arXiv subjects

K. Nam

Publications and source records attributed to K. Nam.

3 recordsLinked to original sources

arXiv2021

Magnetic properties of S = 5/2 anisotropic triangular chain Bi3FeMo2O12

Competing magnetic interactions in low-dimensional quantum magnets can lead to the exotic ground state with fractionalized excitations. Herein, we present our results on an S = 5/2 quasi-one-dimensional spin system Bi3FeMo2O12. The structure of Bi3FeMo2O12 consists of very well separated, infinite zig-zag S = 5/2 spin chains. The observation of a broad maximum around 10 K in the magnetic susceptibility suggests the presence of short-range spin correlations. Magnetic susceptibility data do not fit to S=5/2 uniform spin chain model due to the presence of 2nd nearest-neighbor coupling (J2) along with the 1st nearest-neighbor coupling J1 of the zig-zag chain. The electronic structure calculations infer that the value of J1 is comparable with J2 (J2/J1~1.1) with a negligible inter-chain interaction (J'/J ~ 0.01), implying that Bi3FeMo2O12 is a highly frustrated triangular chain system. The absence of magnetic long-range ordering down to 0.2 K is seen in the heat capacity data, despite a relatively large antiferromagnetic Curie-Weiss temperature of -40 K. The magnetic heat capacity follows nearly a linear behavior at low temperatures indicating that the S = 5/2 anisotropic triangular chain exhibits the gapless excitations.

cond-mat.str-el↗

arXiv2020

Barium Selective Chemosensing by Diazacrown Ether Naphthalimide Turn-on Fluorophores for Single Ion Barium Tagging

Single molecule fluorescence detection of barium is investigated for enhancing the sensitivity and robustness of a neutrinoless double beta decay ($0νββ$) search in $^{136}$Xe, the discovery of which would alter our understanding of the nature of neutrinos and the early history of the Universe. A key developmental step is the synthesis of barium-selective chemosensors capable of incorporation into ongoing experiments in high-pressure $^{136}$Xe gas. Here we report turn-on fluorescent naphthalimide chemosensors containing monoaza- and diaza-crown ethers as agents for single Ba$^{2+}$ detection. Monoaza-18-crown-6 ether naphthalimide sensors showed sensitivity primarily to Ba$^{2+}$ and Hg$^{2+}$, whereas two diaza-18-crown-6 ether naphthalimides revealed a desirable selectivity toward Ba$^{2+}$. Solution-phase fluorescence and NMR experiments support a photoinduced electron transfer mechanism enabling turn-on fluorescence sensing in the presence of barium ions. Changes in ion-receptor interactions enable effective selectivity between competitive barium, mercury, and potassium ions, with detailed calculations correctly predicting fluorescence responses. With these molecules, dry-phase single Ba$^{2+}$ ion imaging with turn-on fluorescence is realized using oil-free microscopy techniques. This represents a significant advance toward a practical method of single Ba$^{2+}$ detection within large volumes of $^{136}$Xe, plausibly enabling a background-free technique to search for the hypothetical process of $0νββ$.

physics.ins-det↗

arXiv2011

The UFFO (Ultra Fast Flash Observatory) Pathfinder: Science and Mission

Hundreds of gamma-ray burst (GRB) optical light curves have been measured since the discovery of optical afterglows. However, even after nearly 7 years of operation of the Swift Observatory, only a handful of measurements have been made soon (within a minute) after the gamma ray signal. This lack of early observations fails to address burst physics at short time scales associated with prompt emissions and progenitors. Because of this lack of sub-minute data, the characteristics of the rise phase of optical light curve of short-hard type GRB and rapid-rising GRB, which may account for ~30% of all GRB, remain practically unknown. We have developed methods for reaching sub-minute and sub-second timescales in a small spacecraft observatory. Rather than slewing the entire spacecraft to aim the optical instrument at the GRB position, we use rapidly moving mirror to redirect our optical beam. As a first step, we employ motorized slewing mirror telescope (SMT), which can point to the event within 1s, in the UFFO Pathfinder GRB Telescope onboard the Lomonosov satellite to be launched in Nov. 2011. UFFO's sub-minute measurements of the optical emission of dozens of GRB each year will result in a more rigorous test of current internal shock models, probe the extremes of bulk Lorentz factors, provide the first early and detailed measurements of fast-rise GRB optical light curves, and help verify the prospect of GRB as a new standard candle. We will describe the science and the mission of the current UFFO Pathfinder project, and our plan of a full-scale UFFO-100 as the next step.

astro-ph.HE↗