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  • 101. Bhat, Pallavi
    et al.
    Subramanian, Kandaswamy
    Brandenburg, Axel
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA. Stockholm University, Sweden.
    A unified large/small-scale dynamo in helical turbulence2016Ingår i: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 461, nr 1, s. 240-247Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We use high resolution direct numerical simulations (DNS) to show that helical turbulence can generate significant large-scale fields even in the presence of strong small-scale dynamo action. During the kinematic stage, the unified large/ small-scale dynamo grows fields with a shapeinvariant eigenfunction, with most power peaked at small scales or large k, as in Subramanian & Brandenburg. Nevertheless, the large-scale field can be clearly detected as an excess power at small k in the negatively polarized component of the energy spectrum for a forcing with positively polarized waves. Its strength (B) over bar, relative to the total rms field Brms, decreases with increasing magnetic Reynolds number, Re-M. However, as the Lorentz force becomes important, the field generated by the unified dynamo orders itself by saturating on successively larger scales. The magnetic integral scale for the positively polarized waves, characterizing the smallscale field, increases significantly from the kinematic stage to saturation. This implies that the small-scale field becomes as coherent as possible for a given forcing scale, which averts the Re-M-dependent quenching of (B) over bar /B-rms. These results are obtained for 1024(3) DNS with magnetic Prandtl numbers of PrM = 0.1 and 10. For PrM = 0.1, B/ Brms grows from about 0.04 to about 0.4 at saturation, aided in the final stages by helicity dissipation. For Pr-M = 10, (B) over bar /B-rms grows from much less than 0.01 to values of the order the 0.2. Our results confirm that there is a unified large/ small-scale dynamo in helical turbulence.

  • 102.
    Bhatnagar, Akshay
    et al.
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA. Indian Institute of Science, India.
    Gupta, A.
    Mitra, Dhrubaditya
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA.
    Perlekar, P.
    Wilkinson, M.
    Pandit, R.
    Deviation-angle and trajectory statistics for inertial particles in turbulence2016Ingår i: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics, ISSN 1539-3755, E-ISSN 1550-2376, Vol. 94, nr 6, artikel-id 063112Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Small particles in suspension in a turbulent fluid have trajectories that do not follow the pathlines of the flow exactly. We investigate the statistics of the angle of deviation φ between the particle and fluid velocities. We show that, when the effects of particle inertia are small, the probability distribution function (PDF) Pφ of this deviation angle shows a power-law region in which Pφ∼φ-4. We also find that the PDFs of the trajectory curvature κ and modulus θ of the torsion have power-law tails that scale, respectively, as Pκ∼κ-5/2, as κ→∞, and Pθ∼θ-3, as θ→∞: These exponents are in agreement with those previously observed for fluid pathlines. We propose a way to measure the complexity of heavy-particle trajectories by the number NI(t,St) of points (up until time t) at which the torsion changes sign. We present numerical evidence that nI(St)≡limt→∞NI(t,St)t∼St-Δ for large St, with Δ≃0.5.

  • 103.
    Bhatnagar, Akshay
    et al.
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA. Stockholm University, Sweden.
    Gupta, Anupam
    Mitra, Dhrubaditya
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA. Stockholm University, Sweden.
    Pandit, Rahul
    Heavy inertial particles in turbulent flows gain energy slowly but lose it rapidly2018Ingår i: Physical review. E, ISSN 2470-0045, E-ISSN 2470-0053, Vol. 97, nr 3, artikel-id 033102Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We present an extensive numerical study of the time irreversibility of the dynamics of heavy inertial particles in three-dimensional, statistically homogeneous, and isotropic turbulent flows. We show that the probability density function (PDF) of the increment, W(tau), of a particle's energy over a time scale tau is non-Gaussian, and skewed toward negative values. This implies that, on average, particles gain energy over a period of time that is longer than the duration over which they lose energy. We call this slow gain and fast loss. We find that the third moment of W(tau) scales as tau(3) for small values of tau. We show that the PDF of power-input p is negatively skewed too; we use this skewness Ir as a measure of the time irreversibility and we demonstrate that it increases sharply with the Stokes number St for small St; this increase slows down at St similar or equal to 1. Furthermore, we obtain the PDFs of t(+) and t(-), the times over which p has, respectively, positive or negative signs, i.e., the particle gains or loses energy. We obtain from these PDFs a direct and natural quantification of the slow gain and fast loss of the energy of the particles, because these PDFs possess exponential tails from which we infer the characteristic loss and gain times t(loss) and t(gain), respectively, and we obtain t(loss) < t(gain) for all the cases we have considered. Finally, we show that the fast loss of energy occurs with greater probability in the strain-dominated region than in the vortical one; in contrast, the slow gain in the energy of the particles is equally likely in vortical or strain-dominated regions of the flow.

  • 104.
    Bhatnagar, Akshay
    et al.
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA.
    Gupta, Anupam
    Mitra, Dhrubaditya
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA.
    Pandit, Rahul
    Perlekar, Prasad
    How long do particles spend in vortical regions in turbulent flows?2016Ingår i: PHYSICAL REVIEW E, ISSN 2470-0045, Vol. 94, nr 5, artikel-id 053119Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We obtain the probability distribution functions (PDFs) of the time that a Lagrangian tracer or a heavy inertial particle spends in vortical or strain-dominated regions of a turbulent flow, by carrying out direct numerical simulations of such particles advected by statistically steady, homogeneous, and isotropic turbulence in the forced, three-dimensional, incompressible Navier-Stokes equation. We use the two invariants, Q and R, of the velocity-gradient tensor to distinguish between vortical and strain-dominated regions of the flow and partition the Q-R plane into four different regions depending on the topology of the flow; out of these four regions two correspond to vorticity-dominated regions of the flow and two correspond to strain-dominated ones. We obtain Q and R along the trajectories of tracers and heavy inertial particles and find out the time t(pers) for which they remain in one of the four regions of the Q-R plane. We find that the PDFs of tpers display exponentially decaying tails for all four regions for tracers and heavy inertial particles. From these PDFs we extract characteristic time scales, which help us to quantify the time that such particles spend in vortical or strain-dominated regions of the flow.

  • 105.
    Bhatnagar, Akshay
    et al.
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA. Stockholm Univ, Roslagstullsbacken 23, S-10691 Stockholm, Sweden..
    Gustavsson, K.
    Gothenburg Univ, Dept Phys, S-41296 Gothenburg, Sweden..
    Mehlig, B.
    Gothenburg Univ, Dept Phys, S-41296 Gothenburg, Sweden..
    Mitra, Dhrubaditya
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA. Stockholm Univ, Roslagstullsbacken 23, S-10691 Stockholm, Sweden..
    Relative velocities in bidisperse turbulent aerosols: Simulations and theory2018Ingår i: Physical review. E, ISSN 2470-0045, E-ISSN 2470-0053, Vol. 98, nr 6, artikel-id 063107Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We perform direct numerical simulations of a bidisperse suspension of heavy spherical particles in forced, homogeneous, and isotropic three-dimensional turbulence. We compute the joint distribution of relative particle distances and longitudinal relative velocities between particles of different inertia. For a pair of particles with small difference in their inertias we compare our results with recent theoretical predictions [Meibohm et al., Phys. Rev. E 96, 061102 (2017)] for the shape of this distribution. We also compute the moments of relative velocities as a function of particle separation and compare with the theoretical predictions. We observe good agreement. For a pair of particles that are very different from each other-one is heavy and the other one has negligible inertia-we give a theory to calculate their root-mean-square relative velocity. This theory also agrees well with the results of our simulations.

  • 106.
    Bhatnagar, Akshay
    et al.
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA. Stockholm Univ, Roslagstullsbacken 23, S-10691 Stockholm, Sweden.
    Gustavsson, K.
    Mitra, Dhrubaditya
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA. Stockholm Univ, Roslagstullsbacken 23, S-10691 Stockholm, Sweden.
    Statistics of the relative velocity of particles in turbulent flows: Monodisperse particles2018Ingår i: Physical review. E, ISSN 2470-0045, E-ISSN 2470-0053, Vol. 97, nr 2, artikel-id 023105Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We use direct numerical simulations to calculate the joint probability density function of the relative distance R and relative radial velocity component V-R for a pair of heavy inertial particles suspended in homogeneous and isotropic turbulent flows. At small scales the distribution is scale invariant, with a scaling exponent that is related to the particle-particle correlation dimension in phase space, D-2. It was argued [K. Gustavsson and B. Mehlig, Phys. Rev. E 84, 045304 (2011); J. Turbul. 15, 34 (2014)] that the scale invariant part of the distribution has two asymptotic regimes: (1) vertical bar V-R vertical bar << R, where the distribution depends solely on R, and (2) vertical bar V-R vertical bar >> R, where the distribution is a function of vertical bar V-R vertical bar alone. The probability distributions in these two regimes are matched along a straight line: vertical bar V-R vertical bar = z*R. Our simulations confirm that this is indeed correct. We further obtain D-2 and z* as a function of the Stokes number, St. The former depends nonmonotonically on St with aminimum at about St approximate to 0.7 and the latter has only a weak dependence on St.

