Effects of a parallel magnetic field on the metal-insulator transition in a dilute two-dimensional electron system

The temperature dependence of conductivity sigma(T) of a two-dimensional electron system in silicon has been studied in parallel magnetic fields B. At B = 0, the system displays a metal-insulator transition at a critical electron density n(c)(0), and dsigma/dT>0 in the metallic phase. At low fields ( B < or approximately equal to 2 T), n(c) increases as n(c)(B)-n(c)(0) proportional, variant Bbeta ( beta approximately 1), and the zero-temperature conductivity scales as sigma(n(s),B,T = 0)/sigma(n(s),0,0) = f(B(beta)/delta(n)), where delta(n) = [n(s)-n(c)(0)]/n(c)(0) and n(s) is electron density, as expected for a quantum phase transition. The metallic phase persists in fields of up to 18 T, consistent with the saturation of n(c) at high fields.     

Novel metallic behavior in two dimensions

Experiments on a sufficiently disordered two-dimensional (2D) electron system in silicon reveal a new and unexpected kind of metallic behavior, where the conductivity decreases as sigma(n(s),T) = sigma(n(s),T = 0)+A(n(s))T(2) (where n(s) is carrier density) to a nonzero value as temperature T-->0. In 2D, the existence of a metal with dsigma/dT>0 is very surprising. In addition, a novel type of a metal-insulator transition obtains, which is unlike any known quantum phase transition in 2D.     

Fluctuation-driven dynamics of the internet topology

We study the dynamics of the Internet topology based on empirical data on the level of the autonomous systems. It is found that the fluctuations occurring in the stochastic process of connecting and disconnecting edges are important features of the Internet dynamics. The network's overall growth can be described approximately by a single characteristic degree growth rate g(eff) approximately 0.016 and the fluctuation strength sigma(eff) approximately 0.14, together with the vertex growth rate alpha approximately 0.029. A stochastic model which incorporates these values and an adaptation rule newly introduced reproduces several features of the real Internet topology such as the correlations between the degrees of different vertices.     

Scaling of the magnetoconductivity of silicon MOSFETs: evidence for a quantum phase transition in two dimensions

For a broad range of electron densities n and temperatures T, the in-plane magnetoconductivity of the two-dimensional system of electrons in silicon MOSFETs can be scaled onto a universal curve with a single parameter H(sigma)(n,T), where H(sigma) obeys the empirical relation H(sigma) = A(n) [Delta(n)(2)+T2](1/2). The characteristic energy k(B)Delta associated with the magnetic field dependence of the conductivity decreases with decreasing density, and extrapolates to 0 at a critical density n(0), signaling the approach to a zero-temperature quantum phase transition. We show that H(sigma) = AT for densities near n(0).     

Gravitational collapse of colloidal gels

We present a unified framework for understanding the compaction of colloidal gels under their own weight. The dynamics of the collapse are determined by the value of the gravitational stress sigma(g), as compared to the yield stress sigma(Y) of the network. For sigma(g)sigma(Y), the network eventually yields, leading to rapid settling. In both cases, the rate of collapse is backflow limited, while its overall magnitude is determined by a balance between gravitational stress and network elastic stress.     

Observation of self-similar behavior of the 3D, nonlinear Rayleigh-Taylor instability

The Rayleigh-Taylor unstable growth of laser-seeded, 3D broadband perturbations was experimentally measured in the laser-accelerated, planar plastic foils. The first experimental observation showing the self-similar behavior of the bubble size and amplitude distributions under ablative conditions is presented. In the nonlinear regime, the modulation sigma(rms) grows as alpha(sigma)gt(2), where g is the foil acceleration, t is the time, and alpha(sigma) is constant. The number of bubbles evolves as N(t) alpha(omegat sq.rt(9) + C)(-4) and the average size evolves as (t) alpha omega(2)gt(2), where C is a constant and omega = 0.83 +/- 0.1 is the measured scaled bubble-merging rate.     

Electron impact ionization of sodium Rydberg atoms below 2 eV

We have observed electron impact ionization of highly excited sodium Rydberg atoms in ns and nd states, n=35-51, below E=2 eV electron kinetic energy with energy resolution 0.25 eV. Measured absolute cross sections near 0 eV range from sigma(35d) approximately 7 x 10(-10) to sigma(50d) approximately 4 x 10(-9) cm(2). The energy dependence is consistent with that of widely used binary encounter approximation cross sections, and sigma(n) follows a power law in n. The measured cross sections are 14 to 24 times larger than theoretically predicted values. This enhancement may signal the effect of large polarizabilities of high Rydberg states not yet accounted for in ionization theories.     

Global gyrokinetic stability of pressure-gradient-driven electromagnetic modes in tokamaks with regions of low shear

Tokamaks with large pressure gradients (alpha(max)) formed in regions of weak magnetic shear are shown to be susceptible to novel, low-n, global, kinetic, electromagnetic modes with a toroidal mode number n in the range 21 on the magnetic axis and alpha(max) near q(min), new, global kinetic infernal modes with a strong mode rotation omega(r) and a finite growth rate gamma<|omega(r)| are found. For equilibria with reverse shear where q(min) is off axis and alpha(max) near q(min), the existence of an unstable low-n global branch of Alfve n ion temperature gradient modes is revealed with an oscillatory gamma as a function of n. The addition of trapped electron dynamics is shown to be further destabilizing.     

