Accepted
, , Received Date: 2024-09-22
Abstract +
In order to examine the influence of wall temperature change on the flow and heat transfer properties of rarefied gas in restricted space, the discrete unified gas kinetic scheme (DUGKS) is used to simulate the thermal creep flows in a square cavity. All the boundaries of the cavity are stationary diffuse reflection walls. The temperature of left wall and right wall are lower, and the temperature of the upper wall and the lower wall are both subjected to periodic variation. The simulation parameters considered in the present work are set as follows: the Knudsen number Kn is in a range 0.01–10, temperature change frequency St in a range of 0.5–5, and temperature change amplitude Ah in a range of 0.1–0.8. The results indicate that the velocity field and temperature field in the cavity exhibit periodic variations. No inverse Fourier heat transfer phenomenon is observed within the parameter ranges studied. The intensity of the thermal creep flow can be increased by increasing the frequency, temperature, and the Knudsen number. This can also raise the temperature jump and velocity slip close to the temperature change walls. Heat transfer lag and a reduction in the heat transfer capability of the wall are caused by increasing St and Kn. When St is small, say, St = 0.5, a complex vortex structure is seen in the cavity. As the value of St rises to 5, the vortex disappears, the gas travels from the variable temperature wall to the horizontal centerline of cavity, and the region close to the middle of the left wall and right wall changes from an endothermic zone to an exothermic zone. Furthermore, the temperature field and velocity field inside the cavity hardly change, but the degree of heat transfer on the wall decreases with the increase of Ah. The main results are shown in the figure attached below. This work provides helpful recommendations for designing the MEMS devices by using pulsed heating.
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Abstract +
During the treatment of subcostal lesions with high intensity focused ultrasound (HIFU), the obstruction by the ribs significantly affects the therapeutic effect, an impact that can be assessed through numerical calculations. In existing studies, ribs are typically regarded as perfect acoustic absorbers, even this assumption could reveal the impact of the ribs on the acoustic field to some extent, it might still underestimate the energy behind the rib cage. To address the shortcomings of current work, this paper proposes an innovative numerical calculation method refraining from regarding ribs as perfect acoustic absorbers. Subsequently, experiments are conducted using ABS plastic rib cage mimic to compare the effectiveness of the two methods, demonstrating that the method proposed in this paper, which avoids the assumption of considering ribs as perfect acoustic absorbers, could better reveal the impacts caused by ribs, and further studies are carried out on the impact of ribs in a multi-layered medium model. In response to the numerical oscillation issues encountered in existing work when dealing with media with high acoustic attenuation coefficients, this paper employs the operator splitting method to enhance the stability of numerical calculations. Furthermore, to tackle the challenges posed by asymmetric acoustic fields in numerical computations, this paper introduces matrix vectorization techniques and achieves stable solutions for the acoustic field under the backward implicit difference scheme. Additionally, a gradual maximum number of harmonics is employed to reduce the computational load when considering nonlinear effects. These improvements in both the numerical calculation model and the corresponding algorithm not only enhance the precision of numerical computations, but also reveal the underestimation of energy behind the ribs due to the assumption of perfect acoustic absorbers, which is significant for optimizing HIFU treatment strategies.
, , Received Date: 2024-10-29
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The exploration of the quantum nature of gravity has always been the focus of academic research. In this work, we consider a double “gravitational cat state” quantum system consisting of a pair of massive particles coupled with gravitational interaction confined in their respective double potential wells. Specifically, we model the double “gravitational cat state” system as a two-qubit system by assuming that the system is initially in the two-qubit Bell state, and investigate the effects of stable classical field and decayed field noise on the quantum speed limit time (QSLT) and trace distance discord (TDD) dynamics of the double “gravitational cat state”. The results show that the QSLT can be controlled by changing the parameters of the system and the environment, and the quantum state dynamic evolution of the system with massive particles can be accelerated. The quantum state evolution can be accelerated by increasing the gravitational coupling intensity between the two massive particles. The decay rate of the decaying field can also regulate the QSLT of the system to a certain extent, so as to accelerate the quantum state evolution, as shown in the attached figure (a). Under the influence of decaying field noise, it is worth noting that the intensity of gravitational coupling affects the frequency of quantum discord oscillations in this two-particle system. The QSLT shows an oscillating trend with time: rapidly increasing to a certain value in a short period of time, then beginning to decline, and then oscillating until it reaches a stable value. That is to say, the evolution of quantum states goes through an oscillatory cycle of first deceleration and then acceleration until the evolution rate becomes stable after a certain period of time. At the same time, there are similar oscillations in the dynamics of quantum discord. Moreover, by comparing these two, it is found that the QSLT decreases in the process of increasing the quantum discord in the system. When the discord oscillation has regularity, the QSLT tends to a certain value, and the quantum discord of the double “gravitational cat state” system has a certain relationship with the QSLT as shown in the attached figure (b). In other words, the quantum discord will affect the rate of quantum state evolution to some extent, and the increase of quantum discord between systems will be more conducive to the evolution of quantum states.
