Self-consistent Band Structure Research Articles

Electronic structure of iron pyrite by the LMTO_ASA method

We obtain the self-consistent band structure and density of states of iron pyrite FeS2 using the linear muffin-tin orbital method in the atomic-sphere approximation (LMTO-ASA). Both the electronic structure and density of states are discussed in detail. The calculated band gap is direct, with a width of 0.90508 eV. These results agree with experimental measurements. The calculated density of states shows that the valence band is fully occupied while the first conduction band is unoccupied.

Quasiparticle self-consistent GW band structures of Mg-IV-N2 compounds: The role of semicore d states

We present improved band structure calculations of the Mg-IV-N2 compounds in the quasiparticle self-consistent GW approximation. Compared to previous calculations (Phys. Rev. B 94, 125201 (2016)) we here include the effects of the Ge-3d and Sn-4d semicore states and find that these tend to reduce the band gap significantly. This places the band gap of MgSnN2 in the difficult to reach green region of the visible spectrum. The stability of the materials with respect to competing binary compounds is also evaluated and details of the valence band maximum manifold splitting and effective masses are provided.

Quasiparticle self-consistent GW electronic band structures of Be-IV-N2 compounds

The electronic band structures of BeSiN2 and BeGeN2 compounds are calculated using the quasiparticle self-consistent GW method. The lattice parameters are calculated for the wurtzite based crystal structure commonly found in other II–IV-N2 compounds with the Pbn21 space group. They are determined both in the local density approximation and generalized gradient approximation, which provide lower and upper limits. At the GGA lattice constants, which gives lattice constants closer to the experimental ones, BeSiN2 is found to have an indirect band gap of 6.88 eV and its direct gap at is 7.77 eV, while in BeGeN2 the gap is direct at and equals 5.03 eV. To explain the indirect gap in BeSiN2 comparisons are made with the parent III-N compound w-BN band structure. The effective mass parameters are also evaluated and found to decrease from BeSiN2 to BeGeN2.

Interlayer charge transfer in n-modulation doped Al1−xGaxAs–GaAs single heterostructures

We studied the interlayer charge transfer in n-modulation doped Al1−xGaxAs–GaAs single heterostructures by photoluminescence and magnetotransport. Photoluminescence contributions from a high-mobility quasi-twodimensional electron system at the interface and GaAs bulk type excitons were analyzed, covering an excitation intensity range of nearly three orders of magnitude. The experiment was compared with selfconsistent band structure calculations that allowed to follow the charge redistribution in the sample. After sample cool down in the dark a parallel conducting quasi-twodimensional channel of low electron mobility appeared in the Al1−xGaxAs-layer, that gained in density up to a saturation value. There was only little interaction between the Al1−xGaxAs and GaAs sides in the persistent regime and a reduction of the depletion charge in the GaAs could largely account for the interface channel density enhancement. Contrary, the negative photoeffect present during continuous illumination strongly depends on the interaction between the interface and the parallel channel, which serves as a temporary sink for excess charges from the GaAs side. When the illumination is switched off the parallel channel acts as a source and the sample essentially returns to the prior illumination state of saturation.

Pressure Induced Structural Phase Transition, Metallization and Superconductivity in KI

The self-consistent band structure calculation for potassium iodide (KI) performed both in NaCl and CsCl structures using the full potential linear muffin-tin orbital (FP-LMTO) method is reported. The equilibrium lattice constant, bulk modulus and the phase transition pressure at which the compound undergoes structural phase transition from NaCl to CsCl are predicted from the total energy calculations. The band structure, density of states (DOS), electronic charge distributions, metallization and superconducting transition temperature (Tc) of KI are obtained as a function of pressure for both in NaCl and CsCl structures. The density of states at the Fermi level (N(EF)) gets enhanced under pressure, which leads to metallization in KI. It is found that, the charge transfer from s and p states to d state causes structural phase transition and superconductivity in KI. The pressure corresponding to structural phase transition from NaCl structure (B1) to the CsCl structure (B2) is 0.02 Mbar in KI. This value is agreement with the experimental value of 0.019 Mbar. In KI, the metallization occurs through indirect closing of the band gap between Γ and H points at the reduced volume V/Vo=0.43 (CsCl structure), the corresponding metallization pressure is 1.228Mbar. Our results completely agree with the experimental observation of 1.31 Mbar. The highest Tc estimated is 2.151 K and the corresponding pressure is 4.945 Mbar in the NaCl structure and 0.107 K in the CsCl structure.

