Showing papers on "Effective mass (solid-state physics) published in 2009"

Graphene: Status and Prospects

Abstract: Graphene is a wonder material with many superlatives to its name. It is the thinnest known material in the universe and the strongest ever measured. Its charge carriers exhibit giant intrinsic mobility, have zero effective mass, and can travel for micrometers without scattering at room temperature. Graphene can sustain current densities six orders of magnitude higher than that of copper, shows record thermal conductivity and stiffness, is impermeable to gases, and reconciles such conflicting qualities as brittleness and ductility. Electron transport in graphene is described by a Dirac-like equation, which allows the investigation of relativistic quantum phenomena in a benchtop experiment. This review analyzes recent trends in graphene research and applications, and attempts to identify future directions in which the field is likely to develop.

Trilayer graphene is a semimetal with a gate-tunable band overlap.

Abstract: Graphene-based materials are promising candidates for nanoelectronic devices because very high carrier mobilities can be achieved without the use of sophisticated material preparation techniques. However, the carrier mobilities reported for single-layer and bilayer graphene are still less than those reported for graphite crystals at low temperatures, and the optimum number of graphene layers for any given application is currently unclear, because the charge transport properties of samples containing three or more graphene layers have not yet been investigated systematically. Here, we study charge transport through trilayer graphene as a function of carrier density, temperature, and perpendicular electric field. We find that trilayer graphene is a semimetal with a resistivity that decreases with increasing electric field, a behaviour that is markedly different from that of single-layer and bilayer graphene. We show that the phenomenon originates from an overlap between the conduction and valence bands that can be controlled by an electric field, a property that had never previously been observed in any other semimetal. We also determine the effective mass of the charge carriers, and show that it accounts for a large part of the variation in the carrier mobility as the number of layers in the sample is varied.

Wave attenuation mechanism in an acoustic metamaterial with negative effective mass density

Abstract: The wave attenuation and energy transfer mechanisms of a metamaterial having a negative effective mass density are studied. The metamaterial considered is represented by a lattice system consisting of mass-in-mass units. The attenuation of wave amplitude for frequencies in the stop band is studied from the energy transfer point of view. It is found that most of the work done by the external force on the lattice system is stored by the internal mass if the forcing frequency is close to the local resonance frequency. However, the energy stored in the internal mass is only temporary; it is taken out by the external force in the form of negative work in a cyclic manner. This behavior is utilized to design metamaterials for preventing stress waves from passing them.

Collective oscillations of an imbalanced Fermi gas: axial compression modes and polaron effective mass.

Abstract: We investigate the low-lying compression modes of a unitary Fermi gas with imbalanced spin populations. For low polarization, the strong coupling between the two spin components leads to a hydrodynamic behavior of the cloud. For large population imbalance we observe a decoupling of the oscillations of the two spin components, giving access to the effective mass of the Fermi polaron, a quasiparticle composed of an impurity dressed by particle-hole pair excitations in a surrounding Fermi sea. We find m*/m = 1.17(10), in agreement with the most recent theoretical predictions.

Feynman path-integral treatment of the BEC-impurity polaron

Abstract: The description of an impurity atom in a Bose-Einstein condensate can be cast in the form of Fr\"ohlich's polaron Hamiltonian, where the Bogoliubov excitations play the role of the phonons. An expression for the corresponding polaronic coupling strength is derived, relating the coupling strength to the scattering lengths, the trap size and the number of Bose condensed atoms. This allows to identify several approaches to reach the strong-coupling limit for the quantum gas polarons, whereas this limit was hitherto experimentally inaccessible in solids. We apply Feynman's path-integral method to calculate for all coupling strengths the polaronic shift in the free energy and the increase in the effective mass. The effect of temperature on these quantities is included in the description. We find similarities to the acoustic polaron results and indications of a transition between free polarons and self-trapped polarons. The prospects, based on the current theory, of investigating the polaron physics with ultracold gases are discussed for lithium atoms in a sodium condensate.

