Increase In Linewidth Research Articles

Abundant cyanopolyynes as a probe of infall in the Serpens South cluster-forming region

We have detected bright HC7N J = 21-20 emission toward multiple locations in the Serpens South cluster-forming region using the K-Band Focal Plane Array at the Robert C. Byrd Green Bank Telescope. HC7N is seen primarily toward cold filamentary structures that have yet to form stars, largely avoiding the dense gas associated with small protostellar groups and the main central cluster of Serpens South. Where detected, the HC7N abundances are similar to those found in other nearby star forming regions. Toward some HC7N `clumps', we find consistent variations in the line centroids relative to NH3 (1,1) emission, as well as systematic increases in the HC7N non-thermal line widths, which we argue reveal infall motions onto dense filaments within Serpens South with minimum mass accretion rates of M ~ 2-5 M_sun Myr^-1. The relative abundance of NH3 to HC7N suggests that the HC7N is tracing gas that has been at densities n ~ 10^4 cm^-3, for timescales t < 1-2 x 10^5 yr. Since HC7N emission peaks are rarely co-located with those of either NH3 or continuum, it is likely that Serpens South is not particularly remarkable in its abundance of HC7N, but instead the serendipitous mapping of HC7N simultaneously with NH3 has allowed us to detect HC7N at low abundances in regions where it otherwise may not have been looked for. This result extends the known star-forming regions containing significant HC7N emission from typically quiescent regions, like the Taurus molecular cloud, to more complex, active environments.

Improved quantification precision of human brain short echo-time (1) H magnetic resonance spectroscopy at high magnetic field: a simulation study.

The gain in quantification precision that can be expected in human brain (1) H MRS at very high field remains a matter of debate. Here, we investigate this issue using Monte-Carlo simulations. Simulated human brain-like (1) H spectra were fitted repeatedly with different noise realizations using LCModel at B0 ranging from 1.5T to 11.7T, assuming a linear increase in signal-to-noise ratio with B0 in the time domain, and assuming a linear increase in linewidth with B0 based on experimental measurements. Average quantification precision (Cramér-Rao lower bound) was then determined for each metabolite as a function of B0 . For singlets, Cramér-Rao lower bounds improved (decreased) by a factor of ∼ B0 as B0 increased, as predicted by theory. For most J-coupled metabolites, Cramér-Rao lower bounds decreased by a factor ranging from B0 to B0 as B0 increased, reflecting additional gains in quantification precision compared to singlets owing to simplification of spectral pattern and reduced overlap. Quantification precision of (1) H magnetic resonance spectroscopy in human brain continues to improve with B0 up to 11.7T although peak signal-to-noise ratio in the frequency domain levels off above 3T. In most cases, the gain in quantification precision is higher for J-coupled metabolites than for singlets.

Finite Element Analysis of Failure in Cu Interconnect Megasonic Cleaning

Megasonic cleaning has been one of the most successful techniques for Cu/low-k interconnects post-CMP cleaning. The structural deformation and stress of Cu and low-k materials in megasonic cleaning are examined with finite element method (FEM). The maximum stress is concentrated in the binding area between Cu and low-k. With decrease of Cu line width, the maximum stress increases and the max value exceeds the yield strength of Cu which results in the plastic deformation. The increasing frequency will change the bubble collision times. Therefore the fatigue is potential. The maximum displacement moves from center to the sides of top surface with increase of line width. When the line width is 25nm, the deformation is the largest.

Intrinsic and Thermal Linewidths of Spin-Transfer-Driven Vortex Self-Oscillations

Narrow linewidth spin transfer driven vortex self-oscillations in Py/Cu/Py spin valves of elliptical cross section (160 nm × 75 nm) are investigated by means of micromagnetic simulations. “GPmagnet”, a high performance GPU based micromagnetic package, is used for this purpose, allowing for 100 KHz spectral resolution at reasonable computational time. The variation of the linewidth with applied current (10 × 107 A/cm2 A/cm2 <; J <; 20 × 107 A/cm2) is analyzed in the temperature range 0 K <; K <; 300 K. Linewidths around 1 MHz are found at T = 0K. At T = 300 K, linewidths remain below 2 MHz for J <; 16 × 107 A/cm2, whereas for larger currents the presence of two oscillation modes lead to a further linewidth increase.

