Search Results (73 total)

The magnetically driven superconductor-insulator transition in amorphous thin films (e.g., InO and Ta) exhibits several mysterious phenomena, such as a putative metallic phase and a huge magnetoresistance peak. Unfortunately, several conflicting categories of theories, particularly quantum-vortex condensation, and normal region percolation, explain key observations equally well. We present a experimental setup, an amorphous thin-film bilayer, where a drag resistance measurement would clarify the role quantum vortices play in the transition, and hence decisively point to the correct picture. We provide a thorough analysis of the device, which shows that the vortex paradigm gives rise to a drag with an opposite sign and orders of magnitude larger than the drag measured if competing paradigms apply.

We establish a general propagation model to describe the spin Hall effect of light beam in left-handed materials (LHMs). A spin-dependent shift of the beam centroid perpendicular to the refractive index gradient for the light beam through an air-LHM interface is demonstrated. For a certain circularly polarized component, whether the transverse shift is positive or negative depends on the magnitude of the refractive index gradient. Very surprisingly, the spin Hall effect in the LHM is unreversed, although the sign of refractive index gradient is reversed. The physics underlying this counterintuitive effect is that the spin angular momentum of photons is unreversed. Further, we reveal that the angular shift in the LHM is reversed due to the negative diffraction. These findings provide alternative evidence for that the linear momentum of photons is reversed, while the spin angular momentum is unreversed in the LHM.

We propose an interlayer correlation of multilayer quantum-dot (QD) distributions from a rigorous strain energy calculation. This leads to a map of correlations, or interlayer alignment, based solely on lateral and vertical spacing (xdist/b versus hdist/h). We identify four distinct correlation regimes—aligned correlation, antialigned correlation, noncorrelation, and the transition zone between the aligned and antialigned correlation. Our prediction matches well with available experimental data for a broad range of semiconductors with low elastic anisotropy [A=2C₄₄/(C₁₁−C₁₂)<2] and can further predict the QD array distribution for those with high elastic anisotropy (A≥2) by a simple shift in hdist/h. The agreement spans both IV-VI and III-V systems. Moreover, the aligned correlation regime produces a large decay in strain energy magnitudes in subsequently grown layers, which may contribute to their observed larger nucleation domains.

Theoretical investigations of hyperfine quenching of the two metastable levels 1s²2s2p  ³P₀ and ³P₂ have been performed along the Be-like isoelectronic sequence for ions between Z=6–22. It is shown that the lifetime of the latter level is sensitive to hyperfine quenching in the beginning of the isoelectronic sequence, but the sensitivity decreases towards the high-Z end. It is also shown that the hyperfine dependent branching fraction between the 2s^{2  1}S₀–2s2p  ³P₂ and the 2s2p  ³P₁–2s2p  ³P₂ transitions can lead to some rather drastic changes in the spectra. Our predicted value of 0.677  s⁻¹ for the hyperfine-induced 2s^{2  1}S₀–2s2p  ³P₀ transition rate for ⁴⁷Ti¹⁸⁺ are in disagreement with a recent experimental value of 0.56(3)  s⁻¹. Our calculated values of this quenching are in agreement with other recent calculations all along the sequence.

Theoretical studies on hyperfine-interaction-dependent lifetimes of the 4s4p  ³P₂ states of Zn-like ions are reported in this paper. A systematic investigation along the isoelectronic sequence shows that the lifetime of the 4s4p  ³P₂ level is very sensitive to hyperfine interaction at the beginning of the sequence, but gets less sensitive going to the higher end. The intensity ratio between the 4s^{2  1}S₀-4s4p  ³P₂ and the 4s4p  ³P₁-4s4p  ³P₂ transitions was also investigated and it is shown that this ratio is very sensitive to hyperfine interaction. In some isotopes the two branches are dominated by transitions from different upper hyperfine levels. Our research also reveals that the hyperfine-induced electric dipole transitions contribute to the transitions of 4s^{2  1}S₀-4s4p  ³P₂ at about the same level as, or even higher than, M2 transitions whenever the nucleus has a spin. This is true all along the isoelectronic sequence.

Raman spectra of single-walled carbon nanotubes (SWNTs) with diameters of 0.6–1.3 nm have been studied under high pressure. A “plateau” in the pressure dependence of the G-band frequencies was observed in all experiments, both with and without pressure transmission medium. Near the onset of the G-band plateau, the corresponding radial breathing mode (RBM) lines become very weak. A strong broadening of the full width at half maximum of the RBMs just before the onset of the G-band plateau suggests that a structural transition starts in the SWNTs. Raman spectra from SWNTs released from different pressures also indicate that a significant structural transition occurs during the G-band plateau process. Simulations of the structural changes and the corresponding Raman modes of a nanotube under compression show a behavior similar to the experimental observations. Based on the experimental results and the theoretical simulation, a detailed model is suggested for the structural transition of SWNTs, corresponding to the experimentally obtained Raman results in the high-pressure domain.

