Observation of backward pulse propagation through a medium with a negative group velocity

The nature of pulse propagation through a material with a negative value of the group velocity has been mysterious, as simple models seem to predict that pulses will propagate "backward" through such a material. Using an erbium-doped optical fiber and measuring the time evolution of the pulse intensity at many points within the fiber, we demonstrate that the peak of the pulse does propagate backward inside the fiber, even though the energy flow is always in the forward direction.     

桂林图书馆西文期刊开发与利用探析

通过对本馆西文期刊订购、入藏、读者利用情况的调查,分析了西文期刊利用率低的主要原因,提出了开发与利用西文期刊的几点建议。     

常见媒体对农互动传播的创新探索与实践

采用互动传播理论,对报刊、广播、电视、网络、手机5种常见媒体对农互动传播的方式方法以及创新探索与实践进行了研究梳理。研究发现,常见媒体对农互动传播的创新探索与实践主要分为两种,一是基于传统传播特点进行强化,二是借助自身优势实现多种媒体间的互动融合。并提出了各媒体只有互相融合,取长补短,其互动传播才能创新、增强和拓展的针对性建议。     

常见媒体对农互动传播的创新探索与实践

采用互动传播理论,对报刊、广播、电视、网络、手机5种常见媒体对农互动传播的方式方法以及创新探索与实践进行了研究梳理。研究发现,常见媒体对农互动传播的创新探索与实践主要分为两种,一是基于传统传播特点进行强化,二是借助自身优势实现多种媒体间的互动融合。并提出了各媒体只有互相融合,取长补短,其互动传播才能创新、增强和拓展的针对性建议。     

Electron coherence in a melting lead monolayer

We used angle-resolved photoemission spectroscopy to measure the electronic dispersion and single-particle spectral function in a liquid metal. A lead monolayer supported on a copper (111) surface was investigated as the temperature was raised through the melting transition of the film. Electron spectra and momentum distribution maps of the liquid film revealed three key features of the electronic structure of liquids: the persistence of a Fermi surface, the filling of band gaps, and the localization of the wave functions upon melting. Distinct coherence lengths for different sheets of the Fermi surface were found, indicating a strong dependence of the localization lengths on the character of the constituent atomic wave functions.     

Cavity quantum electrodynamics: coherence in context

Modern cavity quantum electrodynamics (cavity QED) illuminates the most fundamental aspects of coherence and decoherence in quantum mechanics. Experiments on atoms in cavities can be described by elementary models but reveal intriguing subtleties of the interplay of coherent dynamics with external couplings. Recent activity in this area has pioneered powerful new approaches to the study of quantum coherence and has fueled the growth of quantum information science. In years to come, the purview of cavity QED will continue to grow as researchers build on a rich infrastructure to attack some of the most pressing open questions in micro- and mesoscopic physics.     

Emerging coherence in a population of chemical oscillators

Coherence of interacting oscillating entities has importance in biological, chemical, and physical systems. We report experiments on populations of chemical oscillators and verify a 25-year-old theory of Kuramoto that predicts that global coupling in a set of smooth limit-cycle oscillators with different frequencies produces a phase transition in which some of the elements synchronize. Both the critical point and the predicted dependence of order on coupling are seen in the experiments. We extend the studies both to relaxation and to chaotic oscillators and characterize the quantitative similarities and differences among the types of systems.     

Mesoscopic phase coherence in a quantum spin fluid

Mesoscopic quantum phase coherence is important because it improves the prospects for handling quantum degrees of freedom in technology. Here we show that the development of such coherence can be monitored using magnetic neutron scattering from a one-dimensional spin chain of an oxide of nickel (Y2BaNiO5), a quantum spin fluid in which no classical static magnetic order is present. In the cleanest samples, the quantum coherence length is 20 nanometers, which is almost an order of magnitude larger than the classical antiferromagnetic correlation length of 3 nanometers. We also demonstrate that the coherence length can be modified by static and thermally activated defects in a quantitatively predictable manner.     

Neuronal coherence as a mechanism of effective corticospinal interaction

Neuronal groups can interact with each other even if they are widely separated. One group might modulate its firing rate or its internal oscillatory synchronization to influence another group. We propose that coherence between two neuronal groups is a mechanism of efficient interaction, because it renders mutual input optimally timed and thereby maximally effective. Modulations of subjects' readiness to respond in a simple reaction-time task were closely correlated with the strength of gamma-band (40 to 70 hertz) coherence between motor cortex and spinal cord neurons. This coherence may contribute to an effective corticospinal interaction and shortened reaction times.     

Bone cells: a serum-free medium supports proliferation in primary culture

Bone-cells isolated from embryonic rat calvaria increase in number two-to threefold when cultured at high, but not at low, population densities in a serum-free medium that contains albumin. Cultured cells respond to parathyroid hormone and exhibit a marked rise in alkaline phosphatase activity during proliferation, which suggests the progressive differentiation or preferential growth of osteoblast-like cells.     

Geminga's tails: a pulsar bow shock probing the interstellar medium

We report the X-ray Multimirror Mission-Newton European Photon Imaging Camera observation of two elongated parallel x-ray tails trailing the pulsar Geminga. They are aligned with the object's supersonic motion, extend for approximately 2', and have a nonthermal spectrum produced by electron-synchrotron emission in the bow shock between the pulsar wind and the surrounding medium. Electron lifetime against synchrotron cooling matches the source transit time over the x-ray features' length. Such an x-ray detection of a pulsar bow shock (with no Halpha emission) allows us to gauge the pulsar electron injection energy and the shock magnetic field while constraining the angle of Geminga's motion and the local matter density.     

