arXiv subjects

M. Lee

Publications and source records attributed to M. Lee.

At least 73 records · Page 4Linked to original sources

arXiv2017

Structural and magnetic properties of two branches of the Tripod Kagome Lattice family A$_{2}$RE$_{3}$Sb$_{3}$O$_{14}$ (A = Mg, Zn; RE = Pr, Nd, Gd, Tb, Dy, Ho, Er, Yb)

We present a systematic study of the structural and magnetic properties of two branches of the rare earth Tripod Kagome Lattice (TKL) family A$_{2}$RE$_{3}$Sb$_{3}$O$_{14}$ (A = Mg, Zn; RE = Pr, Nd, Gd, Tb, Dy, Ho, Er, Yb; here, we use abbreviation \textit{A-RE}, as in \textit{MgPr} for Mg$_{2}$Pr$_{3}$Sb$_{3}$O$_{14}$), which complements our previously reported work on \textit{MgDy}, \textit{MgGd}, and \textit{MgEr} \cite{TKL}. The present susceptibility ($χ_{dc}$, $χ_{ac}$) and specific heat measurements reveal various magnetic ground states, including the non-magnetic singlet state for \textit{MgPr}, \textit{ZnPr}; long range orderings (LROs) for \textit{MgGd}, \textit{ZnGd}, \textit{MgNd}, \textit{ZnNd}, and \textit{MgYb}; a long range magnetic charge ordered state for \textit{MgDy}, \textit{ZnDy}, and potentially for \textit{MgHo}; possible spin glass states for \textit{ZnEr}, \textit{ZnHo}; the absence of spin ordering down to 80 mK for \textit{MgEr}, \textit{MgTb}, \textit{ZnTb}, and \textit{ZnYb} compounds. The ground states observed here bear both similarities as well as striking differences from the states found in the parent pyrochlore systems. In particular, while the TKLs display a greater tendency towards LRO, the lack of LRO in \textit{MgHo}, \textit{MgTb} and \textit{ZnTb} can be viewed from the standpoint of a balance among spin-spin interactions, anisotropies and non-Kramers nature of single ion state. While substituting Zn for Mg changes the chemical pressure, and subtly modifies the interaction energies for compounds with larger RE ions, this substitution introduces structural disorder and modifies the ground states for compounds with smaller RE ions (Ho, Er, Yb).

cond-mat.str-el↗

arXiv2017

Global and Brazilian carbon response to El Niño Modoki 2011-2010

The El Niño Modoki in 2010 lead to historic droughts in Brazil. We quantify the global and Brazilian carbon response to this event using the NASA Carbon Monitoring System Flux (CMS-Flux) framework. Satellite observations of CO$_2$, CO, and solar induced fluorescence (SIF) are ingested into a 4D-variational assimilation system driven by carbon cycle models to infer spatially resolved carbon fluxes including net ecosystem exchange, biomass burning, and gross primary productivity (GPP). The global net carbon flux tendency, which is the flux difference 2011-2010 and is positive for net fluxes into the atmosphere, was estimated to be -1.60 PgC between 2011-2010 while the Brazilian tendency was -0.24 $\pm$ 0.11 PgC. This estimate is broadly within the uncertainty of previous aircraft based estimates restricted to the Amazonian basin. The biomass burning tendency in Brazil was -0.24 $\pm$ 0.036 PgC, which implies a near-zero change of the net ecosystem production (NEP). The near-zero change of the NEP is the result of quantitatively comparable increase in GPP (0.34 $\pm$ 0.20) and respiration in Brazil. Comparisons of the component fluxes in Brazil to the global fluxes show a complex balance between regional contributions to individual carbon fluxes such as biomass burning, and their net contribution to the global carbon balance, i.e., the Brazilian biomass burning tendency is a significant contributor to the global biomass burning tendency but the Brazilian net flux tendency is not a dominant contributor to the global tendency. These results show the potential of multiple satellite observations to help quantify the spatially resolved response of productivity and respiration fluxes to climate variability.

physics.ao-ph↗

arXiv2017

ALMA Deep Field in SSA22: Blindly Detected CO Emitters and [CII] Emitter Candidates

