科学家警告:北极冰盖正在迅速缩小

2012年9月,北极海冰的范围下降到自1979年有卫星记录以来的最低点.海冰的面积340万平方公里仅相当于1979-2000年间最小平均覆盖值的一半.根据2011年MIT模型计算,海冰减薄的程度相当于2007年联合国关于气候变迁政府间圆桌会议的估计值的4倍.一些气象学家提醒,没有冰层的北冰洋之夏或许会在近几年出现,将比现有气候模型估计的时间提早10年.     

周期搅动将随机取向的骰子有序排列

正对数千个随机堆放的骰子进行周期旋转搅动可以使其很快变为有序排列,而用常规的摇动方法很难得到这种结果。如果反复轻敲一个装有弹球或者盐粒等物品的盒子,每敲一次会使它们更加致密。然而,需要经过很长时间,并且采用精心的轻敲顺序才能达到最高的堆积密度。一种新方法可以将放满圆筒的骰子很快达到其最高的堆积密度,即将     

低温近场光学显微术对InGaN/GaN多量子阱电致发光温度特性的研究

使用实验室自制的低温近场光学显微镜研究了InGaN/GaN多量子阱发光二极管在室温和液氮 温度下的近场光学像和近场光谱，发现随着温度的降低，不仅近场光学像的光强起伏大大减 小，量子阱发光峰先蓝移后红移，而且在液氮温度下在光子能量更高的位置上出现了新的发 光峰.通过对实验结果的分析，我们将这个新出现的峰归结为p-GaN层中导带底-受主能级间 跃迁形成. 关键词： InGaN/GaN多量子阱 发光二极管 近场光学 低温     

Beating standard quantum limit via two-axis magnetic susceptibility measurement

We report a metrology scheme which measures the magnetic susceptibility of an atomic spin ensemble along the $x$ and $z$ directions and produces parameter estimation with precision beating the standard quantum limit. The atomic ensemble is initialized via one-axis spin squeezing with optimized squeezing time and parameter $\phi$ (to be estimated) assumed as uniformly distributed between 0 and $2\pi$ while fixed in each estimation. One estimation of $\phi$ can be produced with every two magnetic susceptibility data measured along the two axes respectively, which has an imprecision scaling $({1.43\pm0.02})/N^{0.687\pm0.003}$ with respect to the number $N$ of the atomic spins. The measurement scheme is easy to implement and is robust against the measurement fluctuation caused by environment noise and measurement defects.     

一种基于压缩感知的信道缩短滤波器设计

鉴于使用信道缩短滤波器的系统的复杂度会随着滤波器中非零抽头的增加而快速增大, 运用盲自适应子空间追踪(Blind Adaptive Subspace Pursuit, BASP)算法设计了一种稀疏滤波器来减少非零抽头. 首先在最小均方误差(Minimum Mean Square Error, MMSE)准则下, 将信道缩短问题转化为稀疏滤波器的设计问题, 然后通过加入稀疏度盲估计过程, 使滤波器可以自适应地改变稀疏度, 最后在子空间追踪算法(Subspace Pursuit, SP)的框架下实现非零抽头不连续的稀疏滤波器设计. 仿真结果表明, 设计的稀疏滤波器有效地实现了信道缩短, 运用该滤波器的系统可在更低的复杂度下获得更高的精度.     

Probing Lattice Vibrations at SiO_2/Si Surface and Interface with Nanometer Resolution

Recent advances in monochromatic aberration corrected electron microscopy make it possible to detect the lattice vibrations with both high-energy resolution and high spatial resolution. Here, we use sub-10 meV electron energy loss spectroscopy to investigate the local vibrational properties of the SiO_2/Si surface and interface. The energy of the surface mode is thickness dependent, showing a blue shift as z-thickness(parallel to the fast electron beam)of SiO_2 film increases, while the energy of the bulk mode and the interface mode keeps constant. The intensity of the surface mode is well-described by a Bessel function of the second kind. The mechanism of the observed spatially dependent vibrational behavior is discussed and compared with dielectric response theory analysis. Our nanometer scale measurements provide useful information on the bonding conditions at the surface and interface.     

追逐不可预测的猎物

传统追捕理论(pursuit-theory)中的问题之一是一群猎狗追捕一只野兔.该模型的一种常用假设是野兔沿着直线奔跑,或者沿着其他预先设置的路线奔跑.研究人员为这个游戏增加新的悬念:野兔是随机奔跑的.他们研究了野兔路径的随机程度,以及猎狗的体能限制等因素对于猎狗抓住野兔的时间影响.     

Hofstadter butterfly的新拓扑相

许多材料表现出拓扑相.大多数情况下,这种稳定拓扑物态来源于复杂能带的自旋—轨道耦合.但是,对一个在强磁场下的二维晶格而言,简单的能带结构也可以出现拓扑效应.这种磁致的拓扑相源自磁场和晶格的竞争,具有像蝴蝶翅膀 一 样 的 分 形 结 构 , 是 Douglas Hofstadter 在 1976 年提出来的,因此 也 ...     

鉴赏音乐如同品酒:音乐厅的感知式评价

<正>一个音乐厅的音响效果是如何影响听众的感觉?如果运用合适的方法,通过询问听众的感受,我们会获益良多。当幕间休息开始时,我的夫人迫不及待地走向门厅。在饮完一杯葡萄酒后,她不停地讲述我们刚刚听过的音乐是多么棒,多么不可思议地听出小提琴独奏中非常轻柔的音色。我感到愕然。刚才的演出并没有使我感动,尽管那是我所喜欢的让·西贝柳斯的小提琴协奏曲。虽然独奏者表演得很好,但是不知     

Synthesis, Characterization and Magnetic Property Measurements of Zn1－xMnxO Nanoparticles via Vapour Phase Growth

Nanosized Mn-doped ZnO particles were synthesized using a vapour phase growth method. The x-ray analysis revea/s a wurtzite ZnO structure with small expansion of the lattice constants due to the doping of Mn in ZnO. The TEM analysis shows that the nanoparticles have an average diameter around 37 nm, and energy dispersive spectroscopy detection on single nanoparticle indicates that the manganese concentration is around 3.5at.%. Magnetization measurements under field cooling conditions reveal that the as-grown Mn-doped ZnO nanoparticles show paramagnetic behaviour. This work demonstrates that Mn can be doped into nanosized ZnO structures via vapour phase growth, which represents an important step towards the synthesis of nanosized diluted magnetic semiconductors.