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1.
  • Almqvist, Tim, et al. (författare)
  • Domain formation and self-sustained oscillations in quantum cascade lasers
  • 2019
  • Ingår i: European Physical Journal B. - : Springer Science and Business Media LLC. - 1434-6028 .- 1434-6036. ; 92:4
  • Tidskriftsartikel (refereegranskat)abstract
    • Abstract: We study oscillations in quantum cascade lasers due to traveling electric field domains, which are observed both in simulations and experiments. These oscillations occur in a range of negative differential resistance and we clarify the condition determining whether the boundary between domains of different electric field can become stationary. Graphical abstract: [Figure not available: see fulltext.].
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3.
  • Aurell, Erik, et al. (författare)
  • Dynamics and performance of susceptibility propagation on synthetic data
  • 2010
  • Ingår i: European Physical Journal B. - : Springer Science and Business Media LLC. - 1434-6028 .- 1434-6036. ; 77:4, s. 587-595
  • Tidskriftsartikel (refereegranskat)abstract
    • We study the performance and convergence properties of the susceptibility propagation (SusP) algorithm for solving the Inverse Ising problem. We first study how the temperature parameter (T) in a Sherrington-Kirkpatrick model generating the data influences the performance and convergence of the algorithm. We find that at the high temperature regime (T > 4), the algorithm performs well and its quality is only limited by the quality of the supplied data. In the low temperature regime (T < 4), we find that the algorithm typically does not converge, yielding diverging values for the couplings. However, we show that by stopping the algorithm at the right time before divergence becomes serious, good reconstruction can be achieved down to T a parts per thousand 2. We then show that dense connectivity, loopiness of the connectivity, and high absolute magnetization all have deteriorating effects on the performance of the algorithm. When absolute magnetization is high, we show that other methods can be work better than SusP. Finally, we show that for neural data with high absolute magnetization, SusP performs less well than TAP inversion.
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4.
  • Baldissera, Gustavo, et al. (författare)
  • Vacancy induced magnetism in WO3
  • 2013
  • Ingår i: European Physical Journal B. - : Springer Science and Business Media LLC. - 1434-6028 .- 1434-6036. ; 86:6, s. 273-
  • Tidskriftsartikel (refereegranskat)abstract
    • The possibility to obtain ferromagnetic (FM) phase from native defects in WO3 is investigated by theoretically analyzing six different crystalline structures. The local magnetic moment from vacancies is calculated using the projector augmented wave method in combination with the local spin density approximation including a Coulomb correction (LSDA + U) of the W d-states. We find that tungsten vacancies V-W can induce a magnetic phase of similar to 3.5 mu(B)/V-W with a local magnetic moment on the oxygen atoms of at most similar to 1 mu(B)/V-W, whereas corresponding oxygen vacancies V-O have no impact on the magnetic coupling. Intriguingly, although the six crystalline structures have very comparable bonds, the magnetic moment generated by the cation vacancies is different, showing higher local magnetic moments for WO3 structures with low crystalline symmetry. The results indicate that WO3:V-W cannot induce a hole-mediated FM phase, and instead V-W in WO3 induces a local magnetic moment on the unpaired states at surrounding O atoms.
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6.
  • Barra, Adriano, et al. (författare)
  • Mean field spin glasses treated with PDE techniques
  • 2013
  • Ingår i: European Physical Journal B. - : Springer Berlin/Heidelberg. - 1434-6028 .- 1434-6036. ; 86:7
  • Tidskriftsartikel (refereegranskat)abstract
    • Following an original idea of F. Guerra, in this notes we analyze the Sherrington-Kirkpatrick model from different perspectives, all sharing the underlying approach which consists in linking the resolution of the sta- tistical mechanics of the model (e.g. solving for the free energy) to well-known partial differential equation (PDE) problems (in suitable spaces). The plan is then to solve the related PDE using techniques involved in their native field and lastly bringing back the solution in the proper statistical mechanics framework. Within this strand, after a streamlined test-case on the Curie-Weiss model to highlight the methods more than the physics behind, we solve the SK both at the replica symmetric and at the 1-RSB level, obtaining the correct expression for the free energy via an analogy to a Fourier equation and for the self-consistencies with an analogy to a Burger equation, whose shock wave develops exactly at critical noise level (triggering the phase transition).Our approach, beyond acting as a new alternative method (with respect to the standard routes) for tackling the complexity of spin glasses, links symmetries in PDE theory with constraints in statistical mechanics and, as a novel result from the theoretical physics perspective, we obtain a new class of polynomial identities (namely of Aizenman-Contucci type, but merged within the Guerra’s broken replica measures), whose in- terest lies in understanding, via the recent Panchenko breakthroughs, how to force the overlap organization to the ultrametric tree predicted by Parisi. 
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7.
  • Borysov, Stanislav, et al. (författare)
  • U.S. stock market interaction network as learned by the Boltzmann machine
  • 2015
  • Ingår i: European Physical Journal B. - : Springer Berlin/Heidelberg. - 1434-6028 .- 1434-6036. ; 88:12, s. 1-14
  • Tidskriftsartikel (refereegranskat)abstract
    • We study historical dynamics of joint equilibrium distribution of stock returns in the U.S. stock market using the Boltzmann distribution model being parametrized by external fields and pairwise couplings. Within Boltzmann learning framework for statistical inference, we analyze historical behavior of the parameters inferred using exact and approximate learning algorithms. Since the model and inference methods require use of binary variables, effect of this mapping of continuous returns to the discrete domain is studied. The presented results show that binarization preserves the correlation structure of the market. Properties of distributions of external fields and couplings as well as the market interaction network and industry sector clustering structure are studied for different historical dates and moving window sizes. We demonstrate that the observed positive heavy tail in distribution of couplings is related to the sparse clustering structure of the market. We also show that discrepancies between the model’s parameters might be used as a precursor of financial instabilities.
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8.
  • Boström, Mathias, et al. (författare)
  • Casimir attractive-repulsive transition in MEMS
  • 2012
  • Ingår i: European Physical Journal B. - : Springer Science and Business Media LLC. - 1434-6028 .- 1434-6036. ; 85:11, s. 377-
  • Tidskriftsartikel (refereegranskat)abstract
    • Unwanted stiction in micro-and nanomechanical (NEMS/MEMS) systems due to dispersion (van der Waals, or Casimir) forces is a significant hurdle in the fabrication of systems with moving parts on these length scales. Introducing a suitably dielectric liquid in the interspace between bodies has previously been demonstrated to render dispersion forces repulsive, or even to switch sign as a function of separation. Making use of recently available permittivity data calculated by us we show that such a remarkable nonmonotonic Casimir force, changing from attractive to repulsive as separation increases, can in fact be observed in systems where constituent materials are in standard NEMS/MEMS use requiring no special or exotic materials. No such nonmonotonic behaviour has been measured to date. We calculate the force between a silica sphere and a flat surface of either zinc oxide or hafnia, two materials which are among the most prominent for practical microelectrical and microoptical devices. Our results explicate the need for highly accurate permittivity functions of the materials involved for frequencies from optical to far-infrared frequencies. A careful analysis of the Casimir interaction is presented, and we show how the change in the sign of the interaction can be understood as a result of multiple crossings of the dielectric functions of the three media involved in a given set-up.
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9.