Conductivity of a clean one-dimensional wire

We study the low-temperature low-frequency conductivity sigma of an interacting one-dimensional electron system in the presence of a periodic potential. The conductivity is strongly influenced by conservation laws, which, we argue, need to be violated by at least two noncommuting umklapp processes to render sigma finite. The resulting dynamics of the slow modes is studied within a memory matrix approach, and we find an exponential increase as the temperature is lowered, sigma approximately (Deltan)(2)e(T0/(NT)) close to commensurate filling M/N, Deltan = n-M/N<1, and sigma approximately e((T(')(0)/T)(2/3)) elsewhere.     

狭缝微放电等离子体参数的光谱测量

利用平行管水电极介质阻挡放电装置,在氩气和空气混合气体中,得到了狭缝微放电等离子体。利用发射光谱法,研究了此放电中分子振动温度、分子转动温度和电子的平均能量随气体压强的变化。通过氮分子第二正带系(C3Πu→B3Πg)的发射谱线计算了氮分子的振动温度;利用氮分子离子(N2+)的第一负带系(B2Σu+→X2Σg+)的发射谱线计算了氮分子的转动温度;测量了氮分子离子391.4 nm和激发态的氮分子337.1 nm两条发射谱线的相对强度之比,研究了电子能量的变化。结果表明,当压强从60 kPa增大到100kPa,分子振动温度及分子转动温度均减小,氮分子离子谱线与激发态的氮分子谱线的强度之比亦减小。     

Suppressed molecular ionization for a class of diatomics in intense femtosecond laser fields

It is shown that the puzzling experimental observation of suppressed molecular ionization, in intense laser fields, of O2, and its absence in N2, is a symmetry induced dynamical effect. More generally, it is predicted that the ionization signal of the class of homonuclear diatomic molecules having valence orbitals with an antibonding symmetry (e.g., sigma(u), pi(g)) would be suppressed, but not those with a bonding symmetry (e.g., sigma(g), pi(u)). The suppression effect can be visualized as due to an effective destructive interference between the two subwaves of the ionizing electron emerging from the two atomic centers.     

NO分子A^2∑←X^2∏跃迁的激光双光子荧光激发谱

以Nd：YAD脉冲激光器泵浦的光学参量发生器／放大器(OPG／OPA)作激发光源,获得了420～472nm波长范围内NO分子的双光子激光荧光激发谱,并利用此技术对N0分子的能级结构进行了实验研究,将所得谱线峰归属为NO(A^2∑←X^2∏)的跃迁,荧光强度随激光强度的二次方变化关系表明此过程是一双光子激发过程。利用实验所得峰值波长计算了NO(A^2∑)态的基振动频率ωe和平衡位置的力常数k。通过对NO分子A^2∑←X^2∏跃迁的荧光时间分辨光谱进行实验研究,得到266Pa气压下A^2∑(v′=0)态的能级寿命r=53．76ns。测量荧光寿命随气压的变化,利用曲线拟合得到NOA。三(v′=0,1)两振动态的自发辐射寿命和无辐射跃迁驰豫速率常数。     

钠分子B^1∏u与2^1∑g^＋间的碰撞激发转移

The B 1pi(u) electronic state of Na2 was excited by the 441.6 nm He-Cd laser line. The Na atomic transitions and the A 1sigma(u)+ --> X 1sigma(g)+ band of Na2 were recorded. From the intensities and spectra of the Na and Na2 fluorescence several collisional processes in the excited sodium atom-dimer system were identified. The Na atomic lines are the result of collisional energy transfer from Na2 (B 1pi(u)) to Na(3P). Predissociation process may also contribute to atomic fluorescence. The A 1sigma(u)+ --> X 1sigma(g)+ band is interpreted through a populating mechanism involving collisional transfer from B 1pi(u) to 2 1sigma(g)+ followed by a radiative transfer to the A 1sigma(u)+ state. From the decay constants and fluorescence intensities, the rate coefficient at 360 degrees C for collisional energy transfer from Na2 (B 1pi(u)) to Na2 (2 1sigma(g)+) was found to be 5.7 x 10(-10) cm3 x s(-1). The predissociation rate of the B 1pi(u) is 2.7 x 10(6) s(-1).     

Subsampling effects in neuronal avalanche distributions recorded in vivo

Background  

Many systems in nature are characterized by complex behaviour where large cascades of events, or avalanches, unpredictably alternate with periods of little activity. Snow avalanches are an example. Often the size distribution f(s) of a system's avalanches follows a power law, and the branching parameter sigma, the average number of events triggered by a single preceding event, is unity. A power law for f(s), and sigma = 1, are hallmark features of self-organized critical (SOC) systems, and both have been found for neuronal activity in vitro. Therefore, and since SOC systems and neuronal activity both show large variability, long-term stability and memory capabilities, SOC has been proposed to govern neuronal dynamics in vivo. Testing this hypothesis is difficult because neuronal activity is spatially or temporally subsampled, while theories of SOC systems assume full sampling. To close this gap, we investigated how subsampling affects f(s) and sigma by imposing subsampling on three different SOC models. We then compared f(s) and sigma of the subsampled models with those of multielectrode local field potential (LFP) activity recorded in three macaque monkeys performing a short term memory task.     