应用正交试验筛选新台糖22号组培配方

利用正交试验研究不同配方对新台糖22号甘蔗组培苗增殖和促根的影响,以筛选提高甘蔗组培苗生根率及假植成活率较佳的配方。结果表明,较佳的腋芽诱导培养基配方为MS+6-BA 1mg/L+KT 0.5 mg/L+IBA 0.2 mg/L;丛芽分化、增殖培养基配方为MS+6-BA 0.8 mg/L+IBA 0.03 mg/L;促根培养基配方为1/2MS+IBA 0.2 mg/L+NAA 0.1 mg/L。     

六氯苯处理技术现状及其生物降解研究

目的研究不同质量浓度六氯苯(hexaehlorobenzen,HCB)胁迫下狭叶香蒲(Typha angustifolia)根系生物膜菌群结构。方法利用扫描电镜观察不同质量浓度(38和760 μg/L) HCB富营养化水体中香蒲根系生物膜的形态结构;采用高通量测序技术对细菌16S rDNA基因进行序列测定。结果电镜结果显示：根系生物膜附着微生物主要呈椭圆形或杆状。经不同质量浓度HCB处理后,狭叶香蒲根系附着微生物的数量明显增加,随着水体HCB质量浓度升高其根系生物膜中的细菌数量降低。高通量测序结果表明：根系生物膜的优势菌门是变形菌门(Proteobacteria)、放线菌门(Actinobacteria)、绿弯菌门(Chloroflexi)、蓝藻菌门(Cyanobacteria)和厚壁菌门(Firmicutes)等。属水平优势菌群主要包括3类：(1) 高效净化水质的光合菌群,如红微菌属(Rhodomicrobium)、红假单胞菌属(Rhodopseudomonas)、红长命菌属(Rubrivivax)、红芽生菌属(Rhodoblastus)和红细菌属(Rhodobacter)等;(2) 以丛毛单胞菌属(Comamonas)、贪噬菌属(Variovorax)、不动杆菌属(Acinetobacter)、嗜酸菌属(Acidovorax)等为主的有机物降解微生物;(3) 参与脱氮和氮循环的反硝化菌群,如贪噬菌属(Variovorax)、丛毛单胞菌属(Comamonas)、脱硫弧菌属(Desulfovibrio)、红细菌属(Rhodobacter)、不动杆菌属(Acinetobacter)、中慢生根瘤菌属(Mesorhizobium)和红长命菌属(Rubrivivax)等。PICRUSt功能预测结果表明：根系生物膜中菌群的主要代谢功能是碳水化合物、氨基酸以及能量代谢。结论38 µg/L HCB的富营养化水体中狭叶香蒲根系细菌群落的丰富度和多样性均高于760 µg/L HCB的富营养化水体根系细菌群落。此外,属水平优势菌群可能与富营养化水体的水质净化、有机物降解以及脱氮和氮循环有关。研究结果为富营养化水体有机污染物的去除机制提供了理论基础。     

Lead-iron phosphate glass: a stable storage medium for high-level nuclear waste

Results are presented which show that lead-iron phosphate glasses are a promising new waste form for the safe immobilization of both high-level defense and high-level commercial radioactive waste. Relative to the borosilicate nuclear waste glasses that are currently the "reference" waste form for the long-term disposal of nuclear waste, lead-iron phosphate glasses have several distinct advantages: (i) an aqueous corrosion rate that is about 1000 times lower, (ii) a processing temperature that is 100 degrees to 250 degrees C lower, and (iii) a much lower melt viscosity in the temperature range from 800 degrees to 1000 degrees C. Most significantly, the lead-iron phosphate waste form can be processed using a technology similar to that developed for borosilicate nuclear waste glasses.     

Rotational coherence and a sudden breakdown in linear response seen in room-temperature liquids

Highly energized molecules normally are rapidly equilibrated by a solvent; this finding is central to the conventional (linear-response) view of how chemical reactions occur in solution. However, when a reaction initiated by 33-femtosecond deep ultraviolet laser pulses is used to eject highly rotationally excited diatomic molecules into alcohols and water, rotational coherence persists for many rotational periods despite the solvent. Molecular dynamics simulations trace this slow development of molecular-scale friction to a clearly identifiable molecular event: an abrupt liquid-structure change triggered by the rapid rotation. This example shows that molecular relaxation can sometimes switch from linear to nonlinear response.     

Spontaneous Bose coherence of excitons and polaritons

In the past decade, there has been an increasing number of experiments on spontaneous Bose coherence of excitons and polaritons. Four major areas of research are reviewed here: three-dimensional excitons in the bulk semiconductor Cu2O, two-dimensional excitons in coupled quantum wells, Coulomb drag experiments in coupled two-dimensional electron gases, and polaritons in semiconductor microcavities. The unifying theory of all these experiments is the effect of spontaneous symmetry breaking in the Bose-Einstein condensation phase transition.     

A defined medium for a fastidious Spiroplasma

A defined medium (H-1) was developed for cultivation of the suckling mouse cataract agent, Spiroplasma mirum, a fastidious member of the class Mollicutes that causes cataracts and chronic brain infection in inoculated neonate mice. The H-1 medium was used to show the importance of sphingomyelin as a growth factor for the culture of the spiroplasma in vitro. The growth of Spiroplasma mirum and the pathology it induces in sphingomyelin-rich tissues in vivo may be related to this dependency.