## Volume 39 Issue 11

#### Highlighted Articles

Editors' Suggestion

#### Measuring Charge Distribution of Molecular Cations by an Atomic Coulomb Probe Microscope

Xitao Yu, Xiaoqing Hu, Jiaqi Zhou, Xinyu Zhang, Xinning Zhao, Shaokui Jia, Xiaorui Xue, Dianxiang Ren, Xiaokai Li, Yong Wu, Xueguang Ren, Sizuo Luo, and Dajun Ding
Chin. Phys. Lett., 2022, 39 (11): 113301   PDF ( 151 )   HTML ( 69 )
Editors' Suggestion

#### Multiband Dynamics of Extended Harmonic Generation in Solids under Ultraviolet Injection

Yue Lang, Zhaoyang Peng, and Zengxiu Zhao
Chin. Phys. Lett., 2022, 39 (11): 114201   PDF ( 166 )   HTML ( 50 )
Express Letter

#### Optically Detected Magnetic Resonance of Diamond Nitrogen-Vacancy Centers under Megabar Pressures

Jian-Hong Dai, Yan-Xing Shang, Yong-Hong Yu, Yue Xu, Hui Yu, Fang Hong, Xiao-Hui Yu, Xin-Yu Pan, and Gang-Qin Liu
Chin. Phys. Lett., 2022, 39 (11): 117601   PDF ( 319 )   HTML ( 110 )
Review

#### Strongly Lensed Transient Sources: A Review

Kai Liao, Marek Biesiada, and Zong-Hong Zhu
Chin. Phys. Lett., 2022, 39 (11): 119801   PDF ( 148 )   HTML ( 41 )

