Tight-binding Band Structure Research Articles

Physical insights on comparable electron transport in (100) and (110) double-gate fin field-effect transistors

We have investigated the physical mechanisms that result in comparable electron mobility measured from (100) and (110) sidewall double-gate fin field-effect transistors (FinFETs). Using a self-consistent Schrodinger-Poisson simulator coupled with a sp3d5s* tight-binding bandstructure, we have shown that the (100)/〈100〉 and (110)/〈110〉 average conductivity effective mass values are similar. This is explained by the much heavier non-parabolic confinement mass for Δ2 valley of (110) FinFETs, which leads to lower Δ2 energy than Δ4. Thus, for both (100) and (110), the majority of electrons occupy the Δ2 valley with 0.19m0 conductivity effective mass, resulting in comparable electron mobility.

Analytical expression for the RKKY interaction in doped graphene

We obtain an analytical expression for the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction $J$ in electron or hole doped graphene for linear Dirac bands. The results agree very well with the numerical calculations for the full tight-binding band structure in the regime where the linear band structure is valid. The analytical result, expressed in terms of the Meijer G-function, consists of a product of two oscillatory terms, one coming from the interference between the two Dirac cones and the second coming from the finite size of the Fermi surface. For large distances, the Meijer G-function behaves as a sinusoidal term, leading to the result $J \sim R^{-2} k_F \sin (2 k_F R) {1 + \cos[(K-K').R]}$ for moments located on the same sublattice. The $R^{-2}$ dependence, which is the same for the standard two-dimensional electron gas, is universal irrespective of the sublattice location and the distance direction of the two moments except when $k_F =0$ (undoped case), where it reverts to the $R^{-3}$ dependence. These results correct several inconsistencies found in the literature.

Eight-Coordinated Arsenic in the Zintl Phases RbCd4As3 and RbZn4As3: Synthesis and Structural Characterization

Reported are two new series of Zintl phases, ACd(4)Pn(3) and AZn(4)Pn(3) (A = Na, K, Rb, Cs; Pn = As, P), whose structures feature complex atomic arrangements based on four- and eight-coordinated arsenic and phosphorus. A total of 12 compounds have been synthesized from the corresponding elements via high temperature reactions, and their structures have been established by X-ray diffraction. RbCd(4)As(3), KCd(4)As(3), NaCd(4)As(3), NaZn(4)As(3), KCd(4)P(3), and KZn(4)P(3) crystallize with a new rhombohedral structure (space group R3m, Z = 3, Pearson symbol hR24), while the isoelectronic RbZn(4)As(3), CsCd(4)As(3), CsZn(4)As(3), KZn(4)As(3), CsZn(4)P(3), and RbZn(4)P(3) adopt the tetragonal KCu(4)S(3)-type structure (space group P4/mmm, Z = 1, Pearson symbol tP8). Both structures are very closely related to the ubiquitous CaAl(2)Si(2) and ThCr(2)Si(2) structure types, and the corresponding relationships are discussed. The experimental results have been complemented by linear muffin-tin orbital (LMTO) tight-binding band structure calculations. Preliminary transport properties measurements on polycrystalline samples suggest that the compounds of these families could be promising thermoelectric materials.

Transport properties of an armchair boron-nitride nanoribbon embedded between two graphene electrodes

Transport properties of an armchair boron-nitride nanoribbon (ABNNR) between two armchair graphene nanoribbons (AGNRs) have been studied using the tight binding approach in the nearest neighbor approximation. The tight biding parameters are obtained by comparing the tight binding bandstructure of pure and doped hexagonal boron-nitride sheet with first-principle calculations. It is shown that by increasing the ribbon width transmission through the system increases. Also the conductance gap has a damping oscillation with the BNNR width.

Effects of Surface Orientation on the Performance of Idealized III–V Thin-Body Ballistic n-MOSFETs

Ballistic on-currents of thin-body n-channel metal-oxide-semiconductor field-effect transistors (n-MOSFETs) are compared across group IV (Si, Ge) and III-V (InAs, In <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.5</sub> Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.5</sub> As, GaAs, GaSb) materials for different body thickness values, surface orientations, and transport directions under several idealization assumptions. Previous simulation studies have shown that, as oxide capacitance increases, typical III-V channels with (100) surface perform worse than Si in the ballistic limit due to the degraded density-of-states (DOS). In this letter, simulation results based on tight-binding band structure calculations verify a recent proposal that confined III-V n-MOSFETs with small Γ-L separations overcome the DOS bottleneck and deliver high injection velocities, boosting on-current performance. By using the quantized L-valleys, GaSb with (100) or (111) surface orientations shows the best ballistic performance, outperforming all other materials. Although GaAs (100) and InAs or In <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.5</sub> Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.5</sub> As with any surface orientation suffer from the DOS bottleneck, GaAs (111) gives higher ballistic on -currents than Si does.

