Accepted
, , Received Date: 2024-01-05
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Thermoplasmonics originating from the relaxation process of plasmon resonances in nanostructures can be utilized as an efficient and highly localized heat source in solar-hydrogen conversion, but there have been few researches on the interfacial heat transport properties of photoelectrode with the thermoplasmonics effect in a photoelectrochemical water splitting system. In this work, the effects of temperature, interfacial coupling strength and the addition of graphene layers on the interfacial thermal conductance of Au-TiO2 electrodes are investigated by the non-equilibrium molecular dynamics simulation, and the variation of interfacial thermal conductance is analyzed by the phonon density of states. The results show that the interfacial thermal conductivity is increased by 78.55% when the temperature increases from 300 to 800 K. This is related to the fact that more low-frequency phonons participate in the interface heat transport, allowing more heat to be transferred to TiO2 to promote the interface reaction. As the coupling strength of the Au-TiO2 interface increases, the interfacial thermal conductivity of the electrode increases and then tends to stabilize. The interfacial thermal conductivity can be optimized by increasing the degree of overlap of the phonon state densities of Au and TiO2. The addition of a single layer of graphene can increase the interfacial thermal conductivity to 98.072 MW⋅m–2⋅K–1, but the addition of 2 and 3 layers of graphene can hinder interfacial heat transfer in Au and TiO2 due to the interaction between the layers of graphene. When adding graphene layer, medium-frequency phonons and high-frequency phonons are stimulated to participate in the interfacial heat transfer, but with the increase of the graphene layers, the number of low-frequency phonons in a range of 0—30 THz decreases, and these low-frequency phonons make the greatest contribution to the interfacial thermal conductivity. The obtained results are useful in regulating the thermal transport properties of the photoelectrode interface, which can provide new insights into and theoretical basis for the design and construction of composite photoelectrodes.
, , Received Date: 2023-12-23
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Open quantum systems, which are coupled to an external bath, are a critical research field of quantum physics. The steady state, which is a state that any initial state converges after a long time, usually attracts the most interest. In contrast, there are relatively few studies on the nonequilibrium dynamical processes of quantum many-body systems. This is mainly due to the fact that quantum many-body systems generally have interactions, and the Hilbert space required for a complete description of their dynamical processes will grow exponentially with the number of particles and the computational difficulty will increase dramatically as well. Hence the complete description of their dynamical processes has been a difficult problem.With the advancing quantum technologies, there is increasing interest in the nonequilibrium dynamics of open quantum many-body systems. A common phenomenon is that of metastability, where the system initially relaxes into long-lived states, with subsequent converging to the final stationarity at much longer times. In this paper, we establish a low dimensional approximation to describe the metastability dynamics in Markovian open quantum systems, based on the spectra of the Liouvillian super-operator. The separation of time scales implies a splitting in the spectrum, and this spectral division allows us to eliminate the fast decay modes by perturbation method, and then we establish the effective description in the low-lying eigenmodes subspace. Furthermore, we study the dynamics process for the Rydberg atomic systems under electromagnetically induced transparency (EIT) conditions and find that the system can process metastable dynamics if the interactions between the atoms are considered. We compare the effective dynamics in the subspace and the actual dynamics in the full space, and the results show that the effective dynamics works well under the condition that the perturbation approximation holds. Our work provides a feasible idea and method to establish an effective and simplified description of the dynamical process of open quantum many-body systems.
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The transportable quantum memory is a viable solution for realizing the long-distance quantum communication, which requires a storage lifetime of the order of hours. The isotope-enriched 151Eu3+:Y2SiO5 crystals are promising candidates for this application. However, their optical storage efficiency and spin storage lifetime is limited by the wide inhomogeneous linewidth. In this work, we successfully cultivated isotope-enriched 151Eu3+:Y2SiO5 crystals with varying doping concentrations utilizing the Czochralski method. The optical inhomogeneous broadening and spin inhomogeneous broadening are measured by the optical absorption spectroscopy and optically detected magnetic resonance tests, respectively. Notably, in the undoped samples, we identified a baseline level of inhomogeneous broadening linewidths, registering at 390 ± 15 MHz for optical inhomogeneous broadening and 4.6 ± 0.2 kHz for spin inhomogeneous broadening. Our findings reveal that point defects, induced by the doping ions, significantly contribute to the inhomogeneous broadening, at a rate of 0.97 MHz/ppm for optical broadening and 0.014 kHz/ppm for spin broadening. Furthermore, we discussed the impact of dislocations on inhomogeneous broadening and proposed potential strategies to further mitigate these effects. Such advancements hold promise for fostering the development of ultra-long-lifetime transportable quantum memory applications.
