Interrelation between electronic structure and interatomic distances for compounds

The heat capacity was studied for LaMn₂Si₂, La_0.75Y_0.25Mn₂Si₂, La_0.7Y_0.3Mn₂Si₂, YMn₂Si₂ and LaFe₂Si₂ isostructural intermetallic compounds in the temperature range 1.8–360 K. The electronic, magnetic and lattice contributions to the heat capacity of the compounds were determined and analyzed. The interrelation was found between values of the electronic contribution to the heat capacity (density of states at the Fermi level) and crystal lattice parameters of R(Mn,Fe,Ni)₂Si₂ compounds. The electronic contribution and the density of states at Fermi level increase with increasing lattice parameters of the compounds. The change of interlayer Mn–Mn exchange interactions with change of Y concentration in La_1-xY_xMn₂Si₂ compounds is not accompanied by considerable changes in the electronic contribution to the heat capacity and density of states at the Fermi level. The performed analysis of the magnetic contribution shows that no essential differences exist between the behavior of the heat capacity of the compounds with d_Mn–Mnd_c and with d_Mn–Mn<d_c upon various types of the magnetic phase transitions.     

First-principles study of fundamental properties and electronic structure of alloys

We have performed first-principles calculations using full-potential augmented-plane-wave method to investigate the fundamental properties of the Cd_1–xZn_xTe alloys. The composition dependence of the lattice constant and the bulk modulus have been estimated from total energy calculations. By means of the analytical fitting the band structures in the vicinity of the Brillouin center a complete set of effective electron- and hole-masses have also been derived. In order to further understand the effects of the chemical bonding on the above macroscopic properties we then studied the relaxation behaviors and the changes of the electronic states upon alloying for x=0.25 system. The results presented here yield a general understanding of the fundamental properties for the Cd_1–xZn_xTe crystals studies.     

Magnetic ground state and spin waves in A(A = K, Rb or Cs)Fe1.5Se2

We present theoretical results on the ground state phase diagram, spin waves and dynamic structure factor on the J₁-J₂ model. In the reasonable physical parameter region corresponding to AFe_1.5Se₂, the A-collinear antiferromagnetic phase is stable. The spin wave spectra have two acoustic branches and four optical branches for this phase on the rhombus-ordered vacancy lattice, and each of them is twofold degenerate. However, they have one nondegenerate acoustic branch and two nondegenerate optical branches on the square-ordered vacancy lattice. To offer the theoretical guidance for the further experiments, we also discuss the magnetic excitation spectra and the inelastic neutron scattering pattern based on linear spin wave theory.     

Kondo coupling and Curie temperature of in the coherent potential approximation

The ferromagnetism of Ga_1-xMn_xAs is studied in the coherent potential approximation (CPA). In this work, we used the exact Hilbert transformation of the face-centered cubic (fcc) density of states (DOS), which is different from the usual semi-circle DOS employed in previous works. Using Weiss molecular theory, we obtained a nonlinear relation of Curie temperature with respect to Kondo coupling. Our calculated T_C agrees very well with measured values.     

Neutron diffraction study of multipole order in light rare-earth hexaborides

Multipole interactions are known to play a central role in the unconventional properties of light rare-earth hexaborides and especially of CeB₆. Substituting Pr at the Ce sites has the effect of enhancing exchange interactions and changing the symmetry of the local 4f charge distribution, while suppressing the octupole moment. The (T, H) magnetic phase diagrams of the Ce _x Pr_{1 − x} B₆ compounds display a large variety of ordered phases involving magnetic and/or charge degrees of freedom. Here we focus on the compound Ce_0.7Pr_0.3B₆, which is located slightly beyond the Pr concentration where the antiferroquadrupolar phase of pure CeB₆ is suppressed in zero field. The different magnetic structures have been characterized by neutron diffraction and their origin is discussed in connection with recent non-resonant X-ray results by Tanaka et al.      

