Searcharxiv⌕ Search

arXiv subjects

Y. Huang

Publications and source records attributed to Y. Huang.

At least 217 records · Page 12Linked to original sources
arXiv2017

Zoology of a non-local cross-diffusion model for two species

We study a non-local two species cross-interaction model with cross-diffusion. We propose a positivity preserving finite volume scheme based on the numerical method introduced in Ref. [15] and explore this new model numerically in terms of its long-time behaviours. Using the so gained insights, we compute analytical stationary states and travelling pulse solutions for a particular model in the case of attractive-attractive/attractive-repulsive cross-interactions. We show that, as the strength of the cross-diffusivity decreases, there is a transition from adjacent solutions to completely segregated densities, and we compute the threshold analytically for attractive-repulsive cross-interactions. Other bifurcating stationary states with various coexistence components of the support are analysed in the attractive-attractive case. We find a strong agreement between the numerically and the analytically computed steady states in these particular cases, whose main qualitative features are also present for more general potentials.

math.AP↗
arXiv2017

Fano-enhanced pulling and pushing optical force on active plasmonic nanoparticles

We demonstrate tunable pulling and pushing optical forces on plasmonic nanostructures around Fano resonance. The plasmonic nanostructure containing a spherical core with optical gain and a metallic shell shows much larger optical pulling force than a pure gain sphere. One can obtain large field enhancement and giant pulling force at the emerged quadrupole mode. The introduction of optical pump compensate the dissipative loss from metal shell, thus enable the strong coupling between a narrow quadrupole mode and a board dipole mode, giving rise to Fano resonance. The giant negative forces origin from the reversal of electric field at Fano resonance, which lead to pulling force on bound currents and charges. Meanwhile, the separation of the Lorentz force helps to reveal the nature of the pulling forces in gain system. We have shown that by applying the Lorentz force density formula, it is possible to obtain the correct value of the force inside our complex inhomogenous structure made up of dispersive and lossy metamaterial irrespective of the electromagnetic momentum density. Our results provide a practical way to manipulate nanoparticles and give deep insight into light-matter interaction.

physics.optics↗
arXiv2017

Observation of a dispersive charge mode in hole-doped cuprates using resonant inelastic x-ray scattering at the oxygen K edge

We investigate electronic excitations in La2-x(Br,Sr)xCuO4 using resonant inelastic x-ray scattering (RIXS) at the oxygen K edge. RIXS spectra of the hole-doped cuprates show clear momentum dependence below 1 eV. The spectral weight exhibits positive dispersion and shifts to higher energy with increasing hole concentration. Theoretical calculation of the dynamical charge structure factor on oxygen orbitals in a three-band Hubbard model is consistent with the experimental observation of the momentum and doping dependence, and therefore the dispersive mode is ascribed to intraband charge excitations which have been observed in electron-doped cuprates.

cond-mat.str-el↗
arXiv2017

Spectroscopic evidence for bulk-band inversion and three-dimensional massive Dirac fermions in ZrTe5

Three-dimensional topological insulators (3D TIs) represent novel states of quantum matters in which surface states are protected by time-reversal symmetry and an inversion occurs between bulk conduction- and valence-bands. However, the bulk-band inversion which is intimately tied to the topologically nontrivial nature of 3D TIs has rarely been investigated by experiments. Besides, 3D massive Dirac fermions with nearly near band dispersions were seldom observed in TIs. Recently, a van der Waals crystal, ZrTe5, was theoretically predicted to be a TI. Here, we report an infrared transmission study of a high-mobility (~33,000 cm$^2$/(Vs)) multilayer ZrTe5 flake at magnetic fields ($B$) up to 35 T. Our observation of a linear relationship between the zero-magnetic-field optical absorption and the photon energy, a bandgap of ~10 meV and a $\sqrt{B}$ dependence of the Landau level (LL) transition energies at low magnetic fields demonstrates 3D massive Dirac fermions with nearly linear band dispersions in this system. More importantly, the reemergence of the intra-LL transitions at magnetic fields higher than 17 T reveals the energy cross between the two zeroth LLs, which reflects the inversion between the bulk conduction- and valence-bands. Our results not only provide spectroscopic evidence for the TI state in ZrTe5 but also open up a new avenue for fundamental studies of Dirac fermions in van der Waals materials.

cond-mat.mes-hall↗
arXiv2017

Tunable optical bistability and tristability of nonlinear graphene-wrapped dielectric nanoparticles

