5,654 research outputs found
Helicons in Weyl semimetals
Helicons are transverse electromagnetic waves propagating in
three-dimensional (3D) electron systems subject to a static magnetic field. We
present a theory of helicons propagating through a 3D Weyl semimetal. Our
approach relies on the evaluation of the optical conductivity tensor from
semiclassical Boltzmann transport theory, with the inclusion of certain Berry
curvature corrections that have been neglected in the earlier literature (such
as the one due to the orbital magnetic moment). We demonstrate that the axion
term characterizing the electromagnetic response of Weyl semimetals
dramatically alters the helicon dispersion with respect to that in
nontopological metals. We also discuss axion-related anomalies that appear in
the plasmon dispersion relation.Comment: 5 pages, 1 figur
Effect of uniaxial strain on plasmon excitations in graphene
Uniaxial strain is known to modify significantly the electronic properties of
graphene, a carbon single layer of atomic width. Here, we study the effect of
applied strain on the composite excitations arising from the coupling of charge
carriers and plasmons in graphene, i.e. the plasmarons. Specifically, we
predict that the plasmaron energy dispersion, which has been recently observed
experimentally in unstrained graphene, is shifted and broadened by applied
uniaxial strain. Thus, strain constitutes an additional parameter which may be
useful to tune graphene properties in plasmaronic devices.Comment: Invited oral lecture at the 23rd AIRAPT International Conference on
"High Pressure Science and Technology", Mumbai (India), September 25-30,
2011. To be published in J. Phys.: Conf. Series (2012
Cu_{2}O as nonmagnetic semiconductor for spin transport in crystalline oxide electronics
We probe spin transport in Cu_{2}O by measuring spin valve effect in
La_{0.7}Sr_{0.3}MnO_{3}/Cu_{2}O/Co and
La_{0.7}Sr_{0.3}MnO_{3}/Cu_{2}O/La_{0.7}Sr_{0.3}MnO_{3} epitaxial
heterostructures. In La_{0.7}Sr_{0.3}MnO_{3}/Cu_{2}O/Co systems we find that a
fraction of out-of-equilibrium spin polarized carrier actually travel across
the Cu_{2}O layer up to distances of almost 100 nm at low temperature. The
corresponding spin diffusion length dspin is estimated around 40 nm.
Furthermore, we find that the insertion of a SrTiO_{3} tunneling barrier does
not improve spin injection, likely due to the matching of resistances at the
interfaces. Our result on dspin may be likely improved, both in terms of
Cu_{2}O crystalline quality and sub-micrometric morphology and in terms of
device geometry, indicating that Cu_{2}O is a potential material for efficient
spin transport in devices based on crystalline oxides.Comment: 15 pages, 10 figure
Characterization of a high strain composite material
L'Garde has designed and developed a high-strain composite material consisting of car- bon FIbers embedded in a silicone matrix. The behavior of this material is significantly different from standard composites and the paper presents special test methods to measure the properties of this material. It is found that rule of mixtures estimates are quite accurate for the longitudinal moduli in tension and bending, but less accurate for compression. The Poisson's ratio prediction is also not accurate. Regarding the strength of the composite, it is found that conservative predictions of tensile and compressive strengths can be obtained respectively from the Weibull distribution of the strength of a single fiber combined with a simple bundle theory, and the elastic fiber microbuckling stress
Field inoculation of bread wheat with Rhizophagus irregularis under organic farming: variability in growth response and nutritional uptake of eleven old genotypes and a modern variety.
Arbuscular mycorrhizal fungi (AMF) promote crop growth and yield by increasing N and
P uptake and disease resistance, but the role of field AMF inoculation on the uptake of micronutrients,
such as Fe and Zn, and accumulation in plant edible portions is still not clarified. Therefore, we
studied the eect of field inoculation with Rhizophagus irregularis in an organic system on 11 old
genotypes and a modern variety of bread wheat. Inoculation increased root colonization, root
biomass and shoot Zn concentration at early stage and grain Fe concentration at harvest, while
it did not modify yield. Genotypes widely varied for shoot Zn concentration at early stage, and
for plant height, grain yield, Zn and protein concentration at harvest. Inoculation dierentially
modified root AMF community of the genotypes Autonomia B, Frassineto and Bologna. A higher
abundance of Rhizophagus sp., putatively corresponding to the inoculated isolate, was only proved
in Frassineto. The increase of plant growth and grain Zn content in Frassineto is likely linked to
the higher R. irregularis abundance. The AMF role in increasing micronutrient uptake in grain was
proved. This supports the introduction of inoculation in cereal farming, if the variable response of
wheat genotypes to inoculation is considered
Mechanism of stabilization of helix secondary structure by constrained Cα-tetrasubstituted alpha-amino acids
The theoretical basis behind the ability of constrained C\u3b1-tetrasubstituted amino acids (CTAAs) to induce stable helical conformations has been studied through Replica Exchange Molecular Dynamics Potential of Mean Force Quantum Theory of Atoms In Molecules calculations on Ac-l-Ala-CTAA-l-Ala-Aib-l-Ala-NHMe peptide models. We found that the origin of helix stabilization by CTAAs can be ascribed to at least two complementary mechanisms limiting the backbone conformational freedom: steric hindrance predominantly in the (+x,+y,\u2013z) sector of a right-handed 3D Cartesian space, where the z axis coincides with the helical axis and the C\u3b1 of the CTAA lies on the +y axis (0,+y,0), and the establishment of additional and relatively strong C\u2013H\ub7\ub7\ub7O interactions involving the CTAA
