Highlights
Abstract +
The discovery of ultrafast demagnetization has provided a new means for generating ultrafast spin currents by using an ultrashort laser, potentially enabling faster manipulation of material magnetism. This has sparked research on the transport mechanisms of ultrafast spin currents. However, the basic processes are still poorly understood, especially the factors influencing interlayer spin transfer. In this work, a superdiffusive spin transport model is used to investigate the ultrafast spin transport mechanism 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 recent experimental findings are validated. Further analysis shows that reducing the thickness of the Ru spacer layer will significantly enhance the spin current intensity and increase 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. In addition, the spin decay length of hot electron spin currents in the spacer Ru layer is determined to be approximately 0.5 nm. This work 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 ultrafast magnetic control of future spin valve structures and contribute to the development of spintronics in high-speed information processing and storage applications.
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Laser intensity noise suppression in the millihertz frequency band is essential for space-based gravitational wave detection to ensure the sensitivity of the interferometer. Optoelectronic feedback technology is one of the most effective methods of suppressing laser intensity noise. The noise of the photodetector that is the first-stage component in the feedback loop, directly couples into the feedback loop, thus significantly affecting the laser intensity noise. In this paper, starting from the requirement of suppressing laser intensity noise in the 0.1 mHz–1 Hz frequency band for space-based gravitational wave detection, the factors affecting the electronics of photodetectors at extremely low frequencies are analyzed in detail. Using the low dark current characteristic of photodiodes in photovoltaic mode, a zero-bias voltage scheme is adopted to reduce the dark noise of the photodiode. A transimpedance amplification circuit is designed using an integrated operational amplifier with zero offset voltage drift and low-temperature drift metal foil resistors, thereby optimizing the transimpedance capacitor and follower circuit to reduce 1/f noise in the circuit. Active temperature control is employed to stabilize the responsivity of photodiode, and additional measures such as using a homemade low-noise power supply and shielding interference are taken to further reduce the noise. Ultimately, an ultra-low electronic noise photodetector operating in the 0.1 mHz–1 Hz frequency band is developed. A homemade intensity noise evaluation system is used to comprehensively assess the noise both in the time domain and in the frequency domain. The constant noise characteristics of the homemade detector are estimated experimentally. The experimental results show that the electronic noise spectral density of the homemade detector reaches 2×10–6 V/Hz1/2 in the 0.1 mHz–1 Hz frequency band, and the electronic noise of the detector does not vary with optical power. The detector achieves a gain of 35 kV/W at 1064 nm. The noise performance of the detector is two orders of magnitude lower than the laser intensity noise requirement (1×10–4 V/Hz1/2) for space-based gravitational wave detection, providing a critical component and technical support for high-gain optoelectronic feedback control and laser intensity noise suppression in space-based gravitational wave detection.
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Dielectronic recombination (DR) experiments of highly charged ions not only provide essential atomic benchmark data for astrophysical and fusion plasma research but also serve as a stringent test for strong-field quantum electrodynamics (QED) effects, relativistic effects, and electron correlation effects. High-intensity heavy-ion accelerator facility (HIAF), currently under construction at Huizhou, China, will have a high-precision spectrometer ring (SRing) equipped with a 450 kV electron-cooler and an 80 kV ultracold electron-target. This advanced setup facilitates precise measurements of the DR process for highly charged ions in a broad range of center-of-mass energy, from meV to tens of keV. In this work, we carry out the molecular dynamics simulation of the electron beam temperature distribution of the ultracold electron-target at the SRing. The simulation results indicate that after treatment by the designed adiabatic magnetic field and acceleration field, the transverse and longitudinal electron beam temperature generated by the thermionic electron gun can be reduced from 100 meV to below 5 meV and 0.1 meV, respectively. Furthermore, we analyze the influence of this ultracold electron beam temperature on the resonance peak and energy resolution in DR experiment. The resolution gain at the SRing electron-target is particularly pronounced at small electron-ion collision energy, which provides unique experimental conditions for the DR experiments. Taking lithium-like $ {}_{~\,54}^{129}{{\mathrm{X}}{\mathrm{e}}}^{51+} $ and $ {}_{~\,92}^{238}{{\mathrm{U}}}^{89+} $ ions for example, we simulate the DR resonance spectra at the SRing and compare them with the simulated results from the experimental cooler storage ring CSRe. The results reveal that the SRing experiments can resolve fine DR resonance structures with ultra-high energy resolution compared with those from the CSRe. This work lays a solid foundation for precise DR spectroscopy of highly charged ions at the SRing to stringent test of strong field QED effect and extraction nuclear structure information.
