arXiv subjects

C. -C. Hsu

Publications and source records attributed to C. -C. Hsu.

8 recordsLinked to original sources

arXiv2026

Clouds with a silicate lining: Using JWST spectra to probe atmospheric diversity in young AB Dor L dwarfs

We present the first full JWST NIRSpec Prism and MIRI LRS 0.6 - 14 $μ$m (R ~ 100) spectra and analysis of five ~ 133 Myr L dwarf members of the AB Doradus moving group and one probable $\sim 500$ Myr T dwarf of the Oceanus moving group with known inclination angles between ~ $23 - 90^{\circ}$: W0047+68, 2M0355+11, 2M0642+41, W1741-46, 2M2206-42, and 2M2244+20. We construct near-complete spectral energy distributions of each of our objects to measure their bolometric luminosities, and estimate their fundamental parameters ($T_{\text{eff}}$, radius, $M$ and $\log g$). We use cross-sections of relevant gases to identify the species that are present in each atmosphere. Of particular interest is the silicate absorption feature at 8 - 11 $μ$m, which provides insight into the complex cloud structure of brown dwarfs. We examine this silicate absorption feature in detail and also test whether there exists a latitudinal dependence in the silicate absorption feature within a coeval sample of brown dwarfs. Various molecular absorption bands are visible in our spectra, including H$_2$O, CH$_4$, CO and CO$_2$. The shape of the silicate absorption feature varies within our sample, and we find that 4/5 of our L type objects agree with previously observed trends stating that objects viewed equator-on have deeper silicate absorption. We highlight W1741-46 as an outlier in our sample with an unusually strong silicate absorption given its near pole-on orientation. We also present a tentative correlation between the wavelength of peak silicate absorption and inclination, which may suggest variations in cloud chemical composition or physical properties. We find an unexpected spectral diversity within our sample, which motivates future studies on these objects through atmospheric retrievals, which will determine the silicate cloud composition and reveal whether there exists a trend with inclination.

astro-ph.SR↗

arXiv2020

Nanoscale Strain Engineering of Giant Pseudo-Magnetic Fields, Valley Polarization and Topological Channels in Graphene

The existence of nontrivial Berry phases associated with two inequivalent valleys in graphene provides interesting opportunities for investigating the valley-projected topological states. Examples of such studies include observation of anomalous quantum Hall effect in monolayer graphene, demonstration of topological zero modes in molecular graphene assembled by scanning tunneling microscopy, and detection of topological valley transport either in graphene superlattices or at bilayer graphene domain walls. However, all aforementioned experiments involved non-scalable approaches of either mechanically exfoliated flakes or atom-by-atom constructions. Here we report an approach to manipulating the topological states in monolayer graphene via nanoscale strain engineering at room temperature. By placing strain-free monolayer graphene on architected nanostructures to induce global inversion symmetry breaking, we demonstrate the development of giant pseudo-magnetic fields (up to 800 Tesla), valley polarization, and periodic one-dimensional topological channels for protected propagation of chiral modes in strained graphene, thus paving a pathway towards scalable graphene-based valleytronics.

cond-mat.mes-hall↗

arXiv2016

Spectroscopic Studies of the Physical Origin of Environmental Aging Effects on Doped Graphene

The environmental aging effect of doped graphene is investigated as a function of the organic doping species, humidity, and the number of graphene layers adjacent to the dopant by studies of the Raman spectroscopy, x-ray and ultraviolet photoelectron spectroscopy, scanning electron microscopy, infrared spectroscopy, and electrical transport measurements. It is found that higher humidity and structural defects induce faster degradation in doped graphene. Detailed analysis of the spectroscopic data suggest that the physical origin of the aging effect is associated with the continuing reaction of H2O molecules with the hygroscopic organic dopants, which leads to formation of excess chemical bonds, reduction in the doped graphene carrier density, and proliferation of damages from the graphene grain boundaries. These environmental aging effects are further shown to be significantly mitigated by added graphene layers.

cond-mat.mes-hall↗

arXiv2016

Kepler K2 Observations of Sco X-1: Orbital Modulations and Correlations with Fermi GBM and MAXI

