Coherence Length Parameter Research Articles

Field experiment and image reconstruction using a Fourier telescopy imaging system over a 600-m-long horizontal path.

To confirm the effect of uplink atmospheric turbulence on Fourier telescopy (FT), we designed a system for far-field imaging, utilizing a T-type laser transmitting configuration with commercially available hardware, except for a green imaging laser. The horizontal light transmission distance for both uplink and downlink was ∼300 m. For both the transmitting and received beams, the height upon the ground was below 1 m. The imaging laser's pointing accuracy was ∼9.3 μrad. A novel image reconstruction approach was proposed, yielding significantly improved quality and Strehl ratio of reconstructed images. From the reconstruction result, we observed that the tip/tilt aberration is tolerated by the FT system even for Changchun's atmospheric coherence length parameter (r0) below 3 cm. The resolution of the reconstructed images was ∼0.615 μrad.

Magnetic fluctuations on TR3Ba5Cu8Oδ (TR=Ho, Y and Yb) superconducting system

In this work, we report the production of TR3Ba5Cu8Oδ (TR=Ho, Y and Yb) superconducting system using a usual solid state reaction method. The irreversibility line and the analysis of magnetization fluctuations for TR3Ba5Cu8Oδ (TR=Ho, Y and Yb) system were investigated. The curves of magnetization ZFC–FC were measured in magnetic fields of the 100–4000Oe to obtain the values for T⁎ and TC temperatures. The penetration depth and the coherence length parameters as a function of the applied magnetic field were obtained. The data of the magnetization excess ΔM(T, H) was analyzed from the curves of magnetization as a function of logarithm of applied field for different values of temperature in the corresponding range. The Bulavskii, Ledvij and Kogan theory was employed for this purpose which considers fluctuations effects in the free energy and into the equilibrium magnetization.

Sintering behavior of spin-coated FePt and FePtAu nanoparticles

FePt and [FePt]95Au5 nanoparticles with an average size of about 4nm were chemically synthesized and spin coated onto silicon substrates. Samples were subsequently thermally annealed at temperatures ranging from 250to500°C for 30min. Three-dimensional structural characterization was carried out with small-angle neutron scattering (SANS) and small-angle x-ray diffraction (SAXRD) measurements. For both FePt and [FePt]95Au5 particles before annealing, SANS measurements gave an in-plane coherence length parameter a=7.3nm, while SAXRD measurements gave a perpendicular coherence length parameter c=12.0nm. The ratio of c∕a is about 1.64, indicating the as-made particle array has a hexagonal close-packed superstructure. For both FePt and FePtAu nanoparticles, the diffraction peaks shifted to higher angles and broadened with increasing annealing temperature. This effect corresponds to a shrinking of the nanoparticle array, followed by agglomeration and sintering of the nanoparticles, resulting in the eventual loss of positional order with increasing annealing temperature. The effect is more pronounced for FePtAu than for FePt. Dynamic coercivity measurements show that the FePtAu nanoparticles have both higher intrinsic coercivity and higher switching volume at the same annealing temperature. These results are consistent with previous studies that show that additive Au both lowers the chemical ordering temperature and promotes sintering.

Effect of Two Gaps on the Flux-Lattice Internal Field Distribution: Evidence of Two Length Scales inMg1−xAlxB2fromμSR

We have measured the transverse field muon spin precession in the flux-lattice (FL) state of the two-gap superconductor MgB2 and of the electron doped compounds Mg(1-x)AlxB2 in magnetic fields up to 2.8 T. We show the effect of the two gaps on the internal field distribution in the FL, from which we determine two coherence length parameters and the doping dependence of the London penetration depth. This is an independent determination of the complex vortex structure already suggested by the STM observation of large vortices in a MgB2 single crystal. Our data agree quantitatively with STM and we thus validate a new phenomenological model for the internal fields.

Spatial coherence: A new method of quantifying myocardial electrical organization using multichannel epicardial electrograms

A new technique has been developed to quantify the organization of myocardial electrical activity. The spatial coherence technique employs the use of the magnitude-squared coherence (MSC) spectrum to analyze multi-channel electrograms obtained during cardiac mapping. In this study, MSC values for all possible pairs of electrograms recorded from an epicardial plaque consisting of 112 electrodes were computed and systematically integrated to form a three-dimensional coherence surface, that is, a graphical representation of average coherence values versus the spatial orientation of one electrode to another. From this surface, two-dimensional graphs of average coherence versus electrode separation distance were derived, and the data were fitted to an exponentially decaying curve. Two novel parameters indicative of myocardial organization were then extracted from the curves, the coherence length parameter and the coherence plateau parameter. Higher values for these parameters were hypothesized to reflect greater levels of organization of the rhythm. A total of 164 mapping sessions were performed on nine dogs. Three-second data segments of normal sinus rhythm (NSR) and ventricular fibrillation (VF) were analyzed by using spatial coherence. Coherence lengths were found to be significantly longer (24.3 ± 13.4 vs 0.165 ± 0.053 mm, P = .0001) and coherence plateau values significantly higher (0.896 ± 0.104 vs 0.098 ± 0.008, P = .0001) for NSR than for VF. In addition, one instance of ventricular tachycardia, a rhythm more organized than VF, had coherence parameter values greater than those for VF but less than those for NSR (coherence length = 1.23 mm, coherence plateau value = 0.597). These results suggest that spatial coherence may be an effective means of quantifying the electrical organization of a particular cardiac rhythm. The advantages of this technique are (1) it extracts a single parameter from the vast amount of data that is generated in a typical mapping study, thus allowing easy characterization of different cardiac rhythms in terms of their level of organization, and (2) it does not require activation time detection, a process that is difficult when studying arrhythmias such as ventricular fibrillation.

Raman scattering study of lattice disorder in 1-MeV Si-implanted GaAs

We have used Raman scattering to study the lattice disorder created by the implantation of 1-MeV Si ions into GaAs. Using the change in the longitudinal optical (LO) phonon-line position as the signature for lattice damage, combined with chemical etching for controlled layer removal, we monitored the evolution of the disorder depth profile as a function of implantation dose. The shape of the depth profile of the disorder agrees with the theoretical simulation trim for doses of 1×1014 cm−2 or lower. For higher doses a saturation is observed in the amount of residual disorder. This saturation is a manifestation of dynamic annealing occurring during the high-energy implantations, which we attribute to enhanced defect mobility, induced by the transfer of energy to the lattice, in atomic collision cascade processes. In order to correlate the spectral features in the Raman spectra with structural changes in the ion-implanted samples, we characterized the implantation-induced lattice damage using ion-channeling and transmission electron microscopy (TEM) measurements. The residual defects in the MeV-implanted samples are found to consist of dislocation loops and discrete point defects dispersed in an otherwise perfect (although probably strained) crystalline lattice. An average distance between defects was estimated from the channeling and TEM studies, and compared with the coherence-length parameter L used in the ‘‘spatial correlation model,’’ which is commonly used to interpret quantitatively the Raman spectra of ion-implanted materials. Although the model gives a good fit to our data in terms of the position and linewidth of the LO phonon peak, no clear correlation could be established between L and the interdefect separations. We also observed the appearance of the broadbands at about 70, 180, and 245 cm−1, in the Raman spectra, which are commonly attributed to amorphous GaAs, although no trace of amorphous material was detected by the TEM analysis. Our results indicate that the quantitative interpretation of Raman spectra to determine crystalline properties of ion-implanted materials, as well as the assignment of Raman spectral features to particular defect structures, is not unambiguously established yet.