We report the identification of four millimeter line emitting galaxies with the Atacama Large Milli/submillimeter Array (ALMA) in SSA22 Field (ADF22). We analyze the ALMA 1.1 mm survey data, with an effective survey area of 5 arcmin$^2$, a frequency range of 253.1--256.8 and 269.1--272.8 GHz, angular resolution of 0".7 and RMS noise of 0.8 mJy beam$^{-1}$ at 36 km s$^{-1}$ velocity resolution. We detect four line emitter candidates with significance levels above $6 σ$. We identify one of the four sources as a CO(9-8) emitter at $z = 3.1$ in a member of the proto-cluster known in this field. Another line emitter with an optical counterpart is likely a CO(4-3) emitter at $z = 0.7$. The other two sources without any millimeter continuum or optical/near-infrared counterpart are likely to be [CII] emitter candidates at $z = 6.0$ and $6.5$. The equivalent widths of the [CII] candidates are consistent with those of confirmed high-redshift [CII] emitters and candidates, and are a factor of 10 times larger than that of the CO(9-8) emitter detected in this search. The [CII] luminosity of the candidates are $4-7 \times 10^8~\rm L_\odot$. The star formation rates (SFRs) of these sources are estimated to be $10-20~\rm M_{\odot}~yr^{-1}$ if we adopt an empirical [CII] luminosity - SFR relation. One of them has a relatively low-S/N ratio, but shows features characteristic of emission lines. Assuming that at least one of the two candidates is a [CII] emitter, we derive a lower limit of [CII]-based star formation rate density (SFRD) at $z~\sim~6$. The resulting value of $> 10^{-2}$ $\rm M_\odot yr^{-1} Mpc^{-3}$ is consistent with the dust-uncorrected UV-based SFRD. Future millimeter/submillimeter surveys can be used to detect a number of high redshift line emitters, with which to study the star formation history in the early Universe.

astro-ph.GA↗

arXiv2016

Ba8CoNb6O24: a spin-1/2 triangular-lattice Heisenberg antiferromagnet in the 2D limit

The perovskite Ba8CoNb6O24 comprises equilateral effective spin-1/2 Co2+ triangular layers separated by six non-magnetic layers. Susceptibility, specific heat and neutron scattering measurements combined with high-temperature series expansions and spin-wave calculations confirm that Ba8CoNb6O24 is basically a twodimensional (2D) magnet with no detectable spin anisotropy and no long-range magnetic ordering down to 0.06 K. In other words, Ba8CoNb6O24 is very close to be a realization of the paradigmatic spin-1/2 triangular Heisenberg model, which is not expected to exhibit symmetry breaking at finite temperature according to the Mermin and Wagner theorem.

cond-mat.str-el↗

arXiv2016

Canted Magnetic Ground State of Quarter-Doped Manganites $R_{0.75}$Ca$_{0.25}$MnO$_3$ ($R$ = Y, Tb, Dy, Ho, and Er)

Polycrystalline samples of the quarter-doped manganites $R_{0.75}$Ca$_{0.25}$MnO$_3$ ($R$ = Y, Tb, Dy, Ho, and Er) were studied by X-ray diffraction and AC/DC susceptibility measurements. All five samples are orthorhombic and exhibit similar magnetic properties: enhanced ferromagnetism below $T_1$ ($\sim80$ K) and a spin glass (SG) state below $T_{SG}$ ($\sim30$ K). With increasing $R^{3+}$ ionic size, both $T_1$ and $T_{SG}$ generally increase. The single crystal neutron diffraction results on Tb$_{0.75}$Ca$_{0.25}$MnO$_3$ revealed that the SG state is mainly composed of a short-range ordered version of a novel canted (i.e. noncollinear) antiferromagnetic spin state. Furthermore, calculations based on the double exchange model for quarter-doped manganites reveal that this new magnetic phase provides a transition state between the ferromagnetic state and the theoretically predicted spin-orthogonal stripe phase.

cond-mat.mtrl-sci↗

arXiv2016

SXDF-UDS-CANDELS-ALMA 1.5 arcmin$^2$ deep survey

We have conducted 1.1 mm ALMA observations of a contiguous $105'' \times 50''$ or 1.5 arcmin$^2$ window in the SXDF-UDS-CANDELS. We achieved a 5$σ$ sensitivity of 0.28 mJy, providing a flat sensus of dusty star-forming galaxies with $L_{\rm IR} \sim6\times10^{11}$ $L_\odot$ (for $T_{\rm dust}$ =40K) up to $z\sim10$ thanks to the negative K-correction at this wavelength. We detected 5 brightest sources (S/N$>$6) and 18 low-significance sources (5$>$S/N$>$4; these may contain spurious detections, though). One of the 5 brightest ALMA sources ($S_{\rm 1.1mm} = 0.84 \pm 0.09$ mJy) is extremely faint in the WFC3 and VLT/HAWK-I images, demonstrating that a contiguous ALMA imaging survey is able to uncover a faint dust-obscured population that is invisible in deep optical/near-infrared surveys. We found a possible [CII]-line emitter at $z=5.955$ or a low-$z$ CO emitting galaxy within the field, which may allow us to constrain the [CII] and/or the CO luminosity functions across the history of the universe.

