arXiv subjects

S. Yu

Publications and source records attributed to S. Yu.

At least 37 records · Page 2Linked to original sources

arXiv2021

Temperature-dependent energy-level shifts of Spin Defects in hexagonal Boron Nitride

Two-dimensional hexagonal boron nitride (hBN) has attracted large attentions as platforms for realizations for integrated nanophotonics and collective effort has been focused on the spin defect centers. Here, the temperature dependence of the resonance spectrum in the range of 5-600 K is investigated. The zero-field splitting (ZFS) parameter D is found to decrease monotonicly with increasing temperature and can be described by Varshni empirical equation perfectly, while E almost does not change. We systematically study the differences among different hBN nanopowders and provide an evidence of edge effects on ODMR of VB- defects. Considering the proportional relation between D and reciprocal of lattice volume, the thermal expansion might be the dominant reason for energy-level shifts. We also demonstrate that the VB- defects still exist stably at least at 600 K. Moreover, we propose a scheme for detecting laser intensity using the VB- defects in hBN nanopowders, which is based on the obvious dependence of its D value on laser intensity. Our results are helpful to gain insight into the spin properties of VB- and for the realizations of miniaturized, integrated thermal sensor.

quant-ph↗

arXiv2020

Single-photon distributed free-space spectroscopy

Spectroscopy is a well-established nonintrusive tool that has played an important role in identifying substances and quantifying their compositions, from quantum descriptions to chemical and biomedical diagnostics. Challenges exist in accurate measurements in dynamic environments, especially for understanding chemical reactions in arbitrary free-space. We develop a distributed free-space spectroscopy realized by a comb-referenced frequency-scanning single-photon lidar, providing multidimensional (time-range-spectrum) remote sensing. A continuous field experiment over 72 hours is deployed to obtain the spectra of multiple molecules (CO2 and HDO) in free-space over 6 km, with a range resolution of 60 m and a time resolution of 10 min over a spectrum span of 30 GHz. The CO2 and HDO concentrations are retrieved from the spectra acquired. This distributed free-space spectroscopy holds much promise for increasing knowledge of atmospheric environments and chemistry research, especially for complex molecular spectra evolution in any location over large areas.

physics.ao-ph↗

arXiv2020

Solar Flare Prediction Using Magnetic Field Diagnostics Above the Photosphere

In this article, we present the application of the weighted horizontal gradient of magnetic field (WGM) flare prediction method to 3-dimensional (3D) extrapolated magnetic configurations of 13 flaring solar active regions (ARs). The main aim is to identify an optimal height range, if any, in the interface region between the photosphere and lower corona, where the flare onset time prediction capability of WGM is best exploited. The optimal height is where flare prediction, by means of the WGM method, is achieved earlier than at the photospheric level. 3D magnetic structures, based on potential and non-linear force-free field extrapolations, are constructed to study a vertical range from the photosphere up to the low corona with a 45 km step size. The WGM method is applied as a function of height to all 13 flaring AR cases that are subject to certain selection criteria. We found that applying the WGM method between 1000 and 1800 km above the solar surface would improve the prediction of the flare onset time by around 2-8 hours.Certain caveats and an outlook for future work along these lines are also discussed.

astro-ph.SR↗

arXiv2019

Probing Ultrafast Magnetic-Field Generation by Current Filamentation Instability in Femtosecond Relativistic Laser-Matter Interactions

We present experimental measurements of the femtosecond time-scale generation of strong magnetic-field fluctuations during the interaction of ultrashort, moderately relativistic laser pulses with solid targets. These fields were probed using low-emittance, highly relativistic electron bunches from a laser wakefield accelerator, and a line-integrated $B$-field of $2.70 \pm 0.39\,\rm kT\,μm$ was measured. Three-dimensional, fully relativistic particle-in-cell simulations indicate that such fluctuations originate from a Weibel-type current filamentation instability developing at submicron scales around the irradiated target surface, and that they grow to amplitudes strong enough to broaden the angular distribution of the probe electron bunch a few tens of femtoseconds after the laser pulse maximum. Our results highlight the potential of wakefield-accelerated electron beams for ultrafast probing of relativistic laser-driven phenomena.

physics.plasm-ph↗

arXiv2019

Rotational and rotational-vibrational Raman spectroscopy of air to characterize astronomical spectrographs

