arXiv subjects

J. H. Lee

Publications and source records attributed to J. H. Lee.

At least 37 records · Page 2Linked to original sources

arXiv2017

Magnetic Frustration Driven by Itinerancy in Spinel CoV2O4

Localized spins and itinerant electrons rarely coexist in geometrically-frustrated spinel lattices. We show that the spinel CoV2O4 stands at the crossover from insulating to itinerant behavior and exhibits a complex interplay between localized spins and itinerant electrons. In contrast to the expected paramagnetism, localized spins supported by enhanced exchange couplings are frustrated by the effects of delocalized electrons. This frustration produces a non-collinear spin state and may be responsible for macroscopic spin-glass behavior. Competing phases can be uncovered by external perturbations such as pressure or magnetic field, which enhance the frustration.

cond-mat.str-el↗

arXiv2016

Magnetic resonance study of bulk and thin film EuTiO3

Magnetic resonance spectra of EuTiO3 in both bulk and thin film form were taken at temperatures from 3-350 K and microwave frequencies from 9.2-9.8 and 34 GHz. In the paramagnetic phase, magnetic resonance spectra are determined by magnetic dipole and exchange interactions between Eu2+ spins. In the film, a large contribution arises from the demagnetization field. From detailed analysis of the linewidth and its temperature dependence, the parameters of spin-spin interactions were determined: the exchange frequency is 15-15.5 GHz and the estimated critical exponent of the spin correlation length is ~ 0.5. In the bulk samples, the spectra exhibited a distinct minimum in the linewidth at the Néel temperature, T_N = 5.5 K, while the resonance field practically does not change even on cooling below T_N. This is indicative of a small magnetic anisotropy ~ 320 G in the antiferromagnetic phase. In the film, the magnetic resonance spectrum is split below T_N into several components due to excitation of the magnetostatic modes, corresponding to a non-uniform precession of magnetization. Moreover, the film was observed to degrade over two years. This was manifested by an increase of defects and a change in the domain structure. The saturated magnetization in the film, estimated from the magnetic resonance spectrum, was about 900 emu/cm3 or 5.5 mu_B/unit cell at T = 3.5 K.

cond-mat.mtrl-sci↗

arXiv2016

Experimental investigation of a linear-chain structure in the nucleus 14C

It is a well-known fact that a cluster of nucleons can be formed in the interior of an atomic nucleus, and such clusters may occupy molecular-like orbitals, showing characteristics similar to normal molecules consisting of atoms. Chemical molecules having a linear alignment are commonly seen in nature, such as carbon dioxide. A similar linear alignment of the nuclear clusters, referred to as linear-chain cluster state (LCCS), has been studied since the 1950s, however, up to now there is no clear experimental evidence demonstrating the existence of such a state. Recently, it was proposed that an excess of neutrons may offer just such a stabilizing mechanism, revitalizing interest in the nuclear LCCS, specifically with predictions for their emergence in neutron-rich carbon isotopes. Here we present the experimental observation of α-cluster states in the radioactive 14C nucleus. Using the 10Be+α resonant scattering method with a radioactive beam, we observed a series of levels which completely agree with theoretically predicted levels having an explicit linear-chain cluster configuration. We regard this as the first strong indication of the linear-chain clustered nucleus.

nucl-ex↗

arXiv2016

Search for solar axions with CsI(Tl) crystal detectors

The results of a search for solar axions from the Korea Invisible Mass Search (KIMS) experiment at the Yangyang Underground Laboratory are presented. Low-energy electron-recoil events would be produced by conversion of solar axions into electrons via the axio-electric effect in CsI(Tl) crystals. Using data from an exposure of 34,596 $\rm kg \cdot days$, we set a 90 \% confidence level upper limit on the axion-electron coupling, $g_{ae}$, of $1.39 \times 10^{-11}$ for an axion mass less than 1 keV/$\rm c^2$. This limit is lower than the indirect solar neutrino bound, and fully excludes QCD axions heavier than 0.48 eV/$\rm c^2$ and 140.9 eV/$\rm c^2$ for the DFSZ and KSVZ models respectively.

hep-ex↗

arXiv2016

Understanding internal backgrounds of NaI(Tl) crystals toward a 200~kg array for the KIMS-NaI experiment

