K for x=1.0. The compounds (x=0.2–1.0) display the field induced metamagnetic transitions, and the threshold fields first increase and then decrease with an increasing Gd content. The magnetoresistance curves of the Tb_0.4Gd_0.6Mn₆Ge₆ compounds in an applied magnetic field up to 50 kOe are presented and the magnetoresistance effects are related to the metamagnetic transitions.     

First-principles calculations on elasticity of under pressure

First-principles calculations of the crystal structure and the elastic properties of OsN₂ have been carried out with the plane-wave pseudopotential density functional theory method. The calculated values are in very good agreement with experimental data as well as with some of the existing model calculations. The dependence of the elastic constants c_ij, the aggregate elastic moduli (B,G,E), Poisson’s ratio, and the elastic anisotropy on pressure has been investigated. Moreover, the variation of the Debye temperature and the compressional and shear elastic wave velocities with pressure P up to 60 GPa at 0 K have been investigated for the first time.     

Study on the formation during the production of lead-free piezoceramics at the morphotropic phase boundary

Because of the environmental concerns, the manufacture of ceramics based on lead titanate zirconate [Pb(Zr_1−xTi_x)O₃−PZT] has been condemned because of the lead toxicity. In this context, the electromechanical properties of sodium, potassium and lithium niobate [(Na_0.5−x/2K_0.5−x/2Li_x)NbO₃−NKLN] at the morphotropic phase boundary granted these materials the position of most suitable candidate to replace PZT. However, the production of these ceramics is rather critical mainly because of a natural tendency of forming secondary phases. To help with the studies of the synthesis of this lead-free piezoceramic, this work presents an evaluation of the crystallization of the (Na_0.47K_0.47Li_0.06)NbO₃ phase by solid-state reactions. TG-DTA, XRD, dilatometric and ferroelectric hysteresis analyses indicated that a secondary phase (K₃Li₂Nb₅O₁₅) crystallizes at temperatures above 850 C and also during the sintering of the powders compacts at 1080 C. To prevent the formation of this phase, the addition of Na₂Nb₂O₆.nH₂O microfibers obtained through a microwave hydrothermal synthesis was performed in the sintering process. After to this addition, the suppression of the K₃Li₂Nb₅O₁₅ phase occurred and an increase of the NKLN electrical properties was then obtained.     

Tree-level contributions to the rare decays , , and in the Standard Model

The tree-level contributions to the rare decays , , and are analyzed and compared to those occurring in , , and . It is shown that these purely long-distance contributions, arising from the exchange of a charged lepton, can be significant in B⁺ decays for an intermediate τ, potentially blurring the distinction between the modes used to extract B⁺→τ⁺ν_τ and those used to probe the genuine short-distance and FCNC transitions. Numerically, the tree-level contributions are found to account for 98%, 12% and 14% of the total , , and rates, respectively.     

Measurement of at

We present a measurement of the cross section for Z boson production times the branching fraction to tau lepton pairs in collisions at . The measurement is performed in the channel in which one tau lepton decays into a muon and neutrinos, and the other tau lepton decays hadronically or into an electron and neutrinos. The data sample corresponds to an integrated luminosity of 1.0 fb⁻¹ collected with the DØ detector at the Fermilab Tevatron Collider. The sample contains 1511 candidate events with an estimated 20% background from jets or muons misidentified as tau leptons. We obtain , which is consistent with the standard model prediction.     

Effects of nanocracks on the magnetic and electrical properties of single crystals

An investigation of the physical properties of La_0.8Sr_0.2MnO₃ single crystals grown by the molten zone technique is realized close to the metal-to-insulator transition temperature (T_MI). In this paper, we review the effect of the structural defects through magnetotransport and local magnetic microstructures. From electron microscopy observations, some ‘nanocrack’ defects (i.e. defects at a nanometer scale) were found, essentially in the center part of the single crystals. At room temperature, magnetic force microscopy measurements have shown that the absence of defects allowed a magnetic ordering of the domains at the crystal edge, which is the best-crystallized region. In addition, the magnetization loops have permitted us to verify that the crystal was ferromagnetically weaker in the center. On analyzing the electrical resistivity data, we observed in the linear current regime a sensitive variation of the resistivity due to defects, by comparing the center and the edge of the material at T_MI. Additionally, at strong current, non-linearity phenomena have been supposed to be related to local heating. Finally, we discuss the structural disorder effect on the relaxation of the ferromagnetic domains.     

