59 research outputs found
On-Site Repulsion as the Source of Pairing in Carbon Nanotubes and Intercalated Graphite
We show that different non-conventional superconductors have one fundamental
feature in common: pair eigenstates of the Hamiltonian are repulsion-free, the
W=0 pairs. In extended Hubbard models, pairing can occur for resonable
parameter values. For nanotubes the binding energy of the pair depends
strongly on the filling and decreases towards a reduced but nonzero value for
the graphite sheet .Comment: 4 pages, 2 figure
Bouncing transient currents and SQUID-like voltage in nano devices at half filling
Nanorings asymmetrically connected to wires show different kinds of quantum
interference phenomena under sudden excitations and in steady current
conditions. Here we contrast the transient current caused by an abrupt bias to
the magnetic effects at constant current. A repulsive impurity can cause charge
build-up in one of the arms and reverse current spikes.
Moreover, it can cause transitions from laminar current flow to vortices, and
also change the chirality of the vortex. The magnetic behavior of these devices
is also very peculiar. Those nano-circuits which consist of an odd number of
atoms behave in a fundamentally different manner compared to those which
consist of an even number of atoms. The circuits having an odd number of sites
connected to long enough symmetric wires are diamagnetic; they display
half-fluxon periodicity induced by many-body symmetry even in the absence of
electron-phonon and electron-electron interactions. In principle one can
operate a new kind of quantum interference device without superconductors.
Since there is no gap and no critical temperature, one predicts qualitatively
the same behavior at and above room temperature, although with a reduced
current. The circuits with even site numbers, on the other hand, are
paramagnetic.Comment: 7 pages, 10 figures, accepted by Phys. Rev.
"Spin-Disentangled" Exact Diagonalization of Repulsive Hubbard Systems: Superconducting Pair Propagation
By a novel exact diagonalization technique we show that bound pairs propagate
between repulsive Hubbard clusters in a superconducting fashion. The size of
the matrices that must be handled depends on the number of fermion
configurations {\em per spin}, which is of the order of the square root of the
overall size of the Hilbert space. We use CuO units connected by weak O-O
links to model interplanar coupling and c-axis superconductivity in Cuprates.
The numerical evidence on CuO and CuO prompts a new
analytic scheme describing the propagation of bound pairs and also the
superconducting flux quantization in a 3-d geometry.Comment: 5 pages, 3 figure
Symmetric Hubbard Systems with Superconducting Magnetic Response
In purely repulsive, -symmetric Hubbard clusters a correlation effect
produces an effective two-body attraction and pairing; the key ingredient is
the availability of W=0 pairs, that is, two-body solutions of appropriate
symmetry. We study the tunneling of bound pairs in rings of 5-site units
connected by weak intercell links; each unit has the topology of a CuO
cluster and a repulsive interaction is included on every site. Further, we test
the superconducting nature of the response of this model to a threading
magnetic field. We present a detailed numerical study of the two-unit ring
filled with 6 particles and the three-unit ring with 8 particles; in both cases
a lower filling yields normal behavior. In previous studies on 1d Hubbard
chains, level crossings were reported (half-integer or fractional Aharonov-Bohm
effect) which however cannot be due to superconducting pairs. In contrast, the
nontrivial basis of clusters carrying W=0 pairs leads to genuine
Superconducting Flux Quantization (SFQ). The data are understood in terms of a
cell-perturbation theory scheme which is very accurate for weak links. This
low-energy approach leads to an effective hard core boson Hamiltonian which
naturally describes itinerant pairs and SFQ in mesoscopic rings. For the
numerical calculations, we take advantage of a recently proposed exact
diagonalization technique which can be generally applied to many-fermion
problems and drastically reduces the size of the matrices to be handled.Comment: 12 pages, 11 figure
Repulsion-Sustained Supercurrent and Flux Quantization in Rings of Symmetric Hubbard Clusters
We test the response to a threading magnetic field of rings of 5-site
-symmetric repulsive Hubbard clusters connected by weak intercell
links; each 5-site unit has the topology of a CuO cluster and a repulsive
interaction is included on every site. In a numerical study of the three-unit
ring with 8 particles, we take advantage of a novel exact-diagonalization
technique which can be generally applied to many-fermion problems. For O-O
hopping we find Superconducting Flux Quantization (SFQ), but for purely Cu-Cu
links bound pair propagation is hindered by symmetry. The results agree with
W=0 pairing theory.Comment: 4 pages, 2 figure
Magnetically induced pumping and memory storage in quantum rings
Nanoscopic rings pierced by external magnetic fields and asymmetrically
connected to wires behave in sharp contrast with classical expectations. By
studying the real-time evolution of tight-binding models in different
geometries, we show that the creation of a magnetic dipole by a bias-induced
current is a process that can be reversed: connected rings excited by an
internal ac flux produce ballistic currents in the external wires. In
particular we point out that, by employing suitable flux protocols,
single-parameter nonadiabatic pumping can be achieved, and an arbitrary amount
of charge can be transferred from one side to the other. We also propose a set
up that could serve a memory device, in which both the operations of {\it
writing} and {\it erasing} can be efficiently performed.Comment: 5 pages, 6 figures, accepted by Phys. Rev.
Challenges and opportunities in wheat flour, pasta, bread, and bakery product production chains: A systematic review of innovations and improvement strategies to increase sustainability, productivity, and product quality
Magnetic moments in biased quantum circuits
We consider a quantum ring connected to leads and the current which is excited by biasing the circuit in the absence of external magnetic field. The magnetic moment Mring that arises in this way depends on the current distribution inside the ring. We perform a thought experiment in which Mring is determined by measuring the torque due to an infinitesimally small probe magnetic field. This leads to a definition Mring, which is given by the derivative of the grand-canonical energy of the quantum ring with respect to an external magnetic flux in the zero flux limit. We develop the many-body formalism by Green's-function techniques and carry on illustrative model calculations. The resulting theory predicts that at small bias the current in the ring is always laminar, that is, the magnetic moment vanishes in linear-response theory. The approach most naturally lends itself to include induction effects by a self-consistent procedure
Innovative olive tree leaves shredder prototype for the valorization of wasted leaves: An application to high-quality compost production
Extra virgin olive oil is considered worldwide as one of the most important products, a standard bearer of the Mediterranean diet. Despite this, the production chain of extra virgin olive oil generates four times more waste than quantity of oil. For this reason, the disposal of olive mill wastes represents a significant environmental problem in all the Mediterranean countries. In this direction, several innovations and improvement strategies were proposed in the literature to correctly manage these by-products and, in some cases, to valorize them by the recovery of polyphenols and other interesting substances. However, innovations and improvement strategies for the valorization of olive tree leaves are definitely neglected in the literature, thus motivating this work. The aims of this work are as follow: firstly, to develop and test an innovative olive tree leaves shredder prototype to help olive oil millers in the management of waste leaves deriving from pruning and olive oil production; secondly, to test the combination of the minced olive tree leaves with other by-products in the production of high-quality compost. The results showed the effectiveness of the tested olive tree leaves shredder in significantly reducing the volume occupied by the leaves by 40%. Moreover, the mixing of the minced olive tree leaves with other by-products lead to a high-quality compost which, in accordance with Italian legislation, could even be certified and labeled with this denomination. Future research will investigate the quantification of the benefits in terms of environmental impacts using life cycle assessment
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