3,156 research outputs found
Fundamental Principles for Calculating Charged Defect Ionization Energies in Ultrathin Two-Dimensional Materials
Defects in 2D materials are becoming prominent candidates for quantum
emitters and scalable optoelectronic applications. However, several physical
properties that characterize their behavior, such as charged defect ionization
energies, are difficult to simulate with conventional first-principles methods,
mainly because of the weak and anisotropic dielectric screening caused by the
reduced dimensionality. We establish fundamental principles for accurate and
efficient calculations of charged defect ionization energies and electronic
structure in ultrathin 2D materials. We propose to use the vacuum level as the
reference for defect charge transition levels (CTLs) because it gives robust
results insensitive to the level of theory, unlike commonly used band edge
positions. Furthermore, we determine the fraction of Fock exchange in hybrid
functionals for accurate band gaps and band edge positions of 2D materials by
enforcing the generalized Koopmans' condition of localized defect states. We
found the obtained fractions of Fock exchange vary significantly from 0.2 for
bulk -BN to 0.4 for monolayer -BN, whose band gaps are also in good
agreement with experimental results and calculated GW results. The combination
of these methods allows for reliable and efficient prediction of defect
ionization energies (difference between CTLs and band edge positions). We
motivate and generalize these findings with several examples including
different defects in monolayer to few-layer hexagonal boron nitride (-BN),
monolayer MoS and graphane. Finally, we show that increasing the number of
layers of -BN systematically lowers defect ionization energies, mainly
through CTLs shifting towards vacuum, with conduction band minima kept almost
unchanged
Ruthenium atomically dispersed in carbon outperforms platinum toward hydrogen evolution in alkaline media.
Hydrogen evolution reaction is an important process in electrochemical energy technologies. Herein, ruthenium and nitrogen codoped carbon nanowires are prepared as effective hydrogen evolution catalysts. The catalytic performance is markedly better than that of commercial platinum catalyst, with an overpotential of only -12 mV to reach the current density of 10 mV cm-2 in 1 M KOH and -47 mV in 0.1 M KOH. Comparisons with control experiments suggest that the remarkable activity is mainly ascribed to individual ruthenium atoms embedded within the carbon matrix, with minimal contributions from ruthenium nanoparticles. Consistent results are obtained in first-principles calculations, where RuCxNy moieties are found to show a much lower hydrogen binding energy than ruthenium nanoparticles, and a lower kinetic barrier for water dissociation than platinum. Among these, RuC2N2 stands out as the most active catalytic center, where both ruthenium and adjacent carbon atoms are the possible active sites
Fungal loop transfer of nitrogen depends on biocrust constituents and nitrogen form
Besides performing multiple ecosystem services individually and collectively,
biocrust constituents may also create biological networks connecting
spatially and temporally distinct processes. In the fungal loop hypothesis
rainfall variability allows fungi to act as conduits and reservoirs,
translocating resources between soils and host plants. To evaluate the extent
to which biocrust species composition and nitrogen (N)
form influence loops, we created a minor, localized rainfall event containing
15NH4+ and 15NO3−. We then measured the
resulting δ15N in the surrounding dry cyanobacteria- and
lichen-dominated crusts and grass, Achnatherum hymenoides, after
24 h. We also estimated the biomass of fungal constituents using
quantitative PCR and characterized fungal
communities by sequencing the 18S rRNA gene. We found evidence for the
initiation of fungal loops in cyanobacteria-dominated crusts where
15N, from 15NH4+, moved 40 mm h−1 in
biocrust soils with the δ15N of crusts decreasing as the
radial distance from the water addition increased (linear mixed effects model
(LMEM)): R2 = 0.67, F2,12 = 11, P = 0.002). In cyanobacteria crusts, δ15N, from
15NH4+, was diluted as Ascomycota biomass increased (LMEM:
R2 = 0.63, F2,8 = 6.8, P = 0.02), Ascomycota accounted for
82 % (±2.8) of all fungal sequences, and one order, Pleosporales,
comprised 66 % (±6.9) of Ascomycota. The seeming lack of loops in
moss-dominated crusts may stem from the relatively large moss biomass
effectively absorbing and holding N from our minor wet deposition
event. The substantial movement of 15NH4+ may indicate a
fungal preference for the reduced N form during amino acid
transformation and translocation. We found a marginally significant
enrichment of δ15N in A. hymenoides leaves but
only in cyanobacteria biocrusts translocating 15N, offering
evidence of links between biocrust constituents and higher plants. Our
results suggest that minor rainfall events may initiate fungal loops
potentially allowing constituents, like dark septate Pleosporales, to rapidly
translocate N from NH4+ over NO3− through
biocrust networks
Measurement of the Atmospheric Muon Charge Ratio at TeV Energies with MINOS
The 5.4 kton MINOS far detector has been taking charge-separated cosmic ray
muon data since the beginning of August, 2003 at a depth of 2070
meters-water-equivalent in the Soudan Underground Laboratory, Minnesota, USA.
The data with both forward and reversed magnetic field running configurations
were combined to minimize systematic errors in the determination of the
underground muon charge ratio. When averaged, two independent analyses find the
charge ratio underground to be 1.374 +/- 0.004 (stat.) +0.012 -0.010(sys.).
