arXiv subjects

J. H. Kang

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

At least 37 records · Page 2Linked to original sources

arXiv2021

Ultrafast generation and detection of propagating coherent acoustic phonon wave packets in ultra-thin iron pnictide films

We observe pronounced oscillations in differential reflectivity of 9 nm and 60 nm BaFe\textsubscript{2}As\textsubscript{2} (Ba-122) thin films using ultrafast optical spectroscopy. Our studies show that the oscillations result from propagating longitudinal acoustic (LA) phonon wave packets with strong thickness and temperature dependence. Particularly, the experimentally measured oscillation frequency approaches to 50 GHz for the ultra-thin film. Our calculations show that Young's modulus of 9 nm thin film is nearly four times as large as that of 60 nm thin film, consistent with the experiment. The increase in Young's modulus as thickness decrease was attributed to the decrease in parent Ba-122 tetragonality $c/a$ near the film-substrate interface due to material-substrate mismatch effect. %Temperature dependence of LA phonon mode frequency for 9 nm Ba-122 thin film is reported. The temperature-dependent change in LA phonon frequency was attributed to the change in parent Ba-122 othorhombicity $(a-b)/(a+b)$.

cond-mat.mtrl-sci↗

arXiv2020

Light Quantum Control of Persisting Higgs Modes in Iron-Based Superconductors

The Higgs mechanism, i.e., spontaneous symmetry breaking of the quantum vacuum, is a cross-disciplinary principle, universal for understanding dark energy, antimatter and quantum materials, from superconductivity to magnetism. Yet, Higgs modes in one-band superconductors (SCs) are currently under debate due to their competition with charge-density fluctuations. A distinct Higgs mode, controllable by terahertz (THz) laser pulses, can arise in multi-band, unconventional SCs via strong {\em interband} Coulomb interaction, but is yet to be accessed. Here we both discover and demonstrate quantum control of such collective mode in iron-based high-temperature superconductors. Using two-pulse, phase coherent THz spectroscopy, we observe a tunable and coherent 2$Δ_{\mathrm{SC}}$ amplitude oscillation of the complex order parameter in such SC with coupled lower and upper bands. The nonlinear dependence of the amplitude mode oscillations on the THz driving fields is distinct from any one-band and conventional SC results: we observe a large nonlinear change of resonance strength, yet with a persisting mode frequency. We argue that this result provides compelling evidence for a transient coupling between the electron and hole amplitude modes via strong interband coherent interaction. To support this scenario, we perform quantum kinetic modeling of a hybrid Higgs mechanism without invoking extra disorder or phonons. In addition to distinguishing between collective modes and charge fluctuations, the light quantum control of multiband SCs can be extended to probe and manipulate many-body entanglement and hidden symmetries in different quantum materials.

cond-mat.supr-con↗

arXiv2020

Superconductivity in Undoped BaFe2As2 by Tetrahedral Geometry Design

Fe-based superconductors exhibit a diverse interplay between charge, orbital, and magnetic ordering1-4. Variations in atomic geometry affect electron hopping between Fe atoms5,6 and the Fermi surface topology, influencing magnetic frustration and the pairing mechanism through changes of orbital overlap and occupancies7-11. Here, we experimentally demonstrate a systematic approach to realize superconductivity without chemical doping in BaFe2As2, employing geometric design within an epitaxial heterostructure. We control both tetragonality and orthorhombicity in BaFe2As2 through superlattice engineering, which we experimentally find to induce superconductivity when the As-Fe-As bond angle approaches that in a regular tetrahedron. This approach of superlattice design could lead to insights into low dimensional superconductivity in Fe-based superconductors.

cond-mat.supr-con↗

arXiv2019

Discovery of Terahertz Second Harmonic Generation from Lightwave Acceleration of Symmetry--Breaking Nonlinear Supercurrents

