Atmospheric CO2 forces abrupt vegetation shifts locally, but not globally

It is possible that anthropogenic climate change will drive the Earth system into a qualitatively different state. Although different types of uncertainty limit our capacity to assess this risk, Earth system scientists are particularly concerned about tipping elements, large-scale components of the Earth system that can be switched into qualitatively different states by small perturbations. Despite growing evidence that tipping elements exist in the climate system, whether large-scale vegetation systems can tip into alternative states is poorly understood. Here we show that tropical grassland, savanna and forest ecosystems, areas large enough to have powerful impacts on the Earth system, are likely to shift to alternative states. Specifically, we show that increasing atmospheric CO2 concentration will force transitions to vegetation states characterized by higher biomass and/or woody-plant dominance. The timing of these critical transitions varies as a result of between-site variance in the rate of temperature increase, as well as a dependence on stochastic variation in fire severity and rainfall. We further show that the locations of bistable vegetation zones (zones where alternative vegetation states can exist) will shift as climate changes. We conclude that even though large-scale directional regime shifts in terrestrial ecosystems are likely, asynchrony in the timing of these shifts may serve to dampen, but not nullify, the shock that these changes may represent to the Earth system.     

非平衡系统的规格化模型及其临界广义势

非平衡临界的参考态是非平衡定态,在该参考态下存在一个描述系统状态稳定的广义势。本文从一般非平衡系统出发,注意到非平衡临界与平衡临界的差别和联系,建立了非平衡系统的规格化模型,考察了非平衡系统广义势的存在形式,得到了该广义势在非平衡临界点无限小邻域内的具体结构。还讨议了该模型及其广义势在位形空间和动量空间中的粗粒化过程,为对非平衡相变系统临界性质更深一步的考察和研究创造了条件。     

卸料方式对旋风筒分离效率的影响

对旋风筒集料斗的密封卸料方式和翻板阀卸料方式的差异性进行了分析，研究了各种参数对两种卸料方式所得分离效率的影响，讨论了改善翻板阀卸料效果的措施，所得研究结果有助于解决小型旋风筒多级串联系统的正常卸料问题．同时为实际生产过程中，旋风筒分离效率的提高提供了参考依据．     

Scaling of entanglement close to a quantum phase transition

Classical phase transitions occur when a physical system reaches a state below a critical temperature characterized by macroscopic order. Quantum phase transitions occur at absolute zero; they are induced by the change of an external parameter or coupling constant, and are driven by quantum fluctuations. Examples include transitions in quantum Hall systems, localization in Si-MOSFETs (metal oxide silicon field-effect transistors; ref. 4) and the superconductor-insulator transition in two-dimensional systems. Both classical and quantum critical points are governed by a diverging correlation length, although quantum systems possess additional correlations that do not have a classical counterpart. This phenomenon, known as entanglement, is the resource that enables quantum computation and communication. The role of entanglement at a phase transition is not captured by statistical mechanics-a complete classification of the critical many-body state requires the introduction of concepts from quantum information theory. Here we connect the theory of critical phenomena with quantum information by exploring the entangling resources of a system close to its quantum critical point. We demonstrate, for a class of one-dimensional magnetic systems, that entanglement shows scaling behaviour in the vicinity of the transition point.     

光学双稳态方程的定常分岔和临界慢化现象

用新的方法研究吸收型光学双稳态方程的定常分岔问题和临界慢化现象,给出光学双稳态的存在性证明,得到定常分岔图就是熟知的Ｓ－型双稳响应曲线;分析的临界慢化现象,给出了临界慢化时间的估计。     

一类带有扰动的不确定切换系统的鲁棒镇定

研究了一类带有非线性扰动的不确定切换系统的鲁棒镇定问题．利用完备性条件，得到了不确定切换系统渐近稳定的一个充分条件．基于LMI方法，设计出鲁棒状态反馈控制器及相应的切换策略，使得闭环系统在其平衡点处是渐近稳定的．     

地下铲运机转弯侧翻的影响因素分析

为了避免地下铲运机紧急转弯导致侧翻事故发生,根据受力分析,建立了铲运机转弯时的侧翻数学模型。当横向稳定比P〈1时,铲运机稳定;P〉1时,机器发生倾翻;P=1时,地下铲运机处于倾翻临界状态。MATLAB仿真结果表明：减小车身质量、转弯速度、工作面侧倾角、车身重心及车载重心高度可以提高稳定性,同时增加载重质量、增大轮距和转弯半径也可以达到提高稳定性目的,而车身重心偏离值、货载重心偏离值对稳定性的影响不明显。     

