arXiv subjects

Song Li

Publications and source records attributed to Song Li.

At least 109 records · Page 6Linked to original sources

arXiv2019

Topological phase transition induced by magnetic proximity effect in two dimensions

We study the magnetic proximity effect on a two-dimensional topological insulator in a CrI$_3$/SnI$_3$/CrI$_3$ trilayer structure. From first-principles calculations, the BiI$_3$-type SnI$_3$ monolayer without spin-orbit coupling has Dirac cones at the corners of the hexagonal Brillouin zone. With spin-orbit coupling turned on, it becomes a topological insulator, as revealed by a non-vanishing $Z_2$ invariant and an effective model from symmetry considerations. Without spin-orbit coupling, the Dirac points are protected if the CrI$_3$ layers are stacked ferromagnetically, and are gapped if the CrI$_3$ layers are stacked antiferromagnetically, which can be explained by the irreducible representations of the magnetic space groups $C_{3i}^1$ and $C_{3i}^1(C_3^1)$, corresponding to ferromagnetic and antiferromagnetic stacking, respectively. By analyzing the effective model including the perturbations, we find that the competition between the magnetic proximity effect and spin-orbit coupling leads to a topological phase transition between a trivial insulator and a topological insulator.

cond-mat.mtrl-sci↗

arXiv2019

Bottleneck Supervised U-Net for Pixel-wise Liver and Tumor Segmentation

In this paper, we propose a bottleneck supervised (BS) U-Net model for liver and tumor segmentation. Our main contributions are: first, we propose a variation of the original U-Net that incorporates dense modules, inception modules and dilated convolution in the encoding path; second, we propose a bottleneck supervised (BS) U-Net that contains an encoding U-Net and a segmentation U-Net. To train the BS U-Net, the encoding U-Net is first trained to get encodings of the label maps that contain the anatomical information (shape and location). Subsequently, this information is used to guide the training of the segmentation U-Net so as to reserve the anatomical features of the target objects. More specifically, the loss function for segmentation U-Net is set to be the weighted average of the dice loss and the MSE loss between the encodings and the bottleneck feature vectors. The model is applied to a public liver and tumor CT scan dataset. Experimental results show that besides achieving excellent overall segmentation performance, BS U-Net also works great in controlling shape distortion, reducing false positive and false negative cases.

cs.CV↗

arXiv2019

Generalized Elliptical Slice Sampling with Regional Pseudo-priors

In this paper, we propose a MCMC algorithm based on elliptical slice sampling with the purpose to improve sampling efficiency. During sampling, a mixture distribution is fitted periodically to previous samples. The components of the mixture distribution are called regional pseudo-priors because each component serves as the pseudo-prior for a subregion of the sampling space. Expectation maximization algorithm, variational inference algorithm and stochastic approximation algorithm are used to estimate the parameters. Meanwhile, parallel computing is used to relieve the burden of computation. Ergodicity of the proposed algorithm is proven mathematically. Experimental results on one synthetic and two real-world dataset show that the proposed algorithm has the following advantages: with the same starting points, the proposed algorithm can find more distant modes; the proposed algorithm has lower rejection rates; when doing Bayesian inference for uni-modal posterior distributions, the proposed algorithm can give more accurate estimations; when doing Bayesian inference for multi-modal posterior distributions, the proposed algorithm can find different modes well, and the estimated means of the mixture distribution can provide additional information for the location of modes.

stat.CO↗

arXiv2019

Oxygenated (113) diamond surface for nitrogen-vacancy quantum sensors with preferential alignment and long coherence time from first principles

