arXiv subjects

H. He

Publications and source records attributed to H. He.

At least 37 records · Page 2Linked to original sources

arXiv2020

Correlations and incipient antiferromagnetic order within the linear Mn chains of metallic Ti$_4$MnBi$_2$

We report measurements on Ti$_4$MnBi$_2$, where a crystal structure involving linear chains of Mn ions suggests one-dimensional magnetic character. The electrical resistivity is metallic, consistent with the results of electronic structure calculations that find a robust Fermi surface albeit with moderate electronic correlations. Curie-Weiss fit to the magnetic susceptibility finds that the Mn moments are in the low-spin $S = 1/2$ configuration. Neutron diffraction measurements detect weak antiferromagnetic order within the Mn chains, with further evidence for the small staggered moment coming from the entropy associated with the ordering peak in the specific heat as well as from the results of spin-polarized electronic structure calculations. The antiferromagnetic moments are apparently associated with the $d_{x^{2}-y^{2}}$ and $d_{xy}$ orbitals of Mn while the remaining Mn orbitals are delocalized. Strong quantum fluctuations, possibly related to an electronic instability that forms the Mn moment or to the one-dimensional character of Ti$_4$MnBi$_2$, nearly overcome magnetic order.

cond-mat.str-el↗

arXiv2019

Towards quantum-limited coherent detection of terahertz waves in charge-neutral graphene

Spectacular advances in heterodyne astronomy with both the Herschel Space Observatory and Stratospheric Observatory for Far Infrared Astronomy (SOFIA) have been largely due to breakthroughs in detector technology. In order to exploit the full capacity of future THz telescope space missions (e.g. Origins Space Telescope), new concepts of THz coherent receivers are needed, providing larger bandwidths and imaging capabilities with multi-pixel focal plane heterodyne arrays. Here we show that graphene, uniformly doped to the Dirac point, enables highly sensitive and wideband coherent detection of THz signals. With material resistance dominated by quantum localization, and thermal relaxation governed by electron diffusion, proof-of-concept graphene bolometers demonstrate a gain bandwidth of 8 GHz and a mixer noise temperature of 475 K, limited by residual thermal background in our setup. An optimized device will result in a mixer noise temperature as low as 36 K, with the gain bandwidth exceeding 20 GHz, and a Local Oscillator power lower than 100 pW. In conjunction with the emerging quantum-limited amplifiers at the intermediate frequency, our approach promises quantum-limited sensing in the THz domain, potentially surpassing superconducting technologies, particularly for large heterodyne arrays.

cond-mat.mes-hall↗

arXiv2018

Scaling Laws of Ion Acceleration in Ultrathin Foils Driven by Laser Radiation Pressure

Scaling laws of ion acceleration in ultrathin foils driven by radiation pressure of intense laser pulses are investigated by theoretical analysis and two-dimensional particle-in-cell simulations. Considering the instabilities are inevitable during laser plasma interaction, the maximum energy of ions should have two contributions: the bulk acceleration driven by radiation pressure and the sheath acceleration in the moving foil reference induced by hot electrons. A theoretical model is proposed to quantitatively explain the results that the cutoff energy and energy spread are larger than the predictions of "light sail" model, observed in simulations and experiments for a large range of laser and target parameters. Scaling laws derived from this model and supported by the simulation results are verified by the previous experiments.

physics.plasm-ph↗

arXiv2018

Chiral transport and electronic correlations in surface states of HfNiSn single crystals

In most topological insulators, the valence and conduction band appear in reverse or inverted order compared to an equivalent insulator with isolated atoms. Here, we explore a different route towards topologically nontrivial states that may arise from metallic states present on the surface of bulk insulators without such band inversion. High-quality single crystals of HfNiSn show surface transport with weak anti-localization, consistent with a two-dimensional metallic state in the presence of strong spin-orbit coupling. Nonlinear I(V) curves indicate electronic correlations related to a chiral, nonlocal transport component that is qualitatively similar to a quantum Hall edge state, yet in the absence of external magnetic fields. The correlations themselves may play a decisive role in creating an apparent topologically nontrivial state on the HfNiSn surface.

cond-mat.mtrl-sci↗

arXiv2016

Magnetic activity analysis for a sample of G-type main sequence \emph{Kepler} targets

The variation of a stellar light curve owing to the rotational modulation by the magnetic features (starspots and faculae) on the star's surface can be used to investigate the magnetic properties of the host star. In this paper, we use the periodicity and magnitude of the light-curve variation, as two proxies, suggested by (He et al. 2015), to study the stellar magnetic properties for a large sample of G-type main sequence \emph{Kepler} targets, for which the rotation periods recently determined by (McQuillan et al. 2014). By analyzing the correlation between the two magnetic proxies, it is found that: (1) The two proxies are positively correlated for most of the stars in our sample, and the percentages of negative, zero, and positive correlation are $4.27\%$, $6.81\%$, and $88.91\%$, respectively; (2) Negative correlation stars cannot have large magnitude of light-curve variation; (3) with the increase of rotation period, the relative number of positive correlation stars decreases and the negative correlation one increases. These results indicate that the stars with shorter rotation period tend to have positive correlation between the two proxies, and a good portion of the positive correlation stars have larger magnitude of light-curve variation (and hence more intense magnetic activities) than the negative correlation stars.