Capacitance of Graphene/Copper Hybrid Nano Ribbon Interconnect - a First Principle Study

In very large scale integration (VLSI), as the Moore’s law is nearly to end, researchers are in search for alternative interconnect materials with excellent electrical and thermal conductivities. Various emerging reduced dimensional materials are under study to find an alternative to the Cu-based interconnect for post-CMOS technology nodes. In this regard, graphene (G) and carbon nanotubes (CNT) have been studied extensively in last decade. Since graphene and CNT have their own limitations in integrating with present CMOS technology, there is ongoing research to find hybrid materials, which can have simultaneously better thermal and electrical conductivity [1]. Recently, reduction in temperature has been observed in graphene encapsulated Cu wires [2] and potential application of graphene as a heat spreader. Since Cu-based VLSI interconnect is already in use in current CMOS technology it would be easier to integrate graphene as a Cu barrier layer and the heat spreader. Low-cost CVD techniques for large area graphene deposition on Cu film are well known. Graphene deposition on Cu includes an annealing process which eventually increases crystal integrity of Cu with larger grain size. In addition, graphene works as a barrier layer for Cu ion to diffuse into the dielectric. This is why G/Cu hybrid interconnect is a very good case for investigating electronic properties. Motivated by recent experiments, we have studied theoretically G/Cu structure in bulk and in one-dimension (1D) as next generation wire for nano-electronic applications. Our purpose is to investigate electronic properties of this structure with density functional theory (DFT). In this paper, we report density of states (DOS) and the quantum capacitance of (CQ) of G/Cu nanoribbon interconnect. We used DFT to study electronic band structure within Quantum-Espresso (QE) code. QE calculation is based on self–consistent plane-wave with Perdew-Zunger (PZ) pseudo-potential. We have considered one-dimensional slice of G/Cu interconnect of width 5nm, height 0.8nm and a variable length. A single layer of graphene is deposited on top of three atomic layers of Cu {111}. Cu {111} is chosen because this plane is closest to graphene lattice within 2% lattice mismatch. Among these three atomic layers, bottom two layers were fixed to their bulk lattice positions to represent bulk atoms. Top Cu layer and graphene layer were relaxed to find minimum force position. After obtaining atomic positions from relaxation calculations in QE, we performed self-consistent field calculation and band structure calculation. Details of DFT calculation is reported in our earlier work [3]. From electronic band structure, the electron density of states (DOS) is calculated. CQhas been calculated from DOS. C-V characteristics of a 5nm wide G/Cu nanoribbon interconnect (1D) is computed and also compared with G/Cu 2D case. For the purpose of comparison, we calculated quantum capacitance per unit area for both 1D and 2D cases. We have observed significant enhancement of CQ in G/Cu nanoribbon from its 2D counterpart. More electrons tunnel from underlying Cu substrate to graphene layer which cause more carrier density and enhances CQ. Dirac point shifting is also observed which is due to work function difference of G and Cu. Because of Dirac point shifting minimum capacitance point is also shifted from charge neutrality point. Near charge neutrality point, CQis nearly “V” shaped as in graphene but minimum capacitance is non-zero unlike graphene. This calculation is very useful in understanding electrical performances of G/Cu nanoribbon interconnect towards finding a suitable candidate of next generation interconnect materials. Due to enhanced capacitance, beside VLSI interconnect application, this work can be used for charge storage devices also. Acknowledgement:Part of this work is supported by the United States Air Force Research Laboratory under agreement number FA9453-10-1-0002. The U. S. Government is authorized to reproduce and distribute reprints for Government purposes notwithstanding any copyright notation thereon. Portions of this research were conducted with high performance computing resources provided by Louisiana State University (http://www.hpc.lsu.edu). [1] P. Goli, H. Ning, X. Li, C. Y. Lu, K. S. Novoselov, and A. A. Balandin, “Thermal Properties of Graphene–Copper–Graphene Heterogeneous Films,” Nano Letters,vol. 14, no. 3, pp. 1497-1503, April 12, 2014. [2] R. Mehta, S. Chugh, and Z. Chen, “Enhanced Electrical and Thermal Conduction in Graphene-Encapsulated Copper Nanowires,” Nano Letters,vol. 15, no. 3, pp. 2024-2030, March 11, 2015. [3] K. M. Mohsin, A. Srivastava, A. K. Sharma, C. Mayberry, and M. S. Fahad, “Current Transport in Graphene/Copper Hybrid Nano Ribbon Interconnect: A First Principle Study,” ECS Transactions, vol. 75, no. 13, pp. 49-53, August 25, 2016.

