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Chen, Xi

Publications and source records attributed to Chen, Xi.

At least 163 records · Page 9
All sourcesNASA NTRSDOE OSTI
DOE OSTI2017

Detection of HC{sub 3}N maser emission in NGC 253

We report the detection of maser emission from the J=4--3 transition of HC{sub 3}N at 36.4 GHz toward the nearby starburst galaxy NGC 253. This is the first detection of maser emission from this transition in either a Galactic or extragalactic source. The HC{sub 3}N maser emission has a brightness temperature in excess of 2500 K and is offset from the center of the galaxy by approximately 18″ (300 pc), but close to a previously reported class I methanol maser. Both the HC{sub 3}N and methanol masers appear to arise near the interface between the galactic bar and the central molecular zone, where it is thought that molecular gas is being transported inwards, producing a region of extensive low-velocity shocks.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

Growth of a Massive Young Stellar Object Fed by a Gas Flow from a Companion Gas Clump

We present a Submillimeter Array (SMA) observation toward the young massive double-core system G350.69-0.49. This system consists of a northeast (NE) diffuse gas bubble and a southwest (SW) massive young stellar object (MYSO), both clearly seen in the Spitzer images. The SMA observations reveal a gas flow between the NE bubble and the SW MYSO in a broad velocity range from 5 to 30 km s{sup −1} with respect to the system velocity. The gas flow is well confined within the interval between the two objects and traces a significant mass transfer from the NE gas bubble to the SW massive core. The transfer flow can supply the material accreted onto the SW MYSO at a rate of 4.2×10{sup −4} M{sub ⊙} yr{sup −1}. The whole system therefore suggests a mode for the mass growth in the MYSO from a gas transfer flow launched from its companion gas clump, despite the driving mechanism of the transfer flow not being fully determined from the current data.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

High-Yield Synthesis and Optical Properties of Carbon Nanotube Porins

Carbon nanotube porins (CNTPs) are a convenient membrane-based model system for studying nanofluidic transport that replicates a number of key structural features of biological membrane channels. We present a generalized approach for CNTP synthesis using sonochemistry-assisted segmenting of carbon nanotubes. Prolonged tip sonication in the presence of lipid molecules debundles and fragments long carbon nanotube aggregates into stable and water-soluble individual CNTPs with lengths in the range 5–20 nm. We discuss the main parameters that determine the efficiency and the yield of this process, describe the optimized conditions for high-yield CNTP synthesis, and demonstrate that this methodology can be adapted for synthesis of CNTPs of different diameters. We also present the optical properties of CNTPs and show that a combination of Raman and UV–vis–NIR spectroscopy can be used to monitor the quality of the CNTP synthesis. Altogether, CNTPs represent a versatile nanopore building block for creating higher-order functional biomimetic materials.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2017

Obeticholic acid protects against carbon tetrachloride-induced acute liver injury and inflammation

The farnesoid X receptor (FXR) is a ligand-activated transcription factor that plays important roles in regulating bile acid homeostasis. The aim of the present study was to investigate the effects of obeticholic acid (OCA), a novel synthetic FXR agonist, carbon tetrachloride (CCl{sub 4})-induced acute liver injury. Mice were intraperitoneally injected with CCl{sub 4} (0.15 ml/kg). In CCl{sub 4} + OCA group, mice were orally with OCA (5 mg/kg) 48, 24 and 1 h before CCl{sub 4}. As expected, hepatic FXR was activated by OCA. Interestingly, OCA pretreatment alleviated CCl{sub 4}-induced elevation of serum ALT and hepatic necrosis. Moreover, OCA pretreatment inhibited CCl{sub 4}-induced hepatocyte apoptosis. Additional experiment showed that OCA inhibits CCl{sub 4}-induced hepatic chemokine gene Mcp-1, Mip-2 and Kc. Moreover, OCA inhibits CCl{sub 4}-induced hepatic pro-inflammatory gene Tnf-α and Il-1β. By contrast, OCA pretreatment elevated hepatic anti-inflammatory gene Il-4. Further analysis showed that OCA pretreatment inhibited hepatic IκB phosphorylation and blocked nuclear translocation of NF-κB p65 and p50 subunits during CCl{sub 4}-induced acute liver injury. In addition, OCA pretreatment inhibited hepatic Akt, ERK and p38 phosphorylation in CCl{sub 4}-induced acute liver injury. These results suggest that OCA protects against CCl{sub 4}-induced acute liver injury and inflammation. Synthetic FXR agonists may be effective antidotes for hepatic inflammation during acute liver injury. - Highlights: • OCA pretreatment activates hepatic FXR. • FXR activation protects against CCl{sub 4}-induced acute liver injury. • FXR activation inhibits hepatocyte apoptosis during CCl{sub 4}-induced liver injury. • FXR activation differentially regulates hepatic inflammatory genes. • Synthetic FXR agonists are effective antidotes for acute liver injury.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2017

