Charge rearrangement in magnetite: from magnetic field induced easy axis switching to femtoseconds electronic processes

Magnetite is the oldest magnet and the first material where the concept of a strong correlations driven metal–insulator transition was suggested and found at T_V = 124 K in the so-called Verwey phase transformation. Recently, the structure below T_V was solved revealing subtle electronic structure in the form of trimeron lattice that, according to yet another recent communication, may be switched within femtosecond range. In this review article, we argue that the same change of trimeron lattice can be achieved by a magnetic field, in the phenomenon called the easy axis switching. The results of many of our experiments show that although this process is best viewed by magnetization studies, it is also reflected in magnetostriction, causes some changes in electronic transport and can be observed microscopically by NMR that proved electronic order alteration. All those facts suggest that the axis switching process observed and studied by us is intimately linked with the fast change of electronic trimeron order mentioned above.     

基于投影的多元正态性相关与回归检验

考虑多元分布的正态性检验问题,用投影寻踪方法,给出了两个检验统计量,模拟计算也表明所提出的检验统计量具有良好的功效.     

相依索赔Poisson风险模型的Cramer-Lundberg逼近(英文)

本文考虑一类具有相依索赔的Poisson风险模型.利用无穷小方法,得到了破产概率的Cramer-Lundberg逼近及其精确表达式.     

GW方法: 基本原理, 最新进展及其在d-和f-电子体系中的应用

基于格林函数的多体微扰理论提供了描述材料基态和激发态性质的一个严格理论框架. 格林函数依赖于交换关联自能, 后者满足一组复杂的被称为Hedin方程的积分微分方程. GW方法是由对自能算符根据屏蔽库仑作用做多体微扰理论展开到第一项得到, 是目前描述扩展体系准粒子电子激发性质最为准确的第一原理方法. 本文概述了GW方法的基本原理, 并对最新的理论方法进展在一个统一的框架下进行了评述. 最后, 通过对若干典型实例的分析展示了针对d/f-电子体系的GW方法的现状.     

Interpolating between random walk and rotor walk

We introduce a family of stochastic processes on the integers, depending on a parameter and interpolating between the deterministic rotor walk () and the simple random walk (). This p‐rotor walk is not a Markov chain but it has a local Markov property: for each the sequence of successive exits from is a Markov chain. The main result of this paper identifies the scaling limit of the p‐rotor walk with two‐sided i.i.d. initial rotors. The limiting process takes the form , where is a doubly perturbed Brownian motion, that is, it satisfies the implicit equation (1) for all . Here is a standard Brownian motion and are constants depending on the marginals of the initial rotors on and respectively. Chaumont and Doney have shown that Equation 1 has a pathwise unique solution , and that the solution is almost surely continuous and adapted to the natural filtration of the Brownian motion. Moreover, and . This last result, together with the main result of this paper, implies that the p‐rotor walk is recurrent for any two‐sided i.i.d. initial rotors and any .     

The conditioning of least‐squares problems in variational data assimilation

In variational data assimilation a least‐squares objective function is minimised to obtain the most likely state of a dynamical system. This objective function combines observation and prior (or background) data weighted by their respective error statistics. In numerical weather prediction, data assimilation is used to estimate the current atmospheric state, which then serves as an initial condition for a forecast. New developments in the treatment of observation uncertainties have recently been shown to cause convergence problems for this least‐squares minimisation. This is important for operational numerical weather prediction centres due to the time constraints of producing regular forecasts. The condition number of the Hessian of the objective function can be used as a proxy to investigate the speed of convergence of the least‐squares minimisation. In this paper we develop novel theoretical bounds on the condition number of the Hessian. These new bounds depend on the minimum eigenvalue of the observation error covariance matrix and the ratio of background error variance to observation error variance. Numerical tests in a linear setting show that the location of observation measurements has an important effect on the condition number of the Hessian. We identify that the conditioning of the problem is related to the complex interactions between observation error covariance and background error covariance matrices. Increased understanding of the role of each constituent matrix in the conditioning of the Hessian will prove useful for informing the choice of correlated observation error covariance matrix and observation location, particularly for practical applications.     

Infrared study of pressure-induced insulator to metal transitions in vanadium oxide compounds at the SISSI@Elettra beamline

Vanadium oxides display insulator to metal transitions with conductivity jumps up to seven orders of magnitude which makes them very attractive both for technological applications and for the several fundamental issues they raise. We present here an optical study of pressure-induced insulator to metal transitions in VO₂ and V₃O₅, performed at the infrared synchrotron source SISSI@Elettra.     

Small polaron formation in many-particle states of the Hubbard-Holstein model: The one-dimensional case

We investigate polaron formation in a many-electron system in the presence of a local repulsion sufficiently strong to prevent local-bipolaron formation. Specifically, we consider a Hubbard-Holstein model of interacting electrons coupled to dispersionless phonons of frequency . Numerically solving the model in a small one-dimensional cluster, we find that in the nearly adiabatic case , the necessary and sufficient condition for the polaronic regime to occur is that the energy gain in the atomic (i.e., extremely localized) regime overcomes the energy of the purely electronic system . In the antiadiabatic case, , polaron formation is instead driven by the condition of a large ionic displacement (g being the electron-phonon coupling). Dynamical properties of the model in the weak and moderately strong coupling regimes are also analyzed. Received 15 February 1999     

Exponentially correlated Hylleraas-configuration interaction studies of atomic systems. II. Non-relativistic energies of the 1 through 6 states of the Li+ ion

The exponentially correlated Hylleraas-configuration interaction wave function (E-Hy-CI) is a generalization of the Hylleraas-configuration interaction (Hy-CI) in which the single of an Hy-CI wave function is generalized to the generic type . This type of correlation has the right behavior both in the vicinity of the cusp and as goes to infinity; this work shows that wave functions containing both linear and exponential factors converge more rapidly than either one alone for low-lying excited states of ¹S symmetry. E-Hy-CI variational calculations with up to 8568 configurations lead to a nonrelativistic energy of ?7.2799 1341 2669 3059 6491 6759 hartree for the 1 ¹S ground state of the Li+ ion.