1,145 research outputs found
G-quartet biomolecular nanowires
We present a first-principle investigation of quadruple helix nanowires,
consisting of stacked planar hydrogen-bonded guanine tetramers. Our results
show that long wires form and are stable in potassium-rich conditions. We
present their electronic bandstructure and discuss the interpretation in terms
of effective wide-bandgap semiconductors. The microscopic structural and
electronic properties of the guanine quadruple helices make them suitable
candidates for molecular nanoelectronics.Comment: 7 pages, 3 figures, to be published in Applied Physics Letters (2002
Evidence for pseudogap and phase-coherence gap separation by Andreev reflection experiments in Au/La_{2-x}Sr_{x}CuO_4 point-contact junctions
We present new Au/La_{2-x}Sr_{x}CuO_{4} (LSCO) point-contact conductance
measures as a function of voltage and temperature in samples with 0.08 <= x <=
0.2. Andreev reflection features disappear at about the bulk Tc, giving no
evidence of gap for T > Tc. The fit of the normalized conductance at any T < Tc
supports a (s + d)-wave symmetry of the gap, whose dominant low-T s component
follows the Tc(x) curve in contrast with recent angle-resolved photoemission
spectroscopy and quasiparticle tunneling data. These results prove the
separation between pseudogap and phase-coherence superconducting gap in LSCO at
x <= 0.2.Comment: 4 pages, 4 eps figures, 1 table (RevTeX). Labels added to Fig. 1;
Fig. 3 resized; references added; short discussion about ballistic contact
regime adde
First principle theory of correlated transport through nano-junctions
We report the inclusion of electron-electron correlation in the calculation
of transport properties within an ab initio scheme. A key step is the
reformulation of Landauer's approach in terms of an effective transmittance for
the interacting electron system. We apply this framework to analyze the effect
of short range interactions on Pt atomic wires and discuss the coherent and
incoherent correction to the mean-field approach.Comment: 5 pages, 3 figure
Josephson effect in MgB_2 break junctions
We present the first observation of the DC and AC Josephson effect in MgB_2
break junctions. The junctions, obtained at 4.2 K in high-quality, high-density
polycrystalline metallic MgB_2 samples, show a non-hysteretic DC Josephson
effect. By irradiating the junctions with microwaves we observe clear Shapiro
steps spaced by the ideal value. The temperature dependence of the
DC Josephson current and the dependence of the height of the steps on the
microwave power are obtained. These results are a direct prove for the
existence of pairs with charge 2e in this new metallic superconductor and give
evidence of the superconductor-normal metal-superconductor weak link character
of these junctions.Comment: 4 RevTEX pages, 4 eps figure
High availability using virtualization - 3RC
High availability has always been one of the main problems for a data center.
Till now high availability was achieved by host per host redundancy, a highly
expensive method in terms of hardware and human costs. A new approach to the
problem can be offered by virtualization. Using virtualization, it is possible
to achieve a redundancy system for all the services running on a data center.
This new approach to high availability allows the running virtual machines to
be distributed over a small number of servers, by exploiting the features of
the virtualization layer: start, stop and move virtual machines between
physical hosts. The 3RC system is based on a finite state machine, providing
the possibility to restart each virtual machine over any physical host, or
reinstall it from scratch. A complete infrastructure has been developed to
install operating system and middleware in a few minutes. To virtualize the
main servers of a data center, a new procedure has been developed to migrate
physical to virtual hosts. The whole Grid data center SNS-PISA is running at
the moment in virtual environment under the high availability system.Comment: 10 page
Functional characterization of the sea urchin sns chromatin insulator in erythroid cells
Chromatin insulators are regulatory elements that determine domains of genetic functions. We have previously described the characterization of a 265 bp insulator element, termed sns, localized at the 3' end of the early historic H2A gene of the sea urchin Paracentrotus lividus. This sequence contains three cis-acting elements (Box A, Box B, and Box C+T) all needed for the enhancer-blocking activity in both sea urchin and human cells. The goal of this Study was to further characterize the sea urchin sns insulator in the erythroid environment. We employed colony assays in human (K562) and mouse (MEL) erythroid cell lines. We tested the capability of sns to interfere with the communication between the 5HS2 enhancer of the human beta-globin LCR and the gamma-globin promoter. We found that the sns sequence displays directional enhancer-blocking activity. By the use of antibodies against known DNA binding proteins, in electrophoretic mobility shift assays, we demonstrated the binding of the erythroid-specific GATA-1 and the ubiquitous Oct-1 and Sp1 transcription factors. These factors bind to Box A, Box B, and Box C+T, respectively, in both K562 and MEL nuclear extracts. These results may have significant implications for the conservation of insulator function ill evolutionary distant organisms and may prove to be of practical benefit in gene transfer applications for erythroid disorders such as hemoglobinopathies and thalassemias
Point-contact Andreev-reflection spectroscopy in segregation-free Mg_{1-x}Al_{x}B_2 single crystals up to x=0.32
We present new results of point-contact Andreev-reflection (PCAR)
spectroscopy in single-phase Mg_{1-x}Al_{x}B_{2} single crystals with x up to
0.32. Fitting the conductance curves of our point contacts with the two-band
Blonder-Tinkham-Klapwijk model allowed us to extract the gap amplitudes
Delta_{sigma} and Delta_{pi}. The gap values agree rather well with other PCAR
results in Al-doped crystals and polycrystals up to x=0.2 reported in
literature, and extend them to higher Al contents. In the low-doping regime,
however, we observed an increase in the small gap Delta_{pi} on increasing x
(or decreasing the local critical temperature of the junctions, T_{c}^{A})
which is not as clearly found in other samples. On further decreasing T_{c}^{A}
below 30 K, both the gaps decrease and, up to the highest doping level x=0.32
and down to T_{c}^{A}= 12 K, no gap merging is observed. A detailed analysis of
the data within the two-band Eliashberg theory shows that this gap trend can be
explained as being mainly due to the band filling and to an increase in the
interband scattering which is necessary to account for the increase in
Delta_{pi} at low Al contents (x < 0.1). We suggest to interpret the following
decrease of Delta_{pi} for T_{c}^{A} < 30 K as being governed by the onset of
inhomogeneity and disorder in the Al distribution that partly mask the
intrinsic effects of doping and is not taken into account in standard
theoretical approaches.Comment: 22 pages, 9 eps figures, Elsevier style. Theoretical details added in
appendix. Characterization of crystals include
Evidence for Single-gap Superconductivity in Mg(B_{1-x}C_x)_2 Single Crystals with x=0.132 from Point-Contact Spectroscopy
We report the results of the first directional point-contact measurements in
Mg(B_{1-x}C_{x})_2 single crystals with 0.047 <= x <= 0.132. The two-gap
superconductivity typical of MgB_2 persists up to x=0.105. In this region, the
values of the gaps Delta_{sigma} and Delta_{pi} were determined by fitting the
Andreev-reflection conductance curves with a two-band Blonder-Tinkham-Klapwijk
(BTK) model, and confirmed by the single-band BTK fit of the sigma- and pi-band
conductances, separated by means of a magnetic field. At x=0.132, when T_{c}=19
K, we clearly observed for the first time the merging of the two gaps into one
of amplitude Delta~3 meV.Comment: 5 pages, 5 figures. One figure and one panel added; text and
discussion update
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