High-Field Magnetism of theS= 5/2 Triangular-Lattice Antiferromagnet CuFe1-xGaxO2(x=0–0.028)

We have performed multifrequency electron spin resonance (ESR), electric polarization, and magnetization measurements on single crystals of the triangular-lattice antiferromagnet CuFe1-xGaxO2 (x = 0.000–0.028) in high magnetic fields. We compared ESR results with the other results (electric polarization and magnetization measurements) to clarify the origin of the expansion of a ferroelectric and incommensurate (FEIC) phase induced by the doping of nonmagnetic Ga3+ ions. We observed a rapid increase in the ESR linewidth and a large shift of the transition field from a four-sublattice collinear phase to the FEIC phase accompanied with a magnetization jump and a spontaneous electric polarization with slight doping of Ga3+ ions. We have found that the ESR linewidth is highly correlated with this shift. As a result, we have succeeded in explaining the origin of the expansion of the FEIC phase as well as the change of the transition field induced by broadening of the lowest excitation branch with an increase in...

Tunable narrowband terahertz generation in lithium niobate crystals using a binary phase mask

We present a simple scheme of narrowband terahertz (THz) generation by optical rectification in the lithium niobate crystal covered by a binary phase mask. It is shown that a single-domain crystal illumination by spatiotemporal shaped fs-laser pulses is equivalent to the formation of a transversally patterned, quasi-phase-matching structure. Decrease of the optical beam size on the mask leads to an increase of the THz-wave linewidth from 17 GHz to a few THz. The frequency of the generation was tuned in the range of 0.4-1.0 THz by building images of the mask in the crystal with various magnifications. Application results of the presented THz source for measuring transmittance of the superconducting NbN thin film in the 4.2-15 K temperature range are also presented.

Magnetic phase transition for defect induced electron spins from fully metal–semiconductor separated SWCNTs

AbstractESR experiments from PtRhRe grown and highly semiconductor—metal separated single‐walled carbon nanotubes (SWCNTs) were performed at 9.45 GHz and temperatures T between 0.39 and 200 K. No explicit evidence was found for a response from itinerant electrons in the metallic tubes. Rather, in both the metallic (M) and the semiconducting (SC) tubes, the ESR signal originates from quasi‐localized defect spins but interactions with free electrons render the two systems characteristically different. The spin susceptibility was of Curie–Weiss type for T ≥ 10 K. For annealed samples it drops for lower T indicating a transition to a ferromagnetic state. Linewidths decrease and increase with increasing T for M and SC tubes, respectively. As a consequence they cross for the two systems. Interaction of spins with free carriers in M tubes via an RKKY type mechanism and increase of linewidth with temperature for SC tubes due to spin–lattice interaction is suggested to be responsible for this.

Magnetic Properties of Zinc Ferrite Nanoparticles

In the present study, we report the synthesis of Zn-ferrite nanoparticles by a soft chemical coprecipitation method. The magnetic properties of Zn ferrite are sensitive to their average crystallite size ( d) which can be varied by altering the synthesis conditions. The d value was controlled by the pH of the solution which is related to the number of moles of OH- ions (x ). The XRD pattern of as synthesized samples showed single phase spinel structure. The estimated d is in the range of 2 to 12 nm. The magnetization studies showed that the sample with d = 2 nm is paramagnetic whereas the samples with d >; 2 nm exhibited superparamagnetic behavior. The magnetization at 10 kG increases from 2.6 to 21.4 emu/g as d increases from 2 to 12 nm. The temperature dependent studies for the sample with d = 12 nm shows symmetric spectra with single resonance peak of Lorentzian shape. As the temperature decreases an additional peak at lower field side of the main signal appears. The g value and the linewidth (ΔHPP) increase with decrease in temperature indicating the increase in antiferromagnetic (AFM) interactions between the ions in tetrahedral and the octahedral sites.

