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## COVER ARTICLE

2021, 70 (7): 072501. doi: 10.7498/aps.70.20201503
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Radioactive residual nuclides, which are usually closely related to radiation protection and personnel safety, will be generated when target materials are irradiated by high energy particles. Based on different nuclear reaction models, Monte Carlo code is a usual method to obtain residual nuclide production. The simulation accuracy needs to be evaluated by experimental data. In this paper, an irradiation experiment of thin copper target irradiated by 12C6+ particles with energy of 80.5 MeV/u is carried out. The radioactivities and cross-sections of 18 radioactive residual nuclides are obtained by gamma spectrometry analysis. Compared with the Monte Carlo simulation by PHITS, the results show that the spallation model of PHITS has a high reliability in estimating the types of radioactive residual nuclei, and it could be optimized in the aspect of the absolute yield.

## COVER ARTICLE

2021, 70 (6): 068702. doi: 10.7498/aps.70.20201438
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The fabrication of precise arrays of atoms is a key challenge at present. As a kind of biomacromolecule with strict base-pairing and programmable self-assembly ability, DNA is an idea material for directing atom positioning on predefined addresses. Here in this work, we propose the construction of iron atom arrays based on DNA origami templates and illustrate the potential applications in cryptography. First, ferrocene molecule is used as the carrier for iron atom since the cyclopentadienyl groups protect iron from being affected by the external environment. To characterize the iron atom arrays, streptavidins are labelled according to the ferrocene-modified DNA strand through biotin-streptavidin interactions. Based on atomic force microscopy scanning, ferrocene-modified single-stranded DNA sequences prove to be successfully immobilized on predefined positions on DNA origami templates with high yield. Importantly, the address information of iron atoms on origami is pre-embedded on the long scaffold, enabling the workload and cost to be lowered dramatically. In addition, the iron atom arrays can be used as the platform for constructing secure Braille-like patterns with encoded information. The origami assembly and pattern characterizations are defined as encryption process and readout process, respectively. The ciphertext can be finally decoded with the secure key. This method enables the theoretical key size of more than 700 bits to be realized. Encryption and decryption of plain text and a Chinese Tang poem prove the versatility and feasibility of this strategy.

## COVER ARTICLE

2021, 70 (5): 050701. doi: 10.7498/aps.70.20201908
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Laser pumped terahertz (THz) wave up-conversion detection with high sensitivity, fast responsivity and wide frequency band is achieved at room temperature, based on home-made organic nonlinear crystals 4-N,N-dimethylamino-4′-N′-methyl-stilbazolium tosylate (DAST). Green laser pulses pumped KTiOPO4 optical parametric oscillators are utilized as the sources of dual-wavelength near-infrared (NIR) beams (1.3–1.6 μm, for THz-wave difference frequency generation (DFG)) and a single NIR beam (1.2–1.4 μm, for up-conversion detection). The nonlinear medium for both THz-DFG and detection is DAST (grown by CETC-46). A nanosecond-time-resolved THz pulse is obtained with an InGaAs p-i-n photo-diode. The spectrum of the up-converted NIR light is acquired, which allows us to measure the THz frequency indirectly. The sensitivity (also at room temperature) is 4 orders better at 19 THz than the sensitivity of a commercial thermal detector (Golay Cell). The wide frequency band operation is realized with different sets of band-pass filters, which cover the entire range from 3.15 to 29.82 THz except 8.4 THz of the strong absorption peak of DAST. The dynamic range of a THz source based on DFG can be commonly improved by 2–3 orders, by changing the traditional thermal detector with the up-conversion detection. The presented technology can promote the applications of DFG THz source in the fields of high-resolution spectroscopy and imaging.

