Search Results (30 total)

Measurements of the L-shell emission of highly charged gold ions were made under controlled laboratory conditions using the SuperEBIT electron beam ion trap, allowing detailed spectral observations of lines from Fe-like Au⁵³⁺ through Ne-like Au⁶⁹⁺. Using atomic data from the Flexible Atomic Code, we have identified strong 3d_5∕2→2p_3∕2 emission features that can be used to diagnose the charge state distribution in high energy density plasmas, such as those found in the laser entrance hole of hot hohlraum radiation sources. We provide collisional-radiative calculations of the average ion charge ⟨Z⟩ as a function of temperature and density, which can be used to relate charge state distributions inferred from 3d_5∕2→2p_3∕2 emission features to plasma conditions, and investigate the effects of plasma density on calculated L-shell Au emission spectra.

The dielectric dispersion in the uniaxial nematic liquid crystals affects the switching dynamics of the director, as the dielectric torque is determined by not only the present values of the electric field and director but also by their past values. We demonstrate that this “dielectric memory” leads to an unusual contribution to the dielectric torque that is linear in the present field and thus polarity sensitive. This torque can be used to accelerate the “switch-off” phase of director dynamics.

We have investigated the reason for significant discrepancies between the results of two recent, similar computational methods [Zatsarinny , Astron. Astrophys. 426, 699 (2004); Gu, Astrophys. J. 590, 1131 (2003)] for dielectronic recombination (DR) of Mg²⁺. It is found that the choice of orbital description can lead to discrepancies by as much as a factor of 2 between total peak DR rate coefficients resulting from otherwise-identical computations. These unexpected differences are attributed to the large sensitivity to bound-orbital relaxation and continuum-orbital description effects on the computed radiative and autoionizing transitions arising from accidental cancellation. In order to obviate these effects, an approach, using a separate, nonorthogonal orbital basis for each configuration, is employed to yield a DR rate coefficient that we assess to be more reliable than all earlier published results.

The dielectric anisotropy of liquid crystals causes director reorientation in an applied electric field and is thus at the heart of electro-optic applications of these materials. The components of the dielectric tensor are frequency dependent. Until recently, this frequency dependence was not accounted for in a description of director dynamics in an electric field. We theoretically derive the reorienting dielectric torque acting on the director, taking into account the entire frequency spectrum of the dielectric tensor. The model allows one to include the effects of multiple relaxations in both parallel and perpendicular components of the dielectric tensor, thus generalizing a recent model [Y. Yin , Phys. Rev. Lett. 95, 087801 (2005)] limited by the single-relaxation approach. The model predicts the “dielectric memory effect” (DME)—i.e., dependence of the dielectric torque on both the “present” and “past” values of the electric field and the director. The model describes the experimentally observed director reorientation in the case when the rise time of the applied voltage is smaller than the dielectric relaxation time. In typical materials such as pentylcyanobiphenyl (5CB), in which the dielectric anisotropy is positive at low frequencies, the DME slows down the director reorientation in a sharply rising electric field, as the sharp front is perceived as a high-frequency excitation for which the dielectric anisotropy is small or even of a negative sign. In materials that are dielectrically negative, the DME speeds up the response when a sharp pulse is applied.

The linear optical creation of Gaussian cluster states, a potential resource for universal quantum computation, is investigated. First, using Bloch-Messiah reduction, we show how to achieve canonical cluster-state generation, otherwise based on pairwise acting quantum nondemolition gates, by off-line squeezers and beam splitters. Moreover, we find that, in terms of squeezing resources, the canonical states are rather wasteful. Hence we propose a systematic way to create a whole family of cluster-type states, including potentially cheaper states. Any given cluster (or graph) state can be realized this way. As an example, we consider a protocol in which a single-mode quantum state propagates through a multiple-rail cluster. Such a redundant encoding may reduce errors due to finite squeezing.