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The large-spatial-scale stimulated Raman scattering relevant to the SG-III prototype indirect drive parameters is investigated using the code PHANTAM based on ray tracing and convective amplification. The simulations show that strong stimulated Raman side scattering processes occur in both empty and gas-filled hohlraums. The incident laser spot size is found to be the critical factor affecting stimulated Raman side scattering: in constant laser intensity conditions, the convective gain of stimulated Raman side scattering increases with the laser spot size in both types of hohlraums. In our simulations, the wavenumber mismatch leads to a saturation of the convection gain of the stimulated Raman side scattering in empty hohlraum, while in gas-filled hohlraum the convection gain of the stimulated Raman side scattering keeps increasing as the spot size increases. In constant laser power conditions, the convective gain of stimulated Raman side scattering decreases while laser spot size increases, and the convective gain of stimulated Raman side scattering decreases faster in empty hohlraum in our simulations. The convective gain of Raman side scattering can be adjusted by laser spot size.
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Semiconductor disk lasers (SDLs) have advantages of high output power and good beam quality. Its flexible external cavity provides convenience for inserting additional optical element to start mode locking and produce ultra-short pulse train with duration from picosecond to femtosecond. However, the very short lifetime of about a few nanoseconds to tens of nanosecond of the carrier in semiconductor gain medium limits the decrease of pulse repetition rate, thus restrict the increase of peak power of the mode-locked laser pulse to some extent. In this work, by using the relatively shallow In0.2GaAs quantum wells, which has a relatively long carrier lifetime in the active region of gain chip, as well as the particularly designed semiconductor saturable absorption mirror (SESAM) that with a relatively small saturation flux, a passively mode-locked SDL with low repetition rate and high peak power is demonstrated. The used six-mirror cavity has a spot radius of about 200 μm on the chip and a 40 μm spot on the SESAM, and the total cavity length is about of 1.92 m. The SESAM passively mode-locked SDL produces a stable pulses train with the lowest repetition rate of 78 MHz. When the temperature is 12℃ and the transmittance of the output coupler is T = 3%, an average output power of 2.1 W and a pulse duration of 2.08 ps are achieved. The corresponding pulse peak power reaches 12.8 kW, which is about twice of the reported highest peak power in a SESAM mode-locked SDL. When T = 2% and T = 5%, the obtained average output power are 1.34 W and 1.62 W respectively, and the corresponding pulse peak power are 8.17 kW and 9.88 kW. Based on the reported literatures and the results of pulse repetition rate in our experiments, the estimated lifetime of the carriers of the In0.2GaAs quantum wells in the active region of the gain used chip is 16.4 ns. This high peak power mode-locked semiconductor disk laser has important potential applications in biomedical photonics, chemistry, and nonlinear microscopy.
