bluuebunny/crossref_metadata_2025 huggingface dataset:
Delay division multiplexing orthogonal frequency-division multiple access passive optical networks using low-sampling-rate analog-to-digital converter
Authors: Bai Guang-Fu  , Jiang Yang  , Hu Lin  , Tian Jing  , Zi Yue-Jiao  ,   College of Big Data and Information Engineering, Guizhou University, Guiyang 550025, China;,   College of Physics, Guizhou University, Guiyang 550025, China}
Published: December 2020

In traditional orthogonal frequency-division multiple access passive optical networks (OFDMA PON) or time-division multiplexing access (TDMA) based OFDM PONs, analog-to-digital converters (ADCs) with a high sampling rate are required to demodulate high-speed aggregated OFDM data in order to receive a small portion of the downstream data at optical network users (ONUs). Meanwhile, OFDM signal has a higher peak-to-average power ratio (PAPR) than the single carrier signal, which can result in the nonlinear effect. The resulting nonlinearity reduces the received signal performance. To enhance practicability of the present PONs, according to the sub-Nyquist sampling theory, we propose and detail a delay-division-multiplexing (DDM) scheme to enable a FDMA PON with low-sampling-rate ADCs. Based on pre-allocated relative time delays among the ONUs and discrete Fourier transform spread (DFT-S) technique, pre-processed signals sent from an optical line terminal (OLT) can be detected as different downstream signals following spectral aliasing caused by ADCs operating at a sub-Nyquist sampling rate. In the proposed scheme, as the signal distortion introduced by the propagation, aliasing and time shifted sampling is pre-compensated, the DFT and inverse discrete Fourier transform (IDFT) are unnecessary for de-mapping and picking out the signal at ONUs. Therefore, the proposed DDM scheme greatly enhances cost efficiency and enables a reduction in computational complexity. Meanwhile, DFT-S FDMA signal has low PAPR, which relieves the nonlinear effect in signal E/O conversion and transmission. As a result, the proposed scheme benefits the power budget of the OLT and power consumption of the ONUs. In experiment, we demonstrate that each ONU with an ADC operating at 1/2-1/32 of the Nyquist sampling rate is able to receive 1/2-1/32 of the downstream data, with an insignificant performance penalty. Furthermore, the details of the matrices that include channel response, aliasing and time delay are first analyzed. In addition, training symbol is very important for estimating the channel response, and how to derive and design training symbols is the first study to outline the details of this issue. The effects of fiber dispersion and the sampling instant of an ADC on signal performance are also studied. The results show that the signal performance has some degree of tolerance to sampling instant deviation and the power penalty is less than 0.5 dB to achieve a forward error correction limit of 10-3 after 25 km fiber transmission. The theoretical analysis and experimental results indicate that the proposed scheme can simplify the ONU and reduce the cost of the PON.
Mechanism of inverted polymer solar cells based on poly(dopamine)/ZnO as composite cathode buffer layer
Authors: Li Qi  , Zhang Yong  ,   Laboratory of Nanophotonic Functional Materials and Devices, Institute of Optoelectronic Materials and Technology, South China Normal University, Guangzhou 510631, China;,   Guangdong Engineering Technology Research Center of Low Carbon and Advanced Energy Materials, Guangzhou 510631, China}
Published: December 2020

Inverted polymer solar cells with P3HT:PCBM as active layer are fabricated based on poly(dopamine)/ZnO (PDA/ZnO) as composite cathode buffer layer. Effects of PDA/ZnO composite cathode buffer layer with the different self-polymerization times on the device performance are investigated. According to the results, the short circuit current and photoelectric conversion efficiency of polymer solar cells first increase then decrease with the increase of the self-polymerization time of PDA. For 10-min PDA self-polymerization, the photovoltaic performance of the device achieves the optimal values:open circuit voltage 0.66 V, short circuit curent density 9.70 mA/cm2, fill factor 68.06%, and power conversion efficiency 4.35% under irratiation of light with a strength of 100 mW/cm2. We conclude that the improvement of device performance is due to the PDA/ZnO composite cathode buffer layer reduced the contact resistance between the ZnO and ITO, at the same time, the presence of a large number of nitrogen groups in PDA is advantageous for the electronic collection of the inverted polymer solar cells. Meanwhile, polymer solar cell with PDA/ZnO as composite cathode buffer layer also exhibits excelent stability. In addition, PDA has a strong adhesive force that makes the ZnO interface layer on its surface not easy to fall off. This provides a new way of fabricating the flexible polymer solar cell devices.
Simulation analysis of micro-ring resonator based on diamond multilayer waveguide structure
Authors: Li Zhi-Quan  , Bai Lan-Di  , Gu Er-Dan  , Xie Rui-Jie  , Liu Tong-Lei  , Niu Li-Yong  , Feng Dan-Dan  , Yue Zhong  ,   Institute of Electrical Engineering, Yanshan University, Qinhuangdao 066004, China
Published: December 2020

