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Magnetic topological states of matter provide a fertile playground for emerging topological physics and phenomena. The current main focus is on materials whose magnetism stems from 3d magnetic transition elements, e.g., MnBi2Te4, Fe3Sn2 and Co3Sn2S2. In contrast, topological materials with the magnetism from rare earth elements remain largely unexplored. Here we report rare earth antiferromagnet GdAuAl4Ge2 as a candidate magnetic topological metal. Angle resolved photoemission spectroscopy and first-principles calculations have revealed multiple bulk bands crossing the Fermi level and pairs of low energy surface states. According to the parity and Wannier charge center analyses, these bulk bands possess nontrivial Z2 topology, establishing a strong topological insulator state in the nonmagnetic phase. Furthermore, the surface band pairs exhibit strong termination dependence which provides insight into their origin. Our results suggest GdAuAl4Ge2 as a rare earth platform to explore the interplay between band topology, magnetism and f electron correlation, calling for further study targeting on its magnetic structure, magnetic topology state, transport behavior and microscopic properties.

An optical encryption (OE) scheme based on the spread spectrum ghost imaging (SSGI), named SSGI-OE, is proposed to obtain a high security with a smaller key. In the scheme, the randomly selected row number of a Hadamard matrix of order $N$ is used as the secure key, and shared with the authorized user, Bob, through a private channel. Each corresponding row vector of the order $N$ Hadamard matrix is then used as the direct sequence code to modulate a speckle pattern for the ghost imaging system, and an image is encrypted with the help of the SSGI. The measurement results from the bucket detector, named ciphertext, are then transmitted to Bob through a public channel. The illuminating speckle patterns are also shared with Bob by the public channel. With the correct secure key, Bob could reconstruct the image with the aid of the SSGI system, whereas, the unauthorized user, Eve, could not obtain any useful information of the encrypted image. The numerical simulations and experimental results show that the proposed scheme is feasible with a higher security and a smaller key. For the $32 \times 32$ pixels image, the number of bits sent from Alice to Bob by using SSGI-OE ($M=1024,N=2048$) scheme is only $0.0107$ times over the computational ghost imaging optical encryption(CGI-OE) scheme. When the eavesdropping ratio (ER) is less than 40%, the eavesdropper cannot acquire any information of the encrypted image. The extreme circumstance for the proposed SSGI-OE scheme is also discussed, where the eavesdropper begins to extract the information when ER is up to 15%.

Orbital angular momentum (OAM) has the characteristics of mutual orthogonality between modes, and has been applied to underwater wireless optical communication (UWOC) systems to increase the channel capacity. In this work, we propose a diffractive deep neural network (DDNN) based OAM mode recognition scheme, where the DDNN is trained to capture the features of the intensity distribution of the OAM modes and output the corresponding azimuthal indices and radial indices. The results show that the proposed scheme can recognize the azimuthal indices and radial indices of the OAM modes accurately and quickly. In addition, the proposed scheme can resist weak oceanic turbulence (OT), and exhibit excellent ability to recognize OAM modes in a strong OT environment. The DDNN-based OAM mode recognition scheme has potential applications in UWOC systems.

We propose a new full color ghost imaging scheme using both time and code division multiplexing technologies. In the scheme, the speckle patterns of three colors (red, green and blue) are modulated with different time slots and codes. The light intensity is sampled by one bucket detector. Then based on the modulated time slots and codes, we can effectively and simultaneously extract three detection component signals corresponding to three color components of objects from the sampling signal of the bucket detector. Finally, three component images resulting from the three component detection signals can be synthesized into a full color image. The experimental results verify the feasibility of our scheme under the limit of the number of time slots and codes. Moreover, our scheme reduces the number of bucket detectors and can realize high quality imaging even in a noisy environment.