Temperature-dependent band structure ofSrTiO3interfaces

We build a theoretical model for the electronic properties of the two-dimensional (2D) electron gas that forms at the interface between insulating SrTiO$_3$ and a number of polar cap layers, including LaTiO$_3$, LaAlO$_3$, and GdTiO$_3$. The model treats conduction electrons within a tight-binding approximation, and the dielectric polarization via a Landau-Devonshire free energy that incorporates strontium titanate's strongly nonlinear, nonlocal, and temperature-dependent dielectric response. The self-consistent band structure comprises a mix of quantum 2D states that are tightly bound to the interface, and quasi-three-dimensional (3D) states that extend hundreds of unit cells into the SrTiO$_3$ substrate. We find that there is a substantial shift of electrons away from the interface into the 3D tails as temperature is lowered from 300 K to 10 K. This shift is least important at high electron densities ($\sim 10^{14}$ cm$^{-2}$), but becomes substantial at low densities; for example, the total electron density within 4~nm of the interface changes by a factor of two for 2D electron densities $\sim 10^{13}$ cm$^{-2}$. We speculate that the quasi-3D tails form the low-density high-mobility component of the interfacial electron gas that is widely inferred from magnetoresistance measurements.

Current Transport in Graphene/Copper Hybrid Nano Ribbon Interconnect: A First Principle Study

In search of emerging materials replacement of copper interconnect in nanometer CMOS technology nodes, graphene (G) and carbon nanotubes (CNT) have been studied intensively. There is also ongoing research to find hybrid materials, which can have better thermal and electrical conductivity [1]. Recently, reduction in temperature has been observed in graphene encapsulated copper wires [2]. It was argued that this excellent thermal property is inherent to copper not because of graphene. During graphene deposition process, Cu becomes annealed with larger grains. In addition, graphene works as a barrier layer for copper ion to diffuse into dielectric. Motivated by recent experiments we have studied theoretically G/Cu structure in bulk and in one-dimension (1D) as next generation wires for nano-electronic applications. Our purpose is to understand this structure and to study why it outperforms graphene only or Cu only interconnect. In this paper, we report density of states (DOS), current-voltage relation (I-V), and resistance (R) of G/Cu nano ribbon interconnect.We used density functional theory (DFT) with pseudo- potential technique to study electronic band structure within Quantum-Espresso (QE) code. For ballistic transport study, we used Landauer-Buttiker (LB) formalism within Wannier90 Module [3]. We have considered a one-dimensional slice of G/Cu interconnect of width 0.6nm, height 0.8nm and length 10nm. Single layer of graphene is deposited on top of three atomic layers of Cu {111}. Cu {111} is chosen because this plane is closest to graphene lattice within 2% lattice mismatch. Among these three atomic layers, bottom two layers were fixed to their bulk lattice position to represent bulk atoms. Top Cu layer and graphene layer were relaxed to find minimum force position. After obtaining atomic positions from relaxation calculations in QE, we performed self-consistent field calculation and band structure calculation. Bloch states obtained from QE were used as an input for transport calculation in Wannier90 Code. Due to high computational cost, we have limited our study to a 10nm long wire which represent a short local interconnect and a good candidate to study ballistic transport. We believe this geometry will at least serve present purpose to compare performances with graphene only interconnect. I-V characteristics of graphene interconnect and graphene on copper interconnect have been compared. Being one-dimensional wire, we have observed several stairs like and piece wise linear relationships. At zero bias, resistance is found to be the same as quantum resistance for single electronic channel, which is ~h/2e2. Since wire length 10nm is smaller than Cu mean free path (40nm), one should not compare resistivity of G/Cu with Cu only interconnect based on Fuchs-Sondheimer (FS) and Mavadas-Shatzkes (MS) models. For this G/Cu we found resistivity 1.25x10-6 Ω-m, which is one tenth of Cu-bulk resistivity. It is expected by FS & MS theory that Cu resistivity increases with decrease of width. However, in ballistic transport regime FS & MS theories were not applicable. Hence, we have used LB formalism. For comparison, we have studied same geometry for graphene only interconnect using same methodology. From DOS, it is apparent that there is no energy states available near Fermi energy for graphene only interconnect. This is why in I-V characteristics no current is observed near 0V. On other hand, hybrid structure shows better conductivity at low bias than graphene only interconnect. Graphene only nanoribbon might better perform as device not as interconnect. This calculation is useful to understand electrical performances of various nanostructures and to find true contender of next generation interconnect. Acknowledgement: Part of the work is supported by the United States Air Force Research Laboratory under agreement number FA9453-10-1-0002. The U.S. Government is authorized to reproduce and distribute reprints for Government purposes notwithstanding any copyright notation thereon.

