124 research outputs found
The influence of twin boundaries on the Flux Line Lattice structure in YBaCuO: a study by Small Angle Neutron Scattering
The influence of Twin Boundaries (TB) on the Flux Line Lattice(FLL) structure
was investigated by Small Angle Neutron Scattering (SANS). YBaCuO single
crystals possessing different TB densities were studied. The SANS experiments
show that the TB strongly modify the structure of the FLL. The flux lines
meander as soon as the magnetic field makes an angle with the TB direction.
According to the value of this angle but also to the ratio of the flux lines
density over the TB density, one observes that the FLL exhibits two different
unit cells in the plane perpendicular to the magnetic field. One is the
classical hexagonal and anisotropic cell while the other is affected by an
additional deformation induced by the TB. We discuss a possible relation
between this deformation and the increase of the critical current usually
observed in heavily twinned samples.Comment: accepted for publication in Phys Rev
Metastable states of a flux line lattice studied by transport and Small Angle Neutron Scattering
Flux Lines Lattice (FLL) states have been studied using transport
measurements and Small Angle Neutron Scattering in low T materials. In
Pb-In, the bulk dislocations in the FLL do not influence the transport
properties. In Fe doped NbSe, transport properties can differ after a
Field Cooling (FC) or a Zero Field Cooling (ZFC) procedure, as previously
reported. The ZFC FLL is found ordered with narrow Bragg Peaks and is linked to
a linear V(I) curve and to a superficial critical current. The FC FLL pattern
exhibits two Bragg peaks and the corresponding V(I) curve shows a S-shape. This
can be explained by the coexistence of two ordered FLL slightly tilted from the
applied field direction by different superficial currents. These currents are
wiped out when the transport current is increased.Comment: accepted for publication in Phys. Rev.
A study of flux lines lattice order and critical current with time of flight small angle neutron scattering
Small angle neutron scattering (SANS) is an historical technique to study the
flux lines lattice (FLL) in a superconductor. Structural characteristics of the
FLL can be revealed, providing fundamental information for the physics of
vortex lattice.
However, the spatial resolution is limited and all the correlation lengths of
order are difficult to extract with precision.
We show here that a time of flight technique reveals the Bragg peak of the
FLL, and also its translational order with a better resolution.
We discuss the implication of these results for pinning mechanisms in a
Niobium sample.Comment: accepted in PR
Why pinning by surface irregularities can explain the peak effect in transport properties and neutron diffraction results in NbSe2 and Bi-2212 crystals?
The existence of a peak effect in transport properties (a maximum of the
critical current as function of magnetic field) is a well-known but still
intriguing feature of type II superconductors such as NbSe2 and Bi-2212. Using
a model of pinning by surface irregularities in anisotropic superconductors, we
have developed a calculation of the critical current which allows estimating
quantitatively the critical current in both the high critical current phase and
in the low critical current phase. The only adjustable parameter of this model
is the angle of the vortices at the surface. The agreement between the
measurements and the model is really very impressive. In this framework, the
anomalous dynamical properties close to the peak effect is due to co-existence
of two different vortex states with different critical currents. Recent neutron
diffraction data in NbSe2 crystals in presence of transport current support
this point of view
Freezing of Spinodal Decompostion by Irreversible Chemical Growth Reaction
We present a description of the freezing of spinodal decomposition in
systems, which contain simultaneous irreversible chemical reactions, in the
hydrodynamic limit approximation. From own results we conclude, that the
chemical reaction leads to an onset of spinodal decomposition also in the case
of an initial system which is completely miscible and can lead to an extreme
retardation of the dynamics of the spinodal decomposition, with the probability
of a general freezing of this process, which can be experimetally observed in
simultaneous IPN formation.Comment: 10 page
Mineralogy and microporous structure of rocks from a natural CO2 reservoir
Different experimental approaches have been combined to reconstruct the mineral association and microporous structure of rocks from a natural CO2 reservoir.
The fluid reservoir (Caprese Reservoir), was discovered while drilling PSS1 (Pieve Santo Stefano 1) wellbore in San Cassiano Basin (Eastern Tuscany, Central Italy, and consists of sedimentary rocks (Burano Fm.) interbedded with altered volcanic rocks, its depth being about 3,300 m with respect to the land surface. At 3,700 m depth fluid pressure and temperature are 700 bar and 120 \ub0C respectively, with a density for the supercritical CO2\u2013rich fluid of 840 Kg\ub7m-3.
The study was conducted on the volcanic rocks altered by CO2 from the PSS1 wellbore drillcores and on some volcanic rocks unaffected by the presence of CO2. Lastly, rocks from the Burano Formation, unavailable from PSS1, have been sampled on outcrop.