#### Current Issue

GENERAL
 Select Demonstration of Einstein–Podolsky–Rosen Steering with Multiple Observers via Sequential Measurements Jian Li, Yang Zhou, and Qin Wang Chin. Phys. Lett. 2022, 39 (11): 110301 .   DOI: 10.1088/0256-307X/39/11/110301 Abstract   PDF(pc) (3073KB) ( 52 ) PDF(mobile)(3077KB) ( 7 ) HTML ( 19 )  Quantum correlation is a fundamental property that distinguishes quantum systems from classical ones, and is also a fragile resource under projective measurements. Recently, it has been shown that a subsystem in entangled pairs can share nonlocalities with multiple observers in sequence. Here we present a new steering scenario where both subsystems are accessible by multiple observers. Simulation results show that the two qubits in a singlet state can be simultaneously steered separately by two sequential observers.
THE PHYSICS OF ELEMENTARY PARTICLES AND FIELDS
 Select Approach the Gell-Mann–Okubo Formula with Machine Learning Zhenyu Zhang, Rui Ma, Jifeng Hu, and Qian Wang Chin. Phys. Lett. 2022, 39 (11): 111201 .   DOI: 10.1088/0256-307X/39/11/111201 Abstract   PDF(pc) (4396KB) ( 47 ) PDF(mobile)(4400KB) ( 6 ) HTML ( 25 )  Machine learning is a novel and powerful technology and has been widely used in various science topics. We demonstrate a machine-learning-based approach built by a set of general metrics and rules inspired by physics. Taking advantages of physical constraints, such as dimension identity, symmetry and generalization, we succeed to approach the Gell-Mann–Okubo formula using a technique of symbolic regression. This approach can effectively find explicit solutions among user-defined observables, and can be extensively applied to studying exotic hadron spectrum.
ATOMIC AND MOLECULAR PHYSICS
 Select Measuring Charge Distribution of Molecular Cations by an Atomic Coulomb Probe Microscope Xitao Yu, Xiaoqing Hu, Jiaqi Zhou, Xinyu Zhang, Xinning Zhao, Shaokui Jia, Xiaorui Xue, Dianxiang Ren, Xiaokai Li, Yong Wu, Xueguang Ren, Sizuo Luo, and Dajun Ding Chin. Phys. Lett. 2022, 39 (11): 113301 .   DOI: 10.1088/0256-307X/39/11/113301 Abstract   PDF(pc) (3431KB) ( 151 ) PDF(mobile)(3831KB) ( 23 ) HTML ( 69 )  Imaging the charge distributions and structures of molecules and clusters will promote the understanding of the dynamics of the quantum system. Here, we report a method by using an Ar atom as a tip to probe the charge distributions of benzene (Bz) cations in gas phase. Remarkably, the measured charge distributions of Bz$^{+}$ ($\delta_{_{\scriptstyle \rm H}}=0.204$, $\delta_{_{\scriptstyle \rm C}}=-0.037$) and Bz$^{2+}$ ($\delta_{_{\scriptstyle \rm H}}=0.248$, $\delta_{_{\scriptstyle \rm C}}=0.0853$) agree well with the calculated Mulliken distributions, and the structures of Bz$_{2}$ is reconstructed by using the measured charge distributions. The structures of two Bz$_{2}$ isomers (T-shaped and PD isomers) can be resolved from the measured inter-molecular potential $V(R)$ between two Bz ions, and the structures of Bz dimer agree well with the theoretical predictions.
 Select Nonresonant Multiphoton Ionization of Stark Decelerated Molecules by Femtosecond Laser Pulses Congsen Meng, Zhihui Lü, Xiaowei Wang, Dongwen Zhang, Zengxiu Zhao, and Jianmin Yuan Chin. Phys. Lett. 2022, 39 (11): 113701 .   DOI: 10.1088/0256-307X/39/11/113701 Abstract   PDF(pc) (2301KB) ( 46 ) PDF(mobile)(2303KB) ( 6 ) HTML ( 23 )  Nonresonant multiphoton ionization by femtosecond laser pulses can be applied to any molecule virtually, thereby greatly enhancing the scope of Stark decelerated molecules. For comparison, we detect decelerated and trapped ammonia molecules using two different schemes: (i) nonresonant multiphoton ionization using intense femtosecond (fs) pulses in the near infrared, and (ii) resonance-enhanced multiphoton ionization using nanosecond (ns) pulses from a tunable UV laser. The observed number of ions per shot for both schemes is similar. The fs laser detection scheme suffers from an increased background, which can be effectively eliminated by subsequent mass and velocity selection. To determine the detection volume of the ns laser detection scheme, we present measurements in which the decelerated ammonia molecules are bunched to a packet with a longitudinal spread well below $\sim$100 µm. It is concluded that the detection volume for the ns laser detection scheme is 1.5–2 times larger than that of the fs laser detection scheme.
FUNDAMENTAL AREAS OF PHENOMENOLOGY(INCLUDING APPLICATIONS)
