arXiv subjects

Dong Yan

Publications and source records attributed to Dong Yan.

At least 55 records · Page 3Linked to original sources

arXiv2022

Probabilistic buckling of imperfect hemispherical shells containing a distribution of defects

The buckling of spherical shells is plagued by a strong sensitivity to imperfections. Traditionally, imperfect shells tend to be characterized empirically by the knockdown factor, the ratio between the measured buckling strength and the corresponding classic prediction for a perfect shell. Recently, it has been demonstrated that the knockdown factor of a shell containing a single imperfection can be predicted when there is detailed a priori knowledge of the defect geometry. Still, addressing the analogous problem for a shell containing many defects remains an open question. Here, we use finite element simulations, which we validate against precision experiments, to investigate hemispherical shells containing a well-defined distribution of imperfections. Our goal is to characterize the resulting knockdown factor statistics. First, we study the buckling of shells containing only two defects, uncovering nontrivial regimes of interactions that echo existing findings for cylindrical shells. Then, we construct statistical ensembles of imperfect shells, whose defect amplitudes are sampled from a lognormal distribution. We find that a 3-parameter Weibull distribution is an excellent description for the measured statistics of knockdown factors, suggesting that shell buckling can be regarded as an extreme-value statistics phenomenon.

cond-mat.soft↗

arXiv2022

Snap buckling of bistable beams under combined mechanical and magnetic loading

We investigate the mechanics of bistable, hard-magnetic, elastic beams, combining experiments, finite element modeling (FEM), and a reduced-order theory. The beam is made of a hard magneto-rheological elastomer, comprising two segments with antiparallel magnetization along the centerline, and is set into a bistable curved configuration by imposing an end-to-end shortening. Reversible snapping is possible between these two stable states. First, we experimentally characterize the critical field strength for the onset of snapping, at different levels of end-to-end shortening. Second, we perform 3D FEM simulations using the Riks method to analyze high-order deformation modes during snapping. Third, we develop a reduced-order centerline-based beam theory to rationalize the observed magneto-elastic response. The theory and simulations are validated against experiments, with an excellent quantitative agreement. Finally, we consider the case of combined magnetic and mechanical-indentation loading, examining how the applied field affects the bistability and quantifying the maximum load-bearing capacity. Our work provides a set of predictive tools for the rational design of one-dimensional, bistable, magneto-elastic structural elements.

cond-mat.soft↗

arXiv2022

Low-Frequency Noise in Quasi-1D (TaSe$_4$)$_2$I Weyl Semimetal Nanoribbons

We report on low-frequency current fluctuations, i.e. electronic noise, in quasi-one-dimensional (TaSe$_4$)$_2$I Weyl semimetal nanoribbons. It was found that the noise spectral density is of the 1/f type and scales with the square of the current, S~I^2 (f is the frequency). The noise spectral density increases by almost an order of magnitude and develops Lorentzian features near the temperature T~225 K. These spectral changes were attributed to the charge-density-wave phase transition even though the temperature of the noise maximum deviates from the reported Peierls transition temperature in bulk (TaSe$_4$)$_2$I crystals. The noise level, normalized by the channel area, in these Weyl semimetal nanoribbons was surprisingly low, $\sim 10^{-9}$ um$^2$Hz$^{-1}$ at f=10 Hz, when measured below and above the Peierls transition temperature. Obtained results shed light on the specifics of electron transport in quasi-1D topological Weyl semimetals and can be important for their proposed applications as downscaled interconnects.

cond-mat.mes-hall↗

arXiv2022

Tianshou: a Highly Modularized Deep Reinforcement Learning Library

In this paper, we present Tianshou, a highly modularized Python library for deep reinforcement learning (DRL) that uses PyTorch as its backend. Tianshou intends to be research-friendly by providing a flexible and reliable infrastructure of DRL algorithms. It supports online and offline training with more than 20 classic algorithms through a unified interface. To facilitate related research and prove Tianshou's reliability, we have released Tianshou's benchmark of MuJoCo environments, covering eight classic algorithms with state-of-the-art performance. We open-sourced Tianshou at https://github.com/thu-ml/tianshou/.