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Organic ferroelectrics are desirable for the application in the field of wearable electronics due to their eco-friendly process-ability, mechanical flexibility, low processing temperatures, and lightweight. In this work, we used five organic groups as substitution for organic cation and studied the effects of organic cations on the structural stability, electronic structure, mechanical properties and spontaneous polarization of metal-free perovskite A-NH4-(PF6)3(A=MDABCO, CNDABCO, ODABCO, NODABCO, SHDABCO) through first-principles calculations. Firstly, the stability of the five materials was calculated by molecular dynamics simulations, and the energy of all systems is negative and stable after 500 fs, which demonstrated the stability of the five materials in 300 K. The electronic structure calculation shows that the organic perovskite materials have wide band gap with the value of about 7.05 eV. The VBM(Valence Band Maximum) and CBM (Conduction Band Minimum) are occupied by different elements, which is conductive to the separation of electrons and holes. We found that organic cations have an important contribution to the spontaneous polarization of materials, with the contributing over 50%. The presence of hydrogen atoms in the substituting groups (MDABCO, ODABCO) enhances the hydrogen bond interaction between the organic cations and PF6-and increases the displacement of the organic cation, resulting in an increase in the contribution of the polarization of the organic cation to the total polarization. In addition, we observed large piezoelectric strain components, the calculated d33 is 36.5 pC/N for CNDABCO-NH4-(PF6)3, 32.3pC/N for SHNDABCO-NH4-(PF6)3, which is larger than the known d33 of MDABCO-NH4-I3(14pC/N). The calculated d14 is 57.5 pC/N for ODABCO-NH4-(PF6)3, 27.5 pC/N for NODABCO-NH4-(PF6)3. These components are at a high level among known organic perovskite materials and comparable to many known inorganic crystals. The large value of d14 is found to be closely related with the large value of elastic compliance tensor s44. The analysis of Young’s modulus and bulk’s modulus found that these organic perovskite materials have good ductility. These results show that these organic materials are excellent candidates for future environmentally friendly piezoelectric materials.
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Wall-seeping gas film (WSGF) represents a promising method for transition control, drag reduction, and heat reduction in hypersonic vehicles. Experiments were conducted in a Mach 6 hypersonic quiet wind tunnel using nano-tracer planar laser scattering (NPLS) and high-frequency fluctuating pressure measuring techniques. This paper investigates the effects of wall-seeping helium, air, and carbon dioxide gas films under identical volume flow rate conditions on conical boundary layer thickness, disturbance wave structure, wavelength, frequency, amplitude, and nonlinear interaction. The experimental results reveal that WSGF significantly thickens the hypersonic boundary layer, with the thickest position appearing at the downstream boundary of the seeping zone. The boundary layer thickness is thinnest for helium gas film and thickest for carbon dioxide gas film. Generally, air gas film and carbon dioxide gas film induce the appearance of regular, rope-like, and interlaced second-mode waves in advance in the boundary layer. However, under a higher volume flow rate for carbon dioxide gas film, the disturbance wave structure resembles interface fluctuations, with a characteristic wavelength of approximately 18 mm and a peak frequency as low as about 35 kHz, without the rope-like interlaced characteristic. At this time, the influence of shear layer instability becomes significant. The disturbance waves do not exhibit second-mode wave characteristics for wall-seeping helium gas film, whose shape is irregular and undergoes deformation over time and space. Additionally, the power spectral density of wall fluctuating pressure exhibits insignificant variation with volume flow rate and flow direction, which is similar to the characteristics of power spectral density in the laminar boundary layer and has no peak frequency. The wavelength of second-mode waves is about 2~3 times the boundary layer thickness for air gas film while increasing to more than 3 times for carbon dioxide gas film. Compared to air gas film, the application of carbon dioxide gas film results in a smaller peak frequency and bandwidth of disturbance waves, larger characteristic wavelength and amplitude, longer propagation distance, and stronger nonlinear interaction. In the future, attention should be directed towards understanding disturbance wave characteristics in the boundary layer for helium gas film and shear layer instability under larger volume flow rates.