REDSHIFT-INDEPENDENT DISTANCES IN THE NASA/IPAC EXTRAGALACTIC DATABASE: METHODOLOGY, CONTENT, AND USE OF NED-D

Estimates of galaxy distances based on indicators that are independent of cosmological redshift are fundamental to astrophysics. Researchers use them to establish the extragalactic distance scale, to underpin estimates of the Hubble constant, and to study peculiar velocities induced by gravitational attractions that perturb the motions of galaxies with respect to the “Hubble flow” of universal expansion. In 2006 the NASA/IPAC Extragalactic Database (NED) began making available a comprehensive compilation of redshift-independent extragalactic distance estimates. A decade later, this compendium of distances (NED-D) now contains more than 100,000 individual estimates based on primary and secondary indicators, available for more than 28,000 galaxies, and compiled from over 2000 references in the refereed astronomical literature. This paper describes the methodology, content, and use of NED-D, and addresses challenges to be overcome in compiling such distances. Currently, 75 different distance indicators are in use. We include a figure that facilitates comparison of the indicators with significant numbers of estimates in terms of the minimum, 25th percentile, median, 75th percentile, and maximum distances spanned. Brief descriptions of the indicators, including examples of their use in the database, are given in an appendix.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

MOLECULAR LINES OF 13 GALACTIC INFRARED BUBBLE REGIONS

We investigated the physical properties of molecular clouds and star formation (SF) processes around infrared bubbles, which are essentially expanding H ii regions. We performed observations of 13 galactic infrared bubble fields containing 18 bubbles. We observed five molecular lines—{sup 12}CO (J=1→0), {sup 13}CO (J=1→0), C{sup 18}O (J=1→0), HCN (J=1→0), and HCO{sup +} (J=1→0)—and several publicly available surveys were used for comparison: Galactic Legacy Infrared Mid-Plane Survey Extraordinaire, Multiband Imaging Photometer for Spitzer Galactic Plane Survey, APEX Telescope Large Area Survey of the Galaxy, Bolocam Galactic Plane Survey, Very Large Array (VLA) Galactic Plane Survey, Multi-Array Galactic Plane Imaging Survey, and NRAO VLA Sky Survey. We find that these bubbles are generally connected with molecular clouds, most of which are giant. Several bubble regions display velocity gradients and broad-shifted profiles, which could be due to the expansion of bubbles. The masses of molecular clouds within bubbles range from 100 to 19,000 M {sub ⊙}, and their dynamic ages are about 0.3–3.7 Myr, which takes into account the internal turbulence pressure of surrounding molecular clouds. Clumps are found in the vicinity of all 18 bubbles, and molecular clouds near four of these bubbles with larger angular sizes show shell-like morphologies, indicating that either collect-and-collapse or radiation-driven implosion processes may have occurred. Due to the contamination of adjacent molecular clouds, only six bubble regions are appropriate to search for outflows, and we find that four have outflow activities. Three bubbles display ultra-compact H ii regions at their borders, and one is probably responsible for its outflow. In total, only six bubbles show SF activities in the vicinity, and we suggest that SF processes might have been triggered.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

Weak coupling of pseudoacoustic phonons and magnon dynamics in the incommensurate spin-ladder compound S r 14 C u 24 O 41

Intriguing lattice dynamics have been predicted for aperiodic crystals that contain incommensurate substructures. Here we report inelastic neutron scattering measurements of phonon and magnon dispersions in Sr 14 Cu 24 O 41 , which contains incommensurate one-dimensional (1D) chain and two-dimensional (2D) ladder substructures. Two distinct pseudoacoustic phonon modes, corresponding to the sliding motion of one sublattice against the other, are observed for atomic motions polarized along the incommensurate axis. In the long wavelength limit, it is found that the sliding mode shows a remarkably small energy gap of 1.7–1.9 meV, indicating very weak interactions between the two incommensurate sublattices. The measurements also reveal a gapped and steep linear magnon dispersion of the ladder sublattice. The high group velocity of this magnon branch and weak coupling with acoustic and pseudoacoustic phonons can explain the large magnon thermal conductivity in Sr 14 Cu 24 O 41 crystals. In addition, the magnon specific heat is determined from the measured total specific heat and phonon density of states and exhibits a Schottky anomaly due to gapped magnon modes of the spin chains. Furthermore, these findings offer new insights into the phonon and magnon dynamics and thermal transport properties of incommensurate magnetic crystals that contain low-dimensional substructures.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Structures of the Streptococcus sanguinis SrpA Binding Region with Human Sialoglycans Suggest Features of the Physiological Ligand

