Glass transition in fullerenes: mode-coupling theory predictions

We report idealized mode-coupling theory results for the glass transition of ensembles of model fullerenes interacting via phenomenological two-body potentials. Transition lines are found for C60, C70 and C96 in the temperature-density plane. We argue that the observed glass-transition behavior is indicative of kinetic arrest that is strongly driven by the inter-particle attraction in addition to excluded-volume repulsion. In this respect, these systems differ from most standard glass-forming liquids. They feature arrest that occurs at lower densities and that is stronger than would be expected for repulsion-dominated hard-sphere-like or Lennard-Jones-like systems. The influence of attraction increases with increasing the number of carbon atoms per molecule. However, unrealistically large fullerenes would be needed to yield behavior reminiscent of recently investigated model colloids with strong short-ranged attraction (glass-glass transitions and logarithmic decay of time-correlation functions).Comment: 10 pages, 5 figure

The Dynamics of Silica Melts under High Pressure: Mode-Coupling Theory Results

The high-pressure dynamics of a computer-modeled silica melt is studied in the framework of the mode-coupling theory of the glass transition (MCT) using static-structure input from molecular-dynamics (MD) computer simulation. The theory reproduces the experimentally known viscosity minimum (diffusivity maximum) as a function of density or pressure and explains it in terms of a corresponding minimum in its critical temperature. This minimum arises from a gradual change in the equilibrium static structure which shifts from being dominated by tetrahedral ordering to showing the cageing known from high-density liquids. The theory is in qualitative agreement with computer simulation results.Comment: Presented at ESF EW Glassy Liquids under Pressure, to be published in Journal of Physic

Magnetic order in CaFe1-xCoxAsF (x = 0, 0.06, 0.12) superconductor compounds

A Neutron Powder Diffraction (NPD) experiment has been performed to investigate the structural phase transition and magnetic order in CaFe1-xCoxAsF superconductor compounds (x = 0, 0.06, 0.12). The parent compound CaFeAsF undergoes a tetragonal to orthorhombic phase transition at 134(3) K, while the magnetic order in form of a spin-density wave (SDW) sets in at 114(3) K. The antiferromagnetic structure of the parent compound has been determined with a unique propagation vector k = (1,0,1) and the Fe saturation moment of 0.49(5)uB aligned along the long a-axis. With increasing Co doping, the long range antiferromagnetic order has been observed to coexist with superconductivity in the orthorhombic phase of the underdoped CaFe0.94Co0.06AsF with a reduced Fe moment (0.15(5)uB). Magnetic order is completely suppressed in optimally doped CaFe0.88Co0.12AsF. We argue that the coexistence of SDW and superconductivity might be related to mesoscopic phase separation.Comment: 4pages, 4figure

Spitzer Space Telescope evidence in NGC 6791: no super-mass-loss at super-solar metallicity to explain helium white dwarfs?

We use archival Spitzer Space Telescope photometry of the old, super-solar metallicity massive open cluster NGC 6791 to look for evidence of enhanced mass loss, which has been postulated to explain the optical luminosity function and low white dwarf masses in this benchmark cluster. We find a conspicuous lack of evidence for prolificacy of circumstellar dust production that would have been expected to accompany such mass loss. We also construct the optical and infrared luminosity functions, and demonstrate that these fully agree with theoretical expectations. We thus conclude that there is no evidence for the mass loss of super-solar metallicity red giants to be sufficiently high that they can avoid the helium flash at the tip of the red giant branch.Comment: accepted for publication in ApJ Letter

Neutron diffraction study on phase transition and thermal expansion of SrFeAsF

The magnetic ordering and crystal structure of iron pnictide SrFeAsF was investigated by using neutron powder diffraction method. With decreasing temperature, the tetragonal to orthorhombic phase transition is found at 180 K, while the paramagnetic to antiferromagnetic phase transition set in at 133 K. Similar to the parent compound of other iron pnictide system, the striped Fe magnetism is confirmed in antiferromagnetic phase and the Fe moment of 0.58(6) uB aligned along long a axis. The thermal expansion of orthorhombic phase of SrFeAsF is also investigated. Based on the Grueneisen approximation and Debye approximation for internal energy, the volume of SrFeAsF can be well fitted with Debye temperature of 347(5) K. The experimental atomic displacement parameters for different crystallographic sites in SrFeAsF are analyzed with Debye model. The results suggested that the expansion of FeAs layers plays an important role in determining the thermal expansion coefficient.Comment: 4 pages, 3 figure