We report terahertz (THz) second harmonic generation (SHG) in superconductors (SC) with inversion symmetric equilibrium states that forbid even-order nonlinearities. Such SHG signal is observed in single-pulse emission by periodic driving with a multi-cycle THz electric field tuned below the SC energy gap and vanishes above the SC critical temperature. We explain the microscopic physics by a dynamical symmetry breaking principle at sub-THz-cycle by using quantum kinetic modeling of the interplay between strong THz-lightwave nonlinearity and pulse propagation. The resulting non-zero integrated pulse area inside the SC drives lightwave nonlinear supercurrents due to sub--cycle Cooper pair acceleration, in contrast to d.c.-biased superconductors, which can be controlled by the bandstructure and the THz pump field.

cond-mat.supr-con↗

arXiv2018

Competing Orders and Ultrafast Energy Transfer at the Quantum Limit in a Nb$_3$Sn Superconductor Probed by Terahertz Electrodynamics

We report the low-energy electrodynamics of a moderately clean A15 superconductor (SC) following ultrafast excitation to understand and manipulate terahertz (THz) quasi--particle (QP) transport by tuning pump photoexcitation of from competing orders. Using 35-fs optical pulses, we observe a non-thermal enhancement in the low frequency conductivity, opposite to that observed for THz pump, which persists up to an additional critical temperature, above the SC one, from an electronic order in the Martensitic normal state. In the SC state, the fluence dependence of pair breaking kinetics together with an analytic model provides evidence for a `one photon-to-one pair' non-resonant energy transfer during the laser pulse. Such initial transfer of photon energy $\hbarω$ to QPs at the {\em quantum} limit, set by $2Δ_{SC}/\hbarω$=0.33$\%$, is more than one order of magnitude smaller than in previously studied BCS SCs, which we attribute to strong electron--phonon coupling and possible influence of phonon condensation in A15 SCs.

cond-mat.str-el↗

arXiv2018

Search for $CP$ violation in the $D^{+}\toπ^{+}π^{0}$ decay at Belle

We search for $CP$ violation in the charged charm meson decay $D^{+}\toπ^{+}π^{0}$, based on a data sample corresponding to an integrated luminosity of $\rm 921~fb^{-1}$ collected by the Belle experiment at the KEKB $e^{+}e^{-}$ asymmetric-energy collider. The measured $CP$ violating asymmetry is $[+2.31\pm1.24({\rm stat})\pm0.23({\rm syst})]\%$, which is consistent with the standard model prediction and has a significantly improved precision compared to previous results.

hep-ex↗

arXiv2017

Measurements of azimuthal anisotropy and charged-particle multiplicity in $d$$+$Au collisions at $\sqrt{s_{_{NN}}}=$200, 62.4, 39, and 19.6 GeV

We present measurements of the elliptic flow, $v_2$, as a function of transverse momentum ($p_T$), pseudorapidity ($η$), and centrality in $d$$+$Au collisions at $\sqrt{s_{_{NN}}}=$200, 62.4, 39, and 19.6 GeV. The beam-energy scan of $d$$+$Au collisions provides a testing ground for the onset of flow signatures in small collision systems. We measure a nonzero $v_2$ signal at all four collision energies, which, at midrapidity and low $p_T$, is consistent with predictions from viscous hydrodynamic models. Comparisons with calculations from parton transport models (based on the {\sc ampt} Monte Carlo generator) show good agreement with the data at midrapidity to forward ($d$-going) rapidities and low $p_T$. At backward (Au-going) rapidities and $p_T>1.5$ GeV/$c$, the data diverges from {\sc ampt} calculations of $v_2$ relative to the initial geometry, indicating the possible dominance of nongeometry related correlations, referred to as nonflow. We also present measurements of the charged-particle multiplicity ($dN_{\rm ch}/dη$) as a function of $η$ in central $d$$+$Au collisions at the same energies. We find that in $d$$+$Au collisions at $\sqrt{s_{_{NN}}}=$200 GeV the $v_2$ scales with $dN_{\rm ch}/dη$ over all $η$ in the PHENIX acceptance. At $\sqrt{s_{_{NN}}}=$62.4, and 39 GeV, $v_2$ scales with $dN_{\rm ch}/dη$ at midrapidity and forward rapidity, but falls off at backward rapidity. This departure from the $dN_{\rm ch}/dη$ scaling may be a further indication of nonflow effects dominating at backward rapidity.