Colloidal CuFeS2 Nanocrystals: Intermediate Fe d-Band Leads to High Photothermal Conversion Efficiency
We describe the colloidal hot-injection synthesis of phase-pure nanocrystals
(NCs) of a highly abundant mineral, chalcopyrite (CuFeS2). Absorption bands
centered at around 480 and 950 nm, spanning almost the entire visible and near
infrared regions, encompass their optical extinction characteristics. These
peaks are ascribable to electronic transitions from the valence band (VB) to
the empty intermediate band (IB), located in the fundamental gap and mainly
composed of Fe 3d orbitals. Laser-irradiation (at 808 nm) of an aqueous
suspension of CuFeS2 NCs exhibited significant heating, with a photothermal
conversion efficiency of 49%. Such efficient heating is ascribable to the
carrier relaxation within the broad IB band (owing to the indirect VB-IB gap),
as corroborated by transient absorption measurements. The intense absorption
and high photothermal transduction efficiency (PTE) of these NCs in the
so-called biological window (650-900 nm) makes them suitable for photothermal
therapy as demonstrated by tumor cell annihilation upon laser irradiation. The
otherwise harmless nature of these NCs in dark conditions was confirmed by in
vitro toxicity tests on two different cell lines. The presence of the deep Fe
levels constituting the IB is the origin of such enhanced PTE, which can be
used to design other high performing NC photothermal agents.Comment: 12 pages, Chemistry of Materials, 31-May-201
Sensitive determination of lysinoalanine for distinguishing natural from imitation Mozzarella cheese
Abstract A new method of reverse-phase HPLC was used to determine the crosslinked amino acid lysinoalanine in natural Mozzarella cheese, dairy-based substitutes, and related ingredients. Commercial samples manufactured under known conditions or collected at the market were analyzed. The acid-hydrolyzed sample derivatized by 9-fluorenyl-methylchloro-formate was submitted to solid-phase extraction on an amino cartridge to extract selectively the lysinoalanine derivatives that were chromatographed under fluorescence detection. Lysinoalanine was not found ( X ¯ = 1.7 ; n=30) were present in natural Mozzarella cheese. Because of the ingredient characteristics and the more severe thermal processing conditions, the different types of processed cheese and imitation Mozzarella cheese exhibited much higher lysinoalanine contents, ranging from 15 to 421ppm ( X ¯ = 54 ; n=29). Hence, a highly significant distinction between natural Mozzarella cheese and imitations, even those that did not contain added milk protein, could be achieved by the lysinoalanine index. Conversely, the furosine index distinguished the imitation products only when the quantity of reducing sugars allowed the early Maillard reaction to be extensive
Adaptive Reconfiguration of Natural Killer Cells in HIV-1 Infection
Human cytomegalovirus (HCMV) co-infection is highly prevalent within HIV-1 cohorts and is an important cofactor in driving ongoing immune activation, even during effective antiretroviral treatment. HCMV infection has recently been associated with expansion of adaptive-like natural killer (NK) cells, which harbor epigenetic alterations that impact on their cellular function and phenotype. The influence of HCMV co-infection on the considerable heterogeneity among NK cells and their functional responses to different stimuli was assessed in a cohort of HIV-1-infected individuals sampled during different stages of infection, compared with healthy subjects stratified according to HCMV serostatus. Our data demonstrate a reshaping of the NK cell pool in HIV-1 infection of HCMV-seropositive individuals, with an accentuated peripheral transition of CD56dim NK cells toward a mature CD57+ CD85j+ NKG2C+ NKG2A− phenotype. Lack of PLZF further distinguishes adaptive NK cells from other NK cells expressing CD57 or NKG2C. PLZF− NK cells from HIV-infected individuals had high expression of CD2, were Siglec-7 negative and exhibited downregulation of key signaling molecules, SYK and FcεRI-γ, overwhelmingly displaying features of adaptive NK cells that correlated with HCMV serum Ab levels. Notably this adaptive-like signature was detected during early HIV-1 infection and persisted during treatment. Adaptive-like NK cell subsets in HIV-1-infected individuals displayed enhanced IFN-γ production following Fc receptor triggering compared with their conventional NK cell counterparts, and their ability to produce TNF-α and degranulate was preserved. Together, these data suggest that HMCV infection/reactivation, a hallmark of HIV-1 infection, plays a role in driving a relative expansion of NK cells with adaptive features during HIV-1 infection. The identification of selective NK subsets with retained effector activity in HIV-1-infected subjects raises the possibility of developing therapeutic strategies that exploit specific NK subpopulations to achieve better HIV-1 control
Measuring Hall Viscosity of Graphene's Electron Fluid
Materials subjected to a magnetic field exhibit the Hall effect, a phenomenon
studied and understood in fine detail. Here we report a qualitative breach of
this classical behavior in electron systems with high viscosity. The viscous
fluid in graphene is found to respond to non-quantizing magnetic fields by
producing an electric field opposite to that generated by the classical Hall
effect. The viscous contribution is large and identified by studying local
voltages that arise in the vicinity of current-injecting contacts. We analyze
the anomaly over a wide range of temperatures and carrier densities and extract
the Hall viscosity, a dissipationless transport coefficient that was long
identified theoretically but remained elusive in experiment. Good agreement
with theory suggests further opportunities for studying electron
magnetohydrodynamics.Comment: 18 pages, 9 figure
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