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Parameters for aperture calculations at injection for HL-LHC*
https://cds.cern.ch/record/2237427
Accurate evaluations of the margins of available aperture in the LHC and HL-LHC are very important, in order to judge if proposed optics and hardware are adequate, and to push the machine performance. A 2D calculation model was used during the design stage to study the aperture margins, however, the parameters of the model can now be refined based on LHC measurements and operational experience. This has already been carried out for the triplet aperture in the experimental insertions during physics operation [1]. In this report, we study instead the parameter sets for aperture calculations at injection for HL-LHC, and provide an updated set of tolerances as well as a criterion for the allowed aperture.Bruce, RoderikBracco, ChiaraDe Maria, RiccardoGiovannozzi, MassimoRedaelli, StefanoTomas Garcia, RogelioVelotti, Francesco MariaWenninger, JorgFri, 02 Dec 2016 16:20:28 GMT05 Feb 2016CERN-ACC-2016-0328https://cds.cern.ch/record/223742720160328The Polar Nano Regions $\rightleftharpoons$ Relaxor Transition in $Pb_{1-X}(Sc_{1/2}Nb_{1/2})O_{3-X}$; $X$ = bulk concentration of nearest neighbor [Pb-O] divacancies
https://cds.cern.ch/record/2237320
In previous work, molecular dynamics simulations based on a first-principles-derived effective Hamiltonian for $Pb_{1-X}(Sc_{1/2}Nb_{1/2})O_{3-X}$~ (PSN), with nearest-neighbor Pb-O divacancy pairs, was used to calculate $X_{\rm [Pb-O]}$~vs.~T, phase diagrams for PSN with: ideal rock-salt type chemical order; nanoscale chemical short-range order; and random chemical disorder. Here, we show that the phase diagrams should include additional regions in which a glassy relaxor-phase (or state) is predicted. With respect to phase diagram topology, these results strongly support the analogy between relaxors and magnetic spin-glass-systems.Burton, B PCockayne, EricGopman, D BDogan, GunayHood, SarahFri, 02 Dec 2016 12:00:05 GMT01 Dec 2016arXiv:1612.00398https://cds.cern.ch/record/2237320['arXiv:1612.00398']arXiv:1612.00398(Non)-universality of vortex reconnections in superfluids
https://cds.cern.ch/record/2237319
An insight into vortex reconnections in superfluids is presented making use of analytical results and numerical simulations of the Gross--Pitaevskii model. Universal aspects of the reconnection process are investigated by considering different initial vortex configurations and making use of a recently developed tracking algorithm to reconstruct the vortex filaments. We show that about the reconnection event the vortex lines approach and separate always accordingly to the time scaling $ \delta \sim t^{-1/2} $ with pre-factors that depend on the vortex configuration. We also investigate the behavior of curvature and torsion close to the reconnection point, demonstrating analytically that the curvature can exhibit a self-similar behavior that might be broken by the development of shock-like structures in the torsion.Villois, AlbertoKrstulovic, GiorgioProment, DavideFri, 02 Dec 2016 12:00:05 GMT01 Dec 2016arXiv:1612.00386https://cds.cern.ch/record/2237319['arXiv:1612.00386']arXiv:1612.00386Theory of Multiwave Mixing within the Superconducting Kinetic-Inductance Traveling-Wave Amplifier
https://cds.cern.ch/record/2237318
We present a theory of parametric mixing within the coplanar waveguide (CPW) of a superconducting nonlinear kinetic-inductance traveling-wave (KIT) amplifier engineered with periodic dispersion loadings. This is done by first developing a metamaterial band theory of the dispersion-engineered KIT using a Floquet-Bloch construction and then applying it to the description of mixing of the nonlinear RF traveling waves. Our theory allows us to calculate signal gain vs. signal frequency in the presence of a frequency stop gap, based solely on loading design. We present results for both three-wave mixing (3WM), with applied DC bias, and four-wave mixing (4WM), without DC. Our theory predicts an intrinsic and deterministic origin to undulations of 4WM signal gain with signal frequency, apart from extrinsic sources, such as impedance mismatch, and shows that such undulations are absent from 3WM signal gain achievable with DC. Our theory is extensible to amplifiers based on Josephson junctions in a lumped LC transmission line (TWPA).Erickson, Robert PPappas, David PFri, 02 Dec 2016 12:00:05 GMT21 Nov 2016arXiv:1612.00365https://cds.cern.ch/record/2237318['arXiv:1612.00365']arXiv:1612.00365Single-shot measurement of the orbital-angular-momentum spectrum of partially coherent fields
https://cds.cern.ch/record/2237317
