Papers by Keyword: Disorder

Abstract: The polarizability theory of optical activity (OA) is used to investigate the correlation between structure peculiarities and optical properties of crystals of the langasite family. Optical rotation (OR) is calculated as the function of disordering structure parameters for La₃Ga₅SiO₁₄, La₃Ga_5.5Ta_0.5O₁₄ and Ca₃Ga₂Ge₄O₁₄ compounds. Using a single wavelength laser polarimeter the OR was measured on z-cut sample of langasite crystals La₃Ga₅SiO₁₄.

Abstract: The significant alteration of absorption (A) of monolayer graphene under mild oxygen plasma exposure has been observed. The first important effect is the reduction of the excitonic resonance peak at 4.64 eV. Secondly, in the near infrared range, A is gradually suppressed below an exposure-dependent threshold in sense that A << A₀. Quantity A₀ (given by πα and α is the fine structure constant) denotes constant absorption and relates to universal optical conductivity σ₀. The suppression of A₀ can be thought as the weakening of electron-hole interaction as displayed by the reduction of the excitonic resonance peak at 4.64 eV. The weakening of this interaction is due to the disorder introduced by the oxygen plasma exposure.

Abstract: The transport properties of polycrystalline La_0.9Sb_0.1MnO₃ (LSMO) bulk prepared by the solid-state reaction were investigated. We find that transport behaviors heavily depend on the synthesis process. The resistivity of LSMO1 for less rubbing time shows one metal-insulator transition (MIT) peak at temperature of 201 K, while the resistivity of LSMO2 for more rubbing time shows a MIT and a shoulder at about 240 and 140 K, respectively. The magnetoresistance (MR) ratio of LSMO2 reaches 41% under magnetic field of 2 T. Moreover, the MR ratio keeps significant value within broad temperature range. The infrared (IR) absorption spectra of LSMO2 show that the stretch-mode peak split into two Gaussian peaks with the gap about 70 cm^-1. This large splitting indicates there are strong distortion and disorder in LSMO2 sample. The results are interpreted in terms of the disorder system and phase separation in perovskite manganites.

Abstract: The full potential linearized augmented plane wave (FPLAPW) method within generalized gradient approximation (GGA) has been used to investigate the effect of disorder between Ni and Mn atoms on the electronic and magnetic properties of NiCoMnGa quaternary Heusler alloy. We observed that the increase in Mn concentration in NiCoMnGa destroys the half metallicity and decreases the total magnetic moment. Further, the disordered alloy exists in ferrimagnetic (FiM) ground state rather than ferromagnetic (FM) one of ordered system due to antiparallel alignment of extra Mn atom with respect to original Mn atom.

Abstract: Drug discovery from natural products resources have been extensively studied. The most important step in the discovery process is the identification of compounds with interesting biological activity. Single crystal X-ray structure determination is a powerful technique for natural products research and drug discovery in which the detailed three-dimensional structures that emerge can be co-related to the activities of these structures. This article shall present (i) co-crystal structures, (ii) determination of absolute configuration and (iii) the ability to distinguish between whether a natural product compound is a natural product or a natural product artifact. All these three properties are unique to the technique of single crystal X-ray structure determination.

Abstract: Anelastic and dielectric relaxation measurements have been carried out on poled and unpoled samples of the ferroelectric perovskite (Na_0.5Bi_0.5)_1−xBa_xTiO₃ (NBT-xBT), with composition in the range between pure NBT and the morphotropic phase boundary, 0 ≤ x ≤ 0.08. The complex elastic compliance spectra contain clear indications of both the rhombohedral/tetragonal and tetragonal/cubic transitions, allowing the determination of the phase diagram, which is difficult to obtain by diffraction techniques due to the very low distortions in both the tetragonal and rhombohedral phases and to the structural disorder in the Na/Bi sublattice. An extensive study is made for concentrations in the region of the morphotropic boundary (x ∼ 0.06) in order to find possible signatures of monoclinic phase, as for the case of PbZr_1-xTi_xO₃ (PZT). The main features in the anelastic curves are compared with those in the dielectric spectra and are tentatively related to different modes of octahedral rotations and polar cation shifts.

Abstract: Abstract Based on the thin plate theory and the transfer matrix method, the vibration propagation in periodical orthotropic single-leaf rib-stiffened panels is studied. The localization factors of the ordered and disordered periodic systems are calculated. The numerical results show that the anisotropy of the plate affects the propagation of vibration in the pannels. With the relative bending rigidity increasing, or with the relative torsional rigidity increasing, the width of stopbands increases in low dimenionless frequencies. The vibration localization phenomenon occurs in the span-length disordered periodic structures and it is enhanced with the increasing disorder of span-length.

Abstract: The perovskite (Na_1/2Bi_1/2)TiO₃ (NBT) undergoes a series of structural and polar transitions starting from the high temperature paraelectric phase: tetragonal paraelectric, tetragonal antiferroelectric, rhombohedral antiferroelectric and finally rhombohedral ferroelectric, according to neutron diffraction and dielectric spectroscopy. In solid solution with BaTiO₃ (BT) the ferroelectric phase changes from rhombohedral to tetragonal, at the so-called morphotropic phase boundary, and the phases at higher temperature become ill-defined, also because of the large lattice disorder induced by the coexistence of differently charged cations in the same sublattice. Combined dielectric and anelastic spectroscopy measurements are presented, which clarify some issues related to the phase transitions in NBT-BT. The influence of Ba substitution on the tetragonal antiferroelectric phase is determined for the first time, and the possibility that a monoclinic phase, although with very short coherence length, exists near the morphotropic phase boundary is discussed in view of a large maximum of the elastic compliance.

Abstract: The evolution of the magnetism as a function of the particle size is reported here for the ZnFe2O4 nanoparticles. A combination of neutron powder diffraction down to 1.5 K and magnetization measurements suggest that nanoparticles of sizes greater than about 100 nm exhibit bulk like behavior. The size dependence on magnetism can be explained by the rearrangement of the cations on the interstitial sites of the spinel cell.

Abstract: We report on electronic transport in nearly magnetic conductors with strong structural disorder. The initial motivation for this work was a large positive magnetoresistance (MR) found in magnetically ordered ground state of (Y1-xGdx)Co2 alloys. This was a surprising result since a large positive MR is not expected in a system with strong static magnetic or structural disorder. Contemporary theory of magnetotransport and common sense agree that an external magnetic field should suppress magnetic fluctuations, resulting in a negative MR. On the contrary; a positive MR suggests that an external magnetic field enhances static magnetic disorder. It was shown that unusual MR of (Y1-xGdx)Co2 alloys is related to a combination of structural disorder and metamagnetic instability of itinerant Co-3d electrons. The new mechanism of MR is common of a broad class of materials featuring a static magnetic disorder and itinerant metamagnetism. Such systems display a number of unusual properties, among them strong pressure and magnetic field dependencies of resistivity and thermopower, Non-Fermi-Liquid (NFL) behavior of resistivity and, possibly, of thermopower. We review the relevant experimental data, mostly the properties of RCo2-based alloys, and discuss the theoretical model developed for the interpretation of the experimental results. This model includes new mechanism of magnetoresistivity in structurally disordered itinerant magnetic alloys.