Scaling properties of the growing monolayer on the disordered substrate

We present results of theoretical and numerical studies the pattern formation processes in adsorptive system with the disordered substrate, representing high-entropy alloy. The lateral diffusivity of adatoms on the substrate is represented as a quenched spatial disorder. By taking into account a transference of adsorbate between first two layers we construct the reaction-diffusion model, describing evolution of adsorbate concentration on the disordered substrate. It will be shown, that at elevated values of the pressure inside a chamber the quenched spatial disorder will induce pattern formation in the system. We will show that the strength of the spatial disorder can control dynamics of nanostructured film growth, type of surface structures and scaling properties of the growing layer. This study provides an insight into details of self-organization of adatoms on the high-entropy alloys in adsorptive systems.     

Mott-type hopping conduction in the ordered and disordered phases of LiCoO2

The temperature dependence of dc resistance was observed in the range of for layered-rock-salt (hexagonal structure, ordered distribution between Li and Co) and modified-spinel (cubic, random distribution) phases of LiCoO₂. The results suggest Mott-type hopping conduction arising from the localized Co-3d electrons in the valence band. The densities of states (DOS) at the Fermi energy (E_F) estimated from the slope of the resistance curves were 2.0×10²⁰ and 5.5×10¹⁹ cm⁻³ eV⁻¹ for the ordered and disordered phases, respectively. The relatively low DOS at E_F in the disordered phase suggests that E_F approaches the edge of the valence band as a result of the narrowing of Co-t_2g bands due to the higher lattice symmetry in the disordered phase.     

Spin correlations in 2 + ε dimensions - ordered and disordered phase

Closed form expressions for the correlation functions of fixed length spins in 2 and 2 + ? dimensions below and above the critical temperature are obtained by a skeleton graph approach.     

Magnetic,structural and electrical properties of ordered and disordered Co50Fe50 films

Co₅₀Fe₅₀ films with thickness varying from 100 to 500 Å were deposited on a glass substrate by sputtering process, respectively. Two kinds of CoFe films were studied: one was the as-deposited film, and the other the annealed film. The annealing procedure was to keep the films at 400 °C for 5 h in a vacuum of 5×10⁻⁶ mbar. From the X-ray study, we find that the as-deposited film prefers the CoFe(1 1 0) orientation. Moreover, the body-centered cubic (bcc) CoFe(1 1 0) line is split into two peaks: one corresponding to the ordered body-centered tetragonal (bct) phase, and the other, the disordered bcc phase. After annealing, the peak intensity of the ordered bct phase becomes much stronger, while that of the disordered bcc phase disappears. The annealing has also caused the ordered CoFe(2 0 0) line to appear. When the amount of the ordered bct phase in Co₅₀Fe₅₀ is increased, the saturation magnetization (M_s) and coercivity (H_c) become larger, but the electrical resistivity (ρ) decreases. From the temperature coefficient of resistance (TCR) measurement, we learn that the bct grains in the CoFe film start to grow at temperature 82 °C.     

Scaling theory for percolative charge transport in disordered molecular semiconductors

We present a scaling theory for charge transport in disordered molecular semiconductors that extends percolation theory by including bonds with conductances close to the percolating one in the random-resistor network representing charge hopping. A general and compact expression is given for the charge mobility for Miller-Abrahams and Marcus hopping on different lattices with Gaussian energy disorder, with parameters determined from numerically exact results. The charge-concentration dependence is universal. The model-specific temperature dependence can be used to distinguish between the hopping models.     

Resonance field emission for ordered and disordered adsorption of atoms

Model numerical calculations of the enhancement factor of field emission from surfaces, containing ordered and disordered monolayers of adatoms, were carried out using some typical values for the parameters. It was shown that perculiarities of the field emission energy distribution may be used for experimental studies of (“order-disorder” type) phase transitions in monolayers of adsorbates by the method of field emission spectroscopy.     