Based on full-wave scattering theory with self-consistent mean field approximation, we study the optical multi-stability of graphene-wrapped dielectric nanoparticles. We demonstrate that the optical bistability (OB) of the graphene-wrapped nanoparticle exist in both near-field and far-field spectra, and show the optical multi-stability arising from the contributions of higher-order terms of the incident external field. Moreover, both the optical stable region and the switching threshold values can be tuned by changing either the Fermi level or the size of the nanoparticle. Our results promise the graphene-wrapped dielectric nanoparticle a candidate of multi-state optical switching, optical memories and relevant optoelectronic devices.

physics.optics↗
arXiv2017

The Repeating Fast Radio Burst FRB 121102 as Seen on Milliarcsecond Angular Scales

The millisecond-duration radio flashes known as Fast Radio Bursts (FRBs) represent an enigmatic astrophysical phenomenon. Recently, the sub-arcsecond localization (~ 100mas precision) of FRB121102 using the VLA has led to its unambiguous association with persistent radio and optical counterparts, and to the identification of its host galaxy. However, an even more precise localization is needed in order to probe the direct physical relationship between the millisecond bursts themselves and the associated persistent emission. Here we report very-long-baseline radio interferometric observations using the European VLBI Network and the 305-m Arecibo telescope, which simultaneously detect both the bursts and the persistent radio emission at milliarcsecond angular scales and show that they are co-located to within a projected linear separation of < 40pc (< 12mas angular separation, at 95% confidence). We detect consistent angular broadening of the bursts and persistent radio source (~ 2-4mas at 1.7GHz), which are both similar to the expected Milky Way scattering contribution. The persistent radio source has a projected size constrained to be < 0.7pc (< 0.2mas angular extent at 5.0GHz) and a lower limit for the brightness temperature of T_b > 5 x 10^7K. Together, these observations provide strong evidence for a direct physical link between FRB121102 and the compact persistent radio source. We argue that a burst source associated with a low-luminosity active galactic nucleus or a young neutron star energizing a supernova remnant are the two scenarios for FRB121102 that best match the observed data.

astro-ph.HE↗
arXiv2016

Numerical Study of a Particle Method for Gradient Flows

We study the numerical behaviour of a particle method for gradient flows involving linear and nonlinear diffusion. This method relies on the discretisation of the energy via non-overlapping balls centred at the particles. The resulting scheme preserves the gradient flow structure at the particle level, and enables us to obtain a gradient descent formulation after time discretisation. We give several simulations to illustrate the validity of this method, as well as a detailed study of one-dimensional aggregation-diffusion equations.

math.AP↗
arXiv2016

Precision Measurement of the Quadrupole Transition Matrix Element in a Single Trapped $^{40}$Ca$^{+}$

We report the first experimental determination of the $4s \ ^{2}S_{1/2} $ $\leftrightarrow $ $3d \ ^{2}D_{5/2}$ quadrupole transition matrix element in $^{40}$Ca$^+$ by measuring the branching ratio of the $3d \ ^{2}D_{5/2} $ state decaying into the ground state $4s \ ^{2}S_{1/2} $ and the lifetime of the $3d \ ^{2}D_{5/2} $ state, using a technique of highly synchronized measurement sequence for laser control and highly efficient quantum state detection for quantum jumps. The measured branching ratio and improved lifetime are, respectively, 0.9992(80) and 1.1652(46) s, which yield the value of the quadrupole transition matrix element (in absolute value) 9.737(43)~$ea_{0}^{2}$ with the uncertainty at the level of 0.44\%. The measured quadrupole transition matrix element is in good agreement with the most precise many-body atomic structure calculations. Our method can be universally applied to measurements of transition matrix elements in single ions and atoms of similar structure.

physics.atom-ph↗
arXiv2016

Orbital engineering in nickelate heterostructures driven by anisotropic oxygen hybridization rather than orbital energy levels

Resonant inelastic x-ray scattering is used to investigate the electronic origin of orbital polarization in nickelate heterostructures taking $\mathrm{LaTiO_3-LaNiO_3-3x(LaAlO_3)}$, a system with exceptionally large polarization, as a model system. We find that heterostructuring generates only minor changes in the Ni $3d$ orbital energy levels, contradicting the often-invoked picture in which changes in orbital energy levels generate orbital polarization. Instead, O $K$-edge x-ray absorption spectroscopy demonstrates that orbital polarization is caused by an anisotropic reconstruction of the oxygen ligand hole states. This provides an explanation for the limited success of theoretical predictions based on tuning orbital energy levels and implies that future theories should focus on anisotropic hybridization as the most effective means to drive large changes in electronic structure and realize novel emergent phenomena.

