Single Tight-binding Band Research Articles

Quantum dragon solutions for electron transport through nanostructures based on rectangular graphs

Electron transport through nanodevices of atoms in a single-layer rectangular arrangement with free (open) boundary conditions parallel to the direction of the current flow is studied within the single-band tight binding model. The Landauer formula gives the electrical conductance to be a function of the electron transmission probability, , as a function of the energy E of the incoming electron. A quantum dragon nanodevice is one which has a perfectly conducting channel, namely for all energies which are transmitted by the external leads even though there may be arbitrarily strong electron scattering. The rectangular single-layer systems are shown to be able to be quantum dragon devices, both for uniform leads and for dimerized leads. The quantum dragon condition requires appropriate lead-device connections and correlated randomness in the device.

Flat band analogues and flux driven extended electronic states in a class of geometrically frustrated fractal networks

We demonstrate, by explicit construction, that a single band tight binding Hamiltonian defined on a class of deterministic fractals of the b = 3N Sierpinski type can give rise to an infinity of dispersionless, flat-band like states which can be worked out analytically using the scale invariance of the underlying lattice. The states are localized over clusters of increasing sizes, displaying the existence of a multitude of localization areas. The onset of localization can, in principle, be ‘delayed’ in space by an appropriate choice of the energy of the electron. A uniform magnetic field threading the elementary plaquettes of the network is shown to destroy this staggered localization and generate absolutely continuous sub-bands in the energy spectrum of these non-translationally invariant networks.

Graphene nanoribbon thermopower as a tool for molecular spectroscopy

In this work, we study thermoelectric properties of graphene nanoribbons with side-attached organic molecules. By adopting a single-band tight binding Hamiltonian and the Green's function formalism, we calculated the transmission and Seebeck coefficients for different hybrid systems. The corresponding thermopower profiles exhibit a series of sharp peaks at the eigenenergies of the isolated molecule indicating that the system can be proposed as a molecular thermo-device. We have studied the effects of the temperature on the thermoelectric response, and considered random configurations of molecule distributions, in different disorder regimes. We have found that the main features of the thermopower are robust under temperature and disorder.

A non-parabolic single-band tight binding approach: capturing conduction band bending, charge accumulation and quantization at non-polar InN surfaces

A single-band tight binding approach for non-parabolic semiconductors is proposed. The method then is applied to model conduction band bending, surface electron accumulation and quantization at the m-plane surface of InN. The results are shown to compare well with published experimental angle-resolved photoemission spectroscopy data (Colakerol et al 2006 Phys. Rev. Lett. 97 237601). This simple computationally efficient approach is well suited to capture single-band phenomena in a wide variety of non-parabolic materials.

Valuation of magnetic non-collinear effects on FM/trans-PA/FM systems

With Green's function method and single band tight binding Hamiltonian, we calculated coherence transport for trans-polyacetylene (T-PA) systems placed between ferromagnetic electrodes with non-collinear ones. Ferromagnetic electrodes are of two kinds: Fe and Ni. With increasing TMR angle it increases to a maximum at θ=π degree. Electric field on molecule causes a change in the value of TMR. Types of ferromagnetic electrodes also affect the value of TMR, which is caused by density of states related to electrodes.

Transport properties of graphene quantum dots

In this work we present a theoretical study of transport properties of a double crossbar junction composed by segments of graphene ribbons with different widths forming a graphene quantum dot structure. The systems are described by a single-band tight binding Hamiltonian and the Green's function formalism using real space renormalization techniques. We show calculations of the local density of states, linear conductance and I-V characteristics. Our results depict a resonant behavior of the conductance in the quantum dot structures which can be controlled by changing geometrical parameters such as the nanoribbon segments widths and relative distance between them. By applying a gate voltage on determined regions of the structure, it is possible to modulate the transport response of the systems. We show that negative differential resistance can be obtained for low values of gate and bias voltages applied.

Band-gap modulations of double-walled carbon nanotubes under an axial magnetic field

We address here a theoretical study of electronic and transport properties of commensurate double-wall carbon nanotubes (DWCNTs). A single band tight binding hamiltonian is considered and the magnetic field is theoretically described by the Peierls approximation. Weak intershell interactions between a set of neighboring atoms on the walls of the inner and outer tubes are considered. Taking advantage of the real space description, arbitrary configurations of the relative position of the two tubes are taken into account. We study the possibility of Aharonov–Bohm effects in DWCNTs when a magnetic field is applied along the axial direction. The field intensity is found to be more effective than the relative interatomic positions in providing conductance gap modulations.

