| 1. |
- Baggioli, Matteo, et al.
(author)
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Holographic plasmon relaxation with and without broken translations
- 2019
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In: Journal of High Energy Physics (JHEP). - : SPRINGER. - 1126-6708 .- 1029-8479. ; :9
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Journal article (peer-reviewed)abstract
- We study the dynamics and the relaxation of bulk plasmons in strongly coupled and quantum critical systems using the holographic framework. We analyze the dispersion relation of the plasmonic modes in detail for an illustrative class of holographic bottom-up models. Comparing to a simple hydrodynamic formula, we entangle the complicated interplay between the three least damped modes and shed light on the underlying physical processes. Such as the dependence of the plasma frequency and the effective relaxation time in terms of the electromagnetic coupling, the charge and the temperature of the system. Introducing momentum dissipation, we then identify its additional contribution to the damping. Finally, we consider the spontaneous symmetry breaking (SSB) of translational invariance. Upon dialing the strength of the SSB, we observe an increase of the longitudinal sound speed controlled by the elastic moduli and a decrease in the plasma frequency of the gapped plasmon. We comment on the condensed matter interpretation of this mechanism.
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| 2. |
- Thiele, Illia, 1989, et al.
(author)
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Electron Beam Driven Generation of Frequency-Tunable Isolated Relativistic Subcycle Pulses
- 2019
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In: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 122
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Journal article (peer-reviewed)abstract
- © 2019 American Physical Society. We propose a novel scheme for frequency-tunable subcycle electromagnetic pulse generation. To this end a pump electron beam is injected into an electromagnetic seed pulse as the latter is reflected by a mirror. The electron beam is shown to be able to amplify the field of the seed pulse while upshifting its central frequency and reducing its number of cycles. We demonstrate the amplification by means of 1D and 2D particle-in-cell simulations. In order to explain and optimize the process, a model based on fluid theory is proposed. We estimate that using currently available electron beams and terahertz pulse sources, our scheme is able to produce millijoule-strong midinfrared subcycle pulses.
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| 3. |
- Hooton, M.J., et al.
(author)
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Spi-OPS: Spitzer and CHEOPS confirm the near-polar orbit of MASCARA-1 b and reveal a hint of dayside reflection
- 2022
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In: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 658
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Journal article (peer-reviewed)abstract
- Context. The light curves of tidally locked hot Jupiters transiting fast-rotating, early-type stars are a rich source of information about both the planet and star, with full-phase coverage enabling a detailed atmospheric characterisation of the planet. Although it is possible to determine the true spin-orbit angle ψ-a notoriously difficult parameter to measure-from any transit asymmetry resulting from gravity darkening induced by the stellar rotation, the correlations that exist between the transit parameters have led to large disagreements in published values of ψ for some systems. Aims. We aimed to study these phenomena in the light curves of the ultra-hot Jupiter MASCARA-1 b, which is characteristically similar to well-studied contemporaries such as KELT-9 b and WASP-33 b. Methods. We obtained optical CHaracterising ExOPlanet Satellite (CHEOPS) transit and occultation light curves of MASCARA-1 b, and analysed them jointly with a Spitzer/IRAC 4.5 μm full-phase curve to model the asymmetric transits, occultations, and phase-dependent flux modulation. For the latter, we employed a novel physics-driven approach to jointly fit the phase modulation by generating a single 2D temperature map and integrating it over the two bandpasses as a function of phase to account for the differing planet-star flux contrasts. The reflected light component was modelled using the general ab initio solution for a semi-infinite atmosphere. Results. When fitting the CHEOPS and Spitzer transits together, the degeneracies are greatly diminished and return results consistent with previously published Doppler tomography. Placing priors informed by the tomography achieves even better precision, allowing a determination of ψ = 72.1-2.4+2.5 deg. From the occultations and phase variations, we derived dayside and nightside temperatures of 3062-68+66 K and 1720 ± 330 K, respectively.Our retrieval suggests that the dayside emission spectrum closely follows that of a blackbody. As the CHEOPS occultation is too deep to be attributed to blackbody flux alone, we could separately derive geometric albedo Ag = 0.171-0.068+0.066 and spherical albedo As = 0.266-0.100+0.097 from the CHEOPS data, and Bond albedoAB = 0.057-0.101+0.083 from the Spitzer phase curve.Although small, the Ag and As indicate that MASCARA-1 b is more reflective than most other ultra-hot Jupiters, where H- absorption is expected to dominate. Conclusions. Where possible, priors informed by Doppler tomography should be used when fitting transits of fast-rotating stars, though multi-colour photometry may also unlock an accurate measurement of ψ. Our approach to modelling the phase variations at different wavelengths provides a template for how to separate thermal emission from reflected light in spectrally resolved James Webb Space Telescope phase curve data.