astro-ph.GA↗

arXiv2015

PAMELA's measurements of geomagnetically trapped and albedo protons

Data from the PAMELA satellite experiment were used to perform a detailed measurement of under-cutoff protons at low Earth orbits. On the basis of a trajectory tracing approach using a realistic description of the magnetosphere, protons were classified into geomagnetically trapped and re-entrant albedo. The former include stably-trapped protons in the South Atlantic Anomaly, which were analyzed in the framework of the adiabatic theory, investigating energy spectra, spatial and angular distributions; results were compared with the predictions of the AP8 and the PSB97 empirical trapped models. The albedo protons were classified into quasi-trapped, concentrating in the magnetic equatorial region, and un-trapped, spreading over all latitudes and including both short-lived (precipitating) and long-lived (pseudo-trapped) components. Features of the penumbra region around the geomagnetic cutoff were investigated as well. PAMELA observations significantly improve the characterization of the high energy proton populations in near Earth orbits.

astro-ph.EP↗

arXiv2015

Back-Tracing and Flux Reconstruction for Solar Events with PAMELA

The PAMELA satellite-borne experiment is providing first direct measurements of Solar Energetic Particles (SEPs) with energies from $\sim$80 MeV to several GeV in near-Earth space. Its unique observational capabilities include the possibility of measuring the flux angular distribution and thus investigating possible anisotropies related to SEP events. This paper focuses on the analysis methods developed to estimate SEP energy spectra as a function of the particle pitch angle with respect to the Interplanetary Magnetic Field (IMF). The crucial ingredient is provided by an accurate simulation of the asymptotic exposition of the PAMELA apparatus, based on a realistic reconstruction of particle trajectories in the Earth's magnetosphere. As case study, the results of the calculation for the May 17, 2012 event are reported.

astro-ph.IM↗

arXiv2015

PAMELA's measurements of geomagnetic cutoff variations during solar energetic particle events

Data from the PAMELA satellite experiment were used to measure the geomagnetic cutoff for high-energy ($\gtrsim$ 80 MeV) protons during the solar particle events on 2006 December 13 and 14. The variations of the cutoff latitude as a function of rigidity were studied on relatively short timescales, corresponding to single spacecraft orbits (about 94 minutes). Estimated cutoff values were cross-checked with those obtained by means of a trajectory tracing approach based on dynamical empirical modeling of the Earth's magnetosphere. We find significant variations in the cutoff latitude, with a maximum suppression of about 6 deg for $\sim$80 MeV protons during the main phase of the storm. The observed reduction in the geomagnetic shielding and its temporal evolution were compared with the changes in the magnetosphere configuration, investigating the role of IMF, solar wind and geomagnetic (Kp, Dst and Sym-H indexes) variables and their correlation with PAMELA cutoff results.

astro-ph.EP↗

arXiv2015

Solar energetic particle events: trajectory analysis and flux reconstruction with PAMELA

The PAMELA satellite experiment is providing first direct measurements of Solar Energetic Particles (SEPs) with energies from about 80 MeV to several GeV in near-Earth space, bridging the low energy data by other space-based instruments and the Ground Level Enhancement (GLE) data by the worldwide network of neutron monitors. Its unique observational capabilities include the possibility of measuring the flux angular distribution and thus investigating possible anisotropies. This work reports the analysis methods developed to estimate the SEP energy spectra as a function of the particle pitch-angle with respect to the Interplanetary Magnetic Field (IMF) direction. The crucial ingredient is provided by an accurate simulation of the asymptotic exposition of the PAMELA apparatus, based on a realistic reconstruction of particle trajectories in the Earth's magnetosphere. As case study, the results for the May 17, 2012 event are presented.

physics.space-ph↗

arXiv2015

ALMA Deep Field in SSA22: A concentration of dusty starbursts in a z=3.09 protocluster core