The antiferromagnetic (AFM) interlayer coupling effective field in a ferromagnetic/non-magnetic/ferromagnetic (FM/NM/FM) sandwich structure, as a driving force, can dramatically enhance the ferromagnetic resonance (FMR) frequency. Changing the non-magnetic spacer thickness is an effective way to control the interlayer coupling type and intensity, as well as the FMR frequency. In this study, FeCoB/Ru/FeCoB sandwich trilayers with Ru thickness (t Ru) ranging from 1 Å to 16 Å are prepared by a compositional gradient sputtering (CGS) method. It is revealed that a stress-induced anisotropy is present in the FeCoB films due to the B composition gradient in the samples. A t Ru-dependent oscillation of interlayer coupling from FM to AFM with two periods is observed. An AFM coupling occurs in a range of 2 Å ≤ t Ru ≤ 8 Å and over 16 Å, while an FM coupling is present in a range of t Ru < 2 Å and 9 Å ≤ t Ru ≤ 14.5 Å. It is interesting that an ultrahigh optical mode (OM) FMR frequency in excess of 20 GHz is obtained in the sample with t Ru = 2.5 Å under an AFM coupling. The dynamic coupling mechanism in trilayers is simulated, and the corresponding coupling types at different values of t Ru are verified by Layadi’s rigid model. This study provides a controllable way to prepare and investigate the ultrahigh FMR films.

Ghost imaging (GI) offers great potential with respect to conventional imaging techniques. However, there are still some obstacles for reconstructing images with high quality, especially in the case that the orthogonal measurement matrix is impossible to construct. In this paper, we propose a new scheme based on the orthogonal-triangular (QR) decomposition, named QR decomposition ghost imaging (QRGI) to reconstruct a better image with good quality. In the scheme, we can change the randomly non-orthogonal measurement matrix into orthonormal matrix by performing QR decomposition in two cases. (1) When the random measurement matrix is square, it can be firstly decomposed into an orthogonal matrix Q and an upper triangular matrix R . Then let the off-diagonal values of R equal to 0.0, the diagonal elements of R equal to a constant k, where k is the average of all values of the main diagonal, so the resulting measurement matrix can be obtained. (2) When the random measurement matrix is with full rank, we firstly compute its transpose, and followed with above QR operation. Finally, the image of the object can be reconstructed by correlating the new measurement matrix and corresponding bucket values. Both experimental and simulation results verify the feasibility of the proposed QRGI scheme. Moreover, the results also show that the proposed QRGI scheme could improve the imaging quality comparing to traditional GI (TGI) and differential GI (DGI). Besides, in comparison with the singular value decomposition ghost imaging (SVDGI), the imaging quality and the reconstruction time by using QRGI are similar to those by using SVDGI, while the computing time (the time consuming on the light patterns computation) is substantially shortened.

We propose a method for imaging a periodic moving/state-changed object based on computational ghost imaging with Hadamard speckle patterns and a slow bucket detector, named as PO-HCGI. In the scheme, speckle patterns are produced from a part of each row of a Hadamard matrix. Then, in each cycle, multiple speckle patterns are projected onto the periodic moving/state-changed object, and a bucket detector with a slow sampling rate records the total intensities reflected from the object as one measurement. With a series of measurements, the frames of the moving/state-changed object can be obtained directly by the second-order correlation function based on the Hadamard matrix and the corresponding bucket detector measurement results. The experimental and simulation results demonstrate the validity of the PO-HCGI. To the best of our knowledge, PO-HCGI is the first scheme that can image a fast periodic moving/state-changed object by computational ghost imaging with a slow bucket detector.

The transmission loss of photons during quantum key distribution (QKD) process leads to the linear key rate bound for practical QKD systems without quantum repeaters. Phase matching quantum key distribution (PM-QKD) protocol, an novel QKD protocol, can overcome the constraint with a measurement-device-independent structure, while it still requires the light source to be ideal. This assumption is not guaranteed in practice, leading to practical secure issues. In this paper, we propose a modified PM-QKD protocol with a light source monitoring, named PM-QKD-LSM protocol, which can guarantee the security of the system under the non-ideal source condition. The results show that our proposed protocol performs almost the same as the ideal PM-QKD protocol even considering the imperfect factors in practical systems. PM-QKD-LSM protocol has a better performance with source fluctuation, and it is robust in symmetric or asymmetric cases.

