arXiv subjects

Weida Hu

Publications and source records attributed to Weida Hu.

At least 55 records · Page 3Linked to original sources

arXiv2023

The Hubble Deep Hydrogen Alpha (HDH$α$) Project: I. Catalog of Emission-line Galaxies

We present the first results of the Hubble Deep Hydrogen Alpha (HDH$α$) project, which analyzes the space-borne deep H$α$ narrowband imaging data in the GOODS-S region. The HDH$α$ data comprises 72 orbits' images taken with the HST ACS/WFC F658N filter. The exposure time varies across a total area of $\sim$76.1 $\rm{arcmin}^2$, adding up to a total exposure time of 195.7 ks, among which 68.8 ks are spent in the deepest region. These images are aligned, reprojected, and combined to have the same pixel grid as the Hubble Legacy Fields (HLF). The scientific goals of the HDH$α$ include establishing a sample of emission-line galaxies (ELGs) including [O III] emitters at $z\sim$ 0.3, [O II] emitters at $z\sim$ 0.8, and Lyman-$α$ emitters (LAEs) at $z \sim 4.4$, studying the line morphology of ELGs with high resolution imaging data, and statistically analyzing the line luminosity functions and line equivalent-width distributions of ELGs selected with HST. Furthermore, the HDH$α$ project enhances the legacy value of the GOODS-S field by contributing the first HST-based narrowband image to the existing data sets, which includes the HST broadband data and other ancillary data from X-ray to radio taken by other facilities. In this paper, we describe the data reduction process of the HDH$α$, select ELGs based on HST's F658N and broadband data, validate the redshifts of the selected candidates by cross matching with the public spectroscopic catalogs in the GOODS-S, and present a final catalog of the confirmed [O III] emitters at $z\sim$ 0.3, [O II] emitters at $z\sim$ 0.8, and LAEs at $z \sim 4.4$.

astro-ph.GA↗

arXiv2023

Using KCWI to Explore the Chemical Inhomogeneities and Evolution of J1044+0353

J1044+0353 is considered a local analog of the young galaxies that ionized the intergalactic medium at high-redshift due to its low mass, low metallicity, high specific star formation rate, and strong high-ionization emission lines. We use integral field spectroscopy to trace the propagation of the starburst across this small galaxy using Balmer emission- and absorption-line equivalent widths and find a post-starburst population (~ 15 - 20 Myr) roughly one kpc east of the much younger, compact starburst (~ 3 - 4 Myr). Using the direct electron temperature method to map the O/H abundance ratio, we find similar metallicity (1 to 3 sigma) between the starburst and post-starburst regions but with a significant dispersion of about 0.3 dex within the latter. We also map the Doppler shift and width of the strong emission lines. Over scales several times the size of the galaxy, we discover a velocity gradient parallel to the galaxy's minor axis. The steepest gradients (~ 30 $\mathrm{km \ s^{-1} \ kpc^{-1}}$) appear to emanate from the oldest stellar association. We identify the velocity gradient as an outflow viewed edge-on based on the increased line width and skew in a biconical region. We discuss how this outflow and the gas inflow necessary to trigger the starburst affect the chemical evolution of J1044+0353. We conclude that the stellar associations driving the galactic outflow are spatially offset from the youngest association, and a chemical evolution model with a metal-enriched wind requires a more realistic inflow rate than a homogeneous chemical evolution model.

astro-ph.GA↗

arXiv2023

CLASSY VII Lyα Profiles: The Structure and Kinematics of Neutral Gas and Implications for LyC Escape in Reionization-Era Analogs

