Size of the interpolated cation and hole carrier density: two key parameters for the optimisation of colossal magnetoresistive properties of Pr-based manganites

The exploration of the magnetic and transport properties of four series of manganese perovskites, Pr_0.7Ca_0.34−xA_xMnO_3−δ (A=Sr, Ba), Pr_0.7−xLa_xCa_0.3 MnO_3−δ and Pr_0.66Ca_0.34−x Sr_xMnO_3−δ has allowed four phases with colossal magnetoresistive (CMR) properties to be isolated: Pr_0.7Ca_0.25Sr_0.025MnO_3−δ and Pr_0.66Ca_0.26Sr_0.08MnO_3−δ that exhibit a variation of resistance of 2.5. 10⁷% and 10⁹% at μ₀ H=5 T for T=88 K and 50 K respectively, Pr_0.58La_0.12Ca_0.3 MnO_3−δ that exhibits a variation of 6.10⁶% for μ₀ H=5 T at T=80 K and Pr_0.7Ba_0.025Ca_0.275MnO_3−δ for which a resistance variation of 5.10⁹%, at T=50 K, for μ₀ H=5 T is evidenced. for each compound of this series except the barium phase, one observes that the temperature T_max, which corresponds to the resistance maximum on the R(T) curves in zero magnetic field, increases dramatically as the mean size of the interpolated cations increases, and that the CMR effect correlatively decreases dramatically. The comparison of the two series Pr_0.7Ca_0.3−xSr_xMnO_3−δ and Pr_0.66Ca_0.34−xSr_xMnO_3−δ shows also the crucial role of the hole carrier density: for a same mean ionic radius of the interpolated cation T_max is decreased of about 50 K by introducing 0.034 hole per Mn mole.     

Competing ferromagnetic and charge-ordered states in models for manganites: the origin of the colossal magnetoresistance effect

The one-orbital model for manganites with cooperative phonons and superexchange coupling JAF is investigated via large-scale Monte Carlo simulations. The results for two orbitals are also briefly discussed. Focusing on the electron density n=0.75, a regime of competition between ferromagnetic metallic and charge-ordered (CO) insulating states is identified. In the vicinity of the associated bicritical point, colossal magnetoresistance (CMR) effects are observed. The CMR is associated with the development of short-distance correlations among polarons, above the spin ordering temperatures, resembling the charge arrangement of the low-temperature CO state.     

Characteristics of the phase-separated state in manganites: Relationship with transport and magnetic properties

Journal of Experimental and Theoretical Physics - The temperature and magnetic field dependence of the resistivity, magnetoresistance, and magnetic susceptibility of phase-separated manganites in...     

Cumulant expansion for the p–d model: density of states and hole occupation

We propose a nonstandard perturbative solution of the p - d model, based on a generalized cumulant expansion. Using a temperature dependent Green's functions formalism, the behaviour of the density of states and the hole occupation number is investigated in the charge-transfer and in the Mott-Hubbard regime, to describe realistic systems with CuO₂ sheets. A comparison with different analytic and numerical methods is also discussed.     

Role of effective mass in modulating linear and non-linear response properties of single carrier quantum dots: Interplay with system parameters

We explore the pattern of effective mass dependence of linear and non-linear optical (NLO) response of one electron quantum dots harmonically confined in two dimensions. For different combinations of transverse magnetic field strength (ω_c), harmonic confinement potential (ω₀), and anharmonic interaction, the influence of the effective mass (m^*) of the dot on linear (α), and the first (β), and second (γ) NLO responses of the system is computed through linear variational route. The investigation reveals a complicated interplay between the effective mass and the system parameters.     

Threshold and saturation properties of the field-induced twist cell with two parameters weak anchoring boundaries

On the basis of two-parameter formula of weak surface coupling anchoring energy of nematic liquid crystals proposed by Zhao et al recently, the general torque equilibrium equation and boundary conditions of the director have been obtained and the threshold field as well as the saturation field of the field-induced twist cell have been analysed for three kinds of configurations, i.e. homogeneous, splay and Pi cells formed by different rubbing conditions and pretilt angles. The results indicated that the polar anchoring has no effect on the threshold field. It is determined only by the twist anchoring and pretilt angle. But, the polar anchoring and twist anchoring are coupling with each other and have a great influence on the saturation field. The formulae for calculating the threshold field and saturation field are given. These results will be very useful in understanding surface physics and the design of liquid crystal cells.     