  • 107. Bhattacharya, Jyotirmoy
    et al.
    Cremonini, Sera
    Goutéraux, Blaise
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA. Stockholm University, Sweden; Stanford University, United States; Université Paris 7, France.
    Intermediate scalings in holographic RG flows and conductivities2015Ingår i: Journal of High Energy Physics (JHEP), ISSN 1126-6708, E-ISSN 1029-8479, nr 2, artikel-id 035Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We construct numerically finite density domain-wall solutions which interpolate between two AdS(4) fixed points and exhibit an intermediate regime of hyperscaling violation, with or without Lifshitz scaling. Such RG flows can be realized in gravitational models containing a dilatonic scalar and a massive vector field with appropriate choices of the scalar potential and couplings. The infrared AdS(4) fixed point describes a new ground state for strongly coupled quantum systems realizing such scalings, thus avoiding the well-known extensive zero temperature entropy associated with AdS(2) x R-2. We also examine the zero temperature behavior of the optical conductivity in these backgrounds and identify two scaling regimes before the UV CFT scaling is reached. The scaling of the conductivity is controlled by the emergent IR conformal symmetry at very low frequencies, and by the intermediate scaling regime at higher frequencies.

  • 108.
    Bisbas, Thomas G.
    et al.
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA. University College London, UK; Max-Planck-Institut für Extraterrestrische Physik, Germany; University of Florida, USA.
    Haworth, Thomas J.
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA. University of Cambridge, UK.
    Barlow, M. J.
    Viti, S.
    Harries, T. J.
    Bell, T.
    Yates, J. A.
    TORUS-3DPDR: a self-consistent code treating three-dimensional photoionization and photodissociation regions2015Ingår i: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 454, nr 3, s. 2828-2843Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The interaction of ionizing and far-ultraviolet radiation with the interstellar medium is of great importance. It results in the formation of regions in which the gas is ionized, beyond which are photodissociation regions (PDRs) in which the gas transitions to its atomic and molecular form. Several numerical codes have been implemented to study these two main phases of the interstellar medium either dynamically or chemically. In this paper we present TORUS-3DPDR, a new self-consistent code for treating the chemistry of three-dimensional photoionization and photodissociation regions. It is an integrated code coupling the two codes TORUS, a hydrodynamics and Monte Carlo radiation transport code, and 3D-PDR, a PDRs code. The new code uses a Monte Carlo radiative transfer scheme to account for the propagation of the ionizing radiation including the diffusive component as well as a ray-tracing scheme based on the HEALPIX package in order to account for the escape probability and column density calculations. Here, we present the numerical techniques we followed and we show the capabilities of the new code in modelling three-dimensional objects including single or multiple sources. We discuss the effects introduced by the diffusive component of the ultraviolet field in determining the thermal balance of PDRs as well as the effects introduced by a multiple sources treatment of the radiation field. With this new code, three-dimensional synthetic observations for the major cooling lines are possible, for making feasible a detailed comparison between hydrodynamical simulations and observations.

  • 109. Biswas, Tirthabir
    et al.
    Koivisto, Tomi
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA. Stockholm University, Sweden.
    Mazumdar, Anupam
    Atick-Witten Hagedorn conjecture, near scale-invariant matter and blue-tilted gravity power spectrum2014Ingår i: Journal of High Energy Physics (JHEP), ISSN 1126-6708, E-ISSN 1029-8479, nr 8, s. 116-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We will provide an interesting new mechanism to generate almost scale invariant seed density perturbations with a red spectrum, while keeping the gravitational wave spectrum blue-tilted in a stringy thermal contracting phase at temperatures beyond the Hagedorn temperature. This phase is often referred to as the Hagedorn phase where the free energy has been conjectured by Atick and Witten to grow more slowly than ordinary radiation. The primordial fluctuations are created by the statistical thermal fluctuations determined by the partition function, rather than quantum vacuum driven fluid dynamical fluctuations. Our mechanism assumes a non-singular bounce.

  • 110. Bjornson, Kristofer
    et al.
    Balatsky, Alexander V.
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA. os Alamos National Laboratory, United States; ETH Institute for Theoretical Studies, Switzerland.
    Black-Schaffer, Annica M.
    Superconducting order parameter pi-phase shift in magnetic impurity wires2017Ingår i: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 95, nr 10, artikel-id 104521Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    It has previously been found that amagnetic impurity in a conventional s-wave superconductor can give rise to a local pi-phase shift of the superconducting order parameter. By studying a finite wire of ferromagnetic impurities, we are able to trace the origin of the pi-phase shift to a resonance condition for the Bogoliubov-de Gennes quasiparticle states. When nonresonating states localized at the impurity sites are pulled into the condensate for increasing magnetic strength, the superconducting order parameter is reduced in discrete steps, eventually resulting in a pi-phase shift. We also show that for a finite spin-orbit coupling, the pi-phase shift is preserved and occurs in a large portion of the topologically nontrivial phase.

  • 111. Bjornson, Kristofer
    et al.
    Pershoguba, Sergey S.
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA.
    Balatsky, Alexander V.
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA. Los Alamos National Laboratory, United States.
    Black-Schaffer, Annica M.
    Spin-polarized edge currents and Majorana fermions in one- and two-dimensional topological superconductors2015Ingår i: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 92, nr 21, artikel-id 214501Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We investigate the persistent currents, spin-polarized local density of states, and spectral functions of topological superconductors constructed by placing ferromagnetic impurities on top of an s-wave superconductor with Rashba spin-orbit interaction. We solve self-consistently for the superconducting order parameter and investigate both two-dimensional blocks and one-dimensional wires of ferromagnetic impurities, with the magnetic moments pointing both perpendicular and parallel to the surface. We find that the topologically protected edge states of ferromagnetic blocks give rise to spin-polarized edge currents, but that the total persistent current flows in opposite direction to what is expected from the dispersion relation of the edge states. We also show that the Majorana fermions at the end points of one-dimensional wires are spin polarized, which can be directly related to the spin polarization of the edge currents of two-dimensional blocks. Connections are also made to the physics of the Yu-Shiba-Rusinov states for zero-dimensional impurities.

  • 112. Blanchet, L.
    et al.
    Heisenberg, Lavinia
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA. Stockholm University, Sweden.
    Dipolar dark matter with massive bigravity2015Ingår i: Journal of Cosmology and Astroparticle Physics, ISSN 1475-7516, E-ISSN 1475-7516, Vol. 2015, nr 12Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Massive gravity theories have been developed as viable IR modifications of gravity motivated by dark energy and the problem of the cosmological constant. On the other hand, modified gravity and modified dark matter theories were developed with the aim of solving the problems of standard cold dark matter at galactic scales. Here we propose to adapt the framework of ghost-free massive bigravity theories to reformulate the problem of dark matter at galactic scales. We investigate a promising alternative to dark matter called dipolar dark matter (DDM) in which two different species of dark matter are separately coupled to the two metrics of bigravity and are linked together by an internal vector field. We show that this model successfully reproduces the phenomenology of dark matter at galactic scales (i.e. MOND) as a result of a mechanism of gravitational polarisation. The model is safe in the gravitational sector, but because of the particular couplings of the matter fields and vector field to the metrics, a ghost in the decoupling limit is present in the dark matter sector. However, it might be possible to push the mass of the ghost beyond the strong coupling scale by an appropriate choice of the parameters of the model. Crucial questions to address in future work are the exact mass of the ghost, and the cosmological implications of the model.