Spin chirality and electric polarization in multiferroic compounds (, Er)

Polarized neutron diffraction experiments have been performed on multiferroic materials RMn₂O₅ (R=Ho, Er) under electric fields in the ferroelectric commensurate (CM) and the low-temperature incommensurate (LT-ICM) phases, where the former has the highest electric polarization and the latter has reduced polarization. It is found that, after cooling in electric fields down to the CM phase, the magnetic chirality is proportional to the electric polarization. Also we confirmed that the magnetic chirality can be switched by the polarity of the electric polarization in both the CM and LT-ICM phases. These facts suggest an intimate coupling between the magnetic chirality and the electric polarization. However, upon the transition from the CM to LT-ICM phase, the reduction of the electric polarization is not accompanied by any reduction of the magnetic chirality, implying that the CM and LT-ICM phases contain different mechanisms of the magnetoelectric coupling.     

Dramatic change of optical properties at a quantum critical point in the heavy fermion system

Optical properties of have been studied via infrared spectroscopy. For x>0.3, a hump in the optical conductivity σ₁ is observed at about 0.2 eV, resulting from strong hybridization between conduction electrons and Ce 4-f electronic states. For x0.3, in contrast, no such hump is observed. The low frequency plasmon indicating the existence of heavy particles is also observed below coherence temperature T^* for x>0.3.     

Self-similarity of electrostatic fluctuations in a linear magnetised plasma system

In this Letter the long-range time correlations present in the fluctuation data in presence of electrostatic instability in a magnetised dc discharge plasma is presented. The electrostatic instability is generated due to the effect of crossed electric and magnetic field (E×B flow) and has intermediate frequency ranging from 50 to 100 MHz. Hurst exponent, the self-similarity parameter is calculated with the help of different statistical methods suggested by many researchers to determine the long-range time correlation present in fluctuation dynamics in the plasma column. The fluctuation in the ion saturation current is measured by a Langmuir probe for the study and the measurement is done both radially and axially in the plasma system. Estimated results clearly expose the self-similar character of the fluctuations with self-similarity parameters having values from 0.65 to 0.90 through the presence of long-range time correlation.     

Cuprate superconductors in the vicinity of a Pomeranchuk instability

We propose that cuprate superconductors are in the vicinity of a spontaneous d-wave type Fermi surface symmetry breaking, often called a d-wave Pomeranchuk instability. This idea is explored by means of a comprehensive study of magnetic excitations within the slave-boson mean-field theory of the t-J model. We can naturally understand the pronounced xy anisotropy of magnetic excitations in untwinned YBa₂Cu₃O_y and the sizable change of incommensurability of magnetic excitations at the transition temperature to the low-temperature tetragonal lattice structure in La_2-xBa_xCuO₄. In addition, the present theoretical framework allows the understanding of the similarities and differences of magnetic excitations in Y-based and La-based cuprates.     

Phase transitions in the quantum Hall liquid in a quantum wire

Two scenarios for the collapse of the ν=1 quantum Hall liquid (QHL) state, with the effective quantum wire (QW) width defined by the Fermi vector k_F, are studied. Here, ν for the QW is defined as the filling factor of Landau levels (LL) at the center of the QW. In the first one there is no electron redistribution at critical magnetic field , where the Fermi energy, E_F, coincides with the bottom of the empty upper spin-split LL. For the ν=1 state is unstable due to exchange-correlation effects and lateral confinement. In the second scenario, a transition to the ν=2 state occurs, with much smaller width, at . The latter scenario is analyzed in the Hartree–Fock approximation (HFA). Here the Hartree contribution to the total energy affects drastically due to strong electron redistribution in the QW. In both scenarios, the exchange-enhanced g-factor is suppressed at B_cr. The critical fields, activation energy, and optical g-factor obtained in the first scenario are very close to the measured ones.     

Study of structural and electronic transport properties of Ce-doped LaMnO3

The structural and electronic transport properties of La_1−x Ce _x MnO₃ (x=0.0–1.0) have been studied. All the samples exhibit orthorhombic crystal symmetry and the unit cell volume decreases with Ce doping. They also make a metal-insulator transition (MIT) and transition temperature increases with increase in Ce concentration up to 50% doping. The system La_0.5Ce_0.5MnO₃ also exhibits MIT instead of charge-ordered state as observed in the hole doped systems of the same composition.     