We explain the rotational Doppler effect associated with light beams carrying with orbital angular momentum in left-handed materials (LHMs). We demonstrate that the rotational Doppler effect in LHMs is unreversed, which is significantly different from the axial Doppler effect. The physics underlying this intriguing effect is the combined contributions of negative phase velocity and the inverse screw of the wave front. We find that the additional Doppler effect caused by Gouy phase and wave-front curvature should be reversed, because of the negative index. In the normal dispersion region, the rotational Doppler effect induces an upstream energy flow but a downstream momentum flow. In the anomalous dispersion region, however, the rotational Doppler effect produces a downstream energy flow but an upstream momentum flow. We theoretically predict that the rotational Doppler effect can induce a transfer of angular momentum of the LHM to orbital angular momentum of the beam.

An investigation of the lifetime of the 3d⁹4s ³D₃ level along a part of the nickel-like isoelectronic sequence has been performed. The focus of this work has been to evaluate the importance of the hyperfine induced electric quadrupole channel of the 3d^10 1S₀−3d⁹4s ³D₃ transition for isotopes with nuclear spin. Comparisons between the magnetic octupole transition rate and the hyperfine-induced electric quadrupole transition rate shows that the hyperfine quenching is of importance along the whole isoelectronic sequence. At the low-Z end the hyperfine induced electric quadrupole transition channel dominates, being several orders of magnitude larger than the rate for the magnetic octupole transition channel, while for higher Z the two rates are of comparable size.

We investigate the influence of inhomogeneity in the pairing coupling constant U(r⃗) on dirty BCS superconductors, focusing on T_c, the order parameter Δ(r⃗), and the energy gap E_g(r⃗). Within mean-field theory, we find that when the length scale of the inhomogeneity is comparable to or larger than the coherence length, the ratio 2E_g∕T_c is significantly reduced from that of a homogeneous superconductor, while in the opposite limit, this ratio stays unmodified. In two dimensions, when strong phase fluctuations are included, the Kosterlitz-Thouless temperature T_KT is also studied. We find that when the inhomogeneity length scale is much larger than the coherence length, 2E_g∕T_KT can be larger than the usual BCS value. We use our results to qualitatively explain recent experimental observation of a surprisingly low value of 2E_g∕T_c in thin films.

We propose a dynamical gradient network approach to consider the synchronization in the Kuramoto model. Our scheme to adaptively adjust couplings is based on the dynamical gradient networks, where the number of links in each time interval is the same as the number of oscillators, but the links in different time intervals are also different. The gradient network in the (n+1)th time interval is decided by the oscillator dynamics in the nth time interval. According to the gradient network in the (n+1)th time interval, only one inlink’s coupling for each oscillator is adjusted by a small incremental coupling. During the transition to synchronization, the intensities for all oscillators are identical. Direct numerical simulations fully verify that the synchronization in the Kuramoto model is realized effectively, even if there exist delayed couplings and external noise.

We present a detailed investigation of the bulk electronic properties of Y_1−xPr_xBa₂Cu₃O₇ (YPrBCO) using x-ray absorption spectroscopy in the partial fluorescence yield mode (PFY-XAS) at Pr  L₃, Ba  L₃, and Cu  K edges together with Pr  2p_3∕23d_5∕2 and Cu  1s2p resonant inelastic x-ray scatterings. Pr  L₃ PFY-XAS spectra show that Pr is in the mixed-valence state (with the valence number slightly greater than 3) for the whole x=0.2–1 range, while the Pr valence decreases with increasing x. The decrease of the Pr⁴⁺ component upon Pr doping, hinting at a weakening Pr-O hybridization, is inconsistent with the scenario according to which the increase in Pr-O hybridization quenches the superconductivity in YPrBCO for x≥0.6. It rather suggests that the superconductivity may be suppressed by the total increase of the Pr-related states in the electronic structure due to the Pr ion concentration upon doping. No doping dependence is found in the electronic structure of the Ba and Cu sites, consistent with their invariant environment. No temperature dependence in our data is observed within the experimental accuracy.

The multiconfiguration Dirac-Fock (MCDF) method was used to calculate the excitation energies of the levels of ³P and ¹P in the lowest excited configurations for the two homolog elements of No I and Yb I. Also the transition probability of the 7s7p ¹P₁→7s^2 1S₀ transition and the ground-state ionization energy of No I were calculated. The results for Yb I agree very well with the available experiments, with deviations of below 0.6% for the triplets and below 4% for the singlet. The result for the No I excitation energies clears the situation of conflicting results between Borschevsky , 30 056 cm⁻¹ (3.726 eV) [Phys. Rev. A 75, 042514 (2007)], and Fritzsche, 3.36 eV [Eur. Phys. J. D 33, 15 (2005)] for the 7s7p ¹P₁ level, which is planned to be measured in the near future with a newly developed experimental technique by Backe [Eur. Phys. J. D 45, 99 (2007)]. The ionization energy result obtained in this work, 53 701 cm⁻¹, is in excellent agreement with the scaled result of 53 600(600) cm⁻¹.