Using the map of the Soudan rock overburden, the muon momenta as measured
underground were projected to the corresponding values at the surface in the
energy range 1-7 TeV. Within this range of energies at the surface, the MINOS
data are consistent with the charge ratio being energy independent at the two
standard deviation level. When the MINOS results are compared with measurements
at lower energies, a clear rise in the charge ratio in the energy range 0.3 --
1.0 TeV is apparent. A qualitative model shows that the rise is consistent with
an increasing contribution of kaon decays to the muon charge ratio.Comment: 16 pages, 17 figure
Measurement of neutrino velocity with the MINOS detectors and NuMI neutrino beam
The velocity of a ~3 GeV neutrino beam is measured by comparing detection times at the near and far detectors of the MINOS experiment, separated by 734 km. A total of 473 far detector neutrino events was used to measure (v-c)/c=5.12.910-5 (at 68% C.L.). By correlating the measured energies of 258 charged-current neutrino events to their arrival times at the far detector, a limit is imposed on the neutrino mass of mnu<50 MeV/c2 (99% C.L.)
Development of Large-Format Lithium-Ion Cells with Silicon Anode and Low Flammable Electrolyte
NASA is developing safe, high energy and high capacity lithium-ion cell designs and batteries for future missions under NASAs Advanced Space Power System (ASPS) project. Advanced cell components, such as high specific capacity silicon anodes and low-flammable electrolytes have been developed for improving the cell specific energy and enhancing safety. To advance the technology readiness level, we have developed large-format flight-type hermetically sealed battery cells by incorporating high capacity silicon anodes, commercially available lithium nickel, cobalt, aluminum oxide (NCA) cathodes, and low-flammable electrolytes. In this report, we will present the performance results of these various battery cells. In addition, we will also discuss the post-test cell analysis results as well
Nonthermal Electrons at High Mach Number Shocks: Electron Shock Surfing Acceleration
We study the suprathermal electron acceleration mechanism in a perpendicular
magnetosonic shock wave in a high Mach number regime by using a
particle-in-cell simulation. We find that shock surfing/surftron acceleration
producing the suprathermal electrons occurs in the shock transition region
where a series of large amplitude electrostatic solitary waves (ESWs) are
excited by Buneman instability under the interaction between the reflected ions
and the incoming electrons. It is shown that the electrons are likely to be
trapped by ESWs, and during the trapping phase they can be effectively
accelerated by the shock motional/convection electric field. We discuss that
suprathermal electrons can be accelerated up to , where is the ion rest mass energy and is the shock upstream flow velocity.
Furthermore, some of these suprathermal electrons may be effectively trapped
for infinitely long time when Alfv\'en Mach number exceeds several 10,
and they are accelerated up to the shock potential energy determined by the
global shock size.Comment: 21 pages, 6 figure
Resonance fluorescence spectrum of a two-level atom driven by a bichromatic field in a squeezed vacuum
The steady-state resonance fluorescence spectrum of a two-level atom driven by a bichromatic field in a broadband squeezed vacuum is studied. When the carrier frequency of the squeezed vacuum is tuned to the frequency of the central spectral line, anomalous spectral features, such as hole burning and dispersive profiles, can occur at the central line. We show that these features appear for wider, and experimentally more convenient, ranges of the parameters than in the case of monochromatic excitation. ?he absence of a coherent spectral component at the central line makes any experimental attempt to observe these features much easier. We also discuss the general features of the spectrum. When the carrier frequency of the squeezed vacuum is tuned to the first odd or even sidebands, the spectrum is asymmetric and only the sidebands an sensitive to phase. For appropriate choices of the phase the linewidths or only the odd or even sidebands can be reduced. A dressed-stale interpretation is provided
Significance of Thymosin β4 and Implication of PINCH-1-ILK-α-Parvin (PIP) Complex in Human Dilated Cardiomyopathy
Myocardial remodeling is a major contributor in the development of heart failure (HF) after myocardial infarction (MI). Integrin-linked kinase (ILK), LIM-only adaptor PINCH-1, and α-parvin are essential components of focal adhesions (FAs), which are highly expressed in the heart. ILK binds tightly to PINCH-1 and α-parvin, which regulates FA assembly and promotes cell survival via the activation of the kinase Akt. Mice lacking ILK, PINCH or α-parvin have been shown to develop severe defects in the heart, suggesting that these proteins play a critical role in heart function. Utilizing failing human heart tissues (dilated cardiomyopathy, DCM), we found a 2.27-fold (p<0.001) enhanced expression of PINCH, 4 fold for α-parvin, and 10.5 fold (p<0.001) for ILK as compared to non-failing (NF) counterparts. No significant enhancements were found for the PINCH isoform PINCH-2 and parvin isoform β-parvin. Using a co-immunoprecipitation method, we also found that the PINCH-1-ILK-α-parvin (PIP) complex and Akt activation were significantly up-regulated. These observations were further corroborated with the mouse myocardial infarction (MI) and transaortic constriction (TAC) model. Thymosin beta4 (Tβ4), an effective cell penetrating peptide for treating MI, was found to further enhance the level of PIP components and Akt activation, while substantially suppressing NF-κB activation and collagen expression—the hallmarks of cardiac fibrosis. In the presence of an Akt inhibitor, wortmannin, we show that Tβ4 had a decreased effect in protecting the heart from MI. These data suggest that the PIP complex and activation of Akt play critical roles in HF development. Tβ4 treatment likely improves cardiac function by enhancing PIP mediated Akt activation and suppressing NF-κB activation and collagen-mediated fibrosis. These data provide significant insight into the role of the PIP-Akt pathway and its regulation by Tβ4 treatment in post-MI
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