Electronic structure of PbSnS3 and PbGeS3 semiconductor compounds with the mixed cation coordination

The self-consistent band structure calculation of PbSnS3 and PbGeS3 ternary compounds with the mixed cation coordination was performed using the ab initio density functional theory method. It has been found that both compounds are indirect-gap semiconductors. The calculated band-gap widths are Egi = 0.75 eV for PbSnS3 and Egi = 1.96 eV for PbGeS3. It was performed the analysis of partial contributions to the total density of electronic states, which allowed to identify the genetic nature of valence and conduction bands. The feature of the structure of occupied energy bands in these compounds is the presence of isolated quasi-core d-band of lead atoms and the contribution of Pb6s-states of lone-electron pair to the top of valence band as well as to the second occupied subband. The electron density maps in the different planes have been analyzed. They clearly demonstrate presence of covalent-ionic bond character within the infinite chains and ribbons with prevailing charge concentration on Ge–S, Sn–S, Pb–S bonds, and a weak van der Waals bond component with participation of lone-electron pair of lead between layers as well as ribbons.

ELECTRONIC STRUCTURE AND MAGNETIC PROPERTIES OF TIMN3N, TIMN3 AND MNTI3 COMPOUNDS USING TB-LMTO METHOD

Tight Binding Linear Muffin Tin Orbital Method has been employed to obtain the electronic and Magnetic properties of TiMn3N and ordered TiMn3 Compounds. The Equilibrium volume has been found for each compound using self-consistent band structure calculations at several lattice parameters. Obtained results reveals that TiMn3 is ferromagnetic with 3.3624µB as magnetic moment at Mn sites and-1.2174µB at Ti sites. Hence TiMn3 attains the stable magnetic order at Ferromagnetic and Non-Magnetic calculations, while TiMn3N shows the nonmagnetic phase as a stable one during FM and NM calculations.