  • 113. Blanchet, Luc
    et al.
    Heisenberg, Lavinia
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA. Stockholm University, Sweden.
    Dark matter via massive bigravity2015Ingår i: Physical Review D, ISSN 1550-7998, E-ISSN 1550-2368, Vol. 91, nr 10, artikel-id 103518Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    In this work we investigate the existence of relativistic models for dark matter in the context of bimetric gravity, used here to reproduce the modified Newtonian dynamics (MOND) at galactic scales. For this purpose we consider two different species of dark matter particles that separately couple to the two metrics of bigravity. These two sectors are linked together via an internal U(1) vector field, and some effective composite metric built out of the two metrics. Among possible models only certain classes of kinetic and interaction terms are allowed without invoking ghost degrees of freedom. Along these lines we explore the number of allowed kinetic terms in the theory and point out the presence of ghosts in a previous model. Finally, we propose a promising class of ghost-free candidate theories that could provide the MOND phenomenology at galactic scales while reproducing the standard cold dark matter model at cosmological scales.

  • 114.
    Bo, Stefano
    et al.
    KTH, Skolan för datavetenskap och kommunikation (CSC), Beräkningsbiologi, CB. KTH, Centra, Nordic Institute for Theoretical Physics NORDITA.
    Aurell, Erik
    KTH, Skolan för datavetenskap och kommunikation (CSC), Beräkningsbiologi, CB. KTH, Skolan för elektro- och systemteknik (EES), Centra, ACCESS Linnaeus Centre.
    Eichhorn, Ralf
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA.
    Celani, Antonio
    Optimal stochastic transport in inhomogeneous thermal environments2013Ingår i: Europhysics letters, ISSN 0295-5075, E-ISSN 1286-4854, Vol. 103, nr 1, s. 10010-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We consider the optimization of the average entropy production in inhomogeneous temperature environments within the framework of stochastic thermodynamics. For systems modeled by Langevin equations (e.g. a colloidal particle in a heat bath) it has been recently shown that a space-dependent temperature breaks the time reversal symmetry of the fast velocity degrees of freedom resulting in an anomalous contribution to the entropy production of the overdamped dynamics. We show that optimization of entropy production is determined by an auxiliary deterministic problem formally analogous to motion on a curved manifold in a potential. The "anomalous contribution" to entropy plays the role of the potential and the inverse of the diffusion tensor is the metric. We also find that entropy production is not minimized by adiabatically slow, quasi-static protocols but there is a finite optimal duration for the transport process. As an example we discuss the case of a linearly space-dependent diffusion coefficient.

  • 115.
    Bo, Stefano
    et al.
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA. Stockholm University, Sweden.
    Celani, A.
    Detecting Concentration Changes with Cooperative Receptors2015Ingår i: Journal of statistical physics, ISSN 0022-4715, E-ISSN 1572-9613, Vol. 162, nr 5, s. 1365-1382Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Cells constantly need to monitor the state of the environment to detect changes and timely respond. The detection of concentration changes of a ligand by a set of receptors can be cast as a problem of hypothesis testing, and the cell viewed as a Neyman–Pearson detector. Within this framework, we investigate the role of receptor cooperativity in improving the cell’s ability to detect changes. We find that cooperativity decreases the probability of missing an occurred change. This becomes especially beneficial when difficult detections have to be made. Concerning the influence of cooperativity on how fast a desired detection power is achieved, we find in general that there is an optimal value at finite levels of cooperation, even though easy discrimination tasks can be performed more rapidly by noncooperative receptors.

  • 116.
    Bo, Stefano
    et al.
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA.
    Celani, A.
    Multiple-scale stochastic processes: Decimation, averaging and beyond2017Ingår i: Physics reports, ISSN 0370-1573, E-ISSN 1873-6270, Vol. 670, s. 1-59Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The recent experimental progresses in handling microscopic systems have allowed to probe them at levels where fluctuations are prominent, calling for stochastic modeling in a large number of physical, chemical and biological phenomena. This has provided fruitful applications for established stochastic methods and motivated further developments. These systems often involve processes taking place on widely separated time scales. For an efficient modeling one usually focuses on the slower degrees of freedom and it is of great importance to accurately eliminate the fast variables in a controlled fashion, carefully accounting for their net effect on the slower dynamics. This procedure in general requires to perform two different operations: decimation and coarse-graining. We introduce the asymptotic methods that form the basis of this procedure and discuss their application to a series of physical, biological and chemical examples. We then turn our attention to functionals of the stochastic trajectories such as residence times, counting statistics, fluxes, entropy production, etc. which have been increasingly studied in recent years. For such functionals, the elimination of the fast degrees of freedom can present additional difficulties and naive procedures can lead to blatantly inconsistent results. Homogenization techniques for functionals are less covered in the literature and we will pedagogically present them here, as natural extensions of the ones employed for the trajectories. We will also discuss recent applications of these techniques to the thermodynamics of small systems and their interpretation in terms of information-theoretic concepts.

  • 117.
    Bo, Stefano
    et al.
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA. Stockholm Univ, Roslagstullsbacken 23, SE-10691 Stockholm, Sweden..
    Eichhorn, Ralf
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA.
    Driven Anisotropic Diffusion at Boundaries: Noise Rectification and Particle Sorting2017Ingår i: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 119, nr 6, artikel-id 060603Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We study the diffusive dynamics of a Brownian particle in the proximity of a flat surface under nonequilibrium conditions, which are created by an anisotropic thermal environment with different temperatures being active along distinct spatial directions. By presenting the exact time-dependent solution of the Fokker-Planck equation for this problem, we demonstrate that the interplay between anisotropic diffusion and hard-core interaction with the plain wall rectifies the thermal fluctuations and induces directed particle transport parallel to the surface, without any deterministic forces being applied in that direction. Based on current micromanipulation technologies, we suggest a concrete experimental setup to observe this novel noise-induced transport mechanism. We furthermore show that it is sensitive to particle characteristics, such that this setup can be used for sorting particles of different sizes.

  • 118.
    Bo, Stefano
    et al.
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA. Max Planck Inst Phys Komplexer Syst, Nothnitzer Str 38, DE-01187 Dresden, Germany. ; Stockholm Univ, Roslagstullsbacken 23, SE-10691 Stockholm, Sweden.
    Lim, Soon Hoe
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA. Stockholm Univ, Roslagstullsbacken 23, SE-10691 Stockholm, Sweden.
    Eichhorn, Ralf
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA. Stockholm Univ, Roslagstullsbacken 23, SE-10691 Stockholm, Sweden.
    Functionals in stochastic thermodynamics: how to interpret stochastic integrals2019Ingår i: Journal of Statistical Mechanics: Theory and Experiment, ISSN 1742-5468, E-ISSN 1742-5468, nr 8, artikel-id 084005Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    In stochastic thermodynamics standard concepts from macroscopic thermodynamics, such as heat, work, and entropy production, are generalized to small fluctuating systems by defining them on a trajectory-wise level. In Langevin systems with continuous state-space such definitions involve stochastic integrals along system trajectories, whose specific values depend on the discretization rule used to evaluate them (i.e. the 'interpretation' of the noise terms in the integral). Via a systematic mathematical investigation of this apparent dilemma, we corroborate the widely used standard interpretation of heat-and work-like functionals as Stratonovich integrals. We furthermore recapitulate the anomalies that are known to occur for entropy production in the presence of temperature gradients.

  • 119.
    Bo, Stefano
    et al.
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA. Max Planck Institute for the Physics of Complex Systems, Nöthnitzer Str. 38, Dresden, DE-01187, Germany.
    Schmidt, Falko
    Eichhorn, Ralf
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA.
    Volpe, Giovanni
    Measurement of anomalous diffusion using recurrent neural networks2019Ingår i: Physical review. E, ISSN 2470-0045, E-ISSN 2470-0053, Vol. 100, nr 1, artikel-id 010102Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Anomalous diffusion occurs in many physical and biological phenomena, when the growth of the mean squared displacement (MSD) with time has an exponent different from one. We show that recurrent neural networks (RNNs) can efficiently characterize anomalous diffusion by determining the exponent from a single short trajectory, outperforming the standard estimation based on the MSD when the available data points are limited, as is often the case in experiments. Furthermore, the RNNs can handle more complex tasks where there are no standard approaches, such as determining the anomalous diffusion exponent from a trajectory sampled at irregular times, and estimating the switching time and anomalous diffusion exponents of an intermittent system that switches between different kinds of anomalous diffusion. We validate our method on experimental data obtained from subdiffusive colloids trapped in speckle light fields and superdiffusive microswimmers.

