We measure the thermal electron energization in 1D and 2D particleincell simulations of quasiperpendicular, lowbeta (
Ion irradiation is a versatile tool to introduce controlled defects into twodimensional (2D) MoS_{2}on account of its unique spatial resolution and plethora of ion types and energies available. In order to fully realise the potential of this technique, a holistic understanding of ioninduced defect production in 2D MoS_{2}crystals of different thicknesses is mandatory. Xray photoelectron spectroscopy, electron diffraction and Raman spectroscopy show that thinner MoS_{2}crystals are more susceptible to radiation damage caused by 225 keV Xe^{+}ions. However, the rate of defect production in quadrilayer and bulk crystals is not significantly different under our experimental conditions. The rate at which S atoms are sputtered as a function of radiation exposure is considerably higher for monolayer MoS_{2}, compared to bulk crystals, leading to MoO_{3}formation. Pdoping of MoS_{2}is observed and attributed to the acceptor states introduced by vacancies and charge transfer interactions with adsorbed species. Moreover, the outofplane vibrational properties of irradiated MoS_{2}crystals are shown to be strongly thicknessdependent: in mono and bilayer MoS_{2}, the confinement of phonons by defects results in a blueshift of the
 NSFPAR ID:
 10151969
 Publisher / Repository:
 IOP Publishing
 Date Published:
 Journal Name:
 2D Materials
 Volume:
 7
 Issue:
 3
 ISSN:
 20531583
 Page Range / eLocation ID:
 Article No. 035011
 Format(s):
 Medium: X
 Sponsoring Org:
 National Science Foundation
More Like this

Abstract β _{p}= 0.25) collisionless ion–electron shocks with mass ratiom _{i}/m _{e}= 200, fast Mach number –4, and upstream magnetic field angle ${\mathcal{M}}_{\mathrm{ms}}=1$θ _{Bn}= 55°–85° from the shock normal . It is known that shock electron heating is described by an ambipolar, $\stackrel{\u02c6}{\mathit{n}}$ parallel electric potential jump, ΔB ϕ _{∥}, that scales roughly linearly with the electron temperature jump. Our simulations have –0.2 in units of the preshock ions’ bulk kinetic energy, in agreement with prior measurements and simulations. Different ways to measure $\mathrm{\Delta}{\varphi}_{\parallel}/(0.5{m}_{\mathrm{i}}{{u}_{\mathrm{sh}}}^{2})\sim 0.1$ϕ _{∥}, including the use of de Hoffmann–Teller frame fields, agree to tensofpercent accuracy. Neglecting offdiagonal electron pressure tensor terms can lead to a systematic underestimate ofϕ _{∥}in our lowβ _{p}shocks. We further focus on twoθ _{Bn}= 65° shocks: a ( ${\mathcal{M}}_{\mathrm{s}}\phantom{\rule{0.25em}{0ex}}=\phantom{\rule{0.25em}{0ex}}4$ ) case with a long, 30 ${\mathcal{M}}_{\mathrm{A}}\phantom{\rule{0.25em}{0ex}}=\phantom{\rule{0.25em}{0ex}}1.8$d _{i}precursor of whistler waves along , and a $\stackrel{\u02c6}{\mathit{n}}$ ( ${\mathcal{M}}_{\mathrm{s}}\phantom{\rule{0.25em}{0ex}}=\phantom{\rule{0.25em}{0ex}}7$ ) case with a shorter, 5 ${\mathcal{M}}_{\mathrm{A}}\phantom{\rule{0.25em}{0ex}}=\phantom{\rule{0.25em}{0ex}}3.2$d _{i}precursor of whistlers oblique to both and $\stackrel{\u02c6}{\mathit{n}}$ ;B d _{i}is the ion skin depth. Within the precursors,ϕ _{∥}has a secular rise toward the shock along multiple whistler wavelengths and also has localized spikes within magnetic troughs. In a 1D simulation of the , ${\mathcal{M}}_{\mathrm{s}}\phantom{\rule{0.25em}{0ex}}=\phantom{\rule{0.25em}{0ex}}4$θ _{Bn}= 65° case,ϕ _{∥}shows a weak dependence on the electron plasmatocyclotron frequency ratioω _{pe}/Ω_{ce}, andϕ _{∥}decreases by a factor of 2 asm _{i}/m _{e}is raised to the true proton–electron value of 1836. 
