High magnetisation, monodisperse and water-dispersible CoFe@Pt core/shell nanoparticles

High magnetisation and monodisperse CoFe alloy nanoparticles are desired for a wide range of biomedical applications. However, these CoFe nanoparticles are prone to oxidation, resulting in the deterioration of their magnetic properties. In the current work, CoFe alloy nanoparticles were prepared by thermal decomposition of cobalt and iron carbonyls in organic solvents at high temperatures. Using a seeded growth method, we successfully synthesised chemically stable CoFe@Pt core/shell nanostructures. The obtained core/shell nanoparticles have high saturation magnetisation up to 135 emu g−1. The magnetisation value of the core/shell nanoparticles remains 93 emu g−1 after being exposed to air for 12 weeks. Hydrophobic CoFe@Pt nanoparticles were rendered water-dispersible by encapsulating with poly(maleic anhydride-alt-1-octadecene) (PMAO). These nanoparticles were stable in water for at least 3 months and in a wide range of pH from 2 to 11

Plasma-Induced Frequency Chirp of Intense Femtosecond Lasers and Its Role in Shaping High-Order Harmonic Spectral Lines

We investigate the self-phase modulation of intense femtosecond laser pulses propagating in an ionizing gas and its effects on collective properties of high-order harmonics generated in the medium. Plasmas produced in the medium are shown to induce a positive frequency chirp on the leading edge of the propagating laser pulse, which subsequently drives high harmonics to become positively chirped. In certain parameter regimes, the plasma-induced positive chirp can help to generate sharply peaked high harmonics, by compensating for the dynamically-induced negative chirp that is caused by the steep intensity profile of intense short laser pulses.Comment: 5 pages, 5 figure

Fracturing ranked surfaces

Discretized landscapes can be mapped onto ranked surfaces, where every element (site or bond) has a unique rank associated with its corresponding relative height. By sequentially allocating these elements according to their ranks and systematically preventing the occupation of bridges, namely elements that, if occupied, would provide global connectivity, we disclose that bridges hide a new tricritical point at an occupation fraction $p=p_{c}$, where $p_{c}$ is the percolation threshold of random percolation. For any value of $p$ in the interval $p_{c}< p \leq 1$, our results show that the set of bridges has a fractal dimension $d_{BB} \approx 1.22$ in two dimensions. In the limit $p \rightarrow 1$, a self-similar fracture is revealed as a singly connected line that divides the system in two domains. We then unveil how several seemingly unrelated physical models tumble into the same universality class and also present results for higher dimensions

On the selection of AGN neutrino source candidates for a source stacking analysis with neutrino telescopes

The sensitivity of a search for sources of TeV neutrinos can be improved by grouping potential sources together into generic classes in a procedure that is known as source stacking. In this paper, we define catalogs of Active Galactic Nuclei (AGN) and use them to perform a source stacking analysis. The grouping of AGN into classes is done in two steps: first, AGN classes are defined, then, sources to be stacked are selected assuming that a potential neutrino flux is linearly correlated with the photon luminosity in a certain energy band (radio, IR, optical, keV, GeV, TeV). Lacking any secure detailed knowledge on neutrino production in AGN, this correlation is motivated by hadronic AGN models, as briefly reviewed in this paper. The source stacking search for neutrinos from generic AGN classes is illustrated using the data collected by the AMANDA-II high energy neutrino detector during the year 2000. No significant excess for any of the suggested groups was found.Comment: 43 pages, 12 figures, accepted by Astroparticle Physic

Muon Track Reconstruction and Data Selection Techniques in AMANDA

The Antarctic Muon And Neutrino Detector Array (AMANDA) is a high-energy neutrino telescope operating at the geographic South Pole. It is a lattice of photo-multiplier tubes buried deep in the polar ice between 1500m and 2000m. The primary goal of this detector is to discover astrophysical sources of high energy neutrinos. A high-energy muon neutrino coming through the earth from the Northern Hemisphere can be identified by the secondary muon moving upward through the detector. The muon tracks are reconstructed with a maximum likelihood method. It models the arrival times and amplitudes of Cherenkov photons registered by the photo-multipliers. This paper describes the different methods of reconstruction, which have been successfully implemented within AMANDA. Strategies for optimizing the reconstruction performance and rejecting background are presented. For a typical analysis procedure the direction of tracks are reconstructed with about 2 degree accuracy.Comment: 40 pages, 16 Postscript figures, uses elsart.st

The Role of Anisotropy in Distinguishing Domination of Néel or Brownian Relaxation Contribution to Magnetic Inductive Heating: Orientations for Biomedical Applications