  • Boström, Mathias, et al. (författare)
  • Casimir force between atomically thin gold films
  • 2013
  • Ingår i: European Physical Journal B. - : Springer Science and Business Media LLC. - 1434-6028 .- 1434-6036. ; 86:2, s. 43-
  • Tidskriftsartikel (refereegranskat)abstract
    • We have used density functional theory to calculate the anisotropic dielectric functions for ultrathin gold sheets (composed of 1, 3, 6, and 15 atomic layers). Such films are important components in nano-electromechanical systems. When using correct dielectric functions rather than bulk gold dielectric functions we predict an enhanced attractive Casimir-Lifshitz force (at most around 20%) between two atomically thin gold sheets. For thicker sheets the dielectric properties and the corresponding Casimir forces approach those of gold half-spaces. The magnitude of the corrections that we predict should, within the today's level of accuracy in Casimir force measurements, be clearly detectable.
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10.
  • Catalano, Francesco, et al. (författare)
  • Equation of motion truncation scheme based on partial orthogonalization
  • 2021
  • Ingår i: European Physical Journal B. - : Springer Nature. - 1434-6028 .- 1434-6036. ; 94
  • Tidskriftsartikel (refereegranskat)abstract
    • We introduce a general scheme to consistently truncate equations of motion for Green's functions. Our scheme is guaranteed to generate physical Green's functions with real excitation energies and positive spectral weights. There are free parameters in our scheme akin to mean field parameters that may be determined to get as good an approximation to the physics as possible. As a test case we apply our scheme to a two-pole approximation for the 2D Hubbard model. At half-filling we find an insulating solution with several interesting properties: it has low expectation value of the energy and it gives upper and lower Hubbard bands with the full non-interacting bandwidth in the large U limit. Away from half-filling, in particular in the intermediate interaction regime, our scheme allows for several different phases with different number of Fermi surfaces and topologies.Graphic abstract
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12.
  • de Almeida Junior, E. F., et al. (författare)
  • Defects in hexagonal-AlN sheets by first-principles calculations
  • 2012
  • Ingår i: European Physical Journal B. - : Springer. - 1434-6028 .- 1434-6036. ; 85:1
  • Tidskriftsartikel (refereegranskat)abstract
    • Theoretical calculations focused on the stability of an infinite hexagonal AlN (h-AlN) sheet and its structural and electronic properties were carried out within the framework of DFT at the GGA-PBE level of theory. For the simulations, an h-AlN sheet model system consisting in 96 atoms per super-cell has been adopted. For h-AlN, we predict an Al-N bond length of 1.82 angstrom and an indirect gap of 2.81 eV as well as a cohesive energy which is by 6% lower than that of the bulk (wurtzite) AlN which can be seen as a qualitative indication for synthesizability of individual h-AlN sheets. Besides the study of a perfect h-AlN sheet, also the most typical defects, namely, vacancies, anti-site defects and impurities were also explored. The formation energies for these defects were calculated together with the total density of states and the corresponding projected states were also evaluated. The charge density in the region of the defects was also addressed. Energetically, the anti-site defects are the most costly, while the impurity defects are the most favorable, especially so for the defects arising from Si impurities. Defects such as nitrogen vacancies and Si impurities lead to a breaking of the planar shape of the h-AlN sheet and in some cases to the formation of new bonds. The defects significantly change the band structure in the vicinity of the Fermi level in comparison to the band structure of the perfect h-AlN which can be used for deliberately tailoring the electronic properties of individual h-AlN sheets.
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13.
  • Di Marco, Igor, et al. (författare)
  • gamma-Mn at the border between weak and strong correlations
  • 2009
  • Ingår i: European Physical Journal B. - : Springer Science and Business Media LLC. - 1434-6028 .- 1434-6036. ; 72:4, s. 473-478
  • Tidskriftsartikel (refereegranskat)abstract
    • We investigate the role of magnetic fluctuations in the spectral properties of paramagnetic gamma-Mn. Two methods are employed. The Local Density Approximation plus Dynamical Mean-Field Theory together with the numerically exact quantum Monte-Carlo solver is used as a reference for the spectral properties. Then the same scheme is used with the computationally less demanding perturbative spin-polarized fluctuation-exchange solver in combination with the Disordered Local Moment approach, and photoemission spectra are calculated within the one-step model. It is shown that the formation of local magnetic moments in gamma-Mn is very sensitive to the value of Hund's exchange parameter. Comparison with the experimental photoemission spectra demonstrates that gamma-Mn is a strongly correlated system, with the Hubbard band formation, which cannot be described by the perturbative approach. However, minor change of parameters would transform it into a weakly correlated system.
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14.
  • Eremina, R. M., et al. (författare)
  • Magnetization and specific heat of the dimer system CuTe(2)O(5)
  • 2011
  • Ingår i: European Physical Journal B. - : Springer Science and Business Media LLC. - 1434-6028 .- 1434-6036. ; 84:3, s. 391-395
  • Tidskriftsartikel (refereegranskat)abstract
    • We report on magnetization and specific heat measurements on single-crystalline CuTe(2)O(5). The experimental data are directly compared to theoretical results for two different spin structures, namely an alternating spin-chain and a two-dimensional (2D) coupled dimer model, obtained by Das et al. [Phys. Rev. B 77, 224437 (2008)]. While the analysis of the specific heat does not allow to distinguish between the two models, the magnetization data is in good agreement with the 2D coupled dimer model.
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15.
  • Etz, Corina, et al. (författare)
  • First principle study of the magnetism of Sr 2CoMoO 6-δ (δ = 0, 1/2) double perovskites
  • 2006
  • Ingår i: European Physical Journal B. - : Springer Science and Business Media LLC. - 1434-6028 .- 1434-6036. ; 54:4, s. 429-434
  • Tidskriftsartikel (refereegranskat)abstract
    • We investigate the electronic structure of bulk Sr 2CoMoO 6-δ double perovskites using the ab initio Full Potential Linearized Augmented Plane Wave method in order to study their magnetic properties within the GGA and GGA+U methods. We discuss the relative stability of ferromagnetic (FM) and antiferromagnetic (AFM) orders (i) without and with taking into account the observed tilting of the oxygen octahedra and (ii) by introducing oxygen vacancies. We show that a very good agreement with experimental results - AFM order for δ= 0 and FM order for δ= 1/2 - is obtained only when the tilting of the oxygen tetrahedra is taking into account and when the GGA+U method is used
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16.
  • Fedele, R., et al. (författare)
  • How the coherent instabilities of an intense high energy charged particle beam in the presence of nonlocal effects can be explained within the context of the Madelung fluid description
  • 2006
  • Ingår i: European Physical Journal B. - : Springer Science and Business Media LLC. - 1434-6028 .- 1434-6036. ; B49:3, s. 275-281
  • Tidskriftsartikel (refereegranskat)abstract
    • A hydrodynamical description of coherent instabilities that take place in the longitudinal dynamics of a charged-particle coasting beam in a high-energy accelerating machine is presented. This is done within the framework of the Madelung fluid picture provided by the Thermal Wave Model. The well known coherent instability charts in the complex plane of the longitudinal coupling impedance for monochromatic beams are recovered. The results are also interpreted in terms of the deterministic approach to modulational instability analysis usually given for monochromatic large amplitude wave propagation governed by the nonlinear Schrodinger equation. The instability analysis is then extended to a non-monochromatic coasting beam with a given thermal equilibrium distribution, thought of as a statistical ensemble of monochromatic incoherent coasting beams ("white" beam). In this hydrodynamical framework, the phenomenon of Landau damping is predicted without using any kinetic equation governing the phase space evolution of the system.
