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R. Wang

Publications and source records attributed to R. Wang.

At least 91 records · Page 5Linked to original sources
arXiv2019

Symmetry-Protected Ideal Type-II Weyl Phonons in CdTe

Nontrivial low-energy excitations of crystalline solids have insightfully strengthened understanding of elementary particles in quantum field theory. Usually, topological quasiparticles are mainly focused on fermions in topological semimetals. In this work, we alternatively show by first-principles calculations and symmetry analysis that ideal type-II Weyl phonons are present in zinc-blende cadmium telluride (CdTe), a well-known II-VI semiconductor. Importantly, these type-II Weyl phonons originate from the inversion between the longitudinal acoustic and transverse optical branches. Symmetry guarantees the type-II Weyl points to lie along the high-symmetry lines at the boundaries of Brillouin zone even with breaking the inversion symmetry, exhibiting the robustness of protected phonon features. The nontrivial phonon surface states and surface arcs projected on the semi-finite (001) and (111) surfaces are investigated. The phonon surface arcs connecting the Weyl points with opposite chirality, guaranteed to be very long, are clearly visible. This work not only offers a promising candidate for studying type-II Weyl phonons, but also provides a route to realize symmetry-protected nontrivial phonons and related applications in realistic materials.

cond-mat.mtrl-sci↗
arXiv2019

Spectral Energy Distributions of Companion Galaxies to z$\sim$6 Quasars

Massive, quiescent galaxies are already observed at redshift $z\sim4$, i.e. $\sim$1.5 Gyr after the Big Bang. Current models predict them to be formed via massive, gas-rich mergers at $z>6$. Recent ALMA observations of the cool gas and dust in $z\gtrsim$6 quasars have discovered [CII]- and far infrared-bright galaxies adjacent to several quasars. In this work, we present sensitive imaging and spectroscopic follow-up observations, with HST/WFC3, Spitzer/IRAC, VLT/MUSE, Magellan/FIRE and LBT/LUCI-MODS, of ALMA-detected, dust-rich companion galaxies of four quasars at $z\gtrsim 6$, specifically acquired to probe their stellar content and unobscured star formation rate. Three companion galaxies do not show significant emission in the observed optical/IR wavelength range. The photometric limits suggest that these galaxies are highly dust-enshrouded, with unobscured star formation rates SFR$_{\rm UV}<$few M$_{\odot}$ yr$^{-1}$, and a stellar content of $M_{*}<$10$^{10}$ M$_{\odot}$ yr$^{-1}$. However, the companion to PJ167-13 shows bright rest-frame UV emission (F140W AB = 25.48). Its SED resembles that of a star-forming galaxy with a total SFR$\sim$50 M$_{\odot}$ yr$^{-1}$ and $M_{*}\sim 9 \times 10^{9}$ M$_{\odot}$. All the companion sources are consistent with residing on the galaxy main sequence at $z\sim$6. Additional, deeper data from future facilities, such as JWST, are needed in order to characterize these gas-rich sources in the first Gyr of cosmic history.

astro-ph.GA↗
arXiv2019

Robustness of the pumping charge to dynamic disorder

We investigate the effect of disorder on a gapped crystalline system by introducing a class of local quantities for an energy band, which is referred as to band correlation function (BCF) and is the sum of correlation functions for all eigenstates of the band. We show that the BCFs are robust in the presence of disorder if the band gap is not collapse. The eigenstate set of an energy band can be almost completely mapped onto the perturbated eigenstate set, referred as to quasi-closed mapping, when it is sufficiently isolated from other bands. Some features relate to translational symmetry may emerge in a randomly perturbed system. We demonstrate this by simulating numerically the pumping process for a 1D Rice-Mele (RM) model with disorders on the hopping strength and on-site potential. It is shown that the quantized pumping charge is robust against with the dynamic disorder. This result indicates the possibility of measuring the topological invariant in experimental system with imperfection.

cond-mat.mes-hall↗
arXiv2019

Measurement of neutrino-oxygen neutral-current quasi-elastic cross section using atmospheric neutrinos at Super-Kamiokande

Neutral current (NC) interactions of atmospheric neutrinos on oxygen form one of the major backgrounds in the search for supernova relic neutrinos with water-based Cherenkov detectors. The NC channel is dominated by neutrino quasi-elastic (NCQE) scattering off nucleons inside $^{16}$O nuclei. In this paper we report the first measurement of NCQE cross section using atmospheric neutrinos at Super-Kamiokande (SK). The measurement used 2,778 live days of SK-IV data with a fiducial volume of 22.5 kiloton water. Within the visible energy window of 7.5-29.5 MeV, we observed $117$ events compared to the expected $71.9$ NCQE signal and $53.1$ background events. Weighted by the atmospheric neutrino spectrum from 160 MeV to 10 GeV, the flux averaged NCQE cross section is measured to be $(1.01\pm0.17(\text{stat.})^{+0.78}_{-0.30}(\text{sys.}))\times10^{-38}$ cm$^2$.

