arXiv subjects

S. Tan

Publications and source records attributed to S. Tan.

10 recordsLinked to original sources

arXiv2025

CoSEE-Cat: A Comprehensive Solar Energetic Electron event Catalogue obtained from combined in situ and remote-sensing observations from Solar Orbiter -- Catalogue description and first statistical results

(abridged) The acceleration of particles at the Sun and their propagation through interplanetary space are key topics in heliophysics. Specifically, solar energetic electrons (SEEs) measured in situ can be linked to solar flares and coronal mass ejections (CMEs), which can be observed remotely in radio, optical, UV and X-rays. Solar Orbiter, equipped with a wide range of remote-sensing and in situ detectors, provides an excellent opportunity to investigate SEEs and their solar origin from the inner heliosphere. The Comprehensive Solar Energetic Electron event Catalogue (CoSEE-Cat) provides details for all SEE events measured in situ by Solar Orbiter, as well as associated flares, CMEs, and radio bursts. The catalogue contains key parameters of the SEEs, as derived from the Energetic Particle Detector (EPD), complemented with basic parameters of associated X-ray flares provided by STIX, information on eruptive phenomena seen by EUI, characteristics of type III radio bursts observed by RPW, and indications of CME observed by Metis and SoloHI. The conditions in interplanetary space were characterised using SWA and MAG measurements. Finally, data-driven modelling with the Magnetic Connectivity Tool provided an independent estimate of the solar source position of the SEEs. The first data release contains 303 SEE events observed from November 2020 until the end of 2022. In this paper, we describe the catalogue and provide a first statistical analysis. Preliminary results support the flare-related origin of events with an impulsive ion composition and the association of gradual events with extended structures such as CME-driven shocks or erupting flux ropes. We also show that the commonly observed delays of the solar release times of the SEEs relative to the associated X-ray flares and type III radio bursts are at least partially due to propagation effects.

astro-ph.SR↗

arXiv2025

Laser Amplification in $e^{-}$-$\mu^{-}$-ion Plasmas

We investigate laser amplification in $e^{-}$-$\mu^{-}$-ion plasmas, where negative muons partially replace electrons. Theoretical results reveal a hybrid plasma wave, called $\mu$-wave that exhibits ion-acoustic behavior in long-wavelength regime and Langmuir-like behavior in short-wavelength regime. Besides, the Landau damping of $\mu$-wave is smaller than that of Langmuir wave. Particle-in-cell (PIC) simulations confirm the theoretical results of instabilities in$e^{-}$-$\mu^{-}$-ion plasmas. The $\mu$-wave enables efficient laser amplification by suppressing pump-driven spontaneous instabilities through enhanced Landau damping of Langmuir waves. Compared to Raman amplification, $\mu$-wave amplification can maintain the Gaussian waveform of the seed laser, avoiding pulse splitting. Compared to strongcoupling Brillouin amplification, $\mu$-wave amplification exhibits weaker filamentation instability. Our theoretical model can be generalized to other plasma systems containing two species of negatively charged particles, such as two-temperature electron plasmas and negative-ion plasma. These findings establish $e^{-}$-$\mu^{-}$-ion plasma as a promising medium for advanced laser amplification schemes.

physics.plasm-ph↗

arXiv2024

Generalized $T_e$([OIII])-$T_e$(HeI) Discrepancies in Ionized Nebulae: Possible Evidence of Case B Deviations and Temperature Inhomogeneities

The physics of recombination lines (RLs) in the HeI singlet system is expected to be relatively simple, supported by accurate atomic models. We examine the intensities of HeI singlets $\lambda \lambda$3614, 3965, 5016, 6678, 7281 and the triplet HeI $\lambda$5876 in various types of ionized nebulae and compare them with theoretical predictions to test the validity of the ``Case B'' recombination scenario and the assumption of thermal homogeneity. Our analysis includes 85 spectra from Galactic and extragalactic HII regions, 90 from star-forming galaxies, and 218 planetary nebulae, all compiled by the DEep Spectra of Ionized REgions Database Extended (DESIRED-E) project. By evaluating the ratios HeI $\lambda$7281/$\lambda$6678 and HeI $\lambda$7281/$\lambda$5876, we determine $T_e$(HeI) and compare it with direct measurements of $T_e$([OIII] $\lambda$4363/$\lambda$5007). We find that $T_e$(HeI) is systematically lower than $T_e$([OIII]) across most objects and nebula types. Additionally, we identify a correlation between the abundance discrepancy factor (ADF(O$^{2+}$)) and the difference $T_e$([OIII]) - $T_e$(HeI) for planetary nebulae. We explore two potential explanations: photon loss from $n^1P \rightarrow 1^1S$ transitions and temperature inhomogeneities. Deviations from ``Case B'' may indicate photon absorption by HI rather than HeI and/or generalized ionizing photon escape, highlighting the need for detailed consideration of radiative transfer effects. If temperature inhomogeneities are widespread, identifying a common physical phenomenon affecting all ionized nebulae is crucial. Our results suggest that both scenarios can contribute to the observed discrepancies.

