Transitions between secondary structures in isolated polyalanines

Monte Carlo simulations of gas-phase polyalanine peptides have been carried out with the Amber ff96 force field. A low-temperature structural transition takes place between the α-helix stable conformation and β-sheet structures, followed by the unfolding phase change. The transition state ensembles connecting the helix and sheet conformations are investigated by sampling the energy landscape along specific geometric order parameters as putative reaction coordinates, namely the electric dipole μ, the end-to-end distance d, and the gyration radius R_g. By performing series of shooting trajectories, the committor probabilities and their distributions are obtained, revealing that only the electric dipole provides a satisfactory transition coordinate for the α↔β interconversion. The nucleus at the transition is found to have a high helical content.     

Open-system density matrix description of an STM-driven atomic switch: H on Si(100)

Previous experiments indicate that an STM (scanning tunnelling microscope) can be used to switch a hydrogen atom at a partially hydrogen-covered Si(100)-2×1 surface, from one Si atom of a Si dimer to a neighbouring, empty Si site [U.J. Quaade et al., Surf. Sci. 415, L1037, 1998]. It has been suggested that the switching occurs via a transient positive ion resonance state. In an earlier paper, we have examined the switching process for the “above threshold” regime when the bias is large enough to directly populate the positive ion resonance. In the present paper we study the “below threshold” regime instead, where the switching is more appropriately modelled as a ladder climbing over the barrier, in the ground electronic state. For this purpose we solve the Liouville–von Neumann equation in Lindblad form, describing a switching H atom on a Si dimer. STM-induced transition rates between vibrational levels are estimated from cluster calculations, assuming contributions both from a dipole and a resonance scattering mechanism. Vibrational relaxation is also included, as well as finite temperature and field effects. The switching rate in a current regime of about 1 to 10 nA scales highly non-linearly with current, and it is found to be governed by vibrational “ladder climbing” and subsequent tunnelling through the top of the ground state barrier. Multi-phonon processes also play a role. As a result of tunnelling, pronounced isotope effects are observed when replacing H with D. It is further argued that resonance-mediated inelastic scattering dominates over dipole excitation, and that the STM switch is stable also at room temperature.     

Dipoles and band alignment for benzene/Au(111) and C<Subscript>60</Subscript>/Au(111) interfaces

A local orbital DFT-approach combined with a “scissor”-operator is used to obtain the Charge Neutrality Level and the screening parameter in the benzene/Au(111) and C₆₀/Au(111) interfaces. The “pillow” dipole and interface Fermi level are also calculated. The total dipole induced across the interface is compared with the experimental evidences: while the agreement for C₆₀/Au(111) is excellent, for benzene/Au(111), some discrepancies appear that are discussed in the light of other models.     

Director field configurations around a spherical particle in a nematic liquid crystal

We study the director field around a spherical particle immersed in a uniformly aligned nematic liquid crystal and assume that the molecules prefer a homeotropic orientation at the surface of the particle. Three structures are possible: a dipole, a Saturn-ring, and a surface-ring configuration, which we investigate by numerically minimizing the Frank free energy supplemented by a magnetic-field and a surface term. In the dipole configuration, which is the absolutely stable structure for micron-size particles and sufficiently strong surface anchoring, a twist transition is found and analyzed. We show that a transition from the dipole to the Saturn ring configuration is induced by either decreasing the particle size or by applying a magnetic field. The effect of metastability and the occurrence of hysteresis in connection with a magnetic field are discussed. The surface-ring configuration appears when the surface-anchoring strength W is reduced. It is also favored by a large saddle-splay constant K₂₄. A comparison with recent experiments [#!itapdb:Poulin1997!#,#!itapdb:Poulin1998!#] gives a lower bound for W, i.e., for the interface of water and pentylcyanobiphenyl (5CB) in the presence of the surfactant sodium dodecyl sulfate. Received 2 November 1998     

Upconversion due to energy transfer involving Pr<Superscript>3+</Superscript> ions in pairs

