| 1. |
- Cabrera Pacheco, A. J., et al.
(author)
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Asymptotically flat extensions of CMC Bartnik data
- 2017
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In: Classical and quantum gravity. - : Institute of Physics Publishing. - 0264-9381 .- 1361-6382. ; 34:10
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Journal article (peer-reviewed)abstract
- Let g be a metric on the 2-sphere with positive Gaussian curvature and H be a positive constant. Under suitable conditions on (g, H), we construct smooth, asymptotically flat 3-manifolds M with non-negative scalar curvature, with outer-minimizing boundary isometric to and having mean curvature H, such that near infinity M is isometric to a spatial Schwarzschild manifold whose mass m can be made arbitrarily close to a constant multiple of the Hawking mass of . Moreover, this constant multiplicative factor depends only on (g, H) and tends to 1 as H tends to 0. The result provides a new upper bound of the Bartnik mass associated with such boundary data.
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| 2. |
- Chico, Jonathan, et al.
(author)
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First-principles studies of the Gilbert damping and exchange interactions for half-metallic Heuslers alloys
- 2016
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In: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 93:21
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Journal article (peer-reviewed)abstract
- Heusler alloys have been intensively studied due to the wide variety of properties that they exhibit. One of these properties is of particular interest for technological applications, i.e., the fact that some Heusler alloys are half-metallic. In the following, a systematic study of the magnetic properties of three different Heusler families Co(2)MnZ, Co(2)FeZ, and Mn(2)VZ with Z = (Al, Si, Ga, Ge) is performed. A key aspect is the determination of the Gilbert damping from first-principles calculations, with special focus on the role played by different approximations, the effect that substitutional disorder and temperature effects. Heisenberg exchange interactions and critical temperature for the alloys are also calculated as well as magnon dispersion relations for representative systems, the ferromagnetic Co2FeSi and the ferrimagnetic Mn2VAl. Correlation effects beyond standard density-functional theory are treated using both the local spin density approximation including the Hubbard U and the local spin density approximation plus dynamical mean field theory approximation, which allows one to determine if dynamical self-energy corrections can remedy some of the inconsistencies which were previously reported for these alloys.
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| 3. |
- Etz, Corina, et al.
(author)
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Atomistic spin dynamics and surface magnons
- 2015
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In: Journal of Physics. - : IOP Publishing. - 0953-8984 .- 1361-648X. ; 27:24
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Research review (peer-reviewed)abstract
- Atomistic spin dynamics simulations have evolved to become a powerful and versatile tool for simulating dynamic properties of magnetic materials. It has a wide range of applications, for instance switching of magnetic states in bulk and nano-magnets, dynamics of topological magnets, such as skyrmions and vortices and domain wall motion. In this review, after a brief summary of the existing investigation tools for the study of magnons, we focus on calculations of spin-wave excitations in low-dimensional magnets and the effect of relativistic and temperature effects in such structures. In general, we find a good agreement between our results and the experimental values. For material specific studies, the atomistic spin dynamics is combined with electronic structure calculations within the density functional theory from which the required parameters are calculated, such as magnetic exchange interactions, magnetocrystalline anisotropy, and Dzyaloshinskii-Moriya vectors.
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| 4. |
- Karlsson, Dennis, et al.
(author)
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Binder jetting of the AlCoCrFeNi alloy
- 2019
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In: Additive Manufacturing. - : Elsevier. - 2214-8604 .- 2214-7810. ; 27, s. 72-79
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Journal article (peer-reviewed)abstract
- High density components of an AlCoCrFeNi alloy, often described as a high-entropy alloy, were manufactured by binder jetting followed by sintering. Thermodynamic calculations using the CALPHAD approach show that the high-entropy alloy is only stable as a single phase in a narrow temperature range below the melting point. At all other temperatures, the alloy will form a mixture of phases, including a sigma phase, which can strongly influence the mechanical properties. The phase stabilities in built AlCoCrFeNi components were investigated by comparing the as-sintered samples with the post-sintering annealed samples at temperatures between 900 °C and 1300 °C. The as-sintered material shows a dominant B2/bcc structure with additional fcc phase in the grain boundaries and sigma phase precipitating in the grain interior. Annealing experiments between 1000 °C and 1100 °C inhibit the sigma phase and only a B2/bcc phase with a fcc phase is observed. Increasing the temperature further suppresses the fcc phase in favor for the B2/bcc phases. The mechanical properties are, as expected, dependent on the annealing temperature, with the higher annealing temperature giving an increase in yield strength from 1203 MPa to 1461 MPa and fracture strength from 1996 MPa to 2272 MPa. This can be explained by a hierarchical microstructure with nano-sized precipitates at higher annealing temperatures. The results enlighten the importance of microstructure control, which can be utilized in order to tune the mechanical properties of these alloys. Furthermore, an excellent oxidation resistance was observed with oxide layers with a thickness of less than 5 μm after 20 h annealing at 1200 °C, which would be of great importance for industrial applications.
