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H. Park

Publications and source records attributed to H. Park.

At least 37 records · Page 2Linked to original sources
arXiv2021

A compact and stable incidence-plane-rotating second harmonics detector

We describe a compact and stable setup for detecting the optical second harmonics, in which the incident plane rotates with respect to the sample. The setup is composed of rotating Fresnel-rhomb optics and a femtosecond ytterbium-doped fiber-laser source operating at the repetition frequency of 10 MHz. The setup including the laser source occupies an area of 1 m2 and is stable so that the intensity fluctuation of the laser harmonics can be less than 0.2 % for 4 h. We present the isotropic harmonic signal of a gold mirror of 0.5 pW and demonstrate the integrity and sensitivity of the setup. We also show the polarization-dependent six-fold pattern of the harmonics of a few-layer WSe2, from which we infer the degree of local-field effects. Finally, we describe the extendibility of the setup to investigate the samples in various conditions such as cryogenic, strained, ultrafast non-equilibrium, and high magnetic fields.

physics.ins-det↗
arXiv2020

Measurement of time-dependent CP violation parameters in $B^0 \to K_0^SK_0^SK_0^S$ decays at Belle

We measure the time-dependent $C\!P$ violation parameters in $B^{0} \to K^{0}_{S}K^{0}_{S}K^{0}_{S}$ decays using $772 \times 10^6$ $B {\mkern 1.5mu\overline{\mkern-1.5mu B \mkern-0.1mu}\mkern 0.1mu}$ pairs collected at the $Υ(4S)$ resonance with the Belle detector at the KEKB asymmetric-energy $e^+ e^-$ collider. The obtained mixing-induced and direct $C\!P$ asymmetries are $-0.71 \pm 0.23~{\rm(stat)} \pm 0.05~{\rm(syst)}$ and $0.12 \pm 0.16~{\rm(stat)} \pm 0.05~{\rm(syst)}$, respectively. These values are consistent with the Standard Model predictions. The significance of $C\!P$ violation differs from zero by 2.5 standard deviations.

hep-ex↗
arXiv2020

A Silicon Beam Tracker

When testing and calibrating particle detectors in a test beam, accurate tracking information independent of the detector being tested is extremely useful during the offline analysis of the data. A general-purpose Silicon Beam Tracker (SBT) was constructed with an active area of 32.0 x 32.0 mm2 to provide this capability for the beam calibration of the Cosmic Ray Energetics And Mass (CREAM) calorimeter. The tracker consists of two modules, each comprised of two orthogonal layers of 380 μm thick silicon strip sensors. In one module each layer is a 64-channel AC-coupled single-sided silicon strip detector (SSD) with a 0.5 mm pitch. In the other, each layer is a 32-channel DC-coupled single-sided SSD with a 1.0 mm pitch. The signals from the 4 layers are read out using modified CREAM hodoscope front-end electronics with a USB 2.0 interface board to a Linux DAQ PC. In this paper, we present the construction of the SBT, along with its performance in radioactive source tests and in a CERN beam test in October 2006.

physics.ins-det↗
arXiv2020

Major Scientific Challenges and Opportunities in Understanding Magnetic Reconnection and Related Explosive Phenomena in Solar and Heliospheric Plasmas

Magnetic reconnection underlies many explosive phenomena in the heliosphere and in laboratory plasmas. The new research capabilities in theory/simulations, observations, and laboratory experiments provide the opportunity to solve the grand scientific challenges summarized in this whitepaper. Success will require enhanced and sustained investments from relevant funding agencies, increased interagency/international partnerships, and close collaborations of the solar, heliospheric, and laboratory plasma communities. These investments will deliver transformative progress in understanding magnetic reconnection and related explosive phenomena including space weather events.

physics.space-ph↗
arXiv2020

Search for $B^0$ Decays to Invisible Final States ($+γ$) at Belle

We report searches for $B^0\to\rm{invisible}$ and $B^0\to\rm{invisible}+γ$ decays, where the energy of the photon is required to be larger than 0.5 GeV. These results are obtained from a $711\,{\rm fb}^{-1}$ data sample that contains $772 \times 10^6 B\bar{B}$ pairs and was collected near the $Υ\,(4S)$ resonance with the Belle detector at the KEKB $e^+ e^-$ collider. We observe no significant signal for either decay and set upper limits on their branching fractions at $90\%$ confidence level of $\mathcal{B}\,(B^0\to\rm{invisible}) < 7.8\times10^{-5}$ and $\mathcal{B}\,(B^0\to\rm{invisible}+γ) < 1.6\times10^{-5}$.

hep-ex↗
arXiv2020

Clocking Auger Electrons

Intense X-ray free-electron lasers (XFELs) can rapidly excite matter, leaving it in inherently unstable states that decay on femtosecond timescales. As the relaxation occurs primarily via Auger emission, excited state observations are constrained by Auger decay. In situ measurement of this process is therefore crucial, yet it has thus far remained elusive at XFELs due to inherent timing and phase jitter, which can be orders of magnitude larger than the timescale of Auger decay. Here, we develop a new approach termed self-referenced attosecond streaking, based upon simultaneous measurements of streaked photo- and Auger electrons. Our technique enables sub-femtosecond resolution in spite of jitter. We exploit this method to make the first XFEL time-domain measurement of the Auger decay lifetime in atomic neon, and, by using a fully quantum-mechanical description, retrieve a lifetime of $2.2^{ + 0.2}_{ - 0.3}$ fs for the KLL decay channel. Importantly, our technique can be generalised to permit the extension of attosecond time-resolved experiments to all current and future FEL facilities.

physics.atom-ph↗
arXiv2019

Quantum yield enhancement in BDMO-PPV

BDMO-PPV is a photoluminescent semiconducting polymer related to others in the well-studied polyphenylene vinylene family such as MEH-PPV. These materials are known for their instability, degradation, and low efficiency in device operations. We report increased internal and external quantum yield in BDMO-PPV during continuous illumination photodegradation through variations in absorbance, scattering, reflectance, and transmittance of the BDMO-PPV solution. We propose the creation of a highly emissive intermediate photoluminescent state responsible for these increased quantum yields.

physics.chem-ph↗
arXiv2019

An integrated nanophotonic quantum register based on silicon-vacancy spins in diamond

We realize an elementary quantum network node consisting of a silicon-vacancy (SiV) color center inside a diamond nanocavity coupled to a nearby nuclear spin with 100 ms long coherence times. Specifically, we describe experimental techniques and discuss effects of strain, magnetic field, microwave driving, and spin bath on the properties of this 2-qubit register. We then employ these techniques to generate Bell-states between the SiV spin and an incident photon as well as between the SiV spin and a nearby nuclear spin. We also discuss control techniques and parameter regimes for utilizing the SiV-nanocavity system as an integrated quantum network node.

quant-ph↗
arXiv2019

Quantum network nodes based on diamond qubits with an efficient nanophotonic interface

Quantum networks require functional nodes consisting of stationary registers with the capability of high-fidelity quantum processing and storage, which efficiently interface with photons propagating in an optical fiber. We report a significant step towards realization of such nodes using a diamond nanocavity with an embedded silicon-vacancy (SiV) color center and a proximal nuclear spin. Specifically, we show that efficient SiV-cavity coupling (with cooperativity $C >30$) provides a nearly-deterministic interface between photons and the electron spin memory, featuring coherence times exceeding one millisecond. Employing coherent microwave control, we demonstrate heralded single photon storage in the long-lived spin memory as well as a universal control over a cavity-coupled two-qubit register consisting of a SiV and a proximal $^{\mathrm{13}}$C nuclear spin with nearly second-long coherence time, laying the groundwork for implementing quantum repeaters.

quant-ph↗
arXiv2019

Pressure-induced structural dimerization in the hyperhoneycomb iridate $β$-Li$_2$IrO$_3$ at low temperatures

A pressure-induced collapse of magnetic ordering in $β$-Li$_2$IrO$_3$ at $P_m\sim1.5- 2$ GPa has previously been interpreted as evidence for possible emergence of spin liquid states in this hyperhoneycomb iridate, raising prospects for experimental realizations of the Kitaev model. Based on structural data obtained at \emph{room temperature}, this magnetic transition is believed to originate in small lattice perturbations that preserve crystal symmetry, and related changes in bond-directional anisotropic exchange interactions. Here we report on the evolution of the crystal structure of $β$-Li$_2$IrO$_3$ under pressure at low temperatures ($T\leq50$ K) and show that the suppression of magnetism coincides with a change in lattice symmetry involving Ir-Ir dimerization. The critical pressure for dimerization shifts from 4.4(2) GPa at room temperature to $\sim1.5-2$ GPa below 50 K. While a direct $Fddd \rightarrow C2/c$ transition is observed at room temperature, the low temperature transitions involve new as well as coexisting dimerized phases. Further investigation of the Ir ($L_3$/$L_2$) isotropic branching ratio in x-ray absorption spectra indicates that the previously reported departure of the electronic ground state from a $J_{\rm{eff}}=1/2$ state is closely related to the onset of dimerized phases. In essence, our results suggest that the predominant mechanism driving the collapse of magnetism in $β$-Li$_2$IrO$_3$ is the pressure-induced formation of Ir$_2$ dimers in the hyperhoneycomb network. The results further confirm the instability of the $J_{\rm{eff}}=1/2$ moments and related non-collinear spiral magnetic ordering against formation of dimers in the low-temperature phase of compressed $β$-Li$_2$IrO$_3$.

cond-mat.str-el↗
arXiv2019

A Multigroup Moment-Accelerated Deterministic Particle Solver for Time-dependent Thermal Radiative Transfer Problems

We propose an efficient, robust, Lagrangian (characteristic-based) transport solver for the time-dependent thermal radiative Transfer (TRT) applications within the context of a moment-accelerated (High-Order/Low-Order, HOLO) algorithm. This novel transport algorithm inherits the best features of both particle methods (e.g., time accuracy, phase-space adaptivity, positivity) and deterministic, grid-based methods (e.g., no stochastic noise). Particles are evolved by the method of characteristics, with a time-dependent weight that accounts for stiff absorption and reemission process self-consistently. As a result, this approach is able to obtain accurate results while employing large time steps and a moderate number of particles (compared to Implicit Monte Carlo).

physics.comp-ph↗
arXiv2018

Measurement of the branching fraction of $B \rightarrow D^{(*)}π\ellν$ at Belle using hadronic tagging in fully reconstructed events

We report a measurement of the branching fraction of the decay $B \rightarrow D^{(*)}π\ellν$. The analysis uses 772$\times 10^6$ $B\bar{B}$ pairs produced in $e^+e^-\rightarrow Υ(4S)$ data recorded by the Belle experiment at the KEKB asymmetric-energy $e^+e^-$ collider. The tagging $B$ meson in the decay is fully reconstructed in a hadronic decay mode. On the signal side, we reconstruct the decay $B \rightarrow D^{(*)}π\ellν$ $(\ell=e,μ)$. The measured branching fractions are $\mathcal{B}(B^+ \rightarrow D^-π^+ \ell^+ν)$ = [4.55 $\pm$ 0.27 (stat.) $\pm$ 0.39 (syst.)]$\times 10^{-3}$, $\mathcal{B}(B^0 \rightarrow \bar{D}^0π^- \ell^+ν)$ = [4.05 $\pm$ 0.36 (stat.) $\pm$ 0.41 (syst.)]$\times 10^{-3}$, $\mathcal{B}(B^+ \rightarrow D^{*-}π^+ \ell^+ν)$ = [6.03 $\pm$ 0.43 (stat.) $\pm$ 0.38 (syst.)]$\times 10^{-3}$, and $\mathcal{B}(B^0 \rightarrow \bar{D}^{*0}π^- \ell^+ν)$ = [6.46 $\pm$ 0.53 (stat.) $\pm$ 0.52 (syst.)]$\times 10^{-3}$. These are in good agreement with the current world average values.

hep-ex↗
arXiv2018

Nature of the metal-insulator transition in few-unit-cell-thick LaNiO3 films

The nature of the metal insulator transition in thin films and superlattices of LaNiO3 with only few unit cells in thickness remains elusive despite tremendous effort. Quantum confinement and epitaxial strain have been evoked as the mechanisms, although other factors such as growth-induced disorder, cation non-stoichiometry, oxygen vacancies, and substrate-film interface quality may also affect the observable properties in the ultrathin films. Here we report results obtained for near-ideal LaNiO3 films with different thicknesses and terminations grown by atomic layer-by-layer laser molecular beam epitaxy on LaAlO3 substrates. We find that the room-temperature metallic behavior persists until the film thickness is reduced to an unprecedentedly small 1.5 unit cells (NiO2 termination). Electronic structure measurements using x-ray absorption spectroscopy and first-principles calculation suggest that oxygen vacancies existing in the films also contribute to the metal insulator transition.

cond-mat.mtrl-sci↗
arXiv2018

Design and Vertical Tests of SPS-series Double-Quarter Wave (DQW) Cavity Prototypes for the HL-LHC Crab Cavity System

Crab crossing is essential for high-luminosity colliders. The High Luminosity Large Hadron Collider (HL-LHC) will equip one of its Interaction Points (IP1) with Double-Quarter Wave (DQW) crab cavities. A DQW cavity is a new generation of deflecting RF cavities that stands out for its compactness and broad frequency separation between fundamental and first high-order modes. The deflecting kick is provided by its fundamental mode. Each HL-LHC DQW cavity shall provide a nominal deflecting voltage of 3.4 MV, although up to 5.0 MV may be required. A Proof-of-Principle (PoP) DQW cavity was limited by quench at 4.6 MV. This paper describes a new, highly optimized cavity, designated DQW SPS-series, which satisfies dimensional, cryogenic, manufacturing and impedance requirements for beam tests at SPS and operation in LHC. Two prototypes of this DQW SPS-series were fabricated by US industry and cold tested after following conventional SRF surface treatment. Both units outperformed the PoP cavity, reaching a deflecting voltage of 5.3-5.9 MV. This voltage - the highest reached by a DQW cavity - is well beyond the nominal voltage of 3.4 MV and may even operate at the ultimate voltage of 5.0MVwith sufficient margin. This paper covers fabrication, surface preparation and cryogenic RF test results and implications.

physics.acc-ph↗
arXiv2017

Measurement of the Decays $\boldsymbol{B\toη\ellν_\ell}$ and $\boldsymbol{B\toη^\prime\ellν_\ell}$ in Fully Reconstructed Events at Belle

We report branching fraction measurements of the decays $B^+\toη\ell^+ν_\ell$ and $B^+\toη^\prime\ell^+ν_\ell$ based on 711~fb$^{-1}$ of data collected near the $Υ(4S)$ resonance with the Belle experiment at the KEKB asymmetric-energy $e^+e^-$ collider. This data sample contains 772 million $B\bar B$~events. One of the two $B$~mesons is fully reconstructed in a hadronic decay mode. Among the remaining ("signal-$B$") daughters, we search for the $η$~meson in two decay channels, $η\toγγ$ and $η\toπ^+π^-π^0$, and reconstruct the $η^{\prime}$~meson in $η^\prime\toηπ^+π^-$ with subsequent decay of the $η$ into $γγ$. Combining the two $η$ modes and using an extended maximum likelihood, the $B^+\toη\ell^+ν_\ell$ branching fraction is measured to be $(4.2\pm 1.1 (\rm stat.)\pm 0.3 (\rm syst.))\times 10^{-5}$. For $B^+\toη^\prime\ell^+ν_\ell$, we observe no significant signal and set an upper limit of $0.72\times 10^{-4}$ at 90\% confidence level.

hep-ex↗
arXiv2017

The Characterization of new Eu2+ doped TlSr2I5 Scintillator Crystals

TlSr2I5: Eu2+ is a newly discovered scintillator and show promising scintillation properties for X- and γ-rays spectroscopy applications. Two zones vertical Bridgman technique is used for the growth of this scintillator. Luminescence properties of the grown crystals are measured under X-ray excitation at room temperature. Pure and Eu2+ doped crystals contained broad emission bands between 445-670 nm peaking at 528 nm and 430-600 nm peaking at 463 nm, respectively. Energy resolution, light yield and decay time profiles are studied under 662 keV γ-ray excitation using 137Cs radioactive source. Energy resolution of 4.2 % (FWHM) is obtained for 3 mol% Eu2+ doped crystal. For the same sample, light yield of 70,000 ph/MeV is also obtained. Three and two decay constants are observed for the pure and Eu2+ doped samples, respectively at room temperature. Effective Z-number and density are found to be 61 and 5.30 g/cm3, respectively.

physics.ins-det↗