The atomic force microscope (AFM) is a powerful tool for imaging individual biological molecules attached to a substrate and placed in aqueous solution. At present, however, it is limited by the speed at which it can successively record highly resolved images. We sought to increase markedly the scan speed of the AFM, so that in the future it can be used to study the dynamic behavior of biomolecules. For this purpose, we have developed a high-speed scanner, free of resonant vibrations up to 60 kHz, small cantilevers with high resonance frequencies (450 -650 kHz) and small spring constants (150 -280 pN͞nm), an objective-lens type of deflection detection device, and several electronic devices of wide bandwidth. Integration of these various devices has produced an AFM that can capture a 100 ؋ 100 pixel 2 image within 80 ms and therefore can generate a movie consisting of many successive images (80-ms intervals) of a sample in aqueous solution. This is demonstrated by imaging myosin V molecules moving on mica (see http:͞͞www.s.kanazawa-u.ac.jp͞phys͞biophys͞bmvmovie.htm). One of the advantages of the atomic force microscope (AFM) (1) is its capacity to image individual biomolecules in, say, a buffered solution containing ions at physiological concentrations (2, 3). Such capacity suggests that the instrument can be used to record the dynamic behavior of such molecules. In practice, however, only very slow processes can be recorded (2, 4-6), because commercially available AFMs require minutes to form an acceptable image, and many interesting biological processes occur at much higher rates. To understand, and overcome, the factors that limit the scanning rate of an AFM, we begin by considering relations between the characteristics of the constituting components.We consider only the ''tapping mode'' of AFM operation (Digital Instruments, Santa Barbara, CA). This is the mode suitable for imaging biological macromolecules, because vertical oscillation of the cantilever at (or near to) its resonance frequency reduces lateral forces between the tip and the sample (7). The oscillating tip briefly taps the surface at the bottom of each swing, resulting in a decrease in oscillation amplitude. During the x-y scan of the sample stage a feedback loop (see below) keeps this decrease (and hence the tapping force) constant; this is necessary for minimizing the deformation of soft samples. The error signal-the difference between a preset signal and the rms amplitude of the cantilever-is fed into a proportional-integraldifferential (PID) feedback circuit. The PID output is amplified and then sent to the z-piezo actuator; this is repeated until the error signal returns to zero. For the three-dimentional movement of the sample stage to follow the sample topography accurately, the bandwidth of the feedback loop should be comparable to, or larger than, the frequency determined by the x-y scan velocity and the apparent width of the features on the surface. To increase the imaging bandwidth, all elements in the feedback loop have to be optimi...

Although multilayer systems possess global inversion symmetry, some of the layers lack local inversion symmetry because no global inversion centers are present on such layers. Such locally non-centrosymmetric systems exhibit spatially modulated Rashba spin-orbit coupling. In this study, the superconductivity in multilayer models exhibiting inhomogeneous Rashba spin-orbit coupling is investigated. We study the electronic structure, superconducting gap, and spin susceptibility in the superconducting state with mixed parity order parameters. We show the enhancement of the spin susceptibility by Rashba spin-orbit coupling and interpret it on the basis of the crossover from a centrosymmetric superconductor to a non-centrosymmetric superconductor. It is also shown that the spin susceptibility is determined by the phase difference of the order parameter between layers and is nearly independent of the parity mixing of order parameters. An intuitive understanding is given on the basis of the analytic expression of superconducting order parameters in the band basis. The results indicate that not only a broken global inversion symmetry but also a broken local inversion symmetry leads to unique properties of superconductivity. We discuss the superconductivity in artificial superlattices involving CeCoIn5 and multilayer high-Tc cuprates.

In animal fertilization, multiple sperms typically arrive at an egg cell to "win the race" for fertilization. However, in flowering plants, only one of many pollen tubes, conveying plant sperm cells, usually arrives at each ovule that harbors an egg cell. Plant fertilization has thus been thought to depend on the fertility of a single pollen tube. Here we report a fertilization recovery phenomenon in flowering plants that actively rescues the failure of fertilization of the first mutant pollen tube by attracting a second, functional pollen tube. Wild-type (WT) ovules of Arabidopsis thaliana frequently (∼80%) accepted two pollen tubes when entered by mutant pollen defective in gamete fertility. In typical flowering plants, two synergid cells on the side of the egg cell attract pollen tubes, one of which degenerates upon pollen tube discharge. By semi-in vitro live-cell imaging we observed that fertilization was rescued when the second synergid cell accepted a WT pollen tube. Our results suggest that flowering plants precisely control the number of pollen tubes that arrive at each ovule and employ a fertilization recovery mechanism to maximize the likelihood of successful seed set.

In flowering plants, double fertilization is normally accomplished by the first pollen tube, with the fertilized ovule subsequently inhibiting the attraction of a second pollen tube. However, the mechanism of second-pollen-tube avoidance remains unknown. We discovered that failure to fertilize either the egg cell or the central cell compromised second-pollen-tube avoidance in Arabidopsis thaliana. A similar disturbance was caused by disrupting the fertilization-independent seed (FIS) class polycomb-repressive complex 2 (FIS-PRC2), a central cell- and endosperm-specific chromatin-modifying complex for gene silencing. Therefore, the two female gametes have evolved their own signaling pathways. Intriguingly, second-pollen-tube attraction induced by half-successful fertilization allowed the ovules to complete double fertilization, producing a genetically distinct embryo and endosperm. We thus propose that each female gamete independently determines second-pollen-tube avoidance to maximize reproductive fitness in flowering plants.