Size dependent optical band gap of ternary I-III-VI2 semiconductor nanocrystals

Abstract: The size dependent optical band gap of the less-toxic ternary I-III-VI2 chalcopyrite-type semiconductor quantum dots (QDs), CuInS2, CuInSe2, CuGaS2, CuGaSe2, AgInSe2, AgGaS2, and AgGaSe2, were evaluated using the finite-depth-well effective mass approximation calculation. From the comparison of the calculation result with the experimental values for the CuInS2 case, it was shown that the calculation was highly valid to predict the size dependent optical gap of the ternary semiconductor QDs. The optical band gap of the above seven I-III-VI2 QDs covers a wide wavelength range from the near-infrared to ultraviolet. It has been shown that the I-III-VI2 semiconductor QDs have a significant potential as alternatives to the highly toxic cadmium-containing II-VI semiconductor QDs and they are applicable to the wide range of light emitting devices and solar cells.

Analytic model of elastic metamaterials with local resonances

Abstract: A unified analytic model for effective mass density, effective bulk modulus, and effective shear modulus is presented for elastic metamaterials composed of coated spheres embedded in a host matrix. The effective material properties are derived directly from the averages of local momentum, stress, and strain defined in a single doubly coated sphere. It is shown that the effective material parameters predicted by the proposed model are in excellent agreements with the coherent-potential approximation results at low filling fractions where the anisotropy of periodic structures can be neglected for elastic waves. The advantage of the proposed method is that it can reveal clearly the physical mechanism for negative effective material parameters induced by the resonant effect. It is found that negative effective mass density is induced by negative total momentum of the composite for a positive momentum excitation. Negative effective bulk modulus appears for composites with an increasing (decreasing) total volume under a compressive (tensile) stress. Negative effective shear modulus describes composites with axisymmetric deformation under an opposite axisymmetric loading. Numerical examples are also given to illustrate these mechanisms. These findings may be useful in design of elastic metamaterials.

Enhanced effective mass in doped SrTiO3 and related perovskites

Abstract: The effective mass is one of the main factors determining the Seebeck coefficient and electrical conductivity of thermo-electrics. In this ab-initio LDA-GGA study the effective mass is estimated from the curvature of electronic bands by one-band-approximation and is in excellent agreement with experimental data of Nb- and La-doped SrTiO 3 . It is clarified that the deformation of SrTiO 3 crystals has a significant influence on the bandgap, effective electronic DOS-mass and band-mass, but the electronic effect due to the e g -band flattening near the Γ -point due to Nb-doping up to 0.2 at% is the main factor for the effective mass increase. Doping of La shows a linear decrease of the effective mass; this can be explained by the different surroundings of A- and B-sites in perovskite. Substitution with other elements such as Ba on the A-site and V on the B-site in SrTiO 3 increases the effective mass as well.

Making massless Dirac fermions from a patterned two-dimensional electron gas.

Abstract: Analysis of the electronic structure of an ordinary two-dimensional electron gas (2DEG) under an appropriate external periodic potential of hexagonal symmetry reveals that massless Dirac fermions are generated near the corners of the supercell Brillouin zone. The required potential parameters are found to be achievable under or close to laboratory conditions. Moreover, the group velocity is tunable by changing either the effective mass of the 2DEG or the lattice parameter of the external potential, and it is insensitive to the potential amplitude. The finding should provide a new class of systems other than graphene for investigating and exploiting massless Dirac fermions using 2DEGs in semiconductors.

Effect of induced spin-orbit coupling for atoms via laser fields.

Abstract: We propose an experimental scheme to observe spin-orbit coupling effects of a two-dimensional Fermi atomic gas cloud by coupling its internal electronic states (pseudospins) to radiation in a Lambda configuration. The induced spin-orbit coupling can be of the Dresselhaus and Rashba type with a Zeeman term. We show that the optically induced spin-orbit coupling can lead to a spin-dependent effective mass under appropriate conditions with one of them able to be tuned between positive and negative effective masses. As a direct observable we show that in the expansion dynamics of the atomic cloud the initial atomic cloud splits into two clouds for the positive effective mass case regime, and into four clouds for the negative effective mass regime.

Bandgap engineering of graphene: A density functional theory study

Abstract: Three ways of engineering the bandgap of graphene, i.e., surface bonding, isoelectronic codoping, and alternating electrical/chemical environment, are analyzed with the effective mass approximation and density-functional theory calculations. Surface bonding on graphene would lift its top σ valence bands above π valence states, open a sp3 gap, but also bury the linearly dispersive bands into the valence σ bands. Isoelectronic codoping and asymmetric electrical or chemical environment may open the π−π∗ gap of graphene by breaking its sublattice equivalence. The calculated effective mass versus bandgap may provide useful guidance for the future experimental efforts to fabricate graphene-based semiconductors.