## COVER ARTICLE

2021, 70 (4): 047401. doi: 10.7498/aps.70.20201213
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Based on the proximity effect, the exchange interaction at the interface between a ferromagnetic insulator (FI) and a superconductor (S) could enhance the Zeeman splitting of the superconducting quasiparticle density of states. The superconducting electrons feel the exchange field on the surface of the S layer. Therefore, tuning the internal exchange field at the FI/S interface could switch the superconductor from a superconducting state to a normal state, leading to an infinite magnetoresistance in FI/S heterostructure. Here in this work, we fabricate the EuS/Ta heterojunction by the pulsed laser deposition, and perform the magnetotransport measurements. In the EuS/Ta heterojunction, Ta film as a typical BSC supercenter exhibits the superconducting transition under 3.6 K, and the EuS film is ferromagnetic under 20 K. The magnetization of EuS is suppressed by superconductivity of Ta at 0.01 T below 3 K. In addition, the butterfly-type hysteresis loop is observed at 2 K. And the decrease of the saturation magnetization of EuS/Ta heterostructure is observed by comparing with the EuS single layer. It is caused by a reconstruction of homogeneous ferromagnetic order in the EuS ferromagnetic layer due to the proximity effect with the Ta superconducting layer. The above measurement results show that the competition between the ferromagnetism of EuS film and superconductivity of Ta film below Tc of Ta film. If the exchange field of the FI is sufficiently strong, it tries to align the spins of the electrons of a Cooper pair in S layer parallel to each other, thus destroying the superconductivity. Meanwhile, the superconductivity in S layer will be recovered when the exchange field of the FI is weak. The resistance at a specific value of the magnetic field (1 T) steeply drops to zero, and clear hysteresis behavior is observed in EuS/Ta heterostructure, resulting in an infinite magnetoresistance up to 144000%, by tuning the internal exchange field at EuS/Ta interface. Meanwhile, the anomalous Hall effect with hysteresis behavior is observed at 2 K, indicating that the electron in Ta film is spin polarized due to the magnetic proximity effect near the EuS/Ta interface. Our results show that the EuS/Ta heterostructure with infinite magnetoresistance could be a good candidate for spintronic devices.

## COVER ARTICLE

2021, 70 (3): 037401. doi: 10.7498/aps.70.20201291
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$(T_{\rm C}-T)^2$ dependence. At 77 K, the JC of the junction reaches 1.4 × 105 A/cm2, significantly higher than the range of 103–104 A/cm2 as presented by other investigators for YBCO step-edge junctions on MgO substrate with comparable θ of 35°–45°. This indicates a rather strong Josephson coupling of the junction, and by invoking the results of YBCO bicrystal junctions showing similar values of JC, it is tentatively proposed that the presently fabricated junction might be described as an S-s′-S junction with s′ denoting the superconducting region of depressed TC in the vicinity of the step edge or as an S-N-S junction with N denoting a very thin non-superconducting layer. By incorporating the MgO-based YBCO step-edge junction, high-TC radio frequency (RF) SQUID is made. The device shows decent voltage-flux curve and magnetic flux sensitivity of 250 $\text{μ}\Phi_0/{\rm Hz}^{1/2}$ at 1 kHz and 77 K, comparable to the values reported in the literature. To further improve the RF SQUID performance, efforts could be devoted to optimizing the junction parameters such as the junction JC. By using the YBCO step-edge junction on MgO substrate, high-TC direct current SQUID could also be developed, as reported recently by other investigators, to demonstrate the potential of MgO-based step-edge junction in making such a kind of device with superior magnetic flux sensitivity.">The YBa2Cu3O7–δ (YBCO) step-edge Josephson junction on MgO substrate has recently been shown to have important applications in making advanced high-transition temperature (high-TC) superconducting devices such as high-sensitivity superconducting quantum interference device (SQUID), superconducting quantum interference filter, and THz detector. In this paper, we investigate the fabrication and transport properties of YBCO step-edge junction on MgO substrate. By optimizing the two-stage ion beam etching process, steps on MgO (100) substrates are prepared with an edge angle θ of about 34°. The YBCO step-edge junctions are then fabricated by growing the YBCO thin films with a pulsed laser deposition technique and subsequent traditional photolithography. The resistive transition of the junction shows typical foot structure which is well described by the Ambegaokar-Halperin theory of thermally-activated phase slippage for overdamped Josephson junctions. The voltage-current curves with temperature dropping down to 77 K exhibit resistively shunted junction behavior, and the Josephson critical current density JC is shown to follow the $(T_{\rm C}-T)^2$ dependence. At 77 K, the JC of the junction reaches 1.4 × 105 A/cm2, significantly higher than the range of 103–104 A/cm2 as presented by other investigators for YBCO step-edge junctions on MgO substrate with comparable θ of 35°–45°. This indicates a rather strong Josephson coupling of the junction, and by invoking the results of YBCO bicrystal junctions showing similar values of JC, it is tentatively proposed that the presently fabricated junction might be described as an S-s′-S junction with s′ denoting the superconducting region of depressed TC in the vicinity of the step edge or as an S-N-S junction with N denoting a very thin non-superconducting layer. By incorporating the MgO-based YBCO step-edge junction, high-TC radio frequency (RF) SQUID is made. The device shows decent voltage-flux curve and magnetic flux sensitivity of 250 $\text{μ}\Phi_0/{\rm Hz}^{1/2}$ at 1 kHz and 77 K, comparable to the values reported in the literature. To further improve the RF SQUID performance, efforts could be devoted to optimizing the junction parameters such as the junction JC. By using the YBCO step-edge junction on MgO substrate, high-TC direct current SQUID could also be developed, as reported recently by other investigators, to demonstrate the potential of MgO-based step-edge junction in making such a kind of device with superior magnetic flux sensitivity.