nucl-ex↗

arXiv2017

Search for heavy neutrinos at Belle

We report on a search for heavy neutrinos in B-meson decays. The results are obtained using a data sample that contains 772x10^6 BB-bar pairs collected at the Upsilon(4S) resonance with the Belle detector at the KEKB asymmetric-energy e+e- collider. No signal is observed and upper limits are set on mixing of heavy neutrinos with left-handed neutrinos of the Standard Model in the mass range 0.5 - 5.0 GeV/c^2.

hep-ex↗

arXiv2016

Optical Characterization of the BICEP3 CMB Polarimeter at the South Pole

BICEP3 is a small-aperture refracting cosmic microwave background (CMB) telescope designed to make sensitive polarization maps in pursuit of a potential B-mode signal from inflationary gravitational waves. It is the latest in the BICEP/Keck Array series of CMB experiments at the South Pole, which has provided the most stringent constraints on inflation to date. For the 2016 observing season, BICEP3 was outfitted with a full suite of 2400 optically coupled detectors operating at 95 GHz. In these proceedings we report on the far field beam performance using calibration data taken during the 2015-2016 summer deployment season in situ with a thermal chopped source. We generate high-fidelity per-detector beam maps, show the array-averaged beam profile, and characterize the differential beam response between co-located, orthogonally polarized detectors which contributes to the leading instrumental systematic in pair differencing experiments. We find that the levels of differential pointing, beamwidth, and ellipticity are similar to or lower than those measured for BICEP2 and Keck Array. The magnitude and distribution of BICEP3's differential beam mismatch - and the level to which temperature-to-polarization leakage may be marginalized over or subtracted in analysis - will inform the design of next-generation CMB experiments with many thousands of detectors.

astro-ph.IM↗

arXiv2016

Measurement of the Lepton Forward-Backward Asymmetry in $B \rightarrow X_s \ell^+ \ell^-$ Decays with a Sum of Exclusive Modes

We report the first measurement of the lepton forward-backward asymmetry ${\cal A}_{\rm FB}$ as a function of the squared four-momentum of the dilepton system, $q^2$, for the electroweak penguin process $B \rightarrow X_s \ell^+ \ell^-$ with a sum of exclusive final states, where $\ell$ is an electron or a muon and $X_s$ is a hadronic recoil system with an $s$ quark. The results are based on a data sample containing $772\times10^6$ $B\bar{B}$ pairs recorded at the $Υ(4S)$ resonance with the Belle detector at the KEKB $e^+ e^-$ collider. ${\cal A}_{\rm FB}$ for the inclusive $B \rightarrow X_s \ell^+ \ell^-$ is extrapolated from the sum of 10 exclusive $X_s$ states whose invariant mass is less than 2 GeV/$c^2$. For $q^2 > 10.2$ GeV$^2$/$c^2$, ${\cal A}_{\rm FB} < 0$ is excluded at the 2.3$σ$ level, where $σ$ is the standard deviation. For $q^2 < 4.3$ GeV$^2$/$c^2$, the result is within 1.8$σ$ of the Standard Model theoretical expectation.

hep-ex↗

arXiv2015

BICEP2/Keck Array IV: Optical Characterization and Performance of the BICEP2 and Keck Array Experiments

BICEP2 and the Keck Array are polarization-sensitive microwave telescopes that observe the cosmic microwave background (CMB) from the South Pole at degree angular scales in search of a signature of inflation imprinted as B-mode polarization in the CMB. BICEP2 was deployed in late 2009, observed for three years until the end of 2012 at 150 GHz with 512 antenna-coupled transition edge sensor bolometers, and has reported a detection of B-mode polarization on degree angular scales. The Keck Array was first deployed in late 2010 and will observe through 2016 with five receivers at several frequencies (95, 150, and 220 GHz). BICEP2 and the Keck Array share a common optical design and employ the field-proven BICEP1 strategy of using small-aperture, cold, on-axis refractive optics, providing excellent control of systematics while maintaining a large field of view. This design allows for full characterization of far-field optical performance using microwave sources on the ground. Here we describe the optical design of both instruments and report a full characterization of the optical performance and beams of BICEP2 and the Keck Array at 150 GHz.

