Methanogens, microorganisms that produce methane, have long been known to compete for energy resources with microorganisms that respire iron minerals. More recently, it has become clear that iron-respiring microorganisms can also share energy resources with methanogens through what is known as direct interspecies electron transfer. In this case, the iron-respiring microorganism oxidizes some electron donor, or 'food', and then transfers the electrons to the methanogen, which uses them to make methane by reducing carbon dioxide. This pathway has the potential to affect rates of methane formation, but it remains unclear why the groups compete in some environments but cooperate with one another in others. Answering this question would be useful because this knowledge can help us better predict methane emissions from soils and potentially improve our ability to increase methane production for energy capture.
The fast-paced development of laser-wakefield electron acceleration has recently culminated in the generation of electron beams with extreme characteristics, including femtosecond-scale duration, mrad divergence, and high-energy. It is now customary to attain tens to hundreds of pC of charge with an energy of hundreds of MeV per particle with small-scale commercial laser systems, with multi-GeV electron beams being demonstrated from cm-scale accelerators. The interaction of such electron beams with either a solid target or the focus of a second high-power laser can result in the generation of high-quality positron beams and MeV-photon beams. The unrivaled properties of these secondary sources make them ideal for both fundamental and practical applications. In the present article, some of their main characteristics will be discussed, with particular emphasis on their potential applications in fundamental and applied physics. A discussion of potential future developments enabled by near-term laser facilities will also be presented.
Electron Drivers
Figure 2. Effect of the intensity of the scattering laser on the divergence of Compton radiation. Sub-figure (A) shows the increase in divergence along the scattering beam polarization direction with intensity. Sub-figure (B) shows the agreement of the H-D model[76] with the observed divergence change. Sub-figure (C) illustrates how the intensity may be inferred when taking into account a non-diverging electron beam vs. the experimentally attained 10 mrad beam. Figure obtained and modified from Yan et al. [46], with permission from Springer Nature.
Figure 3. Positron generation from a quantum cascade depending on the converter thickness (Data from Sarri et al. [90]). (A) Calculated number of ultra-relativistic (E > 120MeV) leptons leaving the converter, normalized to the number of electrons in the parent beam. (B) Fraction of positrons in the leptonic jet (R=Ne+/(Ne++Ne-). The beam neutrality is first achieved for a converter thickness of 5Lrad.
Figure 4. Experimental measurements of positron generation using laser-driven electrons. (a) Electron, positron and gamma spectra observed from a low power (1.7 TW) laser (Image taken from Gahn et al. [91], with permission from AIP Publishing). (b) Scaling of the positron yield with the converter thickness (l) and with Z2/A (Image taken from Ref. [94], with permission from APS). The quadratic scaling with those parameters indicates that the main positron generation mechanism is the Bethe-Heitler. (c) Scaling of the ratio of positrons in the leptonic beam depending on the thickness of the converter (Image adapted from Sarri et al. [95], including data from Xu et al. [96]). Quasi-neutral beams have been observed for converters of thickness >5Lrad.
Citation: Elliott SS, Breneman A, Colpitts C, Bortnik J, Jaynes A, Halford A, Shumko M, Blum L, Chen L, Greeley A and Turner D (2022) Understanding the properties, wave drivers, and impacts of electron microburst precipitation: Current understanding and critical knowledge gaps. Front. Astron. Space Sci. 9:1062422. doi: 10.3389/fspas.2022.1062422
In this work, we uncover a fundamental physical protocol for imprinting chirality in non-equilibrium spin systems. Taking one-dimensional antiferromagnetic chains as a platform, we explicitly consider the interaction between the electric field of the laser pulse with conduction electrons, which are in turn coupled to localised atomic spins, following the time-dependent coupled evolution of both sub-systems on equal footing. We thereby explicitly demonstrate that this intertwined dynamical process can result in a non-thermal formation of steady chiral states. We show that chirality formation is quite robust against the thermal fluctuations which make the uncovered mechanism relevant for various types of laser experiments performed on magnetic materials. Despite the simple model of the electronic structure we successfully capture the salient features of the underlying process emerging from the interplay between the electronic and magnetic interactions, thus providing a comprehensive strategy for an in-depth exploration of optically-driven chiral magnetism.
a Initial antiferromagnetic configuration in zx plane. Red and blue arrows represent initial up and down moments in the collinear antiferromagnetic ground state of the system. The yellow wavy line denotes the laser pulse polarised along the x-direction. b Following a complex intertwined dynamics of electrons and localised spins, a stable spin spiral state of the system is achieved (see Supplementary Movie 1 and Supplementary Note 2).