We propose an image inversion-based interferometric technique for measuring the orbital-angular- momentum (OAM) spectrum of a field that is an incoherent mixture of OAM eigenstates. The tech- nique directly encodes the angular correlation function of the input field in the azimuthal intensity profile of the output interferogram. We show that for an input field with a symmetric spectrum, a single-shot acquisition of this interferogram is sufficient to measure its OAM spectrum. We first present a proof-of-concept experimental demonstration of this technique for laboratory-produced partially coherent fields with known OAM spectra. We then use this technique to measure the intrinsic OAM spectrum of the signal-idler field from parametric down-conversion over an unprece- dented range of more than 150 modes. This technique therefore surpasses all other existing mea- surement schemes in terms of efficiency and spectral range, and can have significant implications for high-dimensional optical information processing protocols based on the OAM degree of freedom.Kulkarni, GirishSahu, RishabhMagana-Loaiza, Omar SBoyd, Robert WJha, Anand KFri, 02 Dec 2016 12:00:05 GMT01 Dec 2016arXiv:1612.00335https://cds.cern.ch/record/2237317['arXiv:1612.00335']arXiv:1612.00335The Tyranny of Data? The Bright and Dark Sides of Data-Driven Decision-Making for Social Good
https://cds.cern.ch/record/2237316
The unprecedented availability of large-scale human behavioral data is profoundly changing the world we live in. Researchers, companies, governments, financial institutions, non-governmental organizations and also citizen groups are actively experimenting, innovating and adapting algorithmic decision-making tools to understand global patterns of human behavior and provide decision support to tackle problems of societal importance. In this chapter, we focus our attention on social good decision-making algorithms, that is algorithms strongly influencing decision-making and resource optimization of public goods, such as public health, safety, access to finance and fair employment. Through an analysis of specific use cases and approaches, we highlight both the positive opportunities that are created through data-driven algorithmic decision-making, and the potential negative consequences that practitioners should be aware of and address in order to truly realize the potential of this emergent field. We elaborate on the need for these algorithms to provide transparency and accountability, preserve privacy and be tested and evaluated in context, by means of living lab approaches involving citizens. Finally, we turn to the requirements which would make it possible to leverage the predictive power of data-driven human behavior analysis while ensuring transparency, accountability, and civic participation.Lepri, BrunoStaiano, JacopoSangokoya, DavidLetouze, EmmanuelOliver, NuriaFri, 02 Dec 2016 12:00:05 GMT01 Dec 2016arXiv:1612.00323https://cds.cern.ch/record/2237316['arXiv:1612.00323']arXiv:1612.00323First evidence of end-cap control in the synthesis of long-chain polyynes by intense ultrashort laser pulse irradiation
https://cds.cern.ch/record/2237315
Hydrogen- and methyl-capped polyynes were synthesized by irradiating pure liquid toluene with 35 femtosecond, 300 {\mu}J laser pulses having a central wavelength of 800 nm, generated by a regeneratively amplified Ti:sapphire tabletop laser at a repetition rate of 1 kHz. Raman spectroscopy was used to confirm the presence of polyynes in the irradiated samples while high-performance liquid chromatography was used to separate hydrogen-capped polyynes up to C18H2 and methyl-capped polyynes up to HC14CH3. These represent the first such methyl-capped polyynes and the longest hydrogen capped chains synthesized to date by the ultrafast laser based method. Furthermore our results show that choice of the starting solvent molecule directly influences the end caps of the polyynes which can be produced.Ramadhan, AliWesolowski, MichalWakabayashi, TomonariShiromaru, HaruoFujino, TatsuyaKodama, TakeshiDuley, WalterSanderson, JosephFri, 02 Dec 2016 12:00:05 GMT01 Dec 2016arXiv:1612.00320https://cds.cern.ch/record/2237315['arXiv:1612.00320']arXiv:1612.00320Clustering and preferential concentration of finite-size particles in forced homogeneous-isotropic turbulence
https://cds.cern.ch/record/2237314