  • 120. Bogunovic, L.
    et al.
    Fliedner, M.
    Eichhorn, Ralf
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA.
    Wegener, S.
    Regtmeier, J.
    Anselmetti, D.
    Reimann, P.
    Chiral particle separation by a nonchiral microlattice2012Ingår i: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 109, nr 10, s. 100603-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We conceived a model experiment for a continuous separation strategy of chiral molecules (enantiomers) without the need of any chiral selector structure or derivatization agents: Microparticles that only differ by their chirality are shown to migrate along different directions when driven by a steady fluid flow through a square lattice of cylindrical posts. In accordance with our numerical predictions, the transport directions of the enantiomers depend very sensitively on the orientation of the lattice relative to the fluid flow.

  • 121. Bogunovic, L.
    et al.
    Gerkens, S.
    Viefhues, M.
    Regtmeier, J.
    Eichhorn, Ralf
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA.
    Anselmetti, D.
    Valveless fluid actuation: Liebau’s principle fully integrated on the microfluidic scale2013Ingår i: 17th International Conference on Miniaturized Systems for Chemistry and Life Sciences, MicroTAS 2013, 2013, Vol. 1, s. 182-184Konferensbidrag (Refereegranskat)
    Abstract [en]

    In this contribution, we present the integration of valveless impedance pumping on the microfluidic scale. When a fluidic channel is periodically compressed at a point located asymmetrically with respect to an impedance mismatch in the system, an effective fluid flow is the result although no further moving parts such as valves are involved. Such an impedance mismatch could be e.g. a sudden reduction of the channel cross section. Four different types of impedance mismatches are being investigated concerning their response to different applied pressures and compression frequencies .A maximum fluid velocity of 25 }im/s and even a frequency dependent overall inversion of the flow direction could be observed.

  • 122. Bolognesi, Stefano
    et al.
    Chatterjee, Chandrasekhar
    Gudnason, Sven Bjarke
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA. Stockholm University, Sweden.
    Konishi, Kenichi
    Vortex zero modes, large flux limit and Ambjorn-Nielsen-Olesen magnetic instabilities2014Ingår i: Journal of High Energy Physics (JHEP), ISSN 1126-6708, E-ISSN 1029-8479, nr 10, s. 101-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    In the large flux limit vortices become flux tubes with almost constant magnetic field in the interior region. This occurs in the case of non-Abelian vortices as well, and the study of such configurations allows us to reveal a close relationship between vortex zero modes and the gyromagnetic instabilities of vector bosons in a strong background magnetic field discovered by Nielsen, Olesen and Ambjorn. The BPS vortices are exactly at the onset of this instability, and the dimension of their moduli space is precisely reproduced in this way. We present a unifying picture in which, through the study of the linear spectrum of scalars, fermions and W bosons in the magnetic field background, the expected number of translational, orientational, fermionic as well as semilocal zero modes is correctly reproduced in all cases.

  • 123. Bombardelli, D.
    et al.
    Cagnazzo, Alessandra
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA. DESY Hamburg, Theory Group, Germany.
    Frassek, R.
    Levkovich-Maslyuk, F.
    Loebbert, F.
    Negro, S.
    Szecsenyi, I. M.
    Sfondrini, A.
    van Tongeren, S. J.
    Torrielli, A.
    An integrability primer for the gauge-gravity correspondence: an introduction Preface2016Ingår i: Journal of Physics A: Mathematical and Theoretical, ISSN 1751-8113, E-ISSN 1751-8121, Vol. 49, nr 32, artikel-id 320301Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We introduce a series of articles reviewing various aspects of integrable models relevant to the anti-de Sitter (AdS)/conformal quantum field theory (CFT) correspondence. Topics covered in these reviews are: classical integrability, Yangian symmetry, factorized scattering, the Bethe ansatz, the thermodynamic Bethe ansatz, and integrable structures in (conformal) quantum field theory. In the present article we highlight how these concepts have found application in AdS/CFT, and provide a brief overview of the material contained in this series.

  • 124. Bonanno, Alfio
    et al.
    Brandenburg, Axel
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA.
    Del Sordo, Fabio
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA.
    Mitra, Dhrubaditya
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA.
    Breakdown of chiral symmetry during saturation of the Tayler instability2012Ingår i: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics, ISSN 1539-3755, E-ISSN 1550-2376, Vol. 86, nr 1, s. 016313-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We study spontaneous breakdown of chiral symmetry during the nonlinear evolution of the Tayler instability. We start with an initial steady state of zero helicity. Within linearized perturbation calculations, helical perturbations of this initial state have the same growth rate for either sign of helicity. Direct numerical simulations (DNS) of the fully nonlinear equations, however, show that an infinitesimal excess of one sign of helicity in the initial perturbation gives rise to a saturated helical state. We further show that this symmetry breaking can be described by weakly nonlinear finite-amplitude equations with undetermined coefficients which can be deduced solely from symmetry consideration. By fitting solutions of the amplitude equations to data from DNS, we further determine the coefficients of the amplitude equations.

  • 125.
    Bonanno, Alfio
    et al.
    INAF Osservatorio Astrofis Catania, I-95125 Catania, Italy..
    Del Sordo, Fabio
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA. Yale Univ, Dept Geol & Geophys, POB 6666, New Haven, CT 06511 USA.;Stockholm Univ, S-10691 Stockholm, Sweden..
    Analytic mean-field alpha(2)-dynamo with a force-free corona2017Ingår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 605, artikel-id A33Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Context. Stellar dynamos are affected by boundary conditions imposed by stellar coronae. Under some approximations, it is possible to find analytical solutions. Interior dynamo models often consider a current-free corona without taking into account the constraints imposed by the presence of currents in the corona. Aims. We aim to analytically evaluate the effect of coronal currents and of an outer boundary condition on the efficiency of an alpha(2) dynamo. We intend to estimate the change in geometry and dynamo excitation numbers with respect to the current-free case. Methods. We analytically solved the turbulent dynamo induction equation for a homogeneous, non-mirror symmetric turbulence in a spherical domain surrounded by a linear force-free corona with the mean magnetic field B satisfying del x B = beta B. Results. The dynamo number is a decreasing function of beta. Moreover, if the current is parallel to the field (beta > 0), the dynamo number is smaller than in the force- free case. In contrast, for (beta < 0), the dynamo number is greater than in the force-free case. Conclusions. Currents in the corona need to be taken into account because they affect the condition for excitation of a dynamo.

  • 126. Borsato, R.
    et al.
    Ohlsson Sax, Olof
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA.
    Sfondrini, A.
    Stefański, B.
    Torrielli, A.
    On the dressing factors, Bethe equations and Yangian symmetry of strings on AdS(3) x S-3 x T-42017Ingår i: Journal of Physics A: Mathematical and Theoretical, ISSN 1751-8113, E-ISSN 1751-8121, Vol. 50, nr 2, artikel-id 024004Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Integrability is believed to underlie the correspondence with sixteen supercharges. We elucidate the role of massless modes within this integrable framework. Firstly, we find the dressing factors that enter the massless and mixed-mass worldsheet S matrix. Secondly, we derive a set of all-loop Bethe Equations for the closed strings, determine their symmetries and weak-coupling limit. Thirdly, we investigate the underlying Yangian symmetry in the massless sector and show that it fits into the general framework of Yangian integrability. In addition, we compare our S matrix in the near-relativistic limit with recent perturbative worldsheet calculations of Sundin and Wulff.

  • 127.
    Borsato, R.
    et al.
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA.
    Strömwall, J.
    Torrielli, A.
    Q -Poincaré invariance of the AdS3 /CFT2 R -matrix2018Ingår i: Physical Review D: covering particles, fields, gravitation, and cosmology, ISSN 2470-0010, E-ISSN 2470-0029, Vol. 97, nr 6, artikel-id 066001Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We consider the exact R-matrix of AdS3/CFT2, which is the building block for describing the scattering of worldsheet excitations of the light-cone gauge-fixed backgrounds AdS3×S3×T4 and AdS3×S3×S3×S1 with pure Ramond-Ramond fluxes. We show that R is invariant under a "deformed boost" symmetry, for which we write an explicit exact coproduct, i.e. its action on two-particle states. When we include the boost, the symmetries of the R-matrix close into a q-Poincaré superalgebra. Our findings suggest that the recently discovered boost invariance in AdS5/CFT4 may be a common feature of AdS/CFT systems that are treatable with the exact techniques of integrability. With the aim of going towards a universal formulation of the underlying Hopf algebra, we also propose a universal form of the AdS3/CFT2 classical r-matrix. 