## COVER ARTICLE

2021, 70 (2): 027901. doi: 10.7498/aps.70.20201419
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Since the discovery of graphene, two-dimensional (2D) materials have received continuous attention and carried out in-depth exploration and development due to their excellent properties. With the exploration of the preparation of new 2D materials, one began to consider the synergistic effects produced by the in-plane junction and interlayer stacking to compensate for the defects of a single material and obtain some new properties. Matching the lattice structure to achieve specific functionalization, or using van der Waals force to achieve stacking, helps to introduce a new degree of freedom by combining different 2D materials, and open a new window for the research and practical application of 2D materials.From the perspective of atomic manufacturing, in this article we introduce the controllable preparation and optoelectronic applications of 2D planar and van der Waals heterojunction materials. First, we briefly introduce the common 2D materials such as graphene, hexagonal boron nitride, transition metal dichalcogenides and black phosphorus used in the preparation of heterojunctions and related concepts of heterojunctions. Second, we review, in principle, the commonly used characterization methods including scanning probe-based techniques, spectrum-based, electron-based imaging techniques and others. Third, we summarize the preparation methods of planar and vertical heterojunctions. Basically, mechanical transfer method such as wet or dry method can be used to produce various vertical heterostructures of 2D materials, but usually lack the scalability. On the other hand, chemical vapor deposition method provides a scalable route to producing the planar heterostructure and vertical structure of 2D materials. Several strategies have been developed to produce various heterostructures. In addition, the recent development of twist-angle and quasi-crystalline bi-layer graphene is briefly reviewed. Fourth, the properties and applications of 2D van der Waals heterostructures such as field-effect transistor, light emitting diode, solar cell, flexible optoelectronic devices and plasmonic applications are introduced. Finally, the problems in the field are discussed, and the outlook is provided.

## COVER ARTICLE

2020, 69 (24): 248702. doi: 10.7498/aps.69.20201631
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Laws of physics govern all forms of matter movement. However, lives, which are composed of chemical elements which everyone is familiar with, are largely beyond physical description available. This is because the construction of life is not the same as that of general matters, rendering it unknown how physics laws are utilized. In this paper, we present our thinking on the transcriptional apparatus (TA). The TA is a huge molecular machine acting to sense regulatory signals and initiate transcripts at right time and with right rate. The operation of the TA is fundamental to almost all forms of lives. Although great progress has been made in recent years, one often has to face contradictory conclusions from different studies. Additionally, the studies of transcription are divided into several fields, and different fields are increasingly separate and independent. Focusing on eukaryotic transcription, in this review we briefly describe major advances in various fields and present new conflicting view points. Although the structural studies have revealed the main components and architecture of the TA, it is still unclear how the Mediator complex transmits signals from activators to the core transcriptional machinery at the promoter. It is believed that the Mediator functions to recruit RNA polymerase II onto the promoter and promote the entry into transcriptional elongation, which fails to explain how the signal transduction is achieved. On the other hand, the allostery effect of the Mediator allows for signal transmission but is not supported by structural study. It is reported that enhancers, especially supper enhancers, act to recruit activators via forming a so-called liquid drop and phase separation. By contrast, it is suggested that enhancers should cooperate delicately to orchestrate transcription. Results on the kinetics of protein-promoter interaction also contrast with each other, leading to a paradox called “transcriptional clock”. It is then concluded that proteins interact frequently and transiently with promoters and different proteins interact with the promoter at different stages of transcriptional progression. The phenomenon of transcriptional burst questions how the cellular signaling is achieved through such a noisy manner. While the burst frequency or size, or both are potentially modulated by transcriptional activators, more evidence supports the mode of frequency modulation. The technical difficulties in investigating the mechanism of transcription include 1) structural characterization of flexible and/or unstable proteins or protein complexes, 2) measurement of intermolecular kinetics, 3) tracking of single molecule movement, and 4) lack of methodology in theoretical research. We further propose a research strategy based on the ensemble statistical method, and introduce a model for how the TA dynamically operates. The model may act as a benchmark for further investigations. The operating mechanism of the TA should reflect an optimal use of physics laws as a result of long-term biological evolution.