Upconversion luminescence in triply ionized praseodymium-doped TeO₂–Li₂O glass using excitation at ∼590 nm into the ¹D₂ level from a dye laser pumped with the second harmonic of a pulsed Nd:YAG laser has been reported. The mechanism involved in the upconversion emission observed at ∼480 nm indicates that the most important contribution is energy transfer among praseodymium ions in pairs followed by the dipole–dipole interaction. The rate-equation model for the emission at ∼480 nm that provides direct information to determine the energy-transfer rates containing the pair of states involved in the upconversion process has been explored.     

Landau analog levels for dipoles in non-commutative space and phase space

We investigate the analog of Landau quantization, for a neutral polarized particle in the presence of homogeneous electric and magnetic external fields, in the context of non-commutative quantum mechanics. This particle, possessing electric and magnetic dipole moments, interacts with the fields via the Aharonov–Casher and He–McKellar–Wilkens effects. For this model we obtain the Landau energy spectrum and the radial eigenfunctions of the non-commutative space coordinates and non-commutative phase space coordinates. Also we show that the case of non-commutative phase space can be treated as a special case of the usual non-commutative space coordinates.     

Probing for new physics and detecting non-linear vacuum QED effects using gravitational wave interferometer antennas

Low energy non-linear QED effects in vacuum have been predicted since 1936 and have been subject of research for many decades. Two main schemes have been proposed for such a ‘first’ detection: measurements of ellipticity acquired by a linearly polarized beam of light passing through a magnetic field and direct light–light scattering. The study of the propagation of light through an external field can also be used to probe for new physics such as the existence of axion-like particles and millicharged particles. Their existence in nature would cause the index of refraction of vacuum to be different from unity in the presence of an external field and dependent of the polarization direction of the propagating light. The major achievement of reaching the project sensitivities in gravitational wave interferometers such as LIGO and VIRGO has opened the possibility of using such instruments for the detection of QED corrections in electrodynamics and for probing new physics at very low energies. We show that it is possible to distinguish between various scenarios of new physics in the hypothetical case of detecting unexpected values. Considering the design sensitivity in the strain of the near future VIRGO+ interferometer leads to a variable dipole magnet configuration such that B ² D≥13000 T² m  for a ‘first’ vacuum non-linear QED detection.     

Feasibility of narrow-line cooling in optical dipole traps

We have investigated the influence of narrow-line laser cooling on the loading of Ca atoms into optical dipole traps. To describe the narrow-line cooling of alkaline-earth atoms in combination with optical dipole trapping, we have developed a model that takes into account the light shifts of the cooling transition in three dimensions. The model is compared with two experimental realizations of optical dipole traps for calcium at the wavelengths 514 nm and 10.6 μm.     

Collisions between linear polar molecules trapped in a microwave field

The collisions between linear polar molecules, trapped in a microwave field with circular polarization, are theoretically analyzed. We demonstrate that the collisional dynamics is mostly controlled by two ratios ν/B and x=μ₀E₀/ħ B (ν is the microwave frequency, B is the molecular rotational constant, μ₀ is the dipole moment, and E₀ is the electric field amplitude). We discuss the dependence of collision cross sections on these ratios in order to find an advantageous condition for evaporative cooling.     

Scattering of a surface plasmon polariton beam by chains of dipole nanoparticles

Scattering and splitting of surface plasmon polaritons (SPPs) by a chain of strongly interacting nanoparticles located near a metal surface are numerically studied. The applied numerical model is based on the Green’s function formalism and point–dipole approximation for scattering by nanoparticles. Dependencies of the splitting efficiency on the inter-particle distance in the chain and on the angle of incidence of the SPP Gaussian beam are considered. It is found that the splitting efficiency depends on the inter-particle distances especially when the angle between the SPP beam and the chain is relatively small. The role of multiple scattering in the SPP splitting by the chains of nanoparticles is also discussed.     

