Large Longitudinal Field Research Articles

Chirality and helicity of linearly-polarised Laguerre-Gaussian beams of small beam waists

The chirality and helicity of a linearly polarised Laguerre-Gaussian (LG) beam are examined. Such a type of light possesses a large longitudinal field amplitude when it is created with a sufficiently small beam waist and so gives rise to substantial magnitudes of chirality and helicity density distributions. In the simplest case of a doughnut beam of winding number ℓ=1 and another identical to it but for which ℓ=−1, we obtain different chirality and helicity distributions in the focal plane z=0. We also show that this chiral behaviour persists and the patterns evolve so that on planes at z<0 and z>0 the beam convergence phase contributes differently to the changes in the chirality and helicity distributions.

On the importance of frequency-dependent beam parameters for vacuum acceleration with few-cycle radially polarized laser beams.

Tightly focused, ultrashort radially polarized laser beams have a large longitudinal field, which provides a strong motivation for direct particle acceleration and manipulation in a vacuum. The broadband nature of these beams means that chromatic properties of propagation and focusing are important to consider. We show via single-particle simulations that using the correct frequency-dependent beam parameters is imperative, especially as the pulse duration decreases to the few-cycle regime. The results with different spatio-spectral amplitude profiles show either a drastic increase or decrease of the final accelerated electron energy depending on the shape, motivating both proper characterization and potentially a route to optimization.

Evidence for spin liquid ground state in SrDy2O4 frustrated magnet probed by μSR

Muon spin relaxation (μSR) measurements were carried out on SrDy2O4, a frustrated magnet featuring short range magnetic correlations at low temperatures. Zero-field muon spin depolarization measurements demonstrate that fast magnetic fluctuations are present from T = 300 K down to 20 mK. The coexistence of short range magnetic correlations and fluctuations at T = 20 mK indicates that SrDy2O4 features a spin liquid ground state. Large longitudinal fields affect weakly the muon spin depolarization, also suggesting the presence of fast fluctuations. For a longitudinal field of μ0H = 2 T, a non-relaxing asymmetry contribution appears below T = 6 K, indicating considerable slowing down of the magnetic fluctuations as field-induced magnetically-ordered phases are approached.

Squeezing Light in Wires: Fundamental Optical Properties of Si Nanowire Waveguides

Single-mode Si-wire waveguides, fabricated in the Si-on-insulator platform, are the basis for a growing number of potential applications in linear and nonlinear integrated optical devices and systems. This paper reviews the fundamental optical physics and behavior of these waveguides and demonstrates how their reduced transverse dimensions and index contrast lead to a series of unique and distinct modal properties. These properties include readily tunable waveguide dispersion (including dispersion flattening), large longitudinal fields, a decrease in group velocity over those of the bulk materials, and anisotropic nonlinear optical properties. In turn, new devices and device structures are achievable as a result of these features and examples of new device structures are provided.

A novel method for calibration and monitoring of time synchronization of TOF-PET scanners by means of cosmic rays

Abstract All of the present methods for calibration and monitoring of time-of-flight positron emission tomography (TOF-PET) scanner detectors utilize radioactive isotopes, such as 22Na or 68Ge, which are placed or rotate inside the scanner. In this article, we describe a novel method based on the cosmic rays application to the PET calibration and monitoring methods. The concept allows to overcome many of the drawbacks of the present methods and it is well suited for newly developed TOF-PET scanners with a large longitudinal field of view. The method enables also the monitoring of the quality of the scintillator materials and in general allows for the continuous quality assurance of the PET detector performance.

Tailored SERS substrates obtained with cathodic arc plasma ion implantation of gold nanoparticles into a polymer matrix

This manuscript reports on the fabrication of plasmonic substrates using cathodic arc plasma ion implantation, in addition to their performance as SERS substrates. The technique allows for the incorporation of a wide layer of metallic nanoparticles into a polymer matrix, such as PMMA. The ability to pattern different structures using the PMMA matrix is one of the main advantages of the fabrication method. This opens up new possibilities for obtaining tailored substrates with enhanced performance for SERS and other surface-enhanced spectroscopies, as well as for exploring the basic physics of patterned metal nanostructures. The architecture of the SERS-active substrate was varied using three adsorption strategies for incorporating a laser dye (rhodamine): alongside the nanoparticles into the polymer matrix, during the polymer cure and within nanoholes lithographed on the polymer. As a proof-of-concept, we obtained the SERS spectra of rhodamine for the three types of substrates. The hypothesis of incorporation of rhodamine molecules into the polymer matrix during the cathodic arc plasma ion implantation was supported by FDTD (Finite-Difference Time-Domain) simulations. In the case of arrays of nanoholes, rhodamine molecules could be adsorbed directly on the gold surface, then yielding a well-resolved SERS spectrum for a small amount of analyte owing to the short-range interactions and the large longitudinal field component inside the nanoholes. The results shown here demonstrate that the approach based on ion implantation can be adapted to produce reproducible tailored substrates for SERS and other surface-enhanced spectroscopies.

