arXiv subjects

Wenjie Sun

Publications and source records attributed to Wenjie Sun.

29 records · Page 2Linked to original sources

arXiv2022

Unseasonal super ionospheric plasma bubble and scintillations seeded by the 2022 Tonga Volcano Eruption related perturbations

The Hunga-Tonga volcano eruption at 04:14:45 UT on 15 January 2022 produced various waves propagating globally, disturbing the background atmosphere and ionosphere. Coinciding with the arrival of perturbation waves, several equatorial plasma bubbles (EPBs) were consecutively generated at post-sunset hours over the East/Southeast Asian region, with the largest extension to middle latitudes. These EPBs caused intense L-band amplitude scintillations at middle-to-low latitudes, with signal fading depths up to ~16 dB. Considering the very rare occurrence of EPBs during this season in East/Southeast Asian sector and the significantly modulated background ionosphere, we believe that the perturbation waves launched by the volcano eruption triggered the generation of unseasonal super EPBs. The ionospheric perturbations linked with the 2022 Tonga volcano eruption propagated coincidently through the East/Southeast Asia longitude sector near sunset, modulated the equatorial F region bottomside plasma density and acted as the seeding source for the generation of unseasonal super bubbles. Our results implicate that volcano eruption could indirectly affect the satellite communication links in the region more than ten thousand kilometers away.

physics.space-ph↗

arXiv2022

Evidence for quasi-two-dimensional superconductivity in infinite-layer nickelates

After being expected as a promising analogue to cuprates for decades, superconductivity was recently discovered in infinite-layer nickelates, providing new opportunities to explore mechanisms of high-temperature superconductivity. However, in sharp contrast to the single-band quasi-two-dimensional superconductivity in cuprates, nickelates exhibit a multi-band electronic structure and an unexpected isotropic superconductivity as reported recently, which challenges the cuprate-like picture in nickelates. Here, we show the superconductivity in nickelates is actually anisotropic and quasi-two-dimensional in nature, as that in cuprates. By synthesizing high-quality lanthanide nickelate films with enhanced crystallinity and superconductivity ($T_{c}^{onset}$ = 18.8 K, $T_{c}^{zero}$ = 16.5 K), strong anisotropic magnetotransport behaviors have been observed. The quasi-two-dimensional nature is further confirmed by the existence of a cusp-like peak of the angle-dependent $T_{c}$, and a Berezinskii-Kosterlitz-Thouless transition near $T_{c}$. Our work thus suggests a quasi-two-dimensional superconductivity in infinite-layer nickelates, implying a single-3$d_{x^2-y^2}$-band cuprate-like picture may remain valid in these compounds.

cond-mat.supr-con↗

arXiv2021

On trace of Brownian motion on the boundary of a strip

The trace of a Markov process is the time changed process of the original process on the support of the Revuz measure used in the time change. In this paper, we will concentrate on the reflecting Brownian motions on certain closed strips. On one hand, we will formulate the concrete expression of the Dirichlet forms associated with the traces of such reflecting Brownian motions on the boundary. On the other hand, the limits of these traces as the distance between the upper and lower boundaries tends to $0$ or $\infty$ will be further obtained.

math.PR↗

arXiv2021

On a stiff problem in two-dimensional space

In this paper we will study a stiff problem in two-dimensional space and especially its probabilistic counterpart. Roughly speaking, the heat equation with a parameter $\varepsilon>0$ is under consideration: \[ \partial_t u^\varepsilon(t,x)=\frac{1}{2}\nabla \cdot \left(\mathbf{A}_\varepsilon(x)\nabla u^\varepsilon(t,x) \right),\quad t\geq 0, x\in \mathbb{R}^2, \] where $\mathbf{A}_\varepsilon(x)=\text{Id}_2$, the identity matrix, for $x\notin Ω_\varepsilon:=\{x=(x_1,x_2)\in \mathbb{R}^2: |x_2|<\varepsilon\}$ while $$\mathbf{A}_\varepsilon(x):=\begin{pmatrix} a_\varepsilon^- & 0 \\ 0 & a^\shortmid_\varepsilon \end{pmatrix}$$ with two positive constants $a^-_\varepsilon, a^\shortmid_\varepsilon$ for $x\in Ω_\varepsilon$. There exists a diffusion process $X^\varepsilon$ on $\mathbb{R}^2$ associated to this heat equation in the sense that $u^\varepsilon(t,x):=\mathbf{E}^xu^\varepsilon(0,X_t^\varepsilon)$ is its unique weak solution. Note that $Ω_\varepsilon$ collapses to the $x_1$-axis, a barrier of zero volume, as $\varepsilon\downarrow 0$. The main purpose of this paper is to derive all possible limiting process $X$ of $X^\varepsilon$ as $\varepsilon\downarrow 0$. In addition, the limiting flux $u$ of the solution $u^\varepsilon$ as $\varepsilon\downarrow 0 $ and all possible boundary conditions satisfied by $u$ will be also characterized.

math.PR↗

arXiv2021

Epitaxial growth of Ruddlesden-Popper neodymium nickelates Nd$_{n+1}$Ni$_{n}$O$_{3n+1}$ (${n}$ = 1-5)

