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We study in experiment and theory thermal energy and charge transfer close to the quantum limit in a ballistic nanodevice, consisting of multiply connected one-dimensional electron waveguides. The fabricated device is based on an AlGaAs/GaAs heterostructure and is covered by a global top-gate to steer the thermal energy and charge transfer in the presence of a temperature gradient, which is established by a heating current. The estimate of the heat transfer by means of thermal noise measurements shows the device acting as a switch for charge and thermal energy transfer. The wave-packet simulations are based on the multi-terminal Landauer-Büttiker approach and confirm the experimental finding of a mode-dependent redistribution of the thermal energy current, if a scatterer breaks the device symmetry.
The Rosetta probe around comet 67P/Churyumov–Gerasimenko (67P) reveals an anisotropic dust distribution of the inner coma with jet-like structures. The physical processes leading to jet formation are under debate, with most models for cometary activity focusing on localized emission sources, such as cliffs or terraced regions. Here we suggest, by correlating high-resolution simulations of the dust environment around 67P with observations, that the anisotropy and the background dust density of 67P originate from dust released across the entire sunlit surface of the nucleus rather than from few isolated sources. We trace back trajectories from coma regions with high local dust density in space to the non-spherical nucleus and identify two mechanisms of jet formation: areas with local concavity in either two dimensions or only one. Pits and craters are examples of the first case; the neck region of the bi-lobed nucleus of 67P is an example of the latter case. The conjunction of multiple sources, in addition to dust released from all other sunlit areas, results in a high correlation coefficient (~0.8) of the predictions with observations during a complete diurnal rotation period of 67P.
Dust transport and deposition behind larger boulders on the comet 67P/Churyumov–Gerasimenko (67P/C–G) have been observed by the Rosetta mission. We present a mechanism for dust-transport vectors based on a homogeneous surface activity model incorporating in detail the topography of 67P/C–G. The combination of gravitation, gas drag, and Coriolis force leads to specific dust transfer pathways, which for higher dust velocities fuel the near-nucleus coma. By distributing dust sources homogeneously across the whole cometary surface, we derive a global dust-transport map of 67P/C–G. The transport vectors are in agreement with the reported wind-tail directions in the Philae descent area.
Two-dimensional electronic spectroscopy has become one of the main experimental tools for analyzing the dynamics of excitonic energy transfer in large molecular complexes. Simplified theoretical models are usually employed to extract model parameters from the experimental spectral data. Here we show that computationally expensive but exact theoretical methods encoded into a neural network can be used to extract model parameters and infer structural information such as dipole orientation from two dimensional electronic spectra (2DES) or reversely, to produce 2DES from model parameters. We propose to use machine learning as a tool to predict unknown parameters in the models underlying recorded spectra and as a way to encode computationally expensive numerical methods into efficient prediction tools. We showcase the use of a trained neural network to efficiently compute disordered averaged spectra and demonstrate that disorder averaging has non-trivial effects for polarization controlled 2DES.
Branched Flow
(2019)
In many physical situations involving diverse length scales, waves or rays representing them travel through media characterized by spatially smooth, random, modest refractive index variations. "Primary" diffraction (by individual sub-wavelength features) is absent. Eventually the weak refraction leads to imperfect focal "cusps". Much later, a statistical regime characterized by momentum diffusion is manifested. An important intermediate regime is often overlooked, one that is diffusive only in an ensemble sense. Each realization of the ensemble possesses dramatic ray limit structure that guides the waves (in the same sense that ray optics is used to design lens systems). This structure is a universal phenomenon called branched flow. Many important phenomena develop in this intermediate regime. Here we give examples and some of the physics of this emerging field.
Context. Cometary outgassing is induced by the sublimation of ices and the ejection of dust originating from the nucleus. Therefore measuring the composition and dynamics of the cometary gas provides information concerning the interior composition of the body. Nevertheless, the bulk composition differs from the coma composition, and numerical models are required to simulate the main physical processes induced by the illumination of the icy body.
