International Journal of Modern Physics B

This review deals with the state of the art and perspectives of description of nonequilibrium many-body systems using the nonequilibrium Green's function (NGF) method. The basic aim is to describe time evolution of the many-body system from its initial state over its transient dynamics to its long time asymptotic evolution. First, we discuss basic aims of transport theories to motivate the introduction of the NGF techniques. Second, this article summarizes the present view on construction of the electron transport equations formulated within the NGF approach to nonequilibrium. We discuss incorporation of complex initial conditions to the NGF formalism, and the NGF reconstruction theorem, which serves as a tool to derive simplified kinetic equations. Three stages of evolution of the nonequilibrium, the first described by the full NGF description, the second by a non-Markovian generalized master equation and the third by a Markovian master equation will be related to each other.

Bismuth sulfide (Bi₂S₃) exhibits excellent photocatalytic activity under visible light. We perform first-principles, density-function theory (DFT) calculations of the electronic structure for the Bi₂S₃ with native vacancy to facilitate its applications by gaining insight into the role of native defects. We find that the Bi vacancies are effective p-type defects for Bi₂S₃, while the S vacancies induce an intermediate level appearing in the band gap. Besides one Bi vacancy, the native vacancy defect at other four inequivalent positions in Bi₂S₃ leads to a reduction of band gap. Moreover, the change of band gap depends on the position of native vacancy defect. The results indicate that the native defects are the most likely physical cause for the scattered band gaps obtained by experiments. The influence of native vacancy defects on the photocatalytic properties of Bi₂S₃ is also discussed.

Exploration of new rare-earth captured clathrates is an important field of clathrate application. Study on the second phases produced in the synthesis process is crucial for confirming the incorporation of rare earth atoms into the clathrate lattice. In the current study, the Yb-doped clathrate samples with nominal composition of Ba₆Yb₂Ga₁₆Ge₃₀ were prepared by combining arc melting (ARC), melt spinning (MS) and spark plasma sintering (SPS) techniques. By using the TEM with EDS analysis and X-ray diffraction refinement, phase analysis was systemically performed in different steps of the preparation process. Although the clathrate was the main phase in obtained samples, the Yb content in the clathrate phase was found to be much lower than the nominal composition as the Yb atoms strongly tend to create other byproduct phases including Yb₂O₃ — which is the main obstacle in the synthesis of Yb-substituted clathrates. For the dilute Yb-doped germanium clathrate sample a decrease in the thermal conductivity and electrical conductivity was simultaneously observed, therefore, no expected improvement of ZT occurred.

The dielectric properties, electric modulus and ac electrical conductivity (σ_ac) of Al/Co-doped (PVC+TCNQ)/p-Si structures have been investigated in the wide frequency and voltage range of 0.5 kHz–3 MHz and (-4 V)–(9 V), respectively, using the capacitance-voltage (C–V) and conductance-voltage (G/ω–V) measurements at room temperature. The real and imaginary parts of dielectric constant (ε′, ε″), loss tangent (tan δ), σ_ac and the real and imaginary parts of electric modulus (M′, M″) were found strongly function of frequency and applied voltage especially at low frequencies. The ε′–V plot shows an anomalous peak in the forward bias region due to the series resistance (R_s), surface states (N_ss) and interfacial layer (PVC+TCNQ) effects for each frequency and then it goes to negative values known as negative dielectric constant (NDC) at low frequencies (f ≤ 70 kHz). Such observation of NDC is important result because it implies that an increment of bias voltage produces a decrease in the charge on the electrodes. The amount of negativity ε′ value increases with decreasing frequency and this decrement in the NDC corresponds to the increment in the ε″.

The paper deals with low-temperature photoluminescence and deformation luminescence (mechanoluminescence) of a composite material based on fine disperse powder of phosphor SrAl₂O₄:(Eu²⁺, Dy³⁺) and photopolymerizing resin that is transparent in the visible region. New information about the energy levels of impurities and defects was obtained. It has been found that at low temperatures (T = 15 ÷ 200 K) the photoluminescence spectrum of SrAl₂O₄:(Eu²⁺, Dy³⁺) presents two partially overlapping wide bands with the maxima at λ_1max = 446 nm and λ_2max = 517 nm. The strong interaction of energy states of the bands results in temperature quenching of the short-wave band of luminescence (λ_1max = 446 nm) and its complete attenuation at T ≥ 200 K. The results were used to describe the mechanism of persistent luminescence and mechanoluminescence of SrAl₂O₄:(Eu²⁺, Dy³⁺).