Lyman-alpha line profiles are a powerful probe of ISM structure, outflow speed, and Lyman continuum escape fraction. In this paper, we present the Ly$α$ line profiles of the COS Legacy Archive Spectroscopic SurveY, a sample rich in spectroscopic analogs of reionization-era galaxies. A large fraction of the spectra show a complex profile, consisting of a double-peaked Ly$α$ emission profile in the bottom of a damped, Ly$α$ absorption trough. Such profiles reveal an inhomogeneous interstellar medium (ISM). We successfully fit the damped Ly$α$ absorption (DLA) and the Ly$α$ emission profiles separately, but with complementary covering factors, a surprising result because this approach requires no Ly$α$ exchange between high-$N_\mathrm{HI}$ and low-$N_\mathrm{HI}$ paths. The combined distribution of column densities is qualitatively similar to the bimodal distributions observed in numerical simulations. We find an inverse relation between Ly$α$ peak separation and the [O III]/[O II] flux ratio, confirming that the covering fraction of Lyman-continuum-thin sightlines increases as the Ly$α$ peak separation decreases. We combine measurements of Ly$α$ peak separation and Ly$α$ red peak asymmetry in a diagnostic diagram which identifies six Lyman continuum leakers in the CLASSY sample. We find a strong correlation between the Ly$α$ trough velocity and the outflow velocity measured from interstellar absorption lines. We argue that greater vignetting of the blueshifted Ly$α$ peak, relative to the redshifted peak, is the source of the well-known discrepancy between shell-model parameters and directly measured outflow properties. The CLASSY sample illustrates how scattering of Ly$α$ photons outside the spectroscopic aperture reshapes Ly$α$ profiles as the distances to these compact starbursts span a large range.

astro-ph.GA↗

arXiv2023

Discovery of spectacular quasar-driven superbubbles in red quasars

Quasar-driven outflows on galactic scales are a routinely invoked ingredient for galaxy formation models. We report the discovery of ionized gas nebulae as traced by [O III] $λ$5007 AA emission surrounding three luminous red quasars at $z \sim 0.4$ from Gemini Integral Field Unit (IFU) observations. All these nebulae feature unprecedented pairs of "superbubbles" extending $\sim$20 kpc in diameter, and the line-of-sight velocity difference between the red- and blue-shifted bubbles reaches up to $\sim$1200 km s$^{-1}$. Their spectacular dual-bubble morphology (in analogy to the Galactic "Fermi bubbles") and their kinematics provide unambiguous evidence for galaxy-wide quasar-driven outflows, in parallel with the quasi-spherical outflows similar in size from luminous Type-1 and -2 quasars at concordant redshift. These bubble pairs manifest themselves as a signpost of the short-lived superbubble ``break-out'' phase, when the quasar wind drives the bubbles to escape the confinement from the dense environment and plunge into the galactic halo with a high-velocity expansion.

astro-ph.GA↗

arXiv2022

On the Origin of the Strong Optical Variability of Emission-line Galaxies

Emission-line galaxies (ELGs) are crucial in understanding the formation and evolution of galaxies, while little is known about their variability. Here we report the study on the optical variability of a sample of ELGs selected in the COSMOS field, which has narrow-band observations in two epochs separated by $\gtrsim$ 12 years. This sample was observed with Suprime-Cam (SC) and Hyper Suprime-Cam (HSC) on the $Subaru$ telescope in NB816 and $i'/i$ bands, respectively. After carefully removing the wing effect of a narrow-band filter, we check the optical variability in a sample of 181 spectroscopically confirmed ELGs. We find that 0 (0/68) Ha emitters, 11.9% (5/42) [OIII] emitters, and 0 (0/71) [OII] emitters show significant variability ($|Δm_{NB}| \geq 3\,σ_{Δm_{NB,AGN}} = 0.20\, mag$) in the two-epoch narrow-band observations. We investigate the presence of active galactic nucleus (AGN) in this variable ELG (var-ELG) sample with three methods, including X-ray luminosity, mid-infrared activity, and radio-excess. We find zero bright AGN in this var-ELG sample, but cannot rule out the contribution from faint AGN. We find that SNe could also dominate the variability of the var-ELG sample. The merger morphology shown in the HST/F814W images of all the var-ELG sample is in agreement with the enhancement of star formation, i.e., the SNe activity.