Discovery of a huge magnetic field in the very young star NGC 2244-334 in the Rosette Nebula cluster

During a survey of field strengths in upper main sequence stars in open clusters, we observed the star NGC 2244-334 in the Rosette Nebula cluster and discovered an extraordinarily large mean longitudinal field of about −9 kG, the second largest longitudinal field known in a non-degenerate star. This star appears to be a typical Ap He-wk (Si) star of about 4 M� . Spectrum synthesis using a line synthesis code incorporating the effects of the strong magnetic field indicates that He is underabundant by about 1.5 dex, and C, O and Mg by about 0.1-0.4 dex, while Si, Mn and Fe are overabundant by about 1 dex, and Cr and Ti are nearly 2 dex overabundant. Cluster membership for this star is secure, so its age is about 2 × 10 6 yr, which is less than 3% of its main sequence lifetime. This star is one of the very youngest magnetic upper main sequence stars with a well-determined age, and confirms that both magnetic fields and strong chemical peculiarity can appear in stars which are both extremely young and very close to the ZAMS.

Micro-Hall magnetometry studies of thermally assisted and pure quantum tunneling in single molecule magnet Mn 12-acetate

We have studied the crossover between thermally assisted and pure quantum tunneling in single crystals of high spin ( S=10) uniaxial single molecule magnet Mn 12 using micro-Hall effect magnetometry. Magnetic hysteresis experiments have been used to investigate the energy levels that determine the magnetization reversal as a function of magnetic field and temperature. These experiments demonstrate that the crossover occurs in a narrow (∼0.1 K) or broad (∼1 K) temperature interval depending on the magnitude and direction of the applied field. For low external fields applied parallel to the easy axis, the energy levels that dominate the tunneling shift abruptly with temperature. In the presence of a transverse field and/or large longitudinal field these energy levels change with temperature more gradually. A comparison of our experimental results with model calculations of this crossover suggests that there are additional mechanisms that enhance the tunneling rate of low-lying energy levels and broaden the crossover for small transverse fields.

Cohort versus cross-sectional design in large field trials: precision, sample size, and a unifying model.

In planning large longitudinal field trials, one is often faced with a choice between a cohort design and a cross-sectional design, with attendant issues of precision, sample size, and bias. To provide a practical method for assessing these trade-offs quantitatively, we present a unifying statistical model that embraces both designs as special cases. The model takes account of continuous and discrete endpoints, site differences, and random cluster and subject effects of both a time-invariant and a time-varying nature. We provide a comprehensive design equation, relating sample size to precision for cohort and cross-sectional designs, and show that the follow-up cost and selection bias attending a cohort design may outweigh any theoretical advantage in precision. We provide formulae for the minimum number of clusters and subjects. We relate this model to the recently published prevalence model for COMMIT, a multi-site trial of smoking cessation programmes. Finally, we tabulate parameter estimates for some physiological endpoints from recent community-based heart-disease prevention trials, work an example, and discuss the need for compiling such estimates as a basis for informed design of future field trials.

Regular versus chaotic motion of charged particles in non-neutral current sheets

Charged particle motion in reconnecting current sheets (CS) can be both regular and chaotic, depending on the values of transverse (perpendicular to the CS plane) and longitudinal (parallel to the electric field inside the CS) components of the magnetic field. The non-zero transverse field gives rise to chaos, whereas a sufficiently large longitudinal field tends to stabilize the motion. The longitudinal field change in time may be the cause of different regimes of electron acceleration in solar flares and magnetospheric substorms.

On the Mössbauer spectrum of γ-Fe2O3

High quality Mossbauer spectra of γ-Fe2O3 particles have been analyzed in detail to determine the effects of overlapping patterns, quadrupole splitting, and applied magnetic fields. The results are discussed with regard to the use of Mossbauer spectroscopy to measure particle and magnetic moment orientation. Data were collected from two samples at 4.2–315K and with 6.1T longitudinal and 1.65T transverse magnetic fields applied. It is shown that electric field gradient effects alone are insufficient to account for the presence of ΔmI=0 lines when a large longitudinal field is applied.

Magnetic response of spin-density waves in a quasilinear half-filled Hubbard band

The linear and nonlinear response of spin-density waves (SDW) to a magnetic field is studied in a quasi-one-dimensional half-filled Hubbard band within a mean-field approximation. We find that the linear transverse susceptibility remains nearly temperature independent below the SDW transition, while the longitudinal susceptibility drops exponentially with the temperature. At low fields we prove that the free energy is minimum when the field is perpendicular to the spontaneous magnetization and maximum when it is parallel. Regardless of its magnitude, a transverse field is shown to have no significant effect on the temperature dependence of the spontaneous magnetization and the susceptibility in agreement with data. On the other hand, a large longitudinal field destroys the SDW. In this case the free energy is maximum and a much lower energy is obtained by reorienting the spontaneous magnetization perpendicular to the field. The relevance of the results to the organic conductors ${(\mathrm{TMTSF})}_{2}X$ (tetramethyltetraselenafulvalene; $X=\mathrm{P}{\mathrm{F}}_{6} \mathrm{and} \mathrm{As}{\mathrm{F}}_{6}$) is discussed.