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## 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.

## COVER ARTICLE

2020, 69 (20): 207501. doi: 10.7498/aps.69.20201507
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$\langle 111\rangle$ direction of crystal, a well-defined resonance splitting emerges when H > 7 T. Especially, the high-frequency absorption shows pronouncedly nonlinear magnetic field dependence. It is suggested that such anisotropic spin dynamics below Néel temperature be linked with the field-driven quantum criticality unveiled in recent work.">As a typical helimagnet, ZnCr2Se4 possesses fascinating effects including magnetoelectric coupling, magnetostriction, negative thermal expansion, as well as possible diversity in quantum ground states. Here in this work, we investigate magnetic excitation arising from spiral spin structure in ZnCr2Se4 single crystal by using terahertz (THz) time domain spectroscopy (THz-TDS) under magnetic fields up to 10 T and at low temperatures. The magnetic resonance absorption is observed in a sub-THz region as the applied magnetic field is above 4 T, featuring the blue shift with magnetic field increasing. As the THz wave vector ( k ) is vertical to the external magnetic field (H), the single resonance frequency conforms well with the linear Larmor relation, corresponding to a spin structure transformation from helical to ferromagnetic state with magnetic field increasing in ZnCr2Se4. However, in the geometry in which both k and H are along the $\langle 111\rangle$ direction of crystal, a well-defined resonance splitting emerges when H > 7 T. Especially, the high-frequency absorption shows pronouncedly nonlinear magnetic field dependence. It is suggested that such anisotropic spin dynamics below Néel temperature be linked with the field-driven quantum criticality unveiled in recent work.

## COVER ARTICLE

2020, 69 (19): 198501. doi: 10.7498/aps.69.20200742
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The continuous miniaturization and integration of pixelated devices have become a main trend in the field of display. Micro light-emitting diode (micro-LED) display is composed of an array of LEDs that are sub-50-micrometers in length. It has huge advantages in brightness, resolution, contrast, power consumption, lifetime, response speed and reliability compared with liquid crystal display (LCD) and organic LED (OLED) display. Consequently, micro-LED display is regarded as the next-generation display technology with high potential applications, such as virtual reality (VR), augmented reality (AR), mobile phones, tablet computers, high-definition TVs and wearable devices. Currently, the combination of commercial 5G communication technology with VR/AR display, ultra high definition video technologies will further prompt the development of micro-LED display industry. However, some basic scientific and technological problems in micro-LED display remain to be resolved. As the chip size shrinks to below 50 μm, some problems that are not serious for large-sized LEDs appear for micro-LEDs. These problems include crystalline defects, wavelength uniformity, full-color emmision, massively tranferring and testing, etc. In the past two decades, various solutions to those problems have been proposed, which have greatly promoted the progress of micro-LED display. In this paper, an overview of micro-LED display since 2000 is given firstly, which includes the main research results and application achievements. Secondly the issues involved in the wafer epitaxy and chip process of micro-LEDs and possible solutions are discussed based on the display application in detail. The surface state induced by the dangling bonds and dry etching damages are concerned for the nonradiative recombination at a low injection level. The remedies are provided for those surface states, such as atomic-layer deposition and neutral beam etching. Some methods to reduce the threading dislocation and suppress the polarization field are summarized for micro-LED epitaxial growth. Moreover, the GaN-based LEDs on Si (100) substrate are also introduced for the future integration of micro-LEDs into the Si-based integrated circuits. As to the wavelength uniformity, the MOCVD equipment and growth technology including the laser treatment are discussed. In the chip processing part, the full-color display, mass transfer and effective inspection technology are discussed. Assembling RGB individual LEDs, quantum dot phosphor material and nanocoloumn LEDs are different routes for full-color display. Their trends in the future are provided. The pick and place, laser lift-off technologies, are strengthened in the massively transferring for micro-LEDs. In the massively and rapidly inspection technologies, the photoluminscence combined with Raman scattering, the electroluminescence combined with digital camera are discussed. Finally, the summary and outlook in these issues are also provided.

## COVER ARTICLE

2020, 69 (18): 184208. doi: 10.7498/aps.69.20200342
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As a member of the metal phosphorus trichalcogenide family, MPS3 is widely used in nonlinear optics and devices, which can be regarded as a significant benefit for the excellent photonic and optoelectronic properties. In this work, the MnPS3 nanosheet is prepared by the chemical vapor transport method and the MnPS3 saturable absorber is demonstrated by modifying mechanical exfoliation. To the best of our knowledge, the dual-wavelength self-starting mode-locking erbium-doped fiber laser with MnPS3 saturable absorber is demonstrated for the first time. The dual wavelength mode-locked laser with a pulse repetition rate of 5.102 MHz at 1565.19 nm and 1565.63 nm is proposed. Its maximum output power at the dual-wavelength is 27.2 MW. The mode-locked laser can self-start and stably run for more than 280 h.

