Existence of an orientational electric dipolar response in c60 single crystals

The dielectric constant in and conductivity sigma of undoped C(60) single crystals have been measured as a function of temperature, 10 K < T < 330 K, and frequency, 0.2 kilohertz < f < 100 kilohertz. On cooling below the first-order structural phase transition at 260 K, a Debye-like relaxational contribution to the dielectric response is observed, which requires the presence of permanent electric dipoles. The relaxation rate is thermally activated with a broad distribution of energies centered at 270 millielectron volts. The existence of a dipole moment in C(60) is unexpected, because it is precluded by symmetry for the pure ordered cubic phase. These data suggest that the high degree of frozen-in orientational disorder of the C(60) molecules is responsible for the existence of electric dipolar activity.     

Toroidal triblock copolymer assemblies

A stable phase of toroidal, or ringlike, supramolecular assemblies was formed by combining dilute solution characteristics critical for both bundling of like-charged biopolymers and block copolymer micelle formation. The key to toroid versus classic cylinder micelle formation is the interaction of the negatively charged hydrophilic block of an amphiphilic triblock copolymer with a positively charged divalent organic counterion. This produces a self-attraction of cylindrical micelles that leads to toroid formation, a mechanism akin to the toroidal bundling of semiflexible charged biopolymers such as DNA. The toroids can be kinetically trapped or chemically cross-linked. Insight into the mechanism of toroid formation can be gained by observation of intermediate structures kinetically trapped during film casting.     

Simultaneous negative phase and group velocity of light in a metamaterial

We investigated the propagation of femtosecond laser pulses through a metamaterial that has a negative index of refraction for wavelengths around 1.5 micrometers. From the interference fringes of a Michelson interferometer with and without the sample, we directly inferred the phase time delay. From the pulse-envelope shift, we determined the group time delay. In a spectral region, phase and group velocity are negative simultaneously. This means that both the carrier wave and the pulse envelope peak of the output pulse appear at the rear side of the sample before their input pulse counterparts have entered the front side of the sample.     

Defect-induced phase separation in dipolar fluids

A defect-induced, critical phase separation in dipolar fluids is predicted, which replaces the usual liquid-gas transition that is driven by the isotropic aggregation of particles and is absent in dipolar fluids due to strong chaining. The coexisting phases are a dilute gas of chain ends that coexists with a high-density liquid of chain branching points. Our model provides a unified explanation for the branched structures, the unusually low critical temperature and density, and the consequent two-phase coexistence "islands" that were recently observed in experiment and simulation.     

Angular momentum conservation in dipolar energy transfer

Conservation of angular momentum is a familiar tenet in science but has seldom been invoked to understand (or predict) chemical processes. We have developed a general formalism based on Wigner's original ideas concerning angular momentum conservation to interpret the photo-induced reactivity of two molecular donor-acceptor assemblies with physical properties synthetically tailored to facilitate intramolecular energy transfer. Steady-state and time-resolved spectroscopic data establishing excited-state energy transfer from a rhenium(I)-based charge-transfer state to a chromium(III) acceptor can be fully accounted for by F?rster theory, whereas the corresponding cobalt(III) adduct does not undergo an analogous reaction despite having a larger cross-section for dipolar coupling. Because this pronounced difference in reactivity is easily explained within the context of the angular momentum conservation model, this relatively simple construct may provide a means for systematizing a broad range of chemical reactions.     

Ultrastrong coupling of the cyclotron transition of a 2D electron gas to a THz metamaterial

Artificial cavity photon resonators with ultrastrong light-matter interactions are attracting interest both in semiconductor and superconducting systems because of the possibility of manipulating the cavity quantum electrodynamic ground state with controllable physical properties. We report here experiments showing ultrastrong light-matter coupling in a terahertz (THz) metamaterial where the cyclotron transition of a high-mobility two-dimensional electron gas (2DEG) is coupled to the photonic modes of an array of electronic split-ring resonators. We observe a normalized coupling ratio, Ω/ω(c) = 0.58, between the vacuum Rabi frequency, ?, and the cyclotron frequency, ω(c). Our system appears to be scalable in frequency and could be brought to the microwave spectral range with the potential of strongly controlling the magnetotransport properties of a high-mobility 2DEG.     

位于导体壁中的非圆截面等离子体的伸长度对等离子体稳定性的影响

通过引入Cap-cyclide坐标，针对位于导体壁中的非圆截面等离子体的任意伸长度，不使用任何近似的方法，求得了等离子体的稳定解析解。进而利用计算机对解析结果进行数值计算，分析和讨论了位于导体壁中的非圆截面等离子体的伸长度对等离子体稳定性的影响。     

高频引力波在电磁环形腔焦点区域的时空扰动效应

基于弱场条件,计算了在柱坐标下由电磁环形腔中强电磁简正模在其焦点区域产生的引力驻波的第二类克氏联络、里奇张量、曲率标量及其能量密度.研究发现,在焦点区域的引力驻波的能量密度是正定的.此外,讨论了该引力驻波的能量密度和时空的曲率标量的关系.高Q值强场环形微波腔以及受控核聚变的Tokamak装置均具有轴对称的特点,并可作为可能的潜在高频引力波源.     