Theoretical Study on Time Delay of Slow Light by Remodeling SBS Gain and Absorption Spectrum

In this paper, we perform a theoretical study of processes that demonstrates additional time delay obtained by remodeling Brillouin gain spectrum, and we also study the main influence factors on pulse time delay and broadening, as well as analyze the ways which increases the delay, and further suppresses pulse broadening. The results show that by remodeling Brillouin gain spectrum, one obtains 25% delay improvement over the single intrinsic Brillouin gain spectrum; by deriving the expression of broadening factor, one finds that the increase of intrinsic SBS resonance linewidth leads to the decrease of the broadening quantity which corresponds to the maximum delay. Moreover, the increase of the peak power of absorption spectrum further leads to an increase of pulse delay. When the normalized frequency shift is below a certain value, probe pulse can be compressed; When the second-order gain is positive, pulse will be compressed; when the second-order gain is negative, pulse will become broad. DOI: http://dx.doi.org/10.11591/telkomnika.v10i6.1543 Full Text: PDF

Effect of thermal fluctuations in FMR experiments in uniaxial magnetic nanoparticles: Blocked vs. superparamagnetic regimes

We present ferromagnetic resonance (FMR) experiments on two low-interacting nanoparticle systems: Fe3O4 and CoFe2O4 corresponding to low- and high-anisotropy cases, respectively. The spectra have been interpreted in terms of a phenomenological model which applies to the FMR of nanoparticles. The model includes the effect of thermal fluctuations in the FMR covering the range from the superparamagnetic (low-anisotropy-high-temperature) regime to the high-anisotropy-low-temperature situation. We have been able to explain several simultaneous features observed in the FMR spectra of a system of anisotropic nanoparticles when lowering the temperature that include: a decrease of the resonance field with a simultaneous linewidth increase and intensity reduction. These effects had been previously attributed to the existence of a “blocking-temperature” in the FMR. Our interpretation, however, shows that in a magnetic system with easy axes this FMR response originates in the temperature dependence of the dispersion relation. Also, applying the present model to the FMR within the hysteresis cycle it is possible to reproduce the irreversibilities occurring in the resonance spectra. Comparison of FMR and magnetization measurements show that the characteristic FMR time is not related to the inverse microwave frequency.

Mott transition of excitons in GaAs-GaAlAs quantum wells

We investigate the breakup of bound electron–hole pairs, known as Mott transition of excitons, in GaAs-GaAlAs quantum wells with increasing excitation, comparing two different theoretical approaches. Firstly, a thermodynamic approach is used to investigate the ionization equilibrium between electrons, holes and excitons, where the abrupt jump of the degree of ionization from 0 to 1 indicates the Mott density. It is extended to a self-consistent quasi-particle approximation (QPA) for the carrier properties, including dynamical screening of the Coulomb interaction between carriers. Secondly, a spectral approach based on the semiconductor Bloch equations within linear optical response is used, considering the quasi-particle (QP) properties of carriers and the dynamical screening between electron–hole pairs. While the first is effectively a one-particle approach, in the second the whole two-particle spectrum is analyzed. Within the thermodynamic approach, a simple criterion for the Mott transition can be given: namely, if the sum of chemical potentials of carriers, reflecting the effective shrinkage of the band edge, crosses the exciton energy with increasing excitation. We demonstrate that this simple picture cannot be maintained in the two-particle approach. Here, a compact quantity, which describes the behavior of the band edge, does not exist. In fact, the behavior of the single states in the spectrum is generated by the interplay of dynamical screening in the interband self-energy and the effective interaction of the electron–hole pairs. Moreover, the band edge cannot be clearly resolved, since it is merged with excited exciton states (e.g. 2s state), which show up only for densities far below the Mott density. Instead of a Mott density, only a density range can be given, where the Mott transition appears. We demonstrate that a small damping as a prerequisite for the validation of the extended QPA in the thermodynamic approach breaks down, analyzing (i) the dephasing processes with increasing excitation, (ii) the strong increase of the excitonic linewidth and (iii) comparing with the lifetime of carriers in the QP description.

Radiation efficiency of narrowband coherent thermal emitters

The far field radiation efficiency achievable in narrowband thermal emitters is investigated, taking into account the full spatial and spectral variation of the emissivity. A coupled Fabry-Perot cavity model is used to develop an insight into the efficiency variation with cavity coherence and device temperature. It is found that the spatial variation of emissivity has to be explicitly included in the radiation power calculations to accurately estimate the achievable power efficiencies. The calculated radiation efficiencies of an ideal coherent cavity coupled emitter were found to vary from 0.1% to 9%, with a corresponding increase in the emission linewidth from 6.3 nm to 930 nm, and were much lower than that estimated without accounting for effects of spatial coherence. The analysis presented here can be used to determine the optimal operating temperature of a coherent thermal emitter once its emission characteristics and conduction losses are known and it is demonstrated that this optimum temperature is different from the temperature of peak blackbody emission at the resonant absorption wavelength.