astro-ph.GA↗

arXiv2022

LAGER Ly$α$ Luminosity Function at $z\sim7$, Implications for Reionization

We present a new measurement of the Ly$α$ luminosity function at redshift $z=6.9$, finding moderate evolution from $z=5.7$ that is consistent with a fully or largely ionized $z\sim7$ intergalactic medium. Our result is based on four fields of the LAGER (Lyman Alpha Galaxies in the Epoch of Reionization) project. Our survey volume of $6.1\times10^{6}$ Mpc$^{3}$ is double that of the next largest $z\sim 7$ survey. We combine two new LAGER fields (WIDE12 and GAMA15A) with two previously reported LAGER fields (COSMOS and CDFS). In the new fields, we identify $N=95$ new $z=6.9$ Ly$α$ emitters (LAEs); characterize our survey's completeness and reliability; and compute Ly$α$ luminosity functions. The best-fit Schechter luminosity function parameters for all four LAGER fields are in good general agreement. Two fields (COSMOS and WIDE12) show evidence for a bright-end excess above the Schechter function fit. We find that the Ly$α$ luminosity density declines at the same rate as the UV continuum LF from $z=5.7$ to $z=6.9$. This is consistent with an intergalactic medium that was fully ionized as early as redshift $z\sim 7$, or with a volume-averaged neutral hydrogen fraction of $x_{HI} < 0.33$ at $1σ$.

astro-ph.GA↗

arXiv2021

New spectroscopic confirmations of Lyman-$α$ emitters at z $\sim$ 7 from the LAGER survey

We report spectroscopic confirmations of 15 Lyman-alpha galaxies at $z\sim7$, implying a spectroscopic confirmation rate of $\sim$80% on candidates selected from LAGER (Lyman-Alpha Galaxies in the Epoch of Reionization), which is the largest (24 deg$^2$) survey aimed at finding Lyman-alpha emitters (LAEs) at $z\sim7$ using deep narrow-band imaging from DECam at CTIO. LAEs at high-redshifts are sensitive probes of cosmic reionization and narrow-band imaging is a robust and effective method for selecting a large number of LAEs. In this work, we present results from the spectroscopic follow-up of LAE candidates in two LAGER fields, COSMOS and WIDE-12, using observations from Keck/LRIS. We report the successful detection of Ly$α$ emission in 15 candidates (11 in COSMOS and 4 in WIDE-12 fields). Three of these in COSMOS have matching confirmations from a previous LAGER spectroscopic follow-up and are part of the overdense region, LAGER-$z7$OD1. Additionally, two candidates that were not detected in the LRIS observations have prior spectroscopic confirmations from Magellan. Including these, we obtain a spectroscopic confirmation success rate of $\sim$$80$% for LAGER LAE candidates. Apart from Ly$α$, we do not detect any other UV nebular lines in our LRIS spectra; however, we estimate a 2$σ$ upper limit for the ratio of NV/Ly$α$, $f_{NV}/f_{Lyα} \lesssim 0.27$, which implies that ionizing emission from these sources is mostly dominated by star formation. Including confirmations from this work, a total of 33 LAE sources from LAGER are now spectroscopically confirmed. LAGER has more than doubled the sample of spectroscopically confirmed LAE sources at $z\sim7$.

astro-ph.GA↗

arXiv2021

Correlations between H$α$ Equivalent Width and Galaxy Properties at $z = 0.47$: Physical or Selection-driven?