cs.LG↗

arXiv2022

A reduced-order, rotation-based model for thin hard-magnetic plates

We develop a reduced-order model for thin plates made of hard magnetorheological elastomers (hard-MREs), which are materials composed of hard-magnetic particles embedded in a polymeric matrix. First, we propose a new magnetic potential, as an alternative to an existing torque-based 3D continuum theory of hard-MREs, obtained by reformulating the remnant magnetization of a deformed hard-MRE body. Specifically, the magnetizations in the initial and current configurations are related by the rotation tensor decomposed from the deformation gradient, independently of stretching deformation. This description is motivated by recently reported observations in microscopic homogenization simulations. Then, we derive a 2D plate model through the dimensional reduction of our proposed rotation-based 3D theory. For comparison, we also provide a second plate model derived from the existing 3D theory. Finally, we perform precision experiments to thoroughly evaluate the proposed 3D and 2D models on hard-magnetic plates under various magnetic and mechanical loading conditions. We demonstrate that our rotation-based modification of the magnetic potential is crucial in correctly capturing the behavior of plates subjected to an applied field aligned with the magnetization, and undergoing in-plane stretching. In all the tested cases, our rotation-based 3D and 2D models yield predictions in excellent quantitative agreement with the experiments and can thus serve as predictive tools for the rational design of hard-magnetic plate structures.

cond-mat.soft↗

arXiv2022

Rydberg electromagnetically induced transparency and absorption of strontium triplet states in a weak microwave field

We study theoretically laser excitation of Rydberg triplet states of strontium atoms in the presence of weak microwave (MW) fields. Starting from the ground state $5s^2\,^1S_0$, the Rydberg excitation is realized through the metastable, triplet $5s5p\,^3P_1$ state, whose decay rate $γ_2$ is $2π\times 7.5$ kHz, much smaller than the one in the singlet state or alkali atoms. The influences of $γ_2$ on the transparency and absorption spectrum in the electromagnetically induced transparency (EIT), and electromagnetically induced absorption (EIA) regime are examined. Narrow transparent windows (EIT) or absorption peaks (EIA) are found, whose distance in the spectrum depends on the Rabi frequency of the weak MW field. It is found that the spectrum exhibits higher contrast than using the singlet state or alkali atoms in typical situations. Using the metastable intermediate state, we find that resonance fluorescence of Sr gases exhibits very narrow peaks, which are modulated by the MW field. When the MW field is weaker than the probe and control light, the spectrum distance of these peaks are linearly proportional to $Ω_m$. This allows us to propose a new way to sense very weak MW fields through resonance fluorescence. Our study shows that the Sr triplet state could be used to develop the Rydberg MW electrometry that gains unique advantages.

quant-ph↗

arXiv2022

Policy Learning for Robust Markov Decision Process with a Mismatched Generative Model

In high-stake scenarios like medical treatment and auto-piloting, it's risky or even infeasible to collect online experimental data to train the agent. Simulation-based training can alleviate this issue, but may suffer from its inherent mismatches from the simulator and real environment. It is therefore imperative to utilize the simulator to learn a robust policy for the real-world deployment. In this work, we consider policy learning for Robust Markov Decision Processes (RMDP), where the agent tries to seek a robust policy with respect to unexpected perturbations on the environments. Specifically, we focus on the setting where the training environment can be characterized as a generative model and a constrained perturbation can be added to the model during testing. Our goal is to identify a near-optimal robust policy for the perturbed testing environment, which introduces additional technical difficulties as we need to simultaneously estimate the training environment uncertainty from samples and find the worst-case perturbation for testing. To solve this issue, we propose a generic method which formalizes the perturbation as an opponent to obtain a two-player zero-sum game, and further show that the Nash Equilibrium corresponds to the robust policy. We prove that, with a polynomial number of samples from the generative model, our algorithm can find a near-optimal robust policy with a high probability. Our method is able to deal with general perturbations under some mild assumptions and can also be extended to more complex problems like robust partial observable Markov decision process, thanks to the game-theoretical formulation.