We have performed interface-resolved direct numerical simulations of forced homogeneous-isotropic turbulence in a dilute suspension of spherical particles in the Reynolds number range Re-lambda=115-140. The solid-fluid density ratio was set to 1.5, gravity was set to zero, and two particle diameters were investigated corresponding to approximately 5 and 11 Kolmogorov lengths. Note that these particle sizes are clearly outside the range of validity of the point-particle approximation, as has been shown by Homann & Bec (2010). At the present parameter points the global effect of the particles upon the fluid flow is weak. We observe that the dispersed phase exhibits clustering with moderate intensity. The tendency to cluster, which was quantified in terms of the standard deviation of Voronoi cell volumes, decreases with the particle diameter. We have analyzed the relation between particle locations and the location of intense vortical flow structures. The results do not reveal any significant statistical correlation. Contrarily, we have detected a small but statistically significant preferential location of particles with respect to the `sticky points' proposed by Goto & Vassilicos (2008), i.e. points where the fluid acceleration field is acting such as to increase the local particle concentration in one-way coupled point-particle models under Stokes drag. The presently found statistical correlation between the `sticky points' and the particle locations further increases when focusing on regions with high local concentration. Our results suggest that small finite-size particles can be brought together along the expansive directions of the fluid acceleration field, as previously observed only for the simplest model for sub-Kolmogorov particles. We further discuss the effect of density ratio and collective particle motion upon the basic Eulerian and Lagrangian statistics.Uhlmann, MarkusChouippe, AgatheFri, 02 Dec 2016 12:00:04 GMT01 Dec 2016arXiv:1612.00318https://cds.cern.ch/record/2237314['arXiv:1612.00318']arXiv:1612.00318Interactions in an acoustic world
https://cds.cern.ch/record/2237311
The present paper aims to complete an earlier paper where the acoustic world was introduced. This is accomplished by analyzing the interactions which occur between the inhomogeneities of the acoustic medium, which are induced by the acoustic vibrations traveling in the medium. When a wave packet travels in a medium, the medium becomes inhomogeneous. The spherical wave packet behaves like an acoustic spherical lens for the acoustic plane waves. According to the principle of causality, there is an interaction between the wave and plane wave packet. In specific conditions the wave packet behaves as an acoustic black hole.Simaciu, IonDumitrescu, GheorgheBorsos, ZoltanBradac, MarianaFri, 02 Dec 2016 12:00:04 GMT28 Nov 2016arXiv:1612.00294https://cds.cern.ch/record/2237311['arXiv:1612.00294']arXiv:1612.00294Comment on 'Attempts to test an alternative electrodynamic theory of superconductors by low-temperature scanning tunneling and atomic force microscopy'
https://cds.cern.ch/record/2237310
It is argued that alternative electrodynamics of superconductivity proposed by Hirsch lacks mathematical rigour and it is conceptually flawed. Gauge non-invariance of the theory makes justification of the experiment reported in [arXiv:1607.05060] to test its prediction doubtful. It seems even with improved techniques the outcome would remain inconclusive.Tiwari, S CFri, 02 Dec 2016 12:00:04 GMT28 Nov 2016arXiv:1612.00293https://cds.cern.ch/record/2237310['arXiv:1612.00293']arXiv:1612.00293On Cracking of Charged Anisotropic Polytropes
https://cds.cern.ch/record/2237309
Recently in \cite{34}, the role of electromagnetic field on the cracking of spherical polytropes has been investigated without perturbing charge parameter explicitly. In this study, we have examined the occurrence of cracking of anisotropic spherical polytropes through perturbing parameters like anisotropic pressure, energy density and charge. We consider two different types of polytropes in this study. We discuss the occurrence of cracking in two different ways $(i)$ by perturbing polytropic constant, anisotropy and charge parameter $(ii)$ by perturbing polytropic index, anisotropy and charge parameter for each case. We conclude that cracking appears for a wide range of parameters in both cases. Also, our results are reduced to \cite{33} in the absence of charge.Azam, MMardan, S AFri, 02 Dec 2016 12:00:04 GMT28 Nov 2016arXiv:1612.00290https://cds.cern.ch/record/2237309['arXiv:1612.00290']arXiv:1612.00290A testable prediction from entropic gravity
https://cds.cern.ch/record/2237308
