A semimetal model of the normal state magnetic susceptibility and transport properties of Ba(Fe1−xCox)2As2

A simple two-band 3D model of a semimetal is constructed to determine which normal state features of the Ba(Fe_1?xCo_x)₂As₂ superconductors can be qualitatively understood within this framework. The model is able to account in a semiquantitative fashion for the measured magnetic susceptibility, Hall, and Seebeck data, and the low temperature Sommerfeld coefficient for 0 < x < 0.3 with only three parameters for all x. The purpose of the model is not to fit the data but to provide a simple starting point for thinking about the physics of these interesting materials. Although many of the static magnetic properties, such as the increase of the magnetic susceptibility with temperature, are reproduced by the model, none of the spin-fluctuation dynamics are addressed. A general conclusion from the model is that the magnetic susceptibility of most semimetals should increase with temperatures.     

Effects of nematic fluctuations on the elastic properties of iron arsenide superconductors

We demonstrate that the changes in the elastic properties of the FeAs systems, as seen in our resonant ultrasound spectroscopy data, can be naturally understood in terms of fluctuations of emerging nematic degrees of freedom. Both the softening of the lattice in the normal, tetragonal phase as well as its hardening in the superconducting phase are consistently described by our model. Our results confirm the view that structural order is induced by magnetic fluctuations.     

Crowd synchrony and quorum sensing in delay-coupled lasers

Crowd synchrony and quorum sensing arise when a large number of dynamical elements communicate with each other via a common information pool. Previous evidence has shown that this type of coupling leads to synchronization, when coupling is instantaneous and the number of coupled elements is large enough. Here we consider a situation in which the transmission of information between the system components and the coupling pool is not instantaneous. To that end, we model a system of semiconductor lasers optically coupled to a central laser with a delay. Our results show that, even though the lasers are nonidentical due to their distinct optical frequencies, zero-lag synchronization arises. By changing a system parameter, we can switch between two different types of synchronization transition. The dependence of the transition with respect to the delay-coupling parameters is studied.     

On the 2-adic valuation of the cardinality of elliptic curves over finite extensions of $$\mathbb {F}_{\varvec{q}} $$

In this paper we study the difference between the 2-adic valuations of the cardinalities \( \# E( \mathbb {F}_{q^k} ) \) and \( \# E( \mathbb {F}_q ) \) of an elliptic curve E over \( \mathbb {F}_q \). We also deduce information about the structure of the 2-Sylow subgroup \( E[ 2^\infty ]( \mathbb {F}_{q^k} ) \) from the exponents of \( E[ 2^\infty ]( \mathbb {F}_q ) \).     

Iterates and Hypoellipticity of Partial Differential Operators on Non-Quasianalytic Classes

Let P be a linear partial differential operator with constant coefficients. For a weight function ω and an open subset Ω of \mathbbR^N{\mathbb{R}^N} , the class E_P,{w}(W){\mathcal{E}_{P,\{\omega\}}(\Omega)} of Roumieu type involving the successive iterates of the operator P is considered. The completeness of this space is characterized in terms of the hypoellipticity of P. Results of Komatsu and Newberger-Zielezny are extended. Moreover, for weights ω satisfying a certain growth condition, this class coincides with a class of ultradifferentiable functions if and only if P is elliptic. These results remain true in the Beurling case E_P,(w)(W){\mathcal{E}_{P,(\omega)}(\Omega)}.     

流动中边界面的识别

将流场的边界面定义为流动表面,在该表面上剪切率为0,并利用所建议的程序求解.该方法是基于速度向量场的计算,与坐标系的选择无关.     

Synthesis and characterization of long perylenediimide polymer fibers: from bulk to the single-molecule level

The synthesis and characterization of perylenediimide polyisocyanides is reported. In addition to short oligomers, our synthetic approach results in the formation of extremely long, well-defined, and rigid perylenediimide polymers. Ordering and close-packing of the chromophores in these long polymers is guaranteed by attachment to a polyisocyanide backbone with amino acid side chains. Hydrogen bonding interactions between those groups stabilize and rigidify the helical polymer structure. The rodlike nature of the synthesized long perylenediimide pendant polyisocyanides as well as the helical arrangement of the chromophores is demonstrated by means of atomic force microscopy. Remarkably, polymer fibers up to 1 mum in length have been visualized, containing several thousands of perylenediimide molecules. Circular dichroism spectroscopy reveals the chiral organization of the chromophore units in the polymer, whereas absorption and emission measurements prove the occurrence of excited-state interactions between those moieties due to the close packing of the chromophore groups. However, an intricate optical behavior is encountered in bulk as a result of the coexistence of short oligomers and long polymers of perylenediimide, a situation subsequently uncovered by means of single-molecule experiments. Individual long helical perylenediimide polyisocyanides exhibit a typical red-shifted fluorescence spectrum, which, together with depolarized emission continuously decreasing in time, demonstrate that fluorescence arises from multiple excimer-like species in the polymer. Upon continuous irradiation of these long polymers, a fast decay in fluorescence lifetime is observed, a situation explained by photoinduced creation of quenching sites. Radical/ion formation by intramolecular electron transfer between close-by perylenediimide moieties is the most probable mechanism for this process. Appropriate control of the electron-transfer process might open the possibility of applying these polymers as perylenediimide-based supramolecular nanowires.     

Conformational features of an actuator containing calix[4]arene and thiophene: a molecular dynamics study

Molecular dynamics simulations have been performed for poly(calix[4]arene bis(bithiophene)) in dichloromethane solution. This material responds to its electronic structure variations with significant conformational changes, producing contraction-expansion movements. Simulations have been performed for the three states of this molecular actuator (reduced, oxidized-nondeprotonated, and oxidized-deprotonated), a specific force-field being developed for each case. Results, which are fully consistent with previous ab initio quantum mechanical calculations on an isolated actuating unit, have revealed important findings about the dynamics of the system. Analyses of the flexibility/rigidity of the molecular chain with the state, the interaction of the polymer with the solvent molecules and the influence of environmental factors (as the viscosity of solvent, the counterions and the thermal agitation) on the dynamics have provided important insights to the actuation mechanism.