arXiv subjects

J. H. Yang

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

At least 19 recordsLinked to original sources

arXiv2024

A multi-cubic-kilometre neutrino telescope in the western Pacific Ocean

Next-generation neutrino telescopes with significantly improved sensitivity are required to pinpoint the sources of the diffuse astrophysical neutrino flux detected by IceCube and uncover the century-old puzzle of cosmic ray origins. A detector near the equator will provide a unique viewpoint of the neutrino sky, complementing IceCube and other neutrino telescopes in the Northern Hemisphere. Here we present results from an expedition to the north-eastern region of the South China Sea, in the western Pacific Ocean. A favorable neutrino telescope site was found on an abyssal plain at a depth of $\sim$ 3.5km. At depths below 3km, the sea current speed, water absorption and scattering lengths for Cherenkov light, were measured to be $v_{\mathrm{c}}<$10cm/s, $λ_{\mathrm{abs} }\simeq$ 27m and $λ_{\mathrm{sca} }\simeq$ 63m, respectively. Accounting for these measurements, we present the design and expected performance of a next-generation neutrino telescope, TRopIcal DEep-sea Neutrino Telescope (TRIDENT). With its advanced photon-detection technology and large dimensions, TRIDENT expects to observe the IceCube steady source candidate NGC 1068 with 5$σ$ significance within 1 year of operation. This level of sensitivity will open a new arena for diagnosing the origin of cosmic rays and probing fundamental physics over astronomical baselines.

astro-ph.HE↗

arXiv2021

A study of Intrinsic $γ$-ray Emission for {\it Fermi}/LAT-detected BL Lacs

BL Lacs are one subclass of blazars with highly energetic $γ$-ray emission, which is strongly boosted by a relativistic beaming effect. The latest catalogue of the 10 years of {\it Fermi}/LAT data Abdollahi et al. (2020) and the $γ$-ray Doppler factors in Pei et al. (2020) provide us with a large BL Lac sample to study their jet emission morphologies and intrinsic properties. In this paper, we collected a sample of 294 {\it Fermi} BL Lacs and probed the correlations between the $γ$-ray emissions and luminosity distances. Our analyses give following conclusions: (1) the observed $γ$-ray emissions are really boosted by the $γ$-ray Doppler factor, and the intrinsic $γ$-ray emissions are closely correlated with luminosity distances. (2) the morphology of jet emissions for HBLs may be continuous, while that for IBLs may be the case of a moving sphere in the $γ$-ray bands.

astro-ph.HE↗

arXiv2018

First-principles Study on the Magnetic Interactions in Honeycomb Na2IrO3

Honeycomb iridate Na2IrO3, a Jeff=1/2 magnet, is a potential platform for realizing the quantum spin liquid. Many experiments have shown that its magnetic ground state has a zigzag antiferromagnetic (AFM) order. However, there is still a lack of consensus on the theoretical model explaining such order, since its second nearest neighbor (NN) and long-range third NN magnetic interactions are highly unclear. By properly taking into account the orbital moments, achieved through constraining their directions in the first-principles calculations, we obtain that the relative angle between orbital and spin moments is fairly small and in the order of several degrees, which thus validates the Jeff=1/2 state in Na2IrO3. Surprisingly, we find that the long-range third NN Heisenberg interactions are sizable whereas the second NN magnetic interactions are negligible. Using maximally localized Wannier functions, we show that the sizable long-range third NN Heisenberg interaction results from the extended nature of the Jeff=1/2 state. Based on our study, we propose a minimal J1-K1-Γ_1-J3 model in which the magnetic excitations have an intensity peak at 5.6 meV, consistent with the inelastic neutron scattering experiment [Phys. Rev. Lett. 108, 127204 (2012)]. The present work demonstrates again that constraining orbital moments in the first-principles calculations is powerful to investigate the intriguing magnetism in the Jeff=1/2 magnets, and paves the way toward gaining a deep insight into the novel magnetism discovered in the honeycomb Jeff=1/2 magnets.

cond-mat.str-el↗

arXiv2018

Spectral Measurement of the Electron Antineutrino Oscillation Amplitude and Frequency using 500 Live Days of RENO Data

The Reactor Experiment for Neutrino Oscillation (RENO) has been taking electron antineutrino ($\overlineν_{e}$) data from the reactors in Yonggwang, Korea, using two identical detectors since August 2011. Using roughly 500 live days of data through January 2013 we observe 290,775 (31,514) reactor $\overlineν_{e}$ candidate events with 2.8 (4.9)% background in the near (far) detector. The observed visible positron spectra from the reactor $\overlineν_{e}$ events in both detectors show discrepancy around 5 MeV with regard to the prediction from the current reactor $\overlineν_{e}$ model. Based on a far-to-near ratio measurement using the spectral and rate information we have obtained $\sin^2 2 θ_{13} = 0.082 \pm 0.009({\rm stat.}) \pm 0.006({\rm syst.})$ and $|Δm_{ee}^2| =[2.62_{-0.23}^{+0.21}({\rm stat.})_{-0.13}^{+0.12}({\rm syst.})]\times 10^{-3}$eV$^2$.