Structural modifications of GaN after cerium implantation

Among the family of rare earth (RE) dopants, the doping of first member Ce into GaN is the least studied system. This article reports structure properties of Ce-doped GaN realized by technique of ion implantation. Ce ions were implanted into metal organic chemical vapor deposition grown n- and p-GaN/sapphire thin films at doses 3 × 1014 and 2 × 1015 cm−2. X-ray diffraction scans and Raman scattering measurements exhibited expansion of lattice in the implanted portion of the samples. First order Raman scattering spectra show appearance of several disorder-activated Raman scattering modes in addition to typical GaN features. A dose-dependent decrease in intensity of E2 mode was observed in Raman the spectra of the implanted samples. Ultraviolet Raman spectra of implanted samples show complete quenching of photoluminescence emission and appearance of multiple A1(LO) phonon scattering modes up to fifth order. Moreover, a decrease in intensity and an increase in line width of LO modes as a function of wavenumber were observed for implanted samples. Copyright © 2012 John Wiley & Sons, Ltd.

The molecular gas content of the Pipe Nebula

The Pipe Nebula is a molecular cloud hosting the B59 region as its only active star-forming clump. While the particular importance of outflows in active star forming regions is subject of debate, the quiet nature of the gas in B59 makes it a good site to directly see the impact of protostellar feedback on the quiescent dense gas. Using HARP at the JCMT, we mapped the B59 region with the J=3-2 transition of 12CO to study the kinematics and energetics of the outflows, and 13CO and C18O to study the overall dynamics of the ambient cloud, the physical properties of the gas, and the hierarchical structure of the region. The B59 region has a total of 30Msun of cold and quiescent material, mostly gravitationally bound, with narrow line widths throughout. Such low levels of turbulence in non-star-forming sites of B59 are indicative of the intrinsic initial conditions of the cloud. On the other hand, close to the forming protostars the impact of the outflows is observed as a localised increase of both line widths from 0.3 to 1 km/s, and 13CO excitation temperatures by 2-3K. The impact of the outflows is also evident in the low column density material which shows signs of being pushed, shaped and carved by the outflow bow shocks as they pierce their way out of the cloud. Much of this structure is readily apparent in a dendrogram analysis of the cloud. The low mass of B59 together with its intrinsically quiescent gas and small number of protostars, allows the identification of specific regions where the outflows from the embedded sources interact the dense gas. Our study suggests that outflows are an important mechanism for injecting and sustaining supersonic turbulence at sub-parsec size scales. We find that less than half of the outflow energy is deposited as turbulent energy of the gas, however this turbulent energy is sufficient to slow down the collapse of the region.

Linear optical absorption and photoluminescence emission properties of gold nanoparticles prepared by laser ablation technique

Gold nanoparticles of average size varying between 1.1 and 3.3 nm are prepared by 1064 nm Nd:YAG laser ablation of solid gold target kept in ethylene glycol medium. The measured UV-Visible absorption spectra showed the presence of sharp absorption peaks in the UV and in the visible regions due to the interband transition and surface plasmon resonance (SPR) oscillations in Au nanoparticles, respectively. The increase in linewidth of the SPR peaks with the reduction in particle sizes is observed due to intrinsic size effects. The prepared samples exhibit photoluminescence (PL) emissions in the UV-Visible region peaked at ∼354 nm due to the recombination of electrons with holes from sp conduction band to d-band of Au. The peak PL intensity in the sample prepared with 60 minutes of laser ablation time is enhanced by a factor of ∼2.5 compared to that obtained in the sample prepared with a laser-ablation time duration of 15 minutes.

Superconducting fluctuations and anomalous phonon renormalization much above superconducting transition temperature in Ca4Al2O5.7Fe2As2

Raman studies on Ca4Al2O5.7Fe2As2 superconductor in the temperature range of 5 K to 300 K, covering the superconducting transition temperature Tc = 28.3 K, reveal that the Raman mode at ∼230 cm−1 shows a sharp jump in frequency by ∼2% and linewidth increases by ∼175% at To ∼ 60 K. Below To, anomalous softening of the mode frequency and a large decrease by ∼10 cm−1 in the linewidth are observed. These precursor effects at T0 (∼2Tc) are attributed to significant superconducting fluctuations, possibly enhanced due to reduced dimensionality arising from weak coupling between the well separated (∼15 Å) Fe-As layers in the unit cell. A large blue-shift of the mode frequency between 300 K and 60 K (∼7%) indicates strong spin-phonon coupling in this superconductor.

