Unusual Normal and Superconducting State Properties Observed in Hydrothermal Fe_(1-δ)Se Flakes

The electronic and superconducting properties of Fe_(1-δ)Se single-crystal flakes grown hydrothermally are studied by the transport measurements under zero and high magnetic fields up to 38.5 T. The results contrast sharply with those previously reported for nematically ordered Fe Se by chemical-vapor-transport(CVT) growth. No signature of the electronic nematicity, but an evident metal-to-nonmetal crossover with increasing temperature,is detected in the normal state of the present hydrothermal samples. Interestingly, a higher superconducting critical temperature T_c of 13.2 K is observed compared to a suppressed T_c of 9 K in the presence of the nematicity in the CVT Fe Se. Moreover, the upper critical field in the zero-temperature limit is found to be isotropic with respect to the field direction and to reach a higher value of ~42 T, which breaks the Pauli limit by a factor of 1.8.     

Machine learning identification of symmetrized base states of Rydberg atoms

Studying the complex quantum dynamics of interacting many-body systems is one of the most challenging areas in modern physics. Here, we use machine learning (ML) models to identify the symmetrized base states of interacting Rydberg atoms of various atom numbers (up to six) and geometric configurations. To obtain the data set for training the ML classifiers, we generate Rydberg excitation probability profiles that simulate experimental data by utilizing Lindblad equations that incorporate laser intensities and phase noise. Then, we classify the data sets using support vector machines (SVMs) and random forest classifiers (RFCs). With these ML models, we achieve high accuracy of up to 100% for data sets containing only a few hundred samples, especially for the closed atom configurations such as the pentagonal (five atoms) and hexagonal (six atoms) systems. The results demonstrate that computationally cost-effective ML models can be used in the identification of Rydberg atom configurations.     

Nonsymmetric bifurcations of solutions of the 2D Navier–Stokes system

We consider the 2D Navier–Stokes system written for the stream function with periodic boundary conditions and construct a set of initial data such that initial critical points bifurcate from 1 to 2 and then to 3 critical points in finite time. The bifurcation takes place in a small neighborhood of the origin. Our construction does not require any symmetry assumptions or the existence of special fixed points. For another set of initial data we show that 3 critical points merge into 1 critical point in finite time. We also construct a set of initial data so that bifurcation can be generated by the Navier–Stokes flow and do not require the existence of an initial critical point.     

The impact of the third O2 addition reaction network on ignition delay times of neo-pentane

We studied the oxidation of neo-pentane by combining experiments, theoretical calculations, and mechanistic developments to elucidate the impact of the 3rd O₂ addition reaction network on ignition delay time predictions. The experiments are based on photoionization mass spectrometry in jet-stirred and time-resolved flow reactors allowing for sensitive detection of the keto-hydroperoxide (KHP) and keto-dihydroperoxide (KDHP) intermediates. With neo-pentane exhibiting a unique symmetric molecular structure, which consequently results only in single KHP and KDHP isomers, theoretical calculations of ionization and fragment appearance energies and of absolute photoionization cross sections enabled the unambiguous identification and quantification of the KHP intermediate. Its temperature and time-resolved profiles together with calculated and experimentally observed KHP-to-KDHP signal ratios were compared to simulation results based on a newly developed mechanism that describes the 3rd O₂ addition reaction network. A satisfactory agreement has been observed between the experimental data points and the simulation results, thus adding confidence to the model's overall performance. Finally, this mechanism was used to predict ignition delay times reported previously in shock tube and rapid compression machine experiments (J. Bugler et al., Combust. Flame 163 (2016) 138–156). While the model accurately reproduces the experimental data, simulations with and without the 3rd O₂ addition reaction network included reveal only a negligible effect on the predicted ignition delay times at 10 and 20 atm. According to model calculations, low temperatures and high pressures promote the importance of the 3rd O₂ addition reactions.     

