Realistic Tight-binding Model Research Articles

Effects of interface disorder on transmission probability in magnetic multilayer

We present a study of conductance through disordered Fe-Cr[100] and Co-Cu[100] interfaces within a realistic tight-binding model. In our model, the specular and diffuse parts of the conductance are treated on an equal footing. We observe a substantial increase in conductance due to disorder in the channels that are strongly reflected by pure interfaces. We also show how this phenomenon affects the giant magnetoresistance. In addition, we find that the series resistor model may significantly overestimate the resistance if the thickness of the disordered layer is less than several times the electron mean free path.

Electronic spectra of quasi-regular heterostructures: simple versus realistic models

The electronic spectra of quasi-regular systems are investigated via simple one-dimensional, one-band models and compared with those obtained by means of realistic empirical tight-binding models, particularly for the Fibonacci sequence case.

Understanding the quantum size effects in ZnO nanocrystals

In the present work, we report the synthesis of high quality ZnO nanocrystals with sharp absorption edges in four different sizes, namely 3.0, 3.5, 4.7 and 5.4 nm, characterized by X-ray and electron diffraction, as well as transmission electron microscopy. The bandgaps of these samples, in conjunction with further data from the published literature, exhibit a systematic dependence on the nanocrystal size. In absence of any prior reliable theoretical results in the literature to understand this dependence quantitatively, we have analyzed for the first time, the electronic structure of bulk ZnO obtained from the full potential linearized augmented plane wave method using fatbands, density of states and partial density of states. The crystal orbital Hamiltonian population is obtained from linearized Muffin-Tin orbital band structure calculations to understand the range of hopping interactions relevant for an accurate description of the electronic structure. Using these analyses, a realistic tight binding model is proposed. Based on this model, we calculate the variation of the bandgap with the size of ZnO nanocrystals. These theoretical results agree well with all available data over the entire range of sizes, establishing the effectiveness of this approach.

Electronic spectra of quasi-regular heterostructures: simple versus realistic models

The electronic spectra of quasi-regular systems are investigated via simple one-dimensional, one-band models and compared with those obtained by means of realistic empirical tight-binding models, particularly for the Fibonacci sequence case.

Finite-size effects in the conductance and giant magnetoresistance of Fe/Cr and Co/Cu nanowires

We present theoretical studies of the size dependence of the current perpendicular-to-the-plane ballistic conductance and giant magnetoresistance (GMR) in Fe/Cr and Co/Cu nanowires within a realistic tight-binding model. Symmetry properties of the nanowire with respect to transformations of the point group ${C}_{4v}$ are exploited to decompose the problem into several independent parts. This allows us to study mesoscopic-size nanowires which are experimentally achievable. Our calculations predict substantial reduction in GMR due to the finite size of the nanowire.

Density waves in the organic metal $alpha;-(BEDT-TTF)2KHg(SCN)4

Abstract We have investigated possible spin and charge density waves in the organic metal α-(BEDT-TTF) 2 KHg(SCN) 4 . This system shows density wave like transition at T=8 K , and the nature of the density wave is not clarified up to now. Using a realistic tight binding model and an inhomogeneous mean-field theory, we obtained several stable density wave states. Especially, spin density wave states (SDW) are stable only for large on-site Coulomb interaction U⩾400 meV . Their spin moments are tiny and inhomogeneous even in the unit cell. Also charge density wave appears simultaneously with the SDW.

Thermoelectric power and electronic structures of Kondo insulators

Thermoelectric power of the Kondo insulators is analyzed theoretically. It is pointed out that Kelvin's relation found in 1851 must be modified for insulators at low temperatures. A realistic tight-binding model is constructed for the description of the conduction bands of the most typical Kondo insulator YbB 12. It will be a good starting point for the development of a realistic theory of the Kondo insulators with strong correlation.

