arXiv subjects

J. Chen

Publications and source records attributed to J. Chen.

At least 235 records · Page 13Linked to original sources

arXiv2017

Gapless spin-liquid ground state in the $S = 1/2$ kagome antiferromagnet

Frustrated quantum magnetism has moved to the forefront of physics research, posing fundamental questions concerning quantum disordered states, entanglement, topology and the nature of the quantum wavefunction. The defining problem in the field is one of the simplest, the ground state of the nearest-neighbour $S = 1/2$ antiferromagnetic Heisenberg model on the kagome lattice, but has defied all theoretical and numerical methods employed to date. We apply the formalism of tensor-network states (TNS), specifically the method of projected entangled simplex states (PESS), whose combination of a correct accounting for multipartite entanglement and infinite system size provides qualitatively new insight. By studying the ground-state energy, the staggered magnetization we find at all finite tensor bond dimensions and the effects of a second-neighbour coupling, we demonstrate that the ground state is a gapless spin liquid. We discuss the comparison with other numerical studies and the physical interpretation of the gapless ground state.

cond-mat.str-el↗

arXiv2017

Selective decay from a candidate of the $σ$-bond linear-chain state in $^{14}$C

A cluster-transfer experiment $^9$Be($^9$Be,$^{14}$C$^*\rightarrowα$+$^{10}$Be)$α$ was carried out using an incident beam energy of 45 MeV. This reaction channel has a large $Q$-value that favors populating the high-lying states in $^{14}$C and separating various reaction channels. A number of resonant states are reconstructed from the forward emitting $^{10}$Be + $α$ fragments with respect to three sets of well discriminated final states in $^{10}$Be, most of which agree with the previous observations. A state at 22.5(1) MeV in $^{14}$C is found to decay predominantly into the states around 6 MeV in $^{10}$Be daughter nucleus, in line with the unique property of the predicted band head of the $σ$-bond linear-chain molecular states. A new state at 23.5(1) MeV is identified which decays strongly into the first excited state of $^{10}$Be.

nucl-ex↗

arXiv2017

The Extinction Properties of and Distance to the Highly Reddened Type Ia Supernova SN 2012cu

Correction of Type Ia Supernova brightnesses for extinction by dust has proven to be a vexing problem. Here we study the dust foreground to the highly reddened SN 2012cu, which is projected onto a dust lane in the galaxy NGC 4772. The analysis is based on multi-epoch, spectrophotometric observations spanning 3,300 - 9,200 Å, obtained by the Nearby Supernova Factory. Phase-matched comparison of the spectroscopically twinned SN 2012cu and SN 2011fe across 10 epochs results in the best-fit color excess of (E(B-V), RMS) = (1.00, 0.03) and total-to-selective extinction ratio of (RV , RMS) = (2.95, 0.08) toward SN 2012cu within its host galaxy. We further identify several diffuse interstellar bands, and compare the 5780 Å band with the dust-to-band ratio for the Milky Way. Overall, we find the foreground dust-extinction properties for SN 2012cu to be consistent with those of the Milky Way. Furthermore we find no evidence for significant time variation in any of these extinction tracers. We also compare the dust extinction curve models of Cardelli et al. (1989), O'Donnell (1994), and Fitzpatrick (1999), and find the predictions of Fitzpatrick (1999) fit SN 2012cu the best. Finally, the distance to NGC4772, the host of SN 2012cu, at a redshift of z = 0.0035, often assigned to the Virgo Southern Extension, is determined to be 16.6$\pm$1.1 Mpc. We compare this result with distance measurements in the literature.

astro-ph.SR↗

arXiv2016

Bayesian Inference for State Space Models using Block and Correlated Pseudo Marginal Methods

This article addresses the problem of efficient Bayesian inference in dynamic systems using particle methods and makes a number of contributions. First, we develop a correlated pseudo-marginal (CPM) approach for Bayesian inference in state space (SS) models that is based on filtering the disturbances, rather than the states. This approach is useful when the state transition density is intractable or inefficient to compute, and also when the dimension of the disturbance is lower than the dimension of the state. Second, we propose a block pseudo-marginal (BPM) method that uses as the estimate of the likelihood the average of G independent unbiased estimates of the likelihood. We associate a set of underlying uniform of standard normal random numbers used to construct each of the individual unbiased likelihood estimates and then use component-wise Markov Chain Monte Carlo to update the parameter vector jointly with one set of these random numbers at a time. This induces a correlation of approximately 1-1/G between the logs of the estimated likelihood at the proposed and current values of the model parameters. Third, we show for some non-stationary state space models that the BPM approach is much more efficient than the CPM approach, because it is difficult to translate the high correlation in the underlying random numbers to high correlation between the logs of the likelihood estimates. Although our focus has been on applying the BPM method to state space models, our results and approach can be used in a wide range of applications of the PM method, such as panel data models, subsampling problems and approximate Bayesian computation.