We report the results of $1^{\prime}.5 \times3^{\prime}$ mapping at 1.1~mm with the Atacama Large Millimeter/submillimeter Array (ALMA) toward the central region of the $z=3.09$ SSA22 protocluster. By combining our source catalog with archival spectroscopic redshifts, we find that eight submillimeter galaxies (SMGs) with flux densities, $S_{\rm 1.1~mm}=0.7-6.4$~mJy ($L_{\rm IR}\sim10^{12.1}-10^{13.1}L_\odot$) are at $z=3.08-3.10$. Not only are these SMGs members of the protocluster but they in fact reside within the node at the junction of the 50 Mpc-scale filamentary three-dimensional structure traced by Lyman-$α$ emitters (LAEs) in this field. The eight SMGs account for a star formation rate density (SFRD) $\sim$10 $M_\odot$ yr$^{-1}$ Mpc$^{-3}$ in the node, which is two orders of magnitudes higher than the global SFRD at this redshift. We find that four of the eight SMGs host a X-ray luminous active galactic nuclei (AGN). Our results suggest that the vigorous star formation activity and the growth of super massive black holes (SMBHs) occurred simultaneously in the densest regions at $z\sim3$, which may correspond to the most active historical phase of the massive galaxy population found in the core of the clusters in the present universe. Two SMGs are associated with Lyman-$α$ blobs (LABs), implying that the two populations coexist in high density environments for a few cases.

astro-ph.GA↗

arXiv2015

Ground state selection in XY pyrochlore antiferromagnets R$_{2}$Ge$_{2}$O$_{7}$ (R = Er, Yb)

Elastic neutron scattering, ac susceptibility, and specific heat experiments on the pyrochlores Er$_{2}$Ge$_{2}$O$_{7}$ and Yb$_{2}$Ge$_{2}$O$_{7}$ show that both systems are antiferromagnetically ordered in the $Γ_5$ manifold. The ground state is a $ψ_{3}$ phase for the Er sample and a $ψ_{2}$ or $ψ_{3}$ phase for the Yb sample, which suggests "Order by Disorder"(ObD) physics. Furthermore, we unify the various magnetic ground states of all known R$_{2}$B$_{2}$O$_{7}$ (R = Er, Yb, B = Sn, Ti, Ge) compounds through the enlarged XY type exchange interaction $J_{\pm}$ under chemical pressure. The mechanism for this evolution is discussed in terms of the phase diagram proposed in the theoretical study [Wong et al., Phys. Rev. B 88, 144402, (2013)].

cond-mat.str-el↗

arXiv2015

The GALFA-HI Compact Cloud Catalog

We present a catalog of 1964 isolated, compact neutral hydrogen clouds from the Galactic Arecibo L-Band Feed Array Survey Data Release One (GALFA-HI DR1). The clouds were identified by a custom machine-vision algorithm utilizing Difference of Gaussian kernels to search for clouds smaller than 20'. The clouds have velocities typically between |VLSR| = 20-400 km/s, linewidths of 2.5-35 km/s, and column densities ranging from 1 - 35 x 10^18 cm^-2. The distances to the clouds in this catalog may cover several orders of magnitude, so the masses may range from less than a Solar mass for clouds within the Galactic disc, to greater than 10^4 Solar Masses for HVCs at the tip of the Magellanic Stream. To search for trends, we separate the catalog into five populations based on position, velocity, and linewidth: high velocity clouds (HVCs); galaxy candidates; cold low velocity clouds (LVCs); warm, low positive-velocity clouds in the third Galactic Quadrant; and the remaining warm LVCs. The observed HVCs are found to be associated with previously-identified HVC complexes. We do not observe a large population of isolated clouds at high velocities as some models predict. We see evidence for distinct histories at low velocities in detecting populations of clouds corotating with the Galactic disc and a set of clouds that is not corotating.

astro-ph.GA↗

arXiv2015

High pressure floating zone growth and structural properties of ferrimagnetic quantum paraelectric BaFe$_{12}$O$_{19}$

High quality single crystals of BaFe$_{12}$O$_{19}$ were grown using the floating zone technique in flowing oxygen pressurized to 100 atm. Single crystal neutron diffraction was used to determine the nuclear and magnetic structure of BaFe$_{12}$O$_{19}$ at 4 K and 295 K. At both temperatures, there exist local electric dipoles formed by the off-mirror-plane displacements of magnetic Fe$^{3+}$ ions at the bipyramidal sites. The displacement at 4 K is about half of that at room temperature. The temperature dependence of the specific heat shows no anomaly associated with long range polar ordering in the temperature range from 1.90-300 K. The inverse dielectric permittivity, $1/\varepsilon$, along the c-axis shows a $T^2$ temperature dependence between 10 K and 20 K, with a significantly reduced temperature dependence displayed below 10 K. Moreover, as the sample is cooled below 1.4 K there is an anomalous sharp upturn in $1/\varepsilon$. These features resemble those of classic quantum paraelectrics such as SrTiO$_3$. The presence of the upturn in $1/\varepsilon$ indicates that BaFe$_{12}$O$_{19}$ is a critical quantum paraelectric system with Fe$^{3+}$ ions involved in both magnetic and electric dipole formation.