Origin of magnetocrystalline anisotropy oscillations in (001) face-centred-cubic Cothin films and effect of sp–d hybridization

Magnetocrystalline anisotropy (MA) energy of (001) face-centred-cubicCo(N) films is calculatedfor film thicknesses N = 1–28in a realistic tight-binding model with and without sp–d orbital hybridizationincluded. The obtained results show that the average MA energy is not largelyinfluenced by the sp–d hybridization. On the other hand, the oscillation pattern isremarkably changed when the sp–d hybridization is included: in this case the MAenergy has oscillations with a clear period of 2 atomic layers (AL), similar to theprevious ab initio calculations (Szunyogh L, Újfalussy B, Blaas C, Pustugova U,Sommers C and Weinberger P 1997 Phys. Rev. B 56 14036). A careful analysis in k- andN-spacesreveals that the total MA oscillations are a superposition of two oscillatorycontributions: one coming from the neighbourhood of the Γ̄-pointwith period close to 2 AL (regardless whether the sp–d hybridization ispresent or not) and the other originating in the region around the M̄-point. TheM̄-pointcontribution has a larger period and its amplitude is significantly smaller than that of the Γ̄-pointcontribution when the sp–d hybridization is included so that the 2 AL Γ̄-pointcontribution is dominant in this case. The two oscillatory MA contributions areattributed to quantum-well states and the corresponding oscillation periods arerelated to the extremal radii of the minority-spin bulk Co Fermi surface.

Realistic tight-binding model for the electronic structure of II-VI semiconductors

We analyze the electronic structure of group II-VI semiconductors obtained within LMTO approach in order to arrive at a realistic and minimal tight binding model, parameterized to provide an accurate description of both valence and conduction bands. It is shown that a nearest-neighbor $sp^3d^5$ model is fairly sufficient to describe to a large extent the electronic structure of these systems over a wide energy range, obviating the use of any fictitious $s^*$ orbital. The obtained hopping parameters obey the universal scaling law proposed by Harrison, ensuring transferability to other systems. Furthermore, we show that certain subtle features in the bonding of these compounds require the inclusion of anion-anion interactions in addition to the nearest-neighbor cation-anion interactions.

Geometric and Electronic Structure of New Carbon-Network Materials: Nanotube Array on Graphite Sheet

We design a new class of carbon-network materials with a periodically modified graphite sheet. The modified part corresponds to (6,6) carbon-nanotube geometry. Their tube parts form triangular lattice on graphite sheet. On these systems each tube has six heptagons at the bottom, giving rise to a seamless s p 2 -C network with a negative curvature. We consider these nanotube arrays on graphite sheet with three kinds of tube-end geometries and various sizes for both graphite and tube parts. We report their electronic structures obtained by using a realistic tight-binding model, and for selected systems the density-functional theory. Interestingly, results show that most of them are semiconductors although both (6,6) tube and graphite are metallic. The difference in their tube-end geometries and the sizes of graphite and tube parts affect their electronic structures. Some have nearly flat band states around the Fermi level, showing a possibility of ferromagnetic behavior if hole or electron is doped. Some are direct-gap semiconductors whose interband transition is optically allowed. Their typical gap energies are about 1 eV. Therefore they should emit infrared light.

Perpendicular giant magnetoresistance in Fe/Cr nanowires

We present theoretical studies on the size dependence of the current perpendicular-to-the-plane ballistic conductance and giant magnetoresistance (GMR) in Fe/Cr nanowires within a realistic tight-binding model. We find that the conductance of the minority channel in the parallel configuration increases very slowly with the nanowire size. At the same time, the conductance of all other channels reaches the value observed in Fe/Cr multilayers at very small nanowire size. This limits the GMR ratio to only a fraction of the multilayer value for small nanowires.