cond-mat.str-el↗
arXiv2016

Andreev reflection in s-type superconductor proximized 3D topological insulator

We investigate transport and shot noise in lateral N-TI-S contacts, where N is a normal metal, TI is a Bi-based three dimensional topological insulator (3D TI), and S is an s-type superconductor. In normal state, the devices are in the elastic diffusive transport regime, as demonstrated by a nearly universal value of the shot noise Fano factor $F_{\rm N}\approx1/3$ in magnetic field and in reference normal contact. In the absence of magnetic field, we identify the Andreev reflection (AR) regime, which gives rise to the effective charge doubling in shot noise measurements. Surprisingly, the Fano factor $F_{\rm AR}\approx0.22\pm0.02$ is considerably reduced in the AR regime compared to $F_{\rm N}$, in contrast to previous AR experiments in normal metals and semiconductors. We suggest that this effect is related to a finite thermal conduction of the proximized, superconducting TI owing to a residual density of states at low energies.

cond-mat.mes-hall↗
arXiv2016

Quenched Magnon excitations by oxygen sublattice reconstruction in (SrCuO$_2$)$_n$/(SrTiO$_3$)$_2$ superlattices

The recently discovered structural reconstruction in the cuprate superlattice (SrCuO$_2$)$_n$/(SrTiO$_3$)$_2$ has been investigated across the critical value of $n=5$ using resonant inelastic x-ray scattering (RIXS). We find that at the critical value of $n$, the cuprate layer remains largely in the bulk-like two-dimensional structure with a minority of Cu plaquettes being reconstructed. The partial reconstruction leads to quenching of the magnons starting at the $Γ$-point due to the minority plaquettes acting as scattering points. Although comparable in relative abundance, the doped charge impurities in electron-doped cuprate superconductors do not show this quenching of magnetic excitations.

cond-mat.str-el↗
arXiv2016

Epitaxial growth and thermodynamic stability of SrIrO3/SrTiO3 heterostructures

Obtaining high-quality thin films of 5d transition metal oxides is essential to explore the exotic semimetallic and topological phases predicted to arise from the combination of strong electron correlations and spin-orbit coupling. Here, we show that the transport properties of SrIrO3 thin films, grown by pulsed laser deposition, can be optimized by considering the effect of laser-induced modification of the SrIrO3 target surface. We further demonstrate that bare SrIrO3 thin films are subject to degradation in air and are highly sensitive to lithographic processing. A crystalline SrTiO3 cap layer deposited in-situ is effective in preserving the film quality, allowing us to measure metallic transport behavior in films with thicknesses down to 4 unit cells. In addition, the SrTiO3 encapsulation enables the fabrication of devices such as Hall bars without altering the film properties, allowing precise (magneto)transport measurements on micro- and nanoscale devices.

cond-mat.str-el↗
arXiv2016

Quantifying discordance in the 2015 Planck CMB spectrum

We examine the internal consistency of the Planck 2015 cosmic microwave background (CMB) temperature anisotropy power spectrum. We show that tension exists between cosmological constant cold dark matter (LCDM) model parameters inferred from multipoles l<1000 (roughly those accessible to Wilkinson Microwave Anisotropy Probe), and from l>=1000, particularly the CDM density, Omega_ch^2, which is discrepant at 2.5 sigma for a Planck-motivated prior on the optical depth, tau=0.07+/-0.02. We find some parameter tensions to be larger than previously reported because of inaccuracy in the code used by the Planck Collaboration to generate model spectra. The Planck l>=1000 constraints are also in tension with low-redshift data sets, including Planck's own measurement of the CMB lensing power spectrum (2.4 sigma), and the most precise baryon acoustic oscillation (BAO) scale determination (2.5 sigma). The Hubble constant predicted by Planck from l>=1000, H_0=64.1+/-1.7 km/s/Mpc, disagrees with the most precise local distance ladder measurement of 73.0+/-2.4 km/s/Mpc at the 3.0 sigma level, while the Planck value from l<1000, 69.7+/-1.7 km/s/Mpc, is consistent within 1 sigma. A discrepancy between the Planck and South Pole Telescope (SPT) high-multipole CMB spectra disfavors interpreting these tensions as evidence for new physics. We conclude that the parameters from the Planck high-multipole spectrum probably differ from the underlying values due to either an unlikely statistical fluctuation or unaccounted-for systematics persisting in the Planck data.

astro-ph.CO↗
arXiv2016

A universal description of pseudorapidity distributions in both nucleus-nucleus and p-p collisions at now available energy regions