On Landauer vs. Boltzmann and Full Band vs. Effective Mass Evaluation of Thermoelectric Transport Coefficients

Abstract: The Landauer approach to diffusive transport is mathematically related to the solution of the Boltzmann transport equation, and expressions for the thermoelectric parameters in both formalisms are presented. Quantum mechanical and semiclassical techniques to obtain from a full description of the bandstructure, E(k), the number of conducting channels in the Landauer approach or the transport distribution in the Boltzmann solution are developed and compared. Thermoelectric transport coefficients are evaluated from an atomistic level, full band description of a crystal. Several example calculations for representative bulk materials are presented, and the full band results are related to the more common effective mass formalism. Finally, given a full E(k) for a crystal, a procedure to extract an accurate, effective mass level description is presented.

Band parameters for cadmium and zinc chalcogenide compounds

Abstract: Theoretical investigations of the electronic, optical and elastic properties of cadmium and zinc chalcogenides in the zinc-blende structure are performed using a pseudopotential formalism. Our results are in reasonable agreement with the available experimental data. Polynomial expressions are obtained for the electron effective mass and the static dielectric constant as a function of the fundamental energy band-gap. Relations of elastic constants ratio to the ionicity are also examined and discussed.

Hot carrier solar cells operating under practical conditions

Abstract: We theoretically investigated the features of hot carrier solar cells, from which photogenerated carriers are extracted before they are completely thermalized. There are three channels of energy dissipation from photogenerated carriers that lowers the conversion efficiency: thermalization in the absorber, emission from the absorber, and thermodynamically unavoidable heat flux to the ambient. The emission increases with increasing carrier density in the absorber, whereas the heat flux decreases. Previous calculations of the conversion efficiency have been carried out under the supposition of no thermalization of carriers. In this case, the dominant process of energy dissipation is the emission, like conventional solar cells represented by the Shockley and Queisser formula. In practice, the carriers should be extracted to external circuits immediately after photogeneration because they are partially thermalized. This restriction leads to a much smaller carrier density and consequently more significant energy dissipation by heat flux, whereas the influence of the emission is negligible. As a result, the conversion efficiency is considerably lower than the values under the supposition of no thermalization. To suppress the heat flux to improve conversion efficiency, a smaller effective electron mass and a higher carrier temperature are required, as well as more intense irradiation. When the effective electron mass is much smaller than that of holes, the thermalization of holes has little influence on lowering the conversion efficiency.

Bending AdS waves with new massive gravity

Abstract: We study AdS-waves in the three-dimensional new theory of massive gravity recently proposed by Bergshoeff, Hohm, and Townsend. The general configuration of this type is derived and shown to exhibit different branches, with different asymptotic behaviors. In particular, for the special fine tuning m2 = ±1/(2l2), solutions with logarithmic fall-off arise, while in the range -1/(2l^2)$>m2 > −1/(2l2), spacetimes with Schrodinger isometry group are admitted as solutions. Spacetimes that are asymptotically AdS3, both for the Brown-Henneaux and for the weakened boundary conditions, are also identified. The metric function that characterizes the profile of the AdS-wave behaves as a massive excitation on the spacetime, with an effective mass given by meff2 = m2−1/(2l2). For the critical value m2 = −1/(2l2), the value of the effective mass precisely saturates the Breitenlohner-Freedman bound for the AdS3 space where the wave is propagating on. The analogies with the AdS-wave solutions of topologically massive gravity are also discussed. Besides, we consider the coupling of both massive deformations to Einstein gravity and find the exact configurations for the complete theory, discussing all the different branches exhaustively. One of the effects of introducing the Chern-Simons gravitational term is that of breaking the degeneracy in the effective mass of the generic modes of pure New Massive Gravity, producing a fine structure due to parity violation. Another effect is that the zoo of exact logarithmic specimens becomes considerably enlarged.