The H$α$ equivalent width (EW) is an observational proxy for specific star formation rate (sSFR) and a tracer of episodic star-formation activity. Previous assessments show that EW strongly anti-correlates with stellar mass as $M^{-0.25}$ similar to the sSFR -- stellar mass relation. However, such a correlation may be driven/formed by selection effects. In this study, we investigate how H$α$ EWs correlate with galaxy properties and how selection biases could alter such correlations using a $z = 0.47$ narrowband-selected sample of 1572 H$α$ emitters from the Ly$α$ Galaxies in the Epoch of Reionization (LAGER) survey. The sample covers 3 deg$^2$ of COSMOS and $1.1\times10^5$ cMpc$^3$. We assume an intrinsic EW distribution to form mock samples of H$α$ emitters (HAEs) and propagate the selection criteria to match observations, giving us control on how selection biases can affect the underlying results. We find EW intrinsically correlates with stellar mass as $W_0 \propto M^{-0.16\pm0.03}$ and decreases by a factor of $\sim 3$ from $10^{7}$ to $10^{10}$ M$_\odot$. We find low-mass HAEs to be $\sim 320$ times more likely to have rest-frame EW$ > 200$Åcompared to high-mass HAEs. Combining the intrinsic EW -- stellar mass correlation with an observed SMF correctly reproduces the observed H$α$ LF, while not correcting for selection effects underestimates the number of bright HAEs. This suggests that the intrinsic EW -- stellar mass correlation is physically significant and reproduces three statistical distributions of galaxy populations (LF, SMF, EW distribution). At lower masses, we find there are more high-EW outliers compared to high masses, even after taking into account selection effects. Our results suggest that high sSFR outliers indicative of bursty SF activity are intrinsically more prevalent in low-mass HAEs and not a byproduct of selection effects.

astro-ph.GA↗

arXiv2021

A Lyman-α protocluster at redshift 6.9

Protoclusters, the progenitors of the most massive structures in the Universe, have been identified at redshifts of up to 6.6. Besides exploring early structure formation, searching for protoclusters at even higher redshifts is particularly useful to probe the reionization. Here we report the discovery of the protocluster LAGER-z7OD1 at a redshift of 6.93, when the Universe was only 770 million years old and could be experiencing rapid evolution of the neutral hydrogen fraction in the intergalactic medium. The protocluster is identified by an overdensity of 6 times the average galaxy density, and with 21 narrowband selected Lyman-$α$ galaxies, among which 16 have been spectroscopically confirmed. At redshifts similar to or above this record, smaller protogroups with fewer members have been reported. LAGER-z7OD1 shows an elongated shape and consists of two subprotoclusters, which would have merged into one massive cluster with a present-day mass of $3.7 \times 10^{15}$ solar masses. The total volume of the ionized bubbles generated by its member galaxies is found to be comparable to the volume of the protocluster itself, indicating that we are witnessing the merging of the individual bubbles and that the intergalactic medium within the protocluster is almost fully ionized. LAGER-z7OD1 thus provides a unique natural laboratory to investigate the reionization process.

astro-ph.GA↗

arXiv2020

Reversible Engineering of Topological Insulator Surface State Conductivity through Optical Excitation

Despite the broadband response, limited optical absorption at a particular wavelength hinders the development of optoelectronics based on Dirac fermions. Heterostructures of graphene and various semiconductors have been explored for this purpose, while non-ideal interfaces often limit the performance. The topological insulator is a natural hybrid system, with the surface states hosting high-mobility Dirac fermions and the small-bandgap semiconducting bulk state strongly absorbing light. In this work, we show a large photocurrent response from a field effect transistor device based on intrinsic topological insulator Sn-Bi1.1Sb0.9Te2S. The photocurrent response is non-volatile and sensitively depends on the initial Fermi energy of the surface state, and it can be erased by controlling the gate voltage. Our observations can be explained with a remote photo-doping mechanism, in which the light excites the defects in the bulk and frees the localized carriers to the surface state. This photodoping modulates the surface state conductivity without compromising the mobility, and it also significantly modify the quantum Hall effect of the surface state. Our work thus illustrates a route to reversibly manipulate the surface states through optical excitation, shedding light into utilizing topological surface states for quantum optoelectronics.