Abstract A steadystate, semianalytical model of energetic particle acceleration in radiojet shear flows due to cosmicray viscosity obtained by Webb et al. is generalized to take into account more general cosmicray boundary spectra. This involves solving a mixed Dirichlet–Von Neumann boundary value problem at the edge of the jet. The energetic particle distribution function
f _{0}(r ,p ) at cylindrical radiusr from the jet axis (assumed to lie along thez axis) is given by convolving the particle momentum spectrum with the Green’s function ${f}_{0}(\infty ,p\prime )$ , which describes the monoenergetic spectrum solution in which $G(r,p;p\prime )$ as ${f}_{0}\to \delta (pp\prime )$r → ∞ . Previous work by Webb et al. studied only the Green’s function solution for . In this paper, we explore for the first time, solutions for more general and realistic forms for $G(r,p;p\prime )$ . The flow velocity ${f}_{0}(\infty ,p\prime )$ =u u (r ) _{z}is along the axis of the jet (thee z axis). is independent ofu z , andu (r ) is a monotonic decreasing function ofr . The scattering time in the shear flow region 0 < $\tau {(r,p)={\tau}_{0}(p/{p}_{0})}^{\alpha}$r <r _{2}, and , where $\tau {(r,p)={\tau}_{0}(p/{p}_{0})}^{\alpha}{(r/{r}_{2})}^{s}$s > 0 in the regionr >r _{2}is outside the jet. Other original aspects of the analysis are (i) the use of cosmic ray flow lines in (r ,p ) space to clarify the particle spatial transport and momentum changes and (ii) the determination of the probability distribution that particles observed at ( ${\psi}_{p}(r,p;p\prime )$r ,p ) originated fromr → ∞ with momentum . The acceleration of ultrahighenergy cosmic rays in active galactic nuclei jet sources is discussed. Leaky box models for electron acceleration are described. $p\prime $ 
Abstract We report the discovery and confirmation of the Transiting Exoplanet Survey Satellite (TESS) singletransit, warm and dense subSaturn, TIC 139270665 b. This planet is unusually dense for its size: with a bulk density of 2.13 g cm^{−3}(0.645
R _{J}, 0.463M _{J}), it is the densest warm subSaturn of the TESS family. It orbits a metalrich G2 star. We also found evidence of a second planet, TIC 139270665 c, with a longer period of days and minimum mass ${1010}_{220}^{+780}$ of ${M}_{P}\mathrm{sin}i$ ${4.89}_{0.37}^{+0.66}$M _{J}. First clues of TIC 139270665 b’s existence were found by citizen scientists inspecting TESS photometric data from sector 47 in 2022 January. Radial velocity measurements from the Automated Planet Finder combined with TESS photometry and spectral energy distributions viaEXOFASTv2 system modeling suggested a day orbital period for TIC 139270665 b and also showed evidence for the second planet. Based on this estimated period, we mobilized the Unistellar citizen science network for photometric followup, capitalizing on their global distribution to capture a second transit of TIC 139270665 b. This citizen science effort also served as a test bed for an education initiative that integrates young students into modern astrophysics data collection. The Unistellar photometry did not definitively detect a second transit, but did enable us to further constrain the planet’s period. As a transiting, warm, and dense subSaturn, TIC 139270665 b represents an interesting laboratory for further study to enhance our models of planetary formation and evolution. ${23.624}_{0.031}^{+0.030}$ 
Abstract We perform particleincell simulations to elucidate the microphysics of relativistic weakly magnetized shocks loaded with electronpositron pairs. Various external magnetizations
σ ≲ 10^{−4}and pairloading factorsZ _{±}≲ 10 are studied, whereZ _{±}is the number of loaded electrons and positrons per ion. We find the following: (1) The shock becomes mediated by the ion Larmor gyration in the mean field whenσ exceeds a critical valueσ _{L}that decreases withZ _{±}. Atσ ≲σ _{L}the shock is mediated by particle scattering in the selfgenerated microturbulent fields, the strength and scale of which decrease withZ _{±}, leading to lowerσ _{L}. (2) The energy fraction carried by the postshock pairs is robustly in the range between 20% and 50% of the upstream ion energy. The mean energy per postshock electron scales as . (3) Pair loading suppresses nonthermal ion acceleration at magnetizations as low as ${\overline{E}}_{\mathrm{e}}\propto {\left({Z}_{\pm}+1\right)}^{1}$σ ≈ 5 × 10^{−6}. The ions then become essentially thermal with mean energy , while electrons form a nonthermal tail, extending from ${\overline{E}}_{\mathrm{i}}$ to $E\sim {\left({Z}_{\pm}+1\right)}^{1}{\overline{E}}_{\mathrm{i}}$ . When ${\overline{E}}_{\mathrm{i}}$σ = 0, particle acceleration is enhanced by the formation of intense magnetic cavities that populate the precursor during the late stages of shock evolution. Here, the maximum energy of the nonthermal ions and electrons keeps growing over the duration of the simulation. Alongside the simulations, we develop theoretical estimates consistent with the numerical results. Our findings have important implications for models of early gammaray burst afterglows. 
Abstract Polyatomic molecules have been identified as sensitive probes of chargeparity violating and parity violating physics beyond the Standard Model (BSM). For example, many linear triatomic molecules are both lasercoolable and have parity doublets in the ground electronic
state arising from the bending vibration, both features that can greatly aid BSM searches. Understanding the $\tilde{X}{}^{2}{\mathrm{\Sigma}}^{+}(010)$ state is a crucial prerequisite to precision measurements with linear polyatomic molecules. Here, we characterize the fundamental bending vibration of $\tilde{X}{}^{2}{\mathrm{\Sigma}}^{+}(010)$ YbOH using highresolution optical spectroscopy on the nominally forbidden ${}^{174}$ $\tilde{X}{}^{2}{\mathrm{\Sigma}}^{+}(010)$ transition at 588 nm. We assign 39 transitions originating from the lowest rotational levels of the $\to \tilde{A}{}^{2}{\mathrm{\Pi}}_{1/2}(000)$ state, and accurately model the state’s structure with an effective Hamiltonian using bestfit parameters. Additionally, we perform Stark and Zeeman spectroscopy on the $\tilde{X}{}^{2}{\mathrm{\Sigma}}^{+}(010)$ state and fit the moleculeframe dipole moment to $\tilde{X}{}^{2}{\mathrm{\Sigma}}^{+}(010)$ ${D}_{\mathrm{m}\mathrm{o}\mathrm{l}}=2.16(1)$D and the effective electrong factor to . Further, we use an empirical model to explain observed anomalous line intensities in terms of interference from spin–orbit and vibronic perturbations in the excited ${g}_{S}=2.07(2)$ state. Our work is an essential step toward searches for BSM physics in YbOH and other linear polyatomic molecules. $\tilde{A}{}^{2}{\mathrm{\Pi}}_{1/2}(000)$