Theory of diluted magnetic semiconductors: A minimal model

In this article we review our recent theoretical studies on the carrier induced ferromagnetism of DMS. We propose a minimal model, which is composed of a single tight-binding band of carriers and magnetic impurities, which randomly substitute the host sites. Both of nonmagnetic attractive potentials due to impurities and the exchange interactions between carrier and impurity spins are taken into account. We apply the coherent potential approximation in studying this system. The obtained phase diagrams illustrate the characteristic features of the carrier induced ferromagnetism in DMS. The role of nonmagnetic interaction for the enhancement of the Curie temperature is clarified. The results suggest that the ferromagnetism in Ga1−xMnxAs is caused by a double-exchange-like mechanism mediated by valence band holes. We also report the results of numerical simulations of the model.

Intrinsic asymmetries in spin-valves: a tight binding model study

We have calculated the I– V curves, dynamical conductance, and tunneling magnetoresistance (TMR) of 1D magnetic tunneling junction through singleband tight binding model calculations based on the non-equilibrium Green's function approach. The difference in density of state of two ferromagnetic leads and the bias dependence of the propagator cause intrinsic asymmetries in TMR and dynamical conductance at finite bias. Besides, we have displayed that large TMR can be obtained even at high bias for half metallic leads.

An algebraic solution of driven single band tight binding dynamics

The dynamics of the driven single band tight binding model for Wannier–Stark systems is formulated and solved using a dynamical algebra. This Lie algebraic approach is very convenient for evaluating matrix elements and expectation values. A classicalization of the tight binding model is discussed as well as some illustrating examples of Bloch oscillations and dynamical localization effects. It is also shown that a dynamical invariant can be constructed.

Magnetic phase diagram of metallic pyrochlore lattice in the double-exchange model

The magnetic phase diagram of metallic kagomeand pyrochlore lattices at the ground state have been studied theoretically in the double-exchange model using a mean-field approximation. The model includes the hopping of itinerant electrons, which is expressed by a single tight-binding band, antiferromagnetic coupling between localized spins, and ferromagnetic exchange interaction between the localized and itinerant electron spins. The phase diagram was found to be rather insensitive to the ferromagnetic exchange interaction but to show a rich variety in magnetic structure with doping. A crossover between ferromagnetic and spin glass phases observed in RE2Mo2O7 pyrochlore lattices, where RE denotes rare-earth ions, is discussed in view of the calculated results.

Spin and charge transport in the presence of spin-orbit interaction

We present the study of spin and charge transport in nanostructures in the presence of spin-orbit (SO) interaction. Single band tight binding Hamiltonians for Elliot-Yafet and Rashba SO interaction are derived. Using these tight binding Hamiltonians and spin resolved Landauer-Buttiker formula, spin and charge transport is studied. Specifically numerical results are presented for a new method to perform magnetic scanning tunneling microscopy with non-magnetic tip but in the presence of Elliot-Yafet SO interaction. The spin relaxation phenomena in two-dimensional electron gas in the presence of Rashba SO interaction are studied and contrary to naive expectation, it is shown that disorder helps to reduce spin relaxation.

Wave Propagation Through a Coherently Amplifying Random Medium

We report a detailed and systematic numerical study of wave propagation through a coherently amplifying random one-dimensional medium. The coherent amplification is modeled by introducing a uniform imaginary part in the site energies of the disordered single-band tight binding Hamiltonian. Several distinct length scales (regimes), most of them new, are identified from the behavior of transmittance and reflectance as a function of the material parameters. We show that the transmittance is a non-self-averaging quantity with a well defined mean value. The stationary distribution of the super reflection differs qualitatively from the analytical results obtained within the random phase approximation in strong disorder and amplification regime. The study of the stationary distribution of the phase of the reflected wave reveals the reason for this discrepancy. The applicability of random phase approximation is discussed. We emphasize the dual role played by the lasing medium, as an amplifier as well as a reflector.

In-plane penetration-depth anisotropy in a d-wave model.

