25 research outputs found
Coherent transport in linear arrays of quantum dots: the effects of period doubling and of quasi-periodicity
We evaluate the phase-coherent transport of electrons along linear structures
of varying length, which are made from two types of potential wells set in
either a periodic or a Fibonacci quasi-periodic sequence. The array is
described by a tight-binding Hamiltonian and is reduced to an effective dimer
by means of a decimation-renormalization method, extended to allow for
connection to external metallic leads, and the transmission coefficient is
evaluated in a T-matrix scattering approach. Parallel behaviors are found for
the energy dependence of the density of electron states and of the
transmittivity of the array. In particular, we explicitly show that on
increasing its length the periodic array undergoes a metal-insulator transition
near single occupancy per dot, whereas prominent pseudo-gaps emerge away from
the band center in the Fibonacci-ordered array.Comment: 11 pages, 7 figure
Theory of coherent transport by an ultra-cold atomic Fermi gas through linear arrays of potential wells
Growing interest is being given to transport of ultra-cold atomic gases
through optical lattices generated by the interference of laser beams. In this
connection we evaluate the phase-coherent transport of a spin-polarized gas of
fermionic atoms along linear structures made from potential wells set in four
alternative types of sequence. These are periodic chains of either identical
wells or pairs of different wells, and chains of pairs of wells arranged in
either a Fibonacci quasi-periodic sequence or a random sequence. The
transmission coefficient of fermionic matter is evaluated in a T-matrix
scattering approach by describing each array through a tight-binding
Hamiltonian and by reducing it to an effective dimer by means of a
decimation/renormalization method. The results are discussed in comparison with
those pertaining to transport by Fermi-surface electrons coupled to an outgoing
lead and by an atomic Bose-Einstein condensate. Main attention is given to (i)
Bloch oscillations and their mapping into alternating-current flow through a
Josephson junction; (ii) interference patterns that arise on period doubling
and their analogy with beam splitting in optical interferometry; (iii)
localization by quasi-periodic disorder inside a Fibonacci-ordered structure of
double wells; and (iv) Anderson localization in a random structure of double
wells.Comment: 14 pages, 4 figure
Electronic transmission in bent quantum wires
Electronic transmission in bent quantum wires modeled by the tight binding
Hamiltonian, and clamped between ideal, semi-infinite leads is studied. The
effect of `bending' the chain is simulated by introducing a non-zero hopping
between the extremities of the wire. It is seen that the proximity of the two
ends gives rise to Falo line shapes in the transmission spectrum. Transmission
properties for both an ordered lattice and a Fibonacci quantum wire are
discussed. In the quasi-periodic Fibonacci chain, the proximity of the two ends
of the chain closes all the gaps in the spectrum and the spectrum loses it's
Cantor set character.Comment: 6 pages, 6 figures, Fig.2 replaced by the correct versio
Electrical conductance in a single wall carbon nanotube (SWCNT): tight binding model
In this study, we derive analytically Green’s function (GF) formalism to calculate the electrical conductance for an armchair SWCNT in the ballistic regime. We obtain an exact analytical formula for the conductance of the SWCNT, in the tight-binding approach and assuming nearest-neighbor interaction by recursion process in the GF formalism. We show that in the presence of uniform external potential, the number of conductance channels and resonance energy range of the system decrease
The electronic conductance of polypyrrole (PPy) molecular wires and emergence of Fano resonance phenomena
In this paper, we studied the electronic conductance of a polypyrrole polymer, which is embedded between two semi-infinite simple chains by using Green’s function technique in tight-binding approach. We first reduced the center polymer to a one dimensional chain with renormalized onsite and hopping energies by renormalization method. Then, we calculated the system conductivity as a function of incoming electron energy, polymer length and contact hopping terms. The results showed that by increasing polymer length and decreasing contact hopping energies, the conductance decreases in the gap regions. This means that for larger gaps, the electron tunneling happens with more difficulty. Moreover, at the resonance area, due to the existence of nitrogen atom in the polymer cyclic structure, the Fano resonance will emerge. Furthermore, the polymer can behave like a metallic chain by variation of the value of nitrogen on-site term
The phonon and thermal properties of a ladder nanostructure
 In this paper, we study the phonon thermal properties of a ladder nanostructure in harmonic approximation. We present a model consisting of two infinite chains with different masses. Then, we investigate the effect of different masses on the phonon spectrum. Moreover, as a specific case, in the absence of the second neighbor interaction, we calculate the phonon density of states/modes. Finally, we consider the thermal conductivity of the system. The results show that the phonon spectrum shifts down to the lower frequencies by increasing the masses. Furthermore, a frequency gap appears in the phonon spectrum. By increasing the springs constants, the thermal conductance decreases
Electronic transport of molecular nanowires by considering of electron hopping energy between the second neighbors
In this paper, we study the electronic conductance of molecular nanowires by considering the electron hopping between the first and second neighbors with the help Green’s function method at the tight-binding approach. We investigate three types of structures including linear uniform and periodic chains as well as poly(p-phenylene) molecule which are embedded between two semi-infinite metallic leads. The results show that in the second neighbor approximation, the resonance, anti-resonance and Fano phenomena occur in the conductance spectra of these structures. Moreover, a new gap is observed at edge of the lead energy band wich its width depends on the value of the electron hopping energy between the second neighbors. In the systems including intrinsic gap, this hopping energy shifts the gap in the energy spectra