We present a ghost handwritten digit recognition method for the unknown handwritten digits based on ghost imaging (GI) with deep neural network, where a few detection signals from the bucket detector, generated by the cosine transform speckle, are used as the characteristic information and the input of the designed deep neural network (DNN), and the output of the DNN is the classification. The results show that the proposed scheme has a higher recognition accuracy (as high as 98% for the simulations, and 91% for the experiments) with a smaller sampling ratio (say 12.76%). With the increase of the sampling ratio, the recognition accuracy is enhanced. Compared with the traditional recognition scheme using the same DNN structure, the proposed scheme has slightly better performance with a lower complexity and non-locality property. The proposed scheme provides a promising way for remote sensing.

We propose a compressed ghost imaging scheme based on differential speckle patterns, named CGI-DSP. In the scheme, a series of bucket detector signals are acquired when a series of random speckle patterns are employed to illuminate an unknown object. Then the differential speckle patterns (differential bucket detector signals) are obtained by taking the difference between present random speckle patterns (present bucket detector signals) and previous random speckle patterns (previous bucket detector signals). Finally, the image of object can be obtained directly by performing the compressed sensing algorithm on the differential speckle patterns and differential bucket detector signals. The experimental and simulated results reveal that CGI-DSP can improve the imaging quality and reduce the number of measurements comparing with the traditional compressed ghost imaging schemes because our scheme can remove the environmental illuminations efficiently.

Microcrystalline diamond (MCD) films with different grain sizes ranging from 160 nm to 2200 nm are prepared by using a hot filament chemical vapor deposition (HFCVD) system, and the influences of grain size and structural features on optical properties are investigated. The results show that the film with grain size in a range of 160 nm–310 nm exhibits a higher refractive index in a range of (2.77–2.92). With grain size increasing to 620±300 nm, the refractive index shows a value between 2.39 and 2.47, approaching to that of natural diamond (2.37–2.55), and a lower extinction coefficient value between 0.08 and 0.77. When the grain size increases to 2200 nm, the value of refractive index increases to a value between 2.66 and 2.81, and the extinction coefficient increases to a value in a range of 0.22–1.28. Visible Raman spectroscopy measurements show that all samples have distinct diamond peaks located in a range of 1331 cm−1–1333 cm−1, the content of diamond phase increases gradually as grain size increases, and the amount of trans-polyacetylene (TPA) content decreases. Meanwhile, the sp2 carbon clusters content and its full-width-at-half-maximum (FWHM) value are significantly reduced in MCD film with a grain size of 620 nm, which is beneficial to the improvement of the optical properties of the films.

Spin injection, spin diffusion, and spin detection are investigated in Co/Ag/Co lateral spin valves at room temperature. Clear spin accumulation signals are detected by the non-local measurement. By fitting the results to the one-dimensional diffusion equation, ~8.6% spin polarization of the Co/Ag interface and ~ 180 nm spin diffusion length in Ag are obtained. Thermal treatment results show that the spin accumulation signal drastically decreases after 100 °C annealing, and disappears under 200 °C annealing. Our results demonstrate that, compared to the spin diffusion length, the decrease and the disappearance of the spin accumulation signal are mainly dominated by the variation of the interfacial spin polarization of the Co/Ag interface.

