741 research outputs found
Spontaneous polarization and piezoelectricity in boron nitride nanotubes
Ab initio calculations of the spontaneous polarization and piezoelectric
properties of boron nitride nanotubes show that they are excellent
piezoelectric systems with response values larger than those of piezoelectric
polymers. The intrinsic chiral symmetry of the nanotubes induces an exact
cancellation of the total spontaneous polarization in ideal, isolated nanotubes
of arbitrary indices. Breaking of this symmetry by inter-tube interaction or
elastic deformations induces spontaneous polarization comparable to those of
wurtzite semiconductors.Comment: 5 pages in PRB double column format, 3 figure
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
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
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
Systems approaches and algorithms for discovery of combinatorial therapies
Effective therapy of complex diseases requires control of highly non-linear
complex networks that remain incompletely characterized. In particular, drug
intervention can be seen as control of signaling in cellular networks.
Identification of control parameters presents an extreme challenge due to the
combinatorial explosion of control possibilities in combination therapy and to
the incomplete knowledge of the systems biology of cells. In this review paper
we describe the main current and proposed approaches to the design of
combinatorial therapies, including the empirical methods used now by clinicians
and alternative approaches suggested recently by several authors. New
approaches for designing combinations arising from systems biology are
described. We discuss in special detail the design of algorithms that identify
optimal control parameters in cellular networks based on a quantitative
characterization of control landscapes, maximizing utilization of incomplete
knowledge of the state and structure of intracellular networks. The use of new
technology for high-throughput measurements is key to these new approaches to
combination therapy and essential for the characterization of control
landscapes and implementation of the algorithms. Combinatorial optimization in
medical therapy is also compared with the combinatorial optimization of
engineering and materials science and similarities and differences are
delineated.Comment: 25 page
Correlation between mechanical properties and processing conditions in rubber-toughened wood polymer composites
The use of wood fibers is a deeply investigated topic in current scientific research and one of their most common applications is as filler for thermoplastic polymers. The resulting material is a biocomposite, known as a Wood Polymer Composite (WPC). For increasing the sustainability and reducing the cost, it is convenient to increase the wood fiber content as much as possible, so that the polymeric fraction within the composite is thereby reduced. On the other hand, this is often thwarted by a sharp decrease in toughness and processability-a disadvantage that could be overcome by compounding the material with a toughening agent. This work deals with the mechanical properties in tension and impact of polypropylene filled with 50 wt.% wood flour, toughened with different amounts (0%, 10%, and 20%) of a polypropylene-based thermoplastic vulcanizate (TPV). Such properties are also investigated as a function of extrusion processing variables, such as the feeding mode (i.e., starve vs. flood feeding) and screw speed. It is found that the mechanical properties do depend on the processing conditions: the best properties are obtained either in starve feeding conditions, or in flood feeding conditions, but at a low screw speed. The toughening effect of TPV is significant when its content reaches 20 wt.%. For this percentage, the processing conditions are less relevant in governing the final properties of the composites in terms of the stiffness and strength
The effect of magnetic impurities in a two-band superconductor: A point-contact study of Mn-substituted MgB2 single crystals
We present the first results of directional point-contact measurements in
Mg_{1-x}Mn_{x}B_2 single crystals, with x up to 0.015 and bulk T_c down to 13.3
K. The order parameters Delta_{sigma} and Delta_{pi} were obtained by fitting
the conductance curves with the two-band Blonder-Tinkham-Klapwijk (BTK) model.
Both Delta_{pi} and Delta_{sigma} decrease with the critical temperature of the
junctions T_c^A, but remain clearly distinct up to the highest Mn content. Once
analyzed within the Eliashberg theory, the results indicate that spin-flip
scattering is dominant in the sigma band, as also confirmed by first-principle
band structure calculations.Comment: 4 pages, 5 eps figures. New theoretical results added, text and some
figures changed. References adde
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