Raman scattering enables unforeseen uses for the laser guide-star system of the Very Large Telescope. Here, we present the observation of one up-link sodium laser beam acquired with the ESPRESSO spectrograph at a resolution $λ/Δλ\sim 140'000$. In 900s on-source, we detect the pure rotational Raman lines of $^{16}$O$_2$, $^{14}$N$_2$, and $^{14}$N$^{15}$N (tentatively) up to rotational quantum numbers $J$ of 27, 24, and 9, respectively. We detect the $^{16}$O$_2$ fine-structure lines induced by the interaction of the electronic spin \textbf{S} and end-over-end rotational angular momentum \textbf{N} in the electronic ground state of this molecule up to $N=9$. The same spectrum also reveals the $ν_{1\leftarrow0}$ rotational-vibrational Q-branch for $^{16}$O$_2$ and $^{14}$N$_2$. These observations demonstrate the potential of using laser guide-star systems as accurate calibration sources for characterizing new astronomical spectrographs.

astro-ph.IM↗

arXiv2018

Single Crystal Growth and Spin Polarization Measurements of Diluted Magnetic Semiconductor (BaK)(ZnMn)2As2

Recently a new type diluted magnetic semiconductor (BaK)(ZnMn)2As2 (BZA) with high Cure temperature (Tc) was discovered showing independent spin and charge doping mechanism. This makes BZA a promising material for spintronics devices. Here we report for the first time the successful growth of BZA single crystal. An Andreev reflection junction that can be used to evaluate spin polarization was fabricated based on the BZA single crystal, a 66% spin polarization of the BZA single crystal was hence obtained by Andreev reflection spectroscopy analysis.

cond-mat.mtrl-sci↗

arXiv2018

Imaging Spectroscopy of Solar Radio Burst Fine Structures

Solar radio observations provide a unique diagnostic of the outer solar atmosphere. However, the inhomogeneous turbulent corona strongly affects the propagation of the emitted radio waves, so decoupling the intrinsic properties of the emitting source from the effects of radio-wave propagation has long been a major challenge in solar physics. Here we report quantitative spatial and frequency characterization of solar radio burst fine structures observed with the LOw Frequency Array (LOFAR), an instrument with high time resolution that also permits imaging at scales much shorter than those corresponding to radio-wave propagation in the corona. The observations demonstrate that radio-wave propagation effects, and not the properties of the intrinsic emission source, dominate the observed spatial characteristics of radio burst images. These results permit more accurate estimates of source brightness temperatures, and open opportunities for quantitative study of the mechanisms that create the turbulent coronal medium through which the emitted radiation propagates.

astro-ph.SR↗

arXiv2018

On the evolution of pre-flare patterns of a 3-dimensional model of AR 11429

We apply a novel pre-flare tracking of sunspot groups towards improving the estimation of flare onset time by focusing on the evolution of the 3D magnetic field construction of AR 11429. The 3D magnetic structure is based on potential field extrapolation encompassing a vertical range from the photosphere through the chromosphere and transition region into the low corona. The basis of our proxy measure of activity prediction is the so-called weighted horizontal gradient of magnetic field (WG_M) defined between spots of opposite polarities close to the polarity inversion line of an active region. The temporal variation of the distance of the barycenter of the opposite polarities is also found to possess potentially important diagnostic information about the flare onset time estimation as function of height similar to its counterpart introduced initially in an application at the photosphere only in Korsos et al. (2015). We apply the photospheric pre-flare behavioural patterns of sunspot groups to the evolution of their associated 3D-constructed AR 11429 as function of height. We found that at a certain height in the lower solar atmosphere the onset time may be estimated much earlier than at the photosphere or at any other heights. Therefore, we present a tool and recipe that may potentially identify the optimum height for flare prognostic in the solar atmosphere allowing to improve our flare prediction capability and capacity.

astro-ph.SR↗

arXiv2016

The Deep and Transient Universe in the SVOM Era: New Challenges and Opportunities - Scientific prospects of the SVOM mission

