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Measurement of radioactive residual nuclides induced in Cu target by 80.5 MeV/u carbon ions
Zhou Bin, Yu Quan-Zhi, Zhang Hong-Bin, Zhang Xue-Ying, Ju Yong-Qin, Chen Liang, Ruan Xi-Chao
2021, 70 (7): 072501. doi: 10.7498/aps.70.20201503
Abstract +
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.

EDITOR'S SUGGESTION

Effect of pressure on discharge mode transition in radio-frequency capacitively coupled micro-beam plasmas
Huang Jiu-Huan, Gao Fei, Wang You-Nian
2021, 70 (7): 075205. doi: 10.7498/aps.70.20201716
Abstract +
Micro-beam radio-frequency (RF) capacitive discharges have been widely used in the plasma enhanced chemical vapor deposition of nanocrystalline particles such as nano silicon crystal. However, the plasma column shrinks radially at a sufficiently high gas pressures as manifested by their glow not entirely filling the radial cross-section of the discharge tube. This greatly limits the dissociation rate of gas in plasma. In order to obtain the information about the plasma column varying with gas pressure, the formation of different gas discharge mode under different pressure is discussed. In this paper the spatial characteristics of micro-beam RF capacitive discharges are investigated by using an intensified charged-coupled device (ICCD) and a single lens reflex camera (SLR camera). Furthermore, high voltage probe and current probe are used to record the electrical characteristics of the high voltage electrode. The results indicate that in a pure argon discharge, the discharge mode evolves from a glow discharge into a filament discharge with the increase of pressure. As the pressure continues to increase, the filament is split: a single channel of plasma is split into two or more filaments at a certain gas pressure. However, the glow discharge in a mixture of 99% argon and 1% hydrogen at a low pressure is observed: the plasma spreads throughout the tube. As the pressure increases, the filament disappears, and the plasma column still can be observed in the center of quartz tube. The glow shrinks in the radial center at a moderate pressure. At a high pressure, the "annulus" glow discharge is achieved as manifested by a glow ring on the surface of the discharge tube. In addition, in pure hydrogen discharges, the discharge mode evolves from the full-space glow discharge into an "annulus" glow discharge with pressure increasing. Finally, through the interaction between the electron heating by the radio frequency electric field and heat conduction of gas, the filament discharge in a low thermal conduction gas is explained. In addition, special attention is paid to the pure argon filamentation, which is the splitting of a single channel of plasma into two or more smaller filaments as a result of the skin effect.

EDITOR'S SUGGESTION

Epitaxial growth of Ho3Fe5O12 films with perpendicular magnetic anisotropy and spin transport properties in Ho3Fe5O12/Pt heterostructures
Yang Meng, Bai He, Li Gang, Zhu Zhao-Zhao, Zhu Yun, Su Jian, Cai Jian-Wang
2021, 70 (7): 077501. doi: 10.7498/aps.70.20201737
Abstract +
Rare-earth iron garnet films with perpendicular magnetic anisotropy could open new perspectives for spintronics. Holmium iron garnet (Ho3Fe5O12, HoIG) films with thickness ranging from 2 to 100 nm are epitaxially grown on (111) orientated gadolinium gallium garnet single crystal substrate doped with yttrium and scandium (Gd0.63Y2.37Sc2Ga3O12, GYSGG) by ultra-high vacuum magnetron sputtering. A 3-nm Pt film is further deposited on each of the HoIG films. The magnetic anisotropy and magneto-transport properties of heterostructures at room temperature are investigated. It is shown that the HoIG film as thin as 2 nm (less than two unit cells in thickness) exhibits the ferromagnetic properties at room temperature, and perpendicular magnetic anisotropy is achieved in the 2-60 nm thick films, and a maximum effective perpendicular anisotropy field reaches 350 mT due to the strain induced magnetoelastic anisotropy. The HoIG/Pt heterostructure shows significant anomalous Hall effect (AHE) and appreciable spin-Hall magnetoresistance (SMR) and/or anisotropic magnetoresistance (AMR). Remarkably, the AHE starts to decline gradually when the HoIG thickness is less than 4 nm, but the magnetoresistance decreases rapidly with the HoIG layer becoming less than 7 nm in thickness. The fact that the AHE in the heterostructure is less sensitive to the HoIG thickness suggests that the interface effect is more dominant in the AHE mechanism, whereas the bulk magnetic properties of the HoIG plays a more important role for the observed magnetoresistance. In addition, the spin Seebeck effect decreases exponentially with the decrease of HoIG thickness till the ultrathin limit, which was previously validated in the micrometer-thick YIG/Pt stacks in the frame of thermally excited magnon accumulation and propagation. The present results show that the nanometer HoIG/Pt heterostructure with tunable perpendicular magnetic anisotropy and efficient interfacial spin exchange interaction could be a promising candidate for insulating magnet based spintronic devices.