First-principles investigations on thermodynamic properties of the ordered and disordered Si0.5Ge0.5 alloys

The structure, formation energy, and thermodynamic properties of Si_0.5Ge_0.5 alloys are investigated through first-principles method. The ordered and disordered structures of Si_0.5Ge_0.5 compounds are considered. Our results show that thermodynamic instabilities of Si_0.5Ge_0.5 alloys at 0 K can be judged from the calculated formation energy. However, the alloy might be prepared at specified environment owing to the entropy effects considered. Moreover, the temperature dependence of the heat capacity, Debye temperature and thermal expansion coefficient of ordered and disordered structures are discussed.     

Acoustic properties of the disordered relaxor ferroelectric PbSc1/2Ta1/2O3

The behavior of acoustic phonons in crystals of a relaxor ferroelectric, namely, the lead scandium tantalate PbSc_1/2Ta_1/2O₃ (PST), is studied in the vicinity of the diffuse phase transition. The behavior of longitudinal and transverse acoustic phonons in a PST single crystal is examined using Brillouin scattering. The phonon subsystem is found to behave anomalously in the vicinity of T = 297 K, which can probably be assigned to the existence of a phase transition. Analysis of the results obtained yields the values of the elastic moduli C₁₁, C₁₂, and C₄₄ for the cubic phase of the crystal over a wide temperature range.     

Scaling relation for the density of states of a disordered n-orbital model

A tight-binding model of a particle on a d-dimensional lattice with n orbitals per lattice site is considered. It is shown that the density of states obeys a scaling relation at the n = ∞ band edges with characteristic dimension d = 4.     

Damage rates in neutron irradiated feco and feco2v ordered and disordered alloys

Ordered and disordered samples of FeCo and FeCo2V alloys have been irradiated at liquid hydrogen temperature with fission neutrons up to an integrated dose of about 7,2.10¹⁷n/cm² (E > 1 MeV). During the irradiation, the resistivity increases continuously due to point defect production.     

Electronic structure and optical properties of ordered compounds potassium tantalate and potassium niobate and their disordered alloys

The electronic energy band structure, site and angular momentum decomposed density of states (DOS) of cubic perovskite oxides KNbO₃ and KTaO₃ have been obtained from a first principles density functional based full potential linearized augmented plane wave (FLAPW) method within a generalized gradient approximation (GGA). The total DOS in valence region is compared with the experimental photo-emission spectra (PES). The calculated DOS is in good agreement with the experimental energy spectra and the features in the spectra are interpreted by comparison with the projected density of states (PDOS). The valence band PES is mainly composed of Nb-4d/Ta-5d and O 2p states in KNbO₃ and KTaO₃, respectively. Using the PDOS and the band structure we have analyzed the inter-band contribution to the optical properties of these materials. The real and imaginary parts of the dielectric function have been calculated and compared with experimental data. They are found to be in a reasonable agreement. The role of band structure on the optical properties have been discussed.     

Luminescence of Mn ions in ordered and disordered LiAl5O8

The phosphorescence and excitation spectra of Mn ions in the ordered and disordered phases of LiAl₅O₈ have been measured. In both phases Mn²⁺ ions substitute for Al³⁺ ions in two different tetrahedral sites of the LiAl₅O₈ lattice. In both sites in the ordered phase, sharp zero-phonon transitions have been observed in the low temperature phosphorescence and excitation spectra - these transitions were considerably broadened in the disordered phase due to crystal field inhomogeneity in that phase. The deviation from neutrality caused by the Mn²⁺ ions in the ordered phase is largely compensated by Mn⁴⁺ ions occupying octahedral Al³⁺ sites. On disordering, a large proportion of Mn⁴⁺ is reduced to Mn²⁺, while the remainder takes up a site with a higher proportion of Li⁺ ions as next nearest neighbours. This leads to an increase in the ionicity of the Mn⁴⁺ site in the disordered phase and hence to a larger value of the Racah parameter B.     

Solubility and diffusion of H in ordered (L12) and disordered Pd3Mn

Hydrogen isotherms have been measured from 423 to 573 K for the disordered and L1₂ ordered forms of Pd₃Mn and, from these equilibrium isotherms, Δ_HH$\mathrm{\Delta }{H}_{\mathrm{H}}$ and Δ_SH$\mathrm{\Delta }{S}_{\mathrm{H}}$ have been determined over a range of r values where r  =H-to-metal, atom ratio. Δ_HH$\mathrm{\Delta }{H}_{\mathrm{H}}$ values are significantly more negative for the L1₂ ordered form than for the disordered form. H diffusion constants have been determined for the L1₂ and disordered forms of Pd₃Mn from gas phase H permeation measurements through Pd₃Mn membranes (423–573 K). The activation energy for diffusion of H in the L1₂ form is 35.2 kJ/mol H which is more reasonable than the value of 76.3 kJ/mol H previously reported. The diffusion constant is greater for the disordered than for the L1₂ form.     