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In this paper, a novel shared-aperture method of electromagnetic metasurface and antenna is proposed to obtain low radar-cross-section (RCS) performance. This method first designs low-RCS metasurface and conventional antenna independently, and then obtains novel low-RCS antenna by combining this metasurface with conventional antenna based on shared-aperture technique. Besides, current analysis and consequent local structure modification are also conducted to guarantee the antenna’s good radiation performance and broadband RCS reduction in the meantime. Using this method, a dual-layer polarization rotation unit cell is first proposed and its broadband working principle is investigated by both theoretical analysis and numerical comparison. Based on this unit cell, a broadband low-RCS metasurface is constructed. Then an initial shared-aperture metasurface antenna is obtained by substituting the middle cells in the metasurface with conventional patch antenna directly. Through careful analysis of surface current in radiation mode, the gain decrease of this metasurface antenna is revealed. On this basis, a limited removing strategy is put forward and some metasurface cells in the antenna is removed with the aid of current analysis. Consequently, an improved shared-aperture metasurface antenna is proposed. (The design flow of this metasurface antenna is displayed in the following Fig. 1) This improved antenna works from 6.3 GHz to 7.48 GHz, which is a bit wider than conventional patch antenna. Its gain is also higher than conventional antenna with the maximum improvement of 1 dB. Meanwhile, apparent RCS reduction is obtained from 6 GHz to 16 GHz for any polarized incident wave, and the reduction peak is larger than 20 dB. Fabrications and measurements are finally conducted. The good agreements of measured results and numerical calculations are achieved. The well-behaved radiation performance and broadband low-RCS property of this metasurface antenna verify the effectiveness of the proposed method. Different from most reported design method of low-RCS antenna directly from conventional antenna, the proposed method adopts a reverse thinking and converts the scattering optimization to radiation optimization, realizing the integration of metasurface and antenna, and making the design of low-RCS antenna easier and faster.
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To investigate the effect of the interface electronic structure of core-shell quantum dots on the conductivity and space charge characteristics of polyethylene insulation, nanocomposite insulations, namely CdSe@ZnS/LDPE and ZnSe@ZnS/LDPE, are synthesized. The study focuses on elucidating the evolution patterns of DC conductivity and space charge within the nanocomposite insulation, accompanied by an analysis of the impact of the interfacial electronic structure of core-shell quantum dots on the distribution of charge traps. Comparative analysis reveals that, in contrast to LDPE insulation, ZnSe@ZnS/LDPE nanocomposite insulation demonstrates a substantial reduction in DC conductivity by 47.2% and a decrease in space charge accumulation by 40.3% under conditions of elevated temperature and strong electric fields. The trap energy level experiences an increase, signifying a heightened trapping effect on carriers. Leveraging density functional theory, the band structure characteristics of core-shell quantum dots integrated with polyethylene are computationally assessed. The findings underscore that the band misalignment at the core-shell interface and the shell-insulation interface induces shifts in the conduction band bottom and valence band top, respectively. These shifts impose a confinement effect on electrons and holes, with the extent of this effect escalating with the augmented band gap difference between the core layer and the shell layer. Consequently, this phenomenon curtails carrier migration, thereby inhibiting space charge accumulation under conditions of elevated temperature and strong electric fields.
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In the realm of laser fusion research, the precision of shock-timing technology is pivotal for attaining optimal adiabat tuning during the compression phase of fusion capsules, which is crucial for ensuring the high-performance implosion. The current main technological approach for shock-timing experiments is the use of keyhole targets and VISAR diagnostics to measure the shock velocity history. Nonetheless, this approach encounters limitations when scaling down to smaller capsules, primarily due to the reduced effective reflection area available for VISAR diagnostics. This study introduces a novel high-precision shock-timing experimental methodology for a double-step radiation-driven implosion with a 0.375mm radius capsule on a 100 kJ laser facility. By developing a theoretical framework for calculating the intensity of VISAR images with spherical reflective surfaces, an innovative experimental technical route is proposed to utilize the keyhole cone reflection effect to enhance the VISAR diagnostic spatial area, effectively increasing the effective data collection region by nearly threefold for small-scale capsules. The technique has been adeptly applied to measure shock waves in cryogenic liquid-deuterium-filled capsules under shaped implosion experimental conditions, obtaining high-precision shock-timing experimental data. Experimental data reveals that the application of this technology has markedly enhanced both the image quality and the precision of data analysis for shock wave velocity measurements in small-scale capsules. Furthermore, it has been discovered that under similar laser conditions, there exist considerable variations in the shock velocity profiles. Simulation analysis suggests that the differences in the "N+1" reflected shock wave's catching-up behavior, caused by minor variations in laser intensity, are the main reason for the substantial merge velocity differences. It is demonstrated that minor variations in laser parameters can significantly affect the transmission behavior of the shock wave. This experiment highlights the intricate sensitivity of shock wave transmission in the high-performance shaped implosion physics process at the current small capsule scale, and it is essential to conduct shock-timing experiments for precisely tuning the actual shock wave behavior. This research not only lays a robust technical foundation for the advancement of adiabat tuning experiments on China's 100 kJ laser facility but also carries profound implications for the ultra-high pressure physics research based on the spherical convergence effect.