RKKY interaction in graphene from the lattice Green’s function

We study the exchange interaction $J$ between two magnetic impurities in undoped graphene (the Ruderman-Kittel-Kasuya-Yosida [RKKY] interaction) by directly computing the lattice Green's function for the tight-binding band structure for the honeycomb lattice. The method allows us to compute $J$ numerically for much larger distances than can be handled by finite-lattice calculations as well as for small distances. In addition, we rederive the analytical long-distance behavior of $J$ for linearly dispersive bands and find corrections to the oscillatory factor that were previously missed in the literature. The main features of the RKKY interaction in half-filled graphene are that unlike the $J\ensuremath{\propto}(2{k}_{F}R){}^{\ensuremath{-}2}\mathrm{sin}(2{k}_{F}R)$ behavior of an ordinary two-dimensional metal in the long-distance limit, $J$ in graphene falls off as $1/{R}^{3}$, shows the $1+\mathrm{cos}[(K\ensuremath{-}{K}^{\ensuremath{'}})\ifmmode\cdot\else\textperiodcentered\fi{}R]$-type oscillations with additional phase factors depending on the direction, and exhibits a ferromagnetic interaction for moments on the same sublattice and an antiferromagnetic interaction for moments on the opposite sublattices as required by particle-hole symmetry. The computed $J$ with the full band structure agrees with our analytical results in the long-distance limit, including the oscillatory factors with the additional phases.

Multi-Orbital Molecular Compound (TTM-TTP)I3: Effective Model and Fragment Decomposition

The electronic structure of the molecular compound (TTM-TTP)I 3 , which exhibits a peculiar intra-molecular charge ordering, has been studied using multi-configuration ab initio calculations. First we derive an effective Hubbard-type model based on the molecular orbitals (MOs) of TTM-TTP; we set up a two-orbital Hamiltonian for the two MOs near the Fermi energy and determine its full parameters: the transfer integrals, the Coulomb and exchange interactions. The tight-binding band structure obtained from these transfer integrals is consistent with the result of the direct band calculation based on density functional theory. Then, by decomposing the frontier MOs into two parts, i.e., fragments, we find that the stacked TTM-TTP molecules can be described by a two-leg ladder model, while the inter-fragment Coulomb energies are scaled to the inverse of their distances. This result indicates that the fragment picture that we proposed earlier [M.-L. Bonnet et al. : J. Chem. Phys. 132 (2010) 214705] successfully describes the low-energy properties of this compound.

Tight-binding Hamiltonian for LaOFeAs

First-principles electronic-structure calculations have been very useful in understanding some of the properties of the new iron-based superconductors. Further explorations of the role of the individual atomic orbitals in explaining various aspects of research in these materials, including experimental work, would benefit from the availability of a tight-binding (TB) Hamiltonian that accurately reproduces the first-principles band-structure results. In this work we have used the Naval Research Laboratory-TB method to construct a TB Hamiltonian from linearized augmented plane wave (LAPW) results. Our TB model includes the $\text{Fe}\text{ }d$ orbitals and the $p$ orbitals from both As and O for the prototype material LaOFeAs. The resulting TB band structure agrees well with that of the LAPW calculations from 2.7 eV below to 0.8 eV above the Fermi level, ${\ensuremath{\epsilon}}_{F}$, and the Fermi surface matches perfectly to that of the LAPW. The TB densities of states (DOSs) are also in very good agreement with those from the LAPW in the above energy range, including the per orbital decomposition. We use our results to provide insights on the existence of a pseudogap in the DOS just above the Fermi level. We have also performed a separate TB fit to a database of LAPW results as a function of volume and with variations in the As positions. This fit although less accurate regarding the band structure near ${\ensuremath{\epsilon}}_{F}$, reproduces the LAPW total energies very well and has transferability to nonfitted energies.