Structural and electronic properties of carbon nanotube tapers

Carbon nanotube tapers are a set of nanostructures comprised of straight tubular sections with decreasing diameters, joined to each other via conical funnels and terminated with a hemispherical cap. The funnels are formed with the help of topological defects, which minimally include at least one pentagon-heptagon pair. The structural, electronic, and transport properties of tapers are analyzed using realistic tight-binding models. Specifically, it is shown that straight nanotube tapers are monochiral objects. Among a variety of possible taper structures, kinetics of the growth process suggests that the most prevalent tapers will have either zigzag or armchair structures. Their scanning tunneling microscopy (STM) images have been simulated for identification purposes. The STM images of tapers are dominated by a protruding pentagon inherent in the taper structure, which unfortunately does not allow for an easy identification of the chirality of the underlying nanotubes. Turning to transport properties, it is shown that zigzag-based tapers will likely be poor conductors, because of gaps induced by the semiconducting segments. Armchair-based tapers, on the other hand, are characterized by a finite conductance at low bias voltages and make attractive prototypes for nanoscale probes and devices.

Effect of disorder on perpendicular magnetotransport in Co/Cu multilayers

We investigate the spin-dependent conductance of the Co/Cu multilayers in the current-perpendicular-to-the-plane (CPP) geometry. Using a realistic tight-binding model for the electronic band structure of the multilayer, and introducing disorder in the on-site atomic energies we calculate the conductance and giant magnetoresistance (GMR) within the quantum-mechanical linear response theory by performing averaging over random disorder configurations. By varying the thickness and the number of individual layers and the degree of disorder in the multilayers, we analyze factors influencing the CPP GMR. In particular, we show the importance of the thickness-dependent interface resistance, which depends on the mean free path and consequently lies beyond the two-current series-resistor model.

General Green’s-function formalism for transport calculations withspdHamiltonians and giant magnetoresistance in Co- and Ni-based magnetic multilayers

A general Green's-function technique for elastic spin-dependent transport calculations is presented, which (i) scales linearly with system size and (ii) allows straightforward application to general tight-binding Hamiltonians $(spd$ in the present work). The method is applied to studies of conductance and giant magnetoresistance (GMR) of magnetic multilayers in current perpendicular to planes geometry in the limit of large coherence length. The magnetic materials considered are Co and Ni, with various nonmagnetic materials from the $3d,$ $4d,$ and $5d$ transition metal series. Realistic tight-binding models for them have been constructed with the use of density functional calculations. We have identified three qualitatively different cases which depend on whether or not the bands (densities of states) of a nonmagnetic metal (i) form an almost perfect match with one of spin subbands of the magnetic metal (as in Cu/Co spin valves), (ii) have almost pure $\mathrm{sp}$ character at the Fermi level (e.g., Ag), and (iii) have almost pure d character at the Fermi energy (e.g., Pd, Pt). The key parameters which give rise to a large GMR ratio turn out to be (i) a strong spin polarization of the magnetic metal, (ii) a large energy offset between the conduction band of the nonmagnetic metal and one of spin subbands of the magnetic metal, and (iii) strong interband scattering in one of spin subbands of a magnetic metal. The present results show that GMR oscillates with variation of the thickness of either nonmagnetic or magnetic layers, as observed experimentally.

Local approach to electronic excitations in the insulating copper oxidesLa2CuO4andSr2CuO2Cl2

We calculate the spectra of single-particle excitations on the realistic tight-binding models for ${\mathrm{La}}_{2}{\mathrm{CuO}}_{4}$ and ${\mathrm{Sr}}_{2}{\mathrm{CuO}}_{2}{\mathrm{Cl}}_{2},$ which are prototypical antiferromagnetic insulators and the parent compounds of high-${T}_{c}$ superconductors. To deal with strong correlation effects, we calculate the self-energy for the single-particle Green's function by using the local three-body scattering approximation, which considers multiple scatterings between three particles excited on the Hartree-Fock (HF) ground state. The present method is shown to improve greatly the HF results, leading to good agreement with the photoemission experiments; the band gap is reduced from the HF value, the satellite structure appears, and the spectral intensity coming from the Cu $3d$ states is pushed upward in the valence band. The dispersion relation for quasiparticles is obtained in good agreement with the recent angle-resolved photoemission experiment for ${\mathrm{Sr}}_{2}{\mathrm{CuO}}_{2}{\mathrm{Cl}}_{2}.$ The validity of the present theory is demonstrated in the Emery model through a comparison with the exact diagonalization calculations for finite clusters.