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17.
  • Fedele, R, et al. (författare)
  • Stability and collapse of localized solutions of the controlled three-dimensional Gross-Pitaevskii equation
  • 2010
  • Ingår i: European Physical Journal B. - : Springer. - 1434-6028 .- 1434-6036. ; 74:1, s. 97-116
  • Tidskriftsartikel (refereegranskat)abstract
    • On the basis of recent investigations, a newly developed analytical procedure is used for constructing a wide class of localized solutions of the controlled three-dimensional (3D) Gross-Pitaevskii equation (GPE) that governs the dynamics of Bose-Einstein condensates (BECs) in the presence of a spatio-temporally varying external potential. The controlled 3D GPE is decomposed into a two-dimensional (2D) linear Schrodinger equation (called the 'transverse equation') and a one-dimensional (1D) nonlinear Schrodinger equation (called the 'longitudinal equation'), constrained by a variational condition for the controlling potential. The latter corresponds to the requirement for the minimization of the control operation in the transverse plane. Then, the above class of localized solutions are constructed as the product of the solutions of the transverse and longitudinal equations. A consistency condition between the transverse and longitudinal solutions yields a relationship between the transverse and longitudinal restoring forces produced by the external trapping potential well through a 'controlling parameter' (i.e. the average, with respect to the transverse profile, of the nonlinear inter-atomic interaction term of the GPE). It is found that the longitudinal profile supports localized solutions in the form of bright, dark or grey solitons with time-dependent amplitudes, widths and centroids. The related longitudinal phase is varying in space and time with time-dependent curvature radius and wavenumber. In turn, all the above parameters (i.e. amplitudes, widths, centroids, curvature radius and wavenumbers) can be easily expressed in terms of the controlling parameter. It is also found that the transverse profile has the form of Hermite-Gauss functions (depending on the transverse coordinates), and the explicit spatio-temporal dependence of the controlling potential is self-consistently determined. On the basis of these exact 3D analytical solutions, a stability analysis is carried out, focusing our attention on the physical conditions for having collapsing or non-collapsing solutions.
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18.
  • Fröstad, T., et al. (författare)
  • Two-electron quantum dot in tilted magnetic fields : Sensitivity to the confinement model
  • 2013
  • Ingår i: European Physical Journal B. - : Springer Science and Business Media LLC. - 1434-6028 .- 1434-6036. ; 86:10
  • Tidskriftsartikel (refereegranskat)abstract
    • Semiconductor quantum dots are conventionally treated within the effective-mass approximation and a harmonic model potential in the two-dimensional plane for the electron confinement. The validity of this approach depends on the type of the quantum-dot device as well as on the number of electrons confined in the system. Accurate modeling is particularly demanding in the few-particle regime, where screening effects are diminished and thus the system boundaries may have a considerable effect on the confining potential. Here we solve the numerically exact two-electron states in both harmonic and hard-wall model quantum dots subjected to tilted magnetic fields. Our numerical results enable direct comparison against experimental singlet-triplet energy splittings. Our analysis shows that hard and soft wall models produce qualitatively different results for quantum dots exposed to tilted magnetic fields. Hence, we are able to address the sensitivity of the two-body phenomena to the modeling, which is of high importance in realistic spin-qubit design.
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19.
  • Galakhov, AV, et al. (författare)
  • Influence of the Coulomb parameter U on partial densities of states of CuGeO3: Comparison with X-ray spectral data
  • 2004
  • Ingår i: European Physical Journal B. Condensed Matter and Complex Systems. - : Springer Science and Business Media LLC. - 1434-6028. ; 41:3, s. 295-300
  • Tidskriftsartikel (refereegranskat)abstract
    • The electronic structure of the strongly Coulomb correlated cuprate CuGeO3 has been calculated by the local-density-approximation method (LDA+U). The parameter U was varied from 0 to 8 eV. The results of the band-structure calculations are compared with experimental data obtained by means of X-ray photoelectron and resonant X-ray emission spectroscopy methods (Cu Lalpha and O Kalpha X-ray emission spectra). It is established that a LDA+U calculation with U = 4 eV reproduces well the X-ray photoelectron and X-ray resonant emission spectral data.
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20.
  • Garhammer, Julian, et al. (författare)
  • On the challenge to improve the density response with unusual gradient approximations
  • 2018
  • Ingår i: European Physical Journal B. - : SPRINGER. - 1434-6028 .- 1434-6036. ; 91:7
  • Tidskriftsartikel (refereegranskat)abstract
    • Certain excitations, especially ones of long-range charge transfer character, are poorly described by time-dependent density functional theory (TDDFT) when typical (semi-)local functionals are used. A proper description of these excitations would require an exchange-correlation response differing substantially from the usual (semi-) local one. It has recently been shown that functionals of the generalized gradient approximation (GGA) type can yield unusual potentials, mimicking features of the exact exchange derivative discontinuity and showing divergences on orbital nodal surfaces. We here investigate whether these unusual potential properties translate into beneficial response properties. Using the Sternheimer formalism we closely investigate the response obtained with the 2013 exchange approximation by Armiento and Kummel (AK13) and the 1988 exchange approximation by Becke (B88), both of which show divergences on orbital nodal planes. Numerical calculations for Na-2 as well as analytical and numerical calculations for the hydrogen atom show that the response of AK13 behaves qualitatively different from usual semi-local functionals. However, the AK13 functional leads to fundamental instabilities in the asymptotic region that prevent its practical application in TDDFT. Our findings may help the development of future improved functionals. They also corroborate that the frequency-dependent Sternheimer formalism is excellently suited for running and analyzing TDDFT calculations.
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21.
  • Ghosh, S., et al. (författare)
  • Electronic structure and magnetism of disordered bcc Fe alloys
  • 2001
  • Ingår i: European Physical Journal B. - : Springer Science and Business Media LLC. - 1434-6028 .- 1434-6036. ; 23:4, s. 455-461
  • Tidskriftsartikel (refereegranskat)abstract
    • We study the electronic structure and magnetic properties of disordered bcc CoxFe1-x, CrxFe1-x and MnxFe1-x alloys in their ferromagnetic phases using the Augmented Space Recursion (ASR) technique coupled with the tight-binding linearized muffin tin orbital (TB-LMTO) method. We calculate the density of states and magnetic moment of these alloys to show the variation upon alloying Fe with the other neighbouring 3d transition metals using arguments based on charge transfer, exchange splitting and hybridization effects.
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22.
  • Hartmann, Ola, et al. (författare)
  • Magnetic properties of the multiferroic chromium thio-spinels CdCr2S4 and HgCr2S4
  • 2013
  • Ingår i: European Physical Journal B. - : Springer Science and Business Media LLC. - 1434-6028 .- 1434-6036. ; 86:4, s. 148-
  • Tidskriftsartikel (refereegranskat)abstract
    • Powder samples of CdCr2S4 and HgCr2S4 were investigated with respect to their magnetic behavior using muon spin rotation/relaxation spectroscopy (mu SR). In CdCr2S4, a second order magnetic transition at 83 +/- 1 K is confirmed. No influence on the mu SR magnetic parameters due to the simultaneous ferroelectric transition was detected. In the limit T -> 0, persistent spin fluctuation exists as a consequence of magnetic frustration. The proposed structural transition around 150 K is barely visible in the mu SR spectra, indicating that the change in local symmetry is subtle. In HgCr2S4, we see in zero field at 27 +/- 2 K a second order transition from the paramagnetic into a long-range ordered (LRO) magnetic state. This LRO state is characterized in the mu SR spectra by a spontaneous spin precession signal. Applications of moderate magnetic fields (up to 450 mT) destroy the precession signal in favor of a rapidly decaying exponential signal. It reflects short-range order (SRO) of strongly correlated dynamic spins. The magnitude of the field required to induce this change is dependent on temperature. The observation of an increase of magnetic spin fluctuations with applied field suggests that spin dynamics play a key role in the field induced changes of the magnetic properties.
