Observation of mode-like features in tunneling spectra of iron-based superconductors

We report scanning tunneling microscopy/spectroscopy（STM/STS） studies on iron-based superconductors of Ba1-xKxFe2As2 and nearly optimally doped Fe（Te,Se）. Mode-like features were observed universally outside the superconducting gaps in the tunneling spectra, which are similar to our previous observations in other samples and can be ascribed to the interaction between electrons and spin excitations. Furthermore, an almost linear relationship between the superconducting gaps and the superconducting transition temperatures was noted and should also be taken into account in understanding the mechanism of iron-based superconductors.

Advantages of employing multilevel monitoring wells for design of tunnels subjected to multi-aquifer alluvial

For tunnels being excavated through multiple knowledge of the aquifers’hydraulic head becomes essential for determining groundwater inflow into the tunnel and analyzing its stability,specifically using multilevel monitoring systems.In the multi-aquifer alluvial section of the Glas tunnel(Iran),since the hydraulic head calculations were based on the data obtained from single-piezometer boreholes,the excavation risk was assessed to be at high level and the tunnel seemed to be unstable,thus an incorrect conclusion was derived from the misleading data.To take cost mitigation measures into account,it was necessary to calculate the hydraulic head at tunnel level accurately.By installing nested and clustered wells the mean hydraulic head was measured to be 70 m,significantly different from the 90 m previously determined by boreholes.Considering the updated value,the groundwater inflow and bulkhead load,formerly calculated as 0.65 m^(3)/s and 9.5 bars,were determined to be 0.49 m^(3)/s and 7.5 bars,respectively.

Impact of insulator layer thickness on the performance of metal-MgO-ZnO tunneling diodes

The performance of metal-insulator-semiconductor （MIS） type tunneling diodes based on ZnO nanostructures is investigated through modeling. The framework used in this work is the Schr6dinger equation with an effective-mass approximation. The working mechanism of the MIS type tunneling diode is investigated by examining the electron density, electric field, electrostatic potential, and conduction band edge of the device. We show that a valley in the electrostatic potential is formed at the ZnO/MgO interface, which induces an energy barrier at the ZnO side of this interface. Therefore, electrons need to overcome two barriers： the high and narrow MgO barrier, and the barrier from the depletion region induced at the ZnO side of the ZnO/MgO interface. As the MgO layer becomes thicker, the valley in electrostatic potential becomes deeper. At the same time, the barrier induced at the ZnO/MgO interface becomes higher and wider. This leads to a fast decrease in the current passing through the MIS diode. We optimize the thickness of the MgO insulating layer, sandwiched between a ZnO film （in this work we use a single ZnO nanowire） and a metal contact, to achieve maximum performance of the diode, in terms of rectification ratio. An optimal MgO layer thickness of 1.5 nm is found to yield the highest rectification ratio, of approximately 169 times that of a conventional metal-semiconductor-metal Schottky diode. These simulated results can be useful in the design and optimization of ZnO nanodevices, such as light emitting diodes and UV photodetectors.

Robustness evaluation of cutting tool maintenance planning for soft ground tunneling projects

Tunnel boring machines require extensive maintenance and inspection effort to provide a high availability.The cutting tools of the cutting wheel must be changed timely upon reaching a critical condition.While one possible maintenance strategy is to change tools only when it is absolutely necessary,tools can also be changed preventively to avoid further damages.Such different maintenance strategies influence the maintenance duration and the overall project performance.However,determine downtime related to a particular mainte-nance strategy is still a challenging task.This paper shows an analysis of the robustness to achieve the planned project performance of a maintenance strategy considering uncertainties of wear behavior of the cutting tools.A simulation based analysis is presented,imple-menting an empirical wear prediction model.Different strategies of maintenance planning are compared by performing a parameter vari-ation study including Monte-Carlo simulations.The maintenance costs are calculated and evaluated with respect to their robustness.Finally,an improved and robust maintenance strategy has been determined.