cond-mat.mes-hall↗

arXiv2020

Gate-Tunable Semiconductor Heterojunctions from 2D/3D van der Waals Interfaces

Van der Waals (vdW) semiconductors are attractive for highly scaled devices and heterogeneous integration since they can be isolated into self-passivated, two-dimensional (2D) layers that enable superior electrostatic control. These attributes have led to numerous demonstrations of field-effect devices ranging from transistors to triodes. By exploiting the controlled, substitutional doping schemes in covalently-bonded, three-dimensional (3D) semiconductors and the passivated surfaces of 2D semiconductors, one can construct devices that can exceed performance metrics of 'all-2D' vdW heterojunctions. Here, we demonstrate, 2D/3D semiconductor heterojunctions using MoS2 as the prototypical 2D semiconductor laid upon Si and GaN as the 3D semiconductor layers. By tuning the Fermi levels in MoS2, we demonstrate devices that concurrently exhibit over seven orders of magnitude modulation in rectification ratios and conductance. Our results further suggest that the interface quality does not necessarily affect Fermi-level tuning at the junction opening up possibilities for novel 2D/3D heterojunction device architectures.

cond-mat.mes-hall↗

arXiv2020

Chargeable photoconductivity in Van der Waals heterojunctions

Van der Waals (vdW) heterojunctions, based on two-dimensional (2D) materials, show great potential for the development of eco-friendly and high-efficiency nano-devices. Considerable research has been performed and has reported valuable applications of photovoltaic cells, photodetectors, etc. However, simultaneous energy conversion and storage in a single device has not been achieved. Here, we demonstrate a simple strategy to construct a vdW p-n junction between a WSe2 layer and quasi-2D electron gas. After once optical illumination, the device stores the light-generated electrons and holes for up to seven days, and then releases a very large photocurrent of 2.9 mA with bias voltage applied in darkness; this is referred to as chargeable photoconductivity (CPC), which completely differs from any previously observed photoelectric phenomenon. In normal photoconductivity, the recombination of electron-hole pairs takes place at the end of their lifetime, causing a release of heat; in contrast, infinite-lifetime photocarriers can be generated in CPC devices without a thermal loss. The photoelectric conversion and storage are completely self-excited during the charging process. The ratio between currents in full- and empty-energy states below the critical temperature reaches as high as 109, with an external quantum efficiency of 4410000% during optical charging. A theoretical model developed to explain the mechanism of this effect is in good agreement with the experimental data. This work paves a path towards storage-type photoconductors and high-efficiency entropy-decreasing devices.

cond-mat.mtrl-sci↗

arXiv2020

A large, deep 3 deg$^2$ survey of H$α$, [OIII], and [OII] emitters from LAGER: constraining luminosity functions

We present our measurements of the H$α$, [OIII], and [OII] luminosity functions as part of the Lyman Alpha Galaxies at Epoch of Reionization (LAGER) survey using our samples of 1577 $z = 0.47$ H$α$-, 3933 $z = 0.93$ [OIII]-, and 5367 $z = 1.59$ [OII]-selected emission line galaxies in a single 3 deg$^2$ CTIO/Blanco DECam pointing of the COSMOS field. Our observations reach 5$σ$ depths of $8.2\times10^{-18}$ erg s$^{-1}$ cm$^{-2}$ and comoving volumes of $(1-7)\times10^{5}$ Mpc$^3$ making our survey one of the deepest narrowband surveys. We measure the observed luminosity functions and find best-fits of $ϕ^\star = 10^{-3.16\pm0.09}$ Mpc$^{-3}$ and $L^\star = 10^{41.72\pm0.09}$ erg s$^{-1}$ for H$α$, $ϕ^\star = 10^{-2.16^{+0.10}_{-0.12}}$ Mpc$^{-3}$ and $L^\star = 10^{41.38^{+0.07}_{-0.06}}$ erg s$^{-1}$ for [OIII], and $ϕ^\star = 10^{-1.97^{+0.07}_{-0.07}}$ Mpc$^{-3}$ and $L^\star = 10^{41.66\pm0.03}$ erg s$^{-1}$ for [OII], with $α$ fixed to $-1.75$, $-1.6$, and $-1.3$, respectively. An excess of bright $> 10^{42}$ erg s$^{-1}$ [OIII] emitters is observed and may be due to AGN contamination. Dust corrections are applied assuming $A_{\rm{H}α} = 1$ mag. We also design our own empirical rest-frame $g - r$ calibration using SDSS DR12 data, test it against our $z = 0.47$ H$α$ emitters with $z$COSMOS $1$D spectra, and calibrate it for $(g - r)$ between $-0.8$ and $1.3$ mag. Dust and AGN-corrected star formation rate densities (SFRDs) are measured as $\log_{10}ρ_{\rm{SFR}}/(\rm{M}_\odot\ \rm{yr}^{-1}\ \rm{Mpc}^{-3}) = -1.63\pm0.04$, $-1.07\pm0.06$, and $-0.90\pm0.10$ for H$α$, [OIII], and [OII], respectively. We find our [OIII] and [OII] samples fully trace cosmic star formation activity at their respective redshifts in comparison to multi-wavelength SFRDs, while the H$α$ sample traces $\sim 70$ percent of the total $z = 0.47$ SFRD.

