The dynamic energy landscape of dihydrofolate reductase catalysis

We used nuclear magnetic resonance relaxation dispersion to characterize higher energy conformational substates of Escherichia coli dihydrofolate reductase. Each intermediate in the catalytic cycle samples low-lying excited states whose conformations resemble the ground-state structures of preceding and following intermediates. Substrate and cofactor exchange occurs through these excited substates. The maximum hydride transfer and steady-state turnover rates are governed by the dynamics of transitions between ground and excited states of the intermediates. Thus, the modulation of the energy landscape by the bound ligands funnels the enzyme through its reaction cycle along a preferred kinetic path.     

Hydrogen-evolving solar cells

Sunlight is directly converted to chemical energy in hydrogen-evolving photoelectrochemical cells with semiconductor electrodes. Their Gibbs free energy efficiency of solar-to-hydrogen conversion, 13.3 percent, exceeds the solar-to-fuel conversion efficiency of green plants and approaches the solar-to-electrical conversion efficiency of the best p-n junction cells. In hydrogen-evolving photoelectrodes, electron-hole pairs photogenerated in the semiconductor are separated at electrical microcontacts between the semiconductor and group VIII metal catalyst islands. Conversion is efficient when the island diameters are small relative to the wave-lengths of sunlight exciting the semiconductor; when the island spacings are smaller than the diffusion length of electrons at the semiconductor surface; when the height of the potential energy barriers that separate the photogenerated electrons from holes at the semiconductor surface is raised by hydrogen alloying of the islands; when radiationless recombination of electron-hole pairs at the semiconductor-solution interface between the islands is suppressed by controlling the semiconductor surface chemistry; and when the semiconductor has an appropriate band gap (1.0 to 1.8 electron volts) for efficient solar conversion.     

Effects of federal residential energy conservation programs

We used a detailed engineering-economic simulation model of residential energy use to evaluate the effects of nine different residential energy-use "futures." These "futures" are described in terms of annual and cumulative energy use from 1977 through 2000 by fuel, end use, and in aggregate. Outputs from the model also include economic information on the costs to households of fuels, equipment, and thermal improvements to new and existing structures. The major outputs from these nine cases are shown in Tables 1 and 2 and Figs. 6 and 7.     

转反义GVDE基因烟草的叶绿素荧光参数及抗氧化酶活性的变化

 【目的】研究姜紫黄质脱环氧化酶基因（GVDE）的功能。【方法】以烟草为试材,构建反义抑制表达载体。将从姜中克隆的GVDE基因反向转入烟草,对获得的抗性植株用PCR和PCR-Southern检测分析。同时测定烟草叶片荧光参数变化和抗氧化物酶（APX、SOD、POD）活性。【结果】反义基因已整合到烟草的基因组中。转反义GVDE基因烟草的NPQ、Fv/Fm始终低于对照,APX、SOD和POD等抗氧化酶活性也低于对照植株。【结论】在转反义GVDE基因的烟草中,强光下耗散过剩光能的非辐射能量耗散能力明显降低。     

On finding all suboptimal foldings of an RNA molecule

An algorithm and a computer program have been prepared for determining RNA secondary structures within any prescribed increment of the computed global minimum free energy. The mathematical problem of determining how well defined a minimum energy folding is can now be solved. All predicted base pairs that can participate in suboptimal structures may be displayed and analyzed graphically. Representative suboptimal foldings are generated by selecting these base pairs one at a time and computing the best foldings that contain them. A distance criterion that ensures that no two structures are "too close" is used to avoid multiple generation of similar structures. Thermodynamic parameters, including free-energy increments for single-base stacking at the ends of helices and for terminal mismatched pairs in interior and hairpin loops, are incorporated into the underlying folding model of the above algorithm.     

Deconfined quantum critical points

The theory of second-order phase transitions is one of the foundations of modern statistical mechanics and condensed-matter theory. A central concept is the observable order parameter, whose nonzero average value characterizes one or more phases. At large distances and long times, fluctuations of the order parameter(s) are described by a continuum field theory, and these dominate the physics near such phase transitions. We show that near second-order quantum phase transitions, subtle quantum interference effects can invalidate this paradigm, and we present a theory of quantum critical points in a variety of experimentally relevant two-dimensional antiferromagnets. The critical points separate phases characterized by conventional "confining" order parameters. Nevertheless, the critical theory contains an emergent gauge field and "deconfined" degrees of freedom associated with fractionalization of the order parameters. We propose that this paradigm for quantum criticality may be the key to resolving a number of experimental puzzles in correlated electron systems and offer a new perspective on the properties of complex materials.     