Investigations have shown that the collective motion not only appears in nucleus-nucleus but also in p-p collisions. The best tool for depicting such collective motion is relativistic hydrodynamics. In this paper, the collective motion is assumed obeying the hydro model which integrates the features of Landau and Hwa-Bjorken theory and is one of a very few analytically solvable models. The fluid is then supposed freezing out into charged particles from a time-like hypersurface with a fixed time of t_FO . The researches of present paper show that this part of charged particles together with leading particles, which, by conventional definition, carry on the quantum numbers of colliding nucleons and take away the most part of incident energy, can give a proper universal description to the pseudorapidity distributions of charged particles measured in both nucleus-nucleus and p-p collisions at now available energy regions.

nucl-th↗
arXiv2016

GaAs/AlGaAs Nanowire Photodetector

We demonstrate an efficient core-shell GaAs/AlGaAs nanowire photodetector operating at room temperature. The design of this nanoscale detector is based on a type-I heterostructure combined with a metal-semiconductor-metal (MSM) radial architecture, in which built-in electric fields at the semiconductor heterointerface and at the metal/semiconductor Schottky contact promote photogenerated charge separation, enhancing photosensitivity. The spectral photoconductive response shows that the nanowire supports resonant optical modes in the near-infrared region, which lead to large photocurrent density in agreement with the predictions of electromagnetic and transport computational models. The single nanowire photodetector shows remarkable peak photoresponsivity of 0.57 A/W, comparable to large-area planar GaAs photodetectors on the market, and a high detectivity of 7.2 10^10 cm\sqrt{Hz}/W at λ=855 nm. This is promising for the design of a new generation of highly sensitive single nanowire photodetectors by controlling optical mode confinement, bandgap, density of states, and electrode engineering.

cond-mat.mes-hall↗
arXiv2015

Electron correlation effects and scattering rates in Fe$_{1+y}$Te$_{1-x}$Se$_x$ superconductor

Using angle-resolved photoemission spectroscopy we have studied the low-energy electronic structure and the Fermi surface topology of Fe$_{1+y}$Te$_{1-x}$Se$_x$ superconductors. Similar to the known iron pnictides we observe hole pockets at the center and electron pockets at the corner of the Brillouin zone (BZ). However, on a finer level, the electronic structure around the $Γ$- and $Z$-points in $k$-space is substantially different from other iron pnictides, in that we observe two hole pockets at the $Γ$-point, and more interestingly only one hole pocket is seen at the $Z$-point, whereas in $1111$-, $111$-, and $122$-type compounds, three hole pockets could be readily found at the zone center. Another major difference noted in the Fe$_{1+y}$Te$_{1-x}$Se$_x$ superconductors is that the top of innermost hole-like band moves away from the Fermi level to higher binding energy on going from $Γ$ to $Z$, quite opposite to the iron pnictides. The polarization dependence of the observed features was used to aid the attribution of the orbital character of the observed bands. Photon energy dependent measurements suggest a weak $k_z$ dispersion for the outer hole pocket and a moderate $k_z$ dispersion for the inner hole pocket. By evaluating the momentum and energy dependent spectral widths, the single-particle self-energy was extracted and interestingly this shows a pronounced non-Fermi liquid behaviour for these compounds. The experimental observations are discussed in context of electronic band structure calculations and models for the self-energy such as the spin-fermion model and the marginal-Fermi liquid.

cond-mat.supr-con↗
arXiv2015

Computational design of high performance hybrid perovskite on silicon tandem solar cells

In this study, the optoelectronic properties of a monolithically integrated series-connected tandem solar cell are simulated. Following the large success of hybrid organic-inorganic perovskites, which have recently demonstrated large efficiencies with low production costs, we examine the possibility of using the same perovskites as absorbers in a tandem solar cell. The cell consists in a methylammonium mixed bromide-iodide lead perovskite, CH3NH3PbI3(1-x)Br3x (0 < x < 1), top sub-cell and a single-crystalline silicon bottom sub-cell. A Si-based tunnel junction connects the two sub-cells. Numerical simulations are based on a one-dimensional numerical drift-diffusion model. It is shown that a top cell absorbing material with 20% of bromide and a thickness in the 300-400 nm range affords current matching with the silicon bottom cell. Good interconnection between single cells is ensured by standard n and p doping of the silicon at 5.10^19cm-3 in the tunnel junction. A maximum efficiency of 27% is predicted for the tandem cell, exceeding the efficiencies of stand-alone silicon (17.3%) and perovskite cells (17.9%) taken for our simulations, and more importantly, that of the record crystalline Si cells.

cond-mat.mtrl-sci↗