, , Received Date: 2024-09-27
Abstract +
The interaction between an electric field and the energy levels of Rydberg states results in the Stark effect, which can be used for quantum detection by measuring the frequency shift in electromagnetically induced transparency (EIT) spectra. By using the functional relationship between the frequency shift and the electric field, it is possible to measure the electric field in question. However, the mismatch between the probe laser and the polarization direction of the coupled laser leads to errors in the measurement of the frequency shift, affecting the accurate measurement of the electric field. In this work, the Schrödinger equation is solved by perturbation method to derive the functional relationship between the energy offset and the electric field strength. Then, the functional relationship between the energy offset and the electric field strength is brought into the solution of the density matrix equation, and the influences of the polarization direction of the detected light and coupled light on the EIT-Stark mathematical model are analyzed. Then an internal electrode method is used to prevent shielding effects caused by alkali metal atoms adhering to the surface of the atomic vapor cell, thereby enabling the application of the electric field. The calibration of the Rydberg state polarisation rate is achieved by using a standard source and measuring the frequency shift of the EIT spectrum. Finally, the effects of polarisation mismatch on the measurement results of EIT spectrum and the electric field are verified by modulating the laser polarization direction. The experimental data show that when the polarization directions of the probe laser and coupled laser are parallel to each other, it is the most matched polarization direction for the lasers, the peak value of the EIT spectrum is the largest, and the maximum relative error of the electric field measurement is 1.67%. When the angle between the polarisation directions of the probe light and the coupled light laser is 45°, the laser polarisation mismatch is the most severe, the EIT spectral peak is the lowest and the maximum relative error of the electric field measurement is 10.24%.
, , Received Date: 2024-10-14
Abstract +
In this work, the randomness of electrically pumped random laser (RL) from ZnO-based metal-insulator-semiconductor (MIS) structured light-emitting device (LED) is significantly suppressed, by using appropriately patterned hydrothermal ZnO film with large crystal grains as the light-emitting layer. The hydrothermal ZnO film on silicon substrate, with the crystal grains sized over 500 nm, is first patterned into a number of square blocks separated by streets by using laser direct writing photolithography. Based on such a patterned ZnO film, the MIS (Au/SiO2/ZnO) structured LEDs are prepared on silicon substrates. Under the same injection current, the LED with the patterned ZnO film exhibits much fewer RL modes than that with the non-patterned ZnO film and, moreover, the former displays ever-fewer RL modes with the the decrease of block size. Besides, the wavelength of the strongest RL mode from the LED with the patterned ZnO film fluctuates in a much narrower range than that with the non-patterned ZnO film. It is worth mentioning that the LED with the patterned hydrothermal ZnO film can even be pumped into the single-mode RL under the desirable conditions such as low injection current and small patterned blocks. Moreover, the comparative investigation indicates that the LED with the large-grain hydrothermal ZnO film exhibits the smaller RL threshold current than that with the small-grain sputtered ZnO film, and the former has fewer RL modes and a higher output lasing power than the latter under the same injection current. As for the physical mechanism behind the aforementioned results, it is analyzed as follows. Regarding the LED with the patterned ZnO film, on the one hand, due to the limited numbers of crystal grains and grain boundaries within a single block, the multiple optical scattering is remarkably suppressed. Then, the paths through which the net optical gain and therefore the lasing action can be achieved via multiple optical scattering are much fewer than those in the case of the non-patterned ZnO film. On the other hand, due to optical gain competition among different RL modes occurring within the limited space of a single block, the RL modes with significant spatial overlap cannot lase simultaneously. For the two-fold reasons as mentioned above, the LED exhibits ever-fewer RL modes with the decrease of the size of blocks. Moreover, the inter-block optical coupling enables the optical gain competition among different RL modes to be more violent within a single block, leading to further reduction of RL modes.