EDITOR'S SUGGESTION

High-precision dual-wavelength time transfer via1085-km telecommunication fiber link
Chen Fa-Xi, Zhao Kan, Li Bo, Liu Bo, Guo Xin-Xing, Kong Wei-Cheng, Chen Guo-Chao, Guo Bao-Long, Liu Tao, Zhang Shou-Gang
2021, 70 (7): 070702. doi: 10.7498/aps.70.20201277
Abstract +
To reduce the influence of fiber dispersion on accuracy of fiber-based time synchronization, a method of dispersion-error corrected dual-wavelength time synchronization is proposed in this paper. Specificlly, the method is to measure the dispersion coefficient of the fiber link, and then input it to each remote terminal, the time delay error caused by the fiber dispersion is eliminated through the delay phase controller. With the self-developed engineering prototypes, the experimental verifications are subsequently made both in laboratory and real field. Before the test, 16 devices of time synchronization are connected in series for calibration. The time synchronization system is able to keep delay difference within ± 15 ps after being calibrated. In the laboratory, the experimental setup is built by cascading 16 rolls of 50km-long fiber coils, and the total length of the fiber link is 800 km. The experimental results show that the dispersion coefficient of 800 km fiber link is 13.36 ps/(km·nm), and the delay error caused by dispersion is maintained within 10 ps after correction. The stability of the time transfer is 5.7 ps in standard deviation and the time deviation is 1.12 ps at an averaging time of 100000 s. In the real field test, a 1085-km-long field fiber link is utilized, along which 16 self-developed time-frequency transceiversare set at the cascaded fiber-optic stations. After being corrected with a dispersion coefficient of 16.67 ps/(km·nm) for 1085 km urban fiber link, the time transfer is demonstrated to have a dispersion-caused delay error of 60 ps. The experimental results show that the time standard deviation is 18 ps and the time transfer instability is 9.2 ps at an averaging time of 1 s and 5.4 ps at an averaging time of 40000 s. Finally, the time uncertainty of 800-km-long laboratory optical fiber link and 1085-km-long urban optical fiber link are evaluated, and the time uncertainty is 18.4 ps and 63.5 ps, respectively. This work paves the way for constructing the time synchronization fiber network in China. To further reduce the delay error caused by dispersion in a long-distance time transfer link, the more accurate thermal control of the lasers should be adopted to reduce the shifts of forward and backward wavelengths.

EDITOR'S SUGGESTION

Precise ranging for the multi regions of two complex-shape targets by using two chaotic polarization components in the optically pumped spin vertical cavity surface emitting laser with optical injection
Zhong Dong-Zhou, Zeng Neng, Yang Hua, Xu Zhe
2021, 70 (7): 074206. doi: 10.7498/aps.70.20201693
Abstract +
The ranging based on the chaotic lidar (CLR) generated by using the nonlinear dynamic of semiconductor with optical feedback or optical injection exhibits many advantages over the ranging using pulse lasers and CW lasers, such as low probability of intercept, strong anti-interference ability and low cost. Moreover, it has high resolution, benefiting from the broad bandwidth of the optical chaos. Finally, it is easily be generated and controlled due to the sensitivity of chaotic radar to laser parameters. The resolution of the correlated chaotic lidar (CLR) ranging which has been reported in many literatures is largely limited by the bandwidth of the chaotic laser. An ultra-fast chaotic laser with large modulation bandwidth is required to further improve the ranging resolution. The recently proposed optically pumped spin-VCSEL has attractive features such as flexible spin control of lasing output, fast dynamics with femtosecond magnitude and large modulation bandwidth. The ultra-fast chaos radar wave emitted from the optically pumped spin-VCSEL with optical injection or optical feedback is expected to be used for improving the resolution and accuracy of target ranging. In addition, since the multi beams of CLRs were utilized in the previous works, the number of ranging targets is limited to a small number of targets. The reported CLR ranging technology cannot completely detect the distance of different regions in the target, and it is not suitable for the accurate ranging of the whole area in the complex shape target. The detection waveform based on the correlation CLR has not been designed before the target ranging, which affects the further improvement of the resolution and accuracy of the target ranging. To overcome these problems, it is necessary to further explore the theoretical and physical mechanism of the CLR ranging for the multi-region in complex shape target, and explore the new scheme and method for its realization. Motivated by these, in this paper, based on the optically pumped spin vertical cavity surface emitting laser with optical injection, we present a novel scheme for the accurate ranging of the multi regions in two complex shape targets, using two chaotic polarization components modulated by the bipolar sinc waveform. Here, these two modulated chaotic polarization probe waveforms possess the attractive features of the uncorrelation in time and space, fast dynamic with femtosecond magnitude. Utilizing these features, the accurate ranging to the position vectors of the multi regions of two complex-shape targets can be achieved by correlating the multi beams of the time-delay reflected chaotic polarization probe waveforms with their corresponding reference waveforms. The further investigations show that the ranging to the multi-region small targets possesses the very low relative error that is less than 0.94%. If the bandwidths of the photodetectors are large enough, their range resolutions are achieved as high as 0.4 mm, and exhibit excellent strong anti-noise performance and strong stability. The multi area target ranging proposed in our scheme has the following attractive advantages: stable and high range resolution, strong anti-noise ability and very low relative error. These characteristics can meet the needs of the position vector ranging of the multi regions in complex shape targets.