Physics/computer science. Passing messages between disciplines

Problems in computer science, such as error correction in information transfer and "satisfiability" in optimization, show phase transitions familiar from solid-state physics. In his Perspective, Mézard explains how recent advances in these three fields originate in similar "message passing" procedures. The exchange of elaborate messages between different variables and constraints, used in the study of phase transitions in physical systems, helps to make error correction and satisfiability codes more efficient.     

Endotoxin-induced structural transformations in liquid crystalline droplets

The ordering of liquid crystals (LCs) is known to be influenced by surfaces and contaminants. Here, we report that picogram per milliliter concentrations of endotoxin in water trigger ordering transitions in micrometer-size LC droplets. The ordering transitions, which occur at surface concentrations of endotoxin that are less than 10(-5) Langmuir, are not due to adsorbate-induced changes in the interfacial energy of the LC. The sensitivity of the LC to endotoxin was measured to change by six orders of magnitude with the geometry of the LC (droplet versus slab), supporting the hypothesis that interactions of endotoxin with topological defects in the LC mediate the response of the droplets. The LC ordering transitions depend strongly on glycophospholipid structure and provide new designs for responsive soft matter.     

Excitation spectra of circular, few-electron quantum dots

Studies of the ground and excited states in semiconductor quantum dots containing 1 to 12 electrons showed that the quantum numbers of the states in the excitation spectra can be identified and compared with exact calculations. A magnetic field induces transitions between the ground and excited states. These transitions were analyzed in terms of crossings between single-particle states, singlet-triplet transitions, spin polarization, and Hund's rule. These impurity-free quantum dots allow "atomic physics" experiments to be performed in magnetic field regimes not accessible for atoms.     

Spontaneous emission spectrum in double quantum dot devices

A double quantum dot device is a tunable two-level system for electronic energy states. A dc electron current was used to directly measure the rates for elastic and inelastic transitions between the two levels. For inelastic transitions, energy is exchanged with bosonic degrees of freedom in the environment. The inelastic transition rates are well described by the Einstein coefficients, relating absorption with stimulated and spontaneous emission. The most effectively coupled bosons in the specific environment of the semiconductor device used here were acoustic phonons. The experiments demonstrate the importance of vacuum fluctuations in the environment for quantum dot devices and potential design constraints for their use for preparing long-lived quantum states.     

Information and the ecology of scholars

Groups of scholars often behave in ways that are startlingly reminiscent of the behavior of groups of plants and animals in a biological ecosystem. I have tried to show that the parallel may have its basis in two fundamental considerations: (i) the similar properties of stable dissipative structures in a wide variety of contexts and (ii) the mathematical similarity of information to chemical (nutrient) free energy. A particularly intriguing problem is that of successional trends arising from the kinetic limitation of information flow through the intellectual and social structures created by scholars. The "strategy of ecosystem development" may have implications for the strategy of academic administration. In this connection, it is worth bearing in mind Marglef's dictum (6, p. 77): "Probably the hypothesis holds everywhere that the less mature ecosystem feeds the more mature structures around it."     

Packing structures and transitions in liquids and solids

Classification of potential energy minima-mechanically stable molecular packings-offers a unifying principle for understanding condensed phase properties. This approach permits identification of an inherent structure in liquids that is normally obscured by thermal motions. Melting and freezing occur through characteristic sequences of molecular packings, and a defect-softening phenomenon underlies the fact that they are thermodynamically first order. The topological distribution of feasible transitions between contiguous potential minima explains glass transitions and associated relaxation behavior.     

Observation of half-height magnetization steps in Sr2RuO4

Spin-triplet superfluids can support exotic objects, such as half-quantum vortices characterized by the nontrivial winding of the spin structure. We present cantilever magnetometry measurements performed on mesoscopic samples of Sr(2)RuO(4), a spin-triplet superconductor. With micrometer-sized annular-shaped samples, we observed transitions between integer fluxoid states as well as a regime characterized by "half-integer transitions"--steps in the magnetization with half the height of the ones we observed between integer fluxoid states. These half-height steps are consistent with the existence of half-quantum vortices in superconducting Sr(2)RuO(4).     