Aims. The objectives of this study are to bring new constraints on the interior composition of the nucleus of comet 67P/Churyumov-Gerasimenko (hereafter 67P) by comparing the results of a thermophysical model applied to the nucleus of 67P and the coma measurements made by the Reflectron-type Time-Of-Flight (RTOF) mass spectrometer. This last is one of the three instruments of the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis (ROSINA), used during the Rosetta mission.
Methods. Using a thermophysical model of the comet nucleus, we studied the evolution of the stratigraphy (position of the sublimation and crystallisation fronts), the temperature of the surface and subsurface, and the dynamics and spatial distribution of the volatiles (H2O, CO2 and CO). We compared them with the in situ measurements from ROSINA/RTOF and an inverse coma model.
Results. We observed the evolution of the surface and near surface temperature, and the deepening of sublimation fronts. The thickness of the dust layer covering the surface strongly influences the H2O outgassing but not the more volatiles species. The CO outgassing is highly sensitive to the initial CO/H2O ratio, as well as to the presence of trapped CO in the amorphous ice.
Conclusions. The study of the influence of the initial parameters on the computed volatile fluxes and the comparison with ROSINA/RTOF measurements provide a range of values for an initial dust mantle thickness and a range of values for the volatile ratio. These imply the presence of trapped CO. Nevertheless, further studies are required to reproduce the strong change of behaviour observed in RTOF measurements between September 2014 and February 2015.
Two-dimensional electronic spectra (2DES) provide unique ways to track the energy transfer dynamics in light-harvesting complexes. The interpretation of the peaks and structures found in experimentally recorded 2DES is often not straightforward, since several processes are imaged simultaneously. The choice of specific pulse polarization sequences helps to disentangle the sometimes convoluted spectra, but brings along other disturbances. We show by detailed theoretical calculations how 2DES of the Fenna-Matthews-Olson complex are affected by rotational and conformational disorder of the chromophores.
Transient capture of electrons in magnetic fields, or: comets in the restricted three-body problem
(2020)
The motion of celestial bodies in astronomy is closely related to the orbits
of electrons encircling an atomic nucleus. Bohr and Sommerfeld presented a
quantization scheme of the classical orbits to analyze the eigenstates of the
hydrogen atom. Here we discuss another close connection of classical
trajectories and quantum mechanical states: the transient dynamics of objects
around a nucleus. In this setup a comet (or an electron) is trapped for a while
in the vicinity of parent object (Jupiter or an atomic nucleus), but eventually
escapes after many revolutions around the center of attraction.
During a two year period between 2014 and 2016 the coma of comet 67P/Churyumov-Gerasimenko (67P/C-G) has been probed by the Rosetta spacecraft. Density data for 14 gas species was recorded with the COmet Pressure Sensor (COPS) and the Double Focusing Mass Spectrometer (DFMS) being two sensors of the ROSINA instrument. The combination with an inverse gas model yields emission rates on each of 3996 surface elements of a surface shape for the cometary nucleus.
The temporal evolution of gas production, of relative abundances, and peak productions weeks after perihelion are investigated. Solar irradiation and gas production are in a complex relation revealing features differing for gas species, for mission time, and for the hemispheres of the comet. This characterization of gas composition allows one to correlate 67P/C-G to other solar and interstellar comets, their formation conditions and nucleus properties, see [Bodewits D., et al., 2020 Nature Astronomy].
The determination of non-gravitational forces based on precise astrometry is one of the main tools to establish the cometary character of interstellar and solar-system objects. The Rosetta mission to comet 67P/C-G provided the unique opportunity to benchmark Earth-bound estimates of non-gravitational forces with in-situ data. We determine the accuracy of the standard Marsden and Sekanina parametrization of non-gravitational forces with respect to the observed dynamics. Additionally we analyse the rotation-axis changes (orientation and period) of 67P/C-G. This comparison provides a reference case for future cometary missions and sublimation models for non-gravitational forces.