SPECIAL TOPIC—Dynamics of atoms and molecules at extremes
COVER ARTICLE
Analysis of dynamic response and screening effects on electron-ion energy relaxation in dense plasma
2025, 74 (3): 035101.
doi: 10.7498/aps.74.20241588
Abstract +
Accurate knowledge of electron-ion energy relaxation plays a vital role in non-equilibrium dense plasmas with widespread applications such as in inertial confinement fusion, in laboratory plasmas, and in astrophysics. We present a theoretical model for the energy transfer rate of electron-ion energy relaxation in dense plasmas, where the electron-ion coupled mode effect is taken into account. Based on the proposed model, other simplified models are also derived in the approximations of decoupling between electrons and ions, static limit, and long-wavelength limit. The influences of dynamic response and screening effects on electron-ion energy relaxation are analyzed in detail. Based on the models developed in the present work, the energy transfer rates are calculated under different plasma conditions and compared with each other. It is found that the behavior of electron screening in the random phase approximation is significantly different from the one in the long-wave approximation. This difference results in an important influence on the electron-ion energy relaxation and temperature equilibration in plasmas with temperature $T_{\rm{e}} < T_{\rm{i}}$. The comparison of different models shows that the effects of dynamic response, such as dynamic screening and coupled-mode effect, have stronger influence on the electron-ion energy relaxation and temperature equilibration. In the case of strong degeneracy, the influence of dynamic response will result in an order of magnitude difference in the electron-ion energy transfer rate. In conclusion, it is crucial to properly consider the finite-wavelength screening of electrons and the coupling between electron and ion plasmonic excitations in order to determine the energy transfer rate of electron-ion energy relaxation in dense plasma.
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SPECIAL TOPIC—Dynamics of atoms and molecules at extremes
COVER ARTICLE
2024, 73 (24): 240701.
doi: 10.7498/aps.73.20241290
Abstract +
Electron capture in the collision of highly charged ions with atoms and molecules is a fundamental process related to the electron transition between bound states belonging to two atomic-centers. The X-ray emission after electron capture is important for X-ray astrophysical modeling, fusion plasma diagnostics, and ion irradiated biophysics. In the past few decades, momentum-imaging cold-target recoil ion momentum spectroscopy has been a significantly developed technique and widely used to measure the quantum state-selective population in electron capture processes. Based on the cold target recoil ion momentum spectroscopy installed on the 150 kV highly charged ion platform in Fudan University, Shanghai City, China, the state-selectivity of double electron capture in the bombardment of 1.4–20 keV/u Ar8+ on He is measured, and the relative cross sections of the 3l 3l' to 3l 7l' double excited states are obtained. It is found that with the increase of collision energy, more quantum state-selectivity channels are open in the double electron capture of Ar8+-He collision. It is also found that the relative cross section of the quantum state population is strongly dependent on the collision energy of the projectile ion. The present measurements not only enrich the state-selective cross-sectional library and collision dynamics of highly charged ion charge exchange processes, but also provide experimental benchmarks for existing theoretical calculations.