Theory of the time-resolved spectral function of high-temperature superconductors with bosonic modes

We develop a three-temperature model to simulate the time dependence of the electron and phonon temperatures in high-temperature superconductors displaying strong anisotropic electron-phonon coupling. This model not only takes the tight-binding band structure into account but also is valid in the superconducting state. Based on this model, we calculate the time-resolved spectral function via the double-time Green's functions. We find that the dip-hump structure evolves with the time delay. More interestingly, an unusual structure is obtained when the phonons are pumped directly. This finding may serve as a direct evidence for electron-vibration mode coupling.

RKKY coupling in graphene

We study the carrier-mediated exchange interaction, the so-called Ruderman-Kittel-Kasuya-Yoshida (RKKY) coupling, between two magnetic impurity moments in graphene using exact diagonalization on the honeycomb lattice. By using the tight-binding nearest-neighbor band structure of graphene we also avoid the use of a momentum cutoff which plagues perturbative results in the Dirac continuum model formulation. We extract both the short and long impurity-impurity distance behavior and show on a qualitative agreement with earlier perturbative results in the long-distance limit but also report on a few new findings. In the bulk the RKKY coupling is proportional to $1/{|\mathbf{R}|}^{3}$ and displays $[1+\text{cos}(2{\mathbf{k}}_{D}\ensuremath{\cdot}\mathbf{R})]$ -type oscillations. A-A sublattice coupling is always ferromagnetic whereas A-B subattice coupling is always antiferromagnetic and three times as large. We also study the effect of edges in zigzag graphene nanoribbons (ZGNRs). We find that for impurities on the edge the RKKY coupling decays exponentially because of the localized zero-energy edge states and we also conclude that a nonperturbative treatment is essential for these edge impurities. For impurities inside a ZGNR the bulk characteristics are quickly regained.

Multiband effective bond-orbital model for nitride semiconductors with wurtzite structure

A multiband empirical tight-binding model for group-III-nitride semiconductors with a wurtzite structure has been developed and applied to both bulk systems and embedded quantum dots. As a minimal basis set we assume one s-orbital and three p-orbitals, localized in the unit cell of the hexagonal Bravais lattice, from which one conduction band and three valence bands are formed. Non-vanishing matrix elements up to second nearest neighbors are taken into account. These matrix elements are determined so that the resulting tight-binding band structure reproduces the known Gamma-point parameters, which are also used in recent kp-treatments. Furthermore, the tight-binding band structure can also be fitted to the band energies at other special symmetry points of the Brillouin zone boundary, known from experiment or from first-principle calculations. In this paper, we describe details of the parametrization and present the resulting tight-binding band structures of bulk GaN, AlN, and InN with a wurtzite structure. As a first application to nanostructures, we present results for the single-particle electronic properties of lens-shaped InN quantum dots embedded in a GaN matrix.

Anisotropic tight-binding model applied to zigzag ultra-small nanotubes

A single-wall carbon nanotube (SWCNT) can be visualized as a graphene rolled into a cylinder. Tight-binding band structure calculations, with hopping between nearest-neighbor π orbitals only (NNTB), established rules by which both the mode in which the graphene is rolled up and the diameter determine whether the SWCNT is a metal or a semiconductor. However, when the diameter of the SWCNT is ultra-small its large curvature results in the breakage of these rules. In this work, we studied zigzag (n, 0) SWCNTs with diameters smaller than 0.7 nm using a π orbital-only tight-binding model including anisotropy in the hopping between next-nearest-neighbor sites (ANNNTB). Band overlaps were found in the electronic band structures of the zigzag SWCNTs for n=3, 4, 5, and 6, indicating that they are metals. The reason why the band structures of armchair and chiral SWCNTs are less affected by curvature effects becomes clear with the ANNNTB model, as does the reason why non-degenerate states cause band overlaps of the zigzag SWCNTs for n=3, 4, 5, and 6. Our results show that a π orbital-only tight-binding model is able to describe both the band overlaps and gaps obtained by ab initio calculations for zigzag SWCNTs.