Ce4+离子在Ca2SnO4一维结构基质中的电荷迁移光谱研究

利用高温固相反应法制备了Ca₂Sn_1-xCe_xO₄和Ca_2-ySr_ySn_1-xCe_xO₄一维结构发光体. XPS结果显示 Ca₂SnO₄拥有两种结合能分别为5277 eV和5293 关键词： 2Sn_1-xCe_xO₄')" href="#">Ca₂Sn_1-xCe_xO₄ 2-ySr_ySn_1-xCe_xO₄')" href="#">Ca_2-ySr_ySn_1-xCe_xO₄ 一维结构 电荷迁移光谱     

Anomalous vortex dynamics in

We report on measurements of current–voltage (I–V) characteristics for YNi₂B₂C single crystals with weak pinning in various fields at 7.6 K. We find nonmonotonic, N-shaped I–V curves in a certain field region deep in the vortex solid phase. This behavior is anomalous, since there exists an intermediate I region where flow voltage V shows a decrease with increasing I (a driving force). While the exact nature remains unknown, this phenomenon suggests vortex motion (driving I) induced pinning.     

Performance of Nd:YLF laser by using crystal electrooptic Q-switch

The polarization rotating coefficient and the laser-damage threshold of La₃Ga₅SiO₁₄ crystal at 1064 nm are measured, which are 1.1 deg/mm and , respectively. The working principle and the design method of electrooptic Q-switch based on La₃Ga₅SiO₁₄ crystal, which has the optical activity, are reported. The performance of Nd:YLF laser with the electrooptic Q-switch of La₃Ga₅SiO₁₄ crystal was studied. The output pulses with an energy of 379 mJ, a duration of 8.7 ns and repetition of 10 Hz are achieved.     

Spin-glass, antiferromagnetism and kondo behavior in Ce2Au1−x Co x Si3 alloys

Recently, the solid solution Ce₂Au_{1− x Co} xSi₃ has been shown to exhibit many magnetic anomalies associated with the competition between magnetic ordering and the Kondo effect. Here we report high pressure electrical resistivity of Ce₂AuSi₃, ac susceptibility (X) and magnetoresistance of various alloys of this solid solution in order to gain better knowledge of the magnetism of these alloys. High pressure resistivity behavior is consistent with the proposal that Ce₂AuSi₃ lies at the left-hand side of the maximum in Doniach’s magnetic phase diagram. The ac X data reveal that there are in fact two magnetic transitions, one at 2 K and the other at 3 K for this compound, both of which are spin-glass-like. However, as the Co concentration is increased, antiferromagnetism is stabilized for intermediate compositions before attaining non-magnetism for the Co end member.     

Radiation characteristics of Botryococcus braunii, Chlorococcum littorale, and Chlorella sp. used for fixation and biofuel production

This paper reports experimental measurements of the radiation characteristics of green algae used for carbon dioxide fixation via photosynthesis. The generated biomass can be used to produce not only biofuels but also feed for animal and food supplements for human consumptions. Particular attention was paid to three widely used species namely Botryococcus braunii, Chlorella sp., and Chlorococcum littorale. Their extinction and absorption coefficients were obtained from normal–normal and normal–hemispherical transmittance measurements over the spectral range from 400 to 800 nm. Moreover, a polar nephelometer was used to measure the scattering phase function of the microorganisms at 632.8 nm. It was observed that for all strains, scattering dominates over absorption. The magnitudes of the extinction and scattering cross-section are functions of the size, shape, and chlorophyll content of each strain in a non-trivial manner. Absorption peaks at 435, 475, and 676 nm corresponding to chlorophyll a and chlorophyll b. The results can be used for scaling and optimization of CO₂ fixation in ponds or photobioreactors as well as in the development of controlled ecological life support systems.     