## COVER ARTICLE

2020, 69 (23): 238702. doi: 10.7498/aps.69.20200690
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1,3,5-trinitro-1,3,5-triazacyclohexane (RDX) or hexogen, a high-insensitivity explosive, the accurately description of its energy and properties is of fundamental significance in the sense of security and application. Based on the machine learning method, high-dimensional neural network is used to construct potential function of RDX crystal. In order to acquire enough data in neural network learning, based on the four known crystal phases of RDX, the structural global search is performed under different spatial groups to obtain 15199 structure databases. Here in this study, we use nearby atomic environment to build 72 different basis functions as input neurons, in which the 72 different basis functions represent the interaction with nearby atoms for each type of element. Among them, 90% data are randomly set as training set, and the remaining 10% data are taken as test set. To obtain the better training effect, 9 different neural network structures carry out 2000 step iterations at most, thereby the 30-30-10 hidden layer structure has the lower root mean square error (RMSE) after the 1847 iterations compared with the energies from first-principles calculations. Thus, the potential function fitted by 30-30-10 hidden layer network is chosen in subsequent calculations. This constructed potential function can reproduce the first-principles results of test set well, with the RMSE of 59.2 meV/atom for binding energy and 7.17 eV/Å for atomic force. Especially, the RMSE of the four known RDX crystal phases from 1 atm to 6 GPa are 10.0 meV/atom and 1.11 eV/Å for binding energy and atomic force, respectively, indicating that the potential function has a better description of the known structures. Furthermore, we also propose four additional RDX crystal phases with lower enthalpy, which may be alternative crystal phases undetermined in experiment. In addition, based on molecular dynamics simulation with this potential function, the α-phase RDX crystal can stay stable for a few ps, further proving the applicability of our constructed potential function.

## COVER ARTICLE

2020, 69 (22): 226801. doi: 10.7498/aps.69.20201160
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The essence of the scientific problem in all solid-state batteries lies in the properties of the introduced solid electrolyte and the existence of a new solid-solid interface. Starting from the structure-property relationship, the structural evolution of the solid-solid interface and the electrolyte itself, and the matter transport process determine the performance of the all-solid-state battery. With the continuous enrichment of solid electrolyte materials, the current problems in all solid-state batteries are mainly concentrated on the solid-solid interface. The composition and structure at the interface limit the performance of all solid-state batteries. According to the different situations of solid-solid interface contact, this article summarizes and discusses the structure and matter transport at the solid-solid interface in all solid-state batteries according to the three levels of solid-solid interface physical contact, chemical contact and surface modification. Finally, the relationship between local symmetry and material properties under the macroscopic complex system is discussed from the perspective of the functional origin of functional materials.

## COVER ARTICLE

2020, 69 (21): 217705. doi: 10.7498/aps.69.20200540
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Piezoelectric ceramics, as a kind of functional material, can realize the mutual transformation between mechanical energy and electrical energy, and has been widely used in civil and military fields. With the improvement of people's awareness of environment protection and self-health care, the study of lead-free piezoelectric ceramics with excellent performance and environmental friendliness has become an urgent task. Among several kinds of lead-free piezoelectric materials, potassium sodium niobate [(K, Na)NbO3, KNN]-based ceramics has attracted much attention due to its good comprehensive properties, but there have been carried out few studies focusing on the utilization of phase boundary to regulate the properties of high piezoelectric and electrocaloric effect simultaneously. In this work, lead-free 0.944K0.48Na0.52Nb0.95Sb0.05O3 -0.04Bi0.5(Na0.82K0.18)0.5ZrO3-1.6%(AgxNa1–x)SbO3-0.4%Fe2O3 ceramics is prepared via the conventional solid-state method, and the effect of AS/NS ratio on phase structure, electrical properties, and electrocaloric effect are studied. The obtained results show that the ceramics has a multiphase coexistence with “rhombohedral-orthorhombic-tetragonal” (R-O-T) in all compositions. With the increase of AS content, the piezoelectric and ferroelectric properties of the ceramics fluctuate (d33 = 518–563 pC/N, kp = 0.45–0.56; Pmax = 21–23 μC/cm2, Pr = 14–17 μC/cm2). In addition, the electrocaloric effect (ECE) for each of the samples is studied by the indirect method. Broadening temperature span (～90 K) of electrocaloric effect is obtained in the vicinity of O-T phase transition region, while a low ECE value is observed. A stronger ECE peak (ΔTmax > 0.6 K) can be observed when the measurement temperature reaches near the Curie temperature. Consequently, both large piezoelectric property and high electrocaloric performance can be realized in KNN-based ceramics by new phase boundary construction.
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