stat.ME↗

arXiv2016

Charge dynamics of the antiferromagnetically ordered Mott insulator

We study the charge dynamics of the half-filled Hubbard model on the square lattice at zero temperature. We employ a slave-fermion formulation in which the charge degrees of freedom are represented by fermionic holons and doublons and the bosonic spin degrees of freedom are assumed to be described by the antiferromagnetic Heisenberg model. The ground state has long-ranged magnetic (Néel) order and its Mott-insulating characteristics are the consequence of holon-doublon bound-state formation. Within this framework, we calculate the average double occupancy, the electron density of states, and the spectral function in the self-consistent Born approximation. The lower and upper Hubbard bands are reproduced, there is spectral-weight transfer into a coherent quasiparticle band at their lower edges, and the Mott gap, associated with holon-doublon binding, is renormalized due to the interactions of the holons and doublons with the magnons. We compute the zeros of the Green function at the chemical potential to discuss the Luttinger volume and show that the poles of the self-energy reflect the underlying quasiparticle dispersion with a spin-renormalized hopping parameter. Optical conductivity results relate the optical gap to the Mott gap of the insulating system. We conclude that a self-consistent treatment of the spin fluctuation effects on the charge degrees of freedom captures all the essential physics of the antiferromagnetic Mott-Hubbard insulator.

cond-mat.str-el↗

arXiv2016

The decay characteristic of $^{22}$Si and its ground-state mass significantly affected by three-nucleon forces

The decay of the proton-rich nucleus $^{22}$Si was studied by a silicon array coupled with germanium clover detectors. Nine charged-particle groups are observed and most of them are recognized as $β$-delayed proton emission. A charged-particle group at 5600 keV is identified experimentally as $β$-delayed two-proton emission from the isobaric analog state of $^{22}$Al. Another charged-particle emission without any $β$ particle at the low energy less than 300 keV is observed. The half-life of $^{22}$Si is determined as 27.5 (18) ms. The experimental results of $β$-decay of $^{22}$Si are compared and in nice agreement with shell-model calculations. The mass excess of the ground state of $^{22}$Si deduced from the experimental data shows that three-nucleon (3N) forces with repulsive contributions have significant effects on nuclei near the proton drip line.

nucl-ex↗

arXiv2016

Beta-decay study of $T_z=-2$ proton-rich nucleus $^{20}$Mg

The $β$ decay of the drip-line nucleus $^{20}$Mg gives important information on resonances in $^{20}$Na, which are relevant for the astrophysical $rp$-process. A detailed $β$ decay spectroscopic study of $^{20}$Mg was performed by a continuous-implantation method. A detection system was specially developed for charged-particle decay studies, giving improved spectroscopic information including the half-life of $^{20}$Mg, the excitation energies, the branching ratios, and the log $ft$ values for the states in $^{20}$Na populated in the $β$ decay of $^{20}$Mg. A new proton branch was observed and the corresponding excited state in $^{20}$Na was proposed. The large isospin asymmetry for the mirror decays of $^{20}$Mg and $^{20}$O was reproduced, as well. However, no conclusive conclusion can be draw about the astrophysically interesting 2645~keV resonance in $^{20}$Na due to the limited statistics.

nucl-ex↗

arXiv2016

Cluster Decay of the High-lying excited states in $^{14}$C

A cluster-transfer experiment of $^9\rm{Be}(^9\rm{Be},^{14}\rm{C}\rightarrowα+^{10}\rm{Be})α$ at an incident energy of 45 MeV was carried out in order to investigate the molecular structure in high-lying resonant states in $^{14}$C. This reaction is of extremely large $Q$-value, making it an excellent case to select the reaction mechanism and the final states in outgoing nuclei. The high-lying resonances in $^{14}$C are reconstructed for three sets of well discriminated final states in $^{10}$Be. The results confirm the previous decay measurements with clearly improved decay-channel selections and show also a new state at 23.5(1) MeV. The resonant states at 22.4(3) and 24.0(3) MeV decay primarily into the typical molecular states at about 6 MeV in $^{10}$Be, indicating a well developed cluster structure in these high-lying states in $^{14}$C. Further measurements of more states of this kind are suggested.

nucl-ex↗

arXiv2016

Rigidity of down-up algebras with respect to finite group coactions

If a nontrivial finite group coacts on a graded noetherian down-up algebra $A$ inner faithfully and homogeneously, then the fixed subring is not isomorphic to $A$. Therefore graded noetherian down-up algebras are rigid with respect to finite group coactions, in the sense of Alev-Polo. An example is given to show that this rigidity under group coactions does not have all the same consequences as the rigidity under group actions.