astro-ph.SR↗

arXiv2015

Operation of graphene quantum Hall resistance standard in a cryogen-free table-top system

We demonstrate quantum Hall resistance measurements with metrological accuracy in a small cryogen-free system operating at a temperature of around 3.8K and magnetic fields below 5T. Operating this system requires little experimental knowledge or laboratory infrastructure, thereby greatly advancing the proliferation of primary quantum standards for precision electrical metrology. This significant advance in technology has come about as a result of the unique properties of epitaxial graphene on SiC.

cond-mat.mes-hall↗

arXiv2011

Efficient Reinforcement Learning Using Recursive Least-Squares Methods

The recursive least-squares (RLS) algorithm is one of the most well-known algorithms used in adaptive filtering, system identification and adaptive control. Its popularity is mainly due to its fast convergence speed, which is considered to be optimal in practice. In this paper, RLS methods are used to solve reinforcement learning problems, where two new reinforcement learning algorithms using linear value function approximators are proposed and analyzed. The two algorithms are called RLS-TD(lambda) and Fast-AHC (Fast Adaptive Heuristic Critic), respectively. RLS-TD(lambda) can be viewed as the extension of RLS-TD(0) from lambda=0 to general lambda within interval [0,1], so it is a multi-step temporal-difference (TD) learning algorithm using RLS methods. The convergence with probability one and the limit of convergence of RLS-TD(lambda) are proved for ergodic Markov chains. Compared to the existing LS-TD(lambda) algorithm, RLS-TD(lambda) has advantages in computation and is more suitable for online learning. The effectiveness of RLS-TD(lambda) is analyzed and verified by learning prediction experiments of Markov chains with a wide range of parameter settings. The Fast-AHC algorithm is derived by applying the proposed RLS-TD(lambda) algorithm in the critic network of the adaptive heuristic critic method. Unlike conventional AHC algorithm, Fast-AHC makes use of RLS methods to improve the learning-prediction efficiency in the critic. Learning control experiments of the cart-pole balancing and the acrobot swing-up problems are conducted to compare the data efficiency of Fast-AHC with conventional AHC. From the experimental results, it is shown that the data efficiency of learning control can also be improved by using RLS methods in the learning-prediction process of the critic. The performance of Fast-AHC is also compared with that of the AHC method using LS-TD(lambda). Furthermore, it is demonstrated in the experiments that different initial values of the variance matrix in RLS-TD(lambda) are required to get better performance not only in learning prediction but also in learning control. The experimental results are analyzed based on the existing theoretical work on the transient phase of forgetting factor RLS methods.

cs.LG↗

arXiv2005

Progress in Three-Dimensional Coherent X-Ray Diffraction Imaging

The Fourier inversion of phased coherent diffraction patterns offers images without the resolution and depth-of-focus limitations of lens-based tomographic systems. We report on our recent experimental images inverted using recent developments in phase retrieval algorithms, and summarize efforts that led to these accomplishments. These include ab-initio reconstruction of a two-dimensional test pattern, infinite depth of focus image of a thick object, and its high-resolution (~10 nm resolution) three-dimensional image. Developments on the structural imaging of low density aerogel samples are discussed.

physics.optics↗

arXiv2005

High-resolution ab initio three-dimensional X-ray diffraction microscopy

Coherent X-ray diffraction microscopy is a method of imaging non-periodic isolated objects at resolutions only limited, in principle, by the largest scattering angles recorded. We demonstrate X-ray diffraction imaging with high resolution in all three dimensions, as determined by a quantitative analysis of the reconstructed volume images. These images are retrieved from the 3D diffraction data using no a priori knowledge about the shape or composition of the object, which has never before been demonstrated on a non-periodic object. We also construct 2D images of thick objects with infinite depth of focus (without loss of transverse spatial resolution). These methods can be used to image biological and materials science samples at high resolution using X-ray undulator radiation, and establishes the techniques to be used in atomic-resolution ultrafast imaging at X-ray free-electron laser sources.

physics.optics↗

arXiv2004

Use of extended and prepared reference objects in experimental Fourier transform X-ray holography

The use of one or more gold nanoballs as reference objects for Fourier Transform holography (FTH) is analysed using experimental soft X-ray diffraction from objects consisting of separated clusters of these balls. The holograms are deconvoluted against ball reference objects to invert to images, in combination with a Wiener filter to control noise. A resolution of ~30nm, smaller than one ball, is obtained even if a large cluster of balls is used as the reference, giving the best resolution yet obtained by X-ray FTH. Methods of dealing with missing data due to a beamstop are discussed. Practical prepared objects which satisfy the FTH condition are suggested, and methods of forming them described.

physics.optics↗

arXiv2004

Phasing diffuse scattering. Application of the SIR2002 algorithm to the non-crystallographic phase problem