To take advantage of the astrophysical potential of Gamma-Ray Bursts (GRBs), Chinese and French astrophysicists have engaged the SVOM mission (Space-based multi-band astronomical Variable Objects Monitor). Major advances in GRB studies resulting from the synergy between space and ground observations, the SVOM mission implements space and ground instrumentation. The scientific objectives of the mission put a special emphasis on two categories of GRBs: very distant GRBs at z$>$5 which constitute exceptional cosmological probes, and faint/soft nearby GRBs which allow probing the nature of the progenitors and the physics at work in the explosion. These goals have a major impact on the design of the mission: the on-board hard X-ray imager is sensitive down to 4 keV and computes on line image and rate triggers, and the follow-up telescopes on the ground are sensitive in the NIR. At the beginning of the next decade, SVOM will be the main provider of GRB positions and spectral parameters on very short time scale. The SVOM instruments will operate simultaneously with a wide range of powerful astronomical devices. This rare instrumental conjunction, combined with the relevance of the scientific topics connected with GRB studies, warrants a remarkable scientific return for SVOM. In addition, the SVOM instrumentation, primarily designed for GRB studies, composes a unique multi-wavelength observatory with rapid slew capability that will find multiple applications for the whole astronomy community beyond the specific objectives linked to GRBs. This report lists the scientific themes that will benefit from observations made with SVOM, whether they are specific GRB topics, or more generally all the issues that can take advantage of the multi-wavelength capabilities of SVOM.

astro-ph.IM↗

arXiv2014

Widespread Rotationally-Hot Hydronium Ion in the Galactic Interstellar Medium

We present new observations of the (6,6) and (9,9) inversion transitions of the hydronium ion toward Sagittarius B2(N) and W31C. Sensitive observations toward Sagittarius B2(N) show that the high, ~ 500 K, rotational temperatures characterizing the population of the highly-excited metastable H3O+ rotational levels are present over a wide range of velocities corresponding to the Sagittarius B2 envelope, as well as the foreground gas clouds between the Sun and the source. Observations of the same lines toward W31C, a line of sight that does not intersect the Central Molecular Zone, but instead traces quiescent gas in the Galactic disk, also imply a high rotational temperature of ~ 380 K, well in excess of the kinetic temperature of the diffuse Galactic interstellar medium. While it is plausible that some fraction of the molecular gas may be heated to such high temperatures in the active environment of the Galactic center, characterized by high X-ray and cosmic ray fluxes, shocks and high degree of turbulence, this is unlikely in the largely quiescent environment of the Galactic disk clouds. We suggest instead that the highly-excited states of the hydronium ion are populated mainly by exoergic chemical formation processes and temperature describing the rotational level population does not represent the physical temperature of the medium. The same arguments may be applicable to other symmetric top rotors, such as ammonia. This offers a simple explanation to the long-standing puzzle of the presence of a pervasive, hot molecular gas component in the central region of the Milky Way. Moreover, our observations suggest that this is a universal process, not limited to the active environments associated with galactic nuclei.

astro-ph.GA↗

arXiv2012

Volume-limited radio survey of ultracool dwarfs

We conducted a volume-limited survey at 4.9 GHz of 32 nearby ultracool dwarfs with spectral types covering the range M7 -- T8. A statistical analysis was performed on the combined data from the present survey and previous radio observations of ultracool dwarfs. Whilst no radio emission was detected from any of the targets, significant upper limits were placed on the radio luminosities that are below the luminosities of previously detected ultracool dwarfs. Combining our results with those from the literature gives a detection rate for dwarfs in the spectral range M7 -- L3.5 of ~ 9%. In comparison, only one dwarf later than L3.5 is detected in 53 observations. We report the observed detection rate as a function of spectral type, and the number distribution of the dwarfs as a function of spectral type and rotation velocity. The radio observations to date point to a drop in the detection rate toward the ultracool dwarfs. However, the emission levels of detected ultracool dwarfs are comparable to those of earlier type active M dwarfs, which may imply that a mildly relativistic electron beam or a strong magnetic field can exist in ultracool dwarfs. Fast rotation may be a sufficient condition to produce magnetic fields strengths of several hundreds Gauss to several kilo Gauss, as suggested by the data for the active ultracool dwarfs with known rotation rates. A possible reason for the non-detection of radio emission from some dwarfs is that maybe the centrifugal acceleration mechanism in these dwarfs is weak (due to a low rotation rate) and thus cannot provide the necessary density and/or energy of accelerated electrons. An alternative explanation could be long-term variability, as is the case for several ultracool dwarfs whose radio emission varies considerably over long periods with emission levels dropping below the detection limit in some instances.