Streptococcus sanguinis is a leading cause of bacterial infective endocarditis, a life-threatening infection of heart valves. S. sanguinis binds to human platelets with high avidity, and this adherence is likely to enhance virulence. Previous studies suggest that a serine-rich repeat adhesin termed SrpA mediates the binding of S. sanguinis to human platelets via its interaction with sialoglycans on the receptor GPIbα. However, in vitro binding assays with SrpA and defined sialoglycans failed to identify specific high-affinity ligands. To improve our understanding of the interaction between SrpA and human platelets, we determined cocrystal structures of the SrpA sialoglycan binding region (Srp ABR ) with five low-affinity ligands: three sialylated trisaccharides (sialyl-T antigen, 3'-sialyllactose, and 3'-sialyl-N-acetyllactosamine), a sialylated tetrasaccharide (sialyl-Lewis X ), and a sialyl galactose disaccharide component common to these sialoglyans. We then combined structural analysis with mutagenesis to further determine whether our observed interactions between SrpABR and glycans are important for binding to platelets and to better map the binding site for the physiological receptor. Here, we found that the sialoglycan binding site of SrpA BR is significantly larger than the sialoglycans cocrystallized in this study, which suggests that binding of SrpA to platelets either is multivalent or occurs via a larger, disialylated glycan.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2016

Experimental observation of topological Fermi arcs in type-II Weyl semimetal MoTe2

Weyl semimetal is a new quantum state of matter hosting the condensed matter physics counterpart of the relativistic Weyl fermions originally introduced in high-energy physics. The Weyl semimetal phase realized in the TaAs class of materials features multiple Fermi arcs arising from topological surface states and exhibits novel quantum phenomena, such as a chiral anomaly-induced negative magnetoresistance and possibly emergent supersymmetry. Recently it was proposed theoretically that a new type (type-II) of Weyl fermion that arises due to the breaking of Lorentz invariance, which does not have a counterpart in high-energy physics, can emerge as topologically protected touching between electron and hole pockets. In this work, we report direct experimental evidence of topological Fermi arcs in the predicted type-II Weyl semimetal MoTe 2 (refs 23-25). Additionally, the topological surface states are confirmed by directly observing the surface states using bulk- and surface-sensitive angle-resolved photoemission spectroscopy, and the quasi-particle interference pattern between the putative topological Fermi arcs in scanning tunnelling microscopy. By establishing MoTe 2 as an experimental realization of a type-II Weyl semimetal, our work opens up opportunities for probing the physical properties of this exciting new state.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Electron cyclotron heating can drastically alter reversed shear Alfvén eigenmode activity in DIII-D through finite pressure effects

Here, a recent DIII-D experiment investigating the impact of electron cyclotron heating (ECH) on neutral beam driven reversed shear Alfvén eigenmode (RSAE) activity is presented. The experiment includes variations of ECH injection location and timing, current ramp rate, beam injection geometry (on/off-axis), and neutral beam power. Essentially all variations carried out in this experiment were observed to change the impact of ECH on AE activity significantly. In some cases, RSAEs were observed to be enhanced with ECH near the off-axis minimum in magnetic safety factor (${{q}_{\min}}$ ), in contrast to the original DIII-D experiments where the modes were absent when ECH was deposited near ${{q}_{\min}}$ . It is found that during intervals when the geodesic acoustic mode (GAM) frequency at ${{q}_{\min}}$ is elevated and the calculated RSAE minimum frequency, including contributions from thermal plasma gradients, is very near or above the nominal TAE frequency (f TAE ), RSAE activity is not observed or RSAEs with a much reduced frequency sweep range are found. This condition is primarily brought about by ECH modification of the local electron temperature (T e ) which can raise both the local T e at ${{q}_{\min}}$ as well as its gradient. A q-evolution model that incorporates this reduction in RSAE frequency sweep range is in agreement with the observed spectra and appears to capture the relative balance of TAE or RSAE-like modes throughout the current ramp phase of over 38 DIII-D discharges. Detailed ideal MHD calculations using the NOVA code show both modification of plasma pressure and pressure gradient at ${{q}_{\min}}$ play an important role in modifying the RSAE activity. Analysis of the ECH injection near the ${{q}_{\min}}$ case where no frequency sweeping RSAEs are observed shows the typical RSAE is no longer an eigenmode of the system. What remains is an eigenmode with poloidal harmonic content reminiscent of the standard RSAE, but absent of the typical frequency sweeping behavior. The remaining eigenmode is also often strongly coupled to gap TAEs. Analysis with the non-perturbative gyro fluid code TAEFL confirms this change in RSAE activity and also shows a large drop in the resultant mode growth rates.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2016