Magnetic inductive heating (MIH) has been a topic of great interest because of its potential applications, especially in biomedicine. In this paper, the parameters characteristic for magnetic inductive heating power including maximum specific loss power (SLPmax), optimal nanoparticle diameter (Dc) and its width (ΔDc) are considered as being dependent on magnetic nanoparticle anisotropy (K). The calculated results suggest 3 different Néel-domination (N), overlapped Néel/Brownian (NB), and Brownian-domination (B) regions. The transition from NB- to B-region changes abruptly around critical anisotropy Kc. For magnetic nanoparticles with low K (K Kc) are opposite. The decreases of the SLPmax when increasing polydispersity and viscosity are characterized by different rates of d(SLPmax)/dσ and d(SLPmax)/dη depending on each domination region. The critical anisotropy Kc varies with the frequency of an alternating magnetic field. A possibility to improve heating power via increasing anisotropy is analyzed and deduced for Fe3O4 magnetic nanoparticles. For MIH application, the monodispersity requirement for magnetic nanoparticles in the B-region is less stringent, while materials in the N- and/or NB-regions are much more favorable in high viscous media. Experimental results on viscosity dependence of SLP for CoFe2O4 and MnFe2O4 ferrofluids are in good agreement with the calculations. These results indicated that magnetic nanoparticles in the N- and/or NB-regions are in general better for application in elevated viscosity media

Performance of CMS muon reconstruction in pp collision events at sqrt(s) = 7 TeV

The performance of muon reconstruction, identification, and triggering in CMS has been studied using 40 inverse picobarns of data collected in pp collisions at sqrt(s) = 7 TeV at the LHC in 2010. A few benchmark sets of selection criteria covering a wide range of physics analysis needs have been examined. For all considered selections, the efficiency to reconstruct and identify a muon with a transverse momentum pT larger than a few GeV is above 95% over the whole region of pseudorapidity covered by the CMS muon system, abs(eta) < 2.4, while the probability to misidentify a hadron as a muon is well below 1%. The efficiency to trigger on single muons with pT above a few GeV is higher than 90% over the full eta range, and typically substantially better. The overall momentum scale is measured to a precision of 0.2% with muons from Z decays. The transverse momentum resolution varies from 1% to 6% depending on pseudorapidity for muons with pT below 100 GeV and, using cosmic rays, it is shown to be better than 10% in the central region up to pT = 1 TeV. Observed distributions of all quantities are well reproduced by the Monte Carlo simulation.Comment: Replaced with published version. Added journal reference and DO

Performance of CMS muon reconstruction in pp collision events at sqrt(s) = 7 TeV

The performance of muon reconstruction, identification, and triggering in CMS has been studied using 40 inverse picobarns of data collected in pp collisions at sqrt(s) = 7 TeV at the LHC in 2010. A few benchmark sets of selection criteria covering a wide range of physics analysis needs have been examined. For all considered selections, the efficiency to reconstruct and identify a muon with a transverse momentum pT larger than a few GeV is above 95% over the whole region of pseudorapidity covered by the CMS muon system, abs(eta) < 2.4, while the probability to misidentify a hadron as a muon is well below 1%. The efficiency to trigger on single muons with pT above a few GeV is higher than 90% over the full eta range, and typically substantially better. The overall momentum scale is measured to a precision of 0.2% with muons from Z decays. The transverse momentum resolution varies from 1% to 6% depending on pseudorapidity for muons with pT below 100 GeV and, using cosmic rays, it is shown to be better than 10% in the central region up to pT = 1 TeV. Observed distributions of all quantities are well reproduced by the Monte Carlo simulation.Comment: Replaced with published version. Added journal reference and DO

A Magnetic and Crystallographic Study of (Sm/Gd)₂(Fe/Si)₁₇C\u3csub\u3ez\u3c/sub\u3e Solid Solutions

The crystallographic and magnetic properties of SmyGd2-yFe17-xSix (0 ≤ x ≤ 3 and y = 1 and 1.5) solid solutions and their interstitial carbides have been investigated using x-ray diffraction and magnetic measurements. The SmyGd2-yFe17-xSix samples crystallized in the rhombohedral Th2Zn17 structure with less than 5 mol % of impurities. The unit cells of the mixed rare-earth (R) samples are smaller than those of Sm2Fe17 and Gd2Fe17. The carbided samples contain up to a total of 15 mol % of free iron, an iron suicide, and/or cubic Si5C3. The unit cells of the carbided samples are 1%-4% larger than those of the parent samples. For a given silicon concentration, the Curie temperatures (Tc) of SmyGd2-yFe17-xSix intermetallics are higher than those of the two end members. For example, the Tc of SmGdFe17 (280°C) is approximately 160° and 80° higher than that of Sm2Fe17 and Gd2Fe17, respectively. The Tc measured for the SmyGd2-yFe17-xSx samples, 280-290°C, are among the highest values observed for a R2Fe17-xMx intermetallic where M is a substituent other than cobalt. Except in the case of SmGdFe16SiCz (z unknown), the Tc of the carbided samples are 20%-25% higher than those of the parent samples. A Tc of 426°C and a magnetization of 120.6 emu/g observed for SmGdFe16SiCz are the highest values measured for the intermetallics investigated herein. As determined by x-ray diffraction studies of magnetically aligned samples, the easy axis of magnetization is parallel to the c axis