The zero-temperature penetration depth in the CuO{sub 2} plane of YBa{sub 2}Cu{sub 3}O{sub 7} has been found to be very anisotropic. The large measured difference between the {ital a} and {ital b} directions is taken as evidence that a significant part of the condensate resides in the chains. We present a simplified model for this anisotropy in which planes (CuO{sub 2}) and chains (CuO) are described within a single tight-binding band but with different hopping parameters in the {ital a} and {ital b} directions. The pairing is assumed to be due to spin fluctuations in the nearly antiferromagnetic Fermi liquid, but a different mechanism that leads to {ital d}-wave-type pairing would have similar results.

D-wave superconductivity in the nearly antiferromagnetic fermi liquid

Self consistent calculations, based on the 2 dimensional Hubbard model, indicate that the nearly antiferromagnetic Fermi liquid may be a superconductor with dx2-y2 symmetry. The recent observation of a large anisotropy of the in plane penetration depth has been widely interpreted as evidence that a significant part of the condensate in YBa2Cu3O7−gd resides in the chains. This introduces orthorhombic symmetry. Calculations, which remain within a single tight binding band model but with different nearest neighbour hopping in a- and b-directions, can account for the penetration depth anisotropy and also explain the observed size of the D.C. Josephson current between YBa2Cu3O7−gd and Pb in c-axis tunnelling.

Effects of a nonlinear impurity in three-dimensional tight-binding bands

The effects of a single nonlinear impurity of arbitrary strength and nonlinearity in a three-dimensional system described by a single tight-binding band are studied. For given nonlinearity, there exists a threshold strength for the impurity below which no bound states are found. The dependence of the threshold strength on the nonlinearity is calculated. Above the threshold strength, two bound states are found outside the continuum of the band. The energies of the bound states are studied as a function of the impurity strength.

Exchange coupling in magnetic multilayers: effect of partial confinement of carriers

The exchange coupling J(l) between magnetic layers across a non-magnetic spacer is observed to oscillate as a function of the spacer thickness l. In an earlier work a theory of the oscillatory exchange was proposed which shows that the oscillation periods are characteristic of the spacer. The theory relied on the assumption of an infinitely large exchange splitting in the magnetic layers, which leads to complete confinement of magnetic carriers of one spin in the ferromagnetic configuration of the sandwich. While this may be valid for strong ferromagnets such as Co or Ni, the complete confinement model is not a realistic approximation for iron which has holes in its majority-spin band. The theory is now generalized to the case of partial confinement of carriers in the spacer appropriate to a sandwich with weakly ferromagnetic layers. An exactly solvable hole-gas model of the coupling as well as numerical tight-binding results are presented which demonstrate that the oscillation period is unaffected but the amplitude and phase depend critically on the degree of confinement. Asymptotic expansions for J(l) valid at finite temperature and for an arbitrary single tight-binding band are also obtained. They show that the period and temperature dependence of the oscillations are directly related to the properties of the spacer Fermi surface but the amplitude and phase depend also on the exchange splitting in the ferromagnetic layers.

The density of states of a single tight-binding band for a b.c.c. element

An exact expression has been derived for the density of statesn(e) of a single tight-binding band of a b.c.c. element, within the two-centred approximation with nearest-neighbour contribution. Using this expression, a simple approximate relation is suggested forn(e), which can be useful in deriving various properties of transition metals.

Electron density correlation of a superlattice with wave function overlaps

A calculation of RPA electron-electron density correlation function of a type I superlattice is presented. The electron tunneling between adjacent quantum wells is taken into consideration. A formal expression of density correlation function is explicitly given within single band tight binding approximation.

Influence of long-range order on magnon energy in itinerant electron ferromagnetic alloys

The stoichiometric binary alloys AB 3 of fcc structure are considered. The single-band tight binding Hamiltonian with intraatomic Coulomb interaction is used, the hopping integrals are restricted to nearest neighbouring atomic sites and are assumed independent of the species. The atomic potentials and the Coulomb integrals are assumed different for A and B atoms. The long-range order parameter p characterizes the occupancies of the 4 s.c. sublattices, in which the fcc lattice is divided. A specific Ansatz is used to simplify the hopping part of the Hamiltonian, complicated in the Bloch basis. The spin wave stiffness constant D is calculated using the CPA method for calculating the average exchange contribution and the perturbation method to find the magnon scattering contribution. The dependence of D on the long-range order parameter p is calculated for Ni 3Fe and compared with the experimental data.