Abstract +
Thermoreflectance techniques, particularly frequency-domain thermoreflectance (FDTR), play a crucial role in measuring the thermal properties of bulk and thin-film materials. These methods precisely measure thermal conductivity, specific heat capacity, and interfacial thermal conductance by analyzing the surface temperature response signals through thermoreflectance. However, the complex interplay among parameters presents challenges in data analysis, where single-variable analysis often fails to accurately capture intra-layer and inter-layer interactions. In this work, FDTR is used as a case study and the relationships between sensitivity coefficients of various parameters are systematically explored through singular value decomposition (SVD). Specifically, the SVD of sensitivity matrix S of the system's parameters is performed to identify smaller singular values and their corresponding right singular vectors, which are the basis vectors of the null space of matrix S . These vectors reveal the relationships among parameter sensitivities, thereby uncovering the most fundamental combination parameters that determine the thermoreflectance signal. This method not only clarifies the dependency relationships between variables but also determines the maximum number of parameters that can be experimentally extracted, and the parameters that must be known beforehand. To demonstrate the practical value of these combination parameters, this work conducts a detailed analysis of FDTR signals from an aluminum/sapphire sample. Unlike traditional FDTR experiments, which typically fit only the thermal conductivity and interfacial thermal conductance of the substrate, our sensitivity analysis reveals that it is possible to simultaneously determine the thermal conductivity of the metal film, substrate’s thermal conductivity, substrate’s specific heat capacity, and interfacial thermal conductance. The fitting results are consistent with reference values from the literature and measurements from other thermoreflectance techniques, thus validating the effectiveness and reliability of our method. This comprehensive analysis not only deepens the understanding of thermoreflectance phenomena but also provides strong support for the future development of thermal characterization technology and material research, showing the significant potential application of SVD in complex multi-parameter systems.
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Ultrafast scanning electron microscope (USEM) integrates pump-probe technique with microscopic imaging, enabling the visualizing of photon-induced surface charge dynamics with high spatial and temporal resolution. This capability is crucial for high-resolution detection of semiconductor surface states and optoelectronic devices. This work discusses the parametric design of a thermionic emission electron gun that has been modified into a photoemission electron gun, based on a home-built ultrafast scanning electron microscope. Given that the dose of the photoemitting electron beam is usually much lower than that of thermionic emission, the transition to photoemission requires the removal of the self-bias voltage function of the original electron microscope power supply to ensure the normal operation of the Wehnelt electrode. We quantitatively analyze the dependence of bias voltage, cathode, Wehnelt electrode, and anode on the position, size and divergence angle of crossover, which helps to improve the parameter adjustment of the modified electron gun. The analysis results indicate that if the distance between the Wehnelt electrode and the anode is adjusted from 8 to 23 mm, the distance between the filament and wehnelt can be changes from 0.65 to 0.45 mm to cooperate with the bias adjustment, so that the normal use of high-resolution thermionic emission mode, low voltage mode and photoemission mode can be realized. Subsequently, the effect of the mirror’s position on the electron optical path is analyzed. It is found that when the anode is raised 1.4 mm above the mirror, the influence on the electron optical path can be ignored. Additionally, the zero-of-time and temporal broadening of the photo-electron pulse are further simulated. The results indicate that with the increase of bias voltage, the time zero of photoemission will be delayed and the temporal broadening will become larger. This study lays a foundation for the future development of ultrafast electron microscope and the design of photoemission electron sources.
Abstract +
In magnetic atomic gases, the dipolar relaxation process couples the system spin and kinetic degrees of freedom. When the average kinetic energy is significantly lower than the Zeeman splitting, the atoms predominantly occupy the lowest Zeeman state. As the Zeeman splitting approaches the average kinetic energy, some atoms transfer to adjacent Zeeman states through dipolar relaxation, converting kinetic energy into Zeeman energy. By utilizing optical pumping, atoms transferred to higher spin states can be repumped to the ground state, thereby achieving a continuous cooling cycle and effectively lowering the system’s temperature. As the energy removed in a single cooling cycle is much larger than the energy of scattered photons, this demagnetization cooling scheme significantly enhances cooling effciency and reduces atomic loss. In this work, we establish state-coupled equations that incorporate dipolar relaxation and optical pumping to analyze the demagnetization cooling process, modeling the evolution of atom number and temperature during the cooling of $^{164}{\text{Dy}}$ atoms. We develop a strategy to generate an optimal magnetic field waveform by maximizing the demagnetization rate. Based on this strategy, we investigate the influence of crucial experimental parameters on demagnetization cooling and determine their specific ranges for producing large atom number of BEC, including the optical dipole trap frequency, as well as the intensity and polarization purity of the optical pumping light. The results indicate that demagnetization cooling enables the direct preparation of a large number of dysprosium BEC with sub-second timescales, reducing the cooling time by an order of magnitude compared to conventional methods for dysprosium atoms. Furthermore, it could achieve a cooling effciency of $\chi \approx 44.92$, an order of magnitude higher than that of traditional evaporative cooling.
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