| 1. |
- Zouganelis, I., et al.
(författare)
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The Solar Orbiter Science Activity Plan : Translating solar and heliospheric physics questions into action
- 2020
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Ingår i: Astronomy and Astrophysics. - : EDP SCIENCES S A. - 0004-6361 .- 1432-0746. ; 642
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Tidskriftsartikel (refereegranskat)abstract
- Solar Orbiter is the first space mission observing the solar plasma both in situ and remotely, from a close distance, in and out of the ecliptic. The ultimate goal is to understand how the Sun produces and controls the heliosphere, filling the Solar System and driving the planetary environments. With six remote-sensing and four in-situ instrument suites, the coordination and planning of the operations are essential to address the following four top-level science questions: (1) What drives the solar wind and where does the coronal magnetic field originate?; (2) How do solar transients drive heliospheric variability?; (3) How do solar eruptions produce energetic particle radiation that fills the heliosphere?; (4) How does the solar dynamo work and drive connections between the Sun and the heliosphere? Maximising the mission's science return requires considering the characteristics of each orbit, including the relative position of the spacecraft to Earth (affecting downlink rates), trajectory events (such as gravitational assist manoeuvres), and the phase of the solar activity cycle. Furthermore, since each orbit's science telemetry will be downloaded over the course of the following orbit, science operations must be planned at mission level, rather than at the level of individual orbits. It is important to explore the way in which those science questions are translated into an actual plan of observations that fits into the mission, thus ensuring that no opportunities are missed. First, the overarching goals are broken down into specific, answerable questions along with the required observations and the so-called Science Activity Plan (SAP) is developed to achieve this. The SAP groups objectives that require similar observations into Solar Orbiter Observing Plans, resulting in a strategic, top-level view of the optimal opportunities for science observations during the mission lifetime. This allows for all four mission goals to be addressed. In this paper, we introduce Solar Orbiter's SAP through a series of examples and the strategy being followed.
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| 2. |
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| 3. |
- Prokes, K., et al.
(författare)
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Collapsed tetragonal phase in Ca(Fe1-xCox)(2)As-2 stabilized by pressure: Structural studies using single-crystal neutron diffraction
- 2012
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Ingår i: Physical Review B (Condensed Matter and Materials Physics). - 1098-0121. ; 85:10
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Tidskriftsartikel (refereegranskat)abstract
- Ca(Fe1-xCox)(2)As-2 single crystals with concentration x = 0.032, 0.051, and 0.063 were investigated by means of neutron diffraction under hydrostatic pressure. The region of existence of the high-temperature tetragonal, low-temperature orthorhombic, and pressure-induced collapsed tetragonal phases have been determined and the corresponding p-T phase diagrams for the three compositions have been constructed. Structural details of the tetragonal phases at various conditions have been determined. The anomaly marking the orthorhombic-to-tetragonal structural phase transition temperature T-S decreases with increasing Co substitution. Similarly to that, the critical pressure of the tetragonal (or orthorhombic)-to-collapsed-tetragonal transition also decreases with increasing Co doping level. The behavior is highly hysteretic and the differences in critical pressures with increasing and decreasing branches eventually makes the collapsed tetragonal phase stable down to ambient pressure and enables structural studies under identical conditions. Within the error bars on the fourth digit, no modification of the As structural positional parameter is found. Such a result suggests that the change of the As-As bond is caused solely by the reduction of the c-axis lattice parameter.
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| 4. |
- Wolter, A. U. B., et al.