Thickness dependence of amplified spontaneous emission in low-absorbing organic waveguides

The effect of varying film thickness (h) on the amplified spontaneous emission (ASE) properties of 0.5 wt.% perylenediimide-doped polystyrene waveguides is reported. The threshold dependence on h, not previously investigated in detail, is analyzed in terms of the film absorption and photoluminescence, the confinement of the fundamental waveguide mode (TE0), and the presence of high-order modes. For h<400 nm and down to 150 nm, the ASE wavelength blueshifts, while the linewidth and threshold increase. The detrimental ASE operation in very thin films is due to the low absorption as well as to the poor confinement of the TE0 mode.

Comparing localized and nonlocalized dynamic 31P magnetic resonance spectroscopy in exercising muscle at 7T

By improving spatial and anatomical specificity, localized spectroscopy can enhance the power and accuracy of the quantitative analysis of cellular metabolism and bioenergetics. Localized and nonlocalized dynamic 31P magnetic resonance spectroscopy using a surface coil was compared during aerobic exercise and recovery of human calf muscle. For localization, a short echo time single-voxel magnetic resonance spectroscopy sequence with adiabatic refocusing (semi-LASER) was applied, enabling the quantification of phosphocreatine, inorganic phosphate, and pH value in a single muscle (medial gastrocnemius) in single shots (TR = 6 s). All measurements were performed in a 7 T whole body scanner with a nonmagnetic ergometer. From a series of equal exercise bouts we conclude that: (a) with localization, measured phosphocreatine declines in exercise to a lower value (79 ± 7% cf. 53 ± 10%, P = 0.002), (b) phosphocreatine recovery shows shorter half time (t1/2 = 34 ± 7 s cf. t1/2 = 42 ± 7 s, nonsignificant) and initial postexercise phosphocreatine resynthesis rate is significantly higher (32 ± 5 mM/min cf. 17 ± 4 mM/min, P = 0.001) and (c) in contrast to nonlocalized 31P magnetic resonance spectroscopy, no splitting of the inorganic phosphate peak is observed during exercise or recovery, just an increase in line width during exercise. This confirms the absence of contaminating signals originating from weaker-exercising muscle, while an observed inorganic phosphate line broadening most probably reflects variations across fibers in a single muscle. Magn Reson Med, 2012. © 2012 Wiley Periodicals, Inc.

Importance of residual stresses in the Brillouin gain spectrum of single mode optical fibers

Residual stresses inside optical fibers can impact significantly on Brillouin spectrum properties. We have analyzed the importance of internal stresses on the Brillouin Gain Spectrum (BGS) for a conventional G.652 fiber and compared modeling results to measurements. Then the residual internal stresses have been investigated for a set of trench-assisted fibers: fibers are coming from a single preform with different draw tensions. Numerical modeling based on measured internal stresses profiles are compared with corresponding BGS experimental results. Clearly, Brillouin spectrum is shifted linearly versus draw tension with a coefficient of -20MHz/100g and its linewidth increases.

Magnetic and dielectric behavior of terbium substituted Mg1−xTbxFe2O4 ferrites

The effects of concentration of terbium (Tb) on ferromagnetic resonance (FMR), relative initial permeability, morphology, and dielectric properties of MgFe2O4 ferrites were studied. It is found that FMR line-width and FMR position of magnesium–terbium (Mg–Tb) ferrites are found to fluctuate with the increase of Tb-content. The increase in FMR line-width is attributed to the charge transfer mechanism and inhomogeneity in the samples (TbFeO3). The relative initial permeability decreases gradually as a function of Tb-content. The magnetic loss factors of substituted samples exhibit decreasing behavior in the frequency range 1kHz to 10MHz. Terbium substituted samples have lower dielectric constant as compared to the pure MgFe2O4 ferrite. The decrease in dielectric constant is attributed to the retardation in electron exchange mechanism caused by lockup among iron and terbium ions. An irregular behavior of the dielectric loss is found, which is related to the hopping probability of charges in these ferrites. A decrease in the ac conductivity is observed beyond 8MHz and this corresponds to the occurrence of dielectric loss. The low magnetic and dielectric behavior makes these ferrite materials suitable for their applications in microwave devices.