Hall resistivity of granular metals

We calculate the Hall conductivity sigma(xy) and resistivity rho(xy) of a granular system at large tunneling conductance g(T)>1. We show that in the absence of Coulomb interaction the Hall resistivity depends neither on the tunneling conductance nor on the intragrain disorder and is given by the classical formula rho(xy)=H/(n*ec), where n* differs from the carrier density n inside the grains by a numerical coefficient determined by the shape of the grains. The Coulomb interaction gives rise to logarithmic in temperature T correction to rho(xy) in the range Gamma less or similar T less or similar min(g(T)E(c), E(Th)), where Gamma is the tunneling escape rate, E(c) is the charging energy, and E(Th) is the Thouless energy of the grain.     

Entropy production in a two-dimensional reversible Gray-Scott system

The entropy production sigma is calculated in the time evolution processes toward a Turing-like pattern and a chaotic pattern in a two-dimensional reaction-diffusion system. The contributions of reaction and diffusion to the entropy production are evaluated separately. Though its contribution to total sigma is about 5%, the entropy production in diffusion foretells the moving direction of the dots (reaction spots) and the line-shaped patterns. The entropy production of the entire system sigma depicts well the cooperative dynamics and evolution of chaotic dot patterns. It is suggested that sigma can be a scalar measure for quantitative studies of hierarchic pattern dynamics. The relation is also discussed between the bifurcation parameter and the distance from thermodynamic equilibrium.     

Spin correlation and discrete symmetry in spinor Bose-Einstein condensates

We study spin correlations in Bose-Einstein condensates of spin 1 bosons with scatterings dominated by a total spin equal 2 channel. We show that the low energy spin dynamics in the system can be mapped into an o(n) nonlinear sigma model. n = 3 at the zero magnetic field limit and n = 2 in the presence of weak magnetic fields. In an ordered phase, the ground state has a discrete Z2 symmetry and is degenerate under the group [U(1)xS(n-1)]/Z(2). We explore consequences of the discrete symmetry and propose some measurements to probe it.     

电子束在α-磁铁模型中传输模拟与优化

在电子直线加速器设计中,α-磁铁通常被用来压缩脉冲长度,采用热阴极微波电子枪和α-磁铁相结合的设计,可以获得皮秒或亚皮秒量级脉冲长度的电子束。电子束在α-磁铁中的传输过程较为复杂,相关文献介绍的研究成果均是基于理想模型,且有镜板开孔的α-磁铁的束流传输研究,对α-磁铁的使用具有重要的指导作用。本文首先简单介绍了用于高能电子成像装置的α-磁铁模型的设计,对电子束在理想模型与α-磁铁模型中传输模拟结果进行对比分析,验证了所构建模型可以有效地实现脉冲纵向压缩的结论。此外,对以优化角度进入α-磁铁和在空间电荷效应影响下的单束团及多束团束流动力学模拟结果进行了讨论,提出通过减小入射角度来补偿α-磁铁镜板上的束流进出孔对束流传输影响的方案。通过在α-磁铁与电子枪之间增加四极磁铁组,优化了束流发射度和束团尺寸。     

Testing the stability of fundamental constants with the 199Hg+ single-ion optical clock

Over a two-year duration, we have compared the frequency of the 199Hg+ 5d(10)6s (2)S(1/2)(F=0)<-->5d(9)6s(2) (2)D(5/2)(F=2) electric-quadrupole transition at 282 nm with the frequency of the ground-state hyperfine splitting in neutral 133Cs. These measurements show that any fractional time variation of the ratio nu(Cs)/nu(Hg) between the two frequencies is smaller than +/-7 x 10(-15) yr(-1) (1sigma uncertainty). According to recent atomic structure calculations, this sets an upper limit to a possible fractional time variation of g(Cs)(m(e)/m(p))alpha(6.0) at the same level.     

锂分子3^1Ⅱg里德堡态的光谱研究

我们用脉冲光学-光学双共振荧光激发谱（OODR），测量了在35500-38000cm^-1能量范围内的里德堡态，观察到^7Li23^1Ⅱg态的10个振动能，本文观测的3^1Ⅱg态的146条激发谱线进行了归属，得到了新的Dunhm常数，RKR势能曲线以及A^1∑u^ 到3^1Ⅱg态跃迁的夫兰克-康登因子，并且讨论了3^1Ⅱg态的双重分裂以及与附近里德堡态F（4）^1∑g^ ,5^1∑g^ ,6^1∑g^ 和G（2）^1Ⅱg的微扰，发现在我们的实验精度下（0.2cm^-1)3^1Ⅱg态掇重分裂可以忽略。