Mixed Cations and Structural Complexity in (Eu1−xCax)4In3Ge4 and (Eu1−xCax)3In2Ge3—The First Two Members of the Homologous Series A2[n+m]In2n+mGe2[n+m] (n, m = 1, 2, ...∞; A = Ca, Sr, Ba, Eu, or Yb)

Reported are the synthesis and the structural characterization of two members of a new homologous series of polar intermetallic compounds, which exist only with mixed alkaline-earth and rare-earth metal cations. Crystals of (Eu(1-x)Ca(x))(4)In(3)Ge(4) (0.35(1) <or= x <or= 0.70(1)) and (Eu(1-x)Ca(x))(3)In(2)Ge(3) (0.78(1) <or= x <or= 0.90(1)) have been grown using a molten In metal flux and structurally characterized by single-crystal X-ray diffraction. (Eu(1-x)Ca(x))(4)In(3)Ge(4) adopts the monoclinic Mg(5)Si(6)-type structure (space group C2/m, Z = 2, Pearson symbol mS22) with lattice parameters a = 16.874(1)-17.024(2) A, b = 4.496(3)-4.556(1) A, c = 7.473(4)-7.540(1) A, and beta = 107.306(10)-105.631(3) degrees . (Eu(1-x)Ca(x))(3)In(2)Ge(3) crystallizes with a novel orthorhombic structure (space group Pnma, Z = 4, Pearson symbol oP32) with lattice parameters in the ranges a = 7.382(2)-7.4010(9) A, b = 4.452(1)-4.4640(6) A, and c = 23.684(6)-23.734(3) A, depending on the Eu/Ca ratio. The polyanionic substructures in both cases are related and are based on InGe(4) edge-shared tetrahedra, Ge(2) dimers, and bridging In atoms in a nearly square-planar environment. The (Eu(1-x)Ca(x))(4)In(3)Ge(4) structure can be viewed as a 1:1 intergrowth of Mo(2)FeB(2)-like and TiNiSi-like fragments, whereas (Eu(1-x)Ca(x))(3)In(2)Ge(3) can be rationalized as a 2:1 intergrowth of the same structural motifs. Both phases exhibit fairly wide homogeneity ranges and exist only with mixed cations. The experimental results have been complemented by linear muffin-tin orbital tight-binding band structure calculations, as well as an analysis of the observed cationic site preferences.

Polarization-induced switching effect in graphene nanoribbon edge-defect junction

With nonequilibrium Green's function approach combined with density functional theory, we perform an ab initio calculation to investigate transport properties of graphene nanoribbon (GNR) junctions self-consistently. Tight-binding approximation is applied to model the zigzag (ZGNR) electrodes, and its validity is confirmed in comparison to the GAUSSIAN03 periodic boundary condition calculation result of the same system. The origin of abnormal jump points usually appearing in the transmission spectrum is explained with the detailed tight-binding ZGNR band structure. Transport property of an edge-defect ZGNR junction is investigated, and the tunable tunneling current can be sensitively controlled by transverse electric fields.

Crystal and Electronic Structures of the Quasi-Two-Dimensional Organic Conductor α-(BEDT-TTF)2I3 and Its Selenium Analogue α-(BEDT-TSeF)2I3 under Hydrostatic Pressure at Room Temperature

We perform crystal structure analyses of the organic conductor α-(BEDT-TTF) 2 I 3 [where BEDT-TTF denotes bis(ethylene)dithiotetrathiafulvalene] and its selenium analogue α-(BEDT-TSeF) 2 I 3 [where BEDT-TSeF denotes bis(ethylene)dithiotetraselenafulvalene] at room temperature under hydrostatic pressure, where they are expected to have zero-gap points at the Fermi level with a Dirac-cone-type dispersion for a massless particle. We calculate the electronic band structures using a combination of the extended Hückel molecular orbital calculation based on the obtained crystal structures and the tight-binding band structure calculation. We find that I) the obtained Fermi surfaces of both salts under hydrostatic pressure are simply larger than those at ambient pressure owing to an increase in overlap integrals caused by the pressure, and II) electronic structures having zero-gap points with a Dirac-cone-type dispersion at the Fermi level are realized only in α-(BEDT-TTF) 2 I 3 by the introduction of suitable site potentials in wide parameter areas. These results suggest that, to realize such a unique electronic structure in this class of materials, some mechanisms are needed to effectively give the site potentials employed in this study. Examples are electronic correlation effects and I 3 anion potentials.