Action spectra for photoperiodic response in a diapausing mosquito

While 540 nanometers is the most effective wavelength in provoking development, larvae are far more responsive to 540-nanometer light if it is provided immediately preceding rather than followng a white-light photophase which otherwise serves to maintain diapause. This difference in sensitivity is probably due to bleaching and implies that the larvae experience an asymmetric day.     

Lithium blocks a phosphoinositide-mediated cholinergic response in hippocampal slices

The effects of lithium on inositol phosphate metabolism may account for the therapeutic actions of lithium in affective disorder. Muscarinic stimulation of the phosphoinositide system blocks synaptic inhibitory actions of adenosine in the hippocampal slice. At therapeutic concentrations, lithium diminished this muscarinic response, whereas rubidium, which does not affect phosphoinositide metabolism, had no effect. A dampening of phosphoinositide-mediated neurotransmission may explain the normalizing effects of lithium in treating both mania and depression.     

Homeothermic response to reduced ambient temperature in a scarab beetle

Elephant beetles (Megasoma elephas; Scarabaeidae) weighing from 10 to 35 grams, respond homeothermically when ambient temperature is reduced below about 20 degrees C in the laboratory. This metabolic response is not associated with locomotion or any other overt activity. Warming is initiated when the body temperature reaches an apparent set point of 20 degrees to 22 degrees C. Unlike the case for euthermic birds and mammals, energy metabolism and body temperature in these beetles are conspicuously oscillatory, with a given cycle in oxygen consumption peaking before the corresponding cycle in body temperature.     

Classical conditioning of a complex skeletal response

The pigeon's so-called "arbitrary" response of pecking an illuminated disk can be established and maintained by procedures resembling those of classical conditioning. This phenomenon was shown to be independent of the specific signaling relationships between illumination of the pecking disk and presentation of food; it will appear as long as the key is differentially associated with food. When a nondifferential condition is introduced, pecking "extinguishes" even if it has previously been established and even when the new condition involves as much reinforcement as the old one. Reinstating differential conditions reestablishes pecking. The initial conditions determine the speed and apparently the asymptote of pecking rates in the differential condition; initial exposure to a nondifferential procedure retards subsequent acquisition, possibly quite permanently. These findings are discussed in the context of mechanisms of adaptive learning, not involving reward and punishment, which lead to selection of effective behaviors on a nonarbitrary basis.     

Predicting a human gut microbiota's response to diet in gnotobiotic mice

The interrelationships between our diets and the structure and operations of our gut microbial communities are poorly understood. A model community of 10 sequenced human gut bacteria was introduced into gnotobiotic mice, and changes in species abundance and microbial gene expression were measured in response to randomized perturbations of four defined ingredients in the host diet. From the responses, we developed a statistical model that predicted over 60% of the variation in species abundance evoked by diet perturbations, and we were able to identify which factors in the diet best explained changes seen for each community member. The approach is generally applicable, as shown by a follow-up study involving diets containing various mixtures of pureed human baby foods.     

Probing the chiroptical response of a single molecule

Chirally sensitive measurement techniques have generally been restricted to bulk samples. Here, we report the observation of fluorescence-detected circular dichroism (FDCD) from single (bridgedtriarylamine) helicene molecules by using an excitation wavelength (457 nanometers) in the vicinity of an electronic transition that shows circular dichroism in bulk samples. The distributions of dissymmetry (g) parameters by analysis of signals from pure M- and P-type diastereomers are almost perfect mirror images of one another, each spanning a range of both positive and negative values. In addition, we observe a well-defined structure in the histogram of dissymmetry parameters suggestive of specific molecular orientations at the polymer interface. These single-molecule results highlight strong intrinsic circular dichroism responses that can be obscured by cancellation effects in ensemble measurements of a randomly oriented bulk sample.     

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.     

Nucleotide formation as a determinant of 5-Fluorouracil response in mouse leukemias

Survival of mice bearing different transplantable leukemias and treated with 5-fluorouracil was compared with accumulation of drug nucleotides in vitro. There was significant correlation,suggesting that cellular capacity for conversion of the drug to nucleotides is a major determinant of inherent drug sensitivity of these leukemias.     

Late cretaceous precessional cycles in double time: a warm-Earth milankovitch response

Late Cretaceous climatic cycles are reflected in lithological and magnetic variations in carbonate sediments from South Atlantic Deep-Sea Drilling Project site 516F at a paleolatitude of roughly 30 degrees S. Magnetic susceptibility cycles 20 to 60 centimeters in length appear to be controlled by the precession of the equinoxes. Cyclicity is particularly robust within a 24-meter interval in the lower Campanian, where overtone spectral peaks are observed as well as secondary susceptibility maxima within individual precession cycles. One model for this behavior is that sedimentation in the narrow Cretaceous South Atlantic was controlled by equatorial climate dynamics, with the precessional insolation signal rectified by the large land masses surrounding the ocean basin.     

Synthesis of phytoalexins in sorghum as a site-specific response to fungal ingress

Sorghum produces phytoalexins that are 3-deoxyanthocyanidin flavonoids. The compounds inhibit the growth of phytopathogenic fungi in vitro. The phytoalexins appear to be synthesized in subcellular inclusions within a host epidermal cell that is about to be penetrated by a fungus. This site-restricted synthesis suggests that the phytoalexin response occurs initially in the first cells that come under fungal attack and is not simply a response of cells that surround the original infection site.