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| 5. |
- McCormick, Stephen, et al.
(author)
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On a Penrose-Like Inequality in Dimensions Less than Eight
- 2019
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In: International mathematics research notices. - : Oxford University Press. - 1073-7928 .- 1687-0247. ; 2019:7, s. 2069-2084
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Journal article (peer-reviewed)abstract
- On an asymptotically flat manifold M-n with nonnegative scalar curvature, with outer minimizing boundary Sigma, we prove a Penrose-like inequality in dimensions n < 8, under suitable assumptions on the mean curvature and the scalar curvature of Sigma.
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| 6. |
- McCormick, Stephen
(author)
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The Asymptotically Flat Scalar-Flat Yamabe Problem with Boundary
- 2017
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In: Journal of Geometric Analysis. - : SPRINGER. - 1050-6926 .- 1559-002X. ; 27:3, s. 2269-2277
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Journal article (peer-reviewed)abstract
- We consider two cases of the asymptotically flat scalar-flat Yamabe problem on a non-compact manifold with inner boundary in dimension n >= 3. First, following arguments of Cantor and Brill in the compact case, we show that given an asymptotically flat metric g, there is a conformally equivalent asymptotically flat scalar-flat metric that agrees with g on the boundary. We then replace the metric boundary condition with a condition on the mean curvature: given a function f on the boundary that is not too large, we show that there is an asymptotically flat scalar-flat metric, conformally equivalent to g whose boundary mean curvature is given by f. The latter case involves solving an elliptic PDE with critical exponent using the method of sub- and supersolutions. Both results require the usual assumption that the Sobolev quotient is positive.
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| 7. |
- Nilsson, Hans, et al.
(author)
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Evolution of the ion environment of comet 67P during the Rosetta mission as seen by RPC-ICA
- 2017
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In: Monthly notices of the Royal Astronomical Society. - : Oxford University Press. - 0035-8711 .- 1365-2966. ; 469:Suppl_2, s. S252-S261
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Journal article (peer-reviewed)abstract
- Rosetta has followed comet 67P from low activity at more than 3.6 au heliocentric distance to high activity at perihelion (1.24 au) and then out again. We provide a general overview of the evolution of the dynamic ion environment using data from the RPC-ICA ion spectrometer. We discuss where Rosetta was located within the evolving comet magnetosphere. For the initial observations, the solar wind permeated all of the coma. In 2015 mid-April, the solar wind started to disappear from the observation region, to re-appear again in 2015 December. Low-energy cometary ions were seen at first when Rosetta was about 100 km from the nucleus at 3.6 au, and soon after consistently throughout the mission except during the excursions to farther distances from the comet. The observed flux of low-energy ions was relatively constant due to Rosetta's orbit changing with comet activity. Accelerated cometary ions, moving mainly in the antisunward direction gradually became more common as comet activity increased. These accelerated cometary ions kept being observed also after the solar wind disappeared from the location of Rosetta, with somewhat higher fluxes further away from the nucleus. Around perihelion, when Rosetta was relatively deep within the comet magnetosphere, the fluxes of accelerated cometary ions decreased, as did their maximum energy. The disappearance of more energetic cometary ions at close distance during high activity is suggested to be due to a flow pattern where these ions flow around the obstacle of the denser coma or due to charge exchange losses.
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| 8. |
- Nilsson, Hans, et al.