Colossal resistivity change associated with the charge ordered/disordered transition: Monte Carlo study

Earlier theoretical approaches to manganites mainly stem from magnetic framework in which the electronic transports are thought to be spin-dependent and the double exchange plays a vital role. However, quite a number of experimental observations cannot be explained in the magnetic framework, yet. For example, multiplicate insulator-metal transitions and resistivity reduction induced by perturbations other than magnetic field, such as electric current, are not well understood in this framework. Here we present a comprehensive analysis on the magnetic framework and give a Monte Carlo study on the resistivity of a charge ordered/disordered model without accounting for the spin degree of freedom. The result shows a colossal resistivity change as a resultant of the transition between two types of insulated states. This transition has intrinsic difference from the popular insulated-to-metallic transition in the magnetic framework. The present scenario can be used to explain some experimental facts for electronic transports in manganites, which are not accessible in the magnetic framework.     

Colossal resistivity change associated with the charge ordered/disordered transition:Monte Carlo study

Earlier theoretical approaches to manganites mainly stem from magnetic framework in which the electronic transports are thought to be spin-dependent and the double exchange plays a vital role. However, quite a number of experimental observations cannot be explained in the magnetic framework, yet. For example, multiplicate insulator-metal transitions and resistivity reduction induced by perturbations other than magnetic field, such as electric current, are not well understood in this framework. Here we present a comprehensive analysis on the magnetic framework and give a Monte Carlo study on the resistivity of a charge ordered/disordered model without accounting for the spin degree of freedom. The result shows a colossal resistivity change as a resultant of the transition between two types of insulated states. This transition has intrinsic difference from the popular insulated-to-metallic transition in the magnetic framework. The present scenario can be used to explain some experimental facts for electronic transports in manganites, which are not accessible in the magnetic framework.     

Superconducting properties of the atomically disordered MgB2 compound

The effect of disorder induced by neutron irradiation in a nuclear reactor (thermal neutron fluence 1×10¹⁹cm^?2) on the superconducting transition temperature T _c and the upper critical field H _c2 of polycrystalline MgB₂ samples was investigated. Despite the appreciable radiation-induced distortions (more than ten displacements per atom), the initial crystal structure (C32) was retained. The temperature T _c decreased from 38 to 5 K upon irradiation and was practically completely restored after the subsequent annealing at a temperature of 70°C. A weak change in the dH _c2/dT derivative upon irradiation is explained by the fact that the irradiated samples are described by the “pure” limit of the theory of disordered superconductors. The suppression of T _c upon disordering may be due to the isotropization of the originally anisotropic (or multicomponent) superconducting gap or to a decrease in the density of electronic states at the Fermi level.     

Enhanced absorption properties of ordered mesoporous carbon/Co-doped ordered mesoporous carbon double-layer absorbers

Ordered mesoporous carbon (OMC) and metal-doped (M-doped) OMC composites are prepared, and their electromagnetic (EM) parameters are measured. Using the measured EM parameters we calculate the EM wave absorption properties of a double-layer absorber, which is composed of OMC as an absorbing layer and M-doped OMC as the matching layer. The calculated results show that the EM wave absorption performance of OMC/OMC-Co (2.2mm/2.1mm) is improved remarkably. The obtained effective absorption bandwidth is up to 10.3 GHz and the minimum reflection loss reaches 47.6 dB at 14.3 GHz. The enhanced absorption property of OMC/OMC-Co can be attributed to the impedance match between the air and the absorber. Moreover, it can be found that for the absorber with a given matching layer, a larger value of -tanδε (= tan δε absorbing tan δε matching ) can induce better absorption performance, indicating that the difference in impedance between the absorbing layer and the matching layer plays an important role in improving the absorption property of double-layer absorbers.