Electronic properties and structural phase transition in A4 [M4O4] (A=Li, Na, K and Rb; M=Ag and Cu): A first principles study

Self-consistent scalar relativistic band structure calculations for AMO (A=Li, Na, K and Rb; M=Ag and Cu) compounds have been performed using the tight-binding linear muffin-tin orbital (TB-LMTO) method within the local density approximation (LDA). At ambient conditions, these compounds are found to crystallize in tetragonal KAgO-type structure with two different space group I-4m2 and I4/mmm. Nowadays, hypothetical structures are being considered to look for new functional materials. AMO compounds have stoichiometry similar to eight-electron half-Heusler materials of type I–I–VI which crystallizes in cubic (C1b) MgAgAs-type structure with space group F-43m. For all these compounds, by interchanging the positions of atoms in the hypothetical cubic structure, three phases (α, β and γ) are formed. The energy–volume relation for these compounds in tetragonal KAgO-type structure and cubic α, β and γ phases of related structure have been obtained. Under ambient conditions these compounds are more stable in tetragonal KAgO-type (I4/mmm) structure. The total energies calculated within the atomic sphere approximation (ASA) were used to determine the ground state properties such as equilibrium lattice parameters, c/a ratio, bulk modulus, cohesive energy and are compared with the available experimental results. The results of the electronic band structure calculations at ambient condition show that LiCuO and NaMO are indirect band gap semiconductors whereas KMO and RbMO are direct band gap semiconductors. At high pressure the band gap decreases and the phenomenon of band overlap metallization occur. Also these compounds undergo structural phase transition from tetragonal I-4m2 phase to cubic α-phase and transition pressures were calculated.

Consistent LDA’ + DMFT—an unambiguous way to avoid double counting problem: NiO test

We present a consistent way of treating a double counting problem unavoidably arising within the LDA + DMFT combined approach to realistic calculations of electronic structure of strongly correlated systems. The main obstacle here is the absence of systematic (e.g., diagrammatic) way to express LDA (local density approximation) contribution to exchange correlation energy appearing in the density functional theory. It is not clear then, which part of interaction entering DMFT (dynamical mean-field theory) is already taken into account through LDA calculations. Because of that, up to now there is no accepted unique expression for the double counting correction in LDA + DMFT. To avoid this problem we propose here the consistent LDA’ + DMFT approach, where LDA exchange correlation contribution is explicitly excluded for correlated states (bands) during self-consistent band structure calculations. What is left out of Coulomb interaction for those strongly correlated states (bands) is its non-local part, which is not included in DMFT, and the local Hartreelike contribution. Then the double counting correction is uniquely reduced to the local Hartree contribution. Correlations for strongly correlated states are then directly accounted for via the standard DMFT. We further test the consistent LDA’ + DMFT scheme and compare it with conventional LDA + DMFT calculating the electronic structure of NiO. Opposite to the conventional LDA + DMFT our consistent LDA’ + DMFT approach unambiguously produces the insulating band structure in agreement with experiments.

Promising half-metallic ferromagnetism in double perovskites Ba2VTO6 (T=Nb and Mo): Ab-initio LMTO-ASA investigations

The electronic and magnetic properties of new ordered vanadium-based double perovskite oxides Ba2VTO6 (T=Nb and Mo) have been investigated using the ab-initio linear muffin-tin orbitals method with the atomic-sphere approximation (LMTO-ASA). The calculations performed by using the local spin-density approximation with on-site Coulomb interaction (LSDA+U) style. The self-consistent band structure calculations predict half-metallic ferromagnetic (HM-FM) ground states with total spin magnetic moments of 2.1021μB and 3.0633μB per formula unit cell for Ba2VNbO6 and Ba2VMoO6, respectively. HM-FM nature in two compounds organizes from the ddpπ-superexchange interactions 3d-t2g2–O (2p)–4d-t2gn (n=0 or 1), conformity with Zener–Goodenough–Kanamori rules.

Role of point defects on conductivity, magnetism and optical properties in In 2O 3

In order to assess the effects of point defects including transition metal doping on its electronic structure, the self-consistent band structure of the transparent oxide In 2O 3 (in the Ia 3 structure) has been calculated with oxygen vacancies, oxygen and indium interstitial atoms and several transition metal dopants using density functional theory based first principles calculations. An oxygen vacancy alone does not act as a strong native donor but when combined with interstitial indium and (substitutional) transition metal doping, shallow donor levels close to the conduction band are formed. Spin polarized calculations show measurable magnetism in some of the transition metal doped systems while the dielectric functions indicate whether such systems remain transparent among other things.