Piezooptical Properties of Si

The deformation potentials and the piezooptical properties of Si, for strain along the [001] and [111] directions, are investigated with the use of a realistic empirical tight-binding model. The values, as well as their sign, of the deformation potential constants D 1 1 , D 1 5 , D 3 3 , and D 3 5 for the variation of the critical energy E 1 are evaluated for uniaxial and biaxial strain. The sign is found to be negative for D 1 1 , D 3 3 , and D 3 5 , and positive for D 1 5 . The dielectric function of Si under pressure, as well as the reflectivity and the linear piezooptical tensor components P 11 (ω), P 12 (ω) and P 44 (ω) are also evaluated. The main structures in P ij are located around the critical points E 1 and E 2 .

Piezooptical Properties of Si

The deformation potentials and the piezooptical properties of Si, for strain along the [001] and [111] directions, are investigated with the use of a realistic empirical tight-binding model. The values, as well as their sign, of the deformation potential constants D11, D51, D33, and D53 for the variation of the critical energy E1 are evaluated for uniaxial and biaxial strain. The sign is found to be negative for D11, D33, and D53, and positive for D51. The dielectric function of Si under pressure, as well as the reflectivity and the linear piezooptical tensor components P11(ω), P12(ω) and P44(ω) are also evaluated. The main structures in Pij are located around the critical points E1 and E2.

Piezo-optical properties of Ge

The piezo-optical properties of Ge, for pressure along the [001] and [111] directions, are calculated with the use of a realistic empirical tight-binding model. The real and imaginary part of components ${P}_{11}(\ensuremath{\omega})$, ${P}_{12}(\ensuremath{\omega})$, and ${P}_{44}(\ensuremath{\omega})$ of the linear piezo-optical tensor for Ge are evaluated. The main structures in ${P}_{\mathrm{ij}}$ are located around the ${E}_{1}$ and ${E}_{2}$ critical points. Deformation potential constants ${D}_{1}^{1}$, ${D}_{1}^{5}$, ${D}_{3}^{3}$, and ${D}_{3}^{5}$, for the ${E}_{1}\ensuremath{-}{(E}_{1}+{\ensuremath{\Delta}}_{1})$ transitions are also evaluated. The theoretically obtained results are in good agreement with existing experimental values.

Interlayer exchange coupling in Fe/Cr multilayers

We investigate the origin of the long-period oscillation of the interlayer exchange coupling in Fe/Cr trilayer systems. Within the stationary phase approximation the periods of the oscillations are associated with extremal vectors of the Fermi sphere of Cr. Using a realistic tight-binding model with spin-orbit interaction we calculate the coupling strength for each extremal vector based on the spin asymmetry of the reflection amplitude for a propagating state impinging from the Cr to Fe layer. We find that for the (001) and (110) growth directions the biggest coupling strength comes from the extremal vector centered at the ellipsoid N of the Fermi surface of Cr.

Importance of spin-independent scattering potentials in giant magnetoresistance

We demonstrate that the giant magnetoresistance (GMR) effect in magnetic multilayers can be explained quantitatively in terms of the scattering of electrons from a spin-independent random potential that arises from the grown-in defects within the multilayer. We have calculated the GMR ratio for Co4/Cu4 and Fe4/Cr4 (001) multilayered systems within the Kubo–Greenwood formalism assuming that the on-site atomic energies are disordered randomly within a realistic spd tight-binding model. Our predictions are in good agreement with experiment and demonstrate that (i) increasing disorder causes a drop of GMR in multilayers, (ii) GMR for the current perpendicular to the plane is typically by a factor of two higher than GMR for the current in the plane, and (iii) the semiclassical treatment of conductivity applied to magnetic multilayers results in overestimated values of GMR due to the neglect of interband transitions.