A series of Ruddlesden-Popper nickelates, Nd$_{n+1}$Ni$_{n}$O$_{3n+1}$ (${n}$ = 1-5), have been stabilized in thin film form using reactive molecular-beam epitaxy. High crystalline quality has been verified by X-ray diffraction and scanning transmission electron microscopy. X-ray photoelectron spectroscopy indicates the ${n}$-dependent valence states of nickel in these compounds. Metal-insulator transitions show clear ${n}$ dependence for intermediate members (${n}$ = 3-5), and the low-temperature resistivities of which show logarithmic dependence, resembling the Kondo-scattering as observed in the parent compounds of superconducting infinite-layer nickelates.

cond-mat.supr-con↗

arXiv2021

Impact of Cation Stoichiometry on the Crystalline Structure and Superconductivity in Nickelates

The recent discovery of superconductivity in infinite-layer nickelate films has aroused great interest since it provides a new platform to explore the mechanism of high-temperature superconductivity. However, superconductivity only appears in the thin film form and synthesizing superconducting nickelate films is extremely challenging, limiting the in-depth studies on this compound. Here, we explore the critical parameters in the growth of high quality nickelate films using molecular beam epitaxy (MBE). We found that stoichiometry is crucial in optimizing the crystalline structure and realizing superconductivity in nickelate films. In precursor NdNiO3 films, optimal stoichiometry of cations yields the most compact lattice while off-stoichiometry of cations causes obvious lattice expansion, influencing the subsequent topotactic reduction and the emergence of superconductivity in infinite-layer nickelates. Surprisingly, in-situ reflection high energy electron diffraction (RHEED) indicates that some impurity phases always appear once Sr ions are doped into NdNiO3 although the X-ray diffraction (XRD) data are of high quality. While these impurity phases do not seem to suppress the superconductivity, their impacts on the electronic and magnetic structure deserve further studies. Our work demonstrates and highlights the significance of cation stoichiometry in superconducting nickelate family.

cond-mat.supr-con↗

arXiv2020

New quasi-universal relations for static and rapid rotating neutron stars

In the last few decades, lots of universal relations between different global physical quantities of neutron stars have been proposed to constrain the unobservable or hard to be observed properties of neutron stars. But few of them are related to the gravitational redshift or the gravitational binding energy, especially for the fast rotating neutron stars. Here we will focus on the universal relations related to these two quantities. Based on 11 equations of state (EOSs) from the predictions of microscopic nuclear many-body theories for normal or hybrid neutron stars, we proposed a set of new quasi-universal relations under three rotating cases: static, general rotating and Keplerian rotating. These new quasi-universal relations provide a potential way to constrain or estimate the unobservable or hard to be observed properties of neutron stars.

gr-qc↗

arXiv2019

A Fast Matrix-Completion-Based Approach for Recommendation Systems

Matrix completion is widely used in machine learning, engineering control, image processing, and recommendation systems. Currently, a popular algorithm for matrix completion is Singular Value Threshold (SVT). In this algorithm, the singular value threshold should be set first. However, in a recommendation system, the dimension of the preference matrix keeps changing. Therefore, it is difficult to directly apply SVT. In addition, what the users of a recommendation system need is a sequence of personalized recommended results rather than the estimation of their scores. According to the above ideas, this paper proposes a novel approach named probability completion model~(PCM). By reducing the data dimension, the transitivity of the similar matrix, and singular value decomposition, this approach quickly obtains a completion matrix with the same probability distribution as the original matrix. The approach greatly reduces the computation time based on the accuracy of the sacrifice part, and can quickly obtain a low-rank similarity matrix with data trend approximation properties. The experimental results show that PCM can quickly generate a complementary matrix with similar data trends as the original matrix. The LCS score and efficiency of PCM are both higher than SVT.

cs.IR↗

arXiv2018

Effective intervals and regular Dirichlet subspaces

It is shown in [10] that a regular and local Dirichlet form on an interval can be represented by so-called effective intervals with scale functions. This paper focuses on how to operate on effective intervals to obtain regular Dirichlet subspaces. The first result is a complete characterization for a Dirichlet form to be a regular subspace of such a Dirichlet form in terms of effective intervals. Then we give an explicit road map how to obtain all regular Dirichlet subspaces from a local and regular Dirichlet form on an interval, by a series of intuitive operations on the effective intervals in the representation above. Finally applying previous results, we shall prove that every regular and local Dirichlet form has a special standard core generated by a continuous and strictly increasing function.

math.PR↗

arXiv2018

On stiff problems via Dirichlet forms

The stiff problem is concerned with a thermal conduction model with a singular barrier of zero volume. In this paper, we shall build the phase transitions for the stiff problems in one-dimensional space. It turns out that every phase transition definitely depends on the total thermal resistance of the barrier, and the three phases correspond to the so-called impermeable pattern, semi-permeable pattern and permeable pattern of thermal conduction respectively. For each pattern, the related boundary condition of the flux at the barrier is also derived. Mathematically, we shall introduce and explore the so-called snapping out Markov process, which is the probabilistic counterpart of semi-permeable pattern in the stiff problem.

math.PR↗

arXiv2018

Snapping out Walsh's Brownian motion and related stiff problem

Firstly, we shall introduce the so-called snapping out Walsh's Brownian motion and present its relation with Walsh's Brownian motion. Then the stiff problem related to Walsh's Brownian motion will be described and we shall build a phase transition for it. The snapping out Walsh's Brownian motion corresponds to the so-called semi-permeable pattern of this stiff problem.

math.PR↗