Non-local density functional theory for the electronic and structural properties of semiconductors

Calculations have been carried out within the density functional formalism using an exchange-correlation functional with explicit nonlocal dependence on the charge density. The functional includes the effects of the qualitatively different screening in a semiconductor due to the gap in the spectrum. For the test cases of Si and Ge, substantial improvement is found for the minimum gap. The structural properties are in excellent agreement with experiment.     

Size effect in the optical properties of small metallic particles: A solvable model for cubic symmetry

Summary A solvable tight-binding model for the electronic states of cubic crystals of arbitrary size is used to compute the real and the imaginary part of the dielectric function of metallic particles. Two types of microscopic processes are discussed: first-order transitions between electronic states and second-order transitions due to collisions with vibrational states and impurities. Interband and intraband transitions are both taken into account fors andp bands. The results display very strong size effects due to the lack of translational symmetry and to the presence of surface states; the Lorentz-Drude behaviour of the bulk metal is approached as the number of atoms in the particles increases. The size effects in the optical functions of the metallic particles are shown to depend on the environment and this is made to account for the basic features in the optical properties, in particular for the resonance plasma absorption. The case of silver particles in a glassy environment is investigated, experimental trends of the optical absorption with varying particle sizes are explained and reasonable agreement with experimental data is obtained. Based on work supported in part by the Italian Research Council (C.N.R.) through a contract G.N.S.M. and by the French Ministry of National Education (Ministère de l'Education Nationale).     

Mixed valencies: Structure of phase diagrams,lattice properties and the consequences of electron hole symmetry

Symmetry properties and phonon phenomena of mixed valence compounds are discussed within the framework of the periodic Anderson model which was extended to include the interaction of 4f electrons with longitudinal optical phonons. The temperature anomaly in the thermal expansion found in CeSn₃ (positive thermal expansion coefficient) and YbCuAl (negative) is correctly described. Within the model the anomaly is a consequence of the particle hole symmetry of the underlying Hamiltonian. Moreover, the theory gives the positive slope of the phase boundary in the pressure-temperature phase diagram (dP/dT>0), for example for Ce, and predicts a negative slope (dP/dT<0) for Yb compounds.Furthermore, the quite unusual low temperature features of the pressure-temperature phase diagram have been calculated. It is found that the lattice vibrational contribution renormalizes the two essential parameters of the periodic Anderson model. The hybridization energyV ₀ of 4f and 5d–6s states is changed to =V ₀–a–b ² whereas the energy of the 4f stateE ₀ with respect to the 5d band becomes =E ₀–a–b ²., being proportional to the lattice constant, is determined by minimizing the Gibbs free energy, while ² is proportional to the mean square displacement of the rare earth ions. The strong temperature dependence of and ² determines the behaviour of the phase boundary and for large enough coefficientsb andb the phase boundary terminates at two critical points. An argument is given why the unusual low temperature features are more expressed in dirty mixed valence compounds as Sm_1–x Gd _x S than in the pure compound SmS. Furthermore, the theory predicts a quite unusual behaviour of the plasma-like phonon mode in the mixed valence phase: It softens at the critical temperature and at an intermediate temperature.Work performed within the program of the Sonderforschungsbereich 125, Aachen-Jülich-Köln     

Optical properties of radical cation salts derived from the TTF molecule: Evidence for electronic correlations and phase transitions in organic conductors

Following a general presentation of the organic conductors which belong to the series of radical cation salts derived from the tetrathiafulvalene molecule we analyze the characteristic charge transfer absorption bands observed in these mixed valence compounds. Using a Drude-Lorentz model we show that the energy gap present in these materials is basically of the Hubbard type. Nevertheless we observe also in specific cases additional contributions due to either a periodic lattice distortion or a counter-ion electrostatic perturbation. We conclude finally that the electronic correlations are the dominant interactions in these narrow 1d electronic band systems.     

BH~+离子基态及激发态的势能曲线和跃迁性质的研究

利用高精度的多组态相互作用及Davidson修正方法 (MRCI+Q),采用ACV5Z-DK全电子基组计算了BH~+离子的前4个离解通道B~+(~1S_g)+H(~2S_g),B~+(~3Pu)+H(~2S_g),B(~2P_u)+H~+(~1Sg)和B~+(~1Pu)+H(~2Sg)的9个Λ—S态的势能曲线.X~2Σ~+,A~2Π和B~2Σ~+态的光谱常数和已有实验值符合得很好,其中b~4Σ~+,3~2Σ~+,3~2Π和4~2Σ~+态的光谱常数为首次报道,3~2Π和4~2Σ~+态具有双势阱结构.预测了A~2Π和B~2Σ~+态的辐射寿命:τ(A~2Π)=239.2 ns和τ(B~2Σ~+)=431.2 ns.最后在考虑自旋轨道耦合效应下讨论了B~2Σ~+与A~2Π态的势能曲线的相交对激光冷却BH~+离子的影响.     