  • 128.
    Borsato, Riccardo
    et al.
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA.
    Torrielli, A.
    q-Poincaré supersymmetry in AdS5/CFT42018Ingår i: Nuclear Physics B, ISSN 0550-3213, E-ISSN 1873-1562, Vol. 928, s. 321-355Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We consider the exact S-matrix governing the planar spectral problem for strings on AdS5×S5 and N=4 super Yang–Mills, and we show that it is invariant under a novel “boost” symmetry, which acts as a differentiation with respect to the particle momentum. This generator leads us also to reinterpret the usual centrally extended psu(2|2) symmetry, and to conclude that the S-matrix is invariant under a q-Poincaré supersymmetry algebra, where the deformation parameter is related to the ‘t Hooft coupling. We determine the two-particle action (coproduct) that turns out to be non-local, and study the property of the new symmetry under crossing transformations. We look at both the strong-coupling (large tension in the string theory) and weak-coupling (spin-chain description of the gauge theory) limits; in the former regime we calculate the cobracket utilising the universal classical r-matrix of Beisert and Spill. In the eventuality that the boost has higher partners, we also construct a quantum affine version of 2D Poincaré symmetry, by contraction of the quantum affine algebra Uq(sl2ˆ) in Drinfeld's second realisation. 

  • 129.
    Borsato, Riccardo
    et al.
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA. Stockholm University, Sweden.
    Wulff, L.
    On non-abelian T-duality and deformations of supercoset string sigma-models2017Ingår i: Journal of High Energy Physics (JHEP), ISSN 1126-6708, E-ISSN 1029-8479, nr 10, artikel-id 024Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We elaborate on the class of deformed T-dual (DTD) models obtained by first adding a topological term to the action of a supercoset sigma model and then performing (non-abelian) T-duality on a subalgebra g˜ of the superisometry algebra. These models inherit the classical integrability of the parent one, and they include as special cases the so-called homogeneous Yang-Baxter sigma models as well as their non-abelian T-duals. Many properties of DTD models have simple algebraic interpretations. For example we show that their (non-abelian) T-duals — including certain deformations — are again in the same class, where g˜ gets enlarged or shrinks by adding or removing generators corresponding to the dualised isometries. Moreover, we show that Weyl invariance of these models is equivalent to g˜ being unimodular; when this property is not satisfied one can always remove one generator to obtain a unimodular g˜ , which is equivalent to (formal) T-duality. We also work out the target space superfields and, as a by-product, we prove the conjectured transformation law for Ramond-Ramond (RR) fields under bosonic non-abelian T-duality of supercosets, generalising it to cases involving also fermionic T-dualities.

  • 130.
    Borsato, Riccardo
    et al.
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA.
    Wulff, Linus
    Masaryk Univ, Dept Theoret Phys & Astrophys, Brno 61137, Czech Republic..
    Marginal deformations of WZW models and the classical Yang-Baxter equation2019Ingår i: Journal of Physics A: Mathematical and Theoretical, ISSN 1751-8113, E-ISSN 1751-8121, Vol. 52, nr 22, artikel-id 225401Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We show how so-called Yang-Baxter (YB) deformations of sigma models, based on an R-matrix solving the classical Yang-Baxter equation (CYBE), give rise to marginal current-current deformations when applied to the Wess-Zumino-Witten (WZW) model. For non-compact groups these marginal deformations are more general than the ones usually considered, since they can involve a non-Abelian current subalgebra. We classify such deformations of the AdS(3) x S-3 string.

  • 131.
    Borsato, Riccardo
    et al.
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA. Stockholm Univ, NORDITA, Roslagstullsbacken 23, SE-10691 Stockholm, Sweden.;KTH Royal Inst Technol, Roslagstullsbacken 23, SE-10691 Stockholm, Sweden..
    Wulff, Linus
    Masaryk Univ, Dept Theoret Phys & Astrophys, CS-61137 Brno, Czech Republic..
    Non-abelian T-duality and Yang-Baxter deformations of Green-Schwarz strings2018Ingår i: Journal of High Energy Physics (JHEP), ISSN 1126-6708, E-ISSN 1029-8479, nr 8, artikel-id 027Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We perform non-abelian T-duality for a generic Green-Schwarz string with respect to an isometry (super)group G, and we derive the transformation rules for the supergravity background fields. Specializing to G bosonic, or G fermionic but abelian, our results reproduce those available in the literature. We discuss also continuous deformations of the T-dual models, obtained by adding a closed B-field before the dualization. This idea can also be used to generate deformations of the original (un-dualized) model, when the 2-cocycle identified from the closed B is invertible. The latter construction is the natural generalization of the so-called Yang-Baxter deformations, based on solutions of the classical Yang-Baxter equation on the Lie algebra of G and originally constructed for group manifolds and (super)coset sigma models. We find that the deformed metric and B-field are obtained through a generalization of the map between open and closed strings that was used also in the discussion by Seiberg and Witten of non-commutative field theories. When applied to integrable sigma models these deformations preserve the integrability.

  • 132.
    Borysov, Stanislav
    et al.
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA. Singapore Massachusetts Inst Technol MIT Alliance, Singapore.
    Lourenco, Mariana
    Rodrigues, Filipe
    Balatsky, Alexander
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA. Los Alamos Natl Lab, NM USA.
    Pereira, Francisco
    Using Internet Search Queries to Predict Human Mobility in Social Events2016Ingår i: 2016 IEEE 19TH INTERNATIONAL CONFERENCE ON INTELLIGENT TRANSPORTATION SYSTEMS (ITSC), 2016, s. 1342-1347Konferensbidrag (Refereegranskat)
    Abstract [en]

    While our transport systems are generally designed for habitual behavior, the dynamics of large and mega cities systematically push it to its limits. Particularly, transport planning and operations in large events are well known to be a challenge. Not only they imply stress to the system on an irregular basis, their associated mobility behavior is also difficult to predict. Previous studies have shown a strong correlation between number of public transport arrivals with the semi-structured data mined from online announcement websites. However, these models tend to be complex in form and demand substantial information retrieval, extraction and data cleaning work, and so they are difficult to generalize from city to city. In contrast, this paper focuses on enriching previously mined information about special events using automated web search queries. Since this context data comes in unstructured natural language form, we employ supervised topic model to correlate it with real measurements of transport usage. In this way, the proposed approach is more generic and a transit agency can start planning ahead as early as the event is announced on the web. The results show that using information mined from the web search not only shows high prediction accuracy of public transport demand, but also potentially provides interesting insights about popular event categories based on extracted topics.

  • 133.
    Borysov, Stanislav
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Tillämpad fysik, Nanostrukturfysik. KTH, Centra, Nordic Institute for Theoretical Physics NORDITA.
    Roudi, Yasser
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA. The Kavli Institute for Systems Neuroscience, NTNU, Trondheim, Norway.
    Balatsky, Alexander V.
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA. Institute for Materials Science, Los Alamos National Laboratory, Los Alamos, NM, United States.
    U.S. stock market interaction network as learned by the Boltzmann machine2015Ingår i: European Physical Journal B: Condensed Matter Physics, ISSN 1434-6028, E-ISSN 1434-6036, Vol. 88, nr 12, s. 1-14Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    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.

  • 134.
    Borysov, Stanislav S.
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Tillämpad fysik, Nanostrukturfysik. KTH, Centra, Nordic Institute for Theoretical Physics NORDITA. Stockholm University, Sweden.
    Balatsky, Alexander V.
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA. Stockholm University, Sweden.
    Cross-Correlation Asymmetries and Causal Relationships between Stock and Market Risk2014Ingår i: PLoS ONE, ISSN 1932-6203, E-ISSN 1932-6203, Vol. 9, nr 8, s. e105874-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We study historical correlations and lead-lag relationships between individual stock risk (volatility of daily stock returns) and market risk (volatility of daily returns of a market-representative portfolio) in the US stock market. We consider the cross-correlation functions averaged over all stocks, using 71 stock prices from the Standard & Poor's 500 index for 1994-2013. We focus on the behavior of the cross-correlations at the times of financial crises with significant jumps of market volatility. The observed historical dynamics showed that the dependence between the risks was almost linear during the US stock market downturn of 2002 and after the US housing bubble in 2007, remaining at that level until 2013. Moreover, the averaged cross-correlation function often had an asymmetric shape with respect to zero lag in the periods of high correlation. We develop the analysis by the application of the linear response formalism to study underlying causal relations. The calculated response functions suggest the presence of characteristic regimes near financial crashes, when the volatility of an individual stock follows the market volatility and vice versa.