(författare)
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Field-induced quantum criticality in the Kitaev system alpha-RuCl3
- 2017
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Ingår i: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 96:4
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Tidskriftsartikel (refereegranskat)abstract
- alpha-magnetic excitations akin to Kitaev's honeycomb-lattice spin liquid. We have performed a detailed specific-heat investigation at temperatures down to 0.4 K in applied magnetic fields up to 9 T for fields parallel to the ab plane. We find a suppression of the zero-field antiferromagnetic order, together with an increase of the low-temperature specific heat, with increasing field up to mu H-0(c) approximate to 6.9 T. Above H-c, the magnetic contribution to the low-temperature specific heat is strongly suppressed, implying the opening of a spin-excitation gap. Our data point toward a field-induced quantum critical point at H-c; this is supported by universal scaling behavior near H-c. Remarkably, the data also reveal the existence of a small characteristic energy scale well below 1 meV, above which the excitation spectrum changes qualitatively. We relate the data to theoretical calculations based on a J(1)-K-1-Gamma(1)-J(3) honeycomb model.
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| 5. |
- Dufouleur, J., et al.
(författare)
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Suppression of scattering in quantum confined 2D helical Dirac systems
- 2018
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Ingår i: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 97:7
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Tidskriftsartikel (refereegranskat)abstract
- Transport properties of helical Dirac fermions in disordered quantum wires are investigated in the large energy limit. In the quasiballistic regime, the conductance and the Fano factor are sensitive to disorder only when the Fermi energy is close to an opening of a transverse mode. In the limit of a large number of transverse modes, transport properties are insensitive to the geometry of the nanowire or the nature and strength of the disorder but, instead, are dominated by the properties of the interface between the ohmic contact and the nanowire. In the case of a heavily doped Dirac metallic contact, the conductance is proportional to the energy with an average transmission T = pi/4 and a Fano factor of F similar or equal to 0.13. Those results can be generalized to a much broader class of contacts, the exact values of T and F depending on the model used for the contacts. The energy dependence of Aharonov-Bohm oscillations is determined, revealing a damped oscillatory behavior and phase shifts due to the one-dimensional subband quantization and which are not the signature of the nontrivial topology.
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| 6. |
- Emmel, M., et al.
(författare)
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Electronic properties of Co2FeSi investigated by X-ray magnetic linear dichroism
- 2014
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Ingår i: Journal of Magnetism and Magnetic Materials. - : Elsevier BV. - 0304-8853 .- 1873-4766. ; 368, s. 364-373
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Tidskriftsartikel (refereegranskat)abstract
- We present experimental XMLD spectra measured on epitaxial (001)-oriented thin Co2FeSi films, which are rich in features and depend sensitively on the degree of atomic order and interdiffusion from capping layers. Al- and Cr-capped films with different degrees of atomic order were prepared by DC magnetron sputtering by varying the deposition temperatures. The local structural properties of the film samples were additionally investigated by nuclear magnetic resonance (NMR) measurements. The XMLD spectra of the different samples show clear and uniform trends at the L-3,L-2 edges. The Al-capped samples show similar behavior as previous measured XMLD spectra of Co2FeSi0.6Al0.4. Thus, we assume that during deposition Al atoms are being implanted into the subsurface of Co2FeSi. Such an interdiffusion is not observed for the corresponding Cr-capped films, which makes Cr the material of choice for capping Co2FeSi films. We report stronger XMLD intensities at the L-3,L-2 Co and Fe egdes for films with a higher saturation magnetization. Additionally, we compare the spectra with ab initio predictions and obtain a reasonably good agreement. Furthermore, we were able to detect an XMCD signal at the Si Ledge, indicating the presence of a magnetic moment at the Si atoms.
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| 7. |
- Goodacre, Dana, et al.
(författare)
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Water adsorption on vanadium oxide thin films in ambient relative humidity
- 2020
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Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 152:4
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Tidskriftsartikel (refereegranskat)abstract
- In this work, ambient pressure X-ray photoelectron spectroscopy (APXPS) is used to study the initial stages of water adsorption on vanadium oxide surfaces. V 2p, O 1s, C 1s, and valence band XPS spectra were collected as a function of relative humidity in a series of isotherm and isobar experiments. Experiments were carried out on two VO2 thin films on TiO2 (100) substrates, prepared with different surface cleaning procedures. Hydroxyl and molecular water surface species were identified, with up to 0.5 ML hydroxide present at the minimum relative humidity, and a consistent molecular water adsorption onset occurring around 0.01% relative humidity. The work function was found to increase with increasing relative humidity, suggesting that surface water and hydroxyl species are oriented with the hydrogen atoms directed away from the surface. Changes in the valence band were also observed as a function of relative humidity. The results were similar to those observed in APXPS experiments on other transition metal oxide surfaces, suggesting that H2O-OH and H2O-H2O surface complex formation plays an important role in the oxide wetting process and water dissociation. Compared to polycrystalline vanadium metal, these vanadium oxide films generate less hydroxide and appear to be more favorable for molecular water adsorption.