Investigating the suppressive effects of a low-frequency bias-tone on auditory-nerve responses to tail-frequency tones

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2011.Cataloged from PDF version of thesis.Includes bibliographical references (p. 63-64).Sounds are amplified and frequency analyzed in the cochlea and the resulting signals are sent to the brain through the auditory nerve (AN). AN fiber threshold vs. frequency-response characteristics are band-pass shaped, with a sensitive, sharply-tuned tip centered at the "characteristic frequency" (CF) of the fiber. It is now well established that the sharp frequency tuning and high sensitivity of the tip is produced by mechanical feedback amplification from cochlear outer hair cells (OHCs). Specifically, sound vibrates the cochlear basilar membrane and this motion is transmitted to the OHC mechano-electric transduction function which produces a voltage change in the OHCs. This voltage change, through OHC electromotility, feeds back energy that amplifies basilar-membrane motion. For AN fibers with CF > 5 kHz (high-CF fibers), the sharp tuning-curve tip region is flanked on its low-frequency side with a broadly-tuned, less-sensitive "tailfrequency" region where the role of OHC motility has been thought as not significant. Contrary to this prevailing view, it has been experimentally found that the electromotility of OHCs is involved in generating AN responses in a narrow band of frequencies near 2.5 kHz in the tail. The main objective of this thesis was to investigate whether and how the mechano-electric transduction function of OHCs is involved in generating AN responses to tail-frequency tone at 2.5 kHz, and to determine any differences in the details of the mechano-electric transduction function in the tail-frequency responses versus in low-level CF-tone responses. The experimental strategy was to use the suppressive effects of a low-frequency bias-tone on AN responses driven by the active motility of OHCs. AN responses are affected by the mechano-electric transduction function of OHCs such that AN responses to low-level CF-tones are suppressed twice per bias-tone period by high-level bias-tones. This suppression is due to the saturation of the mechano-electric transduction as the bias-tone moves the operating point of OHC mechano-electric transduction into the nonlinear saturation regions twice per period of the bias-tone. Specifically, a bias-tone at 50 Hz was applied together with a second probe-tone, which was either a low-level CF-tone or a nearthreshold tail-frequency tone at 2.5 kHz, in order to characterize and compare the suppressive effects on these two response types. Single AN fiber recordings were made from 27 fibers from 6 cats. As expected, the characteristic pattern of twice-per-bias-tone-period suppression was found from the low-level CF-tone responses from all of the recorded fibers. As for the tail-frequency tone responses, significant suppression was found on 10 of the 27 fibers recorded. Among those 10 fibers, the twice-per-biastone- period suppression pattern associated with the non-linearity of the mechano-electric transduction function of OHCs was found for fibers with CF 15 kHz may be due to the bias-tone sound levels not being high enough. These results directly show that the mechano-electric transduction function of OHCs is involved in generation of AN responses to 2.5 kHz tones for fibers with CF < 15 kHz in a similar way to how it is involved in producing cochlear amplification for low-level CF-tones. Further, comparisons of the details of the suppression pattern, e.g., most importantly the phase of major suppression, did not reveal any significant differences between the two response types. Overall these results indicate the detailed mechanisms of the OHC mechano-electric transduction function that are involved in producing the two responses types are similar. Additionally, the effects of a low-frequency bias-tone on the phase of AN responses to tailfrequency tones were compared with previous work on the effects of medial olivocochlear (MOC) efferent stimulation on the phase of AN responses to tail-frequency tones. Suppression of the gain of the OHC mechano-electric transduction function by a low-frequency bias-tone may affect the phase as well as the rate of AN responses driven by OHCs; however, effects of a low-frequency bias-tone on the phase of AN responses have not been reported in the literature. Bias-tone induced phase shifts were quantified as the difference in the phase of excitation to 2.5 kHz tones between bias-tone levels below and above the suppression threshold. Results were collected from five fibers. The shift in the phase of AN responses to 2.5 kHz tail-frequency tones induced by a bias-tone ranged from -45* (phase lag) to +5* (phase lead). A phase lag was found from 4 of the 5 fibers. These results are in general agreement with the MOC efferent effects on the phase of AN responses which ranged from -80* to +60* with an average of -15*, a phase lag. These results suggest that a lowfrequency bias-tone and MOC efferent stimulation affect the phase of AN responses to a tail-frequency tone through a similar mechanism by lowering the gain of the OHCs.by Hui S. Nam.S.M