We perform first-principles calculations of the lattice dynamics and the thermodynamic properties of LaCl₃ and LaBr₃. Using density-functional perturbation theory, we obtain the Born effective charge tensors, the dielectric permittivity tensors, the phonon frequencies at the Brillouin zone center, and the phonon dispersion curves, as well as corresponding density of states. The Born effective charge and the dielectric permittivity tensors exhibit anisotropy, which are analyzed in detail. The calculated phonon frequencies at the Γ point of the Brillouin zone show good agreement with the experimental values for most vibrational modes. The light yields of LaCl₃:Ce and LaBr₃:Ce are theoretically estimated to be 62 400 and 71  400  photons∕MeV, respectively, on the basis of the calculated values of the dielectric constants and the highest longitudinal optical infrared phonon frequencies. The thermodynamics properties including the phonon contribution to the Helmholtz free energy ΔF, the phonon contribution to the internal energy ΔE, the entropy S, and the constant-volume specific heat C_v are determined within the harmonic approximation based on the calculated phonon dispersion relations.

A detailed large-scale calculation on the resonant excitation rate coefficients from the ground state to the 106 fine-structure levels belonging to 3l¹⁷4l^′ (l=0,1,2; l^′=0,1,2,3) configurations of Ni-like tantalum have been performed using the relativistic distorted-wave approximation. The contributions through all possible Cu-like doubly excited states 3l¹⁷4l^′n^″l^″ and 3l¹⁷5l^′n^″l^″ (n^″≤15, l^″≤8) are calculated. The validity of n^″−3 scaling law is investigated. The radiative damping effects on resonant excitation rates are studied. The significant effects arising from decays to autoionizing levels are also investigated. The contributions from resonant excitation are found to be as important as direct excitation processes for most transitions. In some cases, the resonant excitation can enhance the excitation rate coefficients by an order of magnitude. Large discrepancies between the present resonant excitation rate coefficients with previously published values are found, and the present results should be more reliable and accurate.

We investigate the transport of vortices in superconductors with inhomogeneous pinning under a driving force. The inhomogeneity of pinning is simplified as strong-weak pinning regions. It is demonstrated that the interactions between the vortices captured by strong pinning potentials and the vortices in the weak pinning region cause absolute negative motion (ANM) of vortices: The vortices which are climbing toward the high barriers induced by the strong pinning with the help of driving force move toward the opposite direction of the force and back to their equilibrium positions in the weak pinning region as the force decreases or is withdrawn. Our simulations reveal that the hysteresis of ANM is determined by the competition between the speed of the negative motion which depends on the pinning inhomogeneity in superconductors and the speed of the driving force. Under the conditions of either larger force scanning rate or higher pinning inhomogeneity, a marked ANM and a larger hysteretic speed-force loop could be observed. This indicates that the time window to observe the ANM should be chosen properly. Moreover, the V-I characteristics of Ag-sheathed Bi-2223 tapes are measured, and experimental observations are qualitatively in agreement with the simulation.

With the miniaturization of a solid down to nanometer scale, the elasticity, extensibility, Debye temperature, and specific heat capacity of the solid are no longer constant but change with variation of size. These quantities also change with the temperature of the measurement and the nature of the chemical bond involved. The mechanism behind the intriguing tunability and the interdependence of these quantities remain yet a high challenge. A set of analytical solutions is presented herewith showing that the observed trends could be reproduced by taking the fact of bond order deficiency into consideration. Agreement between predictions and observations clarifies that the shortened and strengthened surface bonds dictate intrinsically the observed tunability, yet atoms in the core interior remain as they are in the bulk. The thermally softening of a specimen arises from bond expansion and bond vibration due to the internal energy increases.

There has long been confusion regarding the origin and temperature dependence of surface energetics and its responsibility for the processes and phenomena at a surface. From the perspective of bonds broken and its consequences on the remaining bonds of the undercoordinated surface atoms, we suggested herewith two essential concepts supplementing to the existing definition of surface energy for clarification purposes. One is the energy-density-gain per unit volume in surface skin and the other is the remaining cohesive-energy per discrete atom upon bond order loss once the surface is formed. The former governs the strength and elasticity while the latter dominates the thermal and structural stability of the surface. The shortened and strengthened bonds between the undercoordinated atoms dictate surface energetics and the effects of thermal expansion and vibration dominate their temperature dependence. Reproduction of the measured size and temperature dependency has led to information about the bond energy, which may go beyond traditional approaches and evidence the validity of the approaches.