## COVER ARTICLE

2020, 69 (17): 172901. doi: 10.7498/aps.69.20200718
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The back-streaming neutron beam line (Back-n) was built in the beginning of 2018, which is part of the China Spallation Neutron Source (CSNS). The Back-n is the first white neutron beam line in China, and its main application is for nuclear data measurement. For most of neutron-induced nuclear reaction measurements based on white neutron facilities, the beam of gamma rays accompanied with neutron beam is one of the most important experimental backgrounds. The back streaming neutron beam is transported directly from the spallation target to the experimental station without any moderator or shielding, the flux of the in-beam gamma rays in the experimental station is much larger than those of these facilities with neutron moderator and shielding. Therefore, it is necessary to consider the influence of in-beam gamma rays on the experimental results. Studies of the in-beam gamma rays are carried out at the back-n. Monte-Carlo simulation is employed to obtain the energy distribution and the time structure of the in-beam gamma rays. According to the simulation results, when the neutron flight time is longer than 1.0 μs the energy distribution of the in-beam gamma rays does not vary with flight time. Therefore, the time structure of these gamma rays can be measured without the correction of the detection efficiency. In this work, the time structure of the in-beam gamma rays in the low neutron energy region is measured by both direct and indirect methods. In the direct measurement, a 6Li loaded ZnS(Ag) scintillator is located on the neutron beam line and the time of flight method is used to determine the time structure of neutrons and gamma rays. The gamma rays are separated from neutrons with pulse-shape discrimination. The black filter method is used to verify the particle discrimination results. In the indirect measurement, the C6D6 scintillation detectors are used to measure the gamma rays scattered off a Pb sample on the way of the neutron beam. The time structure of the in-beam gamma rays is derived from that of the scattered gamma rays. The experimental results are in good agreement with the simulations with the time-of-flight between 12 μs and 2.0 ms. Besides, according to the simulation results, the intensity of the in-beam gamma rays is 1.21 × 106 s–1·cm–2 in the center of the experimental station 2 of Back-n, which is 76.5 m away from the spallation target of CSNS.

## COVER ARTICLE

2020, 69 (16): 167102. doi: 10.7498/aps.69.20200646
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Hybrid organic-inorganic perovskites show large potential applications in solar cells, light emitting diodes and low threshold lasers because of the high tolerance of defects compared with other semiconductor materials. Normally they have been synthesized by dilution method, generating a device with high performance, but they also introduce lots of defects. So far, investigations have been done intensively on ensemble defects both in theory and experiment, but single-defect related trapped excitons are yet to be explored. In this work, we prepared high-quality CH3NH3PbBr3 perovskite nanowires with the length of about 1 μm and the width of several hundred nanometers by “reverse” ligand assisted reprecipitation method, and performed the magneto-photoluminescence measurement of different trapped excitons in single perovskite nanowires at a low temperature with a standard confocal microscopic system. The photoluminescence (PL) peak with narrow linewidth has been observed from trapped excitons with high luminescence intensity and the trapped excitons can be coupled with phonons in different ways. Both Zeeman splittings and diamagnetic effects have been observed in single trapped excitons under the magnetic field, and we found that the different trapped excitons have different Zeeman splittings and diamagnetic effects which is caused by the different defects near the trapped excitons. At the same time, we have extracted the g-factor of the trapped excitons under different magnetic field angles. The extracted exciton g-factors show anisotropic, which can be ascribed to the limitation of the lattice structure of the perovskite and the trapped exciton wave-function anisotropy under a vector magnetic field. Our results demonstrate that trapped excitons with narrow linewidth have very good luminescence properties and studying the magneto-optical properties from single trapped excitons can provide a deep understanding of trapped excitons in perovskites for applications in quantum light sources and spintronics. Furthermore, our results can also provide a possibility to control the electron spin in single-trapped-excitons-based hybrid organic-inorganic perovskites by manipulating the g-factor through an applied vector magnetic field, which promotes the application of the perovskite-based spintronics.

## COVER ARTICLE

2020, 69 (15): 154102. doi: 10.7498/aps.69.20200198
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The topological transitions in two-dimensional photonic crystals (PCs) originate from the opening-closing-reopening of the bandgap, accompanied with the band order inversion. The topological bandgap in magnetic PC can be created by applying a bias magnetic field or deforming the geometry structure of the PC. In this paper, we demonstrate that the direction of the bias magnetic field also plays a key role in modifying the band structure in a two-dimensional magnetic PC. The results show that by reversing the direction of the bias magnetic field, the eigenstates with the same parity may exchange their orders in the band structure. We investigate this type of band order exchange in the applications of constructing topological edge states and its influence on the properties of edge states. We find, for example, reversing the direction of the bias magnetic field can create two almost degenerated topological edge modes, which propagate in the same direction but have opposite orbital angular momenta. The edge modes and their characteristics can be determined by the schematics of the band orders for the photonic crystals on the two sides of the boundary. The relative relationship of the band orders determines the emergence of the topological edge states, the number of edge states, and edge modes’ properties such as the orbital angular momentum and group velocity. Also, it affects the transmission efficiency of the electromagnetic wave on the boundary. The direction effect of the bias magnetic field on band order exchange presented in this paper provides us with a new way to change the feature of topological edge states and helps us to better understand the influence of band order on topological phases of photonic crystals. It may have potential applications, such as in pseudo-spin splitter and reflection-free one-way optical switch.
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