1,057 research outputs found
Critical Field of MGB2 : Crossover from Clean to Dirty Regimes
We have studied the upper critical field, Bc2, in poly-crystalline MgB2
samples in which disorder was varied in a controlled way to carry selectively p
and s bands from clean to dirty limit. We have found that the clean regime
survives when p bands are dirty and s bands are midway between clean and dirty.
In this framework we can explain the anomalous behaviour of Al doped samples,
in which Bc2 decreases as doping increases.Comment: 11 pages, 2 figure
The Crossover beteween Aslamazov-Larkin and Short Wavelength Fluctuations Regimes in HTS Conductivity Experiments
We present paraconductivity (AL) measurements in three different high
temperature superconductors: a melt textured sample, a
epitaxial thin film and a highly textured
tape. The crossovers between different temperature
regimes in excess conductivity have been analysed. The Lawrence-Doniach (LD)
crossover, which separates the 2D and 3D regimes, shifts from lower to higher
temperatures as the compound anisotropy decreases. Once the LD crossover is
overcome, the fluctuation conductivity of the three compounds shows the same
universal behaviour: for all the curves bend down
according to the law. This asymptotic behaviour was
theoretically predicted previously for the high temperature region where the
short wavelength fluctuations (SWF) become important.Comment: 4 pages, Revtex, 1 PostScript figure available upon request
([email protected]); submitted to Physical Review B rapid communication
Decoherent quantum walks driven by a generic coin operation
We consider the effect of different unitary noise mechanisms on the evolution
of a quantum walk (QW) on a linear chain with a generic coin operation: (i)
bit-flip channel noise, restricted to the coin subspace of the QW, and (ii)
topological noise caused by randomly broken links in the linear chain.
Similarities and differences in the respective decoherent dynamics of the
walker as a function of the probability per unit time of a decoherent event
taking place are discussed
Bismuth-Gold absorber for large area TES spiderweb bolometer
Large area spiderweb bolometer of about one centimetre diameter are required for matching multimode or quasi-optical cavities in microwave antenna for CMB measurements as proposed for the Large Scale Polarization Explorer balloon borne sky survey at 140, 220, 250 GHz. Possible applications at low frequencies, 40 GHz or less, in single mode are also foreseen. The main drawback of such large absorber is the achievement of an optimal trade-off among the thermal properties, like fast internal thermal diffusivity, heat capacity and milli-second recovery time and EM characteristics, like the matching impedance and EM power dissipation. In parallel with standard micropatterned gold film absorber deposited onto silicon nitride membrane, we have tested the Bismuth Gold in order to reduce the heat capacity even if with an increase of resistivity. Films of Bismuth Gold may have low resistivity under application of a proper post-production thermal cycle. We present the fabrication method of Bismuth Gold films for our microwave absorbers and the bolometer characterization at low temperature
Enhanced flux pinning in neutron irradiated MgB2
We study the effect of neutron irradiation on the critical current density Jc
of isotopically pure polycrystalline Mg11B2 samples. For fluences in the range
1017-1018 cm-2, Jc is enhanced and its dependence on magnetic field is
significantly improved: we demonstrate that, in this regime, point-like pinning
centers are effectively introduced in the system proportionally to the neutron
fluence. Instead, for larger fluences, a strong suppression of the critical
temperature accompanied by a decrease of both the upper critical field Bc2 and
Jc is found.Comment: 13 pages, 3 igure
High binding yet accelerated guest rotation within a cucurbit[7]uril complex. Toward paramagnetic gyroscopes and rolling nanomachines †
International audienceThe (15-oxo-3,7,11-triazadispiro[5.1.5.3]hexadec-7-yl)oxidanyl, a bis-spiropiperidinium nitroxide derived from TEMPONE, can be included in cucurbit[7]uril to form a strong (K a ∼ 2 × 10 5 M −1) CB[7]@bPTO complex. EPR and MS spectra, DFT calculations, and unparalleled increased resistance (a factor of ∼10 3) toward ascorbic acid reduction show evidence of deep inclusion of bPTO inside CB[7]. The unusual shape of the CB[7]@bPTO EPR spectrum can be explained by an anisotropic Brownian rotational diffusion, the global tumbling of the complex being slower than rotation of bPTO around its " long molecular axis " inside CB[7]. The CB[7] (stator) with the encapsulated bPTO (rotator) behaves as a supramolecular para-magnetic rotor with increased rotational speed of the rotator that has great potential for advanced nano-scale machines requiring wheels such as cucurbiturils with virtually no friction between the wheel and the axle for optimum wheel rotation (i.e. nanopulleys and nanocars)
The Impact of Climate Change on Agriculture and Health Sectors in Tanzania: A review
The impact of climate change in Tanzania is dynamic and differs among regions as they are impacted in different ways. While other regions experience normal rainfall and temperature patterns, others have continued to experience temperature extremes, severe droughts, decline in crops production coupled with food insecurity, extreme weather episodes of heavy rainfall associated with floods, loss of lives and infectious disease outbreaks. Despite the effects of climate change being recognized in the country, awareness is limited among local people, in particular the vulnerable communities. Thus, this review aims to raise awareness by giving a broader picture of impacts of climate change on agriculture and health sector. It reveals that in many parts of Tanzania, agriculture and health sectors may continue to suffer from the effects of climate change aggregated with limited awareness among communities. It is expected, that outbreaks of infectious diseases including malaria and cholera may increase as they correlate positively with high temperatures and rainfall. As a result, health problems and deaths of people, and reduced crops production will continue. Therefore, it is recommended that, the best way to overcome climate change is to invest effectively on the irrigation agriculture; and the health sector's budget should be enough to improve health care services and prepare for outbreaks of climate change sensitive diseases. Most importantly, provision of climate change awareness to the vulnerable communities must be seriously considered. About 50 peer-reviewed articles, government and International reports published between 2000 and 2017 were reviewed
Dynamical Localization in Quasi-Periodic Driven Systems
We investigate how the time dependence of the Hamiltonian determines the
occurrence of Dynamical Localization (DL) in driven quantum systems with two
incommensurate frequencies. If both frequencies are associated to impulsive
terms, DL is permanently destroyed. In this case, we show that the evolution is
similar to a decoherent case. On the other hand, if both frequencies are
associated to smooth driving functions, DL persists although on a time scale
longer than in the periodic case. When the driving function consists of a
series of pulses of duration , we show that the localization time
increases as as the impulsive limit, , is
approached. In the intermediate case, in which only one of the frequencies is
associated to an impulsive term in the Hamiltonian, a transition from a
localized to a delocalized dynamics takes place at a certain critical value of
the strength parameter. We provide an estimate for this critical value, based
on analytical considerations. We show how, in all cases, the frequency spectrum
of the dynamical response can be used to understand the global features of the
motion. All results are numerically checked.Comment: 7 pages, 5 figures included. In this version is that Subsection III.B
and Appendix A on the quasiperiodic Fermi Accelerator has been replaced by a
reference to published wor
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