astro-ph.SR↗

arXiv2012

Nitrogen hydrides in interstellar gas II. Analysis of Herschel/HIFI observations towards W49N and G10.6-0.4 (W31C)

We have used the Herschel-HIFI instrument to observe interstellar nitrogen hydrides along the sight-lines towards W49N and G10.6-0.4 in order to elucidate the production pathways leading to nitrogen-bearing species in diffuse gas. All detections show absorption by foreground material over a wide range of velocities, as well as absorption associated directly with the hot-core source itself. As in the previously published observations towards G10.6-0.4, the NH, NH2 and NH3 spectra towards W49N show strikingly similar and non-saturated absorption features. We decompose the absorption of the foreground material towards W49N into different velocity components in order to investigate whether the relative abundances vary among the velocity components, and, in addition, we re-analyse the absorption lines towards G10.6-0.4 in the same manner. Abundances, with respect to molecular hydrogen, in each velocity component are estimated using CH. The analysis points to a co-existence of the nitrogen hydrides in diffuse or translucent interstellar gas with a high molecular fraction. Towards both sources, we find that NH is always at least as abundant as both o-NH2 and o-NH3, in sharp contrast to previous results for dark clouds. We find relatively constant N(NH)/N(o-NH3) and N(o-NH2)/N(o-NH3) ratios with mean values of 3.2 and 1.9 towards W49N, and 5.4 and 2.2 towards G10.6-0.4, respectively. The mean abundance of o-NH3 is ~2x10^-9 towards both sources. The nitrogen hydrides also show linear correlations with CN and HNC towards both sources, and looser correlations with CH. The upper limits on the NH+ abundance indicate column densities < 2 - 14 % of N(NH). Surprisingly low values of the ammonia ortho-to-para ratio are found in both sources, ~0.5 - 0.7 +- 0.1. This result cannot be explained by current models as we had expected to find a value of unity or higher.

astro-ph.GA↗

arXiv2012

Electron beam induced radio emission from ultracool dwarfs

We present the numerical simulations for an electron-beam-driven and loss-cone-driven electron-cyclotron maser (ECM) with different plasma parameters and different magnetic field strengths for a relatively small region and short time-scale in an attempt to interpret the recent discovered intense radio emission from ultracool dwarfs. We find that a large amount of electromagnetic field energy can be effectively released from the beam-driven ECM, which rapidly heats the surrounding plasma. A rapidly developed high-energy tail of electrons in velocity space (resulting from the heating process of the ECM) may produce the radio continuum depending on the initial strength of the external magnetic field and the electron beam current. Both significant linear polarization and circular polarization of electromagnetic waves can be obtained from the simulations. The spectral energy distributions of the simulated radio waves show that harmonics may appear from 10 to 70$ν_{\rm pe}$ ($ν_{\rm pe}$ is the electron plasma frequency) in the non-relativistic case and from 10 to 600$ν_{\rm pe}$ in the relativistic case, which makes it difficult to find the fundamental cyclotron frequency in the observed radio frequencies. A wide frequency band should therefore be covered by future radio observations.

astro-ph.SR↗

arXiv2012

Superconductivity in SmFe1-xCoxAsO (x = 0.0 to 0.30)

We report synthesis, structural details and magnetization of SmFe1-xCoxAsO with x ranging from 0.0 to 0.30. It is found that Co substitutes fully at Fe site in SmFeAsO in an iso-structural lattice with slightly compressed cell. The parent compound exhibited known spin density wave (SDW) character below at around 140 K. Successive doping of Co at Fe site suppressed the SDW transition for x = 0.05 and later induced superconductivity for x = 0.10, 0.15 and 0.20 respectively at 14, 15.5 and 9K. The lower critical field as seen from magnetization measurements is below 200Oe. The appearance of bulk superconductivity is established by wide open isothermal magnetization M(H) loops. Superconductivity is not observed for higher content of Co i.e. x = 0.30. Clearly the Co substitution at Fe site in SmFe1-xCoxAsO diminishes the Fe SDW character, introduces bulk superconductivity for x between 0.10 and 0.20 and finally becomes non-superconducting for x above 0.20. The Fe2+ site Co3+ substitution injects mobile electrons to the system and superconductivity appears, however direct substitution introduces simultaneous disorder in superconducting FeAs layer and thus superconductivity disappears for higher content of Co.

