arXiv subjects

S. Liu

Publications and source records attributed to S. Liu.

At least 73 records · Page 4Linked to original sources

arXiv2021

HESS J1858+020: A GeV-TeV source possibly powered by CRs from SNR G35.6-0.4

Context: The supernova remnant (SNR) G35.6-0.4 shows a non-thermal radio shell, however, no γ-ray or X-ray counterparts have been found for it thus far. One TeV source, HESS J1858+020, was found near the SNR and this source is spatially associated with some clouds at 3.6 kpc. Aims: To attain a better understanding of the origin of HESS J1858+020, we further investigate the association between SNR cosmic rays (CRs) and the clouds through the Fermi-LAT analysis and hadronic modeling. Methods: We performed the Fermi-LAT analysis to explore the GeV emission in and around the SNR. We explored the SNR physics with previously observed multi-wavelength data. We built a hadronic model using runaway CRs of the SNR to explain the GeV-TeV observation. Results: We found a hard GeV source (SrcX2) that is spatially coincident with both HESS J1858+020 and a molecular cloud complex at 3.6 kpc. In addition, a soft GeV source (SrcX1) was found at the northern edge of the SNR. The GeV spectrum of SrcX2 connects well with the TeV spectrum of HESS J1858+020. The entire γ-ray spectrum ranges from several GeV up to tens of TeV and it follows a power-law with an index of ~2.15. We discuss several pieces of observational evidence to support the middle-aged SNR argument. Using runaway CRs from the SNR, our hadronic model explains the GeV-TeV emission at HESS J1858+020, with a diffusion coefficient that is much lower than the Galactic value.

astro-ph.HE↗

arXiv2021

Efficient domain wall motion in asymmetric magnetic tunnel junctions with vertical current flow

In this paper, we study the domain wall motion induced by vertical current flow in asymmetric magnetic tunnel junctions. The domain wall motion in the free layer is mainly dictated by the current-induced field-like torque acting on it. We show that as we increase the MTJ asymmetry, by considering dissimilar ferromagnetic contacts, a linear-in-voltage field-like torque behavior is accompanied by an enhancement in the domain wall displacement efficiency and a higher degree of bidirectional propagation. Our analysis is based on a combination of a quantum transport model and magnetization dynamics as described by the Landau-Lifshitz-Gilbert equation, along with comparison to the intrinsic characteristics of a benchmark in-plane current injection domain wall device.

cond-mat.mes-hall↗

arXiv2020

Transition between Instability and Seeded Self-Modulation of a Relativistic Particle Bunch in Plasma

We use a relativistic ionization front to provide various initial transverse wakefield amplitudes for the self-modulation of a long proton bunch in plasma. We show experimentally that, with sufficient initial amplitude ($\ge(4.1\pm0.4)$ MV/m), the phase of the modulation along the bunch is reproducible from event to event, with 3 to 7% (of 2$π$) rms variations all along the bunch. The phase is not reproducible for lower initial amplitudes. We observe the transition between these two regimes. Phase reproducibility is essential for deterministic external injection of particles to be accelerated.

physics.plasm-ph↗

arXiv2020

A flux tunable superconducting quantum circuit based on Weyl semimetal MoTe2

Weyl semimetals for their exotic topological properties have drawn considerable attention in many research fields. When in combination with s-wave superconductors, the supercurrent can be carried by their topological surface channels, forming junctions mimic the behavior of Majorana bound states. Here, we present a transmon-like superconducting quantum intereference device (SQUID) consists of lateral junctions made of Weyl semimetal Td-MoTe2 and superconducting leads niobium nitride (NbN). The SQUID is coupled to a readout cavity made of molybdenum rhenium (MoRe), whose response at high power reveal the existence of the constituting Josephson junctions (JJs). The loop geometry of the circuit allows the resonant frequency of the readout cavity to be tuned by the magnetic flux. We demonstrate a JJ made of MoTe2 and a flux-tunable transmon-like circuit based on Weyl materials. Our study provides a platform to utilize topological materials in SQUID-based quantum circuits for potential applications in quantum information processing.

quant-ph↗

arXiv2020

Experimental study of extended timescale dynamics of a plasma wakefield driven by a self-modulated proton bunch

Plasma wakefield dynamics over timescales up to 800 ps, approximately 100 plasma periods, are studied experimentally at the Advanced Wakefield Experiment (AWAKE). The development of the longitudinal wakefield amplitude driven by a self-modulated proton bunch is measured using the external injection of witness electrons that sample the fields. In simulation, resonant excitation of the wakefield causes plasma electron trajectory crossing, resulting in the development of a potential outside the plasma boundary as electrons are transversely ejected. Trends consistent with the presence of this potential are experimentally measured and their dependence on wakefield amplitude are studied via seed laser timing scans and electron injection delay scans.

physics.plasm-ph↗

arXiv2020

First principles analysis of precipitation in Mg-Zn alloys

Precipitation in Mg-Zn alloys was analyzed by means of first principles calculations. Formation energies of symmetrically distinct hcp Mg1-xZnx (0 < x < 1) configurations were determined and potential candidates for Guinier-Preston zones coherent with the matrix were identified from the convex hull. The most likely structures were ranked depending on the interface energy along the basal plane. In addition, the formation energy and vibrational entropic contributions of several phases reported experimentally (Mg4Zn7, MgZn2 cubic, MgZn2 hexagonal, Mg21Zn25 and Mg2Zn11) were calculated. The formation energies of Mg4Zn7, MgZn2 cubic, and MgZn2 hexagonal Laves phases were very close because they were formed by different arrangements of rhombohedral and hexagonal lattice units. It was concluded that β_1^' precipitates were formed by a mixture of all of them. Nevertheless, the differences in the geometrical arrangements led to variations in the entropic energy contributions which determined the high temperature stability. It was found that the MgZn2 hexagonal Laves phase is the most stable phase at high temperature and, thus, β_1^' precipitates tend to transform into the β_2^' (MgZn2 hexagonal) precipitates with higher aging temperature or longer aging times. Finally, the equilibrium βphase (Mg21Zn25) was found to be a long-range order that precipitates the last one on account of the kinetic processes necessary to trigger the transformation from a short-range order phase β_2^' to β.

cond-mat.mtrl-sci↗

arXiv2020

Proton beam defocusing in AWAKE: comparison of simulations and measurements

In 2017, AWAKE demonstrated the seeded self-modulation (SSM) of a 400 GeV proton beam from the Super Proton Synchrotron (SPS) at CERN. The angular distribution of the protons deflected due to SSM is a quantitative measure of the process, which agrees with simulations by the two-dimensional (axisymmetric) particle-in-cell code LCODE. Agreement is achieved for beam populations between $10^{11}$ and $3 \times 10^{11}$ particles, various plasma density gradients ($-20 ÷20\%$) and two plasma densities ($2\times 10^{14} \text{cm}^{-3}$ and $7 \times 10^{14} \text{cm}^{-3}$). The agreement is reached only in the case of a wide enough simulation box (at least five plasma wavelengths).

physics.acc-ph↗

arXiv2020

Global Wheat Head Detection (GWHD) dataset: a large and diverse dataset of high resolution RGB labelled images to develop and benchmark wheat head detection methods

Detection of wheat heads is an important task allowing to estimate pertinent traits including head population density and head characteristics such as sanitary state, size, maturity stage and the presence of awns. Several studies developed methods for wheat head detection from high-resolution RGB imagery. They are based on computer vision and machine learning and are generally calibrated and validated on limited datasets. However, variability in observational conditions, genotypic differences, development stages, head orientation represents a challenge in computer vision. Further, possible blurring due to motion or wind and overlap between heads for dense populations make this task even more complex. Through a joint international collaborative effort, we have built a large, diverse and well-labelled dataset, the Global Wheat Head detection (GWHD) dataset. It contains 4,700 high-resolution RGB images and 190,000 labelled wheat heads collected from several countries around the world at different growth stages with a wide range of genotypes. Guidelines for image acquisition, associating minimum metadata to respect FAIR principles and consistent head labelling methods are proposed when developing new head detection datasets. The GWHD is publicly available at http://www.global-wheat.com/ and aimed at developing and benchmarking methods for wheat head detection.

cs.CV↗

arXiv2020

Prediction of the Al-rich part of the Al-Cu phase diagram using cluster expansion and statistical mechanics

The thermodynamic properties of α-Al and other phases (GP zones, θ'', θ' and θ) in the Al-rich part of the Al-Cu system have been obtained by means of the cluster expansion formalism in combination with statistical mechanics. This information was used to build the Al-rich part of the Al-Cu phase-diagram taking into account vibrational entropic contributions for θ', as those of the other phases were negligible. The simulation predictions of the phase boundaries between α-Al and either θ'', θ' or θ phases as a function of temperature are in good agreement with experimental data and extend the phase boundaries to a wider temperature range. The DFT calculations reveal the presence of a number of metastable Guinier-Preston-zone type configurations that may coexist with α-Al and θ'' at low temperatures. They also demonstrate that θ' is the stable phase below 550K but it is replaced by θ above this temperature due to the vibrational entropic contribution to the Gibbs energy of θ'. This work shows how the combination of cluster expansion and statistical mechanics can be used to expand our knowledge of the phase diagram of metallic alloys and to provide Gibbs free energies of different phases that can be used as input in mesoscale simulations of precipitation.

cond-mat.mtrl-sci↗

arXiv2020

CMOS compatible high-performance nanolasing based on perovskite-SiN hybrid integration

Coherent light sources in silicon photonics are the long-sought holy grail because silicon-based materials have indirect bandgap. Traditional strategies for realizing such sources, e.g., heterogeneous photonic integration, strain engineering and nonlinear process, are technologically demanding. Here, we demonstrate a hybrid lasing device composing of perovskite nanocrystals and silicon nitride nanobeam cavity. We fabricate SiN photonic crystal naonobeam cavities on a solid substrate with significantly improved thermal and mechanical stabilities compared to conventional suspended ones. In addition, adding a PMMA-encapsulation layer on top of the SiN can significantly boost the Q-factor of the cavity mode. By dispersing perovskite nanocrystals as emitters in the PMMA layer, we obtained high-performance coherent emissions in terms of lasing threshold, linewidth and mode volumes. Our work offers a compelling way of creating solution-processed active integrated photonic devices based on the mature platform of silicon photonics for applications in optical information science and photonic quantum technology.

physics.optics↗

arXiv2020

Wave excitation by energetic ring-distributed electron beams in the solar corona

We analyzed properties of waves excited by mildly relativistic electron beams propagating along magnetic field with a ring-shape perpendicular momentum distribution in neutral and current-free solar coronal plasmas. These plasmas are subject to both the beam and the electron cyclotron maser (ECM) instabilities driven by the positive momentum gradient of the ring-beam electron distribution in the directions parallel and perpendicular to the ambient magnetic field, respectively. To explore the related kinetic processes self-consistently, 2.5-dimensional fully kinetic particle-in-cell (PIC) simulations were carried out. To quantify excited wave properties in different coronal conditions, we investigated the dependence of their energy and polarization on the ring-beam electron density and magnetic field. In general, electrostatic waves dominate the energetics of waves and nonlinear waves are ubiquitous. In weakly magnetized plasmas, where the electron cyclotron frequency $ω_{ce}$ is lower than the electron plasma frequency $ω_{pe}$, it is difficult to produce escaping electromagnetic waves with frequency $ω> ω_{pe}$ and small refractive index $|c k / ω| < 1$ ($k$ and $c$ are the wavenumber and the light speed, respectively). Highly polarized and anisotropic escaping electromagnetic waves can, however, be effectively excited in strongly magnetized plasmas with $ω_{ce}/ω_{pe} \geq 1$. The anisotropy of the energy, circular polarization degree (CPD), and spectrogram of these escaping electromagnetic waves strongly depend on the number density ratio of the ring-beam electrons to the background electrons. In particular, their CPDs can vary from left-handed to right-handed with the decrease of the ring-beam density, which may explain some observed properties of solar radio bursts (e.g., radio spikes) from the solar corona.

physics.plasm-ph↗

arXiv2019

Micromagnetometry of two-dimensional ferromagnets

The study of atomically thin ferromagnetic crystals has led to the discovery of unusual magnetic behaviour and provided insight into the magnetic properties of bulk materials. However, the experimental techniques that have been used to explore ferromagnetism in such materials cannot probe the magnetic field directly. Here, we show that ballistic Hall micromagnetometry can be used to measure the magnetization of individual two-dimensional ferromagnets. Our devices are made by van der Waals assembly in such a way that the investigated ferromagnetic crystal is placed on top of a multi-terminal Hall bar made from encapsulated graphene. We use the micromagnetometry technique to study atomically thin chromium tribromide (CrBr3). We find that the material remains ferromagnetic down to monolayer thickness and exhibits strong out-of-plane anisotropy. We also find that the magnetic response of CrBr3 varies little with the number of layers and its temperature dependence cannot be described by the simple Ising model of two-dimensional ferromagnetism.

cond-mat.mes-hall↗

arXiv2019

Coherent optical and spin spectroscopy of nanoscale Pr3+:Y2O3

We investigate the potential for optical quantum technologies of Pr3+:Y2O3 in the form of monodisperse spherical nanoparticles. We measured optical inhomogeneous lines of 27 GHz, and optical homogeneous linewidths of 108 kHz and 315 kHz in particles of 400 nm and 150 nm average diameters respectively for the 1D2(0)--> 3H4(0) transition at 1.4 K. Furthermore, ground state and 1D2 excited state hyperfine structures in Y2O3 are here for the first time determined by spectral hole burning and modeled by complete Hamiltonian calculations. Ground-state spin transitions have energies of 5.99 MHz and 10.42 MHz for which we demonstrate spin inhomogeneous linewidths of 42 and 45 kHz respectively. Spin T2 up to 880 microseconds was obtained for the +-3/2-->+-5/2 transition at 10.42 MHz, a value which exceeds that of bulk Pr3+ doped crystals so far reported. These promising results confirm nanoscale Pr3+:Y2O3 as a very appealing candidate to integrate quantum devices. In particular, we discuss here the possibility of using this material for realizing spin photon interfaces emitting indistinguishable single photons.

quant-ph↗

arXiv2019

Giant oscillations in a triangular network of one-dimensional states in marginally twisted graphene

The electronic properties of graphene superlattices have attracted intense interest that was further stimulated by the recent observation of novel many-body states at "magic" angles in twisted bilayer graphene (BLG). For very small ("marginal") twist angles of 0.1 deg, BLG has been shown to exhibit a strain-accompanied reconstruction that results in submicron-size triangular domains with the Bernal stacking. If the interlayer bias is applied to open an energy gap inside the domain regions making them insulating, marginally-twisted BLG is predicted to remain conductive due to a triangular network of chiral one-dimensional (1D) states hosted by domain boundaries. Here we study electron transport through this network and report giant Aharonov-Bohm oscillations persisting to temperatures above 100 K. At liquid helium temperatures, the network resistivity exhibits another kind of oscillations that appear as a function of carrier density and are accompanied by a sign-changing Hall effect. The latter are attributed to consecutive population of the flat minibands formed by the 2D network of 1D states inside the gap. Our work shows that marginally twisted BLG is markedly distinct from other 2D electronic systems, including BLG at larger twist angles, and offers a fascinating venue for further research.

cond-mat.mes-hall↗

arXiv2019

A near-ideal dechirper for plasma-based electron and positron acceleration using a hollow channel plasma

Plasma-based electron and positron wakefield acceleration has made great strides in the past decade. However one major challenge for its applications to coherent light sources and colliders is the relatively large energy spread of the accelerated beams, currently at a few percent level. This energy spread is usually correlated with particle position in the beam arising from the longitudinal chirp of the wakefield amplitude. Therefore a dechirper is highly desirable for reducing this spread down to $\sim0.1\%$ level, while at the same time for maintaining the emittance of the accelerated beam. Here we propose that a low-density hollow channel plasma can act as a near-ideal dechirper for both electrons and positrons. We demonstrate the concept through large-scale three-dimensional particle-in-cell simulations. We show that the initial positive correlated energy spread (chirp) on the beam exiting a plasma accelerator can be compensated by the nearly linear self-wake induced by the beam in the hollow channel from few percent level down to $\leq 0.1\%$. Meanwhile, the beam emittance can be preserved due to the negligible transverse field inside the channel. This passive method may significantly improve the beam quality of plasma-based accelerators, paving the way for their applications to future compact free electron lasers and colliders.

physics.acc-ph↗

arXiv2019

Strong magnetophonon oscillations in extra-large graphene

Van der Waals materials and their heterostructures offer a versatile platform for studying a variety of quantum transport phenomena due to their unique crystalline properties and the exceptional ability in tuning their electronic spectrum. However, most experiments are limited to devices that have lateral dimensions of only a few micrometres. Here, we perform magnetotransport measurements on graphene/hexagonal boron-nitride Hall bars and show that wider devices reveal additional quantum effects. In devices wider than ten micrometres we observe distinct magnetoresistance oscillations that are caused by resonant scattering of Landau-quantised Dirac electrons by acoustic phonons in graphene. The study allows us to accurately determine graphene's low energy phonon dispersion curves and shows that transverse acoustic modes cause most of phonon scattering. Our work highlights the crucial importance of device width when probing quantum effects and also demonstrates a precise, spectroscopic method for studying electron-phonon interactions in van der Waals heterostructures.

cond-mat.mes-hall↗

arXiv2019

Topological Electronic Structure and Its Temperature Evolution in Antiferromagnetic Topological Insulator MnBi2Te4

Topological quantum materials coupled with magnetism can provide a platform for realizing rich exotic physical phenomena, including quantum anomalous Hall effect, axion electrodynamics and Majorana fermions. However, these unusual effects typically require extreme experimental conditions such as ultralow temperature or sophisticate material growth and fabrication. Recently, new intrinsic magnetic topological insulators were proposed in MnBi2Te4-family compounds - on which rich topological effects could be realized under much relaxed experimental conditions. However, despite the exciting progresses, the detailed electronic structures observed in this family of compounds remain controversial up to date. Here, combining the use of synchrotron and laser light sources, we carried out comprehensive and high resolution angle-resolved photoemission spectroscopy studies on MnBi2Te4, and clearly identified its topological electronic structures including the characteristic gapless topological surface states. In addition, the temperature evolution of the energy bands clearly reveals their interplay with the magnetic phase transition by showing interesting differences for the bulk and surface states, respectively. The identification of the detailed electronic structures of MnBi2Te4 will not only help understand its exotic properties, but also pave the way for the design and realization of novel phenomena and applications.

cond-mat.mtrl-sci↗

arXiv2019

Nature of the Structural Symmetries Associated with Hybrid Improper Ferroelectricity in Ca3X2O7

In hybrid improper ferroelectric systems, polarization arises from the onset of successive nonpolar lattice modes. In this work, measurements and modeling were performed to determine the spatial symmetries of the phases involved in the transitions to these modes. Structural and optical measurements reveal that the tilt and rotation distortions of the MnO6 or TiO6 polyhedra relative to the high symmetry phases driving ferroelectricity in the hybrid improper Ca3X2O7 system (X=Mn and Ti) condense at different temperatures. The tilt angle vanishes abruptly at T$_T$ ~ 400 K for Ca3Mn2O7 (and continuously for X=Ti) and the rotation mode amplitude is suppressed at much higher temperatures T$_R$ ~1060 K. Moreover, Raman measurements in Ca3Mn2O7 under isotropic pressure reveal that the polyhedral tilts can be suppressed by very low pressures (between 1.4 and2.3 GPa) indicating their softness. These results indicate that the Ca3Mn2O7 system provides a new platform for strain engineering of ferroelectric properties in film based systems with substrate induced strain.

cond-mat.mtrl-sci↗