Quasiperiodic magnetic chain as a spin filter for arbitrary spin states

We show that a quasiperiodic magnetic chain comprising magnetic atomic sites sequenced in Fibonacci pattern can act as a prospective candidate for spin filters for particles with arbitrary spin states. This can be achieved by tuning a suitable correlation between the amplitude of the substrate magnetic field and the on-site potential of the magnetic sites, which can be controlled by an external gate voltage. Such correlation leads to a spin filtering effect in the system, allowing one of the spin components to completely pass through the system while blocking the others over the allowed range of energies. The underlying mechanism behind this phenomena holds true for particles with any arbitrary spin states S = 1, 3/2, 2, . . ., in addition to the canonical case of spin-half particles. Our results open up the interesting possibility of designing a spin demultiplexer using a simple quasiperiodic magnetic chain system. Experimental realization of this theoretical study might be possible by using ultracold quantum gases, and can be useful in engineering new spintronic devices.Comment: 8 pages, 5 figures, published versio

Absolutely continuous energy bands and extended electronic states in an aperiodic comb-shaped nanostructure

The nature of electronic eigenstates and quantum transport in a comb-shaped Fibonacci nanostructure model is investigated within a tight-binding framework. Periodic linear chains are side-attached to a Fibonacci chain, giving it the shape of an aperiodic comb. The effect of the side-attachments on the usual Cantor set energy spectrum of a Fibonacci chain is analyzed using the Greens function technique. A special correlation between the coupling of the side-attached chain with the Fibonacci chain and the inter-atomic coupling of the Fibonacci chain results in a dramatic triggering of the fragmented Cantor set energy spectrum into multiple sets of continuous sub-bands of extended eigenstates. The result is valid even for a disordered comb and turns out to be a rare exception of the conventional Anderson localization problem. The electronic transport thus can be made selectively ballistic within desired energy regimes. The number and the width of such continuous sub-bands can be easily controlled by tuning the number of atomic sites in the side-coupled periodic linear chains. This gives us a scope of proposing such aperiodic nanostructures as potential candidates for prospective energy selective nanoscale filtering devices.Comment: 7 pages, 7 figures, Revtex versio

Spin filtering and switching action in a diamond network with magnetic-nonmagnetic atomic distribution

We propose a simple model quantum network consisting of diamond-shaped plaquettes with deterministic distribution of magnetic and non-magnetic atoms in presence of a uniform external magnetic flux in each plaquette and predict that such a simple model can be a prospective candidate for spin filter as well as flux driven spintronic switch. The orientations and the amplitudes of the substrate magnetic moments play a crucial role in the energy band engineering of the two spin channels which essentially gives us a control over the spin transmission leading to a spin filtering effect. The externally tunable magnetic flux plays an important role in inducing a switch on-switch off effect for both the spin states indicating the behavior like a spintronic switch. Even a correlated disorder configuration in the on-site potentials and in the magnetic moments may lead to disorder-induced spin filtering phenomenon where one of the spin channel gets entirely blocked leaving the other one transmitting over the entire allowed energy regime. All these features are established by evaluating the density of states and the two terminal transmission probabilities using the transfer-matrix formalism within a tight-binding framework. Experimental realization of our theoretical study may be helpful in designing new spintronic devices.Comment: 15 Pages, 11 EPS Figures, Revised version, Accepted for publication in Scientific Report

Flat bands in fractal-like geometry

We report the presence of multiple flat bands in a class of two-dimensional (2D) lattices formed by Sierpinski gasket (SPG) fractal geometries as the basic unit cells. Solving the tight-binding Hamiltonian for such lattices with different generations of a SPG network, we find multiple degenerate and non-degenerate completely flat bands, depending on the configuration of parameters of the Hamiltonian. Moreover, we find a generic formula to determine the number of such bands as a function of the generation index $\ell$ of the fractal geometry. We show that the flat bands and their neighboring dispersive bands have remarkable features, the most interesting one being the spin-1 conical-type spectrum at the band center without any staggered magnetic flux, in contrast to the Kagome lattice. We furthermore investigate the effect of the magnetic flux in these lattice settings and show that different combinations of fluxes through such fractal unit cells lead to richer spectrum with a single isolated flat band or gapless electron- or hole-like flat bands. Finally, we discuss a possible experimental setup to engineer such fractal flat band network using single-mode laser-induced photonic waveguides.Comment: 8 pages, 9 figures, accepted versio

Engineering light localization in a fractal waveguide network

We present an exact analytical method of engineering the localization of electromagnetic waves in a fractal waveguide network. It is shown that, a countable infinity of localized electromagnetic modes with a multitude of localization lengths can exist in a Vicsek fractal geometry built with diamond shaped monomode waveguides as the 'unit cells'. The family of localized modes form clusters of increasing size. The length scale at which the onset of localization for each mode takes place can be engineered at will, following a well defined prescription developed within the framework of a real space renormalization group. The scheme leads to an exact evaluation of the wave vector for every such localized state, a task that is non-trivial, if not impossible for any random or deterministically disordered waveguide network.Comment: 8 pages, 4 figures. arXiv admin note: text overlap with arXiv:1204.498

Complete absence of localization in a family of disordered lattices

We present analytically exact results to show that, certain quasi one-dimensional lattices where the building blocks are arranged in a random fashion, can have an absolutely continuous part in the energy spectrum when special correlations are introduced among some of the parameters describing the corresponding Hamiltonians. We explicitly work out two prototype cases, one being a disordered array of a simple diamond network and isolated dots, and the other an array of triangular plaquettes and dots. In the latter case, a magnetic flux threading each plaquette plays a crucial role in converting the energy spectrum into an absolutely continuous one. A flux controlled enhancement in the electronic transport is an interesting observation in the triangle-dot system that may be useful while considering prospective devices. The analytical findings are comprehensively supported by extensive numerical calculations of the density of states and transmission coefficient in each case.Comment: 6 pages, 6 figures, epl draf