Annealing study of amorphous bulk and nanoparticle iron at temperatures from 500 K to 1000 K has been carried out using molecular dynamics (MD) simulations. The simulation is performed for models containing 10⁴ particles Fe at both crystalline and amorphous states. We determine changes of the potential energy, pair radial distribution function (PRDF) and distribution of coordination number (DCN) as a function of annealing time. The calculation shows that the aging slightly reduces the potential energy of system. This result evidences that the amorphous sample undergoes different quasi-equilibrated states during annealing. Similar trend is observed for nanoparticles sample. When the samples are annealed at high temperatures we observe the crystallization in both bulk and nanoparticle. In particular, the system undergoes three stages. At first stage the relaxation proceeds slowly so that the energy of system slightly decreases and the samples structure remains amorphous. Within second stage a structural transformation occurs which significantly changes PRDF and DCN for the relatively short time. The energy of the system is dropped considerably and the amorphous structure transforms into the crystalline. Finally, the crystalline sample undergoes the slow relaxation which reduces the energy of system and eliminates structural defects in crystal lattices.

The new ceramic compounds type BaREBiO₄ (where RE = Sm and Nd) have been reported for the first time. The sample was prepared by solid-state reaction technique. The orthorhombic crystallization with a = 6.429(4) Å, b = 6.536(5) Å, c = 8.648(4) Å for BaSmBiO₄ and a = 6.485(3) Å, b = 6.561(4) Å, c = 8.686(4) Å for BaNdBiO₄ was studied from powder X-ray diffraction studies. The semiconducting nature of the compounds was observed with the energy gap measurement from diffused reflectance spectroscopy (DRS) studies. The paramagnetic nature of the compounds at room temperature and low temperature conditions was identified by magnetization measurements as a function of magnetic fields and temperature. The effective magnetic moment of the samples was found to be 2.126 μ_B and 4.272 μ_B.

A sery of ferrite samples, Ni_0.1Cu_0.2Mg_xZn_0.7-xFe₂O₄, (x = 0.00, 0.15, 0.25, 0.35, 0.45, 0.55 and 0.70) have been prepared by the standard ceramic techniques, sintered at 1200°C for 2 h, and their crystalline structures were investigated by using X-ray diffraction, which confirmed the formation of Ni_0.1Cu_0.2Mg_xZn_0.7-xFe₂O₄ phase. The presence of Mg content increases the crystallinity of the given ferrite which is considered as catalyst for the accomplishment of the solid state reaction. The lattice parameter and crystallite size were calculated from XRD. The cationic distribution was estimated from the theoretical values of lattice parameters. The IR spectra analysis has been studied.

The dielectric constant for the ferrite system Ni–Cu–Zn gradually increases with temperature up to nearly Curie temperature and then increase sharply beyond the T_c. The Curie temperature obtained from the versus T is in agreement with the Curie temperature obtained from other measurements. Dielectric loss tangent (tan δ) has the same behavior as dielectric constant . A minimum value of dielectric loss tangent (tan δ) for the system "Ni–Cu–Zn" was observed at x = 0.45. Magnetic permeability has been studied. The dependence of initial permeability on temperature exhibit a peak which is called Hopkinson peak (HP) in the vicinity of Curie temperature and confirmed the phase purity of ferrite sample. The increasing of porosity and decreasing of grain size play an important role in decreasing μ_i. It is clear that the value of T_c is found to increase by increasing the concentration of diamagnetic substitution Mg for "Ni–Cu–Zn" system.

An epoxy-resin bonding route was used to produce composite rods of the highly magnetostrictive alloys Tb_1-xNd_x(Fe_0.8Co_0.2)_1.93 (0.20 ≤ x ≤ 0.75). The structure, spin configuration, magnetostriction and particle size are investigated by means of X-ray diffraction (XRD), a standard strain technique and scanning electron microscope (SEM). The epoxy-bonded 0–3 type and pseudo 1–3 type composites are successfully fabricated, respectively. XRD analysis shows that the easy magnetization direction (EMD) for the alloy of x = 0.20 lies along 〈111〉 axis. The magnetic curing field makes the particles align as a particulate chain and also causes the particles rotating along its EMD direction. The pseudo 1–3 type epoxy-bonded composite has a larger magnetostriction than that of the 0–3 type composite, which can be attributed to the larger magnetostriction coefficient λ₁₁₁, EMD lying along 〈111〉 direction, the 〈111〉-textured orientation and the chain structure. A large saturation magnetostriction (λ_0S ~ 570 ppm) is achieved for the 1–3 type epoxy/Tb_0.35Nd_0.65(Fe_0.8Co_0.2)_1.93 composite (about 150–250 μm, 10 kOe), which approaches 70% of its monolithic alloy. Furthermore, it only contains 27 vol.% alloy particles in the insulating epoxy matrix and performs a low magnetic anisotropy, which could make it technologically interesting for the field of Nd-containing magnetostrictive materials.