With the development of the technology for fabricating high-quality synthetic diamond and diamond waveguide structures, more and more researchers are being involved in exploring the particular optical properties of diamond for different applications. Because of its high refractive index and nontoxicity to biological species, diamond can be used to make micro-ring resonator to detect the concentration of liquid or gas. In this paper, a single micro-ring resonator model with diamond serving as the core layer is proposed. In the model, the vertical-section of the waveguide adopts a five-layer ridge-type waveguide structure based on As2S3, SiO2 and diamond, i.e. As2S3-SiO2-Diamond-SiO2-As2S3. To investigate the optical properties of the resonator, the vertical-section of the single straight waveguide, the coupling region of the direct waveguide, and the ring waveguide are simulated with the adopted operating wavelength =1550 nm based on the coupling mode theory and micro-ring resonance theory. In addition, the distribution of the field strength for the micro-ring is described at a resonant wavelength of 1543 nm. It is very important to explore the field intensity distribution of the micro-ring for understanding how the light transmits. The transmission characteristics of the micro-ring with the change of the distance between the straight waveguide and the ring waveguide in the coupling region are also simulated. The quality factor and the influence of the coupling coefficient change on the output spectrum are studied by the transfer matrix method and the micro-ring loss is discussed. It is shown that the micro-ring resonator designed with the diamond material has good transmission characteristics. When the resonant wavelength is 1543 nm, the resonant peak reaches more than -12 dB. The quality factor is about 1.54105. When the coupling coefficient k is 0.01, the free spectral range is about 40 nm. The coupling coefficient k is determined by the distance S of the coupling region. The results show that when S is equal to 50 nm, the output spectrum has a good extinction ratio and is better compared with the other values. The error of material processing is mainly affected by size, so the output spectrum near the distance S=50 nm is studied. The result shows that in the tiny change scope, the spectral output peak is linearly related to S. The structure we suggested in this paper expands the application scope of diamond in the field of optics, and provides some guiding significance for developing the optical integrated chips.
Inversion of seabed attenuation by using single mode extracted by warping transform
Authors: Li Jia-Wei  , Lu Li-Cheng  , Guo Sheng-Ming  , Ma Li  ,   Key Laboratory of Underwater Acoustic Environment, Institute of Acoustics, Chinese Acdemy of Sciences, Beijing 100190, China;,   University of Chinese Academy of Sciences, Beijing 100049, China
Published: December 2020

Seabed is an important part of the marine environment and it has a significant influence on sound propagation. Considering the fact that geoacoustic parameters are directly acquired with difficulty and complexity, a lot of researchers have focused on the inversion of them. The seabed attenuation coefficient is insensitivity to the matching field. However it has great effects on the transmission loss, mode amplitude ratios, etc. It can be inverted from measurements of these quantities. In this paper, we present an inversion scheme based on warping transform technique for estimating the seabed attenuation coefficient. It utilizes an equivalent seabed model which is constructed by using a prior and posterior knowledge. The dispersion characteristics of normal modes can be observed using the time-frequency analysis of the explosive signal recorded. The dispersion curve can be used to invert the seabed sound speed and density. The results presented by other scholars in the same circle are cited in this paper that focuses on how to obtain the seabed attenuation. Warping transform technique is used to separate and extract the normal modes. The main advantage of warping transform is that it can transform the time-frequency spectrogram into linear relationship which makes it easier to extract the normal modes. The feature of this paper lies in determining the distance normalized normal mode transmission loss. If the depths of receiving hydrophone and the explosion source are constant, the plot of normalized normal mode transmission loss versus distance is a straight line from the normal modes theory, which can be used to obtain the attenuation factor of real part of pressure. Then the seabed attenuation coefficient of the shallow water acoustic model can be calculated. In order to verify the effectiveness of this method, the warping transformation technology is used to separate and extract the first three modes from the simulated Gaussian pulse signal which is obtained in a simulated environment which is similar to the real marine environment. The extracted results are completely consistent with the numerical results. After that, the impulsive signal data collected in the Yellow Sea are analyzed according to the scheme process, and the relationship between the seabed attenuation and frequency is =0.581fk1.86(dB/m) in a range from 150 Hz to 550 Hz. The results are in good agreement with those obtained by other scholars in the same circle. On the other hand, the inversion results of seabed attenuation from different modes can be used for comparison at the same frequency, which can be a good support for the result.
Simulation of effect of bottom heat source on natural convective heat transfer characteristics in a porous cavity by lattice Boltzmann method
Authors: He Zong-Xu  , Yan Wei-Wei  , Zhang Kai  , Yang Xiang-Long  , Wei Yi-Kun  ,   College of Metrology and Measurement Engineering, China Jiliang University, Hangzhou 310018, China;,   College of Civil Engineering, Shenzhen University, Shenzhen 518060, China;,   Faculty of Mechanical Engineering and Automation, Zhejiang Sci-Tech University, Hangzhou 310018, China
Published: December 2020

