Role of critical spin fluctuations in ultrafast demagnetization of transition-metal rare-earth alloys

Ultrafast magnetization dynamics induced by femtosecond laser pulses have been measured in ferrimagnetic Co0.8Gd0.2, Co.74Tb.26 and Co.86Tb.14 alloys. Using element sensitivity of X-ray magnetic circular dichroism at the Co L3, Tb M5 and Gd M5 edges we evidence that the demagnetization dynamics is element dependent. We show that a thermalization time as fast as 280 fs is observed for the rare-earth in the alloy, when the laser excited state temperature is below the compensation temperature. It is limited to 500 fs when the laser excited state temperature is below the Curie temperature (Tc). We propose critical spin fluctuations in the vicinity of TC as the mechanism which reduces the demagnetization rates of the 4f electrons in transition-metal rare-earth alloys whereas at any different temperature the limited demagnetization rates could be avoided.Comment: 11 pages, 4 figure

Structural dynamics during laser induced ultrafast demagnetization

The mechanism underlying femtosecond laser pulse induced ultrafast magnetization dynamics remains elusive despite two decades of intense research on this phenomenon. Most experiments focused so far on characterizing magnetization and charge carrier dynamics, while first direct measurements of structural dynamics during ultrafast demagnetization were reported only very recently. We here present our investigation of the infrared laser pulse induced ultrafast demagnetization process in a thin Ni film, which characterizes simultaneously magnetization and structural dynamics. This is achieved by employing femtosecond time resolved X-ray resonant magnetic reflectivity (tr-XRMR) as probe technique. The experimental results reveal unambiguously that the sub-picosecond magnetization quenching is accompanied by strong changes in non-magnetic X-ray reflectivity. These changes vary with reflection angle and changes up to 30$\%$ have been observed. Modeling the X-ray reflectivity of the investigated thin film, we can reproduce these changes by a variation of the apparent Ni layer thickness of up to 1$\%$. Extending these simulations to larger incidence angles we show that tr-XRMR can be employed to discriminate experimentally between currently discussed models describing the ultrafast demagnetization phenomenon

Element resolved ultrafast demagnetization rates in ferrimagnetic CoDy

Femtosecond laser induced ultrafast magnetization dynamics have been studied in multisublattice CoxDy1-x alloys. By performing element and time-resolved X-ray spectroscopy, we distinguish the ultrafast quenching of Co3d and Dy4f magnetic order when the initial temperatures are below (T=150K) or above (T=270K) the temperature of magnetic compensation (Tcomp). In accordance with former element-resolved investigations and theoretical calculations, we observe different dynamics for Co3d and Dy4f spins. In addition we observe that, for a given laser fluence, the demagnetization amplitudes and demagnetization times are not affected by the existence of a temperature of magnetic compensation. However, our experiment reveals a twofold increase of the ultrafast demagnetization rates for the Dy sublattice at low temperature. In parallel, we measure a constant demagnetization rate of the Co3d sublattice above and below Tcomp. This intriguing difference between the Dy4f and Co3d sublattices calls for further theoretical and experimental investigations.Comment: 6 Figure, 2 Table

The structure of human 5-lipoxygenase

The synthesis of both proinflammatory leukotrienes and anti-inflammatory lipoxins requires the enzyme 5-lipoxygenase (5-LOX). 5-LOX activity is short-lived, apparently in part because of an intrinsic instability of the enzyme. We identified a 5-LOX-specific destabilizing sequence that is involved in orienting the carboxyl terminus, which binds the catalytic iron. Here, we report the crystal structure at 2.4 angstrom resolution of human 5-LOX stabilized by replacement of this sequence

Hadrons in the Nuclear Medium

Quantum Chromodynamics, the microscopic theory of strong interactions, has not yet been applied to the calculation of nuclear wave functions. However, it certainly provokes a number of specific questions and suggests the existence of novel phenomena in nuclear physics which are not part of the the traditional framework of the meson-nucleon description of nuclei. Many of these phenomena are related to high nuclear densities and the role of color in nucleonic interactions. Quantum fluctuations in the spatial separation between nucleons may lead to local high density configurations of cold nuclear matter in nuclei, up to four times larger than typical nuclear densities. We argue here that experiments utilizing the higher energies available upon completion of the Jefferson Laboratory energy upgrade will be able to probe the quark-gluon structure of such high density configurations and therefore elucidate the fundamental nature of nuclear matter. We review three key experimental programs: quasi-elastic electro-disintegration of light nuclei, deep inelastic scattering from nuclei at $x>1$, and the measurement of tagged structure functions. These interrelated programs are all aimed at the exploration of the quark structure of high density nuclear configurations. The study of the QCD dynamics of elementary hard processes is another important research direction and nuclei provide a unique avenue to explore these dynamics. We argue that the use of nuclear targets and large values of momentum transfer at would allow us to determine whether the physics of the nucleon form factors is dominated by spatially small configurations of three quarks.Comment: 52 pages IOP style LaTex file and 20 eps figure

Conversion of human 5-lipoxygenase to a 15-lipoxygenase by a point mutation to mimic phosphorylation at Serine-663

The enzyme 5-lipoxygenase (5-LOX) initiates biosynthesis of the proinflammatory leukotriene lipid mediators and, together with 15-LOX, is also required for synthesis of the anti-inflammatory lipoxins. The catalytic activity of 5-LOX is regulated through multiple mechanisms, including Ca 2+-targeted membrane binding and phosphorylation at specific serine residues. To investigate the consequences of phosphorylation at S663, we mutated the residue to the phosphorylation mimic Asp, providing a homogenous preparation suitable for catalytic and structural studies. The S663D enzyme exhibits robust 15-LOX activity, as determined by spectrophotometric and HPLC analyses, with only traces of 5-LOX activity remaining; synthesis of the anti-inflammatory lipoxin A4 from arachidonic acid is also detected. The crystal structure of the S663D mutant in the absence and presence of arachidonic acid (in the context of the previously reported Stable-5-LOX) reveals substantial remodeling of helices that define the active site so that the once fully encapsulated catalytic machinery is solvent accessible. Our results suggest that phosphorylation of 5-LOX at S663 could not only down-regulate leukotriene synthesis but also stimulate lipoxin production in inflammatory cells that do not express 15-LOX, thus redirecting lipid mediator biosynthesis to the production of proresolving mediators of inflammation. © FASEB