Dipole squeezing in the multi-photon Jaynes-Cummings model with and without the rotating-wave approximation

By virtue of the numerical method of Graham and Höhnerbach [12], we have investigated the quantum fluctuations of the atomic dipole variables in the multiphoton Jaynes-Cummings model with and without the rotating wave approximation (RWA) when it is restricted to the following initial condition: the atom in its superposition state and the field in the vacuum state. We found that even under the conditions in which the RWA is considered to be valid there are significant effects of virtual-photon field on dipole squeezing predicted in the RWA, and dipole squeezing turns to disappear for sufficiently strong coupling.     

Fractional Aharonov-Bohm oscillation of a two-layer ring with two electrons

When a circular ring suffers a special topological transformation, it becomes a two-layer ring. Due to the special topology of the two-layer ring, orbital angular momenta are allowed to be a half-integer. This would affect the traditional Aharonov-Bohm oscillation (ABO). In this paper, the fractional ABO (FABO) of the ground state energy, persistent current and dipole transition of a two-layer ring with two electrons has been studied. Collective and internal coordinates (θ_C, ϕ) have been introduced. Based on them, a very simple formula for the current has been obtained, the symmetry constraint imposed on the dipole transition has been clarified and a strict relation between the photon energies of the dipole radiation and the persistent current of the ground state has been found. Comparing with the one-layer ring, the period of the fractional ABO of the two-layer ring becomes much shorter.     

Magnetic dipole,electric quadrupole and magnetic octupole moments of the <Emphasis Type="Italic">Δ</Emphasis> baryons in light cone QCD sum rules

Due to the very short life time of the Δ baryons, a direct measurement on the electromagnetic moments of these systems is almost impossible in the experiment and can only be done indirectly. Although only for the magnetic dipole moments of Δ ⁺⁺ and Δ ⁺ systems there are some experimental data, the theoretical, phenomenological and lattice calculations could play crucial role. In the present work, the magnetic dipole (μ _Δ ), electric quadrupole (Q _Δ ) and magnetic octupole (O _Δ ) moments of these baryons are computed within the light cone QCD sum rules. The results are compared with the predictions of the other phenomenological approaches, lattice QCD and existing experimental data.     

Analytic solutions to Maxwell–London equations and levitation force for a general magnetic source in the presence of a long type-II superconducting cylinder

The interaction between a general magnetic source and a long type-II superconducting cylinder in the Meissner or mixed state is studied within the London theory. We first study the Meissner state and solve the Maxwell–London equations when the source is a magnetic monopole located at an arbitrary position. Then the field and supercurrent for a more complicated magnetic charge distribution can be obtained by superposition. A magnetic point dipole with arbitrary direction is studied in detail. It turns out that the levitation force on the dipole contains in general an angular as well as a radial component. By integration we obtain the field and supercurrent when the source is a two-dimensional monopole (a magnetically charged long thread along the axial direction), from which the results for a two-dimensional point dipole easily follow. In the latter case the levitation force points in the radial direction regardless of the orientation of the dipole. The case for a current carrying long straight wire parallel to the cylindrical axis is solved separately. The limit of ideal Meissner state is discussed in most cases. The case of mixed state is discussed briefly. It turns out that vortex lines along the axial direction and vortex rings concentric with the cylinder have no effect outside the cylinder and the levitation forces remain the same as in the case of the Meissner state.     

Highly monochromatic fluorescence via simulation of vacuum-induced coherence

It's shown that the incoherent fluorescence spectrum from a three-level Λ atom with orthogonal atomic transition dipole moments is comprised of a single ultrasharp line by simulating the vacuum-induced coherence (VIC), and the line is on resonance with the driving field. The physical interpretation of the spectral characteristics is given in terms of dressed states.     

