Nuclear moments and optical isotope shift of radioactive 133Ba

Mass separated radioactive ¹³³Ba has been investigated by means of optical spectroscopy. From hyperfine structure and isotope shift we obtain the nuclear magnetic moment μ_I = ?0.769(3) μ_n and the change of the nuclear mean square charge radius $\text{δ〈r}{}^2\text{〉 (}{}^{133}\text{Ba}\text{?}{}^{132}\text{Ba}\text{) = ?0.017(3)}\text{fm}{}^2$.     

Total synthesis of (-)-ratjadone

[reaction in text] A convergent asymmetric synthesis of (-)-ratjadone (1) has been achieved, confirming the assignments of stereochemistry for the naturally occurring antifungal metabolite.     

Cationic Symmetrically and Unsymmetrically Substituted Diboranes and Bis(diboranes) with Direct Boron-Boron Bond: Synthesis by Substitution,Stability and Properties

Starting with diboranes with two electron-rich bridging bicyclic guanidinate substituents, we report in this work the rational synthesis of new dicationic symmetrically- and unsymmetrically-substituted diboranes in S_N1-type substitution reactions in which triflato or bromo substituents are replaced by neutral Lewis bases. The scope of such substitution reactions and their rate are analyzed with different pyridine derivatives of variable Lewis basicity. The first substitution step, leading to a monocationic diborane with one anionic substituent (triflate or bromide) and one neutral Lewis base, proceeds much faster than the second substitution step leading to a dicationic diborane with two neutral Lewis bases. The different time scales for the substitution steps could be used to conveniently synthesize in one-pot reactions several dicationic, unsymmetrically-substituted diboranes with two different neutral Lewis bases.     

Tandem photocycloaddition-retro-mannich fragmentation of enaminones. A route to spiropyrrolines and the tetracyclic core of koumine

[reaction: see text] Intramolecular [2 + 2] photocycloaddition of beta-aminoalkylidene malonates gives transiently a cyclobutane which undergoes retro-Mannich fragmentation to a Delta(1)-pyrroline. The tandem sequence, exemplified in two series based on tryptamine and aminoethyl-1,4-cyclohexadiene, leads to a spiroindolopyrroline skeleton and to the nonindolenine portion of koumine.     

Dynamical Spin Susceptibility in the <Emphasis Type="Italic">t-J</Emphasis> Model: The Memory Function Method

Based on the method of the equations of motion for the relaxation function in terms of Hubbard operators, we evaluate the dynamical spin susceptibility for the t-J model in the paramagnetic phase. Using a Mori-type projection technique, we express the relaxation function in terms of the second-order memory function, which is evaluated in the approximation of coupled modes for hole excitations and spin fluctuations in the fourth order in the hopping parameter t and the exchange interaction J. __________ Translated from Teoreticheskaya i Matematicheskaya Fizika, Vol. 145, No. 2, pp. 240–255, November, 2005.     

Discussion on Ohta et al., “Traveling bands in self-propelled soft particles”

A discussion on the contribution of Ohta and Yamanaka [1] in this special issue, supplemented by new agent-based simulations of band collisions within the standard Vicsek-model.     

Magnetic susceptibility and short-range order in iron pnictides: Anisotropic J1-J2 Heisenberg model

The electron spin relaxation times by piezoelectric and polar optical phonon scattering in GaAs are calculated using the formula derived from the projection-reduction method. The temperature, magnetic field, and electron density dependences of the relaxation time are investigated. The electrons are found to be scattered mostly by piezoelectric phonons at low temperatures and polar optical phonons at high temperatures. The electron density affects the magnetic field dependence of the relaxation time at low temperatures but have only slight affects at high temperatures.     

Spin excitations and thermodynamics of the <Emphasis Type="Italic">t</Emphasis>-<Emphasis Type="Italic">J</Emphasis> model on the honeycomb lattice

We present a spin-rotation-invariant Green-function theory for the dynamic spin susceptibility in the spin-1/2 antiferromagnetic t-J Heisenberg model on the honeycomb lattice. Employing a generalized mean-field approximation for arbitrary temperatures and hole dopings, the electronic spectrum of excitations, the spin-excitation spectrum and thermodynamic quantities (two-spin correlation functions, staggered magnetization, magnetic susceptibility, correlation length) are calculated by solving a coupled system of self-consistency equations for the correlation functions. The temperature and doping dependence of the magnetic (uniform static) susceptibility is ascribed to antiferromagnetic short-range order. Our results on the doping dependencies of the magnetization and susceptibility are analyzed in comparison with previous results for the t-J model on the square lattice.     

Towards a quantitative kinetic theory of polar active matter

A recent kinetic approach for Vicsek-like models of active particles is reviewed. The theory is based on an exact Chapman- Kolmogorov equation in phase space. It can handle discrete time dynamics and “exotic” multi-particle interactions. A nonlocal mean-field theory for the one-particle distribution function is obtained by assuming molecular chaos. The Boltzmann approach of Bertin, et al., Phys. Rev. E 74, 022101 (2006) and J. Phys. A 42, 445001 (2009), is critically assessed and compared to the current approach. In Boltzmann theory, a collision starts when two particles enter each others action spheres and is finished when their distance exceeds the interaction radius. The average duration of such a collision, τ₀, is measured for the Vicsek model with continuous time-evolution. If the noise is chosen to be close to the flocking threshold, the average time between collisions is found to be roughly equal to τ₀ at low densities. Thus, the continuous-time Vicsek-model near the flocking threshold cannot be accurately described by a Boltzmann equation, even at very small density because collisions take so long that typically other particles join in, rendering Boltzmann’s binary collision assumption invalid. Hydrodynamic equations for the phase space approach are derived by means of a Chapman-Enskog expansion. The equations are compared to the Toner-Tu theory of polar active matter. New terms, absent in the Toner-Tu theory, are highlighted. Convergence problems of Chapman-Enskog and similar gradient expansions are discussed.