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| 4. |
- Szabó, G.M., et al.
(author)
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The changing face of AU Mic b: Stellar spots, spin-orbit commensurability, and transit timing variations as seen by CHEOPS and TESS
- 2021
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In: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 654
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Journal article (peer-reviewed)abstract
- AU Mic is a young planetary system with a resolved debris disc showing signs of planet formation and two transiting warm Neptunes near mean-motion resonances. Here we analyse three transits of AU Mic b observed with the CHaracterising ExOPlanet Satellite (CHEOPS), supplemented with sector 1 and 27 Transiting Exoplanet Survey Satellite (TESS) photometry, and the All-Sky Automated Survey from the ground. The refined orbital period of AU Mic b is 8.462995 ± 0.000003 d, whereas the stellar rotational period is Prot = 4.8367 ± 0.0006 d. The two periods indicate a 7:4 spin-orbit commensurability at a precision of 0.1%. Therefore, all transits are observed in front of one of the four possible stellar central longitudes. This is strongly supported by the observation that the same complex star-spot pattern is seen in the second and third CHEOPS visits that were separated by four orbits (and seven stellar rotations). Using a bootstrap analysis we find that flares and star spots reduce the accuracy of transit parameters by up to 10% in the planet-to-star radius ratio and the accuracy on transit time by 3-4 min. Nevertheless, occulted stellar spot features independently confirm the presence of transit timing variations (TTVs) with an amplitude of at least 4 min. We find that the outer companion, AU Mic c, may cause the observed TTVs.
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| 5. |
- Erhart, Paul, 1978
(author)
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A first-principles study of helium storage in oxides and at oxide-iron interfaces
- 2012
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In: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 111:11
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Journal article (peer-reviewed)abstract
- Density-functional theory calculations based on conventional as well as hybrid exchange-correlation functionals have been carried out to study the properties of helium in various oxides (Al2O3, TiO2, Y2O3, YAP, YAG, YAM, MgO, CaO, BaO, SrO) as well as at oxide-iron interfaces. Helium interstitials in bulk oxides are shown to be energetically more favorable than substitutional helium, yet helium binds to existing vacancies. The solubility of He in oxides is systematically higher than in iron and scales with the free volume at the interstitial site nearly independently of the chemical composition of the oxide. In most oxides, He migration is significantly slower and He-He binding is much weaker than in iron. To quantify the solubility of helium at oxide-iron interfaces two prototypical systems are considered (Fe-MgO, Fe-FeO-MgO). In both cases, the He solubility is markedly enhanced in the interface compared to either of the bulk phases. The results of the calculations allow to construct a schematic energy landscape for He interstitials in iron. The implications of these results are discussed in the context of helium sequestration in oxide dispersion strengthened steels, including the effects of interfaces and lattice strain.
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| 6. |
- Gran, Ulf, 1973, et al.
(author)
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Holographic response of electron clouds
- 2019
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In: Journal of High Energy Physics. - : SPRINGER. - 1029-8479 .- 1126-6708. ; 2019:3
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Journal article (peer-reviewed)abstract
- In order to make progress towards more realistic models of holographic fermion physics, we use gauge/gravity duality to compute the dispersion relations for quasinormal modes and collective modes for the electron cloud background, i.e. the non-zero temperature version of the electron star. The results are compared to the corresponding results for the Schwarzschild and Reissner-Nordstrom black hole backgrounds, and the qualitative differences are highlighted and discussed.
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| 7. |
- Labit, B., et al.
(author)
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Dependence on plasma shape and plasma fueling for small edge-localized mode regimes in TCV and ASDEX Upgrade
- 2019
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In: Nuclear Fusion. - : IOP Publishing. - 1741-4326 .- 0029-5515. ; 59:8
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Journal article (peer-reviewed)abstract
- © 2019 Institute of Physics Publishing. All rights reserved. Within the EUROfusion MST1 work package, a series of experiments has been conducted on AUG and TCV devices to disentangle the role of plasma fueling and plasma shape for the onset of small ELM regimes. On both devices, small ELM regimes with high confinement are achieved if and only if two conditions are fulfilled at the same time. Firstly, the plasma density at the separatrix must be large enough (ne,sep/nG ∼ 0.3), leading to a pressure profile flattening at the separatrix, which stabilizes type-I ELMs. Secondly, the magnetic configuration has to be close to a double null (DN), leading to a reduction of the magnetic shear in the extreme vicinity of the separatrix. As a consequence, its stabilizing effect on ballooning modes is weakened.