Role of structural relaxations and vibrational excitations in the high-frequency dynamics of liquids and glasses

We present theoretical investigation on the high-frequency collective dynamics in liquids and glasses at microscopic length scales and terahertz frequency region based on the mode-coupling theory for ideal liquid-glass transition. We focus on recently investigated issues from inelastic-X-ray-scattering and computer-simulation studies for dynamic structure factors and longitudinal and transversal current spectra: the anomalous dispersion of the high-frequency sound velocity and the nature of the low-frequency excitation called the boson peak. It will be discussed how the sound mode interferes with other low-lying modes present in the system. Thereby, we provide a systematic explanation of the anomalous sound-velocity dispersion in systems -- ranging from high temperature liquid down to deep inside the glass state -- in terms of the contributions from the structural-relaxation processes and from vibrational excitations called the anomalous-oscillation peak (AOP). A possibility of observing negative dispersion -- the {\em decrease} of the sound velocity upon increase of the wave number -- is argued when the sound-velocity dispersion is dominated by the contribution from the vibrational dynamics. We also show that the low-frequency excitation, observable in both of the glass-state longitudinal and transversal current spectra at the same resonance frequency, is the manifestation of the AOP. As a consequence of the presence of the AOP in the transversal current spectra, it is predicted that the transversal sound velocity also exhibits the anomalous dispersion. These results of the theory are demonstrated for a model of the Lennard-Jones system.Comment: 25 pages, 22 figure

Colloidal gelation and non-ergodicity transitions

Within the framework of the mode coupling theory (MCT) of structural relaxation, mechanisms and properties of non-ergodicity transitions in rather dilute suspensions of colloidal particles characterized by strong short-ranged attractions are studied. Results building on the virial expansion for particles with hard cores and interacting via an attractive square well potential are presented, and their relevance to colloidal gelation is discussed.Comment: 10 pages, 4 figures; Talk at the Conference: "Unifying Concepts in Glass Physics" ICTP Trieste, September 1999; to be published in J. Phys.: Condens. Matte

Complete Genome Sequence of the Cystic Fibrosis Pathogen Achromobacter xylosoxidans NH44784-1996 Complies with Important Pathogenic Phenotypes

Achromobacter xylosoxidans is an environmental opportunistic pathogen, which infects an increasing number of immunocompromised patients. In this study we combined genomic analysis of a clinical isolated A. xylosoxidans strain with phenotypic investigations of its important pathogenic features. We present a complete assembly of the genome of A. xylosoxidans NH44784-1996, an isolate from a cystic fibrosis patient obtained in 1996. The genome of A. xylosoxidans NH44784-1996 contains approximately 7 million base pairs with 6390 potential protein-coding sequences. We identified several features that render it an opportunistic human pathogen, We found genes involved in anaerobic growth and the pgaABCD operon encoding the biofilm adhesin poly-?-1,6-N-acetyl-D-glucosamin. Furthermore, the genome contains a range of antibiotic resistance genes coding efflux pump systems and antibiotic modifying enzymes. In vitro studies of A. xylosoxidans NH44784-1996 confirmed the genomic evidence for its ability to form biofilms, anaerobic growth via denitrification, and resistance to a broad range of antibiotics. Our investigation enables further studies of the functionality of important identified genes contributing to the pathogenicity of A. xylosoxidans and thereby improves our understanding and ability to treat this emerging pathogen

Spin relaxation: From 2D to 1D

In inversion asymmetric semiconductors, spin-orbit interactions give rise to very effective relaxation mechanisms of the electron spin. Recent work, based on the dimensionally constrained D'yakonov Perel' mechanism, describes increasing electron-spin relaxation times for two-dimensional conducting layers with decreasing channel width. The slow-down of the spin relaxation can be understood as a precursor of the one-dimensional limit

Observation of supercurrent enhancement in SNS junctions by non-equilibrium injection into supercurrent carrying bound Andreev states

We report for the first time enhancement of the supercurrent by means of injection in a mesoscopic three terminal planar SNSNS device made of Al on GaAs. When a current is injected from one of the superconducting Al electrodes at an injection bias $V=\Delta(T)/e$, the DC Josephson current between the other two superconducting electrodes has a maximum, giving evidence for an enhancement due to a non-equilibrium injection into bound Andreev states of the underlying semiconductor. The effect persists to temperatures where the equilibrium supercurrent has vanished.Comment: 7 pages + 3 figures. Resubmitted to Phys. Rev. Lett. Contents change