Shallow nitrogen-vacancy (NV) center in diamond is promising in quantum sensing applications however its sensitivity has been limited by surface terminators and defects. There is an immediate quest to find suitable diamond surfaces for NV sensors. In this work, the surface terminators of (113) diamond to host shallow NV centers are studied by means of first principles calculations. Results indicate that complete oxygen termination of (113) diamond creates positive electron affinity with neither strain on the surface nor in-gap levels. This is a very surprising result as the commonly employed oxygenated (001) diamond surface is often defective due to the disorder created by the strain of ether groups at the surface that seriously undermine the coherence properties of the shallow NV centers. The special atomic configurations on (113) diamond surface are favorable for oxygen bonding, in contrast to (001) and (111) diamond surfaces. These simulations imply that oxygenated diamond (113) surface can be produced by conventional diamond chemical vapor deposition growth. Combining this with the ~73% preferential alignment of as-grown NV centers in (113) oriented diamond, oxygenated (113) diamond is presently supposed to be the most prospective host for NV quantum sensors.

cond-mat.mes-hall↗

arXiv2018

Interfacial spin Hall effect and spin swapping in Fe|Au bilayer from first principles

The interfaces in hybridized structures could give rise to rich phenomena, which open the way to novel devices with extraordinary performance. Here, we investigate the interface-related spin transport properties in Fe|Au bilayer based on first-principle calculation. We find that the spin Hall current in the Au side near the interface flows in the opposite direction to the bulk spin Hall current with the magnitude sensitive to the magnetization direction of Fe. This negative interfacial contribution is attributed to the spin dependent transmission within a few atomic layers, where a strong interfacial Rashba spin-orbit coupling exists. Surprisingly, the interfacial spin Hall currents are found to be not confined at the interface but extend tens of nanometers at low temperature, which is limited by momentum scattering and therefore much shorter than the spin diffusion length. In addition, the interfacial swapping spin currents, as a consequence of the spin precession under the interfacial Rashba field, are also obtained from our calculation and complete the full spin transport picture with all non-vanishing components. Our results suggest the importance of the interface engineering in spin-related transport in ferromagnetic|non-magnetic heterostructures and the possibility of manipulating the interfacial transport by the magnetization orientation of the magnetic layer.

cond-mat.mes-hall↗

arXiv2018

Statefinder diagnostic and constraints on the Palatini f(R) gravity theories

We focus on a series of $f(R)$ gravity theories in Palatini formalism to investigate the probabilities of producing the late-time acceleration for the flat Friedmann-Robertson-Walker (FRW) universe. We apply statefinder diagnostic to these cosmological models for chosen series of parameters to see if they distinguish from one another. The diagnostic involves the statefinder pair $\{r,s\}$, where $r$ is derived from the scale factor $a$ and its higher derivatives with respect to the cosmic time $t$, and $s$ is expressed by $r$ and the deceleration parameter $q$. In conclusion, we find that although two types of $f(R)$ theories: (i) $f(R) = R + αR^m - βR^{-n}$ and (ii) $f(R) = R + α\ln R - β$ can lead to late-time acceleration, their evolutionary trajectories in the $r-s$ and $r-q$ planes reveal different evolutionary properties, which certainly justify the merits of statefinder diagnostic. Additionally, we utilize the observational Hubble parameter data (OHD) to constrain these models of $f(R)$ gravity. As a result, except for $m=n=1/2$ of (i) case, $α=0$ of (i) case and (ii) case allow $Λ$CDM model to exist in 1$σ$ confidence region. After adopting statefinder diagnostic to the best-fit models, we find that all the best-fit models are capable of going through deceleration/acceleration transition stage with late-time acceleration epoch, and all these models turn to de-Sitter point ($\{r,s\}=\{1,0\}$) in the future. Also, the evolutionary differences between these models are distinct, especially in $r-s$ plane, which makes the statefinder diagnostic more reliable in discriminating cosmological models.

gr-qc↗

arXiv2017

Deterministic Browser

Timing attacks have been a continuous threat to users' privacy in modern browsers. To mitigate such attacks, existing approaches, such as Tor Browser and Fermata, add jitters to the browser clock so that an attacker cannot accurately measure an event. However, such defenses only raise the bar for an attacker but do not fundamentally mitigate timing attacks, i.e., it just takes longer than previous to launch a timing attack. In this paper, we propose a novel approach, called deterministic browser, which can provably prevent timing attacks in modern browsers. Borrowing from Physics, we introduce several concepts, such as an observer and a reference frame. Specifically, a snippet of JavaScript, i.e., an observer in JavaScript reference frame, will always obtain the same, fixed timing information so that timing attacks are prevented; at contrast, a user, i.e., an oracle observer, will perceive the JavaScript differently and do not experience the performance slowdown. We have implemented a prototype called DeterFox and our evaluation shows that the prototype can defend against browser-related timing attacks.