cs.LG↗

arXiv2021

Superconductivity in CuIr2-xAlxTe4 telluride chalcogenides

The relationship between charge-density-wave (CDW) and superconductivity (SC), two vital physical phases in condensed matter physics, has always been the focus of scientists' research over the past decades. Motivated by this research hotspot, we systematically studied the physical properties of the layered telluride chalcogenide superconductors CuIr2-xAlxTe4. Through the resistance and magnetization measurements, we found that the CDW order was destroyed by a small amount of Al doping. Meanwhile, the superconducting transition temperature (Tc) kept changing with the change of doping amount and rose towards the maximum value of 2.75 K when x = 0.075. The value of normalized specific heat jump for the highest Tc sample CuIr1.925Al0.075Te4 was 1.53, which was larger than the BCS value of 1.43 and showed that bulk superconducting nature. In order to clearly show the relationship between SC and CDW states, we propose a phase diagram of Tc vs. doping content.

cond-mat.supr-con↗

arXiv2021

A geometrically exact model for thin magneto-elastic shells

We develop a reduced model for hard-magnetic, thin, linear-elastic shells that can be actuated through an external magnetic field, with geometrically exact strain measures. Assuming a reduced kinematics based on the Kirchhoff-Love assumption, we derive a reduced two-dimensional magneto-elastic energy that can be minimized through numerical analysis. In parallel, we simplify the reduced energy by expanding it up to the second-order in the displacement field and provide a physical interpretation. Our theoretical analysis allows us to identify and interpret the two primary mechanisms dictating the magneto-elastic response: a combination of equivalent magnetic pressure and forces at the first order, and distributed magnetic torques at the second order. We contrast our reduced framework against a three-dimensional nonlinear model by investigating three test cases involving the indentation and the pressure buckling of shells under magnetic loading. We find excellent agreement between the two approaches, thereby verifying our reduced model for shells undergoing nonlinear and non-axisymmetric deformations. We believe that our model for magneto-elastic shells will serve as a valuable tool for the rational design of magnetic structures, enriching the set of reduced magnetic models.

cond-mat.soft↗

arXiv2021

A comprehensive framework for hard-magnetic beams: reduced-order theory, 3D simulations, and experiments

Thin beams made of magnetorheological elastomers embedded with hard magnetic particles (hard-MREs) are capable of large deflections under an applied magnetic field. We propose a comprehensive framework, comprising a beam model and 3D finite element modeling (FEM), to describe the behavior of hard-MRE beams under both uniform and constant gradient magnetic fields. First, based on the Helmholtz free energy of bulk (3D) hard-MREs, we perform dimensional reduction to derive a 1D description and obtain the equilibrium equation of the beam through variational methods. In parallel, we extend the existing 3D continuum theory for hard-MREs to the general case of non-uniform fields by incorporating the magnetic body force induced by the field gradient and implementing it in FEM. The beam model and FEM are first validated using experiments and then employed to predict the deflection of a cantilever beam in either a uniform or a constant gradient field. The corresponding parameters governing the magneto-elastic coupling are identified. Then, a set of comparative numerical studies for actuation in different configurations yields additional insight into the beam response. Our study builds on previous work on hard-MRE beams, while providing a more complete framework, both in terms of the methodologies used and the configurations considered, to serve as a valuable predictive toolbox for the rational design of beam-like hard-magnetic structures.

cond-mat.soft↗

arXiv2021

Observation of orbital order in the Van der Waals material 1T-TiSe2

Besides magnetic and charge order, regular arrangements of orbital occupation constitute a fundamental order parameter of condensed matter physics. Even though orbital order is difficult to identify directly in experiments, its presence was firmly established in a number of strongly correlated, three-dimensional Mott insulators. Here, reporting resonant X-ray scattering experiments on the layered Van der Waals compound $1T$-TiSe$_2$, we establish the emergence of orbital order in a weakly correlated, quasi-two-dimensional material. Our experimental scattering results are consistent with first-principles calculations that bring to the fore a generic mechanism of close interplay between charge redistribution, lattice displacements, and orbital order. It demonstrates the essential role that orbital degrees of freedom play in TiSe$_2$, and their importance throughout the family of correlated Van der Waals materials.

