121 research outputs found
Crystalline structure and orientation of gold clusters grown in preformed nanometer-sized pits
Abstract Gold clusters were produced by condensing evaporated gold in nanometer-sized preformed pits on the surface of highly Ž . oriented pyrolytic graphite HOPG . The height of the clusters was 6.7 " 0.7 nm as measured with scanning tunneling microscopy in ultrahigh vacuum, the lateral width was 10.1 " 1.9 nm as determined with transmission electron microscopy Ž . TEM . Using TEM for electron diffraction, we obtained information on the crystalline structure of the clusters. The Ž . intensity of the observed diffraction rings shows the preferential orientation of the clusters with the 111 plane of the gold Ž . lattice parallel to the 0001 surface of HOPG. This was compared to the diffraction pattern of gold clusters produced in the gas phase by inert-gas evaporation and deposited on a flat HOPG surface at room temperature as complete units which showed no preferential orientation. The directional alignment in the surface plane as it is described in the literature for larger gold crystallites grown on a flat HOPG surface is not observed for the nanometer-sized clusters grown in pits
Band-theoretical prediction of magnetic anisotropy in uranium monochalcogenides
Magnetic anisotropy of uranium monochalcogenides, US, USe and UTe, is studied
by means of fully-relativistic spin-polarized band structure calculations
within the local spin-density approximation. It is found that the size of the
magnetic anisotropy is fairly large (about 10 meV/unit formula), which is
comparable with experiment. This strong anisotropy is discussed in view of a
pseudo-gap formation, of which crucial ingredients are the exchange splitting
of U 5f states and their hybridization with chalcogen p states (f-p
hybridization). An anomalous trend in the anisotropy is found in the series
(US>>USe<UTe) and interpreted in terms of competition between localization of
the U 5f states and the f-p hybridization. It is the spin-orbit interaction on
the chalcogen p states that plays an essential role in enlarging the strength
of the f-p hybridization in UTe, leading to an anomalous systematic trend in
the magnetic anisotropy.Comment: 4 pages, 5 figure
Electronic structure of superconducting graphite intercalate compounds: The role of the interlayer state
Although not an intrinsic superconductor, it has been long--known that, when
intercalated with certain dopants, graphite is capable of exhibiting
superconductivity. Of the family of graphite--based materials which are known
to superconduct, perhaps the most well--studied are the alkali metal--graphite
intercalation compounds (GIC) and, of these, the most easily fabricated is the
CK system which exhibits a transition temperature K. By increasing the alkali metal concentration (through high pressure
fabrication techniques), the transition temperature has been shown to increase
to as much as K in CNa. Lately, in an important recent
development, Weller \emph{et al.} have shown that, at ambient conditions, the
intercalated compounds \cyb and \cca exhibit superconductivity with transition
temperatures K and K respectively, in excess
of that presently reported for other graphite--based compounds. We explore the
architecture of the states near the Fermi level and identify characteristics of
the electronic band structure generic to GICs. As expected, we find that charge
transfer from the intercalant atoms to the graphene sheets results in the
occupation of the --bands. Yet, remarkably, in all those -- and only
those -- compounds that superconduct, we find that an interlayer state, which
is well separated from the carbon sheets, also becomes occupied. We show that
the energy of the interlayer band is controlled by a combination of its
occupancy and the separation between the carbon layers.Comment: 4 Figures. Please see accompanying experimental manuscript
"Superconductivity in the Intercalated Graphite Compounds C6Yb and C6Ca" by
Weller et a
Ferromagnetism in Oriented Graphite Samples
We have studied the magnetization of various, well characterized samples of
highly oriented pyrolitic graphite (HOPG), Kish graphite and natural graphite
to investigate the recently reported ferromagnetic-like signal and its possible
relation to ferromagnetic impurities. The magnetization results obtained for
HOPG samples for applied fields parallel to the graphene layers - to minimize
the diamagnetic background - show no correlation with the magnetic impurity
concentration. Our overall results suggest an intrinsic origin for the
ferromagnetism found in graphite. We discuss possible origins of the
ferromagnetic signal.Comment: 11 figure
Advanced glycoxidation and lipoxidation end products (AGEs and ALEs): an overview of their mechanisms of formation
Advanced lipoxidation end products (ALEs) and advanced glycation end products (AGEs) have a pathogenetic role in the development and progression of different oxidative-based diseases including diabetes, atherosclerosis, and neurological disorders. AGEs and ALEs represent a quite complex class of compounds that are formed by different mechanisms, by heterogeneous precursors and that can be formed either exogenously or endogenously. There is a wide interest in AGEs and ALEs involving different aspects of research which are essentially focused on set-up and application of analytical strategies (1) to identify, characterize, and quantify AGEs and ALEs in different pathophysiological conditions ; (2) to elucidate the molecular basis of their biological effects ; and (3) to discover compounds able to inhibit AGEs/ALEs damaging effects not only as biological tools aimed at validating AGEs/ALEs as drug target, but also as promising drugs. All the above-mentioned research stages require a clear picture of the chemical formation of AGEs/ALEs but this is not simple, due to the complex and heterogeneous pathways, involving different precursors and mechanisms. In view of this intricate scenario, the aim of the present review is to group the main AGEs and ALEs and to describe, for each of them, the precursors and mechanisms of formation
- …