Photon-mediated hybridization of frenkel excitons in organic semiconductor microcavities

Coherent excitations of intricate assemblies of molecules play an important role in natural photosynthesis. Microcavities are wavelength-dimension artificial structures in which excitations can be made to couple through their mutual interactions with confined photon modes. Results for microcavities containing two spatially separated cyanine dyes are presented here, where simultaneous strong coupling of the excitations of the individual dyes to a single cavity mode leads to new eigenmodes, described as admixtures of all three states. These "hybrid" exciton-photon structures are of potential interest as model systems in which to study energy capture, storage, and transfer among coherently coupled molecular excitations.     

磁性多层膜的磁化过程的一种唯象理论研究

计算了磁性多层膜的静磁自由能，采用绝对能量极小模型，导出了磁相变方程，获得了磁相图和磁滞回线．结合局域能量极小模型所得到的磁相图和磁滞回线，讨论了实验上所得到的磁滞回线．     

Evidence of soft-mode quantum phase transitions in electron double layers

Inelastic light scattering by low-energy spin-excitations reveals three distinct configurations of spin of electron double layers in gallium arsenide quantum wells at even-integer quantum Hall states. The transformations among these spin states appear as quantum phase transitions driven by the interplay between Coulomb interactions and Zeeman splittings. One of the transformations correlates with the emergence of a spin-flip intersubband excitation at vanishingly low energy and provides direct evidence of a link between quantum phase transitions and soft collective excitations in a two-dimensional electron system.     

黄淮海地区土地利用转型与粮食产量耦合关系研究

土地利用转型包含显性形态和隐性形态的变化,而土地利用强度是土地利用隐性形态的重要指征,既可以反映出人类对土地的干预程度,又可以呈现出特定时期社会经济发展状态。本文以土地利用强度的定量测度为抓手,基于分县统计数据及遥感解译的土地利用数据计算了区域土地利用隐性形态的变化,探讨了区域土地利用转型与粮食产量的耦合关系。研究表明：（1）1990-2010年,黄淮海地区耕地利用强度不断增加,而村庄建设用地利用强度快速减少,且区域时空格局差异明显;（2）耕地利用强度-村庄建设用地利用强度-粮食产量耦合类型时空演化特征明显,“低-低-低”类型主要分布在环渤海湾地区,而“高-高-高”类型主要集聚于冀鲁豫交界地区;（3）土地利用转型推动粮食生产格局不断演化,二者耦合关系模式将呈现出由“拮抗”到“协调”的转化过程。综上所述,土地利用转型过程中,科学优化土地利用转型与粮食生产的耦合关系,对保障区域粮食安全有重要作用。     

Phase transitions of Dirac electrons in bismuth

The Dirac Hamiltonian, which successfully describes relativistic fermions, applies equally well to electrons in solids with linear energy dispersion, for example, in bismuth and graphene. A characteristic of these materials is that a magnetic field less than 10 tesla suffices to force the Dirac electrons into the lowest Landau level, with resultant strong enhancement of the Coulomb interaction energy. Moreover, the Dirac electrons usually come with multiple flavors or valley degeneracy. These ingredients favor transitions to a collective state with novel quantum properties in large field. By using torque magnetometry, we have investigated the magnetization of bismuth to fields of 31 tesla. We report the observation of sharp field-induced phase transitions into a state with striking magnetic anisotropy, consistent with the breaking of the threefold valley degeneracy.     

Entropy in an expanding universe

The question of how the observed evolution of organized structures from initial chaos in the expanding universe can be reconciled with the laws of statistical mechanics is studied, with emphasis on effects of the expansion and gravity. Some major sources of entropy increase are listed. An expanding "causal" region is defined in which the entropy, though increasing, tends to fall further and further behind its maximum possible value, thus allowing for the development of order. The related questions of whether entropy will continue increasing without limit in the future, and whether such increase in the form of Hawking radiation or radiation from positronium might enable life to maintain itself permanently, are considered. Attempts to find a scheme for preserving life based on solid structures fail because events such as quantum tunneling recurrently disorganize matter on a very long but fixed time scale, whereas all energy sources slow down progressively in an expanding universe. However, there remains hope that other modes of life capable of maintaining themselves permanently can be found.