The influence of bottom heat source on natural convective heat transfer characteristics in a two-dimensional square cavity fully filled with a homogeneous porous medium is numerically studied by the lattice Boltzmann method. In this physical model, the upper wall of porous cavity is set to be a cold heat source, and the bottom wall is designed as a local hot heat source. Both the left wall and the right wall are set to be adibatic. Specifically, the effects of both the position and size of bottom heat source on the properties of natural convective heat transfer are analyzed. The numerical results show that the position and size of bottom heat source have great influences on the characteristics of natural convective heat transfer, and there also exist the best position (a=4/16) and optimal size (b=0.75) of the bottom heat source for the maximal convective heat transfer intensity (Numax 10.35) and heat exchange capacity (Qmax 5.69).
Investigation on electrical transport properties of CdZnTe pixel detector
Authors: Nan Rui-Hua  , Wang Peng-Fei  , Jian Zeng-Yun  , Li Xiao-Juan  ,   Shaanxi Key Laboratory of Photoelectric Functional Materials and Devices, School of Materials and Chemical Engineering, Xi'an Technological University, Xi'an 710021, China;,   State Key Laboratory of Solidification Processing, School of Materials Science and Engineering, Northwestern Polytechnical University, Xi'an 710072, China
Published: December 2020

Semi-insulating cadmium zinc telluride (CdZnTe or CZT) is an excellent material candidate for fabricating room-temperature nuclear radiation semiconductor detectors due to its high resistivity and good carrier transport behaviors. It is widely used in nuclear security, nuclear medicine, space science, etc. Nevertheless, the traditional CdZnTe planar detector is subjected to the effect of hole trailing on its hole transport characteristic, where its energy resolution and the photoelectric peak efficiency both decrease, and thus deteriorating the detection performance. In order to eliminate the effect of hole capture, the electrode with pixel structure for CdZnTe detector is designed for detecting single carriers that are only electrons.
In this paper, a 10 mm10 mm2 mm wafer cut from an In doped Cd0.9Zn0.1Te single crystal, grown by the modified vertical Bridgman method, is employed to fabricate a 44 CdZnTe pixel detector, which is composed of 16 small pixel units with an area of 2 mm2 mm. Each of the pixel units is linked up with ASIC multichannel preamplifier and shaping amplifier by flip chip technology. Finally, the signal is treated by an integrated sensing chip. In the first case, the electrical properties and carrier transport properties of CdZnTe pixel detector are characterized by current-voltage (I-V) measurement via an Agilent 4155C semiconductor parameter analyzer and ray energy spectrum response via a standard Multi Channel Analyzer 6560 spectra measurement system, respectively. In the second case, the differences between CdZnTe planar detector and 44 pixel detector in the detection performance are discussed in detail. The results indicate that the bulk resistivity of CdZnTe pixel detector is determined to be about 1.7310 cm by a linear fit of I-V curve. The maximum leakage current of a single pixel is less than 2.2 nA for a bias voltage of 100 V. Furthermore, the carrier transport behaviors are evaluated with the mobility-lifetime product for electron in CdZnTe detector, which is 5.4110-4 cm2V-1 estimated by ray energy spectroscopy response under various bias voltages from 50 to 300 V at room temperature. The energy resolutions of the two CdZnTe detectors can reflect the ability of them to distinguish different energy gays during operation. The best energy resolution of a single pixel in CdZnTe pixel detector for 241Am@59.5 keV ray increases up to 5.78% under a 300 V bias voltage, whereas that of CdZnTe planar detector is only 6.85% in the same conditions. As a consequence, the detection performance of 44 CdZnTe pixel detector is better than that of the planar detector.
Switching characteristics of all-spin logic devices based on graphene interconnects
Authors: Li Cheng  , Cai Li  , Wang Sen  , Liu Bao-Jun  , Cui Huan-Qing  , Wei Bo  ,   College of Science, Air Force Engineering University, Xi'an 710051, China;,   The First Aeronautic Institute, Air Force Engineering University, Xinyang 464000, China
Published: December 2020

Traditional complementary metal-oxide-semiconductor (CMOS) technology has reached nanoscale and its physical limits are determined by atomic theory and quantum mechanics, which results in a series of problems such as deteriorated device reliability, large circuit interconnection delay, and huge static power dissipation. In the past decades, with the discovery of giant magnetoresistance effect and tunnel magnetoresistance effect, spintronics has become a research hotspot in this field. Specially, spin transfer torque effect has been experimentally verified that the magnetization of a ferromagnet layer can be manipulated using spin polarized current rather than an external magnetic field. Spintronics is a new type of electronics which utilizes spin rather than charge as state variable for electrical information processing and storage. As an example, all spin logic (ASL) devices, which stores information by using the magnetization direction of the nanomagnet and communication by using spin current, is generally thought to be a good post-CMOS candidate. Compared with the typical metal material, the graphene material has a large conductivity, long spin relaxation time, and weak spin-orbit interaction. Therefore, the dissipation of spin current in the graphene material is weaker than the counterpart in typical metal when the injected current is identical. In this paper, the switching characteristics of all spin logic device comprised of graphene interconnects are analyzed by using the coupled spin transport and magneto-dynamics model. The results show that comparing with ASL device comprised of copper interconnects, the magnetic moment reversal time of ASL with graphene interconnection is short and the spin current flows into the output magnet is large under the condition of same applied voltage and device size. Meanwhile, the switching delay and the energy dissipation are lower when the interconnects are shorter and narrower. When the critical switching current which is required for the magnetization reversal is applied, the reliable working length of graphene interconnection is significantly longer than that of copper interconnection. So the graphene is the more ideal interconnect material than metal material. Moreover, the switching delay and power dissipation could be further reduced by properly selecting the interconnection dimension. These results mentioned above provide guidelines for the optimization and applications of ASL devices.
Nonlocal resistance in multi-terminal graphene system
Authors: Wang Zi-Bo  , Jiang Hua  , Xie Xin-Cheng  ,   Microsystems and Terahertz Research Center, China Academy of Engineering Physics, Chengdu 610200, China;,   Institute of Electronic Engineering, China Academy of Engineering Physics, Mianyang 621999, China;,   College of Physics, Optoelectronics and Energy, Soochow University, Suzhou 215006, China;,   Institute for Advanced Study of Soochow University, Suzhou 215006, China;,   International Center for Quantum Materials, School of Physics, Peking University, Beijing 100871, China;,   Collaborative Innovation Center of Quantum Matter, Beijing 100871, China
Published: December 2020