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| 8. |
- Haberer, D., et al.
(författare)
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Anisotropic Eliashberg function and electron-phonon coupling in doped graphene
- 2013
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Ingår i: Physical Review B (Condensed Matter and Materials Physics). - 1098-0121. ; 88:8
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Tidskriftsartikel (refereegranskat)abstract
- We investigate, with high-resolution angle-resolved photoemission spectroscopy, the spectral function of potassium-doped quasi-free-standing graphene on Au. Angle-dependent x-ray photoemission and density functional theory calculations demonstrate that potassium intercalates into the graphene/Au interface, leading to an upshift of the K-derived electronic band above the Fermi level. This empty band is what makes this system perfectly suited to disentangle the contributions to electron-phonon coupling coming from the pi band and K-derived bands. From a self-energy analysis we find an anisotropic electron-phonon coupling strength lambda of 0.1 (0.2) for the K Gamma (K M) high-symmetry directions in momentum space, respectively. Interestingly, the high-energy part of the Eliashberg function which relates to graphene's optical phonons is equal in both directions but only in K M does an additional low-energy part appear.
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| 9. |
- Haindl, S., et al.
(författare)
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Thin film growth of Fe-based superconductors : from fundamental properties to functional devices. A comparative review
- 2014
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Ingår i: Reports on progress in physics (Print). - : IOP Publishing. - 0034-4885 .- 1361-6633. ; 77:4, s. 046502-
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Forskningsöversikt (refereegranskat)abstract
- Fe-based superconductors bridge a gap between MgB2 and the cuprate high temperature superconductors as they exhibit multiband character and transition temperatures up to around 55 K. Investigating Fe-based superconductors thus promises answers to fundamental questions concerning the Cooper pairing mechanism, competition between magnetic and superconducting phases, and a wide variety of electronic correlation effects. The question addressed in this review is, however, is this new class of superconductors also a promising candidate for technical applications? Superconducting film-based technologies range from high-current and high-field applications for energy production and storage to sensor development for communication and security issues and have to meet relevant needs of today's society and that of the future. In this review we will highlight and discuss selected key issues for Fe-based superconducting thin film applications. We initially focus our discussion on the understanding of physical properties and actual problems in film fabrication based on a comparison of different observations made in the last few years. Subsequently we address the potential for technological applications according to the current situation.
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| 10. |
- Kuznetsov, E. A., et al.
(författare)
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Strong space plasma magnetic barriers and Alfvenic collapse
- 2007
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Ingår i: JETP Letters. - 0021-3640 .- 1090-6487. ; 85:5, s. 236-241
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Tidskriftsartikel (refereegranskat)abstract
- High-magnitude magnetic barriers in space and solar plasma are proposed to be attributed to the pile up of magnetic field lines and their Alfvenic collapse for MHD flows. The analysis of experimental data from both the Interball and Cluster spacecrafts shows that high-magnitude magnetic structures found in the Earth magnetosheath and near the magnetopause are supported by a nearly thermal transverse plasma flow, with the minimum barrier width being on the order of the ion gyroradius. The collapse termination at such scales can be explained by the balance between the pile up of magnetic field lines and backward finite-gyroradius diffusion. Comparison between the theoretical, modeling, and experimental data shows that the Alfvenic collapse is, in general, a promising mechanism for magnetic field generation and plasma separation.
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