, , Received Date: 2024-07-24
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Graphene, a two-dimensional material characterized by its honeycomb lattice structure, has demonstrated significant potential applications in electronic devices. The topological Anderson insulator (TAI) represents a novel phenomenon where a system transforms into a topological phase induced by disorder. In past studies, TAI is widely found in theoretical models such as the BHZ model and the Kane-Mele model. A common feature is that these models can open topological non-trivial gaps by changing their topological mass terms, but the rise of TAI is independent of the topological status of gaps. In order to investigate whether there is any difference in the disorder-induced phase between topologically trivial and topologically non-trivial cases of the Haldane model in the clean limit, the Haldane model in an infinitely long quasi-one-dimensional ZigZag-edged graphene ribbon is considered in this work. The Hamiltonian and band structure of it are analyzed, and the non-equilibrium Green's function theory is used to calculate the transport properties of ribbons under topologically trivial and non-trivial states versus disorder. The conductance, current density, transport coefficient and localisation length are calculated as parameters characterising the transmission properties. It is found from the analysis of the band structure that the system in either topological trivial or topological non-trivial state has edge states. When the Fermi energy lies in the conduction band, the conductance of the system decreases rapidly under weak disorder intensity and strong disorder intensity, regardless of whether the system is topologically non-trivial or not. At moderate disorder intensities, the conductance of topologically non-trivial systems keeps stable with a value of 1, indicating the appearance of the topological Anderson insulator phase in the system. Meanwhile, for topological trivial systems, the decrease of conductance noticeably slows down. The calculations of local current density show that both systems exhibit robust edge states, with topologically protected edge states showing greater robustness. The analysis of the transmission coefficients of edge state and bulk state indicates that the coexistence of bulk states and robust edge states is the basis for the phenomena observed in the Haldane model. Under weak disorder, bulk states are localized, and the transmission coefficient of edge states decreases due to scattering into the bulk states. Under strong disorder, edge states are localized, resulting in zero conductance. However, at moderate disorder strength, bulk states are annihilated while robust edge states persist, thereby reducing scattering from edge states to bulk states. This enhances the transport stability of the system. The fluctuation of conduction and localisation length reveal that the metal-TAI-normal insulator transition occurs in the Haldane model with topological non-trivial gap and if the system is of cylinder shape, there will be no edge states, the TAI will not occur. For the topological trivial gap case, only metal-normal insulator transition can be clearly identified. Therefore, topologically protected edge states are so robust that they generate a conductance plateau and it is demonstrated that the topologically trivial edge states are robust to a certain extent and can resist this level of disorder. The robustness of edge states is a crucial factor for the occurrence of the TAI phenomenon in the Haldane model.
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Abstract +
Streamer discharge has been widely applied in fields such as sterilization, disinfection, and ozone generation. The secondary discharge process significantly affects the effective ozone production duration and efficiency. However, the mechanism by which oxygen concentration influences secondary discharge characteristics and the yield of target products remains unclear. To address this issue, we developed a fluid-based analysis model of the secondary positive streamer discharge process between needle-plate electrodes under varying oxygen concentrations. This model accounts for the radial electric field and resolves potential non-physical branching issues that may arise in discharge simulations at high oxygen concentrations. In this study, we examine the effect of oxygen concentration on the optical emission characteristics of secondary positive streamers. The optical emission intensity, cathode charge transfer, and the yield of excited-state oxygen atoms ($ \rm O(^3P) $) under different oxygen concentrations are investigated and compared with experimental data. The results indicate that when the oxygen concentration increases from 20% to 90%, the optical emission intensity of the secondary discharge decreases by approximately 0.2%. Meanwhile, the average electron density in the discharge channel decreases by 90%, the electric field intensity changes by less than 10%, and the single discharge duration shortens by 77%. Under these conditions, the proportion of $ \rm O(^3P) $ yield originating from the primary discharge increases from 20% to 38%, and the unit energy yield of excited-state oxygen atoms $ \rm O(^3P) $ rises by 64%. Although the reduced discharge duration lowers the absolute $ \rm O(^3P) $ yield by 50%, the increase in unit energy yield more than compensates for the decrease in single-discharge yield. The decrease in single-discharge yield with increasing oxygen concentration arises from enhanced two- and three-body adsorption effects of oxygen molecules, which reduce the electron density. Additionally, the increased collision probability between electrons and oxygen molecules further influences these characteristic changes.