Since the nonlocal measurement is helpful in discovering nontrivial physics that is too difficult to detect directly, the nonlocal measurement has now become one of the research focuses in condensed matter physics. Recent experiments find the signal of the giant nonlocal resistance in an H-shaped multi-terminal graphene system. After excluding other possible transport mechanisms, such as the classic Ohmic diffusion and the edge states, researchers tend to believe that the nonlocal resistance signal originates from the spin/valley Hall effect existing in graphene sample. Based on the Landauer-Buttiker formula, the numerical results make a relatively perfect match with the experimental data in the same multi-terminal graphene system. However, though the theoretic research has made certain progress in explaining the existence of the nonlocal resistance, it is still difficult to understand some exotic behaviors of the nonlocal resistance, which exhibits properties even contradictory to the known classical theories. For instance, the nonlocal resistance decreases to zero much more rapidly than the local one, and the giant peak of the nonlocal resistance appears inside the energy gap of the graphene. In this review, the experiments focusing on the nonlocal resistance in multi-terminal graphene system are carefully reviewed. Besides, this review also shows the associated theoretic studies, and an overlook of the future study is also provided.
Fabrication of graphene nanostructure and bandgap tuning
Authors: Zhang Hui-Zhen  , Li Jin-Tao  , Lü Wen-Gang  , Yang Hai-Fang  , Tang Cheng-Chun  , Gu Chang-Zhi  , Li Jun-Jie  ,   Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China;,   Collaborative Innovation Center for Nanomaterials and Devices, College of Physics, Qingdao University, Qingdao 266071, China
Published: December 2020

Graphene has potential applications in future microelectronics due to its novel electronic and mechanical properties. However, the lack of the bandgap in graphene poses a challenge and hinders its applications. In order to be able to work in ambient condition, gap engineering of graphene with nanostructure needs about sub-10 nm characteristic size, which increases the difficulty of fabrication and leads to less driving current that can be borne. In this paper, a new method to fabricate sub-10 nm graphene nanostructures is developed. With PMMA/Cr bilayer structure, sub-10 nm graphene nanostructures can be obtained precisely and repeatedly through controlling the etching time. Meanwhile, a new device based on graphene nanoconstrictions connected in parallel is designed and fabricated, whose band gap is bigger than that of graphene nanoribbon and whose characteristic width is the same as that of graphene nanoribbon. With the graphene nanoconstrictions connected in parallel, the band gap of the graphene can be adjusted effectively and the driving current can be significantly increased, which is very important for future practical applications of graphene.
Topological zero-energy modes in time-reversal-symmetry-broken systems
Authors: Zhang Wei-Feng  , Li Chun-Yan  , Chen Xian-Feng  , Huang Chang-Ming  , Ye Fang-Wei  ,   State Key Laboratory of Advanced Optical Communication Systems and Networks, School of Physics and Astronomy, Shanghai Jiao Tong University, Shanghai 200240, China;,   Department of Physics of Zhejiang Normal University, Jinhua 321004, China
Published: December 2020