cs.CR↗

arXiv2016

A Dual Field-of-View Zoom Metalens

Conventional optical zoom system is bulky, expensive and complicated for real time adjustment. Recent progress in the metasurface research has provided a new solution to achieve innovative compact optical systems. In this paper, we propose a highly integrated zoom lens with dual field-of-view (FOV) based on double sided metasurfaces. With silicon nanobrick arrays of spatially varying orientations sitting on both side of a transparent substrate, this ultrathin zoom metalens can be designed to focus an incident circular polarized beam with spin-dependent FOVs without varying the focal plane, which is important for practical applications. The proposed dual FOV zoom metalens, with the advantages such as ultracompactness, flexibility and replicability, can find applications in fields which require ultracompact zoom imaging and beam focusing.

physics.optics↗

arXiv2016

Constraints on non-flat cosmologies with massive neutrinos after Planck 2015

We investigate two dark energy cosmological models (i.e., the $Λ$CDM and $ϕ$CDM models) with massive neutrinos assuming two different neutrino mass hierarchies in both the spatially flat and non-flat scenarios, where in the $ϕ$CDM model the scalar field possesses an inverse power-law potential, $V(ϕ)\propto ϕ^{-α}$ ($α>0$). Cosmic microwave background data from Planck 2015, baryon acoustic oscillations data from 6dFGS, SDSS-MGS, BOSS-LOWZ and BOSS CMASS-DR11, the JLA compilation of Type Ia supernova apparent magnitude observations, and the Hubble Space Telescope $H_0$ prior, are jointly employed to constrain the model parameters. We first determine constraints assuming three species of degenerate massive neutrinos. In the spatially flat (non-flat) $Λ$CDM model, the sum of neutrino masses is bounded as $Σm_ν < 0.165 (0.299)$ eV at 95% confidence level (CL). Correspondingly, in the flat (non-flat) $ϕ$CDM model, we find $Σm_ν < 0.164 (0.301)$ eV at 95% CL. The inclusion of spatial curvature as a free parameter results in a significant broadening of confidence regions for $Σm_ν$ and other parameters. In the scenario where the total neutrino mass is dominated by the heaviest neutrino mass eigenstate, we can obtain the similar conclusions as those obtained in the degenerate neutrino mass scenario. In addition, the results show that the bounds on $Σm_ν$ based on two different neutrino mass hierarchies have insignificant differences in the spatially flat case for both the $Λ$CDM and $ϕ$CDM models, however, the corresponding differences are larger in the non-flat case.

astro-ph.CO↗

arXiv2016

Restricted $q$-Isometry Properties Adapted to Frames for Nonconvex $l_q$-Analysis

This paper discusses reconstruction of signals from few measurements in the situation that signals are sparse or approximately sparse in terms of a general frame via the $l_q$-analysis optimization with $0<q\leq 1$. We first introduce a notion of restricted $q$-isometry property ($q$-RIP) adapted to a dictionary, which is a natural extension of the standard $q$-RIP, and establish a generalized $q$-RIP condition for approximate reconstruction of signals via the $l_q$-analysis optimization. We then determine how many random, Gaussian measurements are needed for the condition to hold with high probability. The resulting sufficient condition is met by fewer measurements for smaller $q$ than when $q=1$. The introduced generalized $q$-RIP is also useful in compressed data separation. In compressed data separation, one considers the problem of reconstruction of signals' distinct subcomponents, which are (approximately) sparse in morphologically different dictionaries, from few measurements. With the notion of generalized $q$-RIP, we show that under an usual assumption that the dictionaries satisfy a mutual coherence condition, the $l_q$ split analysis with $0<q\leq1 $ can approximately reconstruct the distinct components from fewer random Gaussian measurements with small $q$ than when $q=1$