I have shown conceptually that quantum state has a direct relationship to gravitational constant due to entropic force posed by Verlinde's argument and part of the Newton-Schr\"odinger equation (N-S) in the context of gravity induced collapse of the wavefunction via Di\'osi-Penrose proposal. This direct relationship can be used to measure gravitational constant using state-of-the-art mater-wave interferometry to test the entropic gravity argument.Süzen, MehmetFri, 02 Dec 2016 12:00:04 GMT26 Nov 2016arXiv:1612.00288https://cds.cern.ch/record/2237308['arXiv:1612.00288']arXiv:1612.00288Predicting the rise of right-wing populism in response to unbalanced immigration
https://cds.cern.ch/record/2237306
Among the central tenets of globalization is free migration of labor. Although much has been written about its benefits, little is known about the limitations of globalization, including how immigration affects the anti-globalist sentiment. Analyzing polls data, we find that over the last three years in a group of EU countries affected by the recent migrant crisis, the percentage of right-wing (RW) populist voters in a given country depends on the prevalence of immigrants in this country's population and the total immigration inflow into the entire EU. The latter is likely due to the EU resembling a supranational state, where the lack of inner borders causes that "somebody else's problem" easily turns into "my problem". We further find that the increase in the percentage of RW voters substantially surpasses the immigration inflow, implying that if this process continues, RW populism may democratically prevail and eventually lead to a demise of globalization. We present evidence for tipping points in relation to the rise of RW populism. Finally, we model these empirical findings using a complex network framework wherein the success of globalization largely rests on the balance between immigration and immigrant integration.Podobnik, BorisJusup, MarkoStanley, H EugeneFri, 02 Dec 2016 12:00:04 GMT01 Dec 2016arXiv:1612.00270https://cds.cern.ch/record/2237306['arXiv:1612.00270']arXiv:1612.00270Optical Neutrality: Invisibility without Cloaking
https://cds.cern.ch/record/2237305
We show that it is possible to design an invisible wavelength-sized metal-dielectric metamaterial object without evoking cloaking. Our approach is an extension of the neutral inclusion concept by Zhou and Hu [Phys.Rev.E 74, 026607 (2006)] to Mie scatterers. We demonstrate that an increase of metal fraction in the metamaterial leads to a transition from dielectric-like to metal-like scattering, which proceeds through invisibility or optical neutrality of the scatterer. Formally this is due to cancellation of multiple scattering orders, similarly to plasmonic cloaking introduced by Alu and Engheta [Phys.Rev.E 72, 016623 (2005)], but without introduction of the separation of the scatterer into cloak and hidden regions.Hodges, ReedDean, CleonDurach, MaximFri, 02 Dec 2016 12:00:04 GMT01 Dec 2016arXiv:1612.00268https://cds.cern.ch/record/2237305['arXiv:1612.00268']arXiv:1612.00268Kassiopeia: A Modern, Extensible C++ Particle Tracking Package
https://cds.cern.ch/record/2237304
The Kassiopeia particle tracking framework is an object-oriented software package using modern C++ techniques, written originally to meet the needs of the KATRIN collaboration. Kassiopeia features a new algorithmic paradigm for particle tracking simulations which targets experiments containing complex geometries and electromagnetic fields, with high priority put on calculation efficiency, customizability, extensibility, and ease of use for novice programmers. To solve Kassiopeia's target physics problem the software is capable of simulating particle trajectories governed by arbitrarily complex differential equations of motion, continuous physics processes that may in part be modeled as terms perturbing that equation of motion, stochastic processes that occur in flight such as bulk scattering and decay, and stochastic surface processes occuring at interfaces, including transmission and reflection effects. This entire set of computations takes place against the backdrop of a rich geometry package which serves a variety of roles, including initialization of electromagnetic field simulations and the support of state-dependent algorithm-swapping and behavioral changes as a particle's state evolves. Thanks to the very general approach taken by Kassiopeia it can be used by other experiments facing similar challenges when calculating particle trajectories in electromagnetic fields. It is publicly available at https://github.com/KATRIN-Experiment/KassiopeiaFurse, DanielGroh, StefanTrost, NikolausBabutzka, MartinBarrett, John PBehrens, JanBuzinksy, NicholasCorona, ThomasEnomoto, SanshiroErhard, MoritzFormaggio, Joseph AGlück, FerencHarms, FabianHeizmann, FlorianHilk, DanielKäfer, WolfgangKleesiek, MarcoLeiber, BenjaminMertens, SusanneOblath, Noah SRenschler, PascalSchwarz, JohannesSlocum, Penny LWandkowsky, NancyWierman, KevinZacher, MichaelFri, 02 Dec 2016 12:00:03 GMT01 Dec 2016arXiv:1612.00262https://cds.cern.ch/record/2237304['arXiv:1612.00262']arXiv:1612.00262Moment fluid equations for ions in weakly-ionized plasma