EDITOR'S SUGGESTION

Influence of hydrogen-like nucleus mass on electronic state
Liu Zhao-Bin, Li Kai, Zeng Tian-Hai, Wang Feng, Song Xin-Bing, Shao Bin, Zou Jian
2021, 70 (7): 070301. doi: 10.7498/aps.70.20201754
Abstract +
In an isolated two-body composite system, the discussion of how the change of one body affects the state of the other will help to know the relation of a single particle's mixed and pure states. Given 5 one-dimensional hydrogen-like atoms models, each Coulomb interaction potential keeps invariant, while the masses of the nuclei are different. These two-body composite systems stay in their respective entangled state, each electron stays in a mixed state. If we suppose a one-dimensional hydrogen atom model having infinite nuclear mass, the electron stays in a pure state. We select position representation. The wave function of the ground state of the atom has the form of the square root of a δ function. To avoid calculation trouble of δ function, we select the first excited state and the superposed state of the first and the second excited states. We compare the two pure states, the first excited state and the superposed state, with those corresponding mixed states by fidelity and l1 norm coherence, and calculate the purities of the mixed states. The summations become integrations due to the position variable having a continuous eigenvalue spectrum. We investigate the changes in these quantities with the increase of the nuclear mass. The results show that the purities of the mixed states and the fidelities increase with the nuclear mass increasing. However, the coherences of the mixed states decrease with the nuclear mass increasing. This can be explained as that a mixed state with non-zero coherence may approach to a pure eigenstate, while the latter has zero coherence in the eigenspace. These mean that the greater a nuclear mass is, the closer the mixed state approaches to the corresponding pure state. Therefore, the two pure states are the approximations of the corresponding mixed states. The entangled state of the electron and the nucleus is related with the nuclear mass and the Coulomb interaction potential. So, each electron mixed state and its coherence are related with the nucleus and their Coulomb interaction potential. If the nuclear mass is great, the nucleus is approximately motionless or its state is approximately unchanged, and the Coulomb interaction potential approximates to the external Coulomb potential of the electron. The electron approximately stays in a pure state. The state and its coherence are related with the nucleus and the Coulomb interaction. From other point of view, the entangled state of the nucleus and the electron approximates to the product state of the unchanged nucleus state and the electron state. In this case, an electron mixed state approximates to its corresponding pure state, and then these states and their coherences are all related with the nucleus and the Coulomb interaction.

REVIEW

  

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Research progress of neuromorphic computation based on memcapacitors
Ren Kuan, Zhang Ke-Jia, Qin Xi-Zi, Ren Huan-Xin, Zhu Shou-Hui, Yang Feng, Sun Bai, Zhao Yong, Zhang Yong
2021, 70 (7): 078701. doi: 10.7498/aps.70.20201632
Abstract +
The rapid development of artificial intelligence (AI) requires one to speed up the development of the domain-specific hardware specifically designed for AI applications. The neuromorphic computing architecture consisting of synapses and neurons, which is inspired by the integrated storage and parallel processing of human brain, can effectively reduce the energy consumption of artificial intelligence in computing work. Memory components have shown great application value in the hardware implementation of neuromorphic computing. Compared with traditional devices, the memristors used to construct synapses and neurons can greatly reduce computing energy consumption. However, in neural networks based on memristors, updating and reading operations have system energy loss caused by voltage and current of memristors. As a derivative of memristor, memcapacitor is considered as a potential device to realize a low energy consumption neural network, which has attracted wide attention from academia and industry. Here, we review the latest advances in physical/simulated memcapacitors and their applications in neuromorphic computation, including the current principle and characteristics of physical/simulated memcapacitor, representative synapses, neurons and neuromorphic computing architecture based on memcapacitors. We also provide a forward-looking perspective on the opportunities and challenges of neuromorphic computation based on memcapacitors.