cond-mat.str-el↗

arXiv2021

Magneto-active elastic shells with tunable buckling strength

Shell buckling is central in many biological structures and advanced functional materials, even if, traditionally, this elastic instability has been regarded as a catastrophic phenomenon to be avoided for engineering structures. Either way, predicting critical buckling conditions remains a long-standing challenge. The subcritical nature of shell buckling imparts extreme sensitivity to material and geometric imperfections. Consequently, measured critical loads are inevitably lower than classic theoretical predictions. Here, we present a robust mechanism to dynamically tune the buckling strength of shells, exploiting the coupling between mechanics and magnetism. Our experiments on pressurized spherical shells made of a magnetorheological elastomer demonstrate the tunability of their buckling pressure via magnetic actuation. We develop a theoretical model for thin magnetic elastic shells, which rationalizes the underlying mechanism, in excellent agreement with experiments. A dimensionless magneto-elastic buckling number is recognized as the key governing parameter, combining the geometrical, mechanical, and magnetic properties of the system.

cond-mat.soft↗

arXiv2020

Satellite-Terrestrial Channel Characterization in High-Speed Railway Environment at 22.6 GHz

The integration of satellite and terrestrial communication systems plays a vital role in the fifth-generation mobile communication system (5G) for the ubiquitous coverage, reliable service and flexible networking. Moreover, the millimeter wave (mmWave) communication with large bandwidth is a key enabler for 5G intelligent rail transportation. In this paper, the satellite-terrestrial channel at 22.6 GHz is characterized for a typical high-speed railway (HSR) environment. The three-dimensional model of the railway scenario is reconstructed and imported into the Cloud Ray-Tracing (CloudRT) simulation platform. Based on extensive ray-tracing simulations, the channel for the terrestrial HSR system and the satellite-terrestrial system with two weather conditions are characterized, and the interference between them are evaluated. The results of this paper can help for the design and evaluation for the satellite-terrestrial communication system enabling future intelligent rail transportation.

eess.SP↗

arXiv2020

Charge density wave and superconductivity in the family of telluride chalcogenides Zn1-xCuxIr2-yN(N = Al, Ti, Rh)yTe4

The interplay between superconductivity and charge density wave (CDW)/metal-to-insulator transition (MIT) has long been interested and studied in condensed matter physics. Here we study systematically the charge density wave and superconductivity properties in the solid solutions Zn1-xCuxIr2-yN(N = Al, Ti, Rh)yTe4. Resistivity, magnetic susceptibility and specific heat measurements indicate that the CDW state was suppressed immediately while the superconducting critical temperature (Tc) differs from each system. In the Al- and Ti-substitution cases, Tc increase as y increases and reaches a maximum around 2.75 K and 2.84 K respectively at y = 0.075, followed by a decrease of Tc before the chemical phase boundary is reached at y = 0.2. Nevertheless, Tc decreases monotonously with Rh-doping content y increases and disappears above 0.3 with measuring temperature down to 2 K. Surprisingly, in the Zn1-xCuxIr2Te4 solid solution, Tc enhances as x increases and reaches a maximum value of 2.82 K for x = 0.5 but subsequently survives over the whole doping range of 0.00 - 0.9 despite Tc changes slightly with higher doping content, which differs from the observation of zinc doping suppressing the superconductivity quickly in the high Tc cuprate superconductors. The specific heat anomaly at the superconducting transitions for the representative optimal doping samples are all slightly higher than the BCS value of 1.43 and indicate bulk superconductivity in these compounds. Finally, the CDW transition temperature (TCDW) and superconducting transition temperature (Tc) vs. x/y content phase diagrams of Zn1-xCuxIr2-yN(N = Al, Ti, Rh)yTe4 have been established and compared, which offers good opportunity to study the competition between CDW and superconductivity in the telluride chalcogenides.