Abstract +
Phase space reconstruction plays a pivotal role in calculating features of nonlinear systems. By mapping one-dimensional time series onto a high-dimensional phase space using phase space reconstruction techniques, the dynamical characteristics of nonlinear systems can be revealed. However, existing nonlinear analysis methods are primarily based on phase space reconstruction of single-channel data and cannot directly exploit the rich information contained in multi-channel array data. The reconstructed data matrix exhibits structural similarities with multi-channel array data. The relationship between phase space reconstruction and array data structure, as well as the gain in nonlinear features brought by array data, has not been sufficiently studied. This paper employs two classical nonlinear features: multiscale sample entropy and multiscale permutation entropy. Utilizing array multi-channel data to replace the phase space reconstruction step in algorithms to enhance the algorithmic performance. Initially, the relationship between phase space reconstruction parameters and actual array structures is analyzed, and conversion relationships are established. Then, multiple sets of simulated and real-world array data are used to evaluate the performance of the two entropy algorithms. The results show that substituting array data for phase space reconstruction effectively improves the performance of both entropy algorithms. Specifically, the multiscale sample entropy algorithm, when applied to array data, allows for the differentiation of noisy target signals from background noise at low signal-to-noise ratios. Meanwhile, the multiscale permutation entropy algorithm using array data more accurately reveals the complexity structure of signals at different time scales.
, , Received Date: 2024-09-29
Abstract +
Quantum key distribution (QKD) has been extensively studied for practical applications. Advantage distillation (AD) represents a key technique to extract highly correlated bit pairs from weakly correlated ones, thus improving QKD protocol performance, particularly in large-error scenarios. However, its practical implementation remains under-explored. In this study, the AD is integrated into the three-intensity decoy-state BB84 protocol and its performance is demonstrated on a high-speed phase-encoding platform. The experimental system employs an asymmetric Mach-Zehnder interferometer (AMZI) fabricated on a silicon dioxide optical waveguide chip for phase encoding, which is benefited from its low coupling loss and minimum waveguide transmission loss. Phase-randomized weak coherent pulses, generated by a distributed feedback laser at 625 MHz, are modulated into decoy states of varying intensities. The signals are encoded via an AMZI and attenuated to single-photon levels before transmission. At the receiver, another AMZI demodulates the signals detected by avalanche photodiodes in gated mode. Experiments conducted at 50 km and 105 km demonstrate secure key rates of 104 kbits/s and 59 bits/s, respectively. The results at shorter distances closely match theoretical predictions, while slight deviations at 105 km are attributed to signal attenuation and noise. Despite these challenges, the results obtained at 105 km highlight the effectiveness of AD in enhancing secure key rates in the large-error scenario. This study confirms the potential of AD in extending secure communication range of QKD. By leveraging the high integration and scalability of silicon dioxide photonic chips, the proposed system lays a foundation for large-scale QKD deployment, paving the way for developing advanced protocols and real-world quantum networks.