Determination of electron effective mass and electron affinity in HfO2 using MOS and MOSFET structures

Abstract: We present a combined electrical and modeling study to determine the tunneling electron effective mass and electron affinity for HfO 2 . Experimental capacitance–voltage ( C – V ) and current–voltage ( J – V ) characteristics are presented for HfO 2 films deposited on Si(1 0 0) substrates by atomic layer deposition (ALD) and by electron beam evaporation (e-beam), with equivalent oxide thicknesses in the range 10–12.5 A. We extend on previous studies by applying a self-consistent 1D-Schrodinger–Poisson solver to the entire gate stack, including the inter-layer SiO x region – and to the adjacent substrate for non-local barrier tunnelling – self-consistently linked to the quantum-drift-diffusion transport model. Reverse modeling is applied to the correlated gate and drain currents in long-channel MOSFET structures. Values of (0.11 ± 0.03) m 0 and (2.0 ± 0.25) eV are determined for the HfO 2 electron effective mass and the HfO 2 electron affinity, respectively. We apply our extracted electron effective mass and electron affinity to predict leakage current densities in future 32 nm and 22 nm technology node MOSFETs with SiO x thicknesses of 7–8 A and HfO 2 thicknesses of 23–24 A.

Nonequilibrium functional RG with frequency dependent vertex function: A study of the single impurity Anderson model

Abstract: We investigate nonequilibrium properties of the single impurity Anderson model by means of the functional renormalization group (fRG) within Keldysh formalism. We present how the level broadening Γ/2 can be used as flow parameter for the fRG. This choice preserves importan t aspects of the Fermi liquid behaviour that the model exhibits in case of particle-hole symmetry. An approximation scheme for the Keldysh fRG is developed which accounts for the frequency dependence of the two-particle vertex in a way similar but not equivalent to a recently published approximation to the equilibrium Matsubara fRG. Our method turns out to be a flexible tool for the study of weak to intermediate on-site interactions U . 3Γ. In equilibrium we find excellent agreement with NRG results for the linear conductance at finite gate vol tage, magnetic field, and temperature. In nonequilibrium, our results for the current agree well with TD-DMRG. For the nonlinear conductance as function of the bias voltage, we propose reliable results at finite magne tic field and finite temperature. Furthermore, we demonstrate the exponentially small scale of the Kondo temperature to appear in the second order derivative of the self-energy. We show that the approximation is, however, not able to reproduce the scaling of the effective mass at large interactions.

Trends in the electronic structure of dilute nitride alloys

Abstract: The band-anticrossing (BAC) model has been widely applied to analyse the electronic structure of dilute nitride III-V-N alloys such as GaNxAs1−x. The BAC model describes the strong band gap bowing observed at low N composition in GaNxAs1−x in terms of an interaction between the GaAs host matrix conduction band edge and a higher lying band of localized N resonant states. In practice, replacing As by N introduces a range of N-related defect levels, associated with isolated N atoms, N–N pairs and larger clusters of N atoms. We show that the effect of such defect levels on the alloy conduction band structure is strongly dependent on the relative energy of the defect levels and the host conduction band edge. We first consider GaNxAs1−x, where we show that the unexpectedly large electron effective mass and gyromagnetic ratio, and their non-monotonic variation with x, are due to hybridization between the conduction band edge and specific nitrogen states close to the band edge. The N-related defect levels lie below the conduction band edge in GaNxP1−x. We must therefore explicitly treat the interaction between the higher lying GaP host Γ conduction band minimum and defect states associated with a random distribution of N atoms in order to obtain a good description of the lowest conduction states in disordered GaPN alloys. Turning to other alloys, N-related defect levels should generally lie well above the conduction band minimum in InNSb, with the band dispersion of InNSb then well described by a two-level BAC model. Both InP and InAs are intermediate between InSb and GaAs. By contrast, we calculate that N-related defect levels lie close to the conduction band minimum in GaNSb, and will therefore strongly perturb the lowest conduction states in this alloy. Overall, we conclude that the BAC model provides a good qualitative explanation of the electronic properties of dilute nitride alloys, but that it is in many cases necessary to include the details of the distribution of N-related defect levels to obtain a quantitative understanding of the conduction band structure in dilute nitride alloys.