From advance exploration to real time steering of TBMs:A review on pertinent research in the Collaborative Research Center.“Interaction Modeling in Mechanized Tunneling”

This paper reports on planning and construction related results from research performed at the Collaborative Research Center“Interaction Modeling in Mechanized Tunneling”at Ruhr-University Bochum,Germany.Research covers a broad spectrum of topics relevant for mechanized tunneling in soft soil conditions.This includes inverse numerical methods for advance exploration and models for the characterization of the in situ ground conditions,the interaction of the face support and the tail gap grouting with the porous soil,multi-scale models for the design offiber reinforced segmental linings with enhanced robustness,computational methods for the numer-ical simulation of the tunnel advancement,the soil excavation and the material transport in the pressure chamber,logistics processes and risk analysis in urban tunneling.Targeted towards the continuous support of the construction process,a concept for real-time steering support of tunnel boring machines in conjunction with model update procedures and methods of uncertainty quantification is addressed.

A holistic approach for the investigation of lining response to mechanized tunneling induced construction loadings

Design methods for segmental tunnel linings used in mechanized tunnel constructions typically employ numerical bedded beam mod-els and/or classical analytical solutions for the determination of structural forces(i.e.moments and shear and axial forces)and simple load spreading assumptions for the design of the reinforcement in joint areas.However effcient such methods may be,many physical details are often overlooked and/or oversimplified in the process of reducing the actual structure to a structural beam model,e.g.ana-lytically derived loadings are employed,the grouting and ground reactions are reduced to a spring bedding,and the confinement due to grouting at the longitudinal joint is largely not considered in reinforcement design.Such a design process is not able to account for,or predict,the susceptibility of tunnel linings to often observed damages that,although they may not be structurally relevant,lead to ser-viceability or durability issues,such as crack development or chipping at the segment corners.Numerical methods,such as the Finite Element Method,provide an opportunity to model the segmental tunnel lining and its response to the entire TBM construction process and to explicitly model the crack development within individual segments using modern methods to model the discontinuities in struc-tures.In this contribution,a holistic modeling procedure for the representation of the tunnel lining within the tunneling process is pro-posed and compared to traditional lining models.A 3D process oriented Finite Element model is used to calculate the predicted forces on the tunnel lining and the obtained results are compared with those generated by traditional methods.Subsequently,the predicted defor-mations are then transferred to a detailed segment model in which the nonlinear response of the segment at the longitudinal joint is mod-eled using an interface element based approach to simulate concrete cracking.

Optimal measurement design for parameter identification in mechanized tunneling

When performing shallow tunnel construction,settlements on the ground surface often cannot be prevented.Anticipating these sur-face displacements is only possible with profound knowledge of the constitutive parameters of the surrounding soil.Performing inverse analysis on the basis of in situ settlement data is an effcient method for obtaining such information.However,during this process,con-sidering which measurement arrangement can provide the most reliable results is generally neglected.This aspect is addressed in this study by applying the so-called“optimal experimental design”to the mechanized tunnelingfield.A global sensitivity analysis(GSA)isfirstly performed to determine the most relevant model parameters to be identified via back analysis,by employing the considered numerical model and experimental data.Furthermore,the GSA results are utilized to determine where and when measurements should be performed to minimize uncertainty in the identified constitutive parameters.The optimal experimental design(OED)concept is fur-ther applied to evaluate the observation set-up effciency for damage mitigation measures within a representative synthetic example of a tunneling project passing beneath an existing building.Parameter identification based on synthetic noisy experiments is performed to validate the presented method for optimal experimental design.Thus,the soil stiffness and strength parameters are identified according to both an intuitive and the elaborated method,employing the proposed OED strategy and experimental designs,making it possible to assess the feasibility of the OED results.

Fabrication and characteristics of excellent current spreading GaN-based LED by using transparent electrode-insulator-semiconductor structure

GaN-based vertical light-emitting-diodes （V-LEDs） with an improved current injection pattern were fabricated and a novel current injection pattern of LEDs which consists of electrode-insulator-semiconductor （E1S） structure was proposed. The EIS structure was achieved by an insulator layer （20-nm Ta2O5） deposited between the p-GaN and the ITO layer. This kind of EIS structure works through a defect-assisted tunneling mechanism to realize current injection and obtains a uniform current distribution on the chip surface, thus greatly improving the current spreading ability of LEDs. The appearance of this novel current injection pattern of V-LEDs will subvert the impression of the conventional LEDs structure, including simplifying the chip manufacture technology and reducing the chip cost. Under a current density of 2, 5, 10, and 25 A/cm2, the luminous uniformity was better than conventional structure LEDs. The standard deviation of power density distribution in light distribution was 0.028, which was much smaller than that of conventional structure LEDs and illustrated a huge advantage on the current spreading ability of EIS-LEDs.