21 research outputs found
Tuning fulleride electronic structure and molecular ordering via variable layer index
C60 fullerides are uniquely flexible molecular materials that exhibit a rich
variety of behavior, including superconductivity and magnetism in bulk
compounds, novel electronic and orientational phases in thin films, and quantum
transport in a single-C60 transistor. The complexity of fulleride properties
stems from the existence of many competing interactions, such as
electron-electron correlations, electron-vibration coupling, and intermolecular
hopping. The exact role of each interaction is controversial due to the
difficulty of experimentally isolating the effects of a single interaction in
the intricate fulleride materials. Here we report a unique level of control of
the material properties of KxC60 ultra-thin films through well-controlled
atomic layer indexing and accurate doping concentrations. Using STM techniques,
we observe a series of electronic and structural phase transitions as the
fullerides evolve from two-dimensional monolayers to quasi-threedimensional
multilayers in the early stages of layer-by-layer growth. These results
demonstrate the systematic evolution of fulleride electronic structure and
molecular ordering with variable KxC60 film layer index, and shed new light on
creating novel molecular structures and devices.Comment: 16 pages, 4 figures, to appear in Nature Material
Impact of DNA methylation on trophoblast function
The influence of epigenetics is evident in many fields of medicine today. This is also true in placentology, where versatile epigenetic mechanisms that regulate expression of genes have shown to have important influence on trophoblast implantation and placentation. Such gene regulation can be established in different ways and on different molecular levels, the most common being the DNA methylation. DNA methylation has been shown today as an important predictive component in assessing clinical prognosis of certain malignant tumors; in addition, it opens up new possibilities for non-invasive prenatal diagnosis utilizing cell-free fetal DNA methods. By using a well known demethylating agent 5-azacytidine in pregnant rat model, we have been able to change gene expression and, consequently, the processes of trophoblast differentiation and placental development. In this review, we describe how changes in gene methylation effect trophoblast development and placentation and offer our perspective on use of trophoblast epigenetic research for better understanding of not only placenta development but cancer cell growth and invasion as well
BAND-GAP, EXCITONS, AND COULOMB INTERACTION IN SOLID C-60
The band gap of solid C60 is found to be 2.3 ± 0.1 eV. The on-site molecular C60 Coulomb interaction (U) as determined from the KVV C60 Auger spectrum is found to be 1.6 ± 0.2 eV. This value of U is shown to lead to Frenkel-type molecular excitons in the 1.5-2 eV range. These results lead us to suggest that doped C60 should be considered as a highly correlated system with U/W comparable to that in high-Tc cuprates and that stoichiometric K3C60 is a Mott-Hubbard insulator with a gap of about 0.7 eV
Content-based image retrieval based on colour histogram and the discrete cosine transform
We study the static screening in a Hubbard-like model using fixed-node
diffusion Monte Carlo. We find that the random phase approximation is
surprisingly accurate even for metallic systems close to the Mott transition.
As a specific application we discuss the implications of the efficient
screening for the superconductivity in the doped Fullerenes. In the Monte Carlo
calculations we use trial functions with two Gutzwiller-type parameters. To
deal with such trial functions, we introduce a method for efficiently
optimizing the Gutzwiller parameters, both in variational and in fixed-node
diffusion Monte Carlo.Comment: 4 pages LaTeX with 4 eps figures; Computer Simulations Studies in
Condensed Matter Physics XII, Eds.: D.P. Landau, S.P. Lewis, and H.B.
Schuettler, Springer 1999; additional material available at
http://www.mpi-stuttgart.mpg.de/docs/ANDERSEN/fullerene
Bulk superconductivity at 38 K in a molecular system
C60-based solids1 are archetypal molecular superconductors with transition temperatures (Tc) as high as 33 K (refs 2–4). Tc of face-centred-cubic (f.c.c.) A3C60 (A=alkali metal) increases monotonically with inter C60 separation, which is controlled by the A+ cation size. As Cs+ is the largest such ion, Cs3C60 is a key material in this family. Previous studies revealing trace superconductivity in CsxC60 materials have not identified the structure or composition of the superconducting phase owing to extremely small shielding fractions and low crystallinity. Here, we show that superconducting Cs3C60 can be reproducibly isolated by solvent-controlled synthesis and has the highest Tc of any molecular material at 38 K. In contrast to other A3C60 materials, two distinct cubic Cs3C60 structures are accessible. Although f.c.c. Cs3C60 can be synthesized, the superconducting phase has the A15 structure based uniquely among fullerides on body-centred-cubic packing. Application of hydrostatic pressure controllably tunes A15 Cs3C60 from insulating at ambient pressure to superconducting without crystal structure change and reveals a broad maximum in Tc at 7 kbar. We attribute the observed Tc maximum as a function of inter C60separation—unprecedented in fullerides but reminiscent of the atom-based cuprate superconductors—to the role of strong electronic correlations near the metal–insulator transition onset