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The development of AlGaN-based deep ultraviolet light emitting diodes (DUV-LEDs) is currently limited by poor external quantum efficiency (EQE) and wall-plug efficiency (WPE). Internal quantum efficiency (IQE), as an important component of EQE, plays a crucial role in improving the performance of DUV-LEDs. IQE is related to the carrier injection efficiency and the radiation recombination rate in the active region. In order to improve the IQE of AlGaN-based DUV-LEDs, this work proposes a scheme to optimize the period number of superlattice electron barrier layer (SL-EBL) to achieve better carrier injection efficiency and confinement capability. The effect of the period number of SL-EBL on the luminous efficiency, reliability and carrier recombination mechanism of AlGaN-based DUV-LEDs with an emission wavelength of 273 nm was investigated. The experimental results show that the light output power (LOP), external quantum efficiency (EQE) and wall-plug efficiency (WPE) of the DUV-LEDs tend to first increase and then decrease with the period number increase of the SL-EBL, while the leakage current decreases and the reliability is enhanced.The maximum EQE and WPE of the DUV-LED are 3.5% and 3.2%, respectively, at an injection current of 7.5 mA were achieved when the period number of SL-EBL was fixed at 7 (the thickness was 28 nm). Meanwhile, the numerical simulation results show that the electron potential barrier height is boosted with increasing the period number of SL-EBL, and the variation of the hole potential barrier height is negligible. Therefore, increasing the period number of SL-EBL is beneficial for shielding the dislocations and suppressing the leakage of electrons into the p-type layer, which improves the luminous efficiency and reliability of DUV-LEDs. However, when the period number of SL-EBL exceeds 7, the excessively thick hole potential barrier prevents the injection of holes into the activation region and degrades the radiative recombination efficiency. Therefore, EQE and WPE will show an inflection point with the variation of the period number of SL-EBL. In addition, to investigate the carrier recombination mechanism of the active region, the experimental EQE curves are fitted by the ABC model as well as the different slopes in logarithmic light output power-current (L-I) curves are calculated after aging. It can be found that increasing the period number of SL-EBL can effectively suppress the non-radiative combination of carriers in the active region. This investigation can provide an alternative way to enhance the photoelectric performance of DUV-LEDs.
, , Received Date: 2024-01-19
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The high-confinement mode (H-mode) significantly enhances the energy and particle confinement in fusion plasma compared with the low-confinement mode (L-mode), and it is the basic operation scenario for ITER and CFETR. Edge localized mode (ELM) often appears in H-mode, helping to expel impurities to maintain a longer stable state. However, the particle burst and energy burst from ELM eruptions can severely damage the first wall of fusion device, so, it is necessary to control the ELM. Experiments on EAST tokamak and HL-2A tokamak have been conducted with ELM mitigation by lower hybrid wave (LHW), confirming the effect of LHW on ELMs, but the physical mechanism of ELM mitigation by LHW is still not fully understood. In this paper, the influences of LHW injection on the linear and nonlinear characteristics of peeling-ballooning mode (P-B mode) are investigated in the edge pedestal region of H-mode plasma in tokamakby using the BOUT++ code. The simulations take into consideration both the conventional main plasma current driven by LHW and the three-dimensional perturbed magnetic field generated by the scrape-off layer helical current filament (HCF) on the P-B mode. The linear results show that the core plasma current driven by LHW moves the linear toroidal mode spectrum towards higher mode numbers and lower growth rates by reducing the normalized pressure gradient and magnetic shear of the equilibrium. Nonlinear simulations indicate that due to the broadening of the linear mode spectrum, the core current driven by LHW can reduce the pedestal energy loss caused by ELM through globally suppressing different toroidal modes of the P-B mode, and the three-dimensional perturbed magnetic field generated by LHW-driven HCF can reduce the energy loss caused by ELMs through promoting the growth of modes other than the main mode and enhancing the coupling between different modes. It is found in the study that the P-B mode promoted by the three-dimensional perturbed magnetic field generated by HCF has a mode number threshold, and when the dominant mode of the P-B mode is far from the mode number threshold driven by the three-dimensional perturbed magnetic field, the energy loss due to ELMs is more significantly reduced. These results contribute to a more in-depth understanding of the physical mechanism in ELM control experiment by LHW.
