Quadrupolar Arrangement Research Articles

Temperature dependence of screw dislocation mobility on shuffle-set of silicon

Large-scale atomistic simulations are performed in order to observe local behaviors of screw dislocations located on the shuffle set of (111) in single crystal silicon, focusing on the propagation process of the screw dislocations. A quadrupolar arrangement of screw dislocations is utilized to impose the periodic boundary conditions along each of the three spatial directions. With the aid of molecular dynamics simulations, the dislocation mobility is investigated in terms of the critical resolved shear stress. Based on the results from the simulations, we discuss effects of the model size and temperature on the critical resolved shear stress. After choosing the proper model size to reduce undesirable interference between the dislocations, we further estimate the Peierls stress by fitting from a set of the critical resolved shear stresses at various temperatures. Meanwhile, we observe a double kink mechanism in the dislocation propagation which is the most energetically favorable dislocation movement in silicon. We investigate the formation and migration of kink pairs on an undissociated screw dislocation in silicon.

Quadrupolar D-π-A-π-D chromophores with central tetrafluorobenzene acceptor and two peripheral N,N-dimethylamino and methoxy donors

1,2,4,5-Tetrafluorobenzene has been utilized as suitable central acceptor moiety in push–pull chromophores having D-π-A-π-D quadrupolar arrangement. Starting from commercially available 1,4-diiodotetrafluorobenzene, nine novel chromophores with systematically extended π-system were synthesized via cross-coupling reactions. Further electronic tuning has been achieved by variation of the appended donor (N,N-dimethylamino and methoxy groups). Target chromophores were further investigated by X-ray analysis, electrochemical measurements, absorption and emission spectra and theoretical calculations and structure–property relationships were elucidated. Whereas target chromophores showed weak second-order nonlinear responses with β of 0.18–6.09×10−30esu, which is primarily given by their centrosymmetric arrangement, third-order polarizabilities γ of 0.84–22.5×10−27esu notably exceeded the standard Disperse Red 1 (γ=3.47×10−27esu).

Atomistic simulation of single kinks of screw dislocations in α-Fe

We have studied the structure and the formation and migration energies of single kinks in ½〈1 1 1〉 screw dislocations in body-centered cubic iron, by performing static calculations using the Ackland–Mendelev empirical potential, which correctly accounts for the non-degenerate core structure. The methodology for constructing simulation cells with fully periodic boundary conditions based on the quadrupolar arrangement of dislocation dipoles, with a single kink on each dislocation line is presented. The two types of kinks – left and right – are found to have similar widths, namely ∼20 Burgers vectors. The convergences of the formation energies with cell-size along the dislocation line, as well as with the distance between the two dislocations are investigated. A dependence proportional to the inverse of the distance between kinks along the dislocation line is found when kinks overlap. The formation energies of the left and right kinks are significantly different: 0.57 and 0.08 eV, respectively. The Peierls potentials of the second kind are evaluated with the drag method: the energy barriers are found to be lower than 0.1 meV for both kinks.

Bottom-Up Engineering of Peptide Cell Translocators Based on Environmentally Modulated Quadrupole Switches

Designing water-soluble peptides that camouflage their polarity to cross an anhydrous phase may significantly impact drug delivery. We engineered neutrally charged peptides endowed with a conformational switch that enables them to solubilize in both water and lipid. These peptides are capable of translocating without resorting to active internalization mechanisms. Lipid solubility is induced by a quadrupolar arrangement. Our passive translocation motif possesses the highest efficiency and is derived from cellular prions.

Colloidal inclusions in smectic films with spontaneous bend

Inclusions in free-standing smectic films are simple model systems for two-dimensional anisotropic dispersions. From theory and experiment, different topologies of elastic distortions of the embedding liquid crystal are known. Quadrupolar and different dipolar defect configurations in the vicinity of the inclusion are possible, and these configurations determine the type of interactions between the inclusions. The quadrupolar configuration is often energetically preferred. We show, however, that dipolar director configurations around inclusions can be energetically favourable over quadrupolar arrangements in chiral smectics, as a consequence of a spontaneous-bend term in the elastic-energy formulation. As the inclusion size influences the selection of the deformation types, the corresponding spontaneous-bend constant can be estimated for the strong anchoring limit if the c -director fields around inclusions of different diameters are taken into account.

Influence of contact distance on the appearance of vacuum arcs with different AMF contacts

Measurements with vacuum interrupters equipped with bipolar axial magnetic field (AMF) contacts have shown that the post-arc current magnitude and its duration fall with shortened arcing time, and decreasing gap distance, respectively. Simulations of post-arc currents point out that this dependency may result either from different arc diameters or different ion densities around current zero. To clarify this, high-speed movies of vacuum arcs up to 60 kA root mean square were taken, using contacts with a bipolar and quadrupolar axial magnetic field structure. The gap distance was varied by delayed contact opening. Moreover, switching experiments with sealed experimental interrupters under 36 kV conditions with a bipolar AMF arrangement in comparison with a quadrupolar arrangement showed that the bipolar contacts yielded higher interruption capability, in correlation with lower post-arc currents. In order to study the reasons for these phenomena, optical investigations of the arc appearance were carried out under variation of the relevant parameters.

Current zero behavior of vacuum interrupters with bipolar and quadrupolar AMF contacts

Axial magnetic field (AMF) contacts are applied in vacuum circuit breakers to interrupt high short-circuit currents. A magnetic field parallel to the current flow in the arc column improves the breaking capacity by affecting the arc mode. Depending on the design of the contacts, this magnetic field can take different directions over the contact surface. In this paper, measurements of bipolar and quadrupolar AMF contact systems are presented. For bipolar arrangements, the polarity of the field changes once and for quadrupolar arrangements twice. Due to different eddy current paths, the residual magnetic field at current zero is significantly higher for the quadrupolar arrangement. The breaking capability of two contact systems was investigated and completed by measuring the post arc current.

Improved sensitivity of a four-rod capacitor in the study of quadrupolar orientational glasses

The use of a four-rod capacitor to study relaxation effects in quadrupolar orientational glasses is discussed. It is proposed that by altering the signs of the potentials applied to the four-rod capacitor from the standard quadrupolar arrangement, the sensitivity of the experiment may be increased. The study of dielectric relaxation in the microwave region using a conventional parallel-plate capacitor is suggested as a means of studying the vibrations of Ar atoms in the immediate vicinity of N2 molecules in a glassy mixture of N2 and Ar.

Symmetry of Quadrupolar Arrangements in Crystals

We carry out a new group-theoretical classification of all possible static quadrupolar arrangements in a crystal: We define normal quadrupolar configurations and classify them according to the irreducible representations of the space group of the crystal. This method is a generalization of the representation analysis of magnetic structures, and is equivalent to the method recently proposed by Felsteiner, Litvin, and Zak.