二量子比特海森堡XYZ链中的热纠缠

通过分析外磁场、温度、自旋粒子间耦合系数、非对称系数对系统纠缠的影响，研究了均匀磁场中二量子比特海森堡XYZ链中的热纠缠现象．结果表明，系统的基态纠缠随着外磁场B值的增加，保持一段恒定且最大值后，在一个临界点Bc突然下降．耦合系数Z值的增加可以使纠缠无论是在温度方向还是在磁场方向都能扩展．在一定温度条件下，增加非对称系数γ值可以使纠缠在超越临界磁场Bc后的复苏值和范围都增加．这些结果为用磁场、自旋粒子问耦合系数、非对称系数来操纵系统纠缠提供了理论依据．     

用于机械故障诊断的门限自回归模型盲辨识

提出了一种门限自回归(AR)模型的盲辨识算法,并与常用方法进行比较分析。该算法的特点在保证辨识精度上可大大提高其运行速度,而且阶次越高,该算法的优势越明显。将该方法与隐Markov模型结合,以门限自回归模型各区间的AR子模型系数作为特征向量,以隐Markov模型作为分类器,应用到旋转机械升降速过程的故障诊断中。实验结果表明,这种方法有很好的实用性。     

复合脉冲控制阶梯型多态量子系统的转移通道

提出了利用复合脉冲在阶梯型多态量子系统中操控量子态演化和粒子数相干转移的方法.首先通过Morris-Shore变换,借助二能级传播算子描述多态量子系统的演化.然后利用增加脉冲序列数且单个脉冲的相位可控的技术,抑制额外的转移通道,实现高效率高鲁棒性的量子态操控和粒子数转移.最后通过数值模拟研究了额外通道和转移通道的拉比频率比、脉冲面积的变化、单光子失谐偏离零点等因素对转移效率的影响.结果表明:常被用于二能级量子系统的复合脉冲操控方法也可被应用于多态量子系统.通过增加脉冲序列数,可有效减小相关参数的扰动,保持高效率的粒子数转移.多脉冲序列复合脉冲的技术可以解决偏振不纯、激光频率不纯、控制参数扰动等造成的量子态操控效率降低等问题.研究结果对构造量子门、量子模拟等相关研究具有重要意义.     

热力涡轮机械的数字仿真

本文发展了一种积木式的动态仿真程序，它可以仿真由典型设备作任意组合的热力涡轮机械系统。每一个典型设备是一个易于和其它设备相联结的子系统，有一套节省机时的联结方法。本文中所介绍的程序可用于计算非线性系统的大扰动下的过渡过程。文中把催化裂化装置中的能量回收系统作为上述方法的应用实例。     

受井眼约束带接头管柱的纵横弯曲分析

针对受井眼约束带接头管柱的纵横弯曲问题,将管柱变形分成3 类情形：无接触、点接触和连续接触。根据 管柱变形基本方程及接头处管柱的边界条件和连续性条件,分别求解3 种情形下的管柱变形曲线。利用无接触到点 接触以及点接触到连续接触的判别条件,分别求解了接触状态转换的临界参数,并分析了临界参数（φ）随管柱视半径 与接头视半径之比（λ）的变化规律。结果表明,当λ 比1 略大时,φ 对λ 的变化很敏感;当λ 大于2 后,φ 对λ 的变化 不敏感,并且趋于某一稳定值。以λ = 2 为例,详细描述了管柱弯曲接触状态转换的整个过程。计算结果可以为实际 作业中接头对管柱弯曲影响分析提供理论基础。     

A hidden pseudogap under the 'dome' of superconductivity in electron-doped high-temperature superconductors

The ground state of superconductors is characterized by the long-range order of condensed Cooper pairs: this is the only order present in conventional superconductors. The high-transition-temperature (high-T(c)) superconductors, in contrast, exhibit more complex phase behaviour, which might indicate the presence of other competing ground states. For example, the pseudogap--a suppression of the accessible electronic states at the Fermi level in the normal state of high-T(c) superconductors-has been interpreted as either a precursor to superconductivity or as tracer of a nearby ground state that can be separated from the superconducting state by a quantum critical point. Here we report the existence of a second order parameter hidden within the superconducting phase of the underdoped (electron-doped) high-T(c) superconductor Pr2-xCe(x)CuO4-y and the newly synthesized electron-doped material La2-xCe(x)CuO4-y (ref. 8). The existence of a pseudogap when superconductivity is suppressed excludes precursor superconductivity as its origin. Our observation is consistent with the presence of a (quantum) phase transition at T = 0, which may be a key to understanding high-T(c) superconductivity. This supports the picture that the physics of high-T(c) superconductors is determined by the interplay between competing and coexisting ground states.     