Spin and charge order in the CE phase ofLa1−xCaxMnO3

The CE phase of ${\text{La}}_{1\ensuremath{-}x}{\text{Ca}}_{x}{\text{MnO}}_{3}$ is stable for $x\ensuremath{\gtrsim}0.5$ and displays long-range magnetic, charge, and orbital orders. The magnetic order of the Mn spins arises from the competition of the superexchange and double-exchange interactions and the checkerboard charge and the orbital order is the consequence of the Jahn-Teller coupling of the ${e}_{g}$ orbitals to the lattice. Using a mean-field slave-boson approach for the ${e}_{g}$ electrons in two orbitals per site with excluded multiple occupancy and Hund's rule coupling between the ${e}_{g}$ and ${t}_{2g}$ states, we obtain the tight-binding band structure of the CE phase. The unit cell of the CE phase consists of 16 sites. The 32 ${e}_{g}$ bands in the Brillouin zone are grouped into two sets of 16 bands separated by a charge order gap. The phase diagram including the A, B, C, CE, and G phases reproduces the phase sequence observed in ${\text{La}}_{1\ensuremath{-}x}{\text{Ca}}_{x}{\text{MnO}}_{3}$.

Mean-field model for the CE-phase of manganites

The CE phase of La0.5Ca0.5MnO3 displays long-range magnetic, charge and orbital order of the eg states of the Mn ions. The magnetic order of the t2g spins of Mn is determined by the antiferromagnetic superexchange and the ferromagnetic double-exchange. The checkerboard charge and the orbital order is the consequence of the Jahn-Teller coupling of the eg orbitals to the lattice. Using a mean-field slave-boson approach for the eg electrons in two orbitals per site with excluded multiple occupancy and Hund's rule coupling between the eg and t2g states, we obtain the tight-binding band structure of the CE phase. The unit cell of the CE phase consists of 16 sites. The 32 eg bands in the Brillouin zone are grouped into two sets of 16 bands separated by a charge order gap. The charge gap does not directly affect the properties of the compound. If long-range orbital order is included an additional gap opens at the Fermi level.

A nonparabolicity model compared to tight-binding: The case of square silicon quantum wires

This work presents a nonparabolicity (NP) model which is able to improve the effective mass approximation (EMA) for computing transfer characteristics of square silicon quantum wire transistors (SQWT) working in the ballistic regime and subjected to bandstructure effects. The model is found to be treatable within the same transport framework as used in a present 3D EMA Poisson–Schrödinger solver thus keeping a comparable time efficiency. A full-band tight-binding (TB) code provides the bandstructures as well as the transfer characteristics related to a series of SQWTs needed for calibrating the NP model. In comparison with the EMA, the threshold voltage ( V T ) obtained via the NP model is notably closer to the TB data for all wire widths considered in this work. In addition, the NP model is found to satisfactorily predict the increase of the conduction masses belonging to the unprimed conduction valleys of the TB bandstructure.

Monte Carlo simulation of electron mobility in strained Si DG-FETs with TB bandstructure calculation

A low dimensional tight-binding (TB) based bandstructure calculation program is developed for double-gate MOSFETs (DG-FETs) to model the effects of channel orientation, transverse electric field, stress, and geometry-induced quantum confinement. Electron mobility in the strained channel is then evaluated using the Monte Carlo (MC) method, based on the calculated bandstructure. It is concluded that electron mobility is enhanced by the splitting of conduction band valleys and the change of electron effective mass, as a function of layer thickness, crystal orientation and stress in strained Si DG-FETs.

Px,y-orbital counterpart of graphene: Cold atoms in the honeycomb optical lattice

We study the ground-state properties of the interacting spinless fermions in the ${p}_{x,y}$-orbital bands in the two-dimensional honeycomb optical lattice, which exhibit different features from those in the ${p}_{z}$-orbital system of graphene. In addition to two dispersive bands with Dirac cones, the tight-binding band structure exhibits another two completely flat bands over the entire Brillouin zone. With the realistic sinusoidal optical potential, the flat bands acquire a finite but much smaller bandwidth compared to the dispersive bands. The band flatness dramatically enhanced interaction effects giving rise to various charge and bond ordered states at commensurate fillings of $n=\frac{i}{6}(i=1--6)$. At $n=\frac{1}{6}$, the many-body ground states can be exactly solved as the close-packed hexagon states which can be stabilized even in the weakly interacting regime. The dimerization of bonding strength occurs at both $n=\frac{1}{2}$ and $\frac{5}{6}$, and the latter case is accompanied with the charge-density wave of holes. The trimerization of bonding strength and charge inhomogeneity appear at $n=\frac{1}{3},\frac{2}{3}$. These crystalline orders exhibit themselves in the noise correlations of the time-of-flight spectra.