astro-ph.GA↗

arXiv2019

Edge-Epitaxial Growth of InSe Nanowires toward High-Performance Photodetectors

Semiconducting nanowires offer many opportunities for electronic and optoelectronic device applications due to their special geometries and unique physical properties. However, it has been challenging to synthesize semiconducting nanowires directly on SiO2/Si substrate due to lattice mismatch. Here, we developed a catalysis-free approach to achieve direct synthesis of long and straight InSe nanowires on SiO2/Si substrate through edge-homoepitaxial growth. We further achieved parallel InSe nanowires on SiO2/Si substrate through controlling growth conditions. We attributed the underlying growth mechanism to selenium self-driven vapor-liquid-solid process, which is distinct from conventional metal-catalytic vapor-liquid-solid method widely used for growing Si and III-V nanowires. Furthermore, we demonstrated that the as-grown InSe nanowire-based visible light photodetector simultaneously possesses an extraordinary photoresponsivity of 271 A/W, ultrahigh detectivity of 1.57*10^14 Jones and a fast response speed of microsecond scale. The excellent performance of the photodetector indicates that as-grown InSe nanowires are promising in future optoelectronic applications. More importantly, the proposed edge-homoepitaxial approach may open up a novel avenue for direct synthesis of semiconducting nanowire arrays on SiO2/Si substrate.

physics.app-ph↗

arXiv2019

A Dual-gate MoS2 Photodetector Based on Interface Coupling Effect

Two-dimensional (2D) transition metal dichalcogenides (TMDs) based photodetectors have shown great potential for the next generation optoelectronics. However, most of the reported MoS2 photodetectors function under the photogating effect originated from the charge-trap mechanism, which is difficult for quantitative control. Such devices generally suffer from a poor compromise between response speed and responsivity (R) and large dark current. Here, a dual-gated (DG) MoS2 phototransistor operating based on the interface coupling effect (ICE) is demonstrated. By simultaneously applying a negative top-gate voltage (VTG) and positive back-gate voltage (VBG) to the MoS2 channel, the photo-generated holes can be effectively trapped in the depleted region under TG. An ultrahigh R of ~1E5 A/W and detectivity (D*) of ~1E14 Jones have been achieved in several devices with different thickness under Pin of 53 uW/cm2 at VTG=-5 V. Moreover, the response time of the DG phototransistor can also be modulated based on the ICE. Based on these systematic measurements of MoS2 DG phototransistors, the results show that the ICE plays an important role in the modulation of photoelectric performances. Our results also pave the way for the future optoelectrical application of 2D TMDs materials and prompt for further investigation in the DG structured phototransistors.

physics.app-ph↗

arXiv2019

The Ly$α$ Luminosity Function and Cosmic Reionization at $z \sim$ 7.0: a Tale of Two LAGER Fields