hep-ex↗

arXiv2017

The Variability and Period Analysis for the BL Lac AO 0235+164

Variability is one of the extreme observational properties of BL Lacertae objects. AO 0235+164 is a well studied BL Lac through the whole electro-magnetic wavebands. In the present work, we show its optical R band photometric observations carried out during the period of Nov, 2006 to Dec. 2012 using the Ap6E CCD camera attached to the primary focus of the $\rm 70-cm$ meniscus telescope at Abastumani Observatory, Georgia. It shows a large variation of $ΔR$ = 4.88 mag (14.19 - 19.07 mag) and a short time scale of $ΔT_v$ = 73.5 min during our monitoring period. During the period of Dec. 2006 to Nov. 2009, we made radio observations of the source using the 25-m radio telescope at Xinjiang Astronomical Observatory. When a discrete correlation function (DCF) is adopted to the optical and radio observations, we found that the optical variation leads the radio variation by 23.2$\pm$12.9 days.

astro-ph.HE↗

arXiv2017

Intrinsic Correlations for Flaring Blazars Detected by Fermi

Blazars are an extreme subclass of active galactic nuclei. Their rapid variability, luminous brightness, superluminal motion, and high and variable polarization are probably due to a beaming effect. However, this beaming factor (or Doppler factor) is very difficult to measure. Currently, a good way to estimate it is to use the time scale of their radio flares. In this $Letter$, we use multiwavelength data and Doppler factors reported in the literatures for a sample of 86 flaring blazars detected by Fermi to compute their intrinsic multiwavelength data and intrinsic spectral energy distributions, and investigate the correlations among observed and intrinsic data. Quite interestingly, intrinsic data show a positive correlation between luminosity and peak frequency, in contrast with the behavior of observed data, and a tighter correlation between $γ$-ray luminosity and the lower energy ones. For flaring blazars detected by Fermi, we conclude that (1) Observed emissions are strongly beamed; (2) The anti-correlation between luminosity and peak frequency from the observed data is an apparent result, the correlation between intrinsic data being positive; and (3) Intrinsic $γ$-ray luminosity is strongly correlated with other intrinsic luminosities.

astro-ph.HE↗

arXiv2016

The Spectral Energy Distributions of Fermi Blazars

(Abridged) In this paper, multi-wavelength data are compiled for a sample of 1425 Fermi blazars to calculate their spectral energy distributions (SEDs). A parabolic function, $\log(νF_ν) = P_1(\logν- P_2)^2 + P_3,$ is used for SED fitting. Synchrotron peak frequency ($\log ν_p$), spectral curvature ($P_1$), peak flux ($ν_{\rm p}F_{\rm ν_p}$), and integrated flux ($νF_ν$) are successfully obtained for 1392 blazars (461 flat spectrum radio quasars-FSRQs, 620 BL Lacs-BLs and 311 blazars of uncertain type-BCUs, 999 sources have known redshifts). Monochromatic luminosity at radio 1.4 GHz, optical R band, X-ray at 1 keV and $γ$-ray at 1 GeV, peak luminosity, integrated luminosity and effective spectral indexes of radio to optical ($α_{\rm RO}$), and optical to X-ray ($α_{\rm OX}$) are calculated. The "Bayesian classification" is employed to log$ν_{\rm p}$ in the rest frame for 999 blazars with available redshift and the results show that 3 components are enough to fit the $\logν_{\rm p}$ distribution, there is no ultra high peaked subclass. Based on the 3 components, the subclasses of blazars using the acronyms of Abdo et al. (2010a) are classified, and some mutual correlations are also studied. Conclusions are finally drawn as follows: (1) SEDs are successfully obtained for 1392 blazars. The fitted peak frequencies are compared with common sources from samples available (Sambruna et al. 1996, Nieppola et al. 2006, 2008, Abdo et al. 2010a). (2) Blazars are classified as low synchrotron peak sources (LSPs) if $\logν_{\rm p}$(Hz) $\leq 14.0$, intermediate synchrotron peak sources (ISPs) if $14.0 < \logν_{\rm p}$(Hz) $\leq 15.3$, and high synchrotron peak sources (HSPs) if $\logν_{\rm p}$(Hz) $> 15.3$. (3) $γ$-ray emissions are strongly correlated with radio emissions. (...)

astro-ph.HE↗

arXiv2015

Upward curvature of the upper critical field and the V-shaped pressure dependence of $T_c$ in the noncentrosymmetric superconductor PbTaSe$_2$