Speeding up generation of photon Fock state in a superconducting circuit via counterdiabatic driving

Optimal creation of photon Fock states is of importance for quantum information processing and state engineering.Here an efficient strategy is presented for speeding up generation of photon Fock state in a superconducting circuit via counterdiabatic driving.A transmon qubit is dispersively coupled to a quantized electrical field.We address a ∧-configuration interaction between the composite system and classical drivings.Based on two Gaussian-shaped drivings,a single-photon Fock state can be generated adiabatically.Instead of adding an auxiliary counterdiabatic driving,our concern is to modify these two Rabi drivings in the framework of shortcut to adiabaticity.Thus an accelerated operation with high efficiency can be realized in a much shorter time.Compared with the adiabatic counterpart,the shortcut-based operation is significantly insusceptible to decoherence effects.The scheme could offer a promising way to deterministically prepare photon Fock states with superconducting quantum circuits.     

单纯形上Bernstein多项式的一些性质

设Ｂ_m（ｆ，·）为函数ｆ在ｄ维单纯形σ上的ｎ阶Ｂｅｒｎｓｔｅｉｎ多项式，本文对ｆ∈Ｃ^ｒ（σ）及ｆ∈Ｃ^r+2（σ）给出了ｆ的各阶编导数用Ｂ_n（ｆ，·）相应偏导数逼近的误差估计．同时也考虑了整系数Ｂｅｒｎｓｔｅｉｎ多项式的Ｌ_p模估计     

Focused ion beam fabrication and magneto-electrical transport properties of La0.67Ca0.33MnO3 nanobridge

La_0.67Ca_0.33MnO₃ particle films with an average particle size of ~150 nm were grown on single-crystal silicon substrate using pulsed electron deposition technique and then focused ion beam was introduced to fabricate nanobridge in size of 300 × 900 nm on the particle film. The magneto-transport properties of both samples were studied. For the film, there is only one resistance peak at 182 K in temperature-dependent resistance (R–T) curves, which is far lower than ferromagnetic–paramagnetic transition temperature (T _C) of 250 K. When compared to the film, double peaks were observed in both R–T curves and magnetoresistance dependent on temperature (MR–T) curves of the nanobridge, one peak is at 186 K, which is very close to metal–insulator transition temperature (T _P) of film, the other one is at 250 K, which is close to the T _C of film, and these two peaks caused separately by grain and grain boundary (GB), which demonstrated that the electrical transport behavior of grain was separated from that of GB.     

太赫兹大气传输特性实验研究

太赫兹在高速率通信、高分辨力雷达等方面具有广阔的应用前景,而对太赫兹辐射的大气传输特性及其规律进行系统的理论与实验研究则是发展利用该频谱资源的基础条件。对大气作用于太赫兹频段电磁辐射的吸收衰减进行了实验测试研究。通过双光路的差分系统设计,尽可能地减小太赫兹辐射源输出功率不稳定性所产生的系统误差,完成大气模拟环境下太赫兹波段多个频点传输的功率变化测定,以探索大气环境对不同频段THz波衰减的规律性;同时,建立太赫兹大气传输特性动态数据库对海量大气环境数据进行有效地分析,为实验数据处理及理论研究提供支持。收稿日期:; 修订日期:     

宽波段连续调谐全固态CW PPMgLN光学参量振荡器

利用半导体激光泵浦输出1064 nm波长的全固态连续Nd:YVO4激光器作为泵浦源,采用周期调谐和温度调谐组合调谐技术,对基于掺氧化镁周期性极化铌酸锂晶体（MgO:LiNbO3, PPMgLN）准相位匹配（QPM）的全固态连续波（CW）光学参量振荡器（OPO）宽波段无分立连续调谐输出特性进行研究。实验采用连续工作模式和外腔结构,基于多周期PPMgLN晶体的30.2,30.4和30.6 m周期,在改变晶体的极化周期的基础上,同时在30～100 ℃范围内调节晶体工作温度。实验结果表明：CW PPMgLN OPO的泵浦阈值仅为0.22 W;不同极化周期需要的温度调谐范围不同;信号光在1 559.8～1 597.2 nm近红外波段和闲频光在3 187.3~3 347.3 nm中红外波段连续调谐输出。实现了外腔式全固态CW OPO在信号光和闲频光波段的无分立连续调谐输出。     

UPWIND SPLITTING SCHEME FOR CONVECTION-DIFFUSION EQUATIONS

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