hep-ex↗
arXiv2019

Atmospheric Neutrino Oscillation Analysis With Improved Event Reconstruction in Super-Kamiokande IV

A new event reconstruction algorithm based on a maximum likelihood method has been developed for Super-Kamiokande. Its improved kinematic and particle identification capabilities enable the analysis of atmospheric neutrino data in a detector volume 32% larger than previous analyses and increases sensitivity to the neutrino mass hierarchy. Analysis of a 253.9 kton-year exposure of the Super-Kamiokande IV atmospheric neutrino data has yielded a weak preference for the normal hierarchy, disfavoring the inverted hierarchy at 74% assuming oscillations at the best fit of the analysis.

hep-ex↗
arXiv2018

Light Localization and Cooperative Coupling Effects in Aperiodic Vogel Spirals

By using the dyadic Green's matrix spectral method, we demonstrate that aperiodic deterministic Vogel spirals made of electric dipoles support light localization in three dimensions, an effect that does not occur in traditional uniform random media. We discover a light localization transition in Vogel spiral arrays embedded in three-dimensional space by evaluating the Thouless conductance, the level spacing statistics, and by performing a finite-size scaling. This light localization transition is different from the Anderson transition because Vogel spirals are deterministic structures. Moreover, this transition occurs when the vector character of light is fully taken into account, in contrast to what is expected for traditional uniform random media of point-like scatterers. We show that light localization in Vogel arrays is a collective phenomenon that involves the contribution of multiple length scales. Vogel spirals are suitable photonic platforms to localize light thanks to their distinctive structural correlation properties that enable collective electromagnetic excitations with strong light-matter coupling. Our results unveil the importance of aperiodic correlations for the engineering of photonic media with strongly enhanced light-matter coupling compared to the traditional periodic and homogeneous random media.

cond-mat.dis-nn↗
arXiv2018

Dinucleon and Nucleon Decay to Two-Body Final States with no Hadrons in Super-Kamiokande

Using 0.37 megaton$\cdot$years of exposure from the Super-Kamiokande detector, we search for 10 dinucleon and nucleon decay modes that have a two-body final state with no hadrons. These baryon and lepton number violating modes have the potential to probe theories of unification and baryogenesis. For five of these modes the searches are novel, and for the other five modes we improve the limits by more than one order of magnitude. No significant evidence for dinucleon or nucleon decay is observed and we set lower limits on the partial lifetime of oxygen nuclei and on the nucleon partial lifetime that are above $4\times 10^{33}$ years for oxygen via the dinucleon decay modes and up to about $4 \times 10^{34}$ years for nucleons via the single nucleon decay modes.

hep-ex↗
arXiv2018

Ideal Intersecting Nodal Ring Phonons in a Body-Centered Cubic C$_{8}$

Carbon, a basic versatile element in our universe, exhibits rich varieties of allotropic phases, most of which possess promising nontrivial topological fermions. In this work, we identify a distinct topological phonon phase in a realistic carbon allotrope with a body-centered cubic structure, termed bcc-C$_{8}$. We show by symmetry arguments and effective model analysis that there are three intersecting phonon nodal rings perpendicular to each other in different planes. The intersecting phonon nodal rings are protected by time-reversal and inversion symmetries, which quantize the corresponding Berry phase into integer multiples of $π$. Unlike the electron systems, the phonon nodal rings in bcc-C$_{8}$ are guaranteed to remain gapless due to the lack of spin-orbital coupling. The nearly flat drumhead surface states projected on semi-infinite (001) and (110) surfaces of bcc-C$_{8}$ are clearly visible. Our findings not only discover promising nodal ring phonons in a carbon allotrope, but also provide emergent avenues for exploring topological phonons beyond fermionic electrons in carbon-allotropic structures with attractive features.

cond-mat.mtrl-sci↗
arXiv2018

Site selective spin and orbital excitations in Fe3O4

$Fe_3O_4$ is a mixed-valence strongly correlated transition metal oxide which displays the intriguing metal to insulator Verwey transition. Here we investigate the electronic and magnetic structure of $Fe_3O_4$ by a unique combination of high-resolution Fe 2p3d resonant inelastic scattering magnetic circular (RIXS-MCD) and magnetic linear (RIXS-MLD) dichroism. We show that by coupling the site selectivity of RIXS with the magnetic selectivity imposed by the incident polarization handedness, we can unambiguously identify spin-flip excitations and quantify the exchange interaction of the different sublattices. Furthermore, our RIXS-MLD measurements show spin-orbital excitations that exhibit strong polarization and magnetic field dependence. Guided by theoretical simulations, we reveal that the angular dependence arises from a strong interplay between trigonal crystal-field, magnetic exchange and spin-orbit interaction at the nominal $Fe^{2+}$ sites. Our results highlight the capabilities of RIXS magnetic dichroism studies to investigate the ground state of complex systems where in-equivalent sites and bonds are simultaneously present.

