Wavevector-dependent spin filtering and spin transport through magnetic barriers in graphene

We study the spin-resolved transport through magnetic nanostructures in monolayer and bilayer graphene. We take into account both the orbital effect of the inhomogeneous perpendicular magnetic field as well as the in-plane spin splitting due to the Zeeman interaction and to the exchange coupling possibly induced by the proximity of a ferromagnetic insulator. We find that a single barrier exhibits a wavevector-dependent spin filtering effect at energies close to the transmission threshold. This effect is significantly enhanced in a resonant double barrier configuration, where the spin polarization of the outgoing current can be increased up to 100% by increasing the distance between the barriers

Magnetic confinement of massless Dirac fermions in graphene

Due to Klein tunneling, electrostatic potentials are unable to confine Dirac electrons. We show that it is possible to confine massless Dirac fermions in a monolayer graphene sheet by inhomogeneous magnetic fields. This allows one to design mesoscopic structures in graphene by magnetic barriers, e.g. quantum dots or quantum point contacts.Comment: 4 pages, 3 figures, version to appear in PR

Quasi-particle dephasing time in disordered d-wave superconductors

We evaluate the low-temperature cutoff for quantum interference 1/tf induced in a d-wave superconductor by the diffusion enhanced quasiparticle interactions in the presence of disorder. We carry out our analysis in the framework of the non-linear sigma-model which allows a direct calculation of 1/tf, as the mass of the transverse modes of the theory. Only the triplet amplitude in the particle-hole channel and the Cooper amplitude with is pairing symmetry contribute to 1/tf. We discuss the possible relevance of our results to the present disagreement between thermal transport data in cuprates and the localization theory for d-wave quasiparticles

Quantum Otto cycle with inner friction: finite-time and disorder effects

The concept of inner friction, by which a quantum heat engine is unable to follow adiabatically its strokes and thus dissipates useful energy, is illustrated in an exact physical model where the working substance consists of an ensemble of misaligned spins interacting with a magnetic field and performing the Otto cycle. The effect of this static disorder under a finite-time cycle gives a new perspective of the concept of inner friction under realistic settings. We investigate the efficiency and power of this engine and relate its performance to the amount of friction from misalignment and to the temperature difference between heat baths. Finally we propose an alternative experimental implementation of the cycle where the spin is encoded in the degree of polarization of photons.Comment: Published version in the Focus Issue on "Quantum Thermodynamics

Ochrobactrum sp. MPV1 from a dump of roasted pyrites can be exploited as bacterial catalyst for the biogenesis of selenium and tellurium nanoparticles

Background: Bacteria have developed different mechanisms for the transformation of metalloid oxyanions to non-toxic chemical forms. A number of bacterial isolates so far obtained in axenic culture has shown the ability to bioreduce selenite and tellurite to the elemental state in different conditions along with the formation of nanoparticles-both inside and outside the cells-characterized by a variety of morphological features. This reductive process can be considered of major importance for two reasons: firstly, toxic and soluble (i.e. bioavailable) compounds such as selenite and tellurite are converted to a less toxic chemical forms (i.e. zero valent state); secondly, chalcogen nanoparticles have attracted great interest due to their photoelectric and semiconducting properties. In addition, their exploitation as antimicrobial agents is currently becoming an area of intensive research in medical sciences. Results: In the present study, the bacterial strain Ochrobactrum sp. MPV1, isolated from a dump of roasted arsenopyrites as residues of a formerly sulfuric acid production near Scarlino (Tuscany, Italy) was analyzed for its capability of efficaciously bioreducing the chalcogen oxyanions selenite (SeO32-) and tellurite (TeO32-) to their respective elemental forms (Se0 and Te0) in aerobic conditions, with generation of Se- and Te-nanoparticles (Se- and TeNPs). The isolate could bioconvert 2 mM SeO32- and 0.5 mM TeO32- to the corresponding Se0 and Te0 in 48 and 120 h, respectively. The intracellular accumulation of nanomaterials was demonstrated through electron microscopy. Moreover, several analyses were performed to shed light on the mechanisms involved in SeO32- and TeO32- bioreduction to their elemental states. Results obtained suggested that these oxyanions are bioconverted through two different mechanisms in Ochrobactrum sp. MPV1. Glutathione (GSH) seemed to play a key role in SeO32- bioreduction, while TeO32- bioconversion could be ascribed to the catalytic activity of intracellular NADH-dependent oxidoreductases. The organic coating surrounding biogenic Se- and TeNPs was also characterized through Fourier-transform infrared spectroscopy. This analysis revealed interesting differences among the NPs produced by Ochrobactrum sp. MPV1 and suggested a possible different role of phospholipids and proteins in both biosynthesis and stabilization of such chalcogen-NPs. Conclusions: In conclusion, Ochrobactrum sp. MPV1 has demonstrated to be an ideal candidate for the bioconversion of toxic oxyanions such as selenite and tellurite to their respective elemental forms, producing intracellular Se- and TeNPs possibly exploitable in biomedical and industrial applications.[Figure not available: see fulltext.