The effect of bond-order loss of atoms in the surface skin on the thermal conductivity in cylindrical silicon nanowires has been investigated based on the isotropic elastic continuum model. The frequency change of torsional, longitudinal, and flexural modes of phonon vibration have been examined with a core-shell cylindrical rod structure. Results suggest that the thermal conductivity of the wire increases significantly due to the effect of surface bond-order loss that modifies the Young’s modulus in the surface atomic shells, yet atoms in the core region remain as they are in the bulk.

We describe a generalization of the cluster-state model of quantum computation to continuous-variable systems, along with a proposal for an optical implementation using squeezed-light sources, linear optics, and homodyne detection. For universal quantum computation, a nonlinear element is required. This can be satisfied by adding to the toolbox any single-mode non-Gaussian measurement, while the initial cluster state itself remains Gaussian. Homodyne detection alone suffices to perform an arbitrary multimode Gaussian transformation via the cluster state. We also propose an experiment to demonstrate cluster-based error reduction when implementing Gaussian operations.

By implementing a large-area, gain-stabilized microcalorimeter array on an electron beam ion trap, the electron-impact excitation cross sections for the dominant x-ray lines in the Fe XVII spectrum have been measured as a function of electron energy establishing a benchmark for atomic calculations. The results show that the calculations consistently predict the cross section of the resonance line to be significantly larger than measured. The lower cross section accounts for several problems found when modeling solar and astrophysical Fe XVII spectra.

We prove upper and lower bounds relating the quantum gate complexity of a unitary operation, U, to the optimal control cost associated to the synthesis of U. These bounds apply for any optimal control problem, and can be used to show that the quantum gate complexity is essentially equivalent to the optimal control cost for a wide range of problems, including time-optimal control and finding minimal distances on certain Riemannian, sub-Riemannian, and Finslerian manifolds. These results generalize the results of [Nielsen, Dowling, Gu, and Doherty, Science 311, 1133 (2006)], which showed that the gate complexity can be related to distances on a Riemannian manifold.

A system of cascaded qubits interacting via the one-way exchange of photons is studied. While for general operating conditions the system evolves to a superposition of Bell states (a dark state) in the long-time limit, under a particular resonance condition no steady state is reached within a finite time. We analyze the conditional quantum evolution (quantum trajectories) to characterize the asymptotic behavior under this resonance condition. A distinct bimodality is observed: for perfect qubit coupling, the system either evolves to a maximally entangled Bell state without emitting photons (the dark state) or executes a sustained entangled-state cycle—random switching between a pair of Bell states while emitting a continuous photon stream; for imperfect coupling, two entangled-state cycles coexist, between which a random selection is made from one quantum trajectory to another.

The nonlinear and reciprocal properties of magneto-optical (MO) effects in paramagnetic media under a high magnetic field are investigated by theoretical calculations. It is indicated that the Faraday effect in paramagnetic MO media has the obvious nonlinearity and reciprocity when the thermal energy k_BT approaches the effective field energy μ₀μ_BH_i (μ₀μ_BH_e). In addition, the MO effects, especially under an extremely high magnetic field, are found to depend essentially on the effective field H_i except the magnetization M. Our results are shown to be consistent with the experimental data while they are dramatically contrary to the current widely-accepted conclusions of classical MO theories.

An implementation of the Dirac R-matrix theory is presented and applied to the calculation of electron-impact excitation of B-like Fe XXII in the n=2 complex. A detailed comparison between the R-matrix and relativistic distorted-wave (DW) results is given. Contrary to the previous close-coupling studies of this ion, where significantly different background collision strengths are found for several transitions as compared with the DW results, we obtain excellent agreements between the two methods for most transitions, except for the weak transitions to the higher members of the 2p³ configuration, where significant channel-coupling effects are indeed present. We show that the discrepancies found in the previous Dirac R-matrix calculation for dipole transtions at high energies are due to the nonconvergence of partial wave summation, despite the explicit inclusion of partial waves up to l=40. The reason for large differences in the threshold energy region for some transitions between the present and previous Dirac R-matrix results is not clear. We also show that the independent-process, isolated-resonance approximation within the DW framework can describe the near-threshold resonances reasonably well for this ion.