cond-mat.str-el↗
arXiv2018

An Empirical Template Library for FGK and Late--type A Stars Using LAMOST Observed Spectra

We present an empirical stellar spectra library created using spectra from the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) DR5. This library represents a uniform data set ranging from 3750 through 8500K in effective temperature (T_eff), from -2.5 through +1.0 dex in metallicity ([Fe/H]), and from 0 to 5.0 dex in gravity (log g). The spectra in the library have resolution R~1800, with well-calibrated fluxes and rest-framed wavelengths. Using a large number of red stars observed by LAMOST, we generated denser K type templates to fill in data missing from current empirical spectral libraries, particularly the late K type. For K giants, we calibrated the spectroscopic surface gravities against the asteroseismic surface gravities. To verify the reliability of the parameters labeled for this library, we performed an internal cross-validation by using a chi^2 minimization method to interpolate parameters of each individual spectrum using the remaining spectra in the library. We obtained precisions of 41 K, 0.11 dex, and 0.05 dex for T_eff, log g, and [Fe/H], respectively, which means the templates are labeled with correct stellar parameters. Through external comparisons, we confirmed that measurements of the stellar parameters through this library can achieve accuracies of approximately 125K in T_eff, 0.1 dex in [Fe/H] and 0.20 dex in log g without systematic offset. This empirical library is useful for stellar parameter measurements because it has large parameter coverage and full wavelength coverage from 3800 to 8900A.

astro-ph.SR↗
arXiv2018

Non-Darcian flow or fractional derivative ?

Modeling of water and gas flow in low-permeability media is an important topic for a number of engineering such as exploitation of tight gas and disposal of high-level radioactive waste. It has been well documented in the literature that Darcy's linear law of water flux and the hydraulic gradient is not always adequate for low-permeability media. On the basis of the Swartzendruber model, as a non-Darcian model of flow in low-permeability media, a new nonlinear model of water flux and the hydraulic gradient is proposed using fractional derivative. The analytic solution of fractional derivative flow model is presented and all parameters of the fractional derivative flow model are determined by the Levenberg-Marquardt method on the basis of the experimental data of water flow in low-permeability media. It is indicated that the results estimated by the fractional derivative flow model proposed in the paper are in better agreement with the experimental data than the results estimated by the Swartzendruber equation. In addition, a sensitivity study is carried out, showing the effects of fractional derivative order and threshold hydraulic gradient on water flux.

physics.flu-dyn↗
arXiv2018

Three-Dimensional Quantum Anomalous Hall Effect in Ferromagnetic Insulators

The quantum anomalous Hall effect (QAHE) hosts the dissipationless chiral edge states associated with the nonzero Chern number, providing potentially significant applications in future spintronics. The QAHE usually occurs in a two-dimensional (2D) system with time-reversal symmetry breaking. In this work, we propose that the QAHE can exist in three-dimensional (3D) ferromagnetic insulators. By imposing inversion symmetry, we develop the topological constraints dictating the appearance of 3D QAHE based on the parity analysis at the time-reversal invariant points in reciprocal space. Moreover, using first-principles calculations, we identify that 3D QAHE can be realized in a family of intrinsic ferromagnetic insulating oxides, including layered and non-layered compounds that share a centrosymmetric structure with space group $R\bar{3}m$ (No. 166). The Hall conductivity is quantized to be $-\frac{3e^2}{hc}$ with the lattice constant $c$ along $c$-axis. The chiral surface sheet states are clearly visible and uniquely distributed on the surfaces that are parallel to the magnetic moment. Our findings open a promising pathway to realize the QAHE in 3D ferromagnetic insulators.