We study charge delocalization and polarization effects in organic thin films by comparing surface- and bulk-sensitive, high-resolution C  1s photoemission data using tunable synchrotron radiation. In three cases, perylene-tetracarboxylicacid-dianhydride, coronene, and metal-free phthalocyanine, no surface core level shifts are observed (upper limit 100  meV). This unexpected finding shows that polarization effects, and hence polarization differences between surface and bulk, are rather small, and implies that the effective charge is not localized on the core-ionized molecule but delocalized over several neighboring molecules.

A resonant Raman spectroscopy study has been carried out under high pressure, using a diamond anvil cell, on carbon nanotube peapods (C₆₀@SWNT) synthesized in high yield in our laboratory. The Raman signal was excited by a near IR laser (830  nm) to avoid photopolymerization of C₆₀ and thus obtain the intrinsic vibrational information on the C₆₀ molecules in the nanotubes. It is found that the surrounding tubes create an effective pressure on the encapsulated C₆₀ due to tube-fullerene interactions, resulting in a shift of the intrinsic A_g(2) vibrational mode to 1474  cm⁻¹ at ambient pressure. High pressure Raman spectroscopy indicates that (C₆₀)₂ dimers form near 1  GPa, and that a further polymerization of C₆₀ occurs near 23  GPa, creating linear chains of covalently linked C₆₀ molecules in the tubes.

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 propose a procedure to distinguish quasiperiodic from chaotic orbits in short-time series, which is based on the recurrence properties in phase space. The histogram of the return times in a recurrence plot is introduced to disclose the recurrence property consisting of only three peaks imposed by Slater’s theorem. Noise effects on the statistics are studied. Our approach is demonstrated to be efficient in recognizing regular and chaotic trajectories of a Hamiltonian system with mixed phase space.

A Reply to the Comment by Thad G. Walker and Charles J. Goebel.

We report on a large scale calculation concerning the 5s^{2  1}S and 5s5p  ^1,3P levels in ⁸⁸Sr  I, using multiconfiguration Dirac-Fock methods. Our focus is on the ³P₂ meta-stable level, which has been shown to be extremely useful in atom cooling and trapping experiments. The lifetime for this level from our work (1071  s) agrees very well with a previous theoretical value, but not with the available experimental value (520₋₁₄₀⁺³¹⁰  s). The results for all the transitions between the 5s^{2  1}S and 5s5p  ^1,3P levels and within the 5s5p  ^1,3P terms are also given.

We report on the first theoretical investigation of M_F-dependent lifetimes due to interference between a magnetic octupole transition and a hyperfine induced electric quadrupole transition. Extensive multiconfiguration Dirac-Fock calculations are used to model the hyperfine quenching of the magnetic octupole decay of 3d⁹4s ³D₃ and the state mixing between the ³D₃ and ³D₂ due to hyperfine interaction in nickel-like Xe²⁶⁺.

We study the synchronizability of weighted aging scale-free networks with non-normalized and asymmetrical coupling matrices. We found that the synchronizability of such networks is improved when the couplings from older to younger nodes become dominant, where the out degrees of the nodes are heterogeneous and their in degrees are homogeneous, and that the synchronizabilty of the networks is seriously weakened or even lost when the couplings from younger to older nodes become dominant, where the out degrees of the nodes are homogeneous and their in degrees are heterogeneous. We also found that both the heterogeneity of nodes and a smaller average degree can improve the synchronizability of the networks with some appropriately chosen weighting parameter values. We finally show an example of the coupled Lorenz systems for illustration and verification of the theoretical analysis.

Studies of the dynamics of longitudinal space-charge waves in space-charge dominated beams propagating through a transport channel with a long solenoid are performed at the University of Maryland. In this paper, we report some experimental results on the energy modulations converted from density modulations. By changing the working conditions of the electron gun, pure initial density modulations are generated. Energy perturbation waveforms are measured with a high-resolution energy analyzer. The experimental results are compared with both the linear theory and the simulation results. Good agreements are achieved for the relationship between the energy and current perturbation strengths.

Strong dependence of the crystal orientation, morphology, and melting temperature (T_m) on the substrate is observed in the semicrystalline polyethylene thin films. The T_m decreases with the film thickness decrease when the film is thinner than a certain critical thickness, and the magnitude of the depression increases with increasing surface interaction. We attribute the large T_m depression to the decrease in the overall free energy on melting, which is caused by the substrate attraction force to the chains that competes against the interchain force which drives the chains to crystallization.

We present an equation-free dynamic renormalization approach to the computational study of coarse-grained, self-similar dynamic behavior in multidimensional particle systems. The approach is aimed at problems for which evolution equations for coarse-scale observables (e.g., particle density) are not explicitly available. Our illustrative example involves Brownian particles in a 2D Couette flow; marginal and conditional inverse cumulative distribution functions (ICDFs) constitute the macroscopic observables of the evolving particle distributions.