arXiv subjects

J. Yang

Publications and source records attributed to J. Yang.

At least 181 records · Page 10Linked to original sources

arXiv2015

Exciting and Harvesting Vibrational States in Harmonically Driven Granular Chains

This article explores the excitation of different vibrational states in a spatially extended dynamical system through theory and experiment. As a prototypical example, we consider a one-dimensional packing of spherical particles (a so-called granular chain) that is subject to harmonic boundary excitation. The combination of the multi-modal nature of the system and the strong coupling between the particles due to the nonlinear Hertzian contact force leads to broad regions in frequency where different vibrational states are possible. In certain parametric regions, we demonstrate that the Nonlinear Schrodinger (NLS) equation predicts the corresponding modes fairly well. We propose that nonlinear multi-modal systems can be useful in vibration energy harvest- ing and discuss a prototypical framework for its realization. The electromechanical model we derive predicts accurately the conversion from mechanical to electrical energy observed in the experiments.

nlin.PS↗

arXiv2015

New Superconductivity Dome in LaFeAsO$_{1-x}$F$_{x}$ Accompanied by Structural Transition

High temperature superconductivity is often found in the vicinity of antiferromagnetism. This is also true in LaFeAsO$_{1-x}$F$_{x}$ ($x \leq$ 0.2) and many other iron-based superconductors, which leads to proposals that superconductivity is mediated by fluctuations associated with the nearby magnetism. Here we report the discovery of a new superconductivity dome without low-energy magnetic fluctuations in LaFeAsO$_{1-x}$F$_{x}$ with 0.25$\leq x \leq$0.75, where the maximal critical temperature $T_c$ at $x_{opt}$ = 0.5$\sim$0.55 is even higher than that at $x \leq$ 0.2. By nuclear magnetic resonance and Transmission Electron Microscopy, we show that a C4 rotation symmetry-breaking structural transition takes place for $x>$ 0.5 above $T_c$. Our results point to a new paradigm of high temperature superconductivity.

cond-mat.supr-con↗

arXiv2015

A stabilized 18 GHz chip-scale optical frequency comb at 2.8x10-16 relative inaccuracy

Optical frequency combs, coherent light sources that connect optical frequencies with microwave oscillations, have become the enabling tool for precision spectroscopy, optical clockwork and attosecond physics over the past decades. Current benchmark systems are self-referenced femtosecond mode-locked lasers, but four-wave-mixing in high-Q resonators have emerged as alternative platforms. Here we report the generation and full stabilization of CMOS-compatible optical frequency combs. The spiral microcomb's two degrees-of-freedom, one of the comb line and the native 18 GHz comb spacing, are first simultaneously phase-locked to known optical and microwave references. Second, with pump power control, active comb spacing stabilization improves the long-term stability by six orders-of-magnitude, reaching an instrument-limited 3.6 mHz/sqrt(t) residual instability. Third, referencing thirty-three of the nitride frequency comb lines against a fiber comb, we demonstrate the comb tooth-to-tooth frequency relative inaccuracy down to 53 mHz and 2.8x10-16, heralding unprecedented chip-scale applications in precision spectroscopy, coherent communications, and astronomical spectrography.

physics.optics↗

arXiv2015

Nonlinear Low-to-High Frequency Energy Cascades in Diatomic Granular Crystals

We study wave propagation in strongly nonlinear 1D diatomic granular crystals under an impact load. Depending on the mass ratio of the `light' to `heavy' beads, this system exhibits rich wave dynamics from highly localized traveling waves to highly dispersive waves featuring strong attenuation. We experimentally demonstrate the nonlinear resonant and anti-resonant interactions of particles and verify that the nonlinear resonance results in strong wave attenuation, leading to highly efficient nonlinear energy cascading without relying on material damping. In this process, mechanical energy is transferred from low to high frequencies, while propagating waves emerge in both ordered and chaotic waveforms via a distinctive spatial cascading. This energy transfer mechanism from lower to higher frequencies and wavenumbers is of particular significance towards the design of novel nonlinear acoustic metamaterials with inherently passive energy redistribution properties.

cond-mat.soft↗

arXiv2015

Ferromagnetic Spin Fluctuation and Unconventional Superconductivity in Rb$_{2}$Cr$_{3}$As$_{3}$ revealed by $^{75}$As NMR and NQR