The well-known Su-Schrieffer-Heeger (SSH) model predicts that a chain of sites with alternating coupling constant exhibits two topological distinct phases, and at the truncated edge of the topological nontrivial phase there exists topologically protected edge modes. Such modes are named zero-energy modes as their eigenvalues are located exactly at the midgaps of the corresponding bandstructures. The previous publications have reported a variety of photonic realizations of the SSH model, however, all of these studies have been restricted in the systems of time-reversal-symmetry (TRS), and thus the important question how the breaking of TRS affects the topological edge modes has not been explored. In this work, to the best of our knowledge, we study for the first time the topological zero-energy modes in the systems where the TRS is broken. The system used here is semiconductor microcavities supporting exciton-polariton quasi-particle, in which the interplay between the spin-orbit coupling stemming from the TE-TM energy splitting and the Zeeman effect causes the TRS to break. We first study the topological edge modes occurring at the edge of one-dimensional microcavity array that has alternative coupling strengths between adjacent microcavity, and, by rigorously solving the Schrdinger-like equations (see Eq.(1) or Eq.(2) in the main text), we find that the eigen-energies of topological zero-energy modes are no longer pinned at the midgap position:rather, with the increasing of the spin-orbit coupling, they gradually shift from the original midgap position, with the spin-down edge modes moving toward the lower band while the spin-up edge modes moving towards the upper band. Interestingly enough, the mode profiles of these edge modes remain almost unchanged even they are approaching the bulk transmission bands, which is in sharp contrast to the conventional defect modes that have an origin of bifurcation from the Bloch mode of the upper or lower bands. We also study the edge modes in the two-dimensional microcavity square array, and find that the topological zero modes acquire mobility along the truncated edge due to the coupling from the adjacent arrays. Importantly, owing to the breaking of the TRS, a pair of counterpropagating edge modes, of which one has a momentum k and the other has -k, is no longer of energy degeneracy; as a result the scattering between the forward-and backward-propagating modes is greatly suppressed. Thus, we propose the concept of the one-dimensional topological zero-energy modes that are propagating along the two-dimensional lattice edge, with extremely weak backscattering even on the collisions of the topological zero-energy modes with structural defects or disorder.
Variation of thermal expansion at low temperature and phonon relaxation time in graphene with temperature
Authors: Ren Xiao-Xia  , Shen Feng-Juan  , Lin Xin-You  , Zheng Rui-Lun  ,   College of Electronic and Electrical Engineering, Electronic Engineering Research Center of New Energy Storage Devices and Applications, Chongqing University of Arts and Sciences, Chongqing 402160, China;,   School of Mechanical Engineering and Automation, Fuzhou University, Fuzhou 350000, China
Published: December 2020

Considering the anharmonic vibrations and the interactions between electron and phonon of atoms, in this article we study the temperature dependence of Grneisen parameter, thermal expansion coefficient at low temperature and phonon relaxation time by using the theory and method of solid state physics. The influences of the anharmonic vibration of the atom on the above parameters are further discussed. The obtained results are as follows. 1) The thermal expansion coefficient of graphene is a negative value when the temperature drops below room temperature. The absolute value of the thermal expansion coefficient of graphene increases monotonically with the increase of temperature. The thermal expansion coefficient of graphene is-3.64×10-6 K-1 at room temperature. 2) The value of Grneisen parameter is zero in the harmonic approximation. If the anharmonic vibration is considered, the Grneisen parameter will increase slowly with the increase of temperature. Its value is between 1.40 and1.42 and the change is almost linear. And we find that the influence of the second anharmonic term is less than that of the first anharmonic term on Grneisen parameter. 3) The phonon relaxation time decreases with the increase of temperature. The rate changes rapidly at low temperature (T<10 K), then it changes very slowly. The phonon relaxation time is almost inversely proportional to temperature when the temperature is higher than 300 K.
Entangled quantum Otto and quantum Stirling heat engine based on two-spin systems with Dzyaloshinski-Moriya interaction
Authors: Zhao Li-Mei  , Zhang Guo-Feng  ,   School of Physics and Nuclear Energy Engineering, Beihang University, Beijing 100191, China
Published: December 2020

Recently, the influences of the Dzyaloshinski-Moriya (DM) interaction on the performances of the basic thermo-dynamical quantities have attracted a lot of attention. A large number of investigations on the quantum coupling systems with DM interaction have been carried out. However, the specific effects of spin-orbit coupling with the performance on the quantum heat engine have not been taken into account in previous studies. DM interaction is a special kind spin-orbit coupling. To enrich the research of the quantum heat engines, the investigation about the effect of DM interaction on its thermodynamic characteristics should be included. In this study, we construct two entangled quantum engines based on spin-1/2 systems with different DM interactions, with the spin exchange constant and magnetic field fixed. The quantum Otto engine and the quantum Stirling engine are discussed in this article. By numerical calculation, we obtain the expressions for several thermodynamic quantities and plot the isoline maps of the variation of the basic thermodynamic quantities such as heat transfer, work with D1 and D2 and their efficiency in the two engines. The results indicate that the DM interaction plays an important role in the thermodynamic quantities for the quantum Otto engine and the quantum Stirling engine. In addition, the positive work condition is discussed with different DM interactions, with the spin exchange constant and magnetic field. Furthermore fixed, it is found that the efficiency of quantum Otto engine cycle is smaller than the Carnot efficiency while the quantum Stirling cycle can exceed the Carnot efficiency by using the regenerator. Finally, the second law of thermodynamics is shown to be valid in the two entangled quantum systems.
Geometric quantum discord in Tavis-Cummings model
Authors: Cheng Jing  , Shan Chuan-Jia  , Liu Ji-Bing  , Huang Yan-Xia  , Liu Tang-Kun  ,   College of Physics and Electronic Science, Hubei Normal University, Huangshi 435002, China
Published: December 2020