The Korea Invisible Mass Search (KIMS) collaboration has developed low-background NaI(Tl) crystals that are suitable for the direct detection of WIMP dark matter. With experience built on the KIMS-CsI programs, the KIMS-NaI experiment will consist of a 200~kg NaI(Tl) crystal array surrounded by layers of shielding structures and will be operated at the Yangyang underground laboratory. The goal is to provide an unambiguous test of the DAMA/LIBRA's annual modulation signature. Measurements of six prototype crystals show progress in the reduction of internal contaminations of radioisotopes. Based on our understanding of these measurements, we expect to achieve a background level in the final detector configuration that is less than 1~count/day/keV/kg for recoil energies around 2~keV. The annual modulation sensitivity for the KIMS-NaI experiment shows that an unambiguous 7$σ$ test of the DAMA/LIBRA signature would be possible with a 600~kg$\cdot$year exposure with this system.

physics.ins-det↗

arXiv2016

A CaMoO4 Crystal Low Temperature Detector for the AMoRE Neutrinoless Double Beta Decay Search

We report the development of a CaMoO4 crystal low temperature detector for the AMoRE neutrinoless double beta decay (0ν\b{eta}\b{eta}) search experiment. The prototype detector cell was composed of a 216 g CaMoO4 crystal and a metallic magnetic calorimeter. An over-ground measurement demonstrated FWHM resolution of 6-11 keV for full absorption gamma peaks. Pulse shape discrimination was clearly demonstrated in the phonon signals, and 7.6 σ of discrimination power was found for the α and \b{eta}/γ separation. The phonon signals showed rise-times of about 1 ms. It is expected that the relatively fast rise-time will increase the rejection efficiency of two-neutrino double beta decay pile-up events which can be one of the major background sources in 0ν\b{eta}\b{eta} searches.

physics.ins-det↗

arXiv2016

Rapidity and centrality dependence of particle production for identified hadrons in Cu+Cu collisions at $\sqrt{s_{NN}} = 200$ GeV

The BRAHMS collaboration has measured transverse momentum spectra of pions, kaons, protons and antiprotons at rapidities 0 and 3 for Cu+Cu collisions at $\sqrt{s_{NN}} = 200$ GeV. As the collisions become more central the collective radial flow increases while the temperature of kinetic freeze-out decreases. The temperature is lower and the radial flow weaker at forward rapidity. Pion and kaon yields with transverse momenta between 1.5 and 2.5 GeV/c are suppressed for central collisions relative to scaled $p+p$ collisions. This suppression, which increases as the collisions become more central is consistent with jet quenching models and is also present with comparable magnitude at forward rapidity. At such rapidities initial state effects may also be present and persistence of the meson suppression to high rapidity may reflect a combination of jet quenching and nuclear shadowing. The ratio of protons to mesons increases as the collisions become more central and is largest at forward rapidities.

nucl-ex↗

arXiv2015

Technical Design Report for the AMoRE $0νββ$ Decay Search Experiment

The AMoRE (Advanced Mo-based Rare process Experiment) project is a series of experiments that use advanced cryogenic techniques to search for the neutrinoless double-beta decay of \mohundred. The work is being carried out by an international collaboration of researchers from eight countries. These searches involve high precision measurements of radiation-induced temperature changes and scintillation light produced in ultra-pure \Mo[100]-enriched and \Ca[48]-depleted calcium molybdate ($\mathrm{^{48depl}Ca^{100}MoO_4}$) crystals that are located in a deep underground laboratory in Korea. The \mohundred nuclide was chosen for this \zeronubb decay search because of its high $Q$-value and favorable nuclear matrix element. Tests have demonstrated that \camo crystals produce the brightest scintillation light among all of the molybdate crystals, both at room and at cryogenic temperatures. $\mathrm{^{48depl}Ca^{100}MoO_4}$ crystals are being operated at milli-Kelvin temperatures and read out via specially developed metallic-magnetic-calorimeter (MMC) temperature sensors that have excellent energy resolution and relatively fast response times. The excellent energy resolution provides good discrimination of signal from backgrounds, and the fast response time is important for minimizing the irreducible background caused by random coincidence of two-neutrino double-beta decay events of \mohundred nuclei. Comparisons of the scintillating-light and phonon yields and pulse shape discrimination of the phonon signals will be used to provide redundant rejection of alpha-ray-induced backgrounds. An effective Majorana neutrino mass sensitivity that reaches the expected range of the inverted neutrino mass hierarchy, i.e., 20-50 meV, could be achieved with a 200~kg array of $\mathrm{^{48depl}Ca^{100}MoO_4}$ crystals operating for three years.

physics.ins-det↗

arXiv2015

Quantifying noise sources in the KSTAR 2014 Thomson Scattering system from the measured variation on electron temperature

With the Thomson scattering (TS) system in KSTAR, temporal evolution of electron temperature ($T_e$) is estimated using a weighted look-up table method with fast sampling ($1.25$ or $2.5$ GS/s) digitizers during the 2014 KSTAR campaign. Background noise level is used as a weighting parameter without considering the photon noise due to the absence of information on absolute photon counts detected by the TS system. Estimated electron temperature during a relatively quiescent discharge are scattered, i.e., $15$\% variation on $T_e$ with respect to its mean value. We find that this $15$\% variation on $T_e$ cannot be explained solely by the background noise level which leads us to include photon noise effects in our analysis. Using synthetic data, we have estimated the required photon noise level consistent with the observation and determined the dominant noise source in KSTAR TS system.

physics.plasm-ph↗

arXiv2015

Pulse-shape discrimination between electron and nuclear recoils in a NaI(Tl) crystal

We report on the response of a high light-output NaI(Tl) crystal to nuclear recoils induced by neutrons from an Am-Be source and compare the results with the response to electron recoils produced by Compton scattered 662 keV $γ$-rays from a $^{137}$Cs source. The measured pulse-shape discrimination (PSD) power of the NaI(Tl) crystal is found to be significantly improved because of the high light output of the NaI(Tl) detector. We quantify the PSD power with a quality factor and estimate the sensitivity to the interaction rate for weakly interacting massive particles (WIMPs) with nucleons, and the result is compared with the annual modulation amplitude observed by the DAMA/LIBRA experiment. The sensitivity to spin-independent WIMP-nucleon interactions based on 100 kg$\cdot$year of data from NaI detectors is estimated with simulated experiments, using the standard halo model.

astro-ph.IM↗

arXiv2015

Strong competition between orbital-ordering and itinerancy in a frustrated spinel vanadate

The crossover from localized- to itinerant-electron behavior is associated with many intriguing phenomena in condensed-matter physics. In this paper, we investigate the crossover from localized to itinerant regimes in the spinel system Mn$_{1-x}$Co$_x$V$_2$O$_4$. At low Co doping, orbital order (OO) of the localized electrons on the V3+ ions suppresses magnetic frustration by triggering a tetragonal distortion. With Co doping, electronic itinerancy melts the OO and suppresses the structural phase transition while the reduced spin-lattice coupling produces magnetic frustration. Neutron scattering measurements and first-principles-guided spin models reveal that the non-collinear state at high Co doping is produced by weakened local anisotropy and enhanced Co-V spin interactions.

cond-mat.str-el↗

arXiv2015

Optical diode effect in the room-temperature multiferroic BiFeO$_3

Multiferroics permit the magnetic control of the electric polarization and electric control of the magnetization. These static magnetoelectric (ME) effects are of enormous interest: The ability to read and write a magnetic state current-free by an electric voltage would provide a huge technological advantage. Dynamic or optical ME effects are equally interesting because they give rise to unidirectional light propagation as recently observed in low-temperature multiferroics. This phenomenon, if realized at room temperature, would allow the development of optical diodes which transmit unpolarized light in one, but not in the opposite direction. Here, we report strong unidirectional transmission in the room-temperature multiferroic BiFeO$_3$ over the gigahertz--terahertz frequency range. Supporting theory attributes the observed unidirectional transmission to the spin-current driven dynamic ME effect. These findings are an important step toward the realization of optical diodes, supplemented by the ability to switch the transmission direction with a magnetic or electric field.

cond-mat.mtrl-sci↗

arXiv2015

Measurement of the quenching and channeling effects in a CsI crystal used for a WIMP search

We have studied channeling effects in a Cesium Iodide (CsI) crystal that is similar in composition to the ones being used in a search for Weakly Interacting Massive Particles (WIMPs) dark matter candidates, and measured its energy-dependent quenching factor, the relative scintillation yield for electron and nuclear recoils. The experimental results are reproduced with a GEANT4 simulation that includes a model of the scintillation efficiency as a function of electronic stopping power. We present the measured and simulated quenching factors and the estimated effects of channeling.

physics.ins-det↗

arXiv2015

Giant spin-phonon-electronic coupling in a 5d oxide

Enhanced coupling of material properties offers new fundamental insights and routes to multifunctional devices. In this context 5d oxides provide new paradigms of cooperative interactions driving novel emergent behavior. This is exemplified in 5d osmates that host a metal-insulator transition (MIT) driven by magnetic order. Here we consider the most robust case, the 5d perovskite NaOsO3, and reveal a giant coupling between spin and phonon through a frequency shift of Δω=40 cm-1, the largest measured in any material. We identify the dominant octahedral breathing mode and show isosymmetry with spin ordering which induces dynamic charge disproportionation that sheds new light on the MIT. The occurrence of the dramatic spin-phonon-electronic coupling in NaOsO3 is due to a property common to all 5d materials: the large spatial extent of the 5d ion. This allows magnetism to couple to phonons on an unprecedented scale and consequently offers multiple new routes to enhanced coupled phenomena.

cond-mat.str-el↗

arXiv2015

The Q_weak Experimental Apparatus

The Jefferson Lab Q_weak experiment determined the weak charge of the proton by measuring the parity-violating elastic scattering asymmetry of longitudinally polarized electrons from an unpolarized liquid hydrogen target at small momentum transfer. A custom apparatus was designed for this experiment to meet the technical challenges presented by the smallest and most precise ${\vec{e}}$p asymmetry ever measured. Technical milestones were achieved at Jefferson Lab in target power, beam current, beam helicity reversal rate, polarimetry, detected rates, and control of helicity-correlated beam properties. The experiment employed 180 microA of 89% longitudinally polarized electrons whose helicity was reversed 960 times per second. The electrons were accelerated to 1.16 GeV and directed to a beamline with extensive instrumentation to measure helicity-correlated beam properties that can induce false asymmetries. Moller and Compton polarimetry were used to measure the electron beam polarization to better than 1%. The electron beam was incident on a 34.4 cm liquid hydrogen target. After passing through a triple collimator system, scattered electrons between 5.8 degrees and 11.6 degrees were bent in the toroidal magnetic field of a resistive copper-coil magnet. The electrons inside this acceptance were focused onto eight fused silica Cerenkov detectors arrayed symmetrically around the beam axis. A total scattered electron rate of about 7 GHz was incident on the detector array. The detectors were read out in integrating mode by custom-built low-noise pre-amplifiers and 18-bit sampling ADC modules. The momentum transfer Q^2 = 0.025 GeV^2 was determined using dedicated low-current (~100 pA) measurements with a set of drift chambers before (and a set of drift chambers and trigger scintillation counters after) the toroidal magnet.

physics.ins-det↗

arXiv2014

Determination of electron-nucleus collision geometry with forward neutrons

There are a large number of physics programs one can explore in electron-nucleus collisions at a future electron-ion collider. Collision geometry is very important in these studies, while the measurement for an event-by-event geometric control is rarely discussed in the prior deep inelastic scattering experiments off a nucleus. This paper seeks to provide some detailed studies on the potential of tagging collision geometries through forward neutron multiplicity measurements with a zero degree calorimeter. This type of geometry handle, if achieved, can be extremely beneficial in constraining nuclear effects for the electron-nucleus program at an electron-ion collider.

hep-ex↗

arXiv2014

Measurement of Parity-Violating Asymmetry in Electron-Deuteron Inelastic Scattering

The parity-violating asymmetries between a longitudinally-polarized electron beam and an unpolarized deuterium target have been measured recently. The measurement covered two kinematic points in the deep inelastic scattering region and five in the nucleon resonance region. We provide here details of the experimental setup, data analysis, and results on all asymmetry measurements including parity-violating electron asymmetries and those of inclusive pion production and beam-normal asymmetries. The parity-violating deep-inelastic asymmetries were used to extract the electron-quark weak effective couplings, and the resonance asymmetries provided the first evidence for quark-hadron duality in electroweak observables. These electron asymmetries and their interpretation were published earlier, but are presented here in more detail.

nucl-ex↗

arXiv2014

Neutron calibration facility with an Am-Be source for pulse shape discrimination measurement of CsI(Tl) crystals

We constructed a neutron calibration facility based on a 300-mCi Am-Be source in conjunction with a search for weakly interacting massive particle candidates for dark matter. The facility is used to study the response of CsI(Tl) crystals to nuclear recoils induced by neutrons from the Am-Be source and comparing them with the response to electron recoils produced by Compton scattering of 662-keV $γ$-rays from a $^{137}$Cs source. The measured results on pulse shape discrimination (PSD) between nuclear- and electron-recoil events are quantified in terms of quality factors. A comparison with similar result from a neutron reactor demonstrate the feasibility of performing calibrations of PSD measurements using neutrons from a Am-Be source.

physics.ins-det↗