cond-mat.str-el↗
arXiv2018

Magnetic structure of solar flare regions producing hard X-ray pulsations

We present analysis of the magnetic field in seven solar flare regions accompanied by the pulsations of hard X-ray (HXR) emission. These flares were studied by Kuznetsov et al. (2016) (Paper~I), and chosen here because of the availability of the vector magnetograms for their parent active regions (ARs) obtained with the SDO/HMI data. In Paper~I, based on the observations only, it was suggested that a magnetic flux rope (MFR) might play an important role in the process of generation of the HXR pulsations. The goal of the present paper is to test this hypothesis by using the extrapolation of magnetic field with the non-linear force-free field (NLFFF) method. Having done this, we found that before each flare indeed there was an MFR elongated along and above a magnetic polarity inversion line (MPIL) on the photosphere. In two flare regions the sources of the HXR pulsations were located at the footpoints of different magnetic field lines wrapping around the central axis, and constituting an MFR by themselves. In five other flares the parent field lines of the HXR pulsations were not a part of an MFR, but surrounded it in the form of an arcade of magnetic loops. These results show that, at least in the analyzed cases, the "single flare loop" models do not satisfy the observations and magnetic field modeling, while are consistent with the concept that the HXR pulsations are a consequence of successive episodes of energy release and electron acceleration in different magnetic flux tubes (loops) of a complex AR. An MFR could generate HXR pulsations by triggering episodes of magnetic reconnection in different loops in the course of its non-uniform evolution along an MPIL. However, since three events studied here were confined flares, actual eruptions may not be required to trigger sequential particle acceleration episodes in the magnetic systems containing an MFR.

astro-ph.SR↗
arXiv2018

Dynamical topological invariant for non-Hermitian Rice-Mele model

We study a non-Hermitian Rice-Mele model without breaking time-reversal symmetry, with the non-Hermiticity arising from imbalanced hopping rates. The Berry connection, Berry curvature and Chern number are introduced in the context of biorthonormal inner product. It is shown that for a bulk system, although the Berry connection can be complex numbers, the Chern number is still quantized, as topological invariant. For an opened chain system, the mid-gap edge modes are obtained exactly, obeying the bulk-edge correspondence. Furthermore, we also introduce a local current in the context of biorthonormal inner product to measure the pumping charge generated by a cyclic adiabatic evolution. Analytical analysis and numerical simulation of the time evolution of the mid-gap states show that the pumping charge can be a dynamical topological invariant in correspondence with the Chern number. It indicates that the geometric concepts for Hermitian topological insulator can be extended to the non-Hermitian regime.

quant-ph↗
arXiv2018

Precision searches in dijets at the HL-LHC and HE-LHC

This paper explores the physics reach of the High-Luminosity Large Hadron Collider (HL-LHC) for searches of new particles decaying to two jets. We discuss inclusive searches in dijets and b-jets, as well as searches in semi-inclusive events by requiring an additional lepton that increases sensitivity to different aspects of the underlying processes. We discuss the expected exclusion limits for generic models predicting new massive particles that result in resonant structures in the dijet mass. Prospects of the Higher-Energy LHC (HE-LHC) collider are also discussed. The study is based on the Pythia8 Monte Carlo generator using representative event statistics for the HL-LHC and HE-LHC running conditions. The event samples were created using supercomputers at NERSC.

hep-ex↗
arXiv2018

Pulsar glitch and nuclear EoS: Applicability of superfluid model

In this contribution, we have made use of the contemporary equation of states (EOSs) for the complete neutron star structure, and confronted them with one particular glitch constrain for the crustal moment of inertia (MOI). We find that with these EOSs, the radii of three millisecond pulsars selected by NICER: PSR J0437-4715, PSR J1614-2230, PSR J0751+1807, are all around 12.5 km. Also, a star with $M \lesssim 1.55 M_{\odot}$ would fulfill the MOI calculation for glitch constrain with the latest neutron superfluidity density, and the glitch crisis might not be present.

astro-ph.HE↗
arXiv2018

Dynamical bulk-edge correspondence for nodal lines in parameter space

Nodal line in parameter space, at which the energy gap closes up, can either be the boundary separating two topological quantum phases or two conventional phases. We study the topological feature of nodal line in parameter space via a dimerized Kitaev spin chain with staggered transverse field, which can be mapped onto the system of dimerized spinless fermions with p-wave superconductivity. The quantum phase boundaries are straight crossing degeneracy lines in 3D parameter space. We show that the nodal line acts as a vortex filament associated with a vector field, which is generated from the Zak phase of Bogoliubov-de Gennes band. We also investigate the topological invariant in Majorana fermion representation for open chain. The Majorana edge modes are not zero mode, but is still protected by energy gap. The exact mid-gap states of the Majorana lattice allows to obtain the corresponding Majorana probability distribution for whole 3D parameter space, which can establish a scalar field to identify the topological feature of the nodal lines. Furthermore, when we switch on a weak tunneling between two ends of the Majorana lattice, the topological invariants of the nodal lines can be obtained by the pumped Majorana probability from an adiabatic passage encircling the lines. Numerical simulations of quasi-adiabatic time evolution is performed in small system. We compute the current to monitor the transport of Majorana fermion, which exhibits evident character of topological pumping. Our results link the bulk topological invariant to the dynamical behavior of Majorana edge mode, extending the concept of bulk-edge correspondence.

cond-mat.mes-hall↗