Heavy nuclei confinement effect in a pulsed light field

It is a model theoretical work of the applied character in which: “Outside the framework of the dipole approximation (with an accuracy of about v/c) the effective interaction force between stripped uranium nuclei in the presence pulsed field of two laser waves extending towards each other is theoretically studied. It is shown that the effective interaction force between uranium nuclei, can become an attractive force on certain time intervals in the presence of the pulsed laser field. As a result the pulsed laser field can slow down backward motion of nuclei in 7 times.”     

Electron polarization in quantum wells in a strong variable field

The wave function of an electron in a symmetric double quantum well placed in a strong time-periodic electric field is found, expressions for quasienergy functions are derived, and the dependence of the dipole moment on the average electric field is analyzed for the case where the average field remains constant. In the case of slow monotonic variation of the “constant” component of the electric field, the Schro dinger equation is solved by the WKB method. It is found that the dependence of the dipole moment on the average field is of a clearly nonlinear almost-periodic nature and that in the event of adiabatic monotonic variation of the average field there is a periodic relocation of the electron density from well to well with a small frequency proportional to the rate of variation of the average field. Zh. éksp. Teor. Fiz. 116, 217–235 (July 1999)     

Study of low-temperature ordering and crystal structure in FePtBi/Au nanocomposite films

We investigate the frequency behavior of a half-wave dipole antenna placed very close over a 2LC uni-planar compact electromagnetic bandgap (UC-EBG) structure. A very compact and high-gain antenna is realized at 1 GHz. The air distance between the dipole element and the UC-EBG surface is ∼λ ₀/100. We analyse the structure by using the optical model of Hansen, full-wave electromagnetic simulations (EM) and experimental characterizations. The analytical model of Hansen describes accurately the UC-EBG phase contribution to the total radiated field below 1 GHz. Above this frequency, the Hansen analytical model is in discrepancy with the measurements and full-wave simulations, which show split in the radiation patterns. We show that this phenomenon is induced by the power leakage of the fast-wave UC-EBG surface excited by the dipole source inside its leaky wave region. We propose an original model based on the Hansen optical analysis that takes into account the overall phenomena. The model includes the contribution of the weighted fields radiated by the cells of the UC-EBG. This model leads to very good agreements with measurements and full-wave simulations.     

Pre-equilibrium dipole excitation and γ-ray fragment angular correlation in the 32S +74Ge reaction

We report on the results obtained from the study of the ³²S +⁷⁴Ge deep inelastic reaction at incident energy E= 320 MeV. High-energy γ-rays were detected in an array of 6 seven-pack BaF₂ clusters. Coincidence with complex fragments detected in 12 three-stage telescopes ensured the selection of peripheral reaction events. In order to investigate the pre-equilibrium dipole strength excitation two independent analyses were performed. In the first analysis the energy spectra of the γ-rays were evaluated in the statistical model framework while in the second one the γ-ray fragment angular correlation with respect to the nuclear spin vector of the composite system was studied. Both methods indicate the excitation of dipole strength in the highly deformed dinucleus and provide dipole resonance parameter sets that are in good agreement with each other. Received: 24 March 1999 / Revised version: 17 May 1999     

High-resolution photorefractive gratings in nematic liquid crystals sandwiched with photoconductive polymer film

Photorefractive gratings with high grating resolution were observed in the 20 μm thick low-molar-mass nematic liquid crystal (NLC) cell with a separate photoconductive (PC) poly(N-vinylcarbazole) layer. An orientational grating with a grating spacing of 1.9 μm was produced. It is believed that a space–charge field with small fringe spacing forms in the PC layer and its evanescent component penetrates into the NLC layer. The penetrated evanescent field drives the NLC to reorient, and consequently the orientational grating forms. The model indicates that the modulated field exists in several hundred nanometers near the surface, and thus the orientational grating is not full of the NLC film, which is consistent with the observed phenomena of the multiple diffractions. Besides, asymmetric two-beam coupling of 11.2% was achieved for the grating with a grating spacing of 1.9 μm, and a net gain coefficient of larger than 62 cm⁻¹ was obtained.