(author)
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Oxygen ion response to proton bursty bulk flows
- 2016
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In: Journal of Geophysical Research - Space Physics. - : American Geophysical Union (AGU). - 2169-9380 .- 2169-9402. ; 121:8, s. 7535-7546
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Journal article (peer-reviewed)abstract
- We have used Cluster spacecraft data from the years 2001 to 2005 to study how oxygen ions respond to bursty bulk flows (BBFs) as identified from proton data. We here define bursty bulk flows as periods of proton perpendicular velocities more than 100 km/s and a peak perpendicular velocity in the structure of more than 200 km/s, observed in a region with plasma beta above 1 in the near-Earth central tail region. We find that during proton BBFs only a minor increase in the O+ velocity is seen. The different behavior of the two ion species is further shown by statistics of H+ and O+ flow also outside BBFs: For perpendicular earthward velocities of H+ above about 100 km/s, the O+ perpendicular velocity is consistently lower, most commonly being a few tens of kilometers per second earthward. In summary, O+ ions in the plasma sheet experience less acceleration than H+ ions and are not fully frozen in to the magnetic field. Therefore, H+ and O+ motion is decoupled, and O+ ions have a slower earthward motion. This is particularly clear during BBFs. This may add further to the increased relative abundance of O+ ions in the plasma sheet during magnetic storms. The data indicate that O+ is typically less accelerated in association with plasma sheet X lines as compared to H+.
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| 9. |
- Nilsson, Hans, et al.
(author)
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Size of a plasma cloud matters The polarisation electric field of a small-scale comet ionosphere
- 2018
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In: Astronomy and Astrophysics. - : EDP SCIENCES S A. - 0004-6361 .- 1432-0746. ; 616
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Journal article (peer-reviewed)abstract
- Context. The cometary ionosphere is immersed in fast flowing solar wind. A polarisation electric field may arise for comets much smaller than the gyroradius of pickup ions because ions and electrons respond differently to the solar wind electric field. Aims. A situation similar to that found at a low activity comet has been modelled for barium releases in the Earth's ionosphere. We aim to use such a model and apply it to the case of comet 67P Churyumov-Gerasimenko, the target of the Rosetta mission. We aim to explain the significant tailward acceleration of cometary ions through the modelled electric field. Methods. We obtained analytical solutions for the polarisation electric field of the comet ionosphere using a simplified geometry. This geometry is applicable to the comet in the inner part of the coma as the plasma density integrated along the magnetic field line remains rather constant. We studied the range of parameters for which a significant tailward electric field is obtained and compare this with the parameter range observed. Results. Observations of the local plasma density and magnetic field strength show that the parameter range of the observations agree very well with a significant polarisation electric field shielding the inner part of the coma from the solar wind electric field. Conclusions. The same process gives rise to a tailward directed electric field with a strength of the order of 10% of the solar wind electric field. Using a simple cloud model we have shown that the polarisation electric field, which arises because of the small size of the comet ionosphere as compared to the pick up ion gyroradius, can explain the observed significant tailward acceleration of cometary ions and is consistent with the observed lack of influence of the solar wind electric field in the inner coma.
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| 10. |
- Pilkington, Georgia A., et al.
(author)
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Electro-responsivity of ionic liquid boundary layers in a polar solvent revealed by neutron reflectance
- 2018
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In: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 148:19
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Journal article (peer-reviewed)abstract
- Using neutron reflectivity, the electro-responsive structuring of the non-halogenated ionic liquid (IL) trihexyl(tetradecyl)phosphonium-bis(mandelato)borate, [P6,6,6,14][BMB], has been studied at a gold electrode surface in a polar solvent. For a 20% w/w IL mixture, contrast matched to the gold surface, distinct Kiessig fringes were observed for all potentials studied, indicative of a boundary layer of different composition to that of the bulk IL-solvent mixture. With applied potential, the amplitudes of the fringes from the gold-boundary layer interface varied systematically. These changes are attributable to the differing ratios of cations and anions in the boundary layer, leading to a greater or diminished contrast with the gold electrode, depending on the individual ion scattering length densities. Such electro-responsive changes were also evident in the reflectivities measured for the pure IL and a less concentrated (5% w/w) IL-solvent mixture at the same applied potentials, but gave rise to less pronounced changes. These measurements, therefore, demonstrate the enhanced sensitivity achieved by contrast matching the bulk solution and that the structure of the IL boundary layers formed in mixtures is strongly influenced by the bulk concentration. Together these results represent an important step in characterising IL boundary layers in IL-solvent mixtures and provide clear evidence of electro-responsive structuring of IL ions in their solutions with applied potential.
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