  • 135.
    Borysov, Stanislav S.
    et al.
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA.
    Forchheimer, Daniel
    KTH, Skolan för teknikvetenskap (SCI), Tillämpad fysik, Nanostrukturfysik.
    Haviland, David B.
    KTH, Skolan för teknikvetenskap (SCI), Tillämpad fysik, Nanostrukturfysik.
    Dynamic calibration of higher eigenmode parameters of a cantilever in atomic force microscopy by using tip-surface interactions2014Ingår i: Beilstein Journal of Nanotechnology, ISSN 2190-4286, Vol. 5, s. 1899-1904Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We present a theoretical framework for the dynamic calibration of the higher eigenmode parameters (stiffness and optical lever inverse responsivity) of a cantilever. The method is based on the tip-surface force reconstruction technique and does not require any prior knowledge of the eigenmode shape or the particular form of the tip-surface interaction. The calibration method proposed requires a single-point force measurement by using a multimodal drive and its accuracy is independent of the unknown physical amplitude of a higher eigenmode.

  • 136.
    Borysov, Stanislav S.
    et al.
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA.
    Geilhufe, R. Matthias
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA.
    Balatsky, Alexander V.
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA.
    Organic materials database: An open-access online database for data mining2017Ingår i: PLoS ONE, ISSN 1932-6203, E-ISSN 1932-6203, Vol. 12, nr 2, artikel-id e0171501Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We present an organic materials database (OMDB) hosting thousands of Kohn-Sham electronic band structures, which is freely accessible online at http://omdb.diracmaterials.org. The OMDB focus lies on electronic structure, density of states and other properties for purely organic and organometallic compounds that are known to date. The electronic band structures are calculated using density functional theory for the crystal structures contained in the Crystallography Open Database. The OMDB web interface allows users to retrieve materials with specified target properties using non-trivial queries about their electronic structure. We illustrate the use of the OMDB and how it can become an organic part of search and prediction of novel functional materials via data mining techniques. As a specific example, we provide data mining results for metals and semiconductors, which are known to be rare in the class of organic materials.

  • 137.
    Borysov, Stanislav S.
    et al.
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA.
    Olsthoorn, Bart
    Stockholm Univ, Roslagstullsbacken 23, SE-10691 Stockholm, Sweden.;Stockholm Univ, Dept Phys, SE-10691 Stockholm, Sweden..
    Gedik, M. Berk
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA.
    Geilhufe, R. Matthias
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA.
    Balatsky, Alexander V.
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA. Univ Connecticut, Dept Phys, Storrs, CT 06269 USA..
    Online search tool for graphical patterns in electronic band structures2018Ingår i: NPJ COMPUTATIONAL MATERIALS, ISSN 2057-3960, Vol. 4, artikel-id UNSP 46Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Many functional materials can be characterized by a specific pattern in their electronic band structure, for example, Dirac materials, characterized by a linear crossing of bands; topological insulators, characterized by a "Mexican hat" pattern or an effectively free electron gas, characterized by a parabolic dispersion. To find material realizations of these features, manual inspection of electronic band structures represents a relatively easy task for a small number of materials. However, the growing amount of data contained within modern electronic band structure databases makes this approach impracticable. To address this problem, we present an automatic graphical pattern search tool implemented for the electronic band structures contained within the Organic Materials Database. The tool is capable of finding user-specified graphical patterns in the collection of thousands of band structures from high-throughput calculations in the online regime. Using this tool, it only takes a few seconds to find an arbitrary graphical pattern within the ten electronic bands near the Fermi level for 26,739 organic crystals. The source code of the developed tool is freely available and can be adapted to any other electronic band structure database.

  • 138.
    Borysov, Stanislav S.
    et al.
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA.
    Platz, Daniel
    KTH, Skolan för teknikvetenskap (SCI), Tillämpad fysik, Nanostrukturfysik.
    de Wijn, Astrid S.
    Forchheimer, Daniel
    KTH, Skolan för teknikvetenskap (SCI), Tillämpad fysik, Nanostrukturfysik.
    Tolén, Eric A.
    Balatsky, Alexander V.
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA.
    Haviland, David B.
    KTH, Skolan för teknikvetenskap (SCI), Tillämpad fysik, Nanostrukturfysik.
    Reconstruction of tip-surface interactions with multimodal intermodulation atomic force microscopy2013Ingår i: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 88, nr 11, s. 115405-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We propose a theoretical framework for reconstructing tip-surface interactions using the intermodulation technique when more than one eigenmode is required to describe the cantilever motion. Two particular cases of bimodal motion are studied numerically: one bending and one torsional mode, and two bending modes. We demonstrate the possibility of accurate reconstruction of a two-dimensional conservative force field for the former case, while dissipative forces are studied for the latter.

  • 139. Botticella, M. T.
    et al.
    Cappellaro, E.
    Greggio, L.
    Pignata, G.
    Della Valle, M.
    Grado, A.
    Limatola, L.
    Baruffolo, A.
    Benetti, S.
    Bufano, F.
    Capaccioli, M.
    Cascone, E.
    Covone, G.
    De Cicco, D.
    Falocco, Serena
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA.
    Haeussler, B.
    Harutyunyan, V.
    Jarvis, M.
    Marchetti, L.
    Napolitano, N. R.
    Paolillo, M.
    Pastorello, A.
    Radovich, M.
    Schipani, P.
    Tomasella, L.
    Turatto, M.
    Vaccari, M.
    Supernova rates from the SUDARE VST-omegacam search II. Rates in a galaxy sample2017Ingår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 598, artikel-id A50Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Aims. This is the second paper of a series in which we present measurements of the supernova (SN) rates from the SUDARE survey. The aim of this survey is to constrain the core collapse (CC) and Type Ia SN progenitors by analysing the dependence of their explosion rate on the properties of the parent stellar population averaging over a population of galaxies with different ages in a cosmic volume and in a galaxy sample. In this paper, we study the trend of the SN rates with the intrinsic colours, the star formation activity and the masses of the parent galaxies. To constrain the SN progenitors we compare the observed rates with model predictions assuming four progenitor models for SNe Ia with different distribution functions of the time intervals between the formation of the progenitor and the explosion, and a mass range of 8-40 M for CC SN progenitors. Methods. We considered a galaxy sample of approximately 130 000 galaxies and a SN sample of approximately 50 events. The wealth of photometric information for our galaxy sample allows us to apply the spectral energy distribution (SED) fitting technique to estimate the intrinsic rest frame colours, the stellar mass and star formation rate (SFR) for each galaxy in the sample. The galaxies have been separated into star-forming and quiescent galaxies, exploiting both the rest frame U-V vs. V-J colour-colour diagram and the best fit values of the specific star formation rate (sSFR) from the SED fitting. Results. We found that the SN Ia rate per unit mass is higher by a factor of six in the star-forming galaxies with respect to the passive galaxies, identified as such both on the U-V vs. V-J colour-colour diagram and for their sSFR. The SN Ia rate per unit mass is also higher in the less massive galaxies that are also younger. These results suggest a distribution of the delay times (DTD) less populated at long delay times than at short delays. The CC SN rate per unit mass is proportional to both the sSFR and the galaxy mass, confirming that the CC SN progenitors explode soon after the end of the star formation activity. The trends of the Type Ia and CC SN rates as a function of the sSFR and the galaxy mass that we observed from SUDARE data are in agreement with literature results at different redshifts suggesting that the ability of the stellar populations to produce SN events does not vary with cosmic time. The expected number of SNe Ia is in agreement with that observed for all four DTD models considered both in passive and star-forming galaxies so we can not discriminate between different progenitor scenarios. The expected number of CC SNe is higher than that observed, suggesting a higher limit for the minimum progenitor mass. However, at least part of this discrepancy between expected and observed number of CC SNe may reflect a fluctuation due to the relatively poor statistics. We also compare the expected and observed trends of the SN Ia rate with the intrinsic U-J colour of the parent galaxy, assumed to be a tracer of the age distribution. While the slope of the relation between the SN Ia rate and the U-J colour in star-forming galaxies can be well-reproduced by all four DTD models considered, only the steepest of them is able to account for the rates and colour in star-forming and passive galaxies with the same value of the SN Ia production efficiency. The agreement between model predictions and data could be found for the other DTD models, but with a productivity of SN Ia higher in passive galaxies compared to star-forming galaxies.

