No abstract

The twin-image problem, a persistent stagnation mode in iterative projection algorithms (IPAs) for coherent diffraction imaging, occurs when the ideal and twin images appear simultaneously in the reconstruction. Presented here is a methodological framework for IPAs termed the half-cycle multigrid (HMG) for use in phase retrieval to alleviate the twin-image problem during the iterative process. HMG reconstructs the low-frequency phase first to reduce the impact of oscillation caused by phase retrieval in the higher-frequency region of Fourier space during the iteration. The higher-frequency Fourier magnitude is then added to the reconstruction stage by stage using the multigrid method. The unification of phase retrieval orientation in the low-frequency region lays the foundation for that in the whole Fourier space. The reconstruction results of simulated and experimental diffraction patterns demonstrate that HMG effectively reduces the probability of the twin-image problem occurring, enhances the accuracy of low-frequency information, and achieves credible and faithful reconstruction results from noisy diffraction patterns. The combination of HMG with the oversampling smoothness framework allows more reliable reconstruction results, proving that the HMG framework has good extensibility. It is expected that HMG can be combined with other IPAs.

We study the spin-valley dependent transport and magnetoresistance effect in a ferromagnetic MoS2 junction modulated by the off-resonance circularly polarized light. It is found that the conductance could be 100% spin- and valley-polarized simultaneously in the P configuration. Perfect valley polarization can also be realized in the AP configuration. The spin polarization presents a oscillating behavior in AP configuration due to the spin-orbit coupling. Furthermore, the junction achieves a high magnetoresistance that can be controlled by the exchange field and the light.

Coherent diffraction imaging (CDI) is a high-resolution technique that does not require X-ray lenses. With advances in scientific technology, such as synchrotron radiation, X-ray free-electron lasers, and coherent electron sources, CDI has been applied to diverse fields, such as biology, medicine, and semiconductors, as a high-resolution, nondestructive measure. With the rapid increase in demand for these applications, enhancing the efficiency of processing high-volume data has become a significant challenge for promotion. In this study, we proposed an algorithm that combines Kramers–Kronig (KK) relations with oversampling smoothness (OSS). The results were evaluated by introducing an error coefficient. We found that the error of the KK-OSS algorithm is always reduced by approximately 50% compared with the error reduction (ER) algorithm and OSS in real space. In the diffraction space, the error in the KK-OSS decreased by 15%. With 100 iterations, KK-OSS spent 163.1 s on reconstructing most of the sample information, while ER took 258.1 s and the reconstruction was still a random value. In Fraunhofer diffraction, it cost KK-OSS 58.8 s to reconstruct, while OSS took 61.9 s. Therefore, this method can reduce the reconstruction error, shorten the reconstruction time, and improve the efficiency compared with the ER and OSS algorithm using a random phase as the initial value.