astro-ph.IM↗

arXiv2015

Localized High Frequency Electrodynamic Behavior of Optimally-doped Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ Single Crystal films

Localized high frequency (several GHz) electrodynamic properties of a high quality epitaxial, single-crystal Iron-Pnicitde Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ thin film near optimal doping (x=0.08) are measured under a localized and strong RF magnetic field, created by a near-field microwave microscope. Two reflection electrodynamic measurements, including linear and the third harmonic responses, are performed to understand the electromagnetic properties of Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$. Our measurement results show that Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ has have a wide superconducting transition width and may have a multi-gap nature. In addition, based on the $1/T^2$ dependence of the third harmonics signal at lower temperature, Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ shows the possibility of nodal behavior.

cond-mat.supr-con↗

arXiv2014

Measurement of the inclusive semileptonic branching fraction $\mathcal{B}(B_s^0 \rightarrow X^- \ell^+ ν_\ell)$ at Belle

We report a measurement of the inclusive semileptonic $B_s^0$ branching fraction in a $121~fb^{-1}$ data sample collected near the $Υ(5S)$ resonance with the Belle detector at the KEKB asymmetric energy $e^+ e^-$ collider. Events containing $B_s^0$ pairs are selected by reconstructing a tag side $D_s^+$ and identifying a signal side lepton $\ell^+$ ($\ell = e, μ$) that is required to have the same-sign charge to ensure that both originate from different $B_s^0$ mesons. The $B_s^0 \rightarrow X^- \ell^+ ν_\ell$ branching fraction is extracted from the ratio of the measured yields of $D_s^+$ mesons and $D_s^+ \ell^+$ pairs and the known production and branching fractions. The inclusive semileptonic branching fraction is measured to be $[9.6 \pm 0.4\text{(stat)} \pm 0.7\text{(syst)}]~\%$.

hep-ex↗

arXiv2014

Measurement of the Branching Fraction Br(Lambda_c+ -> p K- pi+)

We present the first model-independent measurement of the absolute branching fraction of the Lambda_c+ -> p K- pi+ decay using a data sample of 978 fb^-1 collected with the Belle detector at the KEKB asymmetric-energy e+e- collider. The number of Lambda_c+ baryons is determined by reconstructing the recoiling D(*)- anti-p pi+ system in events of the type e+e- -> D(*)- anti-p pi+ Lambda_c+. The branching fraction is measured to be Br(Lambda_c+ -> p K- pi+) = (6.84 +- 0.24^{+0.21}_{-0.27})%, where the first and second uncertainties are statistical and systematic, respectively.

hep-ex↗

arXiv2014

Search for CP violation in D^0 -> pi^0 pi^0 decays

We search for CP violation in neutral charm meson decays using a data sample with an integrated luminosity of 966 fb^-1 collected with the Belle detector at the KEKB e+e- asymmetric-energy collider. The asymmetry obtained in the rate of D^0 and D^0-bar decays to the pi^0 pi^0 final state, [-0.03+-0.64(stat)+-0.10(syst)]%, is consistent with no CP violation. This constitutes an order of magnitude improvement over the existing result. We also present an updated measurement of the CP asymmetry in the D^0 -> K_S pi^0 decay: A_{CP}(D^0 -> K_S pi^0) = [-0.21+-0.16(stat)+-0.07(syst)]%.

hep-ex↗

arXiv2014

Measurement of Branching Fractions and CP Asymmetries in B -> wK Decays and First Evidence of CP Violation in B0 -> wKS0