We present the largest-ever sample of 79 Ly$α$ emitters (LAEs) at $z\sim$ 7.0 selected in the COSMOS and CDFS fields of the LAGER project (the Lyman Alpha Galaxies in the Epoch of Reionization). Our newly amassed ultradeep narrowband exposure and deeper/wider broadband images have more than doubled the number of LAEs in COSMOS, and we have selected 30 LAEs in the second field CDFS. We detect two large-scale LAE-overdense regions in the COSMOS that are likely protoclusters at the highest redshift to date. We perform injection and recovery simulations to derive the sample incompleteness. We show significant incompleteness comes from blending with foreground sources, which however has not been corrected in LAE luminosity functions in {the} literature. The bright end bump in the Ly$α$ luminosity function in COSMOS is confirmed with 6 (2 newly selected) luminous LAEs (L$_{Lyα}$ $>$ 10$^{43.3}$ erg s$^{-1}$). Interestingly, the bump is absent in CDFS, in which only one luminous LAE is detected. Meanwhile, the faint end luminosity functions from the two fields well agree with each other. The 6 luminous LAEs in COSMOS coincide with 2 LAE-overdense regions, while such regions are not seen in CDFS. The bright-end luminosity function bump could be attributed to ionized bubbles in a patchy reionization. It appears associated with cosmic overdensities, thus supports an inside-out reionization topology at $z$ $\sim$ 7.0, i.e., the high density peaks were ionized earlier compared to the voids. An average neutral hydrogen fraction of $x_{HI}$ $\sim$ 0.2 -- 0.4 is derived at $z\sim$ 7.0 based on the cosmic evolution of the Ly$α$ luminosity function.

astro-ph.GA↗

arXiv2019

Nb$_{2}$SiTe$_{4}$: A Stable Narrow-Gap Two-Dimensional Material with Ambipolar Transport and Mid-Infrared Response

Two-dimensional (2D) materials with narrow band gaps (~0.3 eV) are of great importance for realizing ambipolar transistors and mid-infrared (MIR) detection. However, most of the 2D materials studied so far have band gaps that are too large. A few of them with suitable band gaps are not stable under ambient conditions. In this study, the layered Nb$_{2}$SiTe$_{4}$ is shown to be a stable 2D material with a band gap of 0.39 eV. Field-effect transistors based on few-layer Nb$_2$SiTe$_4$ show ambipolar transport with similar magnitude of electron and hole current and high charge-carrier mobility of ~ 100 cm$^{2}$V$^{-1}$s$^{-1}$ at room temperature. Optoelectronic measurements of the devices show clear response to MIR wavelength of 3.1 $\mathrmμ$m with a high responsivity of ~ 0.66 AW$^{-1}$. These results establish Nb$_{2}$SiTe$_{4}$ as a good candidate for ambipolar devices and MIR detection.

physics.app-ph↗

arXiv2019

Lyman Alpha Galaxies in the Epoch of Reionization (LAGER): Spectroscopic Confirmation of Two Redshift ~ 7.0 Galaxies

We spectroscopically confirmed two narrow-band selected redshift 7.0 Ly$α$ galaxies and studied their restframe UV spectra. The Ly$α$ and other UV nebular lines are very useful to confirm the galactic redshifts and diagnose the different mechanisms driving the ionizing emission. We observed two narrowband-selected $z$=7.0 Ly$α$ candidates in the LAGER Chandra Deep Field South (CDFS) field with IMACS at Magellan telescope and confirmed they are Ly$α$ emitters at $z$=6.924 and 6.931. In one galaxy, we also obtained deep NIR spectroscopy, which yields non-detections of the high-ionization UV nebular lines. We measured upper-limits of the ratios of CIV$λ$1548/Ly$α$, HeII$λ$1640/Ly$α$, OIII]$λ$1660/Ly$α$, and CIII]$λ$1909/Ly$α$ from the NIR spectra. These upper-limits imply that the ionizing emission in this galaxy is dominated by normal star formation instead of AGN.

astro-ph.GA↗