https://cds.cern.ch/record/2237303
A new one-dimensional fluid model for ions in weakly-ionized plasma is proposed. The model differs from the existing ones in two aspects. First, a more accurate approximation of the collision terms in the fluid equations is suggested. For this purpose, the results obtained using the Monte-Carlo kinetic model of the ion swarm experiments are considered. Second, the ion energy equation is taken into account. The fluid equations are closed using a simple model of the ion velocity distribution function. The accuracy of the fluid model is examined by comparing with the results of particle-in-cell/Monte Carlo simulations. In particular, several test problems are considered using a parallel plate model of the capacitively coupled radio-frequency discharge. It is shown that the results obtained using the proposed fluid model are in good agreement with those obtained from the simulations over a wide range of discharge conditions. An approximation of the ion velocity distribution function for the problem under consideration is also discussed.Semenov, I LFri, 02 Dec 2016 12:00:03 GMT01 Dec 2016arXiv:1612.00257https://cds.cern.ch/record/2237303['arXiv:1612.00257']arXiv:1612.00257Effects of spanwise rotation on turbulent channel flow
https://cds.cern.ch/record/2237302
A study of fully developed plane turbulent channel flow subject to spanwise system rotation through direct numerical simulations is presented. In order to study both the influence of the Reynolds number and spanwise rotation on channel flow, the Reynolds number $Re = U_b h/\nu$ is varied between 5000 and $31\,600$ and the rotation number $Ro = 2 \Omega h/U_b$ is varied between 0 and 2.7, where $U_b$ is the mean bulk velocity, $h$ the channel half gap and $\Omega$ the system rotation rate. The mean streamwise velocity profile displays also at higher $Re$ the characteristic linear part with a slope near to $2 \Omega$ and a corresponding linear part in the profiles of the production and dissipation rate of turbulent kinetic energy appears. With increasing $Ro$ a distinct unstable side with large spanwise and wall-normal Reynolds stresses and a stable side with much weaker turbulence develops in the channel. The flow starts to relaminarize on the stable side of the channel and persisting turbulent-laminar patterns appear at higher $Re$. If $Ro$ is further increased the flow on the stable becomes laminar-like while at yet higher $Ro$ the whole flow relaminarizes, although the calm periods might be disrupted by repeating bursts of turbulence, as explained by Brethouwer (2016). The influence of the Reynolds number is notable, in particular on the stable side of the channel where velocity fluctuations are stronger at higher $Re$. Visualizations and spectra show that at lower $Ro$ large counter rotating streamwise roll cells develop on the unstable side. These become less noticeable and disappear when raising $Ro$, especially at higher $Re$. Large-scale structures are present on the unstable side at lower $Ro$ according to spectra, but these structures vanish if $Ro$ is raised.Brethouwer, GeertFri, 02 Dec 2016 12:00:03 GMT01 Dec 2016arXiv:1612.00254https://cds.cern.ch/record/2237302['arXiv:1612.00254']arXiv:1612.00254Neutrons production on the IPHI accelerator for the validation of the design of the compact neutron source SONATE
https://cds.cern.ch/record/2237300
We aim at building an accelerator based compact neutron source which would provide a thermal neutron flux on the order of 4E12 n.s-1.cm-2.sr-1. Such brilliance would put compact neutron sources on par with existing medium flux neutron research reactors. We performed the first neutron production tests on the IPHI proton accelerator at Saclay. The neutron fluxes were measured using gold foil activation and 3He detectors. The measured fluxes were compared with MCNP and GEANT4 Monte Carlo simulations in which the whole experimental setup was modelled. There is a good agreement between the experimental measurements and the Monte-Carlo simulations. The available modelling tools will allow us to optimize the whole Target Moderator Reflector assembly together with the neutron scattering spectrometer geometries.Menelle, AlainOtt, FredericPrunes, FabienHomatter, BenoitAnnighöfer, BurkhardPorcher, FlorenceChauvin, NicolasSchwindling, JérômeLetourneau, AlainMarchix, AnthonyTran, Ngoc-HoangFri, 02 Dec 2016 12:00:03 GMT01 Dec 2016arXiv:1612.00237https://cds.cern.ch/record/2237300['arXiv:1612.00237']arXiv:1612.00237Performances of Neutron Scattering Spectrometers on a Compact Neutron Source