astro-ph.GA↗

arXiv2023

Role of a capping layer on the crystalline structure of Sn thin films grown at cryogenic temperatures on InSb substrates

Metal deposition with cryogenic cooling is widely utilized in the condensed matter community for producing ultra-thin epitaxial superconducting layers on semiconductors. However, a drawback arises when these films return to room temperature, as they tend to undergo dewetting. This issue is mitigated by capping the films with an amorphous layer. In this study, we examined the impact of different in-situ fabricated caps on the structural characteristics of Sn thin films deposited at 80 K on InSb substrates. Regardless of the type of capping, we observed that the films remained smooth upon returning to room temperature and were epitaxial on InSb in the cubic Sn ($\alpha$-Sn) phase. However, we noted a correlation between alumina capping with an electron beam evaporator and an increased presence of tetragonal Sn ($\beta$-Sn) grains. This suggests that heating from the alumina source may contribute to a partial phase transition in the Sn layer. The existence of the $\beta$-Sn phase induced superconducting behavior of the films by percolation effect. This study highlights the potential for modifying the structural properties of cryogenic Sn thin films through in-situ capping, paving the way for precise control in the production of superconducting Sn films for integration into quantum computing platforms.

cond-mat.mtrl-sci↗

arXiv2022

Evidence of $\phi$0-Josephson junction from skewed diffraction patterns in Sn-InSb nanowires

We study Josephson junctions based on InSb nanowires with Sn shells. We observe skewed critical current diffraction patterns: the maxima in forward and reverse current bias are at different magnetic flux, with a displacement of 20-40 mT. The skew is greatest when the external field is nearly perpendicular to the nanowire, in the substrate plane. This orientation suggests that spin-orbit interaction plays a role. We develop a phenomenological model and perform tight-binding calculations, both methods reproducing the essential features of the experiment. The effect modeled is the $\phi$0-Josephson junction with higher-order Josephson harmonics. The system is of interest for Majorana studies: the effects are either precursor to or concomitant with topological superconductivity. Current-phase relations that lack inversion symmetry can also be used to design quantum circuits with engineered nonlinearity.

cond-mat.mes-hall↗

arXiv2022

Planar Josephson Junctions Templated by Nanowire Shadowing

More and more materials, with a growing variety of properties, are built into electronic devices. This is motivated both by increased device performance and by the studies of materials themselves. An important type of device is a Josephson junction based on the proximity effect between a quantum material and a superconductor, useful for fundamental research as well as for quantum and other technologies. When both junction contacts are placed on the same surface, such as a two-dimensional material, the junction is called ``planar". One outstanding challenge is that not all materials are amenable to the standard planar junction fabrication. The device quality, rather than the intrinsic characteristics, may be defining the results. Here, we introduce a technique in which nanowires are placed on the surface and act as a shadow mask for the superconductor. The advantages are that the smallest dimension is determined by the nanowire diameter and does not require lithography, and that the junction is not exposed to chemicals such as etchants. We demonstrate this method with an InAs quantum well, using two superconductors - Al and Sn, and two semiconductor nanowires - InAs and InSb. The junctions exhibit critical current levels consistent with transparent interfaces and uniform width. We show that the template nanowire can be operated as a self-aligned electrostatic gate. Beyond single junctions, we create SQUIDs with two gate-tunable junctions. We suggest that our method can be used for a large variety of quantum materials including van der Waals layers, topological insulators, Weyl semimetals and future materials for which proximity effect devices is a promising research avenue.