The temperature evolution of the upper critical field $H_{c2}(T)$ in the noncentrosymmetric superconductor PbTaSe$_2$ was determined via resistivity measurements down to 0.5 K. A pronounced positive curvature in the $H_{c2}$-$T$ phase diagram was observed in the whole temperature range below $T_c$. The Seebeck coefficient $S(T)$ in the temperature range 5K$\leq$$T$$\leq$350K was found to be negative in sign, modest in magnitude and non-linear in temperature. In addition, the superconducting transition temperature $T_c$ under hydrostatic pressure shows a marked non-monotonic variation, decreasing initially with the applied pressure up to $P_c$$\sim$5-10 kbar but then rising with further pressurization. The underlying physical mechanisms of all these findings have been discussed.

cond-mat.supr-con↗

arXiv2015

Nodeless superconductivity in quasi-one-dimensional Nb2PdS5: A muSR study

Muon spin relaxation and rotation (muSR) measurements have been performed to study the superconducting and magnetic properties of Nb2PdS5. Zero-field muSR data show that no sizeable spontaneous magnetization arises with the onset of superconductivity in Nb2PdS5 which indicates that the time reversal symmetry is probably preserved in the superconducting state of this system. A strong diamagnetic shift is observed in the transverse-field muSR data practically ruling out a dominant triplet-pairing superconducting state in Nb2PdS5. The temperature dependence of magnetic penetration depth evidences the existence of a single s-wave energy gap with a gap value of 1.07(4) meV at zero temperature. The gap to Tc ratio, 2.02(9), indicates that Nb2PdS5 should be considered as a moderately strong-coupling superconductor. The magnetic penetration depth at zero temperature is 785(20) nm, indicating a very low superfluid density consistent with the quasi-one-dimensional nature of this system.

cond-mat.supr-con↗

arXiv2014

Controllable spin-orbit coupling and its influence on the upper critical field in the chemically doped quasi-one-dimensional Nb$_2$PdS$_5$ superconductor

By systematic chemical substitution of Pt and Ni in the newly-discovered superconductor Nb$_2$PdS$_5$ ($T_c\sim$6 K), we study the evolution of its superconducting properties with doping, focussing on the behavior of the upper critical field $H_{c2}$. In contrast to the previous results of Se doping on S sites, superconductivity is found to be rather robust against the Pt and Ni dopants on the one-dimensional Pd chains. Most strikingly, the reduced $H_{c2}$, i.e., the ratio of $H_{c2}/T_c$, is seen to be significantly enhanced by the heavier Pt doping but suppressed in the Ni-doped counterparts, distinct from the nearly constant value in the Se doped samples. Our findings therefore suggest that the upper critical field of this system can be modified in a tunable fashion by chemical doping on the Pd chains with elements of varying mass numbers. The spin-orbit coupling on the Pd sites, by inference, should play an important role in the observed superconductivity and on the large upper critical field beyond the Pauli pair-breaking field.

cond-mat.supr-con↗

arXiv2013

Effect of Selenium doping on the superconductivity of Nb$_2$Pd(S$_{1-x}$Se$_x$)$_5$

We study the isovalent substitution effect by partially introducing Se on S site in the newly discovered superconductor Nb$_2$PdS$_5$ ($T_c\sim$6 K) whose upper critical field is found to be far above its Pauli paramagnetic limit. In this Nb$_2$Pd(S$_{1-x}$Se$_x$)$_5$ (0$\leq$$x$$\leq$0.8) system, superconductivity is systematically suppressed by the Se concentration and ultimately disappears when $x\geq$ 0.5, after which a semiconducting-like ground state emerges. In spite of the considerably reduced $T_c$ with Se doping, the ratio of the upper critical field $H_{c2}$ to $T_c$, remains unaffected. Moreover, the size of the heat capacity jump at $T_c$ is smaller than that expected for a BCS superconductor, implying that a strong-coupling theory cannot be the origin of this large upper critical field. In addition, the low-lying quasiparticle excitations are consistent with a nodeless gap opening over the Fermi surface. These results combined impose severe constraints on any theory of exotic superconductivity in this system.

cond-mat.supr-con↗

arXiv2013

Evidence for two energy gaps and Fermi liquid behavior in SrPt$_2$As$_2$ superconductor

We report a detailed calorimetric study on single crystals of the 5$d$-transition metal pnictide SrPt2As2 with a superconducting critical temperature $T_c$ $\sim$5K. The peculiar field dependence of the electronic specific heat coefficient $γ$ can be decomposed into two linear components. Moreover, the temperature evolution of the electronic specific heat below $T_c$ is best described by a two-gap model. These findings suggest that two energy gaps are associated with the superconductivity. In parallel, we show that the spin-lattice relaxation time $T_1$, through nuclear magnetic resonance measurement, obeys the so-called Korringa relation well. This, along with the $T^2$ dependence of resistivity at low temperatures, points to a Fermi liquid ground state in this material.