 Select Multiband Dynamics of Extended Harmonic Generation in Solids under Ultraviolet Injection Yue Lang, Zhaoyang Peng, and Zengxiu Zhao Chin. Phys. Lett. 2022, 39 (11): 114201 .   DOI: 10.1088/0256-307X/39/11/114201 Abstract   PDF(pc) (1640KB) ( 166 ) PDF(mobile)(1643KB) ( 7 ) HTML ( 50 )  Using one-dimensional semiconductor Bloch equations, we investigate the multiband dynamics of electrons in a cutoff extension scheme employing an infrared pulse with additional UV injection. An extended three-step model is firstly validated to play a dominant role in emitting harmonics in the second plateau. Surprisingly, further analysis employing the acceleration theorem shows that, though harmonics in both the primary and secondary present positive and negative chirps, the positive (negative) chirp in the first region is related to the so-called short (long) trajectory, while that in the second region is emitted through ‘general’ trajectory, where electrons tunneling earlier and recombining earlier contribute significantly. The novel characteristics deepen the understanding of high harmonic generation in solids and may have great significance in attosecond science and reconstruction of band dispersion beyond the band edge.
 Select Dynamic Behavior of Optical Soliton Interactions in Optical Communication Systems Shubin Wang, Guoli Ma, Xin Zhang, and Daiyin Zhu Chin. Phys. Lett. 2022, 39 (11): 114202 .   DOI: 10.1088/0256-307X/39/11/114202 Abstract   PDF(pc) (8351KB) ( 53 ) PDF(mobile)(8427KB) ( 3 ) HTML ( 33 )  In optical systems, it is necessary to investigate the propagation of optical solitons in optical fiber systems for fiber-optic communications. By means of the bilinear method, we obtain the two-soliton solution of the variable coefficient higher-order coupled nonlinear Schrödinger equation. According to the obtained two-soliton solution, a novel two-soliton interaction structure of the system is constructed, and their interactions are studied. Two optical solitons occur with elastic interaction under certain conditions, and their amplitudes, shapes and velocities remain unchanged before and after the action. In addition to the elastic interaction, splitting action and polymerization also occur. The present study on the dynamic behavior of interaction of optical solitons may be valuable for research and applications in optical communication and other fields.
PHYSICS OF GASES, PLASMAS, AND ELECTRIC DISCHARGES
 Select Development and Performance of the First Nb$_3$Sn Thin-Film Cavity via Bronze Process Ming Lu, Didi Luo, Feng Pan, Chunlong Li, Shichun Huang, Ziqin Yang, Andong Wu, Qingwei Chu, Tongtong Zhu, Shuai Wu, Teng Tan, and Hao Guo Chin. Phys. Lett. 2022, 39 (11): 115201 .   DOI: 10.1088/0256-307X/39/11/115201 Abstract   PDF(pc) (8509KB) ( 30 ) PDF(mobile)(8510KB) ( 2 ) HTML ( 14 )  Low-temperature synthesis of Nb$_3$Sn thin-film cavity is of great significance in the field of superconducting radio frequency (SRF). The bronze process can grow only stable Nb$_3$Sn phase at 700 ℃, so it is considered to be the most promising process for low-temperature synthesis of Nb$_3$Sn thin-film cavity. We successfully fabricated the worldwide first Nb$_3$Sn thin-film cavity by bronze process. We technically solved the key problems of precursor preparation, characterized and analyzed the uniformity of the Nb$_3$Sn film, and tested the performance of the cut-out samples and the whole cavity of the Nb$_3$Sn film. It is obtained that the $Q_0$ value of the cavity at 4.2 K is about $1.2\times10^{9}$, which is greater than the performance of the bulk-niobium cavity under the same conditions. This result means that the preparation of Nb$_3$Sn by bronze process has the great potential to more practical copper-based Nb$_3$Sn thin-film cavity, which is expected to achieve a substantial improvement in the performance of SRF cavity and comprehensive engineering applications.
 Select Scaling Laws of Heat Flux Width in the HL-2A Closed Divertor Tokamak Longwen Yan, Jinming Gao, Xianggan Miao, Zhihui Huang, Na Wu, Wenjin Chen, Ting Wu, Weice Wang, Liang Liu, Xiaoxue He, Kaiyang Yi, Yu He, Lin Nie, Zhongbing Shi, and Wulv Zhong Chin. Phys. Lett. 2022, 39 (11): 115202 .   DOI: 10.1088/0256-307X/39/11/115202 Abstract   PDF(pc) (7114KB) ( 18 ) PDF(mobile)(7115KB) ( 5 ) HTML ( 11 )  The scaling law of divertor heat flux width is one of the key topics of magnetic confinement fusion, which is almost inversely proportional to the poloidal magnetic field on some opened divertor tokamaks. This work focuses on the scaling laws of the closed divertor heat flux width in the HL-2A tokamak under different discharge conditions, such as the Ohmic, L- and H-modes. The results indicate that there are basic similarities of the scaling laws of the heat flux width between the opened and closed divertors. However, a larger spreading width in the private flux region is found, which is relevant to a small expansion factor of the magnetic flux in the closed divertor.
CONDENSED MATTER: STRUCTURE, MECHANICAL AND THERMAL PROPERTIES
 Select Lattice Thermal Conductivity of MgSiO$_3$ Perovskite and Post-Perovskite under Lower Mantle Conditions Calculated by Deep Potential Molecular Dynamics Fenghu Yang, Qiyu Zeng, Bo Chen, Dongdong Kang, Shen Zhang, Jianhua Wu, Xiaoxiang Yu, and Jiayu Dai Chin. Phys. Lett. 2022, 39 (11): 116301 .   DOI: 10.1088/0256-307X/39/11/116301 Abstract   PDF(pc) (11630KB) ( 44 ) PDF(mobile)(11718KB) ( 5 ) HTML ( 11 )  Lattice thermal conductivity ($\kappa_{\rm lat}$) of MgSiO$_3$ perovskