Showing papers in "Physica Status Solidi B-basic Solid State Physics in 2006"

Octopus: a tool for the application of time-dependent density functional theory

Abstract: We report on the background, current status, and current lines of development of the octopus project. This program materializes the main equations of density-functional theory in the ground state, and of time-dependent density-functional theory for dynamical effects. The focus is nowadays placed on the optical (i.e. electronic) linear response properties of nanostructures and biomolecules, and on the non-linear response to high-intensity fields of finite systems, with particular attention to the coupled ionic-electronic motion (i.e. photo-chemical processes). In addition, we are currently extending the code to the treatment of periodic systems (both to one-dimensional chains, two-dimensional slabs, or fully periodic solids), magnetic properties (ground state properties and excitations), and to the field of quantum-mechanical transport or “molecular electronics.” In this communication, we concentrate on the development of the methodology: we review the essential numerical schemes used in the code, and report on the most recent implementations, with special attention to the introduction of adaptive coordinates, to the extension of our real-space technique to tackle periodic systems, and on large-scale parallelization. More information on the code, as well as the code itself, can be found at http://www.tddft.org/programs/octopus/. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

PARSEC – the pseudopotential algorithm for real‐space electronic structure calculations: recent advances and novel applications to nano‐structures

Abstract: We describe the formalism, as well as numerical and implementation issues behind PARSEC - the pseudopotential algorithm for real-space electronic structure calculations. Its current capabilities are illustrated via application of PARSEC to numerous problems in nanoscience.

Real-time, real-space implementation of the linear response time-dependent density-functional theory

Abstract: We review our methods to calculate optical response of molecules in the linear response time-dependent density-functional theory. Three distinct formalisms which are implemented in the three-dimensional grid representation are explained in detail. They are the real-time method solving the time-dependent Kohn–Sham equation in the time domain, the modified Sternheimer method which calculates the response to an external field of fixed frequency, and the matrix eigenvalue approach. We also illustrate treatments of the scattering boundary condition, needed to accurately describe photoionization processes. Finally, we show how the real-time formalism for molecules can be used to determine the response of infinite periodic systems. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Effect of surface piezoelectricity on the electromechanical behaviour of a piezoelectric ring

Abstract: A relation on electric field dependent surface stress, or alternatively surface piezoelectricity, is proposed in this paper. Based on such relation, a piezoelectric ring under prescribed potential has been studied and the results show that the surface piezoelectricity may play an important role in the electromechanical behavior of piezoelectric nanostructures. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Realistic investigations of correlated electron systems with LDA + DMFT†

Abstract: Conventional band structure calculations in the local density approximation (LDA) [1–3] are highly successful for many materials, but miss important aspects of the physics and energetics of strongly correlated electron systems, such as transition metal oxides and f-electron systems displaying, e.g., Mott insulating and heavy quasiparticle behavior. In this respect, the LDA + DMFT approach which merges LDA with a modern many-body approach, the dynamical mean-field theory (DMFT), has proved to be a breakthrough for the realistic modeling of correlated materials. Depending on the strength of the electronic correlation, a LDA + DMFT calculation yields the weakly correlated LDA results, a strongly correlated metal, or a Mott insulator. In this paper, the basic ideas and the set-up of the LDA + DMFT(X) approach, where X is the method used to solve the DMFT equations, are discussed. Results obtained with X = QMC (quantum Monte Carlo) and X = NCA (non-crossing approximation) are presented and compared, showing that the method X matters quantitatively. We also discuss LDA + DMFT results for two prime examples of correlated materials, i.e., V2O3 and Ce which undergo a Mott–Hubbard metal–insulator and volume collapse transition, respectively. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Exact exchange for correlated electrons

Abstract: The cover picture is taken from the article by Pavel Novak that was chosen as Editor's Choice of this issue [1]. The figure shows the density of minority spin states in nickel oxide calculated by three commonly used approximations (LSDA, GGA, LDA+U) as well as using the newly proposed ‘Exact Exchange for Correlated Electrons’ (EECE) method. The EECE method treats the interactions between correlated electrons in a Hartree–Fock way, while all other interactions are described by the density functional theory. EECE is a promising starting point for the improvement of orbital-dependent functionals within the density functional theory. Pavel Novak is the head of the ‘Spectroscopy of Magnetic Oxides’ group at the Institute of Physics of ASCR, Prague, Czech Republic. Most of his scientific activity is devoted to the calculation of the electronic structure of solids, but he also closely cooperates with several experimental groups. Particular attention is focused on the nuclear magnetic resonance and electronic structure of magnetic oxides with mixed valency of the cations.

Recent progress with large‐scale ab initio calculations: the CONQUEST code

Abstract: While the success of density functional theory (DFT) has led to its use in a wide variety of fields such as physics, chemistry, materials science and biochemistry, it has long been recognised that conventional methods are very inefficient for large complex systems, because the memory requirements scale as N 2 and the cpu requirements as N 3 (where N is the number of atoms). The principles necessary to develop methods with linear scaling of the cpu and memory requirements with system size (O(N) methods) have been established for more than ten years, but only recently have practical codes showing this scaling for DFT started to appear. We report recent progress in the development of the CONQUEST code, which performs O(N) DFT calculations on parallel computers, and has a demonstrated ability to handle systems of over 10000 atoms. The code can be run at different levels of precision, ranging from empirical tight-binding, through ah initio tight-binding, to full ah initio, and techniques for calculating ionic forces in a consistent way at all levels of precision will be presented. Illustrations are given of practical CONQUEST calculations in the strained Ge/Si(001) system.

Three real-space discretization techniques in electronic structure calculations

Abstract: A characteristic feature of the state-of-the-art of real-space methods in electronic structure calculations is the diversity of the techniques used in the discretization of the relevant partial differential equations. In this context, the main approaches include finite-difference methods, various types of finite-elements and wavelets. This paper reports on the results of several code development projects that approach problems related to the electronic structure using these three different discretization methods. We review the ideas behind these methods, give examples of their applications, and discuss their similarities and differences.

Transparent and flexible carbon nanotube/polypyrrole and carbon nanotube/polyaniline pH sensors

Abstract: Thin film Carbon Nanotube (CNT) networks are used as a conductive, transparent and flexible electrode for electrochemically depositing a conducting polymer on it, polypyrrole or polyaniline in the present work. We analyse the properties of the device as an electrochemical sensor, measuring his pH dependence by recording the open circuit potential in various buffer solutions, ranging from pH 1 to 13. The results show a good sensitivity, linearity and stability in both cases. In the case of CNT/polyaniline, it can be used simply as an optical sensor, as the colour of polyaniline changes with pH. The CNT/polypyrrole and CNT/polyaniline devices could have applications as solid state gas sensor or biosensor deposited on any shape of surface that can be transparent and flexible.

Two-dimensional electrons occupying multiple valleys in AlAs

Abstract: Two-dimensional electrons in AlAs quantum wells occupy multiple conduction-band minima at the X-points of the Brillouin zone. These valleys have large effective mass and g -factor compared to the standard GaAs electrons, and are also highly anisotropic. With proper choice of well width and by applying symmetry-breaking strain in the plane, one can control the occupation of different valleys thus rendering a system with tuneable effective mass, g -factor, Fermi contour anisotropy, and valley degeneracy. Here we review some of the rich physics that this system has allowed us to explore. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Lagrange‐mesh method for quantum‐mechanical problems

Abstract: The Lagrange-mesh method is an approximate variational calculation with the simplicity of a mesh calculation because of the use of a consistent Gauss quadrature. The method can be used for quantum-mechanical bound-state and scattering problems with local and non-local interactions or with a semi-relativistic kinetic energy, and for solving the time-dependent Schrodinger equation. No analytical evaluation of matrix elements is needed. The accuracy is exponential in the number of mesh points and is not significantly reduced with respect to the corresponding variational calculations. Lagrange functions can be based on classical or non-classical orthogonal polynomials as well as on trigonometric and sine functions, as described in examples. Some singularities can be handled with a regularization technique. Applications to three-body systems are discussed for various choices of coordinate systems. The helium atom and hydrogen molecular ion are presented as examples.

Electronic and magnetic structure of ScMnO3

Abstract: The electronic and magnetic structure of the low-temperature phase of ScMnO3 was studied theoretically from first principles The solid phase was modeled with periodic supercells using three different methods: unrestricted Hartree–Fock, B3LYP and BLYP The magnetic coupling constants obtained with these methods were compared with experimental values The effect of lattice relaxation on the coupling constant was investigated (© 2006 WILEY-VCH Verlag GmbH & Co KGaA, Weinheim)

Single walled carbon nanotubes for transport and delivery of biological cargos

Abstract: The innate ability of carbon nanotubes to breach the cell membrane of various types of mammalian cells has been reported. Here, we present a summary of the various applications of carbon nanotubes as a cellular transport and delivery system for functional biological cargos. The internalization of SWNTs for transport and delivery into cells is mediated via endocytosis and does not appear to have any detrimental effect on either the transported cargo or the breached cell. The emergence of SWNT as a new class of cellular transporters holds many exciting promises for SWNT-based systems for drug delivery, protein delivery, gene therapy and cancer therapy applications.

Density functional studies of molecular magnets

Abstract: After a general introduction into the field of molecular magnets the discussion focuses on a more specific description of their most important representative species, single-molecule magnets incorporating transition metal ions. We overview traditional model approaches for the phenomenological description of such systems and outline some ways used to parameterize the corresponding models from experiment and from first-principle calculations. The latter can be either multi-determinantal quantum chemical schemes or those based on the density functional theory. In particular we discuss Heisenberg exchange parameters and magnetic anisotropy constants. As a practical example, an introduction into problems and properties of some single-molecule magnets which gained much attention within last years, namely Mn12-acetate, “Fe8” and “V15” systems, is given. This introduction into systems is followed by a critical comparison of calculation schemes based on the density functional theory that are particularly well suited for the study of molecular magnets. For the above systems we select some benchmark results, obtained by different methods. Finally, we outline our recent progress in the study of other single-molecule magnets, including six-membered “ferric wheels”, “ferric stars” and “Ni4” molecules, which we studied with the use of first-principles methods Siesta and NRLMOL. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Theoretical study of cation doping effect on the electronic conductivity of Li4Ti5O12

Abstract: Based on the electron interference resulting from the electrons reflected at the interfaces of superlattices, the transmission coefficient of electrons passing over barriers of superlattices is discussed. For electrons occupying states above barriers, if these electrons have transmission coefficient of T >= 0.5 passing over the barriers of superlattice, the states above the barriers are defined as the conduction states, and if T < 0.5, they are defined as the local states. All these conduction-states above the barriers compose a series of micro-bands, which are separated by the micro-bands composed of the local states. The width of the conduction- micro-bands is related to the barrier width and electron reflectivity at the interface of the well/barrier. For a fixed barrier width, as the electron reflectivity at interfaces increases, the conduction- micro-band width becomes narrower. The calculated energy spacing between an energy level where electrons have a maximum of transmission coefficient over the barriers and the next level where electrons have a minimum of transmission coefficient is in good agreement with measured data.

Carbon-Nanotube Optoelectronics

Abstract: Semiconducting single-walled carbon nanotubes are direct-gap materials thatprovide ideal systems for the study of photophysics in one-dimension. Whiletheir excited states involve strongly bound 1D excitons, their single atomiclayer structure makes their optical properties especially sensitive to theirenvironment and external fields, thus allowing for their controlled modification. Inthis chapter we review the properties of the excited states of nanotubes,the mechanisms of their production and detection, focusing particularly onelectrically-induced excitation by ambipolar electron-hole recombination and impactexcitation by hot carriers. Radiative decay of photo-excited and electron-excited(electroluminescence) emission as well as the non-radiative decay to free carriersleading to photoconductivity are discussed. The influence of external electricfields and of environmental interactions on excited nanotubes is considered.Finally, the possible technological uses of carbon nanotubes as nanometer scalelight sources and photocurrent and photovoltage detectors are discussed.

Extrinsic defect structure of RE3Al5O12 garnets

Abstract: RE 3 Al 5 O 12 garnets (where RE represents rare earth atoms Lu-Gd and Y) are technologically important, particularly for optical applications. However, the performance of these materials suffers from the existence of point defects, which are responsible for both delayed and reduced light output in Ce:Y 3 Al 5 O 12 (Ce:YAG) scintillators. The complex garnet crystal structure prevents a straightforward description of the defects responsible for decreased performance. In this paper, we employ atomistic simulation techniques to reveal non-intuitive features of the extrinsic defect structure. Specifically: Me 2+ dopants (ranging in ionic size from Mg 2+ -Ba 2+ ) are predicted to reside on RE sites with oxygen vacancies as charge compensating defects and Me 4+ dopants (from Ti 4+ -Pb 4+ ) reside on both RE and Al sites and are predicted to be charge compensated by RE vacancies. These results predict the defects resulting from common Me 2+ impurities, as well as explain why Me 4+ doping is not effective in improving RE 3 Al 5 O 12 scintillator performance. The predicted extrinsic defects in RE 3 Al 5 O 12 associated with Me 2+ (bottom left) and Me 4+ (upper right) doping. Small and large grey atoms represent Al and RE lattice atoms respectively. Oxygen atoms are not shown.

Band structure of functional oxides by screened exchange and the weighted density approximation

Abstract: Most ab-initio calculations of the electronic structure use the local density approximation, which gives good structural data but severely under-estimates the band gaps of semiconductors and insulators. This paper presents calculations of the band structures of some important oxide semiconductors and insulators, using the screened exchange method and the weighted density approximation, which give improved band gaps. The methods are tested on diamond, Si, Ge, MgO, Al 2 O 3 , and SiO 2 and the main interest is for HfO 2 , ZrO 2 , SrTiO 3 , PbTiO 3 , LaAlO 3 , La 2 O 3 , ZrSiO 4 , SnO 2 , CuAlO 2 , and SrCu 2 O 2 .

Single-walled carbon nanotubes /polycarbonate composites : basic electrical and mechanical properties

Abstract: The focus of this paper is to investigate the electrical conductivity and the mechanical properties of single-walled carbon nanotubes/polycarbonate composites. Two different direct incorporating methods were applied. In the first method PC and SWCNTs were mixed with different ratios in a small scale conical twin screw extruder. Here, additional variations in the feeding of the filler material were tested in order to improve the state of dispersion of the melt mixing method. The second one is the coagulation method, here, the filler material was suspended and the polymer material was dissolved in liquid organic substances. The mechanical properties were examined by tension tests and by measuring the Vickers hardness. It should be stressed, that the nanofiller material was provided in large batches (m > 100 g) and extensively mechanically homogenized in order to eliminate quality fluctuations and density differences in the SWCNT occurrences. The filler material was synthesized in a Kratschmer reactor (carbon arc). After homogenization, the nanomaterial was characterized. Based on this the SWCNT content was estimated to be between 30 and 40%, which is usual for arc discharge material.

Optical manipulation of CuCl nanoparticles under an excitonic resonance condition in superfluid helium

Abstract: In order to prepare the optimal condition for the resonant optical manipulation, a lot of CuCl particles with a broad size-distribution ranging from 10 nm to 10 μm have been directly fabricated from a bulk sample by laser ablation in superfluid helium. We irradiated these particles with the laser light covering the excitonic resonance levels of particles smaller than 100 nm in order to transport them onto a target silicon substrate by using resonant radiation force. As a result, we have observed that many particles of from 10 to 50 nm diameters adhere to the substrate. This means that these nanoparticles are transported by the resonant radiation force much stronger than the gravitational force. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Excitons in carbon nanotubes

Abstract: We present two-photon excitation luminescence experiments on carbon nanotubes which show the excitonic origin of the optical excitations. The two-photon allowed exciton state, which has even parity under rotation about the U-axis, is roughly 300 meV above the one-photon active, odd-parity state. This indicates exciton binding energies on the order of 400 meV for nanotubes with diameters around 8 A. Ab-initio calculations of the exciton wavefunctions and energies are in good agreement with our experimental results, confirming the predictions on the symmetry of the exciton states.

The Application of X-ray Diffuse Scattering to the Calculation of the FeAl Equilibrium Diagram

Abstract: Absolute intensity measurements of diffuse X-ray scattering on single crystals of disordered equilibrium FeAl alloys are made at (000), (111), and (1/2,1/2,1/2) points of the reciprocal lattice of b.c.c crystals. These measurements give the opportunity of obtaining the three corresponding Fourier transforms of the interchange energies. According to Khachaturyan's theory these Fourier transforms are the energetic parameters which completely determine the thermodynamics of FeAl alloys in a self-consistent field approximation. For the case of long-range interactions each of the three parameters obtained includes all of the interchange energies. The contribution to the free energy of the exchange interactions of the set of the local spins (So = 1) placed on Fe-atoms was evaluated. This result and obtained experimental values of the three energetic parameters allowed to calculate the FeAl diagram. The calculated diagram is in a very good agreement with the most recent diagram by Swann, Duff, and Fisher obtained by an electron microscopic method (discrepancies with respect to absolute temperature and compositions are everywhere less than 11%) though “adopt” parameters are nowhere used to “tie” the calculated and observed diagram to each other. It is shown that the equilibrium diagram depends essentially on the magnetic contribution to the total free energy. Es wurden Messungen der absoluten Intensitat der diffusen Rontgenstreuung von fehlgeordneten Einkristallen von FeAl-Gleichgewichtslegierungen an (000)-, (111)- und (1/2,1/2,1/2)-Punkten des reziproken Gitters von kubisch-raumzentrierten Kristallen durchgefiihrt. Diese Messungen ergeben die Moglichkeit, die drei entsprechenden Fouriertransformierten der Austanschenergien zu erhalten. In ifbereinstimmung mit der Theorie von Khachaturyan sind diese Fouriertransformierten die Energieparameter, die die Thermodynamik der FeAl-Legierungen in einer selbstkonsistenten Feldnaherung vollstiindig bestimmen. Fur den Fall langreichweitiger Wechselwirkungen schliest jeder der drei erhaltenen Parameter alle Austauschenergien ein. Der Beitrag der Austauschwechselwirkungen des Satzes der lokalen Spins (So = 1) an den Fe-Atomen zur freien Energie wurde berechnet. Dieses Ergebnis und die erhaltenen experimentellen Werte der drei Energieparemeter gestatten, das FeAl-Diagramm zu berechnen. Dieses berechnete Diagramm befindet sich in sehr guter ubereinstimmung mit dem neuesten Diagramm von Swann, Duff und Fisher, das mit einer elektronenmikroskopischen Methode erhalten wurde (Diskrepanzen bezuglich absoluter Temperatur nnd Zusammensetzung sind immer geringer als 1 lye), obwohl nirgendwo „adaptierte” Parameter benutzt wurden, uni die berechneten und beobachteten Diagramme „anzupassen”. Es wird gezeigt, das das Gleichgewichtsdiagramm wesentlich vom magnetischen Beitrag zur gesamten freien Energie abhangt.

N-polarity GaN on sapphire substrate grown by MOVPE

Abstract: N-polar GaN was grown on (0001) sapphire using two-step growth technique by MOVPE. In the growth, the key points were the controlling of the density of nuclei in a buffer layer and promoting lateral growth at high temperature. The grown GaN films had mirror-like smooth surfaces and were proved to have N-polarity using convergent-beam electron diffraction and coaxial impact-collision ion spectroscopy. The FWHM of this GaN in the ω-scan was also much narrower than the previous reports. The density of threading dislocations was much less than for usual Ga-polar GaN grown with a GaN or AlN buffer layer by MOVPE and MBE. In PL at room temperature, the strong edge emission was observed as Ga-polarity. The p-type conduction in Mg-doped GaN was also realized by the same methods as for Ga-polarity.

Theory of excitation transfer in coupled nanostructures – from quantum dots to light harvesting complexes

Abstract: Spatially localized but electromagnetically coupled electrons are model systems for excitation transfer on nanoscales. A unified description of coupled semiconductor quantum dots and coupled cholorophylls in light harvesting complexes is presented, considering Coulomb interaction including excitonic Forster-coupling and (bi-) excitonic shifts as well as electron-phonon interaction. Linear absorption spectra are calculated and principles for the description of third order optical nonlinearities are outlined.

ONETEP: linear‐scaling density‐functional theory with local orbitals and plane waves

Abstract: An overview of the ONETEP (Order-N Electronic Total Energy Package) code is presented, focusing on the twin aims of overall linear scaling and controlled accuracy. The method is outlined, including a description of the density-matrix formulation of density-functional theory, and the optimisation procedures for both the density-kernel and the local orbitals or non-orthogonal generalised Wannier functions. Results of applying the method to a variety of systems are presented to demonstrate the accomplishment of the original aims.

Fabrication of stable dispersions containing up to 70% individual carbon nanotubes in a common organic solvent

Abstract: Applications of single-wall carbon nanotubes (SWNT) are severely restricted, as they exist in rope-like bundles. Many methods have been suggested to de-bundle SWNTs, including both covalent and noncovalent funtionalisation with surfactants, acids, polymers and macromolecules. However, these strategies are unsatisfactory because they result in either the chemical modification of the nanotube or the presence of residual dispersion agents. What is required is the demonstration of stable, exfoliated SWNT dispersions in common solvents. In this work we have demonstrated debundling of single-wall nanotubes by diluting nanotubes dispersions in the solvent N-methyl-2-pyrrolidinone (NMP). Atomic-force-microscopy measurements show a steady decrease in bundle diameter distribution as the concentration is decreased. Photoluminescence and Raman spectroscopy confirmed the presence of individual nanotubes in NMP dispersion.

Raman spectroscopy of open-ended Single Wall Carbon Nanotubes under pressure: effect of the pressure transmitting medium

Abstract: We performed high pressure Raman experiments on purified open-ended single wall carbon nanotubes using different pressure transmitting media (paraffin oil, argon and 4:1 methanol ethanol mixture) and two excitating wavelength (514.5 nm and 632.8 nm). We state that the behavior of the Raman spectrum under pressure is significantly dependent on the pressure transmitting medium. This result points out the relevance of the interactions between the medium and the nanotubes, at the origin of the disagreement between the phase transition sequences reported in literature. The comparaison of tangential modes profiles clearly shows that 4:1 methanol ethanol induces few strain on nanotubes in the opposite of what is observed for argon and paraffin oil pressure transmitting media. These observations are discussed in terms of intercalation and stress due to non hydrostatic conditions.

FP‐LAPW investigations of Zn1–xBexS, Zn1–xBexSe and Zn1–xBexTe ternary alloys

Abstract: The ab initio full potential linearized augmented plane wave (FP-LAPW) method within density functional theory was applied to study the effect of composition on the structural and electronic properties of Zn1–xBexS, Zn1–xBexSe and Zn1–xBexTe ternary alloys. The effect of composition on lattice parameter, bulk modulus, band gap and effective mass was investigated. Deviations of the lattice constant from Vegard's law and the bulk modulus from linear concentration dependence were observed for all three alloys. It was deduced that increasing the Be composition in the alloys increases the hardness of the materials. In addition, the calculated band structures showed that the band gap undergoes a direct-to-indirect transition at a given concentration. Using the approach of Zunger and co-workers, the microscopic origins of band gap bowing have been explained. The electron (hole) effective masses were also calculated. The band gap and effective mass were found to vary non-linearly with Be composition. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Implementation of linear-scaling plane wave density functional theory on parallel computers

Abstract: We describe the algorithms we have developed for linear-scaling plane wave density functional calculations on parallel computers as implemented in the onetep program. We outline how onetep achieves plane wave accuracy with a computational cost which increases only linearly with the number of atoms by optimising directly the single-particle density matrix expressed in a psinc basis set. We describe in detail the novel algorithms we have developed for computing with the psinc basis set the quantities needed in the evaluation and optimisation of the total energy within our approach. For our parallel computations we use the general Message Passing Interface (MPI) library of subroutines to exchange data between processors. Accordingly, we have developed efficient schemes for distributing data and computational load to processors in a balanced manner. We describe these schemes in detail and in relation to our algorithms for computations with a psinc basis. Results of tests on different materials show that onetep is an efficient parallel code that should be able to take advantage of a wide range of parallel computer architectures.