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23.
  • Holme, Petter, 1973-, et al. (författare)
  • Heterogeneous attachment strategies optimize the topology of dynamic wireless networks
  • 2010
  • Ingår i: European Physical Journal B. - : Springer Science and Business Media LLC. - 1434-6028 .- 1434-6036. ; 73:4, s. 597-604
  • Tidskriftsartikel (refereegranskat)abstract
    • In optimizing the topology of wireless networks built of a dynamic set of spatially embedded agents, there are many trade-offs to be dealt with. The network should preferably be as small (in the sense that the average, or maximal, pathlength is short) as possible, it should be robust to failures, not consume too much power, and so on. In this paper, we investigate simple models of how agents can choose their neighbors in such an environment. In our model of attachment, we can tune from one situation where agents prefer to attach to others in closest proximity, to a situation where agents attach to random others regardless of distance (which thus are, on average, further away than the connections to the spatial neighbors). We evaluate this scenario with several performance measures and find that the optimal topologies, for most of the quantities, is obtained for strategies resulting in a mix of most local and a few random connections.
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24.
  • Iqbal, Z., et al. (författare)
  • Evidence for a solid phase of dodecahedral C-20
  • 2003
  • Ingår i: European Physical Journal B. - : Springer Science and Business Media LLC. - 1434-6028 .- 1434-6036. ; 31:4, s. 509-515
  • Tidskriftsartikel (refereegranskat)abstract
    • Evidence is presented for the formation of a solid phase based on the smallest fullerene, C-20, in thin diamond-like carbon films deposited by ultraviolet laser ablation from diamond onto nickel substrates at room temperature in the presence of 10(-4) torr of cyclohexane or benzene. Laser desorption mass spectrometry from the films shows the presence of C-20, C-21 and C-22 species, while micro-Raman spectroscopy and electron diffraction from selected particles together with first principle density-functional calculations, indicate a cubic solid with dodecahedral C-20 Cages as building blocks. Unlike solid C-60 and fully protonated C-20, which are bound by van der Waals forces, the proposed structure is stabilized by linking of the C-20 dodecahedra with bridging carbon atoms at interstitial tetrahedral sites to form a face-centered-cubic lattice with 22 carbon atoms per unit cell.
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25.
  • Ivanov, Sergey, et al. (författare)
  • Cation ordering, ferrimagnetism and ferroelectric relaxor behavior in Pb(Fe1-xScx)(2/3)W1/3O3 solid solutions
  • 2019
  • Ingår i: European Physical Journal B. - : SPRINGER. - 1434-6028 .- 1434-6036. ; 92:8
  • Tidskriftsartikel (refereegranskat)abstract
    • Ceramic samples of the multiferroic perovskite Pb(Fe1-xScx)(2/3)W1/3O3 with 0 <= x <= 0.4 have been synthesized using a conventional solid-state reaction method, and investigated experimentally and theoretically using first-principle calculations. Rietveld analyses of joint synchrotron X-ray and neutron diffraction patterns show the formation of a pure crystalline phase with cubic (Fm3(_)m) structure with partial ordering in the B-sites. The replacement of Fe by Sc leads to the increase of the cation order between the B and B '' sites. As the non-magnetic Sc3+ ions replace the magnetic Fe3+ cations, the antiferromagnetic state of PbFe2/3W1/3O3 is turned into a ferrimagnetic state reflecting the different magnitude of the magnetic moments on the B ' and B '' sites. The materials remain ferroelectric relaxors with increasing Sc content. Results from experiments on annealed and quenched samples show that the cooling rate after high temperature annealing controls the degree of cationic order in Pb(Fe1-xScx)(2/3)W1/3O3 and possibly also in the undoped PbFe2/3W1/3O3.
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26.
  • Johansson, Magnus, et al. (författare)
  • Standing wave instabilities, breather formation and thermalization in a Hamiltonian anharmonic lattice
  • 2002
  • Ingår i: European Physical Journal B. - : Springer Science and Business Media LLC. - 1434-6028 .- 1434-6036. ; 29:2, s. 279-283
  • Tidskriftsartikel (refereegranskat)abstract
    • Modulational instability of travelling plane waves is often considered as the first step in the formation of intrinsically localized modes (discrete breathers) in anharmonic lattices. Here, we consider an alternative mechanism for breather formation, originating in oscillatory instabilities of spatially periodic or quasiperiodic nonlinear standing waves (SWs). These SWs are constructed for Klein-Gordon or Discrete Nonlinear Schrödinger lattices as exact time periodic and time reversible multibreather solutions from the limit of uncoupled oscillators, and merge into harmonic SWs in the small-amplitude limit. Approaching the linear limit, all SWs with nontrivial wave vectors (0 < Q < p) become unstable through oscillatory instabilities, persisting for arbitrarily small amplitudes in infinite lattices. The dynamics resulting from these instabilities is found to be qualitatively different for wave vectors smaller than or larger than p/2, respectively. In one regime persisting breathers are found, while in the other regime the system thermalizes.
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27.
  • Kalvius, G. M., et al. (författare)
  • Frustration driven magnetic states of A-site spinels probed by mu SR
  • 2010
  • Ingår i: European Physical Journal B. - : Springer Science and Business Media LLC. - 1434-6028 .- 1434-6036. ; 77:1, s. 87-100
  • Tidskriftsartikel (refereegranskat)abstract
    • The normal A-site spinels MnAl2O4, FeAl2O4, CoAl2O4, as well as related mixed (Mn0.5Fe0.5Al2O4) and partially inverted (Fe1.4Al1.6O4) spinels have been studied by mu SR. The magnetic ions are subject to magnetic frustration by competing interactions. In all materials and at all temperatures the mu SR spectra consist of two signals suggesting a bimodal distribution of the fluctuation rates of magnetic moments. A characteristic temperature T (M) is found in each compound, representing either a magnetic phase transition into a long-range ordered state (MnAl2O4, Fe1.4Al1.6O4) or the formation of a spin liquid phase (FeAl2O4, CoAl2O4, Mn0.5Fe0.5Al2O4). The magnetic ground state of MnAl(2)O(4)aEuroEuroshows coexistence of antiferromagnetic and spin liquid phases. In FeAl(2)O(4)aEuroEuroand CoAl(2)O(4)aEuroEurolong-range order is suppressed altogether, the ground state can be characterized as a fast relaxing spin liquid coexisting with a small fraction of paramagnetic spins. The partial replacement of Mn by Fe in Mn(0.5)Fe(0.5)Al(2)O(4)aEuroEuroprevents long-range order and leads to a spin liquid state in the low temperature limit. The partial occupancy of B-sites by magnetic ions in Fe(1.4)Al(1.6)O(4)aEuroEurostrengthens the exchange coupling, allowing the formation of long-range magnetic order at a rather high temperature (similar to 100 K). Magnetic phase diagrams are presented demonstrating that for the studied compounds the magnetic properties are determined by the degree of frustration.
