Atom-scale manipulation of Si(111)7 × 7 by STM: dependence on polarity and amplitude of voltage pulses
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Polarity (international relations)
Atomic units
Atomic scale modification of surfaces may trigger off dynamical processes. This is shown here by using the tip of a scanning tunneling microscope to extract an individual atom in a controlled manner from a predefined site of the reconstructed Ge(111)- $c$(2 $\ifmmode\times\else\texttimes\fi{}$8) surface. By thermally activated hopping of neighboring adatoms to the vacant site, the produced single-atom vacancy diffuses on the surface. From statistical analysis of the adatom configurations observed around the vacancy, we determine tiny ( $<$0.15 eV) differences in free energy between the configurations. A linear increase of the configuration free energy with the number of adatoms located at metastable ${T}_{4}$ sites is reported.
Metastability
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Many studies have revealed that confined water chain flipping is closely related to the spatial size and even quantum effects of the confinement environment. Here, we show that these are not the only factors that affect the flipping process of a confined water chain. First-principles calculations and analyses confirm that quantum tunnelling effects from the water chain itself, especially resonant tunnelling, enhance the hydrogen bond rotation process. Importantly, resonant tunnelling can result in tunnelling rotation of hydrogen bonds with a probability close to 1 with only 0.597 eV provided energy. Compared to sequential tunnelling, resonant tunnelling dominants water chain flipping at temperatures up to 20 K higher. Additionally, the ratio of the resonant tunnelling probability to the thermal disturbance probability at 200 K is at least ten times larger than that of sequential tunnelling, which further illustrates the enhancement of hydrogen bond rotation brought about by resonant tunnelling.
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Polarity (international relations)
Sentiment Analysis
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Polarity memory often commonly stabilizes a polarity order in eukaryotic cells and self-propelled disks. This seemingly does not hold for dense cases: the polarity memory of dense cells stabilizes the order whereas that of dense self-propelled disks destabilizes their order. As the origin of this difference in the stabilization of the order from the aspect of eukaryotic cells, we consider the inhibition of polarity fluctuation, owing to polarity memory in dense cells. We examine this inhibition in model cells and evaluate the polarity fluctuation defined as the variance from the polarity order. We show that the polarity fluctuation is inhibited by the polarity memory averaging out configuration fluctuation. Therefore, the order is stabilized only against cell configuration fluctuation, but probably not against the internal polarity fluctuation considered in self-propelled disks. This result implies that the difference in the polarity order reflects the difference in fluctuations between these systems.
Polarity (international relations)
Cell polarity
Polarity reversal
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Scanning tunneling microscopes (STM) are used extensively for studying and manipulating matter at the atomic scale. In spite of the critical role of the STM tip, procedures for controlling the atomic-scale shape of STM tips have not been rigorously justified. Here, we present a method for preparing tips in situ while ensuring the crystalline structure and a reproducibly prepared tip structure up to the second atomic layer. We demonstrate a controlled evolution of such tips starting from undefined tip shapes.
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Tunnel effect
Tunnel junction
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Beyond the double barrier diode as the paradigm of resonant tunnelling, a range of novel tunnelling phenomena have been investigated with judiciously designed multilayer semiconductor structures. We describe three such examples: tunnelling between two superlattices, tunnelling through indirect gap semiconductor layers, and the optical detection of tunnelling between quantum wells.
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The authors have investigated the influence of far infrared radiation on the tunnel current of GaAs/(GaAl)As double-barrier resonant tunnelling devices. From the analysis of the far infrared radiation response, as a function of bias voltage, for different radiation energies, an upper limit for the scattering time in the well is determined. The scattering time is found to be shorter than the tunnelling time, which indicates that tunnelling is mainly sequential in the samples studied.
Tunnel effect
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