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| 8. |
- Alegret, Joan, 1977
(author)
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Numerical Simulations of Plasmonic Nanostructures
- 2008
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Doctoral thesis (other academic/artistic)abstract
- This thesis focuses on the study of metallic nanostructures that support plasmons. Special emphasis is devoted to two specific numerical methods that allow us to predict plasmon characteristics: the discrete dipole approximation (DDA) and the Green's tensor (GT) method.DDA is an approximate method that produces fast and accurate results, but it can only be applied to systems in which the nanostructure is situated in a homogeneous background. In this thesis, DDA has been applied to predict the field enhancement and field decay around nano-rings, showing that the structure is well suited for biosensing; to obtain the spectral characteristics of silver trimers, showing that the actual plasmon modes are closely related to symmetry-adapted coordinates derived from group-theory; and to calculate the optical forces between two spherical particles illuminated by a plane wave, showing that the illumination wavelength determines the separation between the particles.The GT method, on the other hand, is an exact method, in the sense that the system can be solved to arbitrary precision depending on the size of the discretization elements. Its major drawback is the long time it takes to perform the calculations. To tis end, this thesis introduces a novel algorithm, called the top-down extended meshing algorithm (TEMA), that speeds up GT calculations by reducing the number of elements in the discretization process. This decreases the total time needed to perform the calculations, while keeping the precision of the result essentially unaltered. The GT method with TEMA meshes has successfully been used to study single holes of different sizes and shapes (circular and ellipsoidal) in the near- and far-field regime, as well as hole pairs as a function of their separation distance. The results compare very well with experiments, demonstration that the GT method is well suited for predicting the behavior of nano-holes.
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| 9. |
- Andrén, Daniel, 1991, et al.
(author)
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Microscopic metavehicles powered and steered by embedded optical metasurfaces
- 2021
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In: Nature Nanotechnology. - : Springer Science and Business Media LLC. - 1748-3387 .- 1748-3395. ; 16:9, s. 970-974
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Journal article (peer-reviewed)abstract
- Nanostructured dielectric metasurfaces offer unprecedented opportunities to manipulate light by imprinting an arbitrary phase gradient on an impinging wavefront(1). This has resulted in the realization of a range of flat analogues to classical optical components, such as lenses, waveplates and axicons(2-6). However, the change in linear and angular optical momentum(7) associated with phase manipulation also results in previously unexploited forces and torques that act on the metasurface itself. Here we show that these optomechanical effects can be utilized to construct optical metavehicles-microscopic particles that can travel long distances under low-intensity plane-wave illumination while being steered by the polarization of the incident light. We demonstrate movement in complex patterns, self-correcting motion and an application as transport vehicles for microscopic cargoes, which include unicellular organisms. The abundance of possible optical metasurfaces attests to the prospect of developing a wide variety of metavehicles with specialized functional behaviours.
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| 10. |
- Bahrova, O. M., et al.
(author)
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Cooling of nanomechanical vibrations by Andreev injection
- 2022
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In: Low Temperature Physics. - : AIP Publishing. - 1063-777X .- 1090-6517. ; 48:6, s. 476-482
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Journal article (peer-reviewed)abstract
- A nanoelectromechanical weak link composed of a carbon nanotube suspended between two normal electrodes in a gap between two superconducting leads is considered. The nanotube is treated as a movable single level quantum dot in which the position-dependent superconducting order parameter is induced due to the Cooper pair tunneling. We show that electron tunneling processes significantly affect the state of the mechanical subsystem. We found that at a given direction of the applied voltage between the electrodes, the stationary state of the mechanical subsystem has a Boltzmann form with an effective temperature dependent on the parameters of the device. As this takes place, the effective temperature can reach significantly small values (cooling effect). We also demonstrate that nanotube fluctuations strongly affect the dc current through the system. The latter can be used to probe the predicted effects in an experiment. Published under an exclusive license by AIP Publishing.
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