cs.IT↗

arXiv2015

Comparison of cosmological models using standard rulers and candles

In this paper, we used standard rulers and standard candles (separately and jointly) to explore five popular dark energy models under assumption of spatial flatness of the Universe. As standard rulers, we used a data set comprising 118 galactic-scale strong lensing systems (individual standard rulers if properly calibrated for the mass density profile) combined with BAO diagnostics (statistical standard ruler). Supernovae Ia served asstandard candles. Unlike in the most of previous statistical studies involving strong lensing systems, we relaxed the assumption of singular isothermal sphere (SIS) in favor of its generalization: the power-law mass density profile. Therefore, along with cosmological model parameters we fitted the power law index and its first derivative with respect to the redshift (thus allowing for mass density profile evolution). It turned out that the best fitted $γ$ parameters are in agreement with each other irrespective of the cosmological model considered. This demonstrates that galactic strong lensing systems may provide a complementary probe to test the properties of dark energy. Fits for cosmological model parameters which we obtained are in agreement with alternative studies performed by the others. Because standard rulers and standard candles have different parameter degeneracies, combination of standard rulers and standard candles gives much more restrictive results for cosmological parameters. At last, we attempted at model selection based on information theoretic criteria (AIC and BIC). Our results support the claim, that cosmological constant model is still the best one and there is no (at least statistical) reason to prefer any other more complex model.

astro-ph.CO↗

arXiv2015

Cosmological evolution of generalized non-local gravity

We construct a class of generalized non-local gravity (GNLG) model which is the modified theory of general relativity (GR) obtained by adding a term $m^{2n-2} R\Box^{-n}R$ to the Einstein-Hilbert action. Concretely, we not only study the gravitational equation for the GNLG model by introducing auxiliary scalar fields, but also analyse the classical stability and examine the cosmological consequences of the model for different exponent $n$. We find that the half of the scalar fields are always ghost-like and the exponent $n$ must be taken even number for a stable GNLG model. Meanwhile, the model spontaneously generates three dominant phases of the evolution of the universe, and the equation of state parameters turn out to be phantom-like. Furthermore, we clarify in another way that exponent $n$ should be even numbers by discuss the spherically symmetric static solutions in Newtonian gauge. It is worth stressing that the results given by us can include ones in refs. [28, 34] as the special case of $n=2$.

gr-qc↗

arXiv2014

Interfacial Ionic Liquids: Connecting Static and Dynamic Structures

It is well-known that room temperature ionic liquids (RTILs) often adopt a charge-separated layered structure, i.e., with alternating cation- and anion-rich layers, at electrified interfaces. However, the dynamic response of the layered structure to temporal variations in applied potential is not well understood. We used in situ, real-time X-ray reflectivity (XR) to study the potential-dependent electric double layer (EDL) structure of an imidazolium-based RTIL on charged epitaxial graphene during potential cycling as a function of temperature. The results suggest that the graphene-RTIL interfacial structure is bistable in which the EDL structure at any intermediate potential can be described by the combination of two extreme-potential structures whose proportions vary depending on the polarity and magnitude of the applied potential. This picture is supported by the EDL structures obtained by fully atomistic molecular dynamics (MD) simulations at various static potentials. The potential-driven transition between the two structures is characterized by an increasing width but with an approximately fixed hysteresis magnitude as a function of temperature. The results are consistent with the coexistence of distinct anion and cation adsorbed structures separated by an energy barrier (~0.15 eV).

cond-mat.soft↗

arXiv2014

Quasi-monoenergetic Electron Beams from Laser-plasma Acceleration by Ionization-induced Injection in Low- density Pure Nitrogen

We report a laser wakefield acceleration of electron beams up to 130 MeV from laser-driven 4-mm long nitrogen gas jet. By using a moderate laser intensity (3.5*10^18 W.cm^(-2)) and relatively low plasma densities (0.8*10^18 cm^(-3) to 2.7*10^18 cm^(-3)) we have achieved a stable regime for laser propagation and consequently a stable generation of electron beams. We experimentally studied the dependence of the drive laser energy on the laser-plasma channel and electron beam parameters. The quality of the generated electron beams is discussed within the framework of the ionization-induced injection mechanism.