Quantum entanglement plays a key role in quantum information and quantum computation and thus attracts much attention in many branches of physics both in theory and in experiment. But recent studies revealed that some separable states (non-entangled state) may speed up certain tasks over their classical counterparts and may also possess certain kinds of quantum correlations. For example, geometric quantum discord, which is a more general quantum correlation measure than entanglement, can be nonzero for some separable states. From a practical point of view, it is proposed that the geometric quantum discord be responsible for the power of many quantum information processing tasks. In order to capture such correlations, Ollivier and Zurek introduced quantum discord, which measures the discrepancy between two natural yet different quantum analogues of two classically equivalent expressions of mutual information. However, the calculation of quantum discord is based on numerical maximization procedure, and there are few analytical expressions even for a two-qubit state. In order to obtain the analytical results of quantum discord, a geometric measure of quantum discord which measures the quantum correlations through the minimum Hilbert-Schmidt distance between the given state and zero discord state is introduced. Geometric quantum discord is defined as an effective measure of quantum correlation, and the geometric quantum discord through the minimal distance between the quantum state and the set of zero-discord states in a bipartite quantum system can be worked out. In this paper, by using the geometric quantum discord measurement method, the geometric quantum discord in Tavis-Cummings model is investigated, and the influences of the initial state purity, entanglement degree, dipole-dipole coupling intensity between two atoms, and field in the Fock state on the evolution characteristic of geometric quantum discord are analyzed. The results show that the geometric quantum discord appears periodically. It initially decreases to a minimum value, and then turns out to be increased for different initial states. The rigorous analysis and numerical results reveal that when we take a suitable initial state, the geometric quantum discord of two atoms can be kept in correlation. When the atoms are in the different initial states, the quantum properties of the system are significant. The photon number of the field can lead the quantum discord to be weakened. Geometric quantum discord can be increased by increasing the cavity photon number and the dipole-dipole coupling intensity. Geometric quantum discord can be enhanced obviously by increasing the strength of the dipole-dipole coupling interaction. The conclusions may conduce to the understanding of quantum correlation for the other systems from the view of geometric quantum discord.
Control of skyrmion movement in nanotrack by using periodic strain
Authors: Xuan Sheng-Jie  , Liu Yan  ,   College of Sciences, Northeastern University, Shenyang 110819, China
Published: December 2020

Magnetic skyrmions are a topologically stable and particle-like chiral spin configuration. They are appealing because of their potential applications in racetrack memory and other spintronic devices. These applications are strongly dependent on the skyrmion motion in confined geometry. Therefore, it is important to study the moving behaviors of skyrmions in a nanotrack to make them have more practical applications. Mechanical strain and stress have been demonstrated theoretically and experimentally to be able to effectively control the skyrmion phase. It can stabilize the skyrmion lattice in a broad range, and change the shape of the skyrmion crystal. In this paper, we study the moving behaviors of ferromagnetic skyrmions and antiferromagnetic skyrmions under the action of sinusoidally distributed strain in a nanotrack by using micromagnetic simulation. We assume that strain is uniaxial and perpendicular to the plane of the nanotrack. Its strength varies sinusoidally along the x-axis. Meanwhile, we apply an in-pane current along the nanotrack to drive the skyrmion moving towards the right side. We first find that there is a threshold current density that is defined as the minimum current that can drive skyrmion moving continuously. When the current density is larger than the threshold current density, the skyrmion can move continuously in the nanotrack. The threshold current density increases with the amplitude of strain increasing, but decreases with the period of strain increasing. Second, we find that the trajectory of skyrmion changes under the action of the sinusoidal distributed strains. For ferromagnetic skyrmion, its trajectory changes from straight line to periodic wavy line. Also, we find that the longitudinal velocity of skyrmion is affected by the boundary of the nanotrack. When the skyrmion is close to the upper boundary of the nanotrack, the longitudinal velocity increases sharply and it will form a peak in the velocity curve, but when the skyrmion is close to the lower boundary of the nanotrack, the longitudinal velocity decreases and it will form a valley in the velocity curve. The transverse velocity of skyrmion relates to the strain gradient. It is inversely proportional to the strain gradient. For antiferromagnetic skyrmion, we find that the movement trajectory of antiferromagnetic skyrmion does not change under the stress control. However, its diameter and velocity change periodically. Its velocity can vary between 103 m/s and 0. Our results demonstrate that the sinusoidal strain can control the skyrmion motion. This work may provide guidance in designing and developing of the spintronic devices based on magnetic skyrmions.
Influence of background gas on two-dimensional metal evaporation
Authors: Lu Xiao-Yong  , Zhang Xiao-Zhang  ,   Department Engineering of Physics, Tsinghua University, Beijing 100084, China
Published: December 2020