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28.
  • Kartsev, Alexey, et al. (författare)
  • Nonadiabatic Van der Pol oscillations in molecular transport
  • 2014
  • Ingår i: European Physical Journal B. Condensed Matter and Complex Systems. - : Springer Science and Business Media LLC. - 1434-6028. ; 87:1
  • Tidskriftsartikel (refereegranskat)abstract
    • The force exerted by the electrons on the nuclei of a current-carrying molecular junction can be manipulated to engineer nanoscale mechanical systems. In the adiabatic regime a peculiarity of these forces is negative friction, responsible for Van der Pol oscillations of the nuclear coordinates. In this work we study the robustness of the Van der Pol oscillations against high-frequency sources. For this purpose we go beyond the adiabatic approximation and perform full Ehrenfest dynamics simulations. The numerical scheme implements a mixed quantum-classical algorithm for open systems and is capable to deal with arbitrary time-dependent driving fields. We find that the Van der Pol oscillations are extremely stable. The nonadiabatic electron dynamics distorts the trajectory in the momentum-coordinate phase space but preserves the limit cycles in an average sense. We further show that high-frequency fields change both the oscillation amplitudes and the average nuclear positions. By switching the fields off at different times one obtains cycles of different amplitudes which attain the limit cycle only after considerably long times.
  •  
29.
  • Kim, Beom Jun, et al. (författare)
  • Self organized scale-free networks from merging and regeneration
  • 2005
  • Ingår i: European Physical Journal B. - Berlin / Heidelberg : Springer. - 1434-6028 .- 1434-6036. ; 43:3, s. 369-372
  • Tidskriftsartikel (refereegranskat)abstract
    • We propose that the ubiquitous scale free nature of many real world networks may emerge from a steady state process where nodes are created and merged randomly. The merging may be viewed as an optimization of efficiency by minimizing redundancy.
  •  
30.
  • Kochetov, A. E., et al. (författare)
  • Interaction of Au nanowires with impurities
  • 2008
  • Ingår i: European Physical Journal B. - : Springer Science and Business Media LLC. - 1434-6028 .- 1434-6036. ; 61:4, s. 441-444
  • Tidskriftsartikel (refereegranskat)abstract
    • We report the results of our first-principles studies of the interaction between an infinite monoatomic gold nanowire and a carbon-monoxide molecule. We show that the gold monoatomic nanowire is capable of absorbing the CO molecule at the distances of about 1.8 angstrom and forms a bond with the carbon atom. Further, we find that dissociation of the CO molecule as the source of gold nanowire contamination with carbon, which is widely discussed in literature as the possible reason for the striking stability of gold nanowires under stretching, is thermodynamically unfavored.
  •  
31.
  • Koplak, Oksana V., et al. (författare)
  • Spectroscopic fingerprints for charge localization in the organic semiconductor (DOEO)(4)[HgBr4]center dot TCE
  • 2015
  • Ingår i: European Physical Journal B. Condensed Matter and Complex Systems. - : Springer Science and Business Media LLC. - 1434-6028. ; 88:5
  • Tidskriftsartikel (refereegranskat)abstract
    • Changes of the electronic structure accompanied by charge localization and a transition to an antiferromagnetic ground state were observed in the organic semiconductor (DOEO)(4)[HgBr4]center dot TCE. Localization starts in the temperature region of about 150 K and the antiferromagnetic state occurs below 60 K. The magnetic moment of the crystal contains contributions of inclusions (droplets), and individual paramagnetic centers formed by localized holes and free charge carriers at 2 K. Two types of inclusions of 100-400 nm and 2-5 nm sizes were revealed by transmission electron microscopy. Studying the temperatureand angular dependence of electron spin resonance (ESR) spectra revealed fingerprints of antiferromagnetic contributions as well as paramagnetic resonance spectra of individual localized charge carriers. The results point on coexistence of antiferromagnetic long and short range order as evident from a second ESR line. Photoelectron spectroscopy in the VUV, soft and hard X-ray range shows temperature-dependent effects upon crossing the critical temperatures around 60 K and 150 K. The substantially different probing depths of soft and hard X-ray photoelectron spectroscopy yield information on the surface termination. The combined investigation using complementary methods at the same sample reveals the close relation of changes in the transport properties and in the energy distribution of electronic states.
  •  
32.
  • Kotani, A., et al. (författare)
  • Spectator and participator processes in the resonant photon-in and photon-out spectra at the Ce L-3 edge of CeO2
  • 2012
  • Ingår i: European Physical Journal B. - : Springer Science and Business Media LLC. - 1434-6028 .- 1434-6036. ; 85:8, s. 257-
  • Tidskriftsartikel (refereegranskat)abstract
    • We study both theoretically and experimentally the photon-in and photon-out spectra of CeO2, which are caused by the Ce 2p to Ce 5d excitation followed by the three different de-excitation channels: (i) Ce 3d to Ce 2p (denoted by 3d-RXES), (ii) O 2p to Ce 2p (v-RXES), and (iii) Ce 5d to Ce 2p (RIXS). In 3d- and v-RXES, the 5d electron plays a role of a spectator, but in RIXS it is a participator. By extending our single impurity Anderson model (SIAM), which was used recently for our calculations of v-and 3d-RXES spectra of CeO2, we study the polarization dependence in the spectator and participator spectra, and we perform more detailed calculations for 3d- and v-RXES spectral features, as well as new calculations for the RIXS spectrum with charge transfer excitations. The polarization dependence is different for the spectator and participator spectra; we have no polarization correlation between the incident and emitted photons for the spectator spectra but a strong polarization correlation for the participator spectrum. The theoretical calculations predict that the charge transfer excitations in RIXS occur in the transfer-energy range overlapped with v-RXES, but the RIXS and v-RXES spectra can be discriminated by taking advantage of the different polarization dependence. The overlapped RIXS and v-RXES spectra are observed successfully by our experiments and well reproduced by our SIAM calculations.
  •  
33.
  • Levamäki, Henrik, et al. (författare)
  • Gradient-level and nonlocal density functional descriptions of Cu-Au intermetallic compounds
  • 2018
  • Ingår i: European Physical Journal B. - : SPRINGER. - 1434-6028 .- 1434-6036. ; 91:6
  • Tidskriftsartikel (refereegranskat)abstract
    • We use three gradient level and two nonlocal density functional approximations to study the thermodynamic properties of Cu-Au compounds. It is found that a well-designed gradient level approximation (quasi non-uniform approximation, QNA) reproduces the experimental equilibrium volumes and the formation energies of L12 and L10 phases. On the other hand, QNA predicts a non-existent beta(2) phase, which can be remedied only when employing the nonlocal hybrid-level Heyd-Scuseria-Ernzerhof (HSE06) or Perdew-Burke-Ernzerhof (PBE0) approximations. Gradient-level approximations lead to similar electronic structures for the Cu-Au compounds whereas hybrids shift the d-band towards negative energies and account for the complex d-d hybridization more accurately.
  •  
34.