https://cds.cern.ch/record/2237299
There is currently a big effort put into the operation and construction of world class neutron scattering facilities (SNS and SNS-TS2 in the US, J-PARC in Japan, ESS in Europe, CSS in China, PIK in Russia). On the other hand, there exists a network of smaller neutron scattering facilities which play a key role in creating a large neutron scattering community who is able to efficiently use the existing facilities. With the foreseen closure of the ageing nuclear research reactors, especially in Europe there is a risk of seeing a shrinking of the community who would then be able to use efficiently the world class facilities. There is thus a reflection being conducted in several countries for the replacement of smaller research reactors with low energy accelerator based sources. We consider here a reference design for a compact neutron source based on existing accelerator components. We estimate the performances of various types of neutron scattering instruments built around such a source. The results suggest that nowadays state of the art neutron scattering experiments could be successfully performed on such a compact source and that it is thus a viable replacement solution for neutron research reactors.Fabrèges, XavierMenelle, AlainOtt, FrédéricChauvin, NicolasSchwindling, JérômeLetourneau, AlainMarchix, AnthonyFri, 02 Dec 2016 12:00:03 GMT01 Dec 2016arXiv:1612.00232https://cds.cern.ch/record/2237299['arXiv:1612.00232']arXiv:1612.00232CC2 oscillator strengths within the local framework for calculating excitation energies (LoFEx)
https://cds.cern.ch/record/2237298
In a recent work [Baudin and Kristensen, J. Chem. Phys. 144, 224106 (2016)], we introduced a local framework for calculating excitation energies (LoFEx), based on second-order approximated coupled cluster (CC2) linear-response theory. LoFEx is a black-box method in which a reduced excitation orbital space (XOS) is optimized to provide coupled cluster (CC) excitation energies at a reduced computational cost. In this article, we present an extension of the LoFEx algorithm to the calculation of CC2 oscillator strengths. Two different strategies are suggested, in which the size of the XOS is determined based on the excitation energy or the oscillator strength of the targeted transitions. The two strategies are applied to a set of medium-sized organic molecules in order to assess both the accuracy and the computational cost of the methods. The results show that CC2 excitation energies and oscillator strengths can be calculated at a reduced computational cost, provided that the targeted transitions are local compared to the size of the molecule. To illustrate the potential of LoFEx for large molecules, both strategies have been successfully applied to the lowest transition of the bivalirudin molecule (4255 basis functions) and compared with time-dependent density functional theory.Baudin, PabloKjærgaard, ThomasKristensen, KasperFri, 02 Dec 2016 12:00:03 GMT01 Dec 2016arXiv:1612.00230https://cds.cern.ch/record/2237298['arXiv:1612.00230']arXiv:1612.00230Plasma Dipole Oscillation Excited by Trapped Electrons Leading to Bursts of Coherent Radiation
https://cds.cern.ch/record/2237297
Plasma dipole oscillation (PDO) depicted as harmonic motion of a spatially localized block of electrons has, until now, been hypothetical. In practice, the plasma oscillation occurs always as a part of a plasma wave. Studies on radiation burst from plasmas have focused only on coupling of the plasma wave and electromagnetic wave. Here we show that a very-high-field PDO can be generated by the electrons trapped in a moving train of potential wells. The electrons riding on the potential train coherently construct a local dipole moment by charge separation. The subsequent PDO is found to persist stably until its energy is emitted entirely via coherent radiation. In our novel method, the moving potentials are provided by two slightly-detuned laser pulses colliding in a non-magnetized plasma. The radiated energy reaches several millijoules in the terahertz spectral region. The proposed method provides a way of realizing the PDO as a new radiation source in the laboratory. PDO as a mechanism of astrophysical radio-bursts is discussed.Kwon, Kyu BeenKang, TeyounSong, Hyung SeonKim, Young-KukErsfeld, BernhardJaroszynski, Dino AHur, Min SupFri, 02 Dec 2016 12:00:03 GMT01 Dec 2016arXiv:1612.00229https://cds.cern.ch/record/2237297['arXiv:1612.00229']arXiv:1612.00229Torsional and rotational coupling in non-rigid molecules