The spatial distributions of macroscopic parameters such as density, bulk velocity and temperature of the metal vapor have influences on the photo ionization yield of target isotope and the utilization ratio of material, which is related to the separation efficiency and the cost of atomic vapor laser isotope separation. To study this problem more practically, a system of binary gas Bhatnagar-Gross-Krook (BGK) model equations, which describe both the metal vapor and the background gas, is established. The physical characteristics are dealt with by dimensionless method for simplifying the calculations. The model equations are discretized by one-order upwind difference and then are solved by iteration method for obtaining stable results. The computational grids are adjusted to the flow field in order to acquire modest computational cost and accurate result simultaneously. Non-uniform grids in the phase space and in the velocity space are constructed to match the macroscopic parameter gradient and the form of the velocity distribution, respectively. The macroscopic parameters in the cases of different background gas densities, temperatures of tail plate and absorptivities are obtained for studying the influence of the background gas. The results show that with the increase of density of the background gas, the density and temperature of the metal vapor increase, the bulk velocities in the x and z$ direction decrease obviously in the domain far from the evaporation source, while the macroscopic parameters that are close to the evaporation source hardly change. As a result, the evaporation rate is not affected. Meanwhile, a circulation of the background gas is driven by the metal vapor, which in turn affects the diffusion of the metal vapor. Besides, as the temperature of tailing plate rises, the influence of the background gas on the macroscopic parameters of the metal vapor weakens. However, the rise of the temperature of tail plate leads the absorptivity of metal vapor to decrease, which enlarges the influence of the background gas. Therefore, it is appropriate to adjust the temperature of the tail plate based on the relationship between the absorptivity of metal vapor and the temperature. The results of theoretical calculation can serve as a reference for designing the vacuum and laser spot of the separation device.
Theoretical analysis of new optical microcavity
Authors: Gu Hong-Ming  , Huang Yong-Qing  , Wang Huan-Huan  , Wu Gang  , Duan Xiao-Feng  , Liu Kai  , Ren Xiao-Min  ,   State Key Laboratory of Information Photonics and Optical Communications, Beijing University of Posts and Telecommunications, Beijing 100876, China
Published: December 2020

Optical microcavity can confine light into a small volume by resonant recirculation. Devices based on optical microcavities are already indispensable for a wide range of applications and studies. They not only apply to traditional optics, but also have broad application prospects in quantum information and integrated optoelectronic chips. In quantum optical devices, microcavity can cause atoms or quantum dots to emit spontaneous photons in a desired direction or can provide an environment where dissipative mechanisms such as spontaneous emission are overcome so that quantum entanglement of radiation and matter is possible. For better application in quantum communication, optical microcavity needs to have a high quality factor and a low mode volume. Considering the beam coupling, spot shape and experimental production and others, the Fabry-Perot (F-P) microcavity has been widely applied to the field of optoelectronics. However, the Q-factor of the F-P microcavity is generally low, and the mode volume is large, so it needs to be improved.In addition, high Q-factor microcavity can also play a large role in detecting particles and biological macromolecules.In this paper, through the theory of wave optics, the eigenmodes of a new type of cone-top cylindrical optical micro-cavity are analyzed, and the resonant wavelength expression of the resonant cavity is obtained. We discuss the effects of the top mirror angle on the resonator performance and application of COMSOL simulation software to verify the proposed cone-top cylindrical microcavity. The optimized design and simulation results show that the quality factor of the new resonator can be increased by 22.4% to 49928.5 and the effective mode volume of the resonator can be reduced by 47.8% compared with the traditional parallel resonator. In this case, the corresponding new cavity length is 4.51 μm and the diameter is 3.13 μm. In this article its fabrications are also discussed.
Effects of plastic deformation in current collector on lithium diffusion and stress in bilayer lithium-ion battery electrode
Authors: Song Xu  , Lu Yong-Jun  , Shi Ming-Liang  , Zhao Xiang  , Wang Feng-Hui  ,   Bio-inspired and Advanced Energy Research Center, School of Mechanics, Civil Engineering and Architecture, Northwestern Polytechnical University, Xi'an 710129, China
Published: December 2020

Lithium-ion batteries (LIBs) have already become indispensable energy storage devices, as they can meet urgent requirements for higher energy and power density in the applications ranging from portable electronics to electric vehicles. However, in the process of charging and discharging of LIB, the diffusion-induced stress associated with inhomogeneous Li concentration in the electrode may cause the electrode material to damage, and then further degrade storage capacity and cycling performance of LIB. Therefore, it is important to quantitatively understand the mechanism relating to the stress evolution in electrode during electrochemical cycling, which will be conducive to developing effective methods of relieving the diffusion induced stress. In this work, a bilayer electrode model is proposed by taking into account Li diffusion, built-in stress, concentration-dependent material properties and elastoplastic deformation of current collector. Based on the established model, the influences of the possible plastic deformation in the current collector on the lithium diffusion and stress evolution of bilayer electrode during charging are investigated. The numerical results show that the plastic deformation of current collector can weaken the constraint between current collector and active layer, which leads to a smaller electrode curvature and more homogeneous lithium concentration in the active layer. The relaxation effect of the plastic deformation not only significantly relieves the stresses at the bottom and top surface of active layer, but also promotes the diffusion of lithium into active layer, which can improve the structural reliability of the electrode and increase the effective capacity of the active layer. Furthermore, the influences of the yield strength and plastic modulus of the current collector are discussed. The results indicate that the constraint between the current collector and active layer becomes weaker with reducing yield strength and plastic modulus of current collector, respectively. In other words, the further stress relaxation in the electrode indicates that the capacity can be enhanced upon reducing the yield strength and plastic modulus of current collector, respectively. Considering our results, it is expected that a bilayer electrode composed of the current collector with smaller mechanical strength enjoys simultaneous improvement in battery usable capacity and structural reliability. Consequently, the results of this paper provide a route to improving the cycle performance of bilayer lithium-ion battery electrode.
Effects of process parameters on melt-crystal interface in Czochralski silicon crystal growth
Authors: Zhang Ni1\2  , Liu Ding1\2  , Feng Xue-Liang  ,   National and Local Joint Engineering Research Center of Crystal Growth Equipment and System Integration, Xi'an University of Technology, Xi'an 710048, China;,   Shaanxi Key Laboratory of Complex System Control and Intelligent Information Processing, Xi'an 710048, China
Published: December 2020