  • Mahnke, R., et al. (författare)
  • Application of thermodynamics to driven systems
  • 2007
  • Ingår i: European Physical Journal B. - : Springer Science and Business Media LLC. - 1434-6028 .- 1434-6036. ; 57:4, s. 463-471
  • Tidskriftsartikel (refereegranskat)abstract
    • Application of thermodynamics to driven systems is discussed. As particular examples, simple traffic flow models are considered. On a microscopic level, traffic flow is described by Bando's optimal velocity model in terms of accelerating and decelerating forces. It allows to introduce kinetic, potential, as well as total energy, which is the internal energy of the car system in view of thermodynamics. The latter is not conserved, although it has certain value in any of two possible stationary states corresponding either to fixed point or to limit cycle in the space of headways and velocities. On a mesoscopic level of description, the size n of car cluster is considered as a stochastic variable in master equation. Here n = 0 corresponds to the fixed-point solution of the microscopic model, whereas the limit cycle is represented by coexistence of a car cluster with n > 0 and free flow phase. The detailed balance holds in a stationary state just like in equilibrium liquid-gas system. It allows to define free energy of the car system and chemical potentials of the coexisting phases, as well as a relaxation to a local or global free energy minimum. In this sense the behaviour of traffic flow can be described by equilibrium thermodynamics. We find, however, that the chemical potential of the cluster phase of traffic flow depends on an outer parameter - the density of cars in the free-flow phase. It allows to distinguish between the traffic flow as a driven system and purely equilibrium systems.
  •  
35.
  • Manikandan, Sreekanth K., et al. (författare)
  • Asymptotics of work distributions in a stochastically driven system
  • 2017
  • Ingår i: European Physical Journal B. - : Springer Science and Business Media LLC. - 1434-6028 .- 1434-6036. ; 90:12
  • Tidskriftsartikel (refereegranskat)abstract
    • We determine the asymptotic forms of work distributions at arbitrary times T, in a class of driven stochastic systems using a theory developed by Nickelsen and Engel (EN theory) [D. Nickelsen and A. Engel, Eur. Phys. J. B 82 , 207 (2011)], which is based on the contraction principle of large deviation theory. In this paper, we extend the theory, previously applied in the context of deterministically driven systems, to a model in which the driving is stochastic. The models we study are described by overdamped Langevin equations and the work distributions in path integral form, are characterised by having quadratic augmented actions. We first illustrate EN theory, for a deterministically driven system - the breathing parabola model, and show that within its framework, the Crooks fluctuation theorem manifests itself as a reflection symmetry property of a certain characteristic polynomial, which also determines the exact moment-generating-function at arbitrary times. We then extend our analysis to a stochastically driven system, studied in references [S. Sabhapandit, EPL 89 , 60003 (2010); A. Pal, S. Sabhapandit, Phys. Rev. E 87 , 022138 (2013); G. Verley, C. Van den Broeck, M. Esposito, New J. Phys. 16 , 095001 (2014)], for both equilibrium and non-equilibrium steady state initial distributions. In both cases we obtain new analytic solutions for the asymptotic forms of (dissipated) work distributions at arbitrary T. For dissipated work in the steady state, we compare the large T asymptotic behaviour of our solution to the functional form obtained in reference [New J. Phys. 16 , 095001 (2014)]. In all cases, special emphasis is placed on the computation of the pre-exponential factor and the results show excellent agreement with numerical simulations. Our solutions are exact in the low noise (beta -> (infinity)) limit.
  •  
36.
  • Mathieu, Roland, et al. (författare)
  • Enhancement of antiferromagnetic interaction and transition temperature in M3TeO6 systems (M = Mn, Co, Ni, Cu)
  • 2013
  • Ingår i: European Physical Journal B. - : Springer Science and Business Media LLC. - 1434-6028 .- 1434-6036. ; 86:8, s. 361-
  • Tidskriftsartikel (refereegranskat)abstract
    • Several M3TeO6 (M = Mn, Co, Ni, Cu) oxides order antiferromagnetically at low temperatures (less than or similar to 60 K), while displaying interesting dielectric properties at high temperatures (ferroelectricity below 1000 K in M = Ni case). We have investigated and analyzed the structural and magnetic properties of Mn-doped Co3TeO6 and Ni3TeO6, which order antiferromagnetically at temperatures higher than their undoped counterparts.
  •  
37.
  • Moret, Roger, et al. (författare)
  • High-pressure synthesis, structural and Raman studies of a two-dimensional polymer crystal of C60.
  • 2000
  • Ingår i: European Physical Journal B. - : Springer. - 1434-6028 .- 1434-6036. ; 15:2, s. 253-263
  • Tidskriftsartikel (refereegranskat)abstract
    • Two-dimensional polymerisation of a C60 single crystal has been obtained under high-pressure high temperature conditions (700 K - 2 GPa). Crystalline order is preserved but the crystal splits into variants (orientational domains). The analysis of X-ray diffraction and Raman spectroscopy data reveals that the polymer crystal is primarily tetragonal with some admixture of rhombohedral phase. Furthermore, Raman spectroscopy gives evidence for additional C60-C60 dimers, which are probably disordered. For the tetragonal phase, it is shown that successive polymer layers are rotated by about the stacking axis, according to the P42/mmc space group symmetry. The structure of the rhombohedral phase is also clarified. The role of the interlayer interactions in stabilising the two-dimensional polymer phases of C60 is discussed.
  •  
38.
  • Moret, Roger, et al. (författare)
  • Single-crystal structural study of the pressure-temperature induced dimerization of C60
  • 2004
  • Ingår i: European Physics Journal B. - Berlin : Springer. - 1434-6028 .- 1434-6036. ; 37:1, s. 25-37
  • Tidskriftsartikel (refereegranskat)abstract
    • We present a study by Raman spectroscopy and X-ray diffraction/diffuse scattering of C60 single-crystals treated at high-pressure and high-temperature. This allowed us to obtain structural information on the C60 dimer state which can be considered as an intermediate state in the polymerization process. In the 1-6 GPa pressure range the crystals are primarily formed of dimers with additional minor fractions of monomers, 1D and 2D polymers, as shown by the analysis of the Raman spectra. The dimers are disordered within an average cubic lattice derived from that of the monomer. Single-crystal diffraction patterns reveal a characteristic diffuse scattering intensity distribution which has been simulated by calculating the diffuse scattering produced by dimer and trimer model structures. Satisfactory agreement is obtained for random positional and orientational disorder of the C60-C60 dimers although a small concentration of similarly disordered trimers is likely. In a first approximation the dimer/trimer disorder can be considered as random but various inter-dimer correlations are probably present, as discussed.
  •  
39.