, , Received Date: 2024-01-13
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China Spallation Neutron Source (CSNS) I project passed the national acceptance in 2018, and current beam power has reached 140 kW. In order to further improve the output neutron strength of the target station moderator, a 500 kW power upgrade plan has been proposed for CSNS II. The target station is an important part of the spallation neutron source. In the target station, a large number of neutrons are produced by the spallation reaction between high energy protons and the target, these neutrons are moderated by the moderator and become neutrons for neutron scattering experiments. During operation, the target and other key components such as the target container, the moderator reflector container, and the proton beam window are irradiated by high-flux and high-energy particles for a long time, which will result in serious radiation damage. It is important to assess the accumulated radiation damage during operation to determine the service life of each component. At present, the physical quantities used to evaluate the radiation damage degree of materials include displacement per atom (DPA), H and He production. In this work, the displacement damage cross sections of protons and neutrons and the H, He production cross sections for W, SS316 and Al-6061 materials are obtained by using PHITS. The effects of the Norgett-Robinson-Torrens (NRT) model and athermal recombination corrected (ARC) model on the calculation of displacement damage are analyzed. The results show that the cross section calculated based on ARC model is lower than that based on NRT model, because the NRT model does not take into account the resetting of the atoms before reaching thermodynamic equilibrium. On this basis, DPA, H and He production of the key components of the target station operating for 5000h at a power of 500 kW are calculated by combining the baseline model of the second phase target station of the spallation neutron source in China. The results show that the yields of NRT-dpa, ARC-dpa, H and He produced by irradiation are 8.01dpa/y (In this paper, 1y = 2500 MW·h), 2.39 dpa/y, 5110 appm/y and 884 appm/y, respectively. The radiation damage values of the target vessel are 5.34dpa/y, 1.92dpa/y, 2180 appm/y and 334 appm/y, respectively. The radiation damage values of the moderators and reflectors are 3.78 dpa/y, 1.77 dpa/y, 124 appm/y, and 36.7 appm/y. The radiation damage values of the proton beam window are 0.35 dpa/y, 0.19 dpa/y, 962 appm/y, and 216 appm/y. Subsequently, the life of each component is estimated by analyzing the radiation damage. These results are very important for analyzing the radiation damage of these parts, and constructing reasonable maintenance programs.
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High performance non-reciprocal photonic devices can improve the efficiency of optical quantum manipulation, information processing, and quantum simulation effectively. The enhanced optical signal can simultaneously amplify the weak signal output by the quantum system and isolate the sensitive quantum system from the back-scattered external noise, which is the core technology of high-performance photonic devices. In our previous work (Opt. Express 31, 38228), we have achieved dynamic control of unidirectional reflection amplification based on four-wave mixing gain and utilizing the linear variation of coupling field intensity with position. In this article, we ingeniously design a simple three-level closed loop coherent gain atomic system, innovatively setting the intensity of coupling field varying with position by step shape to break the spatial symmetry of probe susceptibility, and achieving perfect non-reciprocal reflection amplification. In contrast, the stepped variation of coupling field intensity is easier to adjust in experiments, this can reduce the difficulty in the experiment greatly. Specifically, the system introduces phase modulation. By changing the phase, the frequency region of probe gain and absorption can be switched, which makes the modulation of reflection amplification more flexible.
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