183 research outputs found
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
From Klein to anti-Klein tunneling in graphene tuning the Rashba spin-orbit interaction or the bilayer coupling
We calculate the transmission coefficient for a particle crossing a potential
barrier in monolayer graphene with Rashba spin-orbit coupling and in bilayer
graphene. We show that in both the cases one can go from Klein tunneling
regime, characterized by perfect normal transmission, to anti-Klein tunneling
regime, with perfect normal reflection, by tuning the Rashba spin-orbit
coupling for a monolayer or the interplane coupling for a bilayer graphene. We
show that the intermediate regime is characterized by a non-monotonic behavior
with oscillations and resonances in the normal transmission amplitude as a
function of the coupling and of the potential parameters.Comment: 9 pages, 5 figure
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.
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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
Elastic constant dishomogeneity and dependence of the broadening of the dynamical structure factor in disordered systems
We propose an explanation for the quadratic dependence on the momentum ,
of the broadening of the acoustic excitation peak recently found in the study
of the dynamic structure factor of many real and simulated glasses. We ascribe
the observed law to the spatial fluctuations of the local wavelength of
the collective vibrational modes, in turn produced by the dishomegeneity of the
inter-particle elastic constants. This explanation is analitically shown to
hold for 1-dimensional disordered chains and satisfatorily numerically tested
in both 1 and 3 dimensions.Comment: 4 pages, RevTeX, 5 postscript figure
High frequency sound waves in vitreous silica
We report a molecular dynamics simulation study of the sound waves in
vitreous silica in the mesoscopic exchanged momentum range. The calculated
dynamical structure factors are in quantitative agreement with recent
experimental inelastic neutron and x-ray scattering data. The analysis of the
longitudinal and transverse current spectra allows to discriminate between
opposite interpretations of the existing experimental data in favour of the
propagating nature of the high frequency sound waves.Comment: 4 pages, Revtex, 4 ps figures; to be published in Phys. Rev. Lett.,
February 198
The Raman coupling function in amorphous silica and the nature of the long wavelength excitations in disordered systems
New Raman and incoherent neutron scattering data at various temperatures and
molecular dynamic simulations in amorphous silica, are compared to obtain the
Raman coupling coefficient and, in particular, its low frequency
limit. This study indicates that in the limit
extrapolates to a non vanishing value, giving important indications on the
characteristics of the vibrational modes in disordered materials; in particular
our results indicate that even in the limit of very long wavelength the local
disorder implies non-regular local atomic displacements.Comment: Revtex, 4 ps figure
Frustration and sound attenuation in structural glasses
Three classes of harmonic disorder systems (Lennard-Jones like glasses,
percolators above threshold, and spring disordered lattices) have been
numerically investigated in order to clarify the effect of different types of
disorder on the mechanism of high frequency sound attenuation. We introduce the
concept of frustration in structural glasses as a measure of the internal
stress, and find a strong correlation between the degree of frustration and the
exponent alpha that characterizes the momentum dependence of the sound
attenuation . In particular, alpha decreases from
about d+1 in low-frustration systems (where d is the spectral dimension), to
about 2 for high frustration systems like the realistic glasses examined.Comment: Revtex, 4 pages including 4 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. \ua9 2016 American Physical Society
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