In our work, we introduce an alternative paradigm to imprint chirality in magnets with a finite laser pulse. Our results show that the laser-driven magnetisation dynamics is preceded by the electronic excitation with a lag time depending on the initial magnetic configuration. By establishing a bridge between the quantum evolution of electronic states and classical spin dynamics, we present a comprehensive picture of the laser-mediated formation of large-scale chiral states. We highlight that the uncovered mechanism is distinctly different from that which is commonly used to interpret laser excited dynamics in terms of transfer of effective temperature between electronic to spin degrees of freedom which is also known as two temperature or three temperature model (considering the phonon modes)53,54. Although these phenomenological models work nicely for ultrafast demagnetisation, they fail to capture the physics of ultrafast generation of chirality.
To show this, we artificially simulate the effect of heating of the electronic sub-system. Due to the presence of a large gap and the fact that the laser pulse only changes the occupation of a small range of eigenstates within a specific energy window, it is not possible to construct a thermal distribution to capture these phenomena. Therefore we replace the laser with a change of occupation by hand at time t0 (Fig. 7a) with a random phase factor to simulate the effect of thermal transition and let the system evolve (Fig. 7b). We observe that with thermal excitation the system forms multiple domains rather than a smooth spiral (Fig. 7c). In accord to existing knowledge, such formation can change the effective magnetic order which can be observed in experiments on ultrafast demagnetisation. However, the large root mean square deviation of δθ and large values of χdev (Fig. 7b) compared to laser-excited dynamics shows that the thermal excitation does not guide the system to a spiral formation. This has been further clarified by showing the end configuration of the evolution (Fig. 7c and Supplementary Note 8).
with angular frequency ω and a standard deviation s. We consider a linearly polarised pulse with polarisation along \(\hatx\) axis and propagating along \(\hatz\) axis. The impact of this electric field can be modelled as an Peierls phase \(\rme^\rmi\frace\hslash \boldsymbolA(t)\cdot \boldsymbold_ij\) where A(t) is the time-dependent vector potential such that \(\mathcalE(t)=-\partial \boldsymbolA(t)/\partial t\), e is the electronic charge and dij is the vector connecting site i to j. This results in the following time-dependent Hamiltonian
By solving Eqs. (7) and (8) simultaneously we are thereby able to study the effect of the laser pulse on the magnetisation dynamics as mediated by excited electronic states. In the past, a similar approach has been adopted to study the quantum evolution of magnetic system41,42,43 and current-driven magnetisation dynamics56,57, however, a steady chirality formation was not observed. Ishihara and Ono43 observe a transient skyrmion-like configuration during a periodically-driven laser-assisted transition from a ferromagnetic to an antiferromagnetic state, stable on the scale of hundred femtoseconds. Contrary to that, as discussed below, we observe the formation of steady chiral states which can survive up to several picoseconds and can be generated with a finite pulse.
Magnetotail reconnection plays a crucial role in explosive energy conversion in geospace. Because of the lack of in-situ spacecraft observations, the onset mechanism of magnetotail reconnection, however, has been controversial for decades. The key question is whether magnetotail reconnection is externally driven to occur first on electron scales or spontaneously arising from an unstable configuration on ion scales. Here, we show, using spacecraft observations and particle-in-cell (PIC) simulations, that magnetotail reconnection starts from electron reconnection in the presence of a strong external driver. Our PIC simulations show that this electron reconnection then develops into ion reconnection. These results provide direct evidence for magnetotail reconnection onset caused by electron kinetics with a strong external driver. 2ff7e9595c
Comments