Dipole

About: Dipole is a research topic. Over the lifetime, 52856 publications have been published within this topic receiving 1100945 citations.

Polarizable Atomic Multipole Water Model for Molecular Mechanics Simulation

Abstract: A new classical empirical potential is proposed for water. The model uses a polarizable atomic multipole description of electrostatic interactions. Multipoles through the quadrupole are assigned to each atomic center based on a distributed multipole analysis (DMA) derived from large basis set molecular orbital calculations on the water monomer. Polarization is treated via self-consistent induced atomic dipoles. A modified version of Thole's interaction model is used to damp induction at short range. Repulsion−dispersion (vdW) effects are computed from a buffered 14−7 potential. In a departure from most current water potentials, we find that significant vdW parameters are necessary on hydrogen as well as oxygen. The new potential is fully flexible and has been tested versus a variety of experimental data and quantum calculations for small clusters, liquid water, and ice. Overall, excellent agreement with experimental and high level ab initio results is obtained for numerous properties, including cluster st...

Spatial variation of currents and fields due to localized scatterers in metallic conduction

Abstract: Localized scatterers can be expected to give rise to spatial variations in the electric field and in the current distribution. The transport equation allowing for spatial variations is solved by first considering the homogeneous transport equation which omits electric fields. The homogeneous solution gives the purely diffusive motion of current carriers and involves large space charges. The electric field is then found, and approximate space charge neutrality is restored, by adding a particular solution of the transport equation in which the electric field is associated only with space charge but not with a current. The presence of point scatterers leads to a dipole field about each scatterer. The spatial average of a number of these dipole fields is the same as that obtained by the usual approach which does not explicitly consider the spatial variation. Infinite plane obstacles with a reflection coefficient r are also considered. These produce a resistance proportional to r/(1-r).

Optical Dipole Traps for Neutral Atoms

Abstract: Publisher Summary This chapter discusses optical dipole traps for neutral atoms Methods for storage and trapping of charged and neutral particles have very often served as the experimental key to great scientific advances, covering physics in the vast energy range from elementary particles to ultracold atomic quantum matter It describes the basic physics of dipole trapping in fardetuned light, the typical experimental techniques and procedures, and the different trap types currently available, along with their specific features In the experiments discussed, optical dipole traps have already shown great promise for a variety of different applications Of particular importance is the trapping of atoms in the absolute internal ground state, which cannot be trapped magnetically In this state, inelastic binary collisions are completely suppressed for energetic reasons In this respect, an ultracold cesium gas represents a particularly interesting situation, because Bose–Einstein condensation seems attainable only for the absolute ground state Therefore, an optical trap may be the only way to realize a quantum-degenerate gas of Cs atoms Further, optical dipole traps can be seen as storage devices at the low end of the presently explorable energy scale Future experiments exploiting the particular advantages of these traps can reveal interesting new phenomena

Magnetic monopoles in spin ice

Abstract: We are familiar with elementary particles that carry either negative or positive electric charge, such as electrons and protons, but there is no evidence of elementary particles with a net magnetic charge. Magnets tend to come with inseparable north and south poles, and there are no known magnetic monopoles despite concerted efforts to find them. But an intriguing theoretical study now proposes that magnetic monopoles may exist, not as elementary particles, but as emergent particles in exotic condensed matter magnetic systems such as 'spin ice'. The theory, based on an analogy to fractional electric charges seen, for example, in quantum Hall systems in two dimensions, can explain a mysterious phase transition that has been observed experimentally in spin ice. The cover, by Alessandro Canossa, depicts a magnetic monopole (red sphere) emerging from break-up of the dipole moment (arrows) of the underlying electronic degrees of freedom in spin ice. A theoretical study proposes that magnetic monopoles may appear not as elementary but as emergent particles in complex, strongly-correlated magnetic systems such as spin ice, in analogy to fractional electric charges in quantum Hall systems. This theory explains a mysterious phase transition in spin ice that has been observed experimentally. Electrically charged particles, such as the electron, are ubiquitous. In contrast, no elementary particles with a net magnetic charge have ever been observed, despite intensive and prolonged searches (see ref. 1 for example). We pursue an alternative strategy, namely that of realizing them not as elementary but rather as emergent particles—that is, as manifestations of the correlations present in a strongly interacting many-body system. The most prominent examples of emergent quasiparticles are the ones with fractional electric charge e/3 in quantum Hall physics2. Here we propose that magnetic monopoles emerge in a class of exotic magnets known collectively as spin ice3,4,5: the dipole moment of the underlying electronic degrees of freedom fractionalises into monopoles. This would account for a mysterious phase transition observed experimentally in spin ice in a magnetic field6,7, which is a liquid–gas transition of the magnetic monopoles. These monopoles can also be detected by other means, for example, in an experiment modelled after the Stanford magnetic monopole search8.

First-principles study of metal adatom adsorption on graphene

Abstract: The adsorption of 12 different metal adatoms on graphene is studied using first-principles density-functional theory with the generalized gradient approximation. The adsorption energy, geometry, density of states (DOS), dipole moment, and work function of each adatom-graphene system are calculated. For the adatoms studied from groups I--III of the Periodic Table, the results are consistent with ionic bonding, and the adsorption is characterized by minimal change in the graphene electronic states and large charge transfer. For transition, noble, and group IV metals, the calculations are consistent with covalent bonding, and the adsorption is characterized by strong hybridization between adatom and graphene electronic states. For ionically bonded adatoms, the charge transfer is calculated quantitatively using two methods, one based on the DOS and the other based on the real-space-charge density. A variation in dipole moments and work-function shifts across the different adatoms is observed. In particular, the work-function shift shows a general correlation with the induced interfacial dipole of the adatom-graphene system and the ionization potential of the isolated atom.