A two-dimensional axisymmetric immersed boundary thermal lattice Boltzmann (IB-TLB) model is presented to study the phase transition in Czochralski silicon crystal growth for improving the morphology of the melt-crystal interface and the crystal quality. Specifically, the Euler grid and the Lagrange grid are established, respectively. The melt-crystal interface is considered as an immersed boundary, and it is described by a series of Lagrange nodes. In this paper, the melt-crystal interface is tracked by the immersed boundary method, and the melt flow and heat transfer are simulated by the lattice Boltzmann method. The D2Q9 model is adopted to solve the axial velocity, radial velocity, swirling velocity and temperature of the melt. The finite difference method is used to solve the temperature distribution of the crystal. Then the solid-liquid transition in crystal growth with moving boundary is solved by the proposed IB-TLB model. The proposed model is validated by the solid-liquid phase transition benchmark. In addition, the flatness of the melt-crystal interface is evaluated by the mean value of the absolute value of the interface deviation and the standard deviation of the interface deviation. The effects of the process parameters on the morphology of melt-crystal interface, melt flow structure and temperature distribution are analyzed. The results show that the morphology of the melt-crystal interface is relevant to the interaction of the crystal pulling rate, the crystal rotation parameter, and the crucible rotation parameter. When the crystal and crucible rotate together, the deviation and fluctuation of the melt-crystal interface can be effectively adjusted, whether they rotate in the same direction or rotate in the opposite directions. And a flat melt-crystal interface can be obtained by appropriately configurating the ratio of crystal rotation parameter to crucible rotation parameter. Finally, according to a series of computations, it is found that when the crucible and crystal rotate in the opposite directions, the crystal rotation parameter and the crucible rotation parameter satisfy a functional relation, with a flat interface maintained. The obtained relationship has a certain reference for adjusting and improving the crystal growth parameters in practice.
Graphene-based broadband all-optical spatial modulator
Authors: Mo Jun  , Feng Guo-Ying  , Yang Mo-Chou  , Liao Yu  , Zhou Hao  , Zhou Shou-Huan1\2  ,   Institute of Laser and Micro/Nano Engineering, College of Electronics and Information Engineering, Sichuan University, Chengdu 610064, China;,   North China Research Institute of Electro-Optics, Beijing 100015, China
Published: December 2020

In this paper, the all-optical spatial modulation of monolayer graphene-coated microfiber is proposed. Graphene is used as a saturable absorber wrapped on the microfiber produced by heating the carbon dioxide laser. When the signal light travels along the microfiber, part of the light will pass along the surface of the microfiber in the form of an evanescent field, and it will be absorbed by the graphene. Simultaneously we shoot the 808 nm pump light into the micro-nanofiber wrapped by the graphene vertically from the space. According to graphene characteristic of preferential absorption, the absorption of the signal light is controlled by the pump light, thus the broadband all-optical space modulation is realized. In a conventional graphene microfiber all-optical modulator, signal light and pump light are generally input into a microfiber via a coupler. However, the mode of operation of pump light and graphene in all-optical spatial modulation are different from those of the traditional modulation, the pump light works on the graphene outside the microfiber, which realizes the separation of the pump light and the signal light. The output signal does not need to be optically filtered for the pump light to obtain the modulated signal. The output signal light of the spatial all-optical modulator has the characteristics of “clean”. We also verify this in experiment. In addition, the pump light is vertically incident from space, the effect of the graphene length on the modulation is not considered and the modulation time is only related to the relaxation time of graphene, which is helpful in improving the response time. Modulation experiments include static spectral modulation and dynamic frequency modulation. In the static spectral modulation, the broad spectrum signal has a maximum modulation depth of 6 dB at 1095 nm when the pump power is 569 mW. The relationship among pump power, wavelength and modulation depth is also analyzed. The higher the pump power, the higher the modulation depth will be; with the same pump power, the modulation depth of long wave length is higher than that of short wave. In the dynamic modulation experiment with the modulation bandwidth~50 nm and the modulation rate~1.5 kHz, the influence of pump light and signal light on output dynamic signal are studied, the feasibility of all-optical space modulation based on graphene is verified experimentally. The composite waveguide of all-optical spatial modulator opens the door to micro-nano ultrafast signal, processing in a more flexible and efficient way.