REVIEW

  

EDITOR'S SUGGESTION

Research progress of Cu2Se thin film thermoelectric properties
Yang Liang-Liang, Qin Yuan-Hao, Wei Jiang-Tao, Song Pei-Shuai, Zhang Ming-Liang, Yang Fu-Hua, Wang Xiao-Dong
2021, 70 (7): 076802. doi: 10.7498/aps.70.20201677
Abstract +
Thermoelectric (TE) materials can directly realize the mutual conversion between heat and electricity, and it is an environmentally friendly functional material. At present, the thermoelectric conversion efficiencies of thermoelectric materials are low, which seriously restricts the large-scale application of thermoelectric devices. Therefore, finding new materials with better thermoelectric properties or improving the thermoelectric properties of traditional thermoelectric materials has become the subject of thermoelectric research. Thin film materials, compared with bulk materials, possess both the two-dimensional macroscopic properties and one-dimensional nanostructure characteristics, which makes it much easier to study the relationships between physical mechanisms and properties. Besides, thin film are also suitable for the preparation of wearable electronic devices. This article summarizes five different preparation methods of Cu2Se thin films, i.e. electrochemical deposition, thermal evaporation, spin coating, sputtering, and pulsed laser deposition. In addition, combing with typical examples, the characterization methods of the film are summarized, and the influence mechanism of each parameter on the thermoelectric performance from electrical conductivity, Seebeck coefficient and thermal conductivity is discussed. Finally, the hot application direction of Cu2Se thin film thermoelectrics is also introduced.

EDITOR'S SUGGESTION

Radiation properties of radiative shock in xenon
Zhao Duo, Li Shou-Xian, An Jian-Zhu, Wu Yong, Wu Ze-Qing, Li Qiong, Wang Fang, Meng Guang-Wei
2021, 70 (7): 075201. doi: 10.7498/aps.70.20200944
Abstract +
Radiative shock is an important phenomenon both in astrophysics and in inertial confinement fusion. In this paper, the radiation properties of X-ray heated radiatve shock in xenon is studied with the simulation method. The radiative shock is described by a one-dimensional, multi-group radiation hydrodynamics model proposed by Zinn [Zinn J 1973 J. Comput. Phys. 13 569]. To conduct computation, the opacity and equation-of-state data of xenon are calculated and put into the model. The reliabilities of the model and the physical parameters of xenon are verified by comparing the temperature and velocity of the radiative shock calculated by the model with those measured experimentally. The evolution of the radiative shock involves abundant physical processes. The core of the xenon can be heated up to 100 eV, resulting in a thermal wave and forming an expanding high-temperature-core. Shortly, the hydrodynamic disturbances reach the thermal wave front, generating a shock. As the thermal wave slows down, the shock gradually exceeds the high-temperature-core, forming a double-step distribution in the temperature profile. The time evolution of the effective temperature of the radiative shock shows two maximum values and one minimum value, and the radiation spectra often deviate from blackbody spectrum. By analyzing the radiation and absorption properties at different positions of the shock, it can be found that the optical property of the shock is highly dynamic and can generate the above-mentioned radiation characteristics. When the radiative shock is just formed, the radiation comes from the shock surface and the shock precursor has a significant absorption of the radiation. As the shock temperature falls during expansion, the shock precursor disappears and the radiation inside the shock can come out owing to absorption coefficient decreases. When the shock becomes transparent, the radiation surface reaches the outside edge of the high-temperature-core. Then, the temperature of the high-temperature-core decreases further, making this region also optically thin, and the radiation from the inner region can come out. Finally, the radiation strength falls because of temperature decreasing.

SPECIAL TOPIC—Toward making functional devices at an atomic scale: Fundamentals and frontiers

  

COVER ARTICLE

Constructions of iron atoms arrays based on DNA origami templates for cryptography applications
Fan Hong-Jian, Li Jiang, Wang Li-Hua, Fan Chun-Hai, Liu Hua-Jie
2021, 70 (6): 068702. doi: 10.7498/aps.70.20201438
Abstract +
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.
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