We report $^{75}$As nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) studies on the superconductor Rb$_{2}$Cr$_{3}$As$_{3}$ with a quasi one-dimensional crystal structure. Below $T\sim$ 100 K, the spin-lattice relaxation rate (1/$T_{1}$) divided by temperature, 1/$T_{1}T$, increases upon cooling down to $T_{\rm c}$ = 4.8 K, showing a Curie-Weiss-like temperature dependence. The Knight shift also increases with decreasing temperature. These results suggest ferromagnetic spin fluctuation. In the superconducting state, 1/$T_{1}$ decreases rapidly below $T_{\text{c}}$ without a Hebel-Slichter peak, and follows a $T^5$ variation below $T\sim$ 3 K, which point to unconventional superconductivity with point nodes in the gap function.

cond-mat.supr-con↗

arXiv2015

Cavity linewidth narrowing with dark-state polaritons

We perform a quantum-theoretical treatment of cavity linewidth narrowing with intracavity electromagnetically induced transparency (EIT). By means of intracavity EIT, the photons in the cavity are in the form of cavity polaritons: bright-state polariton and dark-state polariton. Strong coupling of the bright-state polariton to the excited state induces an effect known as vacuum Rabi splitting, whereas the dark-state polariton decoupled from the excited state induce a narrow cavity transmission window. Our analysis would provide a quantum theory of linewidth narrowing with a quantum field pulse at the single-photon level.

quant-ph↗

arXiv2015

Unusual ferromagnetic critical behavior owing to short-range antiferromagnetic correlations in antiperovskite Cu1-xNMn3+x(0.1<x<0.4)

For ferromagnets, varying from simple metals to strongly correlated oxides,the critical behaviors near the Curie temperature (TC) can be grouped into several universal classes. In this paper, we report an unusual critical behavior in manganese nitrides Cu1-xNMn3+x(0.1<x<0.4). Although the critical behavior below TC can be well described by mean field (MF) theory, robust critical fluctuations beyond the expectations of any universal classes are observed above TC in x=0.1. The critical fluctuations become weaker when x increases, and the MF-like critical behavior is finally restored at x=0.4. In addition, the paramagnetic susceptibility of all the samples deviates from the Curie-Weiss (CW) law just above TC. This deviation is gradually smeared as x increases. The short-range antiferromagnetic ordering above TC revealed by our electron spin resonance measurement explains both the unusual critical behavior and the breakdown of the CW law.

cond-mat.str-el↗

arXiv2015

A low-phase-noise 18 GHz Kerr frequency microcomb phase-locked over 65 THz

Laser frequency combs are coherent light sources that simultaneously provide pristine frequency spacings for precision metrology and the fundamental basis for ultrafast and attosecond sciences. Recently, nonlinear parametric conversion in high-Q microresonators has been suggested as an alternative platform for optical frequency combs, though almost all in 100 GHz frequencies or more. Here we report a low-phase-noise on-chip Kerr frequency comb with mode spacing compatible with high-speed silicon optoelectronics. The waveguide cross-section of the silicon nitride spiral resonator is designed to possess small and flattened group velocity dispersion, so that the Kerr frequency comb contains a record-high number of 3,600 phase-locked comb lines. We study the single-sideband phase noise as well as the long-term frequency stability and report the lowest phase noise floor achieved to date with -130 dBc/Hz at 1 MHz offset for the 18 GHz Kerr comb oscillator, along with feedback stabilization to achieve frequency Allan deviations of 7x10-11 in 1 s. The reported system is a promising compact platform for achieving self-referenced Kerr frequency combs and also for high-capacity coherent communication architectures.

physics.optics↗

arXiv2015

The SFXC software correlator for Very Long Baseline Interferometry: Algorithms and Implementation

In this paper a description is given of the SFXC software correlator, developed and maintained at the Joint Institute for VLBI in Europe (JIVE). The software is designed to run on generic Linux-based computing clusters. The correlation algorithm is explained in detail, as are some of the novel modes that software correlation has enabled, such as wide-field VLBI imaging through the use of multiple phase centres and pulsar gating and binning. This is followed by an overview of the software architecture. Finally, the performance of the correlator as a function of number of CPU cores, telescopes and spectral channels is shown.

astro-ph.IM↗

arXiv2015

Mode-locked ultrashort pulse generation from on-chip normal dispersion microresonators

We describe the generation of stable mode-locked pulse trains from on-chip normal dispersion microresonators. The excitation of hyper-parametric oscillation is facilitated by the local dispersion disruptions induced by mode interactions. The system is then driven from hyper-parametric oscillation to the mode-locked state with over 200 nm spectral width by controlled pump power and detuning. With the continuous-wave driven nonlinearity, the pulses sit on a pedestal, akin to a cavity soliton. We identify the importance of pump detuning and wavelength-dependent quality factors in stabilizing and shaping the pulse structure, to achieve a single pulse inside the cavity. We examine the mode locking dynamics by numerically solving the master equation and provide analytic solutions under appropriate approximations.

physics.optics↗

arXiv2014

Tunable narrow band source via the strong coupling between optical emitter and nanowire surface plasmons

The spectrum width can be narrowed to a certain degree by decreasing the coupling strength for the two-level emitter coupled to the propagating surface plasmon. But the width can not be narrowed any further because of the loss of the photon out of system by spontaneous emission from the emitter. Here we propose a new scheme to construct a narrow-band source via a one-dimensional waveguide coupling with a three-level emitter. It is shown that the reflective spectrum width can be narrowed avoiding the impact of the loss. This approach opens up the possibility of plasmonic ultranarrow single-photon source.

quant-ph↗

arXiv2014

Highly Nonlinear Wave Propagation in Elastic Woodpile Periodic Structures

In the present work, we experimentally implement, numerically compute with and theoretically analyze a configuration in the form of a single column woodpile periodic structure. Our main finding is that a Hertzian, locally-resonant, woodpile lattice offers a test bed for the formation of genuinely traveling waves composed of a strongly-localized solitary wave on top of a small amplitude oscillatory tail. This type of wave, called a nanopteron, is not only motivated theoretically and numerically, but are also visualized experimentally by means of a laser Doppler vibrometer. This system can also be useful for manipulating stress waves at will, for example, to achieve strong attenuation and modulation of high-amplitude impacts without relying on damping in the system.

cond-mat.other↗

arXiv2014

Time-Periodic Solutions of Driven-Damped Trimer Granular Crystals

In this work, we consider time-periodic structures of trimer granular crystals consisting of alternate chrome steel and tungsten carbide spherical particles yielding a spatial periodicity of three. The configuration at the left boundary is driven by a harmonic in-time actuation with given amplitude and frequency while the right one is a fixed wall. Similar to the case of a dimer chain, the combination of dissipation, driving of the boundary, and intrinsic nonlinearity leads to complex dynamics. For fixed driving frequencies in each of the spectral gaps, we find that the nonlinear surface modes and the states dictated by the linear drive collide in a saddle-node bifurcation as the driving amplitude is increased, beyond which the dynamics of the system become chaotic. While the bifurcation structure is similar for solutions within the first and second gap, those in the first gap appear to be less robust. We also conduct a continuation in driving frequency, where it is apparent that the nonlinearity of the system results in a complex bifurcation diagram, involving an intricate set of loops of branches, especially within the spectral gap. The theoretical findings are qualitatively corroborated by the experimental full-field visualization of the time-periodic structures.

nlin.PS↗

arXiv2014

Time- and space-variant wave transmission in helicoidal phononic crystals

We present a dynamically tunable mechanism of wave transmission in 1D helicoidal phononic crystals in a shape similar to DNA structures. These helicoidal architectures allow slanted nonlinear contact among cylin- drical constituents, and the relative torsional movements can dynamically tune the contact stiffness between neighboring cylinders. This results in cross-talking between in-plane torsional and out-of-plane longitudinal waves. We numerically demonstrate their versatile wave mixing and controllable dispersion behavior in both wavenumber and frequency domains. Based on this principle, a suggestion towards an acoustic configuration bearing parallels to a transistor is further proposed, in which longitudinal waves can be switched on/off through torsional waves.

nlin.PS↗

arXiv2014

A Comprehensive Radio View of the Extremely Bright Gamma-Ray Burst 130427A

GRB 130427A was extremely bright as a result of occurring at low redshift whilst the energetics were more typical of high-redshift gamma-ray bursts (GRBs). We collected well-sampled light curves at 1.4 and 4.8 GHz of GRB 130427A with the Westerbork Synthesis Radio Telescope (WSRT); and we obtained its most accurate position with the European Very Long Baseline Interferometry Network (EVN). Our flux density measurements are combined with all the data available at radio, optical and X-ray frequencies to perform broadband modeling in the framework of a reverse-forward shock model and a two-component jet model, and we discuss the implications and limitations of both models. The low density inferred from the modeling implies that the GRB 130427A progenitor is either a very low-metallicity Wolf-Rayet star, or a rapidly rotating, low-metallicity O star. We also find that the fraction of the energy in electrons is evolving over time, and that the fraction of electrons participating in a relativistic power-law energy distribution is less than 15%. We observed intraday variability during the earliest WSRT observations, and the source sizes inferred from our modeling are consistent with this variability being due to interstellar scintillation effects. Finally, we present and discuss our limits on the linear and circular polarization, which are among the deepest limits of GRB radio polarization to date.

astro-ph.HE↗

arXiv2014

Multiple-Candidate Successive Interference Cancellation with Widely-Linear Processing for MAI and Jamming Suppression in DS-CDMA Systems

In this paper, we propose a widely-linear (WL) receiver structure for multiple access interference (MAI) and {jamming signal (JS)} suppression in direct-sequence code-division multiple-access (DS-CDMA) systems. A vector space projection (VSP) scheme is also considered to cancel the {JS} before detecting the desired signals. We develop a novel multiple-candidate successive interference cancellation (MC-SIC) scheme which processes two consecutive user symbols at one time to process the unreliable estimates and a number of selected points serve as the feedback candidates for interference cancellation, which is effective for alleviating the effect of error propagation in the SIC algorithm. Widely-linear signal processing is then used to enhance the performance of the receiver in non-circular modulation scheme. By bringing together the techniques mentioned above, a novel interference suppression scheme is proposed which combines the widely-linear multiple-candidate SIC (WL-MC-SIC) minimum mean-squared error (MMSE) algorithm with the VSP scheme to suppress MAI and {JS} simultaneously. Simulations for binary phase shift keying (BPSK) modulation scenarios show that the proposed structure achieves a better MAI suppression performance compared with previously reported SIC MMSE receivers at lower complexity and a superior {JS} suppression performance.

cs.NI↗

arXiv2014

CuSe-based layered compound Bi$_{2}$YO$_{4}$Cu$_{2}$Se$_{2}$ as a quasi-two-dimensional metal

We have investigated the physical properties of a new layered oxyselenide Bi$_{2}$YO$_{4}$Cu$_{2}$Se$_{2}$, which crystallizes in an unusual intergrowth structure with Cu$_{2}$Se$_{2}$ and Bi$_{2}$YO$_{4}$ layers. Electric transport measurement indicates that Bi$_{2}$YO$_{4}$Cu$_{2}$Se$_{2}$ behaves metallic. Thermal transport and Hall measurements show that the type of the carriers is hole-like and it may be a potential thermoelectric material at high temperatures. First principle calculations are in agreement with experimental results and show that Bi$_{2}$YO$_{4}$Cu$_{2}$Se$_{2}$ is a quasi-2D metal. Further theoretical investigation suggests the ground states of the Bi$_{2}$YO$_{4}$Cu$_{2}$Se$_{2}$-type can be tuned by designing the blocking layers, which will enrich the physical properties of these compounds.

cond-mat.mtrl-sci↗

arXiv2014

VLBI observations of the radio quasar J2228+0110 at z=5.95 and other field sources in multiple-phase-centre mode

A patch of sky in the SDSS Stripe 82 was observed at 1.6 GHz with Very Long Baseline Interferometry (VLBI) using the European VLBI Network (EVN). The data were correlated at the EVN software correlator at JIVE (SFXC). There are fifteen known mJy/sub-mJy radio sources in the target field defined by the primary beam size of a typical 30-m class EVN radio telescope. The source of particular interest is a recently identified high-redshift radio quasar J222843.54+011032.2 (J2228+0110) at redshift z=5.95. Our aim was to investigate the milli-arcsecond (mas) scale properties of all the VLBI-detectable sources within this primary beam area with a diameter of 20 arcmin. The source J2228+0110 was detected with VLBI with a brightness temperature T_b>10^8 K, supporting the active galactic nucleus (AGN) origin of its radio emission, which is conclusive evidence that the source is a radio quasar. In addition, two other target sources were also detected, one of them with no redshift information. Their brightness temperature values (T_b >10^7 K) measured with VLBI suggest a non-thermal synchrotron radiation origin for their radio emission. The detection rate of 20% is broadly consistent with other wide-field VLBI experiments carried out recently. We also derived the accurate equatorial coordinates of the three detected sources using the phase-referencing technique. This experiment is an early attempt of a wide-field science project with SFXC, paving the way for the EVN to conduct a large-scale VLBI survey in the multiple-phase-centre mode.

astro-ph.GA↗