Adiabatic and non-adiabatic corrections to properties of the hydrogen molecular cation and its isotopomers: dissociation energies and bond lengths

A study is presented of adiabatic and non-adiabatic corrections to the dissociation energies and bond lengths of H⁺ ₂, D⁺ ₂ and HD⁺ in vibration-rotation levels of their ground electronic states, with particular attention to isotopic scaling. In previous work (MOSS, R. E., 1999, Molec. Phys., 97, 1) on rotationless levels, an anomalous non-adiabatic correction to the bond length was found for v = 20, N = 0 of HD⁺. Other levels close to dissociation are identified that display anomalous non-adiabatic corrections to the dissociation energies and to the bond lengths. The source of these anomalies is discussed.     

Delay aversion but preference for large and rare rewards in two choice tasks: implications for the measurement of self-control parameters

Background  

Impulsivity is defined as intolerance/aversion to waiting for reward. In intolerance-to-delay (ID) protocols, animals must choose between small/soon (SS) versus large/late (LL) rewards. In the probabilistic discount (PD) protocols, animals are faced with choice between small/sure (SS) versus large/luck-linked (LLL) rewards. It has been suggested that PD protocols also measure impulsivity, however, a clear dissociation has been reported between delay and probability discounting.     

Model for increased efficiency of CIGS solar cells by a stepped distribution of carrier density and Ga in the absorber layer

In this paper, several structures for multilayer Cu(In1-xGax) Se2 (CIGS) thin film solar cells are proposed to achieve high conversion efficiency. All of the modeling and simulations were based on the actual data of experimentally produced CIGS cells reported in the literature. In standard CIGS cells with a single absorber layer, the effects of acceptor density and Ga content on device performance were studied, and then optimized for maximum conversion efficiency. The same procedure was performed for cells with two and three sectioned CIGS absorber layers in which Cu and/or Ga contents were varied within each consecutive section. This produces an internal additional electric field within the absorber layer, which resulted in an increase in carrier collection for longer wavelength photons, and hence, improvement in the conversion efficiency of the cell. An increase of approximately 3% in efficiency is predicted for cells with two layer absorbers. For multilayer cells in which Cu and Ga distribution were stepped simultaneously, the improvement could be approximately 3.5%. This improvement is due to; enhanced carrier collection for longer-wavelength photons, and reduced recombination at the heterojunction and back regions of the cell. These results are confirmed by the physics of the cells.     

Characterization of PEI-coated superparamagnetic iron oxide nanoparticles for transfection: Size distribution,colloidal properties and DNA interaction

Superparamagnetic iron oxide nanoparticles (SPIONs) were coated with polyethylenimine. Here, we briefly describe the synthesis as well as DNA:PEI:SPION complexes and the characterization of the compounds according to their particle size, ζ-potential, morphology, DNA complexing ability, magnetic sedimentation, and colloidal stability. PEI coating of SPIONs led to colloidally stable beads even in high salt concentrations over a wide pH range. DNA plasmids and PCR products encoding for green fluorescent protein were associated with the described beads. The complexes were added to cells and exposed to permanent and pulsating magnetic fields. Presence of these magnetic fields significantly increased the transfection efficiency.     

On the magnetic and thermodynamic properties of Americium-II: A hybrid density functional theoretic study

The ground states of the actinides have been matters of considerable controversies, theory often contradicting experiment specifically in regards to the magnetic nature. To resolve this discrepancy, we present here hybrid density functional theory (HYB-DFT) based studies of the structural, magnetic, electronic, and thermodynamic properties of Americium-II. Three configurations of non-magnetic (NM), anti-ferromagnetic (AFM), and ferromagnetic (FM) with and without spin-orbit coupling (SOC) have been considered to determine the ground state of Am-II. We find that the experimental NM ground state configuration is indeed obtained for Am-II at a level of 40% Hartree-Fock (HF) exchange with SOC and the computed structural properties and the electronic density of states are in good agreement with experimental observations. We also find that HBY-DFT with NSOC fails to predict the correct magnetic and electronic structures for Am-II, indicating the importance of the inclusion of SOC for studies of strongly correlated materials. The phonon related thermodynamic properties of Am-II are presented for the NM ground state configuration and the computed heat capacity and entropy are found to be in good agreement with the experimental measurements. The lattice constant, bulk modulus, heat capacity, and entropy of AM-II are predicted to be 9.44 a.u., 21.7 GPa, , and , respectively.