Quasi-one-dimensional system as a high-temperature superconductor

It is well-known that quasi-one-dimensional superconductors suffer from the pairing fluctuations that significantly reduce the superconducting temperature or even completely suppress any coherent behavior. Here we demonstrate that a coupling to a robust pair condensate changes the situation dramatically. In this case the quasi-one-dimensional system can be a high temperature superconductor governed by the proximity to the Lifshitz transition at which the Fermi level approaches the lower edge of the single-particle spectrum.Comment: 5 pages, 1 figur

CLAVA MULTICORNIS (HYDROZOA): UN NUOVO MODELLO SPERIMENTALE DI CNIDARIO.

The organization of the cnidarian nervous system has been widely documented in polyps and medusae, but little is known about the nervous system of planula larvae, which give rise to adult forms after settling and metamorphosis. We described histological and cytological features of the nervous system in planulae of the hydrozoan Clava multicornis. These planulae crawl on the substrate by means of directional, coordinated ciliary movement coupled to lateral muscular bending movements associated with positive phototaxis. Histological analysis shows pronounced anteroposterior regionalization of the planula\u2019s nervous system, with different neural cell types highly concentrated at the anterior pole. Levels of nervous system complexity are uncovered by neuropeptide-specific immunocytochemistry, which reveals distinct neural subsets having specific molecular phenotypes. During metamorphosis, the larva attaches to the substrate with the anterior end that develops into the basal foot region of the polyp, while the posterior end forms the oral region. To elucidate the fate of the neural cells in C. multicornis, we investigated the distribution pattern of GLWamide and RFamide positive sensory cells at different stages of metamorphosis. We observed that immunoreactivity was still present at the anterior end of the larva during early settlement, but gradually disappeared through the following stages of metamorphosis. Only at later stages, positive cells appeared around the mouth of the newly formed polyp. By TUNEL assay, apoptotic nuclei were identified in the anterior end of the settled larva, in the same region occupied by sensory cells. These results suggest that at least part of the neurons of the larva degenerates during metamorphosis by apoptosis and that at least part of the adult nervous system is formed by de novo differentiation. The WNT and FGF signaling pathways are involved in determining the body axis in several animal models. Moreover, the way of FGF is involved in cell differentiation. The lithium chloride acts by activating the path of canonical WNT signal. SU5402 is an antagonist of FGF receptors and inhibits the FGF signaling pathway. To obtain information regarding the presence of these pathways in cnidarians we analyzed the effects of treatment with lithium chloride and SU5402 on the embryonic development of Clava multicornis. The results obtained showed that the treated animals maintained a length comparable to the control specimens, while changing the shape of the two poles of the animals. In fact, the larvae treated with lithium chloride assume a rod-shaped instead of the typical club-shaped, while the larvae treated with SU5402 are wedge-shaped. Later in the treated animals have been seen cells containing GLWamide and RFamide peptides by immunolocalization. There are differences in the position of cells and their number in the larvae treated with lithium chloride than in those treated with SU5402. This shows that the two substances interfere with the proper development of the animal. The effects of these drugs have had on Clava multicornis suggest the likely presence of the signaling pathway of WNT and FGF in these animals. Retinoic acid (RA) is a well-known gradient morphogen governing the spatial patterning of anteroposterior elements in vertebrates and in other chordates. Components of RA machinery, such as synthetic and catalytic enzymes and receptors, have been isolated in a number of organisms, from nematods to echinoderms, suggesting that the morphogenetic role of this molecule is not exclusive to chordates.. We tested the effects of RA and of a RA antagonist on C. multicornis development by analyzing the nervous system organization and the photoresponsive behavior of larvae exposed to RA during their embryogenesis. After the exposure, the nervous system became completely disorganized, leading to the displacement of RFamide-IR cells. Moreover, RA treated larvae did not exhibit the typical phototropic behavior of control specimens. Our results suggest that RA can alter the normal development of nervous elements in this hydroid species, supporting the hypothesis that the morphogenetic activity of RA predates the origin of chordates. The regenerative capacity of cnidarians is remarkable. In this work we analyzed the regenerative processes of gastric columns of Clava multicornis, treating the latter with increasing concentrations of RA in order to evaluate their effects. Concentrations of RA 10-7 M did not have a material impact on the regeneration of amputated portions of the polyp, which regenerates similarly to control animals. The 10-6 M RA primarily involves the regeneration of stolons. Together these observations imply that the nervous system of the planula of Clava multicornis manifests a remarkable level of histological, cytological, and functional organization, the features of which may be reminiscent of those present in early bilaterian animals. In fact, these results open up important perspectives on the evolutionary significance of the antagonism between RA and FGF / WNT in basal metazoans