cond-mat.supr-con↗

arXiv2012

Synthesis and Physical Properties of FeSe1/2Te1/2 Superconductor

One of the most important properties of very recently reported FeSe based superconductors is the robustness of their superconductivity under applied magnetic field. The synthesis and control of superconductivity in FeSe based compounds is rather a difficult task. Synthesis and physical property characterization for optimized superconductivity of FeSe1/2Te1/2 at 13 K is reported here. The compound crystallized in a tetragonal structure with lattice parameters a = 3.8008(10) and c = 6.0187 (15) A. Magnetization measurements indicated bulk superconductivity with lower critical field (Hc1) of around 180 Oe. By applying Ginzburg Landau (GL) theory, the Hc2(0) value is estimated to be = 1840 kOe for the 90% of resistive transition. A heat capacity measurement revealed bulk superconductivity by a hump at Tc near 13 K, and an expected decrease was observed under an applied magnetic field.

cond-mat.supr-con↗

arXiv2011

Comparative analysis of two formation scenarios of bursty radio emission from ultracool dwarfs

Recently, a number of ultracool dwarfs have been found to produce periodic radio bursts with high brightness temperature and polarization degree; the emission properties are similar to the auroral radio emissions of the magnetized planets of the Solar System. We simulate the dynamic spectra of radio emission from ultracool dwarfs. The emission is assumed to be generated due to the electron-cyclotron maser instability. We consider two source models: the emission caused by interaction with a satellite and the emission from a narrow sector of active longitudes; the stellar magnetic field is modeled by a tilted dipole. We have found that for the dwarf TVLM 513-46546, the model of the satellite-induced emission is inconsistent with the observations. On the other hand, the model of emission from an active sector is able to reproduce qualitatively the main features of the radio light curves of this dwarf; the magnetic dipole seems to be highly tilted (by about 60 degrees) with respect to the rotation axis.

astro-ph.SR↗

arXiv2011

Herschel observations of Extra-Ordinary Sources: Methanol as a probe of physical conditions in Orion KL

We have examined methanol emission from Orion KL with of the {\em Herschel}/HIFI instrument, and detected two methanol bands centered at 524 GHz and 1061 GHz. The 524 GHz methanol band (observed in HIFI band 1a) is dominated by the isolated $Δ$J$=$0, K$=-4\rightarrow$-3, v$_t$$=$0 Q branch, and includes 25 E-type and 2 A-type transitions. The 1061 GHz methanol band (observed in HIFI band 4b) is dominated by the $Δ$J$=$0, K$=7\rightarrow$6, v$_t$$=$0 Q branch transitions which are mostly blended. We have used the isolated E-type v$_t$$=$0 methanol transitions to explore the physical conditions in the molecular gas. With HIFI's high velocity resolution, the methanol emission contributed by different spatial components along the line of sight toward Orion KL (hot core, low velocity flow, and compact ridge) can be distinguished and studied separately. The isolated transitions detected in these bands cover a broad energy range (upper state energy ranging from 80 K to 900 K), which provides a unique probe of the thermal structure in each spatial component. The observations further show that the compact ridge is externally heated. These observations demonstrate the power of methanol lines as probes of the physical conditions in warm regions in close proximity to young stars.

astro-ph.SR↗

arXiv2010

Magnetic and electrical properties and carrier doping effects on the Fe-based host compound Sr4Sc2Fe2As2O6

Additional charge carriers were introduced to the iron oxyarsenide Sr4Sc2Fe2As2O6 under a high-pressure condition, followed by measurements of electrical resistivity, Hall coefficient, and magnetic susceptibility. The host compound Sr4Sc2Fe2As2O6 shows metallic conductivity down to ~200 K and turns to show a semiconducting-like conductivity accompanied by a positive magneto-resistance (22% at 70 kOe). Although the carrier density is comparable at 300 K (5.9x1021 cm-3) with that of the other Fe-based superconductors, no superconductivity appears down to 2 K. This is primarily because the net carrier density decreases over 3 orders of magnitude on cooling and additionally a possible magnetic order at ~120 K prevents carriers from pairing. The properties were altered largely by introducing the additional carriers.

cond-mat.supr-con↗