cond-mat.supr-con↗

arXiv2020

Electromagnetically Induced Transparency of Interacting Rydberg Atoms with Two-Body dephasing

We study electromagnetically induced transparency of a ladder type configuration in ultracold atomic gases, where the upper level is an electronically highly excited Rydberg state. The strong two-body interaction in the Rydberg state leads to the excitation blockade, where all but one atoms are shifted out of resonance such that the transmission of the probe light is affected. We show that molecular coupling in the Rydberg state causes an effective, two-body dephasing. The presence of the two-body dephasing leads to a similar blockade effect. Hence the overall blockade effect is enhanced by the two-body dephasing. Through numerical and approximately analytical calculations, we find that the transmission is reduced drastically by the presence of two-body dephasing in the transparent window, which is accompanied by strong photon-photon anti-bunching. Around the Autler-Townes splitting, the photon bunching is amplified by the two-body dephasing.

physics.optics↗

arXiv2020

Stable $\mathbb{I}$-free lattices and the two-variable algebraic $p$-adic $L$-functions in residually reducible Hida deformation

This paper is a continuation of the author's previous work, where we studied the variation of the number of isomorphic classes of $G_{\mathbb{Q}}$-stable lattices in p-adic families of residually reducible ordinary Galois representations. In this paper, we study the number of isomorphic classes of $G_{\mathbb{Q}}$-stable free lattices in a residually reducible Hida deformation and the variation of their related two-variable algebraic $p$-adic $L$-functions. This result gives an answer of a question asked by Ochiai.

math.NT↗

arXiv2019

Bayesian linear inverse problems in regularity scales

We obtain rates of contraction of posterior distributions in inverse problems defined by scales of smoothness classes. We derive abstract results for general priors, with contraction rates determined by Galerkin approximation. The rate depends on the amount of prior concentration near the true function and the prior mass of functions with inferior Galerkin approximation. We apply the general result to non-conjugate series priors, showing that these priors give near optimal and adaptive recovery in some generality, Gaussian priors, and mixtures of Gaussian priors, where the latter are also shown to be near optimal and adaptive. The proofs are based on general testing and approximation arguments, without explicit calculations on the posterior distribution. We are thus not restricted to priors based on the singular value decomposition of the operator. We illustrate the results with examples of inverse problems resulting from differential equations.

math.ST↗

arXiv2019

Superconductivity in the Ru-Doped CuIr2Te4 Telluride Chalcogenide

Here we report the effect of structural and superconductivity properties on Ru doped CuIr2Te4 telluride chalcogenide. XRD results suggest that the CuIr2-xRuxTe4 maintain the disordered trigonal structure with space group P3m1 (No. 164) for x less than 0.3. The lattice constants, a and c, both decrease with increasing Ru content. Temperature-dependent resistivity, magnetic susceptibility and specific-heat measurements are performed to characterize the superconducting properties systematically. Our results suggest that the optimal doping level for superconductivity in CuIr2-xRuxTe4 is x = 0.05, where Tc is 2.79 K with the Sommerfeld constant gamma of 11.52 mJ mol-1 K-2 and the specific-heat anomaly at the superconducting transition, is approximately 1.51, which is higher than the BCS value of 1.43, indicating CuIr1.95Ru0.05Te4 is a strongly electron-phonon coupled superconductor. The values of lower critical filed and upper critical field calculated from isothermal magnetization and magneto-transport measurements are 0.98 KOe and 2.47 KOe respectively, signifying that the compound is clearly a type-II superconductor. Finally, a dome-like shape superconducting Tcs vs. x content phase diagram is established, where the charge density wave disappears at x = 0.03 while superconducting transition temperature (Tc) rises until it reaches its peak at x = 0.05, then, with decreasing when x reaches 0.3. This feature of the competition between CDW and the superconductivity could be caused by tuning the Fermi surface and density of states with Ru chemical doping.

cond-mat.supr-con↗