  • Moskalenko, S. A., et al. (författare)
  • Effects of Rashba spin-orbit coupling, Zeeman splitting and gyrotropy in two-dimensional cavity polaritons under the influence of the Landau quantization
  • 2015
  • Ingår i: European Physical Journal B. - : Springer. - 1434-6028 .- 1434-6036. ; 88:9
  • Tidskriftsartikel (refereegranskat)abstract
    • We consider the energy spectrum of the two-dimensional cavity polaritons under the influence of a strong magnetic and electric fields perpendicular to the surface of the GaAs-type quantum wells (QWs) with p-type valence band embedded into the resonators. As the first step in this direction the Landau quantization (LQ) of the electrons and heavy-holes (hh) was investigated taking into account the Rashba spin-orbit coupling (RSOC) with third-order chirality terms for hh and with nonparabolicity terms in their dispersion low including as well the Zeeman splitting (ZS) effects. The nonparabolicity term is proportional to the strength of the electric field and was introduced to avoid the collapse of the semiconductor energy gap under the influence of the third order chirality terms. The exact solutions for the eigenfunctions and eigenenergies were obtained using the Rashba method [E.I. Rashba, Fiz. Tverd. Tela 2, 1224 (1960) [Sov. Phys. Solid State 2, 1109 (1960)]]. On the second step we derive in the second quantization representation the Hamiltonians describing the Coulomb electron-electron and the electron-radiation interactions. This allow us to determine the magnetoexciton energy branches and to deduce the Hamiltonian of the magnetoexciton-photon interaction. On the third step the fifth order dispersion equation describing the energy spectrum of the cavity magnetoexciton-polariton is investigated. It takes into account the interaction of the cavity photons with two dipole-active and with two quadrupole-active 2D magnetoexciton energy branches. The cavity photons have the circular polarizations σk ± oriented along their wave vectors k, which has the quantized longitudinal component kz = ± π/Lc, where Lc is the resonator length and another small transverse component k∥ oriented in the plane of the QW. The 2D magnetoexcitons are characterized by the in-plane wave vectors k∥ and by circular polarizations σM arising in the p-type valence band with magnetic momentum projection M = ± 1 on the direction of the magnetic field. The selection rules of the exciton-photon interaction have two origins. The first one, of geometrical-type, is expressed through the scalar products of the two-types circular polarizations. They depend on the in-plane wave vectors k∥ even in the case of dipole-active transitions, because the cavity photons have an oblique incidence to the surface of the QW. Another origin is related with the numbers ne and nh of the LQ levels of electrons and heavy-holes taking part in the magnetoexciton formation. So, the dipole-active transitions take place for the condition ne = nh, whereas in the quadrupole-active transitions the relation is ne = nh ± 1. It was shown that the Rabi frequency ΩR of the polariton branches and the magnetoexciton oscillator strength fosc increase in dependence on the magnetic field strength B as ΩR ~ √B, and fosc ~ B. The optical gyrotropy effects may be revealed if changing the sign of the photon circular polarization at a given sign of the wave vector longitudinal projection kz or equivalently changing the sign of the longitudinal projection kz at the same selected light circular polarization.
  •  
40.
  • Moskalenko, S. A., et al. (författare)
  • Nambu-Goldstone modes of the two-dimensional Bose-Einstein condensed magnetoexcitons
  • 2012
  • Ingår i: European Physical Journal B. - : Springer Science and Business Media LLC. - 1434-6028 .- 1434-6036. ; 85:10, s. 359-
  • Tidskriftsartikel (refereegranskat)abstract
    • The collective elementary excitations of two-dimensional magnetoexcitons in a Bose-Einstein condensate (BEC) with wave vector κ = 0 were investigated in the framework of the Bogoliubov theory of quasiaverages. The Hamiltonian of the electrons and holes lying in the lowest Landau levels (LLLs) contains supplementary interactions due to virtual quantum transitions of the particles to the excited Landau levels (ELLs) and back. As a result, the interaction between the magnetoexcitons with κ = 0 does not vanish and their BEC becomes stable. The equations of motion for the exciton operators d(P) and d†(P) are interconnected with equations of motion for the density operators ρ(P) and D(P). Instead of a set of two equations of motion, as in the case of usual Bose gas, corresponding to normal and abnormal Green's functions, we have a set of four equations of motion. This means we have to deal simultaneously with four branches of the energy spectrum, the two supplementary branches being the optical plasmon branch represented by the operator ρ(P) and the acoustical plasmon branch represented by the operator D(P). The perturbation theory on the small parameter v 2(1 - v 2), where v 2 is the filling factor and (1 - v 2) is the phase space filling factor was developed. The energy spectrum contains only one gapless, true Nambu-Goldstone (NG) mode of the second kind with dependence ω(κ) ≈ κ 2 at small values κ describing the optical-plasmon-type oscillations. There are two exciton-type branches corresponding to normal and abnormal Green's functions. Both modes are gapped with roton-type segments at intermediary values of the wave vectors and can be named as quasi-NG modes. The fourth branch is the acoustical plasmontype mode with absolute instability in the region of small and intermediary values of the wave vectors. All branches have a saturation-type dependencies at great values of the wave vectors. The number and the kind of the true NG modes is in accordance with the number of the broken symmetry operators. The comparison of the results concerning two Bose-Einstein condensates namely of the coplanar magnetoexcitons and of the quantum Hall excitons in the bilayer electron systems reveals their similarity.
  •  
41.
  • Nicolis, S. C., et al. (författare)
  • Self-organization, collective decision making and resource exploitation strategies in social insects
  • 2008
  • Ingår i: European Physical Journal B. - : Springer Science and Business Media LLC. - 1434-6028 .- 1434-6036. ; 65:3, s. 379-385
  • Tidskriftsartikel (refereegranskat)abstract
    • Amplifying communications are a ubiquitous characteristic of group-living animals. This work is concerned with their role in the processes of food recruitment and resource exploitation by social insects. The collective choices made by ants faced with different food sources are analyzed using both a mean field description and a stochastic approach. Emphasis is placed on the possibility of optimizing the recruitment and exploitation strategies through an appropriate balance between individual variability, cooperative interactions and environmental constraints.
  •  
42.
  • Ou, Haiyan, et al. (författare)
  • Advances in wide bandgap SiC for optoelectronics
  • 2014
  • Ingår i: European Physical Journal B. - : Springer Science and Business Media LLC. - 1434-6028 .- 1434-6036. ; 87:3, s. 58-
  • Tidskriftsartikel (refereegranskat)abstract
    • Silicon carbide (SiC) has played a key role in power electronics thanks to its unique physical properties like wide bandgap, high breakdown field, etc. During the past decade, SiC is also becoming more and more active in optoelectronics thanks to the progress in materials growth and nanofabrication. This paper will review the advances in fluorescent SiC for white light-emitting diodes, covering the polycrystalline doped SiC source material growth, single crystalline epitaxy growth of fluorescent SiC, and nanofabrication of SiC to enhance the extraction efficiency for fluorescent SiC based white LEDs.
  •  
43.
  • Rui, Yikang, et al. (författare)
  • Exploring the patterns and evolution of self-organized urban street networks through modeling
  • 2013
  • Ingår i: European Physical Journal B. - : Springer Science and Business Media LLC. - 1434-6028 .- 1434-6036. ; 86:74, s. 1-8
  • Tidskriftsartikel (refereegranskat)abstract
    • As one of the most important subsystems in cities, urban street networks have recently been well studied by using the approach of complex networks. This paper proposes a growing model for self-organized urban street networks. The model involves a competition among new centers with different values of attraction radius and a local optimal principle of both geometrical and topological factors. We find that with the model growth, the local optimization in the connection process and appropriate probability for the loop construction well reflect the evolution strategy in real-world cities. Moreover, different values of attraction radius in centers competition process lead to morphological change in patterns including urban network, polycentric and monocentric structures. The model succeeds in reproducing a large diversity of road network patterns by varying parameters. The similarity between the properties of our model and empirical results implies that a simple universal growth mechanism exists in self-organized cities.
  •  
44.