NA60 results on thermal dimuons

Abstract: The NA60 experiment at the CERN SPS has measured muon pairs with unprecedented precision in 158A GeV In-In collisions. A strong excess of pairs above the known sources is observed in the whole mass region 0.2 rho -> mu+mu- annihilation. The associated rho spectral function shows a strong broadening, but essentially no shift in mass. For M>1 GeV, the excess is found to be prompt, not due to enhanced charm production, with pronounced differences to Drell-Yan pairs. The slope parameter Teff associated with the transverse momentum spectra rises with mass up to the rho, followed by a sudden decline above. The rise for M 1 GeV and its relation to parton-hadron duality is discussed in detail, suggesting a dominantly partonic emission source in this region. A comparison of the data to the present status of theoretical modeling is also contained. The accumulated empirical evidence, including also a Planck-like shape of the mass spectra at low pT and the lack of polarization, is consistent with a global interpretation of the excess dimuons as thermal radiation. We conclude with first results on omega in-medium effects.

Understanding the p-Type Conduction Properties of the Transparent Conducting Oxide CuBO2: A Density Functional Theory Analysis

Abstract: Discovering new candidate p-type transparent conducting oxides has become a major goal for material scientists. Recently delafossite CuBO2 has been proposed as a promising candidate, showing good room temperature electrical conductivity and excellent transparency [Appl. Phys. Lett. 2007, 91, 092123]. In this article we report a density functional theory investigation of CuBO2, examining the geometry and electronic structure using GGA corrected for on-site Coulomb interactions (GGA + U) and a hybrid density functional (HSE06). From analysis of the calculated band structure, density of states, and optical absorption, we predict an indirect fundamental band gap of ∼3.1 eV and a direct optical band gap of ∼3.6 eV. The hole effective mass at the valence band maximum indicates the potential for good p-type conductivity, consistent with the reported experimental results. These results are discussed in relation to other delafossite oxides.

Muonium as a model for interstitial hydrogen in the semiconducting and semimetallic elements

Abstract: Although the interstitial hydrogen atom would seem to be one of the simplest defect centres in any lattice, its solid state chemistry is in fact unknown in many materials, not least amongst the elements. In semiconductors, the realization that hydrogen can profoundly influence electronic properties even as a trace impurity has prompted its study by all available means-but still only in the functionally important or potentially important materials-for the elements, Si, Ge and diamond. Even here, it was not studies of hydrogen itself but of its pseudo-isotope, muonium, that first provided the much needed microscopic pictures of crystallographic site and local electronic structure-now comprehensively confirmed by ab initio computation and such data as exists for monatomic, interstitial hydrogen centres in Si. Muonium can be formed in a variety of neutral paramagnetic states when positive muons are implanted into non-metals. The simple trapped atom is commonly only metastable. It coexists with or reacts to give defect centres with the unpaired electron in somewhat more extended orbitals. Indications of complete delocalization into effective mass states are discussed for B, α-Sn, Bi and even Ge, but otherwise all the muonium centres seen in the elemental semiconductors are deep and relatively compact. These are revealed, distinguished and characterized by μSR spectroscopy-muon spin rotation and resonance informing on sites and spin-density distributions, muon spin relaxation on motional dynamics and charge-state transitions. This Report documents the progress of μSR studies for all the semiconductors and semimetals of the p-block elements, Groups III-VI of the Periodic Table. The striking spectra and originally unanticipated results for Group IV are for the most part well known but deserve summarizing and updating; the sheer diversity of muonium states found is still remarkable, especially in carbon allotropes. The interplay of crystallographic site and charge state in Si and Ge at high temperatures, or under illumination, reflects the capture and loss of charge carriers that should model the electrical activity of monatomic hydrogen but still challenges theoretical descriptions. Spin-flip scattering of conduction electrons by the paramagnetic centres is revealed in heavily doped n-type material, as well as some modification of the local electronic structures. The corresponding spectroscopy for the solid elements of Groups III, V and VI is rather less well known and is reviewed here for the first time; a good deal of previously unpublished data is also included. Theoretical expectations and computational modelling are sparse, here. Recent results for B suggest a relatively shallow centre with molecular character; P and As show deeper quasi-atomic states, but still with substantial overlap of spin density onto surrounding host atoms. Particular attention is paid to the chalcogens. Muonium centres in Te show charge-state transitions already around room temperature; the identification of those in S and Se has been complicated by unusual spin dynamics of a different character, here attributed to spin-orbit coupling and interstitial reorientation. In the metals and semimetals, muonium is not formed as a paramagnetic centre. Here the implanted muons mimic interstital protons and interest shifts to a variety of other topics, including aspects of charge screening (α-Sn, Sb, Bi), site preference and quantum mobility (Al, β-Sn, Pb). The post-transition metals receive only a brief mention, by way of contrast with the nonmetals. Systematic studies of local susceptibility via measurements of muon Knight shifts extends in favourable cases to revealing the elusive high-field Condon domains (Al, Sn, Pb, Bi). Some new information is available on the superconducting phases. Appendices include a derivation of the spin Hamiltonian for paramagnetic muonium centres or molecular radicals having varying admixtures of orbital angular momentum, including the extreme case of orbital degeneracy, and examine the consequences of significant spin-orbit coupling for μSR spectroscopy and muon spin relaxation. This is the framework for the tentative assignments made here for the muonium defect centres formed in sulphur and selenium, namely diatomic species resembling the chalcogen monohydrides. Equally, it provides guidelines for eventual solid-state detection of OMu-the elusive muoniated hydroxyl radical.