https://cds.cern.ch/record/2237296
We analyze theoretically the interplay between the torsional and the rotational motion of an aligned biphenyl-like molecule. To do so, we consider a transition between two electronic states with different internal torsional potentials, induced by means of a resonant laser pulse. The change in the internal torsional potential provokes the motion of the torsional wavepacket in the excited electronic state, modifying the structure of the molecule, and hence, its inertia tensor. We find that this process has a strong impact on the rotational wave function, displaying different behavior depending on the electronic states involved and their associated torsional potentials. We describe the dynamics of the system by considering the degree of alignment and the expectations values of the angular momentum operators for the overall rotation of the molecule.Omiste, Juan JMadsen, Lars BojerFri, 02 Dec 2016 12:00:03 GMT01 Dec 2016arXiv:1612.00224https://cds.cern.ch/record/2237296['arXiv:1612.00224']arXiv:1612.00224Field- and damping-like spin-transfer torque in magnetic multilayers
https://cds.cern.ch/record/2237294
We investigate the spin-transfer torque in a magnetic multilayer structure by means of a spin-diffusion model. The torque in the considered system, consisting of two magnetic layers separated by a conducting layer, is caused by a perpendicular-to-plane current. We compute the strength of the field-like and the damping-like torque for different material parameters and geometries. Our studies suggest that the field-like torque highly depends on the exchange coupling strength of the itinerant electrons with the magnetization both in the pinned and the free layer. While a low coupling leads to very high field-like torques, a high coupling leads to low or even negative field-like torques. The dependence of the different torque terms on system parameters is considered very important for the development of applications such as STT MRAM and spin-torque oscillators.Abert, ClaasSepehri-Amin, HosseinBruckner, FlorianVogler, ChristophHayashi, MasamitsuSuess, DieterFri, 02 Dec 2016 12:00:03 GMT01 Dec 2016arXiv:1612.00194https://cds.cern.ch/record/2237294['arXiv:1612.00194']arXiv:1612.00194Learning Potential Energy Landscapes using Graph Kernels
https://cds.cern.ch/record/2237293
Recent machine learning methods make it possible to model potential energy of atomic configurations with chemical-level accuracy (as calculated from ab-initio calculations) and at speeds suitable for molecular dynamics simulation. Best performance is achieved when the known physical constraints are encoded in the machine learning models. For example, the atomic energy is invariant under global translations and rotations; it is also invariant to permutations of same-species atoms. Although simple to state, these symmetries are complicated to encode into machine learning algorithms. In this paper, we present a machine learning approach based on graph theory that naturally incorporates translation, rotation, and permutation symmetries. Specifically, we use a random walk graph kernel to measure the similarity of two adjacency matrices, each of which represents a local atomic environment. We show on a standard benchmark that our Graph Approximated Energy (GRAPE) method is competitive with state of the art kernel methods. Furthermore, the GRAPE framework is flexible and admits many possible extensions.Ferré, GHaut, TBarros, KFri, 02 Dec 2016 12:00:03 GMT01 Dec 2016arXiv:1612.00193https://cds.cern.ch/record/2237293['arXiv:1612.00193']arXiv:1612.00193Encoding Orbital Angular Momentum of Lights in Magnets
https://cds.cern.ch/record/2237292
Breaking the diffraction limit and focusing laser beams to nanometre-scale are becoming possible with the help of recent developments in plasmonics. This allows us to explore nano-scale physics induced by laser beams with complex spatial profiles like optical vortex, or laser carrying orbital angular momentum (OAM). On the basis of numerical calculations using Landau-Lifshitz-Gilbert equation, here we propose two OAM-dependent phenomena induced by optical vortex, spin waves with multipolar and spiral wave fronts, and ultrafast generation of various kinds of topological defects in chiral magnets. We show that by using Tera Hz optical vortex, we can generate skyrmionic defects with timescale orders of magnitude shorter than by other known schemes.Fujita, HiroyukiSato, MasahiroFri, 02 Dec 2016 12:00:02 GMT01 Dec 2016arXiv:1612.00176https://cds.cern.ch/record/2237292['arXiv:1612.00176']arXiv:1612.00176