Structure and optical properties of multi-quantum wells grown on Si(111) substrates

A series of ZnO/Zn_1−xMg_xO multi-quantum wells (MQWs) were grown on Si(111) substrates by pulsed laser deposition (PLD), with different well widths and depths. Transmission electron microscopy (TEM) analysis revealed that all the samples exhibited a good periodic structure with clear interfaces. Moreover, temperature- dependent behavior of excitonic photoluminescence (PL) was investigated in order to investigate the mechanism of the carrier dynamics. In the temperature range studied, the luminescence was dominated by localized exciton (LE) emission, which was significantly blueshifted with respect to the ZnO single layer, due to the well-known quantum-size effect. The blueshift values increased with elevating Mg compositions in barrier layers or decreasing well thickness, being consistent with the calculated results. The relevant activation energies were deduced, varying in a range of 6.2 to 22 meV.     

Molecular dynamics study of structural changes in freezing clusters

Structural changes of Cu₅₁–Cu₅₆ clusters during freezing have been studied by employing molecular dynamics(MD) simulations. With the growth of the crystallization phase, local patterns in different regions determined by the atom density profiles present different characteristic for these studied clusters. While the atom movements in the even-number clusters result in the formation of initial two-shell, then three-shell, and finally four-shell structures, the atoms in the odd-number clusters, with one exception of the Cu₅₁ cluster, first move into the central region and the outermost parts, then follow the movement from the inner parts into the outer parts. The crystallization temperature of these clusters has an oscillatory increase behavior for odd- and even- numbered copper atoms with increasing cluster size.     

Search for the rare decays , , and

Rare decay modes , J/ψ→D⁻π⁺+c.c., and are searched for using events collected with the BESII detector at the BEPC. No signal above background is observed. We present upper limits on the branching fractions of , B(J/ψ→D⁻π⁺)<7.5×10⁻⁵, and at the 90% confidence level.     

Far-infrared dielectric functions and phonon spectra of alloys determined by spectroscopic ellipsometry

We have used spectroscopic ellipsometry to determine the complex dielectric function of a series of ternary Be_xZn_1−xTe thin films grown by molecular beam epitaxy. The II–VI semiconductor alloys were grown on InP substrates that had an InGaAs buffer layer. After the growth, X-ray diffraction experiments were performed in order to determine the alloy concentration. A standard inversion technique was used to obtain the dielectric functions from the measured ellipsometric spectra, obtained between 2000 nm (5000 cm⁻¹) and 40,000 nm (250 cm⁻¹). By modelling the dielectric function as a collection of oscillators, representing longitudinal and transverse optical phonons of the Be_xZn_1−xTe lattice, we were able to recover the phonon spectra for this alloy system. It is argued that the additional phonon modes that are obtained from ellipsometry are best understood from the recently-proposed percolation model.     

Strain characteristics and superelastic response of single crystals

The intermartensitic transformation, in a two-step complete thermoelastic martensitic transformation in Ni_53.2Mn_22.6Ga_24.2 single crystals, provides a much larger strain than that of the martensitic transformation. With a biasing magnetic field, the intermartensitic transformation strain is inhibited and the martensitic transformation strain is enhanced. Compressive stress–strain characteristics can be affected greatly by a static magnetic field. At low deformation temperature, the irreversible transformation strain induced by the stress becomes reversible, when a static magnetic field is applied. Further, the magnitude of the stress necessary for rearrangement of martensitic variants is dependent on the direction of the biasing magnetic field. Moreover, a well-defined character of the twin-boundary motion, similar to the soliton motion, has been observed upon loading or unloading.