We present a measurement of the branching fractions and charge-parity-(CP-) violating parameters in B -> omega K decays. The results are obtained from the final data sample containing 772 x 10^(6) BBbar pairs collected at the Y(4S) resonance with the Belle detector at the KEKB asymmetric-energy e+e- collider. We obtain the branching fractions B(B0 -> omega K0) = (4.5 +/- 0.4 (stat) +/- 0.3 (syst)) x 10^(-6), B(B- -> omega K-) = (6.8 +/- 0.4 (stat) +/- 0.4 (syst)) x 10^(-6), which are in agreement with their respective current world averages. For the CP violating parameters, we obtain A(B0 -> omega KS) = -0.36 +/- 0.19 (stat) +/- 0.05 (syst), S(B0 -> omega KS) = +0.91 +/- 0.32 (stat) +/- 0.05 (syst), A(B- -> omega K-) = -0.03 +/- 0.04 (stat) +/- 0.01 syst), where A and S represent the direct and mixing-induced CP asymmetry, respectively. We find no evidence of CP violation in the decay channel B-->omega K-; however, we obtain the first evidence of CP violation in the B0 -> omega KS decay channel at the level of 3.1 standard deviations.

hep-ex↗

arXiv2014

Measurements of the masses and widths of the $Σ_{c}(2455)^{0/++}$ and $Σ_{c}(2520)^{0/++}$ baryons

We present measurements of the masses and decay widths of the baryonic states $Σ_{c}(2455)^{0/++}$ and $Σ_{c}(2520)^{0/++}$ using a data sample corresponding to an integrated luminosity of 711 fb$^{-1}$ collected with the Belle detector at the KEKB $e^{+}e^{-}$ asymmetric-energy collider operating at the $Υ(4S)$ resonance. We report the mass differences with respect to the $Λ_{c}^{+}$ baryon $M(Σ_{c}(2455)^{0})-M(Λ_{c}^{+}) = 167.29\pm0.01\pm0.02$ MeV/$c^{2}$, $M(Σ_{c}(2455)^{++})-M(Λ_{c}^{+}) = 167.51\pm0.01\pm0.02$ MeV/$c^{2}$, $M(Σ_{c}(2520)^{0})-M(Λ_{c}^{+}) = 231.98\pm0.11\pm0.04$ MeV/$c^{2}$, $M(Σ_{c}(2520)^{++})-M(Λ_{c}^{+}) = 231.99\pm0.10\pm0.02$ MeV/$c^{2}$, and the decay widths $Γ(Σ_{c}(2455)^{0}) = 1.76\pm0.04^{+0.09}_{-0.21}$ MeV/$c^{2}$, $Γ(Σ_{c}(2455)^{++}) = 1.84\pm0.04^{+0.07}_{-0.20}$ MeV/$c^{2}$, $Γ(Σ_{c}(2520)^{0}) = 15.41\pm0.41^{+0.20}_{-0.32}$ MeV/$c^{2}$, $Γ(Σ_{c}(2520)^{++}) = 14.77\pm0.25^{+0.18}_{-0.30}$ MeV/$c^{2}$, where the first uncertainties are statistical and the second are systematic. The isospin mass splittings are measured to be $M(Σ_{c}(2455)^{++})-M(Σ_{c}(2455)^{0})=0.22\pm0.01\pm0.01$ MeV/$c^{2}$ and $M(Σ_{c}(2520)^{++})-M(Σ_{c}(2520)^{0})=0.01\pm0.15\pm0.03$ MeV/$c^{2}$. These results are the most precise to date.

hep-ex↗

arXiv2014

Electrodynamics of superconducting pnictide superlattices

It has been recently reported (S. Lee et al., Nature Materials 12, 392, 2013) that superlattices where layers of the 8% Co-doped BaFe2As2 superconducting pnictide are intercalated with non superconducting ultrathin layers of either SrTiO3 or of oxygen-rich BaFe2As2, can be used to control flux pinning, thereby increasing critical fields and currents, without significantly affecting the critical temperature of the pristine superconducting material. However, little is known about the electron properties of these systems. Here we investigate the electrodynamics of these superconducting pnictide superlattices in the normal and superconducting state by using infrared reflectivity, from THz to visible range. We find that multi-gap structure of these superlattices is preserved, whereas some significant changes are observed in their electronic structure with respect to those of the original pnictide. Our results suggest that possible attempts to further increase the flux pinning may lead to a breakdown of the pnictide superconducting properties.

cond-mat.supr-con↗