Focus is on rocks samples corresponding to the depth 3,864-3,871 m with respect to PSS1 log, which have been investigated with SEM-EDS and XRD for mineralogical characterization. Moreover, Small Angle Neutron Scattering (SANS) experiments at LLB (Saclay, France) served for microporous structure investigation of PSS1 rocks, and other volcanic rocks from Eastern Alps (IG1) and the Albani Hills (IG2 and IG3) unaffected by CO2, as well as Burano Formation rocks from outcrop
Optimal MHC-II-restricted tumor antigen presentation to CD4+ T helper cells: the key issue for development of anti-tumor vaccines
Present immunoprevention and immunotherapeutic approaches against cancer suffer from the limitation of being not βsterilizingβ procedures, as very poor protection against the tumor is obtained. Thus newly conceived anti-tumor vaccination strategies are urgently needed. In this review we will focus on ways to provide optimal MHC class II-restricted tumor antigen presentation to CD4+ T helper cells as a crucial parameter to get optimal and protective adaptive immune response against tumor. Through the description of successful preventive or therapeutic experimental approaches to vaccinate the host against the tumor we will show that optimal activation of MHC class II-restricted tumor specific CD4+ T helper cells can be achieved in various ways. Interestingly, the success in tumor eradication and/or growth arrest generated by classical therapies such as radiotherapy and chemotherapy in some instances can be re-interpreted on the basis of an adaptive immune response induced by providing suitable access of tumor-associated antigens to MHC class II molecules. Therefore, focussing on strategies to generate better and suitable MHC class IIβrestricted activation of tumor specific CD4+ T helper cells may have an important impact on fighting and defeating cancer
Retrotransposon-Induced Heterochromatin Spreading in the Mouse Revealed by Insertional Polymorphisms
The βarms raceβ relationship between transposable elements (TEs) and their host has promoted a series of epigenetic silencing mechanisms directed against TEs. Retrotransposons, a class of TEs, are often located in repressed regions and are thought to induce heterochromatin formation and spreading. However, direct evidence for TEβinduced local heterochromatin in mammals is surprisingly scarce. To examine this phenomenon, we chose two mouse embryonic stem (ES) cell lines that possess insertionally polymorphic retrotransposons (IAP, ETn/MusD, and LINE elements) at specific loci in one cell line but not the other. Employing ChIP-seq data for these cell lines, we show that IAP elements robustly induce H3K9me3 and H4K20me3 marks in flanking genomic DNA. In contrast, such heterochromatin is not induced by LINE copies and only by a minority of polymorphic ETn/MusD copies. DNA methylation is independent of the presence of IAP copies, since it is present in flanking regions of both full and empty sites. Finally, such spreading into genes appears to be rare, since the transcriptional start sites of very few genes are less than one Kb from an IAP. However, the B3galtl gene is subject to transcriptional silencing via IAP-induced heterochromatin. Hence, although rare, IAP-induced local heterochromatin spreading into nearby genes may influence expression and, in turn, host fitness
Genome-Wide Assessments Reveal Extremely High Levels of Polymorphism of Two Active Families of Mouse Endogenous Retroviral Elements
Endogenous retroviral elements (ERVs) in mice are significant genomic mutagens, causing βΌ10% of all reported spontaneous germ line mutations in laboratory strains. The majority of these mutations are due to insertions of two high copy ERV families, the IAP and ETn/MusD elements. This significant level of ongoing retrotranspositional activity suggests that inbred mice are highly variable in content of these two ERV groups. However, no comprehensive genome-wide studies have been performed to assess their level of polymorphism. Here we compared three test strains, for which sufficient genomic sequence is available, to each other and to the reference C57BL/6J genome and detected very high levels of insertional polymorphism for both ERV families, with an estimated false discovery rate of only 0.4%. Specifically, we found that at least 60% of IAP and 25% of ETn/MusD elements detected in any strain are absent in one or more of the other three strains. The polymorphic nature of a set of 40 ETn/MusD elements found within gene introns was confirmed using genomic PCR on DNA from a panel of mouse strains. For some cases, we detected gene-splicing abnormalities involving the ERV and obtained additional evidence for decreased gene expression in strains carrying the insertion. In total, we identified nearly 700 polymorphic IAP or ETn/MusD ERVs or solitary LTRs that reside in gene introns, providing potential candidates that may contribute to gene expression differences among strains. These extreme levels of polymorphism suggest that ERV insertions play a significant role in genetic drift of mouse lines
- β¦