  • 140. Bourdin, P. -A
    et al.
    Brandenburg, Axel
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA.
    Magnetic Helicity from Multipolar Regions on the Solar Surface2018Ingår i: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 869, nr 1, artikel-id 3Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The emergence of dipolar magnetic features on the solar surface is an idealization. Most of the magnetic flux emergence occurs in complex multipolar regions. Here, we show that the surface pattern of magnetic structures alone can reveal the sign of the underlying magnetic helicity in the nearly force-free coronal regions above. The sign of the magnetic helicity can be predicted to good accuracy by considering the three-dimensional position vectors of three spots on the sphere ordered by their relative strengths at the surface and compute from them the skew product. This product, which is a pseudoscalar, is shown to be a good proxy for the sign of the coronal magnetic helicity.

  • 141. Bourdin, P.
    et al.
    Singh, Nishant K.
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA.
    Brandenburg, Axel
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA.
    Magnetic Helicity Reversal in the Corona at Small Plasma Beta2018Ingår i: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 869, nr 1, artikel-id 2Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Solar and stellar dynamos shed small-scale and large-scale magnetic helicity of opposite signs. However, solar wind observations and simulations have shown that some distance above the dynamo both the small-scale and large-scale magnetic helicities have reversed signs. With realistic simulations of the solar corona above an active region now being available, we have access to the magnetic field and current density along coronal loops. We show that a sign reversal in the horizontal averages of the magnetic helicity occurs when the local maximum of the plasma beta drops below unity and the field becomes nearly fully force free. Hence, this reversal is expected to occur well within the solar corona and would not directly be accessible to in situ measurements with the Parker Solar Probe or SolarOrbiter. We also show that the reversal is associated with subtle changes in the relative dominance of structures with positive and negative magnetic helicity.

  • 142. Boyko, D.
    et al.
    Balatsky, Alexander V.
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA. Los Alamos National Laboratory, United States.
    Haraldsen, J. T.
    Evolution of magnetic Dirac bosons in a honeycomb lattice2018Ingår i: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 97, nr 1, artikel-id 014433Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We examine the presence and evolution of magnetic Dirac nodes in the Heisenberg honeycomb lattice. Using linear spin theory, we evaluate the collinear phase diagram as well as the change in the spin dynamics with various exchange interactions. We show that the ferromagnetic structure produces bosonic Dirac and Weyl points due to the competition between the interactions. Furthermore, it is shown that the criteria for magnetic Dirac nodes are coupled to the magnetic structure and not the overall crystal symmetry, where the breaking of inversion symmetry greatly affects the antiferromagnetic configurations. The tunability of the nodal points through variation of the exchange parameters leads to the possibility of controlling Dirac symmetries through an external manipulation of the orbital interactions.

  • 143.
    Bracco, Andrea
    et al.
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA.
    Candelaresi, S.
    Univ Dundee, Div Math, Dundee DD1 4HN, Scotland..
    Del Sordo, F.
    Univ Crete, FORTH, Phys Dept, Iraklion, Greece..
    Brandenburg, Axel
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA.
    Is there a left-handed magnetic field in the solar neighborhood?: Exploring helical magnetic fields in the interstellar medium through dust polarization power spectra2019Ingår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 621, artikel-id A97Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Context. The analysis of the full-sky Planck polarization data at 850 mu m revealed unexpected properties of the E- and B-mode power spectra of dust emission in the interstellar medium (ISM). The positive cross-correlations over a wide range of angular scales between the total dust intensity, T, and both E and (most of all) B modes has raised new questions about the physical mechanisms that affect dust polarization, such as the Galactic magnetic field structure. This is key both to better understanding ISM dynamics and to accurately describing Galactic foregrounds to the polarization of the cosmic microwave background (CMB). In particular, in the quest to find primordial B modes of the CMB, the observed positive cross-correlation between T and B for interstellar dust requires further investigation towards parity-violating processes in the ISM. Aims. In this theoretical paper we investigate the possibility that the observed cross-correlations in the dust polarization power spectra, and specifically the one between T and B, can be related to a parity-odd quantity in the ISM such as the magnetic helicity. Methods. We produce synthetic dust polarization data, derived from 3D analytical toy models of density structures and helical magnetic fields, to compare with the E and B modes of observations. We present several models. The first is an ideal fully helical isotropic case, such as the Arnold-Beltrami-Childress field. Second, following the nowadays favored interpretation of the T-E signal in terms of the observed alignment between the magnetic field morphology and the filamentary density structure of the diffuse ISM, we design models for helical magnetic fields wrapped around cylindrical interstellar filaments. Lastly, focusing on the observed T-B correlation, we propose a new line of interpretation of the Planck observations advocating the presence of a large-scale helical component of the Galactic magnetic field in the solar neighborhood. Results. Our analysis shows that: I) the sign of magnetic helicity does not affect E and B modes for isotropic magnetic-field configurations; II) helical magnetic fields threading interstellar filaments cannot reproduce the Planck results; and III) a weak helical left-handed magnetic field structure in the solar neighborhood may explain the T-B correlation seen in the Planck data. Such a magnetic-field configuration would also account for the observed large-scale T-E correlation. Conclusions. This work suggests a new perspective for the interpretation of the dust polarization power spectra that supports the imprint of a large-scale structure of the Galactic magnetic field in the solar neighborhood.

  • 144. Bradač, M.
    et al.
    Garcia-Appadoo, D.
    Huang, K. -H
    Vallini, Livia
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA. Dipartimento di Fisica e Astronomia, Italy.
    Finney, E. Q.
    Hoag, A.
    Lemaux, B. C.
    Schmidt, K. B.
    Treu, T.
    Carilli, C.
    Dijkstra, M.
    Ferrara, A.
    Fontana, A.
    Jones, T.
    Ryan, R.
    Wagg, J.
    Gonzalez, A. H.
    ALMA [C II] 158 μm Detection of a Redshift 7 Lensed Galaxy behind RX J1347.1-11452017Ingår i: Astrophysical Journal Letters, ISSN 2041-8205, E-ISSN 2041-8213, Vol. 836, nr 1, artikel-id L2Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We present the results of ALMA spectroscopic follow-up of a z =6.766 Lyα emitting galaxy behind the cluster RX J1347.1-1145. We report the detection of [C ii] 158 μm line fully consistent with the Lyα redshift and with the peak of the optical emission. Given the magnification of μ =5.0 ±0.3, the intrinsic (corrected for lensing) luminosity of the [C ii] line is L [C ii] = 1.4+0.2-0.3 × 107 L⊙, roughly ∼5 times fainter than other detections of z ∼ 7 galaxies. The result indicates that low L [C ii] in z ∼ 7 galaxies compared to the local counterparts might be caused by their low metallicities and/or feedback. The small velocity offset (δv = 20+140-40 Km s-1) between the Lyα and [C ii] line is unusual, and may be indicative of ionizing photons escaping.

  • 145.
    Brandenburg, A.
    et al.
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA. Univ Colorado, Lab Atmospher & Space Phys, Boulder, CO 80303 USA.;Stockholm Univ, SE-10691 Stockholm, Sweden.;Univ Colorado, JILA, Boulder, CO 80303 USA.;Univ Colorado, Dept Astrophys & Planetary Sci, Boulder, CO 80303 USA.;Stockholm Univ, Dept Astron, SE-10691 Stockholm, Sweden..
    Haugen, N. E. L.
    SINTEF Energy Res, N-7465 Trondheim, Norway.;NTNU, Dept Energy & Proc Engn, N-7491 Trondheim, Norway..
    Li, Xiang-Yu
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA. Univ Colorado, Lab Atmospher & Space Phys, Boulder, CO 80303 USA.;Stockholm Univ, SE-10691 Stockholm, Sweden.;Stockholm Univ, Dept Meteorol, Stockholm, Sweden.;Stockholm Univ, Bolin Ctr Climate Res, Stockholm, Sweden..
    Subramanian, K.
    Inter Univ Ctr Astron & Astrophys, Post Bag 4,Pune Univ Campus, Pune 411007, Maharashtra, India..
    Varying the forcing scale in low Prandtl number dynamos2018Ingår i: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 479, nr 2, s. 2827-2833Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Small-scale dynamos are expected to operate in all astrophysical fluids that are turbulent and electrically conducting, for example the interstellar medium, stellar interiors, and accretion discs, where theymay also be affected by or competing with large-scale dynamos. However, the possibility of small-scale dynamos being excited at small and intermediate ratios of viscosity to magnetic diffusivity (the magnetic Prandtl number) has been debated, and the possibility of them depending on the large-scale forcing wavenumber has been raised. Here, we show, using four values of the forcing wavenumber, that the small-scale dynamo does not depend on the scale separation between the size of the simulation domain and the integral scale of the turbulence, i.e. the forcing scale. Moreover, the spectral bottleneck in turbulence, which has been implied as being responsible for raising the excitation conditions of small-scale dynamos, is found to be invariant under changing the forcing wavenumber. However, when forcing at the lowest few wavenumbers, the effective forcing wavenumber that enters in the definition of the magnetic Reynolds number is found to be about twice the minimum wavenumber of the domain. Our work is relevant to future studies of small-scale dynamos, of which several applications are being discussed.