math.RA↗

arXiv2016

Effect of further-neighbor interactions on the magnetization behaviors of the Ising model on a triangular lattice

In this work, we study the magnetization behaviors of the classical Ising model on the triangular lattice using Monte Carlo simulations, and pay particular attention to the effect of further-neighbor interactions. Several fascinating spin states are identified to be stabilized in certain magnetic field regions, respectively, resulting in the magnetization plateaus at 2/3, 5/7, 7/9 and 5/6 of the saturation magnetization MS, in addition to the well known plateaus at 0, 1/3 and 1/2 of MS. The stabilization of these interesting orders can be understood as the consequence of the competition between Zeeman energy and exchange energy.

cond-mat.str-el↗

arXiv2016

Heisenberg antiferromagnet on the Husimi lattice

We perform a systematic study of the antiferromagnetic Heisenberg model on the Husimi lattice using numerical tensor-network methods based on Projected Entangled Simplex States (PESS). The nature of the ground state varies strongly with the spin quantum number, $S$. For $S = 1/2$, it is an algebraic (gapless) quantum spin liquid. For $S = 1$, it is a gapped, non-magnetic state with spontaneous breaking of triangle symmetry (a trimerized simplex-solid state). For $S = 2$, it is a simplex-solid state with a spin gap and no symmetry-breaking; both integer-spin simplex-solid states are characterized by specific degeneracies in the entanglement spectrum. For $S = 3/2$, and indeed for all spin values $S \ge 5/2$, the ground states have $120$-degree antiferromagnetic order. In a finite magnetic field, we find that, irrespective of the value of $S$, there is always a plateau in the magnetization at $m = 1/3$.

cond-mat.str-el↗

arXiv2016

Anti-sites disordering suppression of the possible phase transition in Mn2CrGa

Theoretical and experimental characterizations of Mn2CrGa compound in regard to the possibility of phase transformation have been carried out in this work. Under a high ordering L21 structure, this compound has the potential to be a martensite phase transition material. However, experimental results show a severe disordering took place in this system, which forbids the occurring of the phase transition. This work provides important reference for the design of new phase transition materials in Heusler alloys.

cond-mat.mtrl-sci↗

arXiv2015

Improving Cosmological Distance Measurements Using Twin Type Ia Supernovae

We introduce a method for identifying "twin" Type Ia supernovae, and using them to improve distance measurements. This novel approach to Type Ia supernova standardization is made possible by spectrophotometric time series observations from the Nearby Supernova Factory (SNfactory). We begin with a well-measured set of supernovae, find pairs whose spectra match well across the entire optical window, and then test whether this leads to a smaller dispersion in their absolute brightnesses. This analysis is completed in a blinded fashion, ensuring that decisions made in implementing the method do not inadvertently bias the result. We find that pairs of supernovae with more closely matched spectra indeed have reduced brightness dispersion. We are able to standardize this initial set of SNfactory supernovae to 0.083 +/- 0.012 magnitudes, implying a dispersion of 0.072 +/- 0.010 magnitudes in the absence of peculiar velocities. We estimate that with larger numbers of comparison SNe, e.g, using the final SNfactory spectrophotometric dataset as a reference, this method will be capable of standardizing high-redshift supernovae to within 0.06-0.07 magnitudes. These results imply that at least 3/4 of the variance in Hubble residuals in current supernova cosmology analyses is due to previously unaccounted-for astrophysical differences among the supernovae

astro-ph.CO↗

arXiv2015

Understanding and modeling turbulent fluxes and entrainment in a gravity current

We present an experimental study of the mixing processes in a gravity current flowing on an inclined plane. The turbulent transport of momentum and density can be described in a very direct and compact form by a Prandtl mixing length model: the turbulent vertical fluxes of momentum and density are found to scale quadratically with the vertical mean gradients of velocity and density. The scaling coefficient, the square of the mixing length, is approximately constant over the mixing zone of the stratified shear layer. We show how, in different flow configurations, this length can be related to the shear length of the flow ($ε$/$\partial$ z u^3)^1/2. We also study the fluctuations of the momentum and density turbulent fluxes, showing how they relate to mixing and to the entrainment/detrainment balance. We suggest a quantitative measure of local entrainment and detrainment derived from observed conditional correlations of density flux and density or vertical velocity fluctuations.