cond-mat.supr-con↗

arXiv2012

Beaming Effect in Fermi Blazars

The γ-ray loud blazars (flat spectrum radio quasars--FSRQs and BL Lacertae objects-BLs) are very bright in the γ-ray bands, which is perhaps associated with a beaming effect. Therefore, one can expect that the γ-ray luminosity is correlated with the beaming factor. In this paper, we investigated the relation between the radio Doppler factors and the gamma-ray luminosities. Our analysis suggests that the γ-ray luminosity be strongly correlated with the factor of δ_R for the whole sample, FSRQs, and BLs. When the effect of a common redshift is excluded, the correlation still exists for the FSRQs sub-sample suggesting that the γ-rays are strongly beamed. However, the partial correlation analysis does not show a correlation for the small BL Lac sample.

astro-ph.HE↗

arXiv2012

Strong Dzyaloshinskii-Moriya Interaction and Origin of Ferroelectricity in Cu2OSeO3

By performing density functional calculations, we investigate the origin of the skyrmion state and ferroelectricity in Cu2OSeO3. We find that the Dzyaloshinskii-Moriya interactions between the two different kinds of Cu ions are extremely strong and induce the helical ground state and the skyrmion state in the absence and presence of magnetic field, respectively. On the basis of the general model for the spin-order induced polarization, we propose that the ferroelectric polarization of Cu2OSeO3 in the collinear ferrimagnetic state arises from an unusual mechanism, i.e., the single-spin-site contribution due to the spin-orbit coupling.

cond-mat.str-el↗

arXiv2012

Strong single-ion anisotropy and anisotropic interactions of magnetic adatoms induced by topological surface states

The nature of the magnetism brought about by Fe adatoms on the surface of the topological insulator Bi2Se3 was examined in terms of density functional calculations. The Fe adatoms exhibit strong easy-axis magnetic anisotropy in the dilute adsorption limit due to the topological surface states (TSS). The spin exchange J between the Fe adatoms follows a Ruderman-Kittel-Kasuya-Yosida (RKKY) behavior with substantial anisotropy, and the Dzyaloshinskii-Moriya (DM) interaction between them is quite strong with |D/J|~0.3 under the mediation by the TSS, and can be further raised to ~0.6 by an external electric field. The apparent single-ion anisotropy of a Fe adatom is indispensable in determining the spin orientation.

cond-mat.mtrl-sci↗

arXiv2011

Sequential vibrational resonance in multistable systems

The phenomenon of sequential vibrational resonance existed in a multistable system that is excited by both high- and low-frequency signals is reported. By the method of direct separation of motions, the theoretical investigation on vibrational resonance is conducted in both cases of underdamped and overdamped, and the analytical predictions are in good agreement with the numerical simulations. In view of the theoretical results, the zero-order Bessel function related to the high-frequency signal is included in the renormalized resonant frequency, and which leads to the appearance of the sequential vibrational resonance. In the case of underdamped system with small damping coefficient, the resonance occurs in a series of discrete parameter regions. The sequential vibrational resonance is different from the traditional multiple vibrational resonance, because its appearance is much more regular. The results in this work may be helpful in the field of signal processing electronics, especially for dealing with the very-low-frequency signal.

nlin.CD↗

arXiv2009

Charge carrier localization induced by excess Fe in the Fe1+y(Te,Se) superconductor system

We have investigated the effect of Fe nonstoichiometry on properties of the Fe1+y(Te, Se) superconductor system by means of resistivity, Hall coefficient, magnetic susceptibility, and specific heat measurements. We find that the excess Fe at interstitial sites of the (Te, Se) layers not only suppresses superconductivity, but also results in a weakly localized electronic state. We argue that these effects originate from the magnetic coupling between the excess Fe and the adjacent Fe square planar sheets, which favors a short-range magnetic order.

cond-mat.supr-con↗

arXiv2009

Weak anisotropy of the superconducting upper critical field in Fe1.11Te0.6Se0.4 single crystals

We have determined the resistive upper critical field Hc2 for single crystals of the superconductor Fe1.11Te0.6Se0.4 using pulsed magnetic fields of up to 60T. A rather high zero-temperature upper critical field of mu0Hc2(0) approx 47T is obtained, in spite of the relatively low superconducting transition temperature (Tc approx 14K). Moreover, Hc2 follows an unusual temperature dependence, becoming almost independent of the magnetic field orientation as the temperature T=0. We suggest that the isotropic superconductivity in Fe1.11Te0.6Se0.4 is a consequence of its three-dimensional Fermi-surface topology. An analogous result was obtained for (Ba,K)Fe2As2, indicating that all layered iron-based superconductors exhibit generic behavior that is significantly different from that of the high-Tc cuprates.

cond-mat.supr-con↗