Intersubband optoelectronics in the InGaAs/GaAsSb material system

In this article the authors report on a novel material system for optoelectronic intersubband devices. Superlattices of In0.53Ga0.47As/GaAs0.51Sb0.49 were grown by molecular beam epitaxy. Layer thickness and quality was investigated by high-resolution x-ray diffraction measurements and high-resolution transmission electron microscopy images. Intersubband absorption measurements on In0.53Ga0.47As/GaAs0.51Sb0.49 superlattices, revealed at room temperature transition energies from 213 to 107 meV for In0.53Ga0.47As well widths of 4.5–11.5 nm at room temperature. These results were used to fit parameters for self-consistent superlattice band structure calculations. Finally, quantum cascade lasers with an emission wavelength of 11.3 μm and quantum well infrared photodetectors with a peak response near 5.5 μm were realized in this material system.

Electronic structure and magnetic properties of RuFe 3N nitride

Self-consistent band structure calculations were performed on nitride RuFe 3N in order to investigate its magnetic and ground state properties. The Linear Muffin-Tin Orbital (LMTO) method was employed and calculations were performed at several lattice parameters so as to obtain the RuFe 3N equilibrium volume. Nonmagnetic and ferromagnetic LMTO calculations have shown that the RuFe 3N stable stage is ferromagnetic with constant lattice equilibrium of 7.2502 atomic units (a.u.). At equilibrium volume the LMTO calculations have given magnetic moments of 1.25 and 1.63 μ B at Ru and Fe sites, respectively, and no magnetic moment at N sites. The analysis of states density at equilibrium volume as well as the results for charge transfer illustrates why this ruthenium nitride is ferromagnetic. The LMTO calculations anticipate that the magnetic moment, the hyperfine field (the Fermi contact) and the isomer shift show a strong dependence on the lattice spacing.

Electronic structure of CuCl

Self-consistent energy band structure obtained by two different ab initio methods (LCAO and LMTO) shows that CuCl is a direct gap semiconductor. No indirect gap is found at ambient or high pressures as required by Rusakov's model and Abrikosov's speculations invoking Herring's excitonium state as the origin of superdiamagnetism or high-temperature superconductivity in CuCl. Band gap predictions are compared with several existing experiments including new infrared absorption measurements we have performed and report here.

A self-consistent relativistic APW method with the spin-orbit interaction treated as a perturbation

A simple method for including spin-orbit corrections in self-consistent relativistic APW band structure calculations is presented in which the spin-dependent part of the APW matrix is treated as a perturbation. The solution of the spin-independent problem is used in a way that requires little additional time or coding to include spin. The full calculational procedure is discussed and calculations of spin-orbit splitting in TaC are described to illustrate the method.

Calculations of X-Ray band spectra: Application to vanadium

Results of self-consistent band structure calculations are used to evaluate the K, L, and M x-ray emission and absorption spectra of vanadium metal. The K and L spectra are in generally good agreement with experiment in the overall shapes and widths. Variations between the calculations and measurements are similar in magnitude to variations between measured spectra from different laboratories. The calculated M spectra are too narrow, in poor agreement with experiment. The present one-electron approach appears inadequate for spectra in which significant interaction occurs between a weakly bound core-level vacancy and the partially filled d band.

Maximally localized Wannier functions for GW quasiparticles

We review the formalisms of the self-consistent GW approximation to many-body perturbation theory and of the generation of optimally localized Wannier functions from groups of energy bands. We show that the quasiparticle Bloch wave functions from such GW calculations can be used within this Wannier framework. These Wannier functions can be used to interpolate the many-body band structure from the coarse mesh of Brillouin-zone points on which it is known from the initial calculation to the usual symmetry lines, and we demonstrate that this procedure is accurate and efficient for the self-consistent GW band structure. The resemblance of these Wannier functions to the bond orbitals discussed in the chemical community led us to expect differences between density-functional and many-body functions that could be qualitatively interpreted. However, the differences proved to be minimal in the cases studied. Detailed results are presented for ${\text{SrTiO}}_{3}$ and solid argon.