First-principle study of hydrogenation on monolayer MoS{sub 2}

The structural and electronic properties of hydrogenation on 1H-MoS{sub 2} and 1T-MoS{sub 2} have been systematically explored by using density functional theory (DFT) calculations. Our calculated results indicate an energetically favorable chemical interaction between H and MoS{sub 2} monolayer for H adsorption when increasing concentration of H atoms. For 1H-MoS{sub 2}, single H atom adsorption creates midgap approaching the Fermi level which increases the n-type carrier concentration effectively. As a consequence, its electrical conductivity is expected to increase significantly. For 1T-MoS{sub 2}, H atoms adsorption can lead to the opening of a direct gap of 0.13 eV compared to the metallic pristine 1T-MoS{sub 2}.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Discovery of robust in-plane ferroelectricity in atomic-thick SnTe

Stable ferroelectricity with high transition temperature in nanostructures is needed for miniaturizing ferroelectric devices. Here, we report the discovery of the stable in-plane spontaneous polarization in atomic-thick tin telluride (SnTe), down to a 1–unit cell (UC) limit. The ferroelectric transition temperature T c of 1-UC SnTe film is greatly enhanced from the bulk value of 98 kelvin and reaches as high as 270 kelvin. Moreover, 2- to 4-UC SnTe films show robust ferroelectricity at room temperature. The interplay between semiconducting properties and ferroelectricity in this two-dimensional material may enable a wide range of applications in nonvolatile high-density memories, nanosensors, and electronics.

36 MATERIALS SCIENCE↗
DOE OSTI2016

DIII-D Electron Cyclotron Heating System Status and Upgrades

The DIII-D Electron Cyclotron Heating (ECH) system consists of six 110 GHz gyrotrons with corrugated coaxial 31.75 mm waveguide transmission lines and steerable launching mirrors. The system has been gradually updated, leading to increased experimental flexibility and a high system reliability of 91% in the past year. Operationally, the gyrotrons can generate up to a total of 4.8 MW of rf power for pulses up to 5 seconds. The maximum ECH energy injected into the DIII-D is 16.6 MJ. The HE1,1 mode content is over 85% for all the lines, and the transmission coefficient is better than -1.1 dB for all the transmission lines, close to the theoretical value. A new depressed collector gyrotron was recently installed and was injecting up to 640 kW of power into the plasma during 2014-2015 tokamak operations. Four dual waveguide launchers, which can steer the RF beams ±20 degrees poloidally and toroidally, are used for real-time neoclassical tearing mode control and suppression. The launchers now have increased poloidal scanning speed and beam positioning accuracy of ~±2 mm at the plasma center. Two more gyrotrons are expected to be installed and operational in 2015- 2016. The first is a repaired 110 GHz, 1 MW gyrotron that had a gun failure after more than 11 years of operation at DIII-D. The second is a newly designed depressed collector tube in the 1.5 MW class, operating at 117.5 GHz, manufactured by Communications and Power Industries (CPI).

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2016

High Volumetric Capacity Three-Dimensionally Sphere-Caged Secondary Battery Anodes

High volumetric energy density secondary batteries are important for many applications, which has led to considerable efforts to replace the low volumetric capacity graphite-based anode common to most Li-ion batteries with a higher energy density anode. Because most high capacity anode materials expand significantly during charging, such anodes must contain sufficient porosity in the discharged state to enable the expansion, yet not excess porosity, which lowers the overall energy density. Here, we present a high volumetric capacity anode consisting of a three-dimensional (3D) nanocomposite formed in only a few steps which includes both a 3D structured Sn scaffold and a hollow Sn sphere within each cavity where all the free Sn surfaces are coated with carbon. Here, the anode exhibits a high volumetric capacity of ~1700 mA h cm –3 over 200 cycles at 0.5C, and a capacity greater than 1200 mA h cm –3 at 10C. Importantly, the anode can even be formed into a commercially relevant ~100 μm thick form. When assembled into a full cell the anode shows a good compatibility with a commercial LiMn 2 O 4 cathode. In situ TEM observations confirm the electrode design accommodates the necessary volume expansion during lithiation.

25 ENERGY STORAGE↗