Atmospheric turbulence (AT) induced crosstalk can significantly impair the performance of a free-space optical (FSO) communication link using orbital angular momentum (OAM) multiplexing. In this paper, we propose a multiple-user detection (MUD) turbulence mitigation scheme in an OAM-multiplexed FSO communication link. First, we present a MUD equivalent communication model for an OAM-multiplexed FSO communication link under AT. In the equivalent model, each input bit stream represents one user's information. The deformed OAM spatial modes caused by AT, instead of the pure OAM spatial modes, are used as information carriers, and the overlapping between the deformed OAM spatial modes are computed as the correlation coefficients between the users. Then, we present a turbulence mitigation scheme based on MUD idea to enhance AT tolerance of the OAM-multiplexed FSO communication link. In the proposed scheme, the crosstalk caused by AT is used as a useful component to deduce users' information. The numerical results show that the performance of the OAM-multiplexed communication link has greatly improved by the proposed scheme. When the turbulence strength is 1 × 10−15 m−2/3, the transmission distance is 1000 m and the channel signal-to-noise ratio (SNR) is 26 dB, the bit-error-rate (BER) performance of four spatial multiplexed OAM modes lm = +1,+2,+3,+4 are all close to 10−5, and there is a 2–3 fold increase in the BER performance in comparison with those results without the proposed scheme. In addition, the proposed scheme is more effective for an OAM-multiplexed FSO communication link with a larger OAM mode topological charge interval. The proposed scheme is a promising direction for compensating the interference caused by AT in the OAM-multiplexed FSO communication link.

In this paper, we propose a measurement-device-independent quantum-key-distribution (MDI-QKD) protocol using orbital angular momentum (OAM) in free space links, named the OAM-MDI-QKD protocol. In the proposed protocol, the OAM states of photons, instead of polarization states, are used as the information carriers to avoid the reference frame alignment, the decoy-state is adopted to overcome the security loophole caused by the weak coherent pulse source, and the high efficient OAM-sorter is adopted as the measurement tool for Charlie to obtain the output OAM state. Here, Charlie may be an untrusted third party. The results show that the authorized users, Alice and Bob, could distill a secret key with Charlie's successful measurements, and the key generation performance is slightly better than that of the polarization-based MDI-QKD protocol in the two-dimensional OAM cases. Simultaneously, Alice and Bob can reduce the number of flipping the bits in the secure key distillation. It is indicated that a higher key generation rate performance could be obtained by a high dimensional OAM-MDI-QKD protocol because of the unlimited degree of freedom on OAM states. Moreover, the results show that the key generation rate and the transmission distance will decrease as the growth of the strength of atmospheric turbulence (AT) and the link attenuation. In addition, the decoy states used in the proposed protocol can get a considerable good performance without the need for an ideal source.

The influence of vacancy defect on the doping of silicon nanowires is systematically studied by the first-principles calculations. The atomic structures and electronic properties of vacancies and vacancy—boron (vacancy—phosphor) complexes in H-passivated silicon nanowire with a diameter of 2.3 nm are explored. The results of geometry optimization indicate that a central vacancy can exist stably, while the vacancy at the edge of the nanowire undergoes a local surface reconstruction, which results in the extradition of the vacancy out of the nanowire. Total-energy calculations indicate that the central vacancy tends to form a vacancy—dopant defect pair. Further analysis shows that n-type doping efficiency is strongly inhibited by the unintentional vacancy defect. In contrast, the vacancy defect has little effect on p-type doping. Our results suggest that the vacancy defect should be avoided during the growth and the fabrication of devices.

For modern processes at deep sub-micron technology nodes, yield design, especially the design at the layout stage is an important way to deal with the problem of manufacturability and yield. In order to reduce the yield loss caused by redundancy material defects, the choice of nets to be optimized at first is an important step in the process of layout optimization. This paper provides a new sensitivity model for a short net, which is net-based and reflects the size of the critical area between a single net and the nets around it. Since this model is based on a single net and includes the information of the surrounding nets, the critical area between the single net and surrounding nets can be reduced at the same time. In this way, the efficiency of layout optimization becomes higher. According to experimental observations, this sensitivity model can be used to choose the position for optimization. Compared with the chip-area-based and basic-layout-based sensitivity models, our sensitivity model not only has higher efficiency, but also confirms that choosing the net to be optimized at first improves the design.