physics.plasm-ph↗

arXiv2014

Enhanced electron yield from a laser-plasma accelerator using high-Z gas jet targets

An investigation of the multi-hundred MeV electron beam yield (charge) form helium, nitrogen, neon and argon gas jet plasmas in a laser-plasma wakefield acceleration experiment was carried out. The charge measurement has been made via imaging the electron beam intensity profile on a fluorescent screen into a 14-bit charge coupled device (CCD) which was cross-calibrated with nondestructive electronics-based method. Within given laser and plasma parameters, we found that laser-driven low Z- gas jet targets generate high-quality and well-collimated electron beams with reasonable yields at the level of 10-100 pC. On the other hand, filamentary electron beams which were observed from high-Z gas jets at higher densities reached much higher yield. Evidences for cluster formation were clearly observed in high-Z gases, especially in the argon gas jet target where we received the highest yield of ~ 3 nC

physics.plasm-ph↗

arXiv2014

Enhanced laser-driven electron beam acceleration due to ionization-induced injection

We report an overall enhancement of a laser wakefield acceleration (LWFA) using the ionization injection in a mixture of 0.3 % nitrogen gas in 99.7 % helium gas. Upon the interaction of 30 TW, 30 fs laser pulses with a gas jet of the above gas mixture, > 300 MeV electron beams were generated at a helium plasma densities of 3.3-8.5*10^18 cm^{-3}. Compared with the electron self-injection in pure helium gas jet, the ionization injection has led to the generation of electron beams with higher energies, higher charge, lower density threshold for trapping, and a narrower energy spread without dark current (low energy electrons) or multiple bunches. It is foreseen that further optimization of such a scheme is expected to bring the electron beam energy-spread down to 1 %, making them suitable for driving ultra-compact free-electron lasers

physics.plasm-ph↗

arXiv2014

Generation of electron beams from a laser-based advanced accelerator at Shanghai Jiao Tong University

At Shanghai Jiao Tong University, we have established a research laboratory for advanced acceleration research based on high-power lasers and plasma technologies. In a primary experiment based on the laser wakefield acceleration (LWFA) scheme, multi-hundred MeV electron beams having a reasonable quality are generated using 20-40 TW, 30 femtosecond laser pulses interacting independently with helium, neon, nitrogen and argon gas jet targets. The laser-plasma interaction conditions are optimized for stabilizing the electron beam generation from each type of gas. The electron beam pointing angle stability and divergence angle as well as the energy spectra from each gas jet are measured and compared.

physics.plasm-ph↗

arXiv2013

Sparse Recovery with Coherent Tight Frames via Analysis Dantzig Selector and Analysis LASSO

This article considers recovery of signals that are sparse or approximately sparse in terms of a (possibly) highly overcomplete and coherent tight frame from undersampled data corrupted with additive noise. We show that the properly constrained $l_1$-analysis, called analysis Dantzig selector, stably recovers a signal which is nearly sparse in terms of a tight frame provided that the measurement matrix satisfies a restricted isometry property adapted to the tight frame. As a special case, we consider the Gaussian noise. Further, under a sparsity scenario, with high probability, the recovery error from noisy data is within a log-like factor of the minimax risk over the class of vectors which are at most $s$ sparse in terms of the tight frame. Similar results for the analysis LASSO are showed. The above two algorithms provide guarantees only for noise that is bounded or bounded with high probability (for example, Gaussian noise). However, when the underlying measurements are corrupted by sparse noise, these algorithms perform suboptimally. We demonstrate robust methods for reconstructing signals that are nearly sparse in terms of a tight frame in the presence of bounded noise combined with sparse noise. The analysis in this paper is based on the restricted isometry property adapted to a tight frame, which is a natural extension to the standard restricted isometry property.

cs.IT↗