cond-mat.mes-hall↗

arXiv2019

Neutron scattering study of commensurate magnetic ordering in single crystal CeSb$_2$

Temperature and field-dependent magnetization $M(H,T)$ measurements and neutron scattering study of a single crystal CeSb$_2$ are presented. Several anomalies in the magnetization curves have been confirmed at low magnetic field, i.e., 15.6 K, 12 K, and 9.8 K. These three transitions are all metamagnetic transitions (MMT), which shift to lower temperatures as the magnetic field increases. The anomaly at 15.6 K has been suggested as paramagnetic (PM) to ferromagnetic (FM) phase transition. The anomaly located at around 12 K is antiferromagnetic-like transition, and this turning point will clearly split into two when the magnetic field $H\geq0.2$ T. Neutron scattering study reveals that the low temperature ground state of CeSb$_2$ orders antiferromagnetically with commensurate propagation wave vectors $\textbf{k}=(-1,\pm1/6,0)$ and $\textbf{k}=(\pm1/6,-1,0)$, with N\'eel temperature $T_N\sim9.8$ K. This transition is of first-order, as shown in the hysteresis loop observed by the field cooled cooling (FCC) and field cooled warming (FCW) processes.

cond-mat.mtrl-sci↗

arXiv2018

Enhanced energy gain in a dielectric laser accelerator using a tilted pulse front laser

Using an 800 nm, 45 fs pulse-front-tilted laser we demonstrate a record 315 keV energy gain in a dual grating dielectric laser accelerator (DLA) and average accelerating gradients of 560 MV/m over 0.5 mm. These results open a new regime in DLA characterized by significant evolution of the beam distribution in the longitudinal phase space, corresponding to >1/4 of a synchrotron oscillation. By tilting the laser wavefront we control the resonant velocity of the DLA and observe a net energy gain, indicating that a tapered optical phase could be used to achieve very high energy gain.

physics.acc-ph↗

arXiv2013

LISA-2020: An Intermediate Scale Space Gravitational Wave Observatory for This Decade

Over the last three decades, an exceptionally good science case has been made for pursuing gravitational wave (GW) astronomy. This has engendered a worldwide effort to detect the extremely weak signals generated by expected sources. With the next round of upgrades the ground based instruments are likely to make the first detections of the sources, and a new era of astronomy will begin, possibly as early as 2017. Inconveniently, due to seismic noise and baseline length issues, the low frequency (<10Hz) part of the spectrum, where the most interesting events are expected, will not be accessible. The space-based detector, LISA1, was conceived to fill this gap extending the observational capability to about 10-4 Hz. Due to mission cost growth and severe budget constraints, a flight prior to 2030 now seems very unlikely. This paper examines the case for a scaled down mission that is comparable in cost and duration to medium scale astrophysics missions such as the 1978 ($630M) Einstein (HEAO 2) x-ray Observatory2, the 1989 ($680M) COBE Cosmic Background Explorer3, and the 1999 ($420M) FUSE Far Ultraviolet Spectroscopic Explorer4. We find that a mission of this class is possible if the measurement requirements are somewhat relaxed and a baseline smaller than LISA is used. It appears that such a mission could be launched by 2020 using a conventional program development plan, possibly including international collaboration. It would enable the timely development of this game-changing field of astrophysics, complementing the expected ground results with observations of massive black hole collisions. It would also serve as a stepping stone to LISA, greatly reducing the risk profile of that mission.

gr-qc↗

arXiv2011

Coexistence of superconductivity and magnetism in K0.8Fe2Se1.4S0.4

High-quality single crystals of K0.8Fe2Se1.4S0.4 are successfully synthesized by self-flux method with the superconducting transition temperatures Tconset = 32.8 K and Tczero = 31.2 K. In contrast to external pressure effect on superconductivity, the substitution of S for Se does not suppress Tc, which suggests that chemical doping may mainly modulate the anion height from Fe-layer rather than compressing interlayer distance. The investigation of the micromagnetism by electron spin resonance shows clear evidence for strong spin fluctuation at temperatures above Tc. Accompanied by the superconducting feature spectra, a novel resonance signal develops gradually upon cooling below Tc, indicating the coexistence of superconductivity and magnetism in K0.8Fe2Se1.4S0.4 crystal.

cond-mat.supr-con↗