We present a multi-wavelength study of the low-mass X-ray binary Sco X-1 using Kepler K2 optical data and Fermi GBM and MAXI X-ray data. We recover a clear sinusoidal orbital modulation from the Kepler data. Optical fluxes are distributed bimodally around the mean orbital light curve, with both high and low states showing the same modulation. The high state is broadly consistent with the flaring branch of the Z diagram and the low state with the normal branch. We see both rapid optical flares and slower dips in the high state, and slow brightenings in the low state. High state flares exhibit a narrow range of amplitudes with a striking cut-off at a maximum amplitude. Optical fluxes correlate with X-ray fluxes in the high state, but in the low state they are anti-correlated. These patterns can be seen clearly in both flux-flux diagrams and cross-correlation functions and are consistent between MAXI and GBM. The high state correlation arises promptly with at most a few minutes lag. We attribute this to thermal reprocessing of X-ray flares. The low state anti-correlation is broader, consistent with optical lags of between zero and 3 ~min, and strongest with respect to high energy X-rays. We suggest that the decreases in optical flux in the low state may reflect decreasing efficiency of disc irradiation, caused by changes in the illumination geometry. These changes could reflect the vertical extent or covering factor of obscuration or the optical depth of scattering material.

astro-ph.HE↗

arXiv2015

Nano-scale strain engineering of graphene and graphene-based devices

Structural distortions in nano-materials can induce dramatic changes in their electronic properties. This situation is well manifested in graphene, a two-dimensional honeycomb structure of carbon atoms with only one atomic layer thickness. In particular, strained graphene can result in both charging effects and pseudo-magnetic fields, so that controlled strain on a perfect graphene lattice can be tailored to yield desirable electronic properties. Here we describe the theoretical foundation for strain-engineering of the electronic properties of graphene, and then provide experimental evidences for strain-induced pseudo-magnetic fields and charging effects in monolayer graphene. We further demonstrate the feasibility of nanoscale strain engineering for graphene-based devices by means of theoretical simulations and nano-fabrication technology.

cond-mat.mes-hall↗

arXiv2015

Single-step deposition of high-mobility graphene at reduced temperatures

Current methods of chemical vapor deposition (CVD) of graphene on copper are complicated by multiple processing steps and by high temperatures required in both preparing the copper and inducing subsequent film growth. Here we demonstrate a plasma-enhanced CVD chemistry that enables the entire process to take place in a single step, at reduced temperatures (< 420 C), and in a matter of minutes. Growth on copper foils is found to nucleate from arrays of well-aligned domains, and the ensuing films possess sub-nanometre smoothness, excellent crystalline quality, low strain, few defects and room temperature electrical mobility up to (6.0 +- 1.0) x 10^{4} cm^{2}V^{-1}s^{-1}, better than that of large, single-crystalline graphene derived from thermal CVD growth. These results indicate that elevated temperatures and crystalline substrates are not necessary for synthesizing high-quality graphene.

cond-mat.mtrl-sci↗

arXiv2014

Experimental study of electric breakdowns in liquid argon at centimeter scale

In this paper we present results on measurements of the dielectric strength of liquid argon near its boiling point and cathode-anode distances in the range of 0.1 mm to 40 mm with spherical cathode and plane anode. We show that at such distances the applied electric field at which breakdowns occur is as low as 40 kV/cm. Flash-overs across the ribbed dielectric of the high voltage feed-through are observed for a length of 300 mm starting from a voltage of 55 kV. These results contribute to set reference for the breakdown-free design of ionization detectors, such as Liquid Argon Time Projection Chambers (LAr TPC).

physics.ins-det↗

arXiv2009

The June 2008 flare of Markarian 421 from optical to TeV energies

We present optical to very-high energy (VHE) gamma-ray observations of Mrk 421 between 2008 May 24 and June 23. A high-energy (HE) gamma-ray signal was detected by AGILE-GRID during June 9-15, brighter than the average flux observed by EGRET in Mrk 421 by a factor of approx. 1.5. In 20-60 keV X-rays, a large-amplitude 5-day flare (June 9-15) was resolved with a maximum flux of approx. 55 mCrab. SuperAGILE, RXTE/ASM and Swift/BAT data show a clearly correlated flaring structure between soft and hard X-rays, with a high flux/amplitude variability in hard X-rays. Hints of the same flaring behavior is also detected in the simultaneously recorded GASP-WEBT optical data. A target of opportunity observation by Swift near the flare maximum on June 12-13 revealed the highest 2-10 keV flux ever observed (>100 mCrab) and a peak synchrotron energy of approx. 3 keV, a large shift from typical values of 0.5-1 keV. Observations at VHE (E>200 GeV) gamma-rays during June 6-8 show the source flux peaking in a bright state, well correlated with the simultaneous peak in the X-rays. The gamma-ray flare can be interpreted within the framework of the Synchrotron Self Compton model in terms of a rapid acceleration of leptons in the jet.

astro-ph.CO↗