  • Rybski, D., et al. (författare)
  • Communication activity in social networks : growth and correlations
  • 2011
  • Ingår i: European Physical Journal B. - : Springer Science and Business Media LLC. - 1434-6028 .- 1434-6036. ; 84:1, s. 147-159
  • Tidskriftsartikel (refereegranskat)abstract
    • We investigate the timing of messages sent in two online communities with respect to growth fluctuations and long-term correlations. We find that the timing of sending and receiving messages comprises pronounced long-term persistence. Considering the activity of the community members as growing entities, i.e. the cumulative number of messages sent (or received) by the individuals, we identify non-trivial scaling in the growth fluctuations which we relate to the long-term correlations. We find a connection between the scaling exponents of the growth and the long-term correlations which is supported by numerical simulations based on peaks over threshold. In addition, we find that the activity on directed links between pairs of members exhibits long-term correlations, indicating that communication activity with the most liked partners may be responsible for the long-term persistence in the timing of messages. Finally, we show that the number of messages, M, and the number of communication partners, K, of the individual members are correlated following a power-law, K similar to M (lambda) , with exponent lambda a parts per thousand 3 / 4.
  •  
45.
  • Rönnow, Daniel, et al. (författare)
  • Isotope effects on the electronic critical points of germanium : Ellipsometric investigation of the E1 and E1+ Delta(1) transitions
  • 1998
  • Ingår i: European Physical Journal B. - : Springer Science and Business Media LLC. - 1434-6028 .- 1434-6036. ; 5:1, s. 29-35
  • Tidskriftsartikel (refereegranskat)abstract
    • Within the past years the optical excitations of electrons have been measured for semiconductor samples of different isotope compositions. The isotope shift observed have been compared with calculations of the effects of electron-phonon interaction on the electronic band structure. While qualitative agreement has been obtained, some discrepancies remain especially concerning the E1 and E1 + Delta(1) transitions. We have remeasured the effect of isotope mass on the E1 and E1 + Delta(1) transitions of germanium with several isotopic compositions. The results, obtained by means of spectroscopic ellipsometry, confirm that the real part of the gap self-energies induced by electron-phonon interaction is larger than found from band structure calculations, while the imaginary part agrees with those calculations, which are based on a pseudopotential band structure and a bond charge model for the lattice dynamics. Our results agree with predictions based on the measured temperature dependence of the gaps. We compare our data for E1 and E1 + Delta(1) with results for the lowest direct (E0) and indirect (E9) gaps. The measured values of Δ0 and Δ1 increase noticeably with increasing isotope mass. Similar effects have been observed in the temperature dependence of Delta(1) in alpha-Sn and GaSb. A microscopic explanation for this effect is not available.
  •  
46.
  • Rössler, Thomas, et al. (författare)
  • A v0-representability issue in lattice ensemble-DFT and its signature in lattice TDDFT
  • 2018
  • Ingår i: European Physical Journal B. - : Springer Science and Business Media LLC. - 1434-6028 .- 1434-6036. ; 91, s. 1-8
  • Tidskriftsartikel (refereegranskat)abstract
    • We study a small Anderson-impurity cluster using lattice density functional methods, and try todetermine the exact exchange-correlation (XC) potential via reverse engineering. In doing so we nd singlet{triplet degenerate interacting ground states which cannot be v0-represented in an ensemble-DFT sense. Wealso nd that it is possible to represent a triplet ground state as a pure state, but not the singlet. We further investigate this behavior within time-dependent density-functional theory. Starting from a v0-representableground state and entering the degeneracy region via a time-dependent perturbation, we determine via reverse-engineering the time-dependent XC potential for progressively slower perturbations. This analysis shows that, even in a constant external eld, the XC potential must retain a time-oscillating pattern to ensure a density constant in time, a hint of the underlying representability issue in the ground state.
  •  
47.
  • Schiessling, J, et al. (författare)
  • Insulating surface layer on single crystal K3C60
  • 2004
  • Ingår i: European Physical Journal B. Condensed Matter and Complex Systems. - : Springer Science and Business Media LLC. - 1434-6028. ; 41:4, s. 435-438
  • Tidskriftsartikel (refereegranskat)abstract
    • Using angle-dependent photoemission spectra of core and valence levels we show that metallic, single crystal K3C60 is terminated by an insulating or weakly-conducting surface layer. We attribute this to the effects of strong intermolecular correlations combined with the average surface charge state. Several controversies on the electronic structure are thereby resolved.
  •  
48.
  • Seevinck, Michael P., et al. (författare)
  • Comment on "A local realist model for correlations of the singlet state" by K. De Raedt, K. Keimpema, H. De Raedt, K. Michielsen and S. Miyashita
  • 2007
  • Ingår i: The European Physical Journal B. - : Springer Science and Business Media LLC. - 1434-6028 .- 1434-6036. ; 58:1, s. 51-53
  • Tidskriftsartikel (refereegranskat)abstract
    • De Raedt et al. [Eur. Phys. J. B 53, 139 (2006)] have claimed to provide a local realist model for correlations of the singlet state in the familiar Einstein-Podolsky-Rosen-Bohm (EPRB) experiment when time-coincidence is used to decide which detection events should count in the analysis, and furthermore that this suggests that it is possible to construct local realistic models that can reproduce the quantum mechanical expectation values. In this letter we show that these conclusions cannot be upheld since their model exploits the so-called coincidence-time loophole. When this is properly taken into account no startling conclusions can be drawn about local realist modelling of quantum mechanics.
  •  
49.
  • Smirnova, E. A., et al. (författare)
  • Electronic topological transitions and phase stability in the fcc Al-Zn alloys
  • 2002
  • Ingår i: European Physical Journal B. - : Springer Science and Business Media LLC. - 1434-6028 .- 1434-6036. ; 30:1, s. 57-66
  • Tidskriftsartikel (refereegranskat)abstract
    • We report on a detailed investigation of the phase equilibria and the Fermi surface in the Al-Zn system. Our calculation are based on the density functional theory and we use the linear muffin-tin orbital method and the Green's function technique. The calculated free energies of alloy formation exhibit the existence of a miscibility gap between the alloys containing approximately 10 and 55 at.% of Zn, in agreement with the phase diagram of the Al-Zn system. Seven electronic topological transitions (ETT) were found in Al-Zn system within the stability range of the fcc solid solution. A relation between these ETT and the phase stability of the fcc Al-Zn solid solutions is established. We show that extremum points on the concentration dependencies of the thermodynamic properties of Al-Zn alloys can be explained by band-filling effects.
  •  
50.
  • Talyzin, Alexandr, et al. (författare)
  • Structural and magnetic properties of polymerized C60 with Fe
  • 2007
  • Ingår i: European Physical Journal B. - : Springer Science and Business Media LLC. - 1434-6028 .- 1434-6036. ; 55:1, s. 57-62
  • Tidskriftsartikel (refereegranskat)abstract
    • We provide evidence that high-pressure high-temperature (2.5 GPa and 1040 K) treatment of mixtures of iron with fullerene powders leads to the complete transformation of iron into iron carbide Fe3C. The comparison of the magnetic properties (Curie temperature and magnetic moment) of the here studied samples and those for the ferromagnetic polymer Rh-C-60 indicates that the main ferromagnetic signal reported in those samples is due to Fe3C and not related to the ferromagnetism of carbon as originally interpreted. Taking into account the results obtained in this study the original paper on "Magnetic carbon" [Nature 413, 716 (2001)] was recently retracted.
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