Magnetic barriers and confinement of Dirac-Weyl quasiparticles in graphene

We discuss the properties of the two-dimensional massless Dirac-Weyl quasiparticles realized in graphene monolayers in the presence of inhomogeneous magnetic fields. We show that in contrast to electrostatic barriers, appropriate magnetic barriers are able to confine these quasiparticles. This allows for a novel way of designing mesoscopic structures (e.g., quantum dots, quantum point contacts) in graphene

Bound states for massive Dirac fermions in graphene in a magnetic step field

We calculate the spectrum of massive Dirac fermions in graphene in the presence of an inhomogeneous magnetic field modeled by a step function. We find an analytical universal relation between the bandwidths and the propagating velocities of the modes at the border of the magnetic region, showing how by tuning the mass term one can control the speed of these traveling edge states.Comment: 7 pages, 3 figure

Violation of cluster decomposition and absence of light cones in local integer and half-integer spin chains

We compute the ground-state correlation functions of an exactly solvable chain of integer spins, recently introduced in [R. Movassagh and P. W. Shor, arXiv:1408.1657], whose ground state can be expressed in terms of a uniform superposition of all colored Motzkin paths. Our analytical results show that for spin s≥2 there is a violation of the cluster decomposition property. This has to be contrasted with s=1, where the cluster property holds. Correspondingly, for s=1 one gets a light-cone profile in the propagation of excitations after a local quench, while the cone is absent for s=2, as shown by time dependent density-matrix renormalization group. Moreover, we introduce an original solvable model of half-integer spins, which we refer to as Fredkin spin chain, whose ground state can be expressed in terms of superposition of all Dyck paths. For this model we exactly calculate the magnetization and correlation functions, finding that for s=1/2, a conelike propagation occurs, while for higher spins, s≥3/2, the colors prevent any cone formation and clustering is violated, together with square root deviation from the area law for the entanglement entropy