组合结构化合物Ho2Ni7-xFex(x=0-3.0)的晶体结构、结构转变和磁性

在金属间化合物的结构演变中,原子尺寸因素起着重要的作用.由于密堆积效应,不同原子半径比的元素往往形成不同的结构.而自由电子填充于原子构成的晶体结构的间隙中,它对化合物的结构也有影响.基于组合结构化合物Ho2Ni7-xFex,结合原子尺寸与自由电子对晶体结构的不同影响,文章探讨一种单位体积内自由电子浓度的经验方法来判断Ho2Ni7-xFex化合物中两种异构体间的转变.随着Fe含量的增加,Ho2Ni7-xFex化合物先结晶成Gd2Co7型三方结构,然后结晶成Ce2Ni7型六方结构.利用Rietveld精修技术和磁测量,获得了化合物的晶体结构参数和饱和磁化强度.化合物晶胞常数随Fe含量增加而增加,饱和磁化强度则随之减少(dMs/dx=-2).分析结果表明,单位体积内自由电子浓度更高,化合物形成三方结构,反之则形成六方结构.     

Structural and optical properties of thermally evaporated thin films

Chalcogenide glass Se₅₅Ge₃₀As₁₅ have amorphous structure in both as-deposited and annealed conditions. The optical properties of the as-deposited and annealed films were studied using spectrophotometric measurements of transmittance, T(λ), and reflectance, R(λ), at normal incidence of light in the wavelength range 200–2500 nm. Neither annealing temperature nor film thickness can influence spectral response on refractive index and absorption index of films. The type of electronic transition responsible for optical properties is indirectly allowed transition with energy gap of 1.94 eV and phonon energy of 40 meV. The dispersion of the refractive index is discussed in terms of the single oscillator Wemple–Didomenico (WD) model. The width of band tails of localized states into the gap (ΔE), the single oscillator energy (E_o), the dispersion energy (E_d), the optical dielectric constant (ε_∞), the lattice dielectric constant (ε_L), the plasma frequency (ω_p) and the free charge carrier concentration (N) were estimated.     

Integrable structure of melting crystal model with two -parameters

This paper explores integrable structures of a generalized melting crystal model that has two q-parameters q₁,q₂. This model, like the ordinary one with a single q-parameter, is formulated as a model of random plane partitions (or, equivalently, random 3D Young diagrams). The Boltzmann weight contains an infinite number of external potentials that depend on the shape of the diagonal slice of plane partitions. The partition function is thereby a function of an infinite number of coupling constants t₁,t₂,… and an extra one Q. There is a compact expression of this partition function in the language of a 2D complex free fermion system, from which one can see the presence of a quantum torus algebra behind this model. The partition function turns out to be a tau function (times a simple factor) of two integrable structures simultaneously. The first integrable structure is the bigraded Toda hierarchy, which determines the dependence on t₁,t₂,…. This integrable structure emerges when the q-parameters q₁,q₂ take special values. The second integrable structure is a q-difference analogue of the 1D Toda equation. The partition function satisfies thisq-difference equation with respect to Q. Unlike the bigraded Toda hierarchy, this integrable structure exists for any values of q₁,q₂.     

Nuclear isospin mixing and elastic parity-violating electron scattering

The influence of nuclear isospin mixing on parity-violating elastic electron scattering is studied for the even–even, N=Z nuclei ¹²C, ²⁴Mg, ²⁸Si, and ³²S. Their ground-state wave functions have been obtained using a self-consistent axially-symmetric mean-field approximation with density-dependent effective two-body Skyrme interactions. Some differences from previous shell-model calculations appear for the isovector Coulomb form factors which play a role in determining the parity-violating asymmetry. To gain an understanding of how these differences arise, the results have been expanded in a spherical harmonic oscillator basis. Results are obtained not only within the plane-wave Born approximation, but also using the distorted-wave Born approximation for comparison with potential future experimental studies of parity-violating electron scattering. To this end, for each nucleus the focus is placed on kinematic ranges where the signal (isospin-mixing effects on the parity-violating asymmetry) and the experimental figure-of-merit are maximized. Strangeness contributions to the asymmetry are also briefly discussed, since they and the isospin mixing contributions may play comparable roles for the nuclei being studied at the low momentum transfers of interest in the present work.