对象参数摄动时分散控制系统的镇定

在工程技术领域里，有许多系统是需要采用分散控制的。本文对一类分散控制系统，讨论了对象参数有摄动时系统的镇定问题，得到了一个充分条件。应用这个条件，有可能通过对系统不存在参数摄动时的控制律的修正，而使分散控制系统在参数摄动时可仍保持稳定。     

用状态方程模拟注气对凝析气藏采收率的影响

提高凝析油采收率和整体开发效益是凝析气藏开发的目标,注气是防止凝析油析出从而提高凝析油采收率的较好方法。以一个真实的凝析气藏为例,使用自行开发的PVTCOG软件和PR状态方程研究和对比了凝析气藏定容衰竭不同阶段,注干气、氮气及二氧化碳对露点的影响、对注气时机、对凝析油储量和凝析油采收率的影响。研究表明不同注入气影响露点的趋势不同,随注入气增加凝析油储量下降,但凝析油采收率上升,注入时机不一定是在高于露点压力时最好。     

一类复杂动力系统的参数辨析

对一类复杂系统(Lorenz混沌和Rssler超混沌)的未知参数提出一种简便的辨析方法.首先,通过对系统实行反馈控制使其到达任意不动点,然后,通过求解不动点的平衡方程,解析地得到系统未知参数的表达式.以Lorenz混沌为例将该方法和线性参数观测器的方法进行对比,可以更快地实现该系统全部未知参数的准确快速辨析.在系统参数发生阶跃变化的情况下,该方法可很好地实现对参数高精度的快速辨析.数值计算结果和理论分析一致.该方法可以推广到其他复杂动力系统的参数结构辨析研究.     

一类线性比式和问题的鲁棒算法

对广泛应用于工程设计、非线性系统稳定性分析等实际问题中的一类线性比式和问题(P1)提出了一种鲁棒全局优化算法,这种方法求得的最优解具有稳定性好,可行等特点.从理论上证明了该算法的收敛性,最终数值实验表明了提出方法的可行性.     

团簇Co2FeB2和CoFe2B2稳定性的研究

为深入了解非晶态Co-Fe-B合金的性质,本文从能量学视角,对团簇Co2FeB2和CoFe2B2各构型所占比例定量分析,探究其稳定性,发现团簇Co2FeB2的结合能和吉布斯自由能变化量随构型能量增加出现剧变点,临界能量约为463.061a.u,主要存在构型为能量低于临界值的两种戴帽三角锥和一种四角锥构型。团簇CoFe2B2的结合能和吉布斯自由能变不存在剧变点,有多种异构体共存。高Co含量的团簇有较小的结合能和吉布斯自由能变化量,稳定性弱,此结论符合相关文献报道。     

Generic mechanism for generating a liquid-liquid phase transition

Recent experimental results indicate that phosphorus--a single-component system--can have a high-density liquid (HDL) and a low-density liquid (LDL) phase. A first-order transition between two liquids of different densities is consistent with experimental data for a variety of materials, including single-component systems such as water, silica and carbon. Molecular dynamics simulations of very specific models for supercooled water, liquid carbon and supercooled silica predict a LDL-HDL critical point, but a coherent and general interpretation of the LDL-HDL transition is lacking. Here we show that the presence of a LDL and a HDL can be directly related to an interaction potential with an attractive part and two characteristic short-range repulsive distances. This kind of interaction is common to other single-component materials in the liquid state (in particular, liquid metals), and such potentials are often used to describe systems that exhibit a density anomaly. However, our results show that the LDL and HDL phases can occur in systems with no density anomaly. Our results therefore present an experimental challenge to uncover a liquid-liquid transition in systems like liquid metals, regardless of the presence of a density anomaly.