physics.flu-dyn↗

arXiv2015

Semantic Modelling with Long-Short-Term Memory for Information Retrieval

In this paper we address the following problem in web document and information retrieval (IR): How can we use long-term context information to gain better IR performance? Unlike common IR methods that use bag of words representation for queries and documents, we treat them as a sequence of words and use long short term memory (LSTM) to capture contextual dependencies. To the best of our knowledge, this is the first time that LSTM is applied to information retrieval tasks. Unlike training traditional LSTMs, the training strategy is different due to the special nature of information retrieval problem. Experimental evaluation on an IR task derived from the Bing web search demonstrates the ability of the proposed method in addressing both lexical mismatch and long-term context modelling issues, thereby, significantly outperforming existing state of the art methods for web document retrieval task.

cs.IR↗

arXiv2014

Confirmation of a Star Formation Bias in Type Ia Supernova Distances and its Effect on Measurement of the Hubble Constant

Previously we used the Nearby Supernova Factory sample to show that SNe~Ia having locally star-forming environments are dimmer than SNe~Ia having locally passive environments.Here we use the \constitution\ sample together with host galaxy data from \GALEX\ to independently confirm that result. The effect is seen using both the SALT2 and MLCS2k2 lightcurve fitting and standardization methods, with brightness differences of $0.094 \pm 0.037\ \mathrm{mag}$ for SALT2 and $0.155 \pm 0.041\ \mathrm{mag}$ for MLCS2k2 with $R_V=2.5$. When combined with our previous measurement the effect is $0.094 \pm 0.025\ \mathrm{mag}$ for SALT2. If the ratio of these local SN~Ia environments changes with redshift or sample selection, this can lead to a bias in cosmological measurements. We explore this issue further, using as an example the direct measurement of $H_0$. \GALEX{} observations show that the SNe~Ia having standardized absolute magnitudes calibrated via the Cepheid period--luminosity relation using {\textit{HST}} originate in predominately star-forming environments, whereas only ~50% of the Hubble-flow comparison sample have locally star-forming environments. As a consequence, the $H_0$ measurement using SNe~Ia is currently overestimated. Correcting for this bias, we find a value of $H_0^{corr}=70.6\pm 2.6\ \mathrm{km\ s^{-1}\ Mpc^{-1}}$ when using the LMC distance, Milky Way parallaxes and the NGC~4258 megamaser as the Cepheid zeropoint, and $68.8\pm 3.3\ \mathrm{km\ s^{-1}\ Mpc^{-1}}$ when only using NGC~4258. Our correction brings the direct measurement of $H_0$ within $\sim 1\,σ$ of recent indirect measurements based on the CMB power spectrum.

astro-ph.CO↗

arXiv2014

Proton radiography of magnetic field produced by ultra-intense laser irradiating capacity-coil target

Ultra-intense ultra-short laser is firstly used to irradiate the capacity-coil target to generate magnetic field. The spatial structure and temporal evolution of huge magnetic fields were studied with time-gated proton radiography method. A magnetic flux density of 40T was measured by comparing the proton deflection and particle track simulations. Although the laser pulse duration is only 30fs, the generated magnetic field can last for over 100 picoseconds. The energy conversion efficiency from laser to magnetic field can reach as high as ~20%. The results indicate that tens of tesla (T) magnetic field could be produced in many ultra intense laser facilities around the world, and higher magnetic field could be produced by picosecond lasers.

physics.plasm-ph↗

arXiv2014

A metric space for type Ia supernova spectra

We develop a new framework for use in exploring Type Ia Supernova (SN Ia) spectra. Combining Principal Component Analysis (PCA) and Partial Least Square analysis (PLS) we are able to establish correlations between the Principal Components (PCs) and spectroscopic/photometric SNe Ia features. The technique was applied to ~120 supernova and ~800 spectra from the Nearby Supernova Factory. The ability of PCA to group together SNe Ia with similar spectral features, already explored in previous studies, is greatly enhanced by two important modifications: (1) the initial data matrix is built using derivatives of spectra over the wavelength, which increases the weight of weak lines and discards extinction, and (2) we extract time evolution information through the use of entire spectral sequences concatenated in each line of the input data matrix. These allow us to define a stable PC parameter space which can be used to characterize synthetic SN Ia spectra by means of real SN features. Using PLS, we demonstrate that the information from important previously known spectral indicators (namely the pseudo-equivalent width (pEW) of Si II 5972 / Si II 6355 and the line velocity of S II 5640 / Si II 6355) at a given epoch, is contained within the PC space and can be determined through a linear combination of the most important PCs. We also show that the PC space encompasses photometric features like B or V magnitudes, B-V color and SALT2 parameters c and x1. The observed colors and magnitudes, that are heavily affected by extinction, cannot be reconstructed using this technique alone. All the above mentioned applications allowed us to construct a metric space for comparing synthetic SN Ia spectra with observations.

astro-ph.SR↗