Atomic data (energy levels, radiative rates, electron collision strengths, and excitation rate coefficients) of 107 fine-structure levels of the (1s²2s²2p⁶)3s²3p⁶3d¹⁰, 3s²3p⁶3d⁹4l, 3s²3p⁵3d¹⁰4l, and 3s3p⁶3d¹⁰4l  (l=s,p,d,f) configurations of nickel-like Ta ions are calculated using the distorted-wave approximation. Coupled with these atomic data, a hydrodynamic code MED103 is used to optimize the nickel-like Ta x-ray laser at 4.48  nm. Using the optimized drive-pulse configuration, an experiment is conducted to generate the nickel-like Ta x-ray laser at 4.48  nm.

Dielectronic satellite spectra of heliumlike argon, recorded with a high-resolution x-ray crystal spectrometer at the National Spherical Torus Experiment, were found to be inconsistent with existing predictions resulting in unacceptable values for the power balance and suggesting the unlikely existence of non-Maxwellian electron energy distributions. These problems were resolved with calculations from a new atomic code. It is now possible to perform reliable electron-temperature measurements and to eliminate the uncertainties associated with determinations of non-Maxwellian distributions.

We propose a scheme to unconditionally entangle the internal states of atoms trapped in separate high-finesse optical cavities. The scheme uses the technique of quantum reservoir engineering in a cascaded cavity-QED setting, and for ideal (lossless) coupling between the cavities generates an entangled pure state. Highly entangled states are also shown to be possible for realizable cavity-QED parameters and with nonideal coupling.

Engineering of stop gaps between higher photonic bands provides an alternative to miniaturization of photonic crystals. Femtosecond laser microfabrication of highly correlated void channel polymer microstructures results in photonic crystals with large stop gaps and a multitude of higher-order gaps in the mid- and near-infrared spectral regions. The gap wavelengths obey Bragg’s law. Consistent with theory, varying the woodpile structure unit cell allows for tuning the number of higher-order gaps, and transitions from mere resonant Bragg scattering to stop band total reflection are observed.

Solid C₆₀ was stored in NO under high pressure and the gas molecules NO were found to diffuse into the octahedral interstitial sites in its fcc crystal lattice. Its ¹³C nuclear magnetic resonance magic angle spinning spectra composed of a primary resonance at 143.7 ppm, accompanied by two minor peaks shifted 0.4 and 0.8 ppm downfield, respectively. The dopant was found to depress its phase-transition temperature at 260 K in pure C₆₀, and to reduce substantially the drop Δɛ^′ at the phase-transition temperature. Furthermore, the spectral features associated with relaxation during glass transition at lower temperature, as observed in impedance spectra, were smeared. The fraction of P orientation below T_c has been calculated from the impedance data. These results show that a nearly completely P oriented phase had occurred in (NO)_0.1C₆₀, and this phase is favored by a negative pressure on C₆₀ lattice exerted by NO, as well as the electrostatic interaction between the two.

Native and rare-earth-doped point-defects in β-PbF₂ are studied in the framework of the pair-potential approximation coupled with the shell model description of the lattice ions. For the perfect lattice, a new set of potential parameters are obtained which reproduce structure, elastic and dielectric constants of PbF₂ very well. The calculated formation energies for native defects suggest that the anion Frenkel disorder is preferred over the cation Frenkel and Schottky-like disorder in PbF₂. The computed temperature behavior of the ionic conductivity agrees very well with the available experimental data. In the rare-earth doped PbF₂, a site preference of the charge-compensating fluorine interstitial appears to change from nearest to next-nearest neighbor with the increase in the rare-earth ionic radius.

An earlier electron-beam ion-trap (EBIT) lifetime measurement of the Ne⁸⁺ 1s2s ³S₁ level has been improved upon, reducing the uncertainties to less than the scatter in the existing theoretical calculations. The new result, 91.7±0.4 μs, agrees with the previous value, but is more precise by a factor of 4. The new value distinguishes among theoretical values, as agreement is obtained only with those calculations that employ “exact” nonrelativistic or relativistic wave functions. Routes to measurements with even higher accuracy are discussed.