  • 146.
    Brandenburg, Axel
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA. Stockholm Univ, Roslagstullsbacken 23, SE-10691 Stockholm, Sweden.;Stockholm Univ, AlbaNova Univ Ctr, Dept Astron, SE-10691 Stockholm, Sweden.;Univ Colorado, JILA, Boulder, CO 80303 USA.;Univ Colorado, Lab Atmospher & Space Phys, Boulder, CO 80303 USA.;Carnegie Mellon Univ, McWilliams Ctr Cosmol, Pittsburgh, PA 15213 USA.;Carnegie Mellon Univ, Dept Phys, Pittsburgh, PA 15213 USA..
    A Global Two-scale Helicity Proxy from pi-ambiguous Solar Magnetic Fields2019Ingår i: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 883, nr 2, artikel-id 119Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    If the alpha effect plays a role in the generation of the Sun's magnetic field, the field should show evidence of magnetic helicity of opposite signs at large and small length scales. Measuring this faces two challenges: (i) in weak-field regions, horizontal field measurements are unreliable because of the pi ambiguity, and (ii) one needs a truly global approach to computing helicity spectra in the case where one expects a sign reversal across the equator at all wavenumbers. Here we develop such a method using spin-2 spherical harmonics to decompose the linear polarization in terms of the parity-even and parity-odd E and B polarizations, respectively. Using simple one- and two-dimensional models, we show that the product of the spectral decompositions of E and B, taken at spherical harmonic degrees that are shifted by one, can act as a proxy of the global magnetic helicity with a sign that represents that in the northern hemisphere. We then apply this method to the analysis of solar synoptic vector magnetograms, from which we extract a pseudo-polarization corresponding to a "pi-ambiguated" magnetic field, i.e., a magnetic field vector that has no arrow. We find a negative sign of the global EB helicity proxy at spherical harmonic degrees of around 6. This could indicate a positive magnetic helicity at large length scales, but the spectrum fails to capture clear evidence of the well-known negative magnetic helicity at smaller scales. This method might also be applicable to stellar and Galactic polarization data.

  • 147.
    Brandenburg, Axel
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA. Stockholm Univ, Roslagstullsbacken 23, SE-10691 Stockholm, Sweden.;Stockholm Univ, AlbaNova Univ Ctr, Dept Astron, SE-10691 Stockholm, Sweden.;Univ Colorado, JILA, Boulder, CO 80303 USA.;Univ Colorado, Lab Atmospher & Space Phys, Boulder, CO 80303 USA.;Carnegie Mellon Univ, McWilliams Ctr Cosmol, Pittsburgh, PA 15213 USA.;Carnegie Mellon Univ, Dept Phys, Pittsburgh, PA 15213 USA..
    Ambipolar diffusion in large Prandtl number turbulence2019Ingår i: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 487, nr 2, s. 2673-2684Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We study the effects of ambipolar diffusion (AD) on hydromagnetic turbulence. We consider the regime of large magnetic Prandtl number, relevant to the interstellar medium. In most of the cases, we use the single-fluid approximation where the drift velocity between charged and neutral particles is proportional to the Lorentz force. In two cases we also compare with the corresponding two-fluid model, where ionization and recombination are included in the continuity and momentum equations for the neutral and charged species. The magnetic field properties are found to be well represented by the single-fluid approximation. We quantify the effects of AD on total and spectral kinetic and magnetic energies, the ohmic and AD dissipation rates, the statistics of the magnetic field, the current density, and the linear polarization as measured by the rotationally invariant E and B mode polarizations. We show that the kurtosis of the magnetic field decreases with increasing AD. The E mode polarization changes its skewness from positive values for small AD to negative ones for large AD. Even when AD is weak, changes in AD have a marked effect on the skewness and kurtosis of E, and only a weak effect on those of B. These results open the possibility of employing E and B mode polarizations as diagnostic tools for characterizing turbulent properties of the interstellar medium.

  • 148.
    Brandenburg, Axel
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA. University of Colorado, United States.
    Analytic solution of an oscillatory migratory α2 stellar dynamo2017Ingår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 598, artikel-id A117Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Context. Analytic solutions of the mean-field induction equation predict a nonoscillatory dynamo for homogeneous helical turbulence or constant α effect in unbounded or periodic domains. Oscillatory dynamos are generally thought impossible for constant α. Aims. We present an analytic solution for a one-dimensional bounded domain resulting in oscillatory solutions for constant α, but different (Dirichlet and von Neumann or perfect conductor and vacuum) boundary conditions on the two boundaries. Methods. We solve a second order complex equation and superimpose two independent solutions to obey both boundary conditions. Results. The solution has time-independent energy density. On one end where the function value vanishes, the second derivative is finite, which would not be correctly reproduced with sine-like expansion functions where a node coincides with an inflection point. The field always migrates away from the perfect conductor boundary toward the vacuum boundary, independently of the sign of α. Conclusions. The obtained solution may serve as a benchmark for numerical dynamo experiments and as a pedagogical illustration that oscillatory migratory dynamos are possible with constant α.

  • 149.
    Brandenburg, Axel
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA. Stockholm Univ, KTH Royal Inst Technol, NORDITA, Stockholm, Sweden..
    Magnetic helicity and fluxes in an inhomogeneous alpha(2) dynamo2018Ingår i: Astronomical Notes - Astronomische Nachrichten, ISSN 0004-6337, E-ISSN 1521-3994, Vol. 339, nr 9-10, s. 631-640Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Much work on turbulent three-dimensional dynamos has been done using triply periodic domains, in which there are no magnetic helicity fluxes. Here, we present simulations where the turbulent intensity is still nearly homogeneous, but now there is a perfect conductor boundary condition on one end and a vertical field or pseudovacuum conditions on the other. This leads to migratory dynamo waves. Good agreement with a corresponding analytically solvable alpha(2) dynamo is found. Magnetic helicity fluxes are studied in both types of models. It is found that at moderate magnetic Reynolds numbers, most of the magnetic helicity losses occur on large scales. Whether this changes at even larger magnetic Reynolds numbers, as required for alleviating the catastrophic dynamo quenching problem, remains stillunclear.

  • 150.
    Brandenburg, Axel
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA. Stockholm University, Sweden.
    Magnetic Prandtl number dependence of the kinetic-to-magnetic dissipation ratio2014Ingår i: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 791, nr 1, artikel-id 12Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Using direct numerical simulations of three-dimensional hydromagnetic turbulence, either with helical or non-helical forcing, we show that the kinetic-to-magnetic energy dissipation ratio always increases with the magnetic Prandtl number, i.e., the ratio of kinematic viscosity to magnetic diffusivity. This dependence can always be approximated by a power law, but the exponent is not the same in all cases. For non-helical turbulence, the exponent is around 1/3, while for helical turbulence it is between 0.6 and 2/3. In the statistically steady state, the rate of energy conversion from kinetic into magnetic by the dynamo must be equal to the Joule dissipation rate. We emphasize that for both small-scale and large-scale dynamos, the efficiency of the energy conversion depends sensitively on the magnetic Prandtl number, and thus on the microphysical dissipation process. To understand this behavior, we also study shell models of turbulence and one-dimensional passive and active scalar models. We conclude that the magnetic Prandtl number dependence is qualitatively best reproduced in the one-dimensional model as a result of dissipation via localized Alfvén kinks.

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