Quantum oscillations in the parent pnictide BaFe2As2: Itinerant electrons in the reconstructed state

Abstract: The low-energy quasiparticle dynamics are an essential ingredient to many theories of superconductivity. The Fepnictide superconductors however, show evidence for electron itineracy 1–4 and local magnetism, 5–8 making it difficult to know which theoretical framework is most appropriate for understanding these compounds. 9 Establishing the nature of the itinerancy and magnetism in the parent compounds is therefore of fundamental importance. In the present paper we report quantum oscillation QO measurements in BaFe2As2, consistent with density-functional calculations of the antiferromagnetic ground state. We find that the spin-density wave instability does not fully gap the Fermi surface just as recently predicted 10 and quasiparticle coherence persists in the magnetically ordered ground state. In the measurements reported here on BaFe 2 As 2 we use two separate techniques, torque magnetometry and a radiofrequency contactless conductivity technique using a tunnel diode oscillator TDO, both of which have been used recently to observe oscillations in the closely related compounds LaFePO Ref. 1 and SrFe2As2. 2 We observe three small pockets comprising 1.7%, 0.7%, and 0.3%, of the paramagnetic Brillouin zone that associated with the tetragonal state and produce band-structure calculations of a reconstructed state which are in broad agreement. Furthermore we map the topology of these small pockets and extract their effective mass. The present measurements illustrate that itinerant electrons play a fundamental role in the ordered state of ternary Fe pnictides and place important limits on the topology and size of the Fermi surface in the antiferromagnetic state.

Donor-impurity related binding energy and photoinization cross-section in quantum dots: electric and magnetic fields and hydrostatic pressure effects

Abstract: We have studied the behavior of the binding energy and photoionization cross-section of a donor-impurity in cylindrical-shape GaAs-Ga0.7Al0.3As quantum dots, under the effects of hydrostatic pressure and in-growth direction applied electric and magnetic fields. We have used the variational method under the effective mass and parabolic band approximations. Parallel and perpendicular polarizations of the incident radiation and several values of the quantum dot geometry have also been considered. Our results show that the photoionization cross-section growths as the hydrostatic pressure is increased. For parallel polarization of the incident radiation, the photoionization cross-section decreases when the impurity is shifted from the center of the dot. In the case of perpendicular polarization of the incident radiation, the photoionization cross-section increases when the impurity is shifted in the radial direction of the dot. For on-axis impurities the transitions between the ground state of the impurity and the ground state of the quantum dot are forbidden. In the low pressure regime (less than 13.5 kbar) the impurity binding energy growths linearly with pressure, and in the high pressure regime (higher than 13.5 kbar) the binding energy growths up to a maximum and then decreases. Additionally, we have found that the applied electric and magnetic fields may favor the increase or decrease in binding energy, depending on the impurity position.

Strain-modulated electronic properties of Ge nanowires: A first-principles study

Abstract: We used density-functional theory based first-principles simulations to study the effects of uniaxial strain and quantum confinement on the electronic properties of germanium nanowires along the [110] direction, such as the energy gap and the effective masses of the electron and hole. The diameters of the nanowires being studied are up to $50\text{ }\text{\AA{}}$. As shown in our calculations, the Ge [110] nanowires possess a direct band gap, in contrast to the nature of an indirect band gap in bulk. We discovered that the band gap and the effective masses of charge carries can be modulated by applying uniaxial strain to the nanowires. These strain modulations are size dependent. For a smaller wire $(\ensuremath{\sim}12\text{ }\text{\AA{}})$, the band gap is almost a linear function of strain; compressive strain increases the gap while tensile strain reduces the gap. For a larger wire $(20\text{ }\text{\AA{}}--50\text{ }\text{\AA{}})$, the variation in the band gap with respect to strain shows nearly parabolic behavior: compressive strain beyond $\ensuremath{-}1%$ also reduces the gap. In addition, our studies showed that strain affects effective masses of the electron and hole very differently. The effective mass of the hole increases with a tensile strain while the effective mass of the electron increases with a compressive strain. Our results suggested both strain and size can be used to tune the band structures of nanowires, which may help in design of future nanoelectronic devices. We also discussed our results by applying the tight-binding model.

Electronic structure and optical properties of ZnSiO3 and Zn2SiO4

Abstract: The electronic structure and optical properties of orthorhombic, monoclinic, and rhombohedral (corundum type) modifications of ZnSiO3, and of rhombohedral, tetragonal, and cubic (spinel type) modifications of Zn2SiO4 have been studied using ab initio density functional theory calculations. The calculated fundamental band gaps for the different polymorphs and compounds are in the range 2.22–4.18 eV. The lowest conduction band is well dispersive similar to that found for transparent conducting oxides such as ZnO. This band is mainly contributed by Zn 4s electrons. The carrier effective masses were calculated and compared with those for ZnO. The topmost valence band is much less dispersive and contributed by O 2p and Zn 3d electrons. From the analysis of charge density, charges residing in each site, and electron localization function, it is found that ionic bonding is mainly ruling in these compounds. The calculated optical dielectric tensors show that the optical properties of ZnSiO3 and Zn2SiO4 are almost...

(Cu2S2)(Sr3Sc2O5)−A Layered, Direct Band Gap, p-Type Transparent Conducting Oxychalcogenide: A Theoretical Analysis.

Abstract: Development of a p-type TCO to rival the high-performance n-type TCOs presently utilized in many applications is one of the grand challenges for materials scientists. However, most of the p-type TCOs fabricated to date have suffered from limited hole mobilities, low conductivities, and indirect band gaps. Recently, [Cu2S2][Sr3Sc2O5] has been identified as a possible p-type TCO material, with improved hole mobility. In this article, we study the geometry and electronic structure of [Cu2S2][Sr3Sc2O5] using both GGA + U and HSE06 . We show conclusively that [Cu2S2][Sr3Sc2O5] is a direct band gap material, with a hole effective mass at the valence band maximum that indicates the potential for good p-type conductivity, consistent with the reported experimental results. These results are discussed in relation to other p-type TCO materials.

Correlations between thermoelectric properties and effective mass caused by lattice distortion in Al-doped ZnO ceramics

Abstract: Doped ZnO has practical applications in the industry for thermoelectric generation, owing to its stability at high temperatures. However, the efficiency of energy conversion is not sufficient. In this work, we have focused on an experimental evidence of a first-principles prediction in ab-plane tensile strain and the effective mass behavior in ZnO ceramics. The results showed a systematic c-axis compression of the lattice up to c/a = 1.6010 with increase in the Al additive concentration. It was found that this lattice compression induced an increase in effective mass (m*) from 0.27 to 0.30 m0, leading to the enhancement in the Seebeck coefficient normalized by carrier concentration. Besides, both carrier concentration and Hall mobility increased with increase in Al additive concentration. It was concluded that in the ion-doped ZnO system, a high compression of c/a ratio due to heavy doping could be a key to improving the power factor.

Any l-state improved quasi-exact analytical solutions of the spatially dependent mass Klein–Gordon equation for the scalar and vector Hulthén potentials

Abstract: We present a new approximation scheme for the centrifugal term to obtain a quasi-exact analytical bound state solution within the framework of the position-dependent effective mass radial Klein?Gordon equation with the scalar and vector Hulth?n potentials in any arbitrary D dimension and orbital angular momentum quantum numbers l. The Nikiforov?Uvarov (NU) method is used in the calculations. The relativistic real energy levels and corresponding eigenfunctions for the bound states with different screening parameters have been given in a closed form. It is found that the solutions in the case of constant mass and in the case of s-wave (l=0) are identical with the ones obtained in the literature.