Nonexponential relaxation and quantum tunnel splitting in the molecular magnet Fe8

Magnetic relaxation in molecular magnets under a sweeping field is studied by taking into account local stray fields. It is found that the randomness of local stray field leads to a distribution of the relaxation rate which subsequently makes the relaxation deviate from the exponential law as predicted by the Landau-Zener model such that the Landau-Zener method needs to be revised to deduce an exact tunneling splitting. The tunneling splitting and distribution width of local stray fields are derived from the experimental data for molecular magnets Fe8.published_or_final_versio

Enhanced superconducting proximity effect in strongly correlated heterostructures

The electronic properties of a strongly correlated heterostructure consisting of t-J layer and metallic layer have been investigated by using the Gutzwiller projected mean-field approximation. Considering the proximity effect due to the large pseudogap energy scale of t-J layer, a large superconducting gap could be induced on the metallic layer. This enhanced superconducting gap may be even larger than that of the t-J layer. Related physical quantities including spectral functions and density of states are obtained. The consequences of these results on experiments are discussed. © 2010 The American Physical Society.published_or_final_versio

40-Hz coherent oscillations in neuronal systems

A characteristic coherent oscillation with a frequency of 40 Hz which relates to some functions of the brain is shown to be intrinsic due to the nonlinear firing of spikes of neurons when the network is situated in a stimulus-induced oscillatory state. This oscillation mode is associated with a synchronized firing pattern, and is robust to a variety of couplings between the neurons and to a wide range of external stimuli.published_or_final_versio

Vortex charges in high-temperature superconductors

The vortex charge in high-temperature superconductors was investigated. It was found that the vortex charge was negative when a sufficient strength of antiferromagnetic (AF) order was induced inside the vortex core. The vortex charge at optimal doping was studied as a function of magnetic field. The results showed that the AF order was absent inside the vortex core for small Coulomb repulsion.published_or_final_versio

Dominant bidding strategy in Mobile App advertising auction

The widespread use of intelligent mobile phone has promoted prosperity of mobile App advertising in recent years. Based on existing bidding status, this paper presents the dominant bidding strategy for mobile advertising auction. Firstly, our study characterizes multiple Nash Equilibria resulting from different bidding strategies in wGSP (weighted Generalized Second-Price) auction. Further more, we prove that advertiser’s rank and utility will not decrease by using the dominant bidding strategy. We also consider the situation where the reserve price is set by the mobile advertising platform. It turns out that that advertiser’s payment will be no less than reserve price. Finally, a practical implementation for a virtual market simulates the dynamic bidding process in real world environments.published_or_final_versio

Reflection seismic waveform tomography of physical modelling data

Waveform tomography is commonly tested using numerically generated synthetic seismic data, before the method is applied to field seismic data. However, there are often noticeable differences between idealized synthetic data and real field data, and many factors in the field data, such as noise, irregular source/receiver geometry, affect the inversion solutions. For exploring the potential of reflection seismic waveform tomography, we presented a more realistic test than the synthetic data test, by applying it to physical modelling data, to reconstruct a laboratorial model with complex velocity variation. First, we provided a formulation of the perfectly matched layer absorbing boundary condition, associated with the second-order acoustic wave equation, in order to suppress artificial reflections from subsurface model boundaries in seismic waveform simulation and tomography. Then, we demonstrated the successful implementation of a layer-striping inversion scheme applicable to reflection seismic waveform tomography. Finally, we confirmed the effectiveness of frequency grouping, rather than a single frequency at each iteration, a strategy specifically for the frequency-domain waveform tomography

Mesoporous PbI2 assisted growth of large perovskite grains for efficient perovskite solar cells based on ZnO nanorods

Perovskite solar cells (PSCs) have attracted great attention due to their low cost and high power conversion efficiency (PCE). However, the defects and grain boundaries in perovskite films dramatically degrade their performance. Here, we show a two-step annealing method to produce mesoporous PbI2 films for growth of continuous, pinhole-free perovskite films with large grains, followed by additional ethanol vapor annealing of perovskite films to reduce the defects and grain boundaries. The large perovskite grains dramatically suppress the carrier recombination, and consequently we obtain ZnO-nanorod-based PSCs that exhibit the best efficiency of 17.3%, with high reproducibility

Solution processed PCBM-CH3NH3PbI3 heterojunction photodetectors with enhanced performance and stability

In this work, an anti-solvent process was used to fabricate a perovskite-PCBM bulk heterojunction, in which PCBM diffused in CH3NH3PbI3 and passivated grain boundary defects. Different concentrations of PCBM were studied in this paper. When a low concentration (5 mg/ml, 10 mg/ml or 15 mg/ml) of PCBM was used, the PCBM-CH3NH3PbI3 transition layer provided efficient electron collection. With the increase of the concentration of PCBM, a thicker PCBM layer was formed in the bulk heterojunction. Such a thick PCBM resulted in rough perovskite morphology and low photo-carrier collection efficiency. SEM images and metallographic microscope images confirmed that the PCBM upper layer gradually covered grain boundaries of perovskite films with the increase of the PCBM concentration. On the other hand, low concentrations of PCBM improved the light absorption and crystallinity of CH3NH3PbI3 films. The PCBM/perovskite heterojunction exhibited a high UV responsivity of 0.18 AW-1 and a response time less than 123 ms. This research provides a way to improve the quality of perovskite films and the performance of perovskite photodetectors

Enhanced efficiency and environmental stability of planar perovskite solar cells by suppressing photocatalytic decomposition

The environmental instability of perovskite solar cells caused by the ultraviolet photocatalytic effect of metal oxide layers is a critical issue that must be solved. In this paper, we report improved environmental stability of ZnO film-based planar heterojunction perovskite solar cells, by suppressing photocatalytic activities induced by the ZnO electron transfer layer. The photovoltaic performance and stability in an ambient environment under continuous illumination are effectively improved by applying an aluminum oxide interlayer on the ZnO film to suppress the photocatalytic degradation of perovskites. The highest efficiency of solar cells has increased from 14.62% to 17.17%, and after 250 h of continuous exposure under full spectrum simulated sunlight in air, the efficiency remains as high as 15.03%. The results suggest that effective suppression of photocatalytic degradation of perovskites with a modified electron transfer layer is a new solution to improve the long-term environmental stability of perovskite solar cells