, , Received Date: 2024-10-27
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, , Received Date: 2024-09-09
Abstract +
Permutation time irreversibility is an important method to quantify the nonequilibrium characteristics; however, ordinal pattern is a coarse-graining alternative and cannot accurately represent detailed structural information. In this paper, a fuzzy permutation time irreversibility (fpTIR) is proposed by measuring the difference between vector elements based on a negative exponential function. Amplitude permutation of vector is constructed and its membership degree is calculated, then the difference in probability distribution between the forward sequence and backward sequence is measured for fpTIR. For comparison, Shannon entropy is calculated as the average amount of information in the fuzzy permutation probability distribution, i.e. fuzzy permutation entropy (fPEn), to measure the complexity of the system. According to the surrogate theory, mode series are generated by logistic, Henon, and first-order autoregressive systems to verify the fpTIR, which is then used to analyze heart rates of congestive heart failure, healthy elderly and healthy young subjects from PhysioNet database. The results suggest that fpTIR effectively measures the nonequilibrium characteristic of system and improves the accuracy of heart rate analysis. Since fpTIR and fPEn are different in analyzing probability distributions, they have discrepancies in chaotic series and even opposite results in the heart rate signals, where the results of fpTIR are consistent with theory of complexity loss in aging and disease. In conclusion, the fpTIR not only accurately characterizes the structure of sequences and enhances the effect of the nonequilibrium analysis of complex systems, but also provides a new perspective and theoretical basis for exploring complex systems from the perspectives of nonequilibrium dynamics and entropy complexity.
, , Received Date: 2024-10-23
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, , Received Date: 2024-09-27
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The effect of sawteeth on plasma performance and transport in the plasma of tokamak is an important problem in the fusion field. Sawtooth oscillations can trigger off heat and turbulence pulses that propagate into the edge plasma, and thus enhancing the edge shear flow and inducing a transition from low confinement mode to high confinement mode. The influences of turbulence spreading and symmetry breaking on edge shear flow with sawtooth crashes are observed in the J-TEXT tokamak. The edge plasma turbulence and shear flow are measured using a fast reciprocating electrostatic probe array. The experimental data are analyzed using some methods such as conditional average and probability distribution function. After sawtooth crashes, the heat and turbulence pulses in the core propagate to the edge, with the turbulence pulse being faster than the heat pulse. The attached figures (a)–(e) show the core electron temperature, and the edge electron temperature, turbulence intensity, turbulence drive and spreading rates, Reynolds stress and its gradient, and shearing rates, respectively. After sawtooth crashes, the edge electron temperature increases and the edge turbulence is enhanced, with turbulence preceding temperature. The enhanced edge turbulence is mainly composed of two parts: the turbulence driven by local gradient and the turbulence spreading from core to edge. The development of the estimated turbulence spreading rate is prior to that of the turbulence driving rate. The increase in the turbulence intensity can cause the turbulent Reynold stress and its gradient to increase, thereby enhancing shear flows and radial electric fields. Turbulence spreading leads the edge Reynolds stresses to develop and the shear flow to be faster than edge electron temperature. The Reynolds stress arises from the symmetry breaking of the turbulence wave number spectrum. After sawtooth collapses, the joint probability density function of radial wave number and poloidal wave number of turbulence intensity becomes highly skewed and anisotropic, exhibiting strong asymmetry, which can be seen in attached figures (f) and (g). The development of turbulence spreading flux at the edge is also prior to the particle flux driven by turbulence, indicating that turbulent energy transport is not simply accompanied by turbulent particle transport. These results show that the turbulence spreading and symmetry breaking can enhance turbulent Reynolds stress, thereby driving shear flows, after sawtooth has crashed.
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The discovery of ultrafast demagnetization has introduced a new approach for generating ultrafast spin currents using an ultrashort laser, potentially enabling faster manipulation of material magnetism. This has sparked research into the transport mechanisms of ultrafast spin currents. However, the underlying processes remain poorly understood, particularly the factors influencing interlayer spin transfer. This study employs a superdiffusive spin transport model to investigate the ultrafast spin transport mechanisms in the Ni/Ru/Fe spin valve system, with a particular focus on how interlayer spin transfer affects the ultrafast magnetization dynamics of the ferromagnetic layer. First, by calculating the laser-induced magnetization dynamics of the Ni/Ru/Fe system under different magnetization alignments, the study validates recent experimental findings. Further analysis reveals that reducing the thickness of the Ru spacer layer significantly enhances the spin current intensity and increases the demagnetization difference in the Fe layer, confirming the key role of the hot electron spin current generated by the Ni layer in interlayer spin transport. Additionally, the spin decay length of hot electron spin currents in the spacer Ru layer is determined to be approximately 0.5 nm. This study also shows that laser-induced transient magnetization enhancement can be achieved by adjusting the relative laser absorption in the films. These results provide theoretical support for the future ultrafast magnetic control of spin valve structures and contribute to the advancement of spintronics in high-speed information processing and storage applications.
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