Results 1 - 10 of 24670
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[en] This paper suggests a model for a bubble universe using buildable false vacuum bubbles. We study the causal structures of collapsing false vacuum bubbles using double-null simulations. False vacuum bubbles violate the null energy condition and emit negative energy along the outgoing direction through semi-classical effects. If there are a few collapsing false vacuum bubbles and they emit negative energy to a certain region, then the region can be approximated by a negative energy bath, which means that the region is homogeneously filled by negative energy. If a false vacuum bubble is generated in the negative energy bath and the tension of the bubble effectively becomes negative in the bath, then the bubble can expand and form an inflating bubble universe. This scenario uses a set of assumptions different from those in previous studies because it does not require tunneling to unbuildable bubbles.
[en] We report on a multiple-state switching behavior in the tip height or tunneling current of scanning tunneling microscopy on the Si(111)- 7 x 7 surface. This switching is caused by displacement of silicon adatoms under the influence of energetic tunneling electrons. When the tip is fixed over a center adatom, five well-defined levels appear in the measured tip height and tunneling current. These levels are attributed to different electronic structures, depending on the configuration of the center adatoms in the unit cell. We also demonstrate manipulations of the center adatoms by controlling the sample bias.
[en] Large size shield tunnels have been constructed with the increasing demands, because it offers cost efficiency in urban areas. One major problem created by shield tunnelling is that ground settlement occurs, which is crucial to existing structures. A detailed case study of surface settlement induced by the operation of a large-diameter earth pressure balance (EPB) shield tunnel in Beijing was presented. This study provides important new insights into the values of settlement trough parameters of empirical methods. The narrower transverse settlement trough was observed in large size tunnels. The development of longitudinal settlement can be divided into four stages, and methods were further proposed to assess the associated four components of volume loss. Results demonstrate that the proposed methods can efficiently predict the volume loss and the associated ground settlement within a reasonable accuracy by comparing with the collected field observations. In addition, a bandwidth of achievable surface settlement for future large EPB shield tunnels is suggested via performing analysis on real settlements.
[en] Emerging applications of transient electromagnetic methods (TEM) in tunnelling require higher resolution on the distributions and shapes of low resistivity bodies, such as karst water and karst pipes, using multi-component and multi-array receivers. However, there are no apparent resistivity definitions for both vertical and horizontal components with offsets inside the loop. Although the raw field can show the differences of the earth electric structure, it is not straightforward. Apparent resistivity is very convenient and easy for engineers. We have developed a method for multi-component and multi-array TEM which can be applied in tunnelling and defined the expressions of apparent resistivity. This method takes advantage of the difference in resolution among components. A homogeneous half-space model and four typical three-layered models are used to test the effectiveness of the new definition. A field case history is carried out and analysed to demonstrate the viability of this technique. The results suggest that it is feasible to use the technique in tunnelling, especially for identifying the spatial distribution of karst water and karst pipes. (paper)
[en] The aim of this paper is to determine the face stability of a circular tunnel driven by a pressurized shield. The analysis is performed by a novel visual platform named as TBM Studio, which is developed to perform automatic analytical and visualization analysis on the tunnel face stability during shield tunnelling in soft grounds. Data modeling, analysis, and visualization are programmed separately in different C# projects which are integrated together. The hydrogeological profile of the analysis domain can be automatically generated, and the analysis cross sections can be automatically identified. Several user-friendly interfaces are created to facilitate data input, model calculation, and result visualization. The platform provides users with different analytical and empirical methods to calculate face support pressure and associated ground settlement. The data structure and database are established based on object-oriented programming method. Data-driven dynamic modeling schemes are developed for 3D visualization of the ground and the tunnel, which is efficient and flexible. The application example indicates that visualization analysis by TBM Studio is useful for practicing engineers to select the most appropriate face support pressure calculation model, to determine the safe range of face support pressure, and to identify the critical sections for shield tunnelling prior to excavation.
[en] A scanning tunneling microscope was used to perform inelastic electron tunneling spectroscopy on single Cu(II) etioporphyrin-I molecules. The resulting vibrational spectra showed an increase in ac tunneling conductance at 359 mV. This change was attributed to the vibrational excitation of the methene bridge C-H stretch mode. Vibrational microscopy was used to image the spatial distribution of the inelastic tunneling channels involved in this excitation. (c) 2000 American Institute of Physics
[en] Combining the features of graphene, spintronics and superlattices, this study reports huge tunnel magnetoresistance achieved using graphene-based magnetic tunnel junctions with superlattice barriers. Our calculation results show that the tunnel magnetoresistance of as much as 10"7% is obtained using a magnetic tunnel junction with a six-cell superlattice barrier. This magnetoresistance increases as the number of cells in the superlattice barrier increases. This remarkable value can be linked to the artificially tailored band structures of the superlattice barriers. This generic nature provides new opportunities for other spintronics applications, using graphene superlattices. (paper)
[en] Tunneling in densely populated urban areas frequently brings about under-crossing the existing tunnels, which may induce serviceability problems to the existing tunnels. In order to obtain accurate prediction of ground settlement, the current study synthetically analyzes the coupling effects of new tunnels, in-between soil and the existing tunnel, and proposes a new method for predicting the ground settlement caused by construction of a new subway twin tunnel under-passing an existing tunnel. This paper classifies the influence zone caused by construction of the twin tunnels under-passing existing tunnel into strong, weak and no influence zones based on synthetically considering the coupling effects of front-tunnel, in-between soil, rear-tunnel, and the existing tunnel. Further, by extending the proposed method and applying it to the case of twin-tunneling under-crossing an existing tunnel, detail processes for predicting the ground settlement are presented for the cases of strong and weak influence zones. Two case studies were carried out to verify the proposed method, and the results show that the predicting settlements well agree with the measured data.
[en] We discuss charge oscillation both in Josephson junctions and in normal tunnel junctions taking an approach close to that of Ben-Jacob and Gefen. We discuss interband transitions due to Zener tunneling within the framework of a master equation approach. We show that the Zener tunneling leads to a new type of noise that has long time correlations and that does not satisfy the fluctuation-dissipation relations. We give a summary and point out possible experimental verification of our predictions and discuss the effect of quasi-particle tunneling on charge oscillations in Josephson junctions. (orig./BUD)
[en] Scanning tunneling microscopy (STM) is used for replacement patterning of self-assembled monolayers (SAMs) of thiols on a sub-10 nm scale. Contrasting other schemes of scanning probe patterning of SAMs, the exchange of molecules relies on differences in conductance and, thus, occurs under tunneling conditions where the resolution of the tip is maintained. Exchange takes place at the boundary between different thiols but only when the tip moves from areas of lower to higher conductance. In combination with SAMs which exhibit excellent structural quality, patterns with a contour definition of ± 1 molecule, lines as thin as 2.5 nm and islands with an area of less than 20 nm2 are straightforwardly produced. It is suggested that the shear force exerted onto the molecules with the lower conductance triggers displacement of the one with higher conductance.