cond-mat.mtrl-sci↗

arXiv2015

PAMELA's Measurements of Magnetospheric Effects on High Energy Solar Particles

The nature of particle acceleration at the Sun, whether through flare reconnection processes or through shocks driven by coronal mass ejections (CMEs), is still under scrutiny despite decades of research. The measured properties of solar energetic particles (SEPs) have long been modeled in different particle-acceleration scenarios. The challenge has been to disentangle to the effects of transport from those of acceleration. The Payload for Antimatter Matter Exploration and Light-nuclei Astrophysics (PAMELA) instrument, enables unique observations of SEPs including composition and the angular distribution of the particles about the magnetic field, i.e. pitch angle distribution, over a broad energy range (>80 MeV) -- bridging a critical gap between space-based measurements and ground-based. We present high-energy SEP data from PAMELA acquired during the 2012 May 17 SEP event. These data exhibit differential anisotropies and thus transport features over the instrument rigidity range. SEP protons exhibit two distinct pitch angle distributions; a low-energy population that extends to 90° and a population that is beamed at high energies (>1 GeV), consistent with neutron monitor measurements. To explain a low-energy SEP population that exhibits significant scattering or redistribution accompanied by a high-energy population that reaches the Earth relatively unaffected by dispersive transport effects, we postulate that the scattering or redistribution takes place locally. We believe these are the first comprehensive measurements of the effects of solar energetic particle transport in the Earth's magnetosheath.

astro-ph.SR↗

arXiv2014

FACT -- The G-APD revolution in Cherenkov astronomy

Since two years, the FACT telescope is operating on the Canary Island of La Palma. Apart from its purpose to serve as a monitoring facility for the brightest TeV blazars, it was built as a major step to establish solid state photon counters as detectors in Cherenkov astronomy. The camera of the First G-APD Cherenkov Telesope comprises 1440 Geiger-mode avalanche photo diodes (G-APD), equipped with solid light guides to increase the effective light collection area of each sensor. Since no sense-line is available, a special challenge is to keep the applied voltage stable although the current drawn by the G-APD depends on the flux of night-sky background photons significantly varying with ambient light conditions. Methods have been developed to keep the temperature and voltage dependent response of the G-APDs stable during operation. As a cross-check, dark count spectra with high statistics have been taken under different environmental conditions. In this presentation, the project, the developed methods and the experience from two years of operation of the first G-APD based camera in Cherenkov astronomy under changing environmental conditions will be presented.

astro-ph.IM↗

arXiv2013

FACT - The First G-APD Cherenkov Telescope: Status and Results

The First G-APD Cherenkov telescope (FACT) is the first telescope using silicon photon detectors (G-APD aka. SiPM). It is built on the mount of the HEGRA CT3 telescope, still located at the Observatorio del Roque de los Muchachos, and it is successfully in operation since Oct. 2011. The use of Silicon devices promises a higher photon detection efficiency, more robustness and higher precision than photo-multiplier tubes. The FACT collaboration is investigating with which precision these devices can be operated on the long-term. Currently, the telescope is successfully operated from remote and robotic operation is under development. During the past months of operation, the foreseen monitoring program of the brightest known TeV blazars has been carried out, and first physics results have been obtained including a strong flare of Mrk501. An instantaneous flare alert system is already in a testing phase. This presentation will give an overview of the project and summarize its goals, status and first results.

astro-ph.IM↗

arXiv2013

FACT: Towards Robotic Operation of an Imaging Air Cherenkov Telescope

The First G-APD Cherenkov Telescope (FACT) became operational at La Palma in October 2011. Since summer 2012, due to very smooth and stable operation, it is the first telescope of its kind that is routinely operated from remote, without the need for a data-taking crew on site. In addition, many standard tasks of operation are executed automatically without the need for manual interaction. Based on the experience gained so far, some alterations to improve the safety of the system are under development to allow robotic operation in the future. We present the setup and precautions used to implement remote operations and the experience gained so far, as well as the work towards robotic operation.

astro-ph.IM↗