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J. Yoon

Publications and source records attributed to J. Yoon.

9 recordsLinked to original sources
arXiv2017

Capacitive Deionization -- defining a class of desalination technologies

Over the past decade, capacitive deionization (CDI) has realized a surge in attention in the field of water desalination and can now be considered as an important technology class, along with reverse osmosis and electrodialysis. While many of the recently developed technologies no longer use a mechanism that follows the strict definition of the term "capacitive", these methods nevertheless share many common elements that encourage treating them with similar metrics and analyses. Specifically, they all involve electrically driven removal of ions from a feed stream, storage in an electrode (i.e., ion electrosorption) and release, in charge/discharge cycles. Grouping all these methods in the technology class of CDI makes it possible to treat evolving new technologies in standardized terms and compare them to other technologies in the same class.

physics.app-ph↗
arXiv2017

Personalized Survival Predictions for Cardiac Transplantation via Trees of Predictors

Given the limited pool of donor organs, accurate predictions of survival on the wait list and post transplantation are crucial for cardiac transplantation decisions and policy. However, current clinical risk scores do not yield accurate predictions. We develop a new methodology (ToPs, Trees of Predictors) built on the principle that specific predictors should be used for specific clusters within the target population. ToPs discovers these specific clusters of patients and the specific predictor that perform best for each cluster. In comparison with current clinical risk scoring systems, our method provides significant improvements in the prediction of survival time on the wait list and post transplantation. For example, in terms of 3 month survival for patients who were on the US patient wait list in the period 1985 to 2015, our method achieves AUC of 0.847, the best commonly used clinical risk score (MAGGIC) achieves 0.630. In terms of 3 month survival/mortality predictions (in comparison to MAGGIC), holding specificity at 80.0 percents, our algorithm correctly predicts survival for 1,228 (26.0 percents more patients out of 4,723 who actually survived, holding sensitivity at 80.0 percents, our algorithm correctly predicts mortality for 839 (33.0 percents) more patients out of 2,542 who did not survive. Our method achieves similar improvements for other time horizons and for predictions post transplantation. Therefore, we offer a more accurate, personalized approach to survival analysis that can benefit patients, clinicians and policymakers in making clinical decisions and setting clinical policy. Because risk prediction is widely used in diagnostic and prognostic clinical decision making across diseases and clinical specialties, the implications of our methods are far reaching.

stat.AP↗
arXiv2016

The role of binaries in the enrichment of the early Galactic halo. III. Carbon-enhanced metal-poor stars -- CEMP-s stars

Detailed spectroscopic studies of metal-poor halo stars have highlighted the important role of carbon-enhanced metal-poor (CEMP) stars in understanding the early production and ejection of carbon in the Galaxy and in identifying the progenitors of the CEMP stars among the first stars formed after the Big Bang. Recent work has also classified the CEMP stars by absolute carbon abundance, A(C), into high- and low-C bands, mostly populated by binary and single stars, respectively. Our aim is to determine the frequency and orbital parameters of binary systems among the CEMP-s stars, which exhibit strong enhancements of neutron-capture elements associated with the s-process. This allows us to test whether local mass transfer from a binary companion is necessary and sufficient to explain their dramatic carbon excesses. Eighteen of the 22 stars exhibit clear orbital motion, yielding a binary frequency of 82+-10%, while four stars appear to be single (18+-10%). We thus confirm that the binary frequency of CEMP-s stars is much higher than for normal metal-poor giants, but not 100% as previously claimed. Secure orbits are determined for 11 of the binaries and provisional orbits for six long-period systems (P > 3,000 days), and orbital circularisation time scales are discussed. The conventional scenario of local mass transfer from a former AGB binary companion does appear to account for the chemical composition of most CEMP-s stars. However, the excess of C and s-process elements in some single CEMP-s stars was apparently transferred to their natal clouds by an external (distant) source. This finding has important implications for our understanding of carbon enrichment in the early Galactic halo and some high-redshift DLA systems, and of the mass loss from extremely metal-poor AGB stars. Abridged.

astro-ph.SR↗
arXiv2015

The role of binaries in the enrichment of the early Galactic halo. II. Carbon-Enhanced Metal-Poor Stars - CEMP-no stars

The detailed composition of most metal-poor halo stars has been found to be very uniform. However, a fraction of 20-70% (increasing with decreasing metallicity) exhibit dramatic enhancements in their abundances of carbon - the so-called carbon-enhanced metal-poor (CEMP) stars. A key question for Galactic chemical evolution models is whether this non-standard composition reflects that of the stellar natal clouds, or is due to local, post-birth mass transfer of chemically processed material from a binary companion; CEMP stars should then all be members of binary systems. Our aim is to determine the frequency and orbital parameters of binaries among CEMP stars with and without over-abundances of neutron-capture elements - CEMP-s and CEMP-no stars, respectively - as a test of this local mass-transfer scenario. This paper discusses a sample of 24 CEMP-no stars, while a subsequent paper will consider a similar sample of CEMP-s stars. Most programme stars exhibit no statistically significant radial-velocit variation over this period and appear to be single, while four are found to be binaries with orbital periods of 300-2,000 days and normal eccentricity; the binary frequency for the sample is 17+-9%. The single stars mostly belong to the recently-identified ``low-C band'', while the binaries have higher absolute carbon abundances. We conclude that the nucleosynthetic process responsible for the strong carbon excess in these ancient stars is unrelated to their binary status; the carbon was imprinted on their natal molecular clouds in the early Galactic ISM by an even earlier, external source, strongly indicating that the CEMP-no stars are likely bona fide second-generation stars. We discuss potential production sites for carbon and its transfer across interstellar distances in the early ISM, and implications for the composition of high-redshift DLA systems. Abridged.

astro-ph.SR↗
arXiv2015

The role of binaries in the enrichment of the early Galactic halo. I. r-process-enhanced metal-poor stars

The detailed chemical composition of most metal-poor halo stars has been found to be highly uniform, but a minority of stars exhibit dramatic enhancements in their abundances of heavy neutron-capture elements and/or of carbon. The key question for Galactic chemical evolution models is whether these peculiarities reflect the composition of the natal clouds, or if they are due to later mass transfer of processed material from a binary companion. If the former case applies, the observed excess of certain elements was implanted within selected clouds in the early ISM from a production site at interstellar distances. Our aim is to determine the frequency and orbital properties of binaries among these chemically peculiar stars. This information provides the basis for deciding whether mass transfer from a binary companion is necessary and sufficient to explain their unusual compositions. This paper discusses our study of a sample of 17 moderately (r-I) and highly (r-II) r-process-element enhanced VMP and EMP stars. High-resolution, low signal-to-noise spectra of the stars were obtained at roughly monthly intervals over 8 years with the FIES spectrograph at the Nordic Optical Telescope. From these spectra, radial velocities with an accuracy of ~100 m/s were determined by cross-correlation against an optimized template. 14 of the programme stars exhibit no significant RV variation over this period, while 3 are binaries with orbits of typical eccentricity for their periods, resulting in a normal binary frequency of ~18+-6% for the sample. Our results confirm our preliminary conclusion from 2011, based on partial data, that the chemical peculiarity of the r-I and r-II stars is not caused by any putative binary companions. Instead, it was imprinted on the natal molecular clouds of these stars by an external, distant source. Models of the ISM in early galaxies should account for such mechanisms.

astro-ph.SR↗
arXiv1999

From the magnetic-field-driven transitions to the zero-field transition in two-dimensions

For more than a decade it was widely accepted that two-dimensional electrons are insulating at zero temperature and at zero magnetic-field. Experimentally it was demonstrated that, when placed in a strong perpendicular magnetic field, the insulating phase turns into a quantum-Hall state. While this transition was in accordance with existing theoretical models (KLZ), the density driven metal-insulator transition at zero magnetic-field, recently observed in high-quality two-dimensional systems, was unforeseen and, despite considerable amount of effort, its origins are still unknown. In order to improve our understanding of the zero magnetic-field transition, we conducted a study of the insulator to quantum-Hall transition in low-density, two-dimensional, hole system in GaAs that exhibits the zero magnetic-field metal-insulator transition. We found that, in the low field insulating phase, upon increasing the carrier density towards the metal-insulator transition, the critical magnetic-field of the insulator to quantum-Hall transition decreases and converges to the zero magnetic-field metal-insulator transition. This implies a common origin for both the finite magnetic-field and the zero magnetic-field transitions.

cond-mat↗
arXiv1998

Microwave Resonance and Carrier-Carrier Interaction in Two-Dimensional Hole Systems at High Magnetic Field

Microwave frequency conductivity, Re($σ_{xx}$), of high quality two-dimensional hole systems (2DHS) in a large perpendicular magnetic field (B) is measured with the carrier density ($n_s$) of the 2DHS controlled by a backgate bias. The high B insulating phase of the 2DHS exhibits a microwave resonance that remains well-defined, but shifts to higher peak frequency ($f_{pk}$) as $n_s$ is reduced. Over a wide range of $n_s, f_{pk} \propto n_s^{-1/2}$ is observed for the two samples we measured. The data clearly indicate that both carrier-carrier interactions and disorder are indispensable in determining the dynamics of the insulator. The $n_s$ dependence of $f_{pk}$ is consistent with a weakly pinned Wigner crystal in which domain size increases with $n_s$, due to larger carrier-carrier interaction.

cond-mat.mes-hall↗
arXiv1998

Novel Properties of The Apparent Metal-Insulator Transition in Two-Dimensional Systems

The low-temperature conductivity of low-density, high-mobility, two-dimensional hole systems in GaAs was studied. We explicitly show that the metal-insulator transition, observed in these systems, is characterized by a well-defined critical density, p_0c. We also observe that the low-temperature conductivity of these systems depends linearly on the hole density, over a wide density range. The high-density linear conductivity extrapolates to zero at a density close to the critical density.

cond-mat.mes-hall↗