A new phasing algorithm has been used to determine the phases of diffuse elastic X-ray scattering from a non-periodic array of gold balls of 50 nm diameter. Two-dimensional real-space images, showing the charge-density distribution of the balls, have been reconstructed at 50 nm resolution from transmission diffraction patterns recorded at 550 eV energy. The reconstructed image fits well with scanning electron microscope (SEM) image of the same sample. The algorithm, which uses only the density modification portion of the SIR2002 program, is compared with the results obtained via the Gerchberg-Saxton-Fienup HIO algorithm. In this way the relationship between density modification in crystallography and the HiO algorithm used in signal and image processing is elucidated.

physics.optics↗

arXiv2004

SPEDEN: Reconstructing single particles from their diffraction patterns

Speden is a computer program that reconstructs the electron density of single particles from their x-ray diffraction patterns, using a single-particle adaptation of the Holographic Method in crystallography. (Szoke, A., Szoke, H., and Somoza, J.R., 1997. Acta Cryst. A53, 291-313.) The method, like its parent, is unique that it does not rely on ``back'' transformation from the diffraction pattern into real space and on interpolation within measured data. It is designed to deal successfully with sparse, irregular, incomplete and noisy data. It is also designed to use prior information for ensuring sensible results and for reliable convergence. This article describes the theoretical basis for the reconstruction algorithm, its implementation and quantitative results of tests on synthetic and experimentally obtained data. The program could be used for determining the structure of radiation tolerant samples and, eventually, of large biological molecular structures without the need for crystallization.

physics.optics↗

arXiv2003

X-ray image reconstruction from a diffraction pattern alone

A solution to the inversion problem of scattering would offer aberration-free diffraction-limited 3D images without the resolution and depth-of-field limitations of lens-based tomographic systems. Powerful algorithms are increasingly being used to act as lenses to form such images. Current image reconstruction methods, however, require the knowledge of the shape of the object and the low spatial frequencies unavoidably lost in experiments. Diffractive imaging has thus previously been used to increase the resolution of images obtained by other means. We demonstrate experimentally here a new inversion method, which reconstructs the image of the object without the need for any such prior knowledge.

physics.optics↗

arXiv2003

Coherent X-ray Diffractive Imaging; applications and limitations

The inversion of a diffraction pattern offers aberration-free diffraction-limited 3D images without the resolution and depth-of-field limitations of lens-based tomographic systems, the only limitation being radiation damage. We review our experimental results, discuss the fundamental limits of this technique and future plans.

physics.optics↗

arXiv2002

Magnetic Order and Dynamics in an Orbitally Degenerate Ferromagnetic Insulator

Neutron scattering was used to determine the spin structure and the magnon spectrum of the Mott--Hubbard insulator YTiO$_3$. The magnetic structure is complex, comprising substantial G-type and A-type antiferromagnetic components in addition to the predominant ferromagnetic component. The magnon spectrum, on the other hand, is gapless and nearly isotropic. We show that these findings are inconsistent with the orbitally ordered states thus far proposed for YTiO$_3$ and discuss general implications for a theoretical description of exchange interactions in orbitally degenerate systems.

cond-mat.str-el↗

arXiv2002

Structural and Magnetic Instabilities of La$_{2-x}$Sr$_x$CaCu$_2$O$_6$

A neutron scattering study of nonsuperconducting La$_{2-x}$Sr$_x$CaCu$_2$O$_6$ (x=0 and 0.2), a bilayer copper oxide without CuO chains, has revealed an unexpected tetragonal-to-orthorhombic transition with a doping dependent transition temperature. The predominant structural modification below the transition is an in-plane shift of the apical oxygen. In the doped sample, the orthorhombic superstructure is strongly disordered, and a glassy state involving both magnetic and structural degrees of freedom develops at low temperature. The spin correlations are commensurate.

cond-mat.supr-con↗

arXiv2002

Magnetic Resonant Mode in the Single-Layer High Temperature Superconductor Tl$_2$Ba$_2$Cu$_{6+δ}$

An unusual spin excitation mode observed by neutron scattering has inspired numerous theoretical studies of the interplay between charged quasiparticles and collective spin excitations in the copper oxide high temperature superconductors. The mode has thus far only been observed in materials with crystal structures consisting of copper oxide bilayers, and it is notably absent in the single-layer compound La$_{2-x}$Sr$_{x}$CuO$_{4+δ}$. Neutron scattering data now show that the mode is present in Tl$_2$Ba$_2$Cu$_{6+δ}$, a single-layer compound with T$_c$ $\sim$ 90 K, thus demonstrating that it is a generic feature of the copper oxide superconductors, independent of the layer sequence. This restricts the theoretical models for the origin of the resonant mode and its role in the mechanism of high temperature superconductivity.

cond-mat.supr-con↗

arXiv2001

Coherent molecular solitons in Bose-Einstein condensates

We analyze the coherent formation of molecular BEC from an atomic BEC, using a parametric field theory approach. We point out the transition between a quantum soliton regime, where atoms couple in a local way - to a classical soliton domain, where a stable coupled-condensate soliton can form in three dimensions. This gives the possibility of an intense, stable atom-laser output.

cond-mat.stat-mech↗