101 research outputs found
Detection of topological phase transitions through entropy measurements: the case of germanene
We propose a characterization tool for studies of the band structure of new
materials promising for the observation of topological phase transitions. We
show that a specific resonant feature in the entropy per electron dependence on
the chemical potential may be considered as a fingerprint of the transition
between topological and trivial insulator phases. The entropy per electron in a
honeycomb two-dimensional crystal of germanene subjected to the external
electric field is obtained from the first principle calculation of the density
of electronic states and the Maxwell relation. We demonstrate that, in
agreement to the recent prediction of the analytical model, strong spikes in
the entropy per particle dependence on the chemical potential appear at low
temperatures. They are observed at the values of the applied bias both below
and above the critical value that corresponds to the transition between the
topological insulator and trivial insulator phases, while the giant resonant
feature in the vicinity of zero chemical potential is strongly suppressed at
the topological transition point, in the low temperature limit. In a wide
energy range, the van Hove singularities in the electronic density of states
manifest themselves as zeros in the entropy per particle dependence on the
chemical potential.Comment: 8 pages, 5 figures; final version published in PR
In-situ Magnesium Diboride Superconducting Thin Films grown by Pulsed Laser Deposition
Superconducting thin films of MgB2 were deposited by Pulsed Laser Deposition
on magnesium oxide and sapphire substrates. Samples grown at 450C in an argon
buffer pressure of about 10-2 mbar by using a magnesium enriched target
resulted to be superconducting with a transition temperature of about 25 K.
Film deposited from a MgB2 sintered pellet target in ultra high vacuum
conditions showed poor metallic or weak semiconducting behavior and they became
superconducting only after an ex-situ annealing in Mg vapor atmosphere. Up to
now, no difference in the superconducting properties of the films obtained by
these two procedures has been evidenced.Comment: 10 pages, 4 figure
High quality MgB2 thin films in-situ grown by dc magnetron sputtering
Thin films of the recently discovered magnesium diboride (MgB2) intermetalic
superconducting compound have been grown using a magnetron sputtering
deposition technique followed by in-situ annealing at 830 C. High quality films
were obtained on both sapphire and MgO substrates. The best films showed
maximum Tc = 35 K (onset), a transition width of 0.5 K, a residual resistivity
ratio up to 1.6, a low temperature critical current density Jc > 1 MA/cm2 and
anisotropic critical field with gamma = 2.5 close to the values obtained for
single crystals. The preparation technique can be easily scaled to produce
large area in-situ films.Comment: 7 pages, 4 figure
2N+4-rule and an atlas of bulk optical resonances of zigzag graphene nanoribbons
Development of on-chip integrated carbon-based optoelectronic nanocircuits requires fast and non-invasive structural characterization of their building blocks. Recent advances in synthesis of single wall carbon nanotubes and graphene nanoribbons allow for their use as atomically precise building blocks. However, while cataloged experimental data are available for the structural characterization of carbon nanotubes, such an atlas is absent for graphene nanoribbons. Here we theoretically investigate the optical absorption resonances of armchair carbon nanotubes and zigzag graphene nanoribbons continuously spanning the tube (ribbon) transverse sizes from 0.5(0.4) nm to 8.1(12.8) nm. We show that the linear mapping is guaranteed between the tube and ribbon bulk resonance when the number of atoms in the tube unit cell is 2 N+ 4 , where N is the number of atoms in the ribbon unit cell. Thus, an atlas of carbon nanotubes optical transitions can be mapped to an atlas of zigzag graphene nanoribbons
Work function, deformation potential, and collapse of Landau levels in strained graphene and silicene
We perform a systematic {\it ab initio} study of the work function and its
uniform strain dependence for graphene and silicene for both tensile and
compressive strains. The Poisson ratios associated with armchair and zigzag
strains are also computed. Based on these results, we obtain the deformation
potential, crucial for straintronics, as a function of the applied strain.
Further, we propose a particular experimental setup with a special strain
configuration that generates only the electric field, while the pseudomagnetic
field is absent. Then, applying a real magnetic field, one should be able to
realize experimentally the spectacular phenomenon of the collapse of Landau
levels in graphene or related two-dimensional materials.Comment: 9 pages, 7 figures; final version published in PR
Exploring the interdependencies of research funders in the UK
Investment in medical research is vital to the continuing improvement of the UK's health and wealth. It is through research that we expand our understanding of disease and develop new treatments for patients. Medical research charities currently contribute over ÂŁ1 billion annually to medical research in the UK, of which over ÂŁ350 million is provided by Cancer Research UK. Many charities,
including Cancer Research UK, receive no government funding for their research
activity.
Cancer Research UK is engaged in a programme of work in order to better understand the medical research funding environment and demonstrate the importance of sustained investment. A key part of that is the Office of Health
Economicsâ (OHE) 2011 report âExploring the interdependency between public and charitable medical researchâ. This study found that there are substantial
benefits, both financial and qualitative, from the existence of a variety of funders and that reductions in the level of government financial support for medical
research are likely to have broader negative effects.
This contributed to other evidence which found that the activities and funding of the charity, public and private sectors respectively are complementary, i.e. mutually reinforcing, rather than duplicative or merely substituting for one another.
âExploring the interdependencies of research funders in the UKâ by the Office of Health Economics (OHE) and SPRU: Science and Technology Policy Research at the University of Sussex, represents a continued effort to build the evidence base around the funding of medical research.
This report uncovers the extent to which funders of cancer research are interdependent, nationally and internationally. Key figures show that two
thirds of publications acknowledging external support have relied on multiple funders, while just under half benefited from overseas funding, and almost a fifth are also supported by industry. In addition the analysis
shows that the general public would not want tax funding of cancer research to be reduced, but would not donate enough to charities to compensate for any such reduction
Anisotropy in c-oriented MgB2 thin films grown by Pulsed Laser Deposition
The electronic anisotropy in MgB2, is still a not completely clear topic;
high quality c-oriented films are suitable systems to investigate this
property. In this work we present our results on MgB2 superconducting thin
films grown on MgO and sapphire substrates. The films are deposited in high
vacuum, at room temperature, by Laser Ablation, starting from two different
targets: pure Boron and stoichiometric MgB2. In both cases, to obtain and
crystallize the superconducting phase, an ex-situ annealing in magnesium vapor
is needed. The films were characterized by Synchrotron radiation diffraction
measurements; the films turned out to be strongly c-oriented, with the c-axis
perpendicular to the film surface and an influence of the substrate on
crystallographic parameters is observed. Resisivity measurements with the
magnetic field perpendicular and parallel directions to the film surface
evidenced an anisotropic upper critical field behavior. The Hc2 ratios (h)
resulted in the range 1.2-1.8; this difference will be discussed also in
comparison with the literature data.Comment: presented at ISS 2001,Kobe,Japan. submitted to Physica
Growth methods of c-axis oriented MgB2 thin films by pulsed laser deposition
High quality MgB2 thin films have been obtained by pulsed laser deposition
both on MgO and on Al2O3 substrates using different methods. In the standard
two-step procedure, an amorphous precursor layer is deposited at room
temperature starting both from stoichiometric target and from boron target:
after this first step, it is annealed in magnesium atmosphere in order to
crystallize the superconducting phase. The so obtained films show a strong
c-axis orientation, evidenced by XRD analysis, a critical temperature up to 38
K and very high critical fields along the basal planes, up to 22T at 15K. Also
an in situ one step technique for the realization of superconducting MgB2 thin
films has been developed. In this case, the presence of an argon buffer gas
during deposition is crucial and we observe a strong dependence of the quality
of the deposited film on the background gas pressure. The influence of the Ar
atmosphere has been confirmed by time and space-resolved spectroscopy
measurements on the emission spectrum of the plume. The Ar pressure modifies
strongly the plasma kinetics by promoting excitation and ionization of the
plume species, especially of the most volatile Mg atoms, increasing their
internal energy.Comment: Paper presented at Boromag Workshop, Genoa 17-19 June 2002, in press
on SUS
Growth of c-oriented MgB2 thin films by Pulsed Laser Deposition: structural characterization and electronic anisotropy
MgB2 thin films were deposited using Pulsed Laser Deposition (PLD) and
ex-situ annealing in Mg atmosphere. The films presented critical temperatures
up to 36K and turned out to be preferentially c-oriented both on Al2O3 (r-cut)
and MgO(100) substrates. Synchrotron analyses gave also some indications of in
plane texturing. The films exhibit very fine grain size (1200angstromin the
basal plane and 100angstrom along c-axis) but the general resistivity behavior
and the remarkable extension of the irreversible region confirm that the grains
boundaries are not barriers for supercurrents. Upper critical field
measurements with the magnetic field perpendicular and parallel with respect to
the film surface evidenced a field anisotropy ratio of 1.8. The Hc2 values are
considerably higher with respect to the bulk ones, namely when the field lies
in the basal plane, and the field-temperature phase diagram for the two
magnetic field orientations suggest the possibility of strongly enhancing the
pinning region by means of texturing.Comment: 20 pages, 6 figure
On farm agronomic and first environmental evaluation of oil crops for sustainable bioenergy chains.
Energy crops, and in particular oil crops, could be an important occasion for developing new non food production rows for a new multi-functional agriculture in Italy. In this view, the use of local biomass is a fundamental starting point for the development of a virtuous energy chain that should pursue not only agricultural profitability, but also chain sustainability and that is less dependent on the global market, characterized by instability in terms of biomass availability and price. From this perspective, particular attention must be paid to crop choice on the basis of its rusticity and of its adaptability to local growing conditions and to low input cropping systems. In this context, alike woody and herbaceous biomasses, oil crops such as sunflower and rapeseed should be able to support local agricultural bioenergy chain in Italy. In addition, in a local bioenergy chain, the role of the farmers should not be limited just to grain production; but also grain processing should be performed at farm or consortium level in oilseed extraction plants well proportioned to the cropped surface. In this way, by means of a simple power generator, farmer could thus produce its own thermal and electric energy from the oil, maximizing his profit. This objective could also be achieved through the exploitation of the total biomass, including crop residues and defatted seed meals, that may be considered as fundamental additional economic and/or environmental benefits of the chain. This paper reports some results of three-years on-farm experiments on oil crop chain carried out in the framework of "Bioenergie" project, that was focused to enhance farmers awareness of these criteria and to the feasibility at open field scale of low-input cultivation of rapeseed, sunflower and Brassica carinata in seven Italian regions. In several on-farm experiences, these crops produced more than 800 kg ha-1 of oil with good energy properties. Defatted seed meals could be interesting as organic fertilizers and, in the case of B. carinata, as a biofumigant amendment that could offer a total or partial alternative to some chemicals in agriculture. Furthermore, biomass soil incorporation could contribute to C sequestration, catching CO2 from atmosphere and sinking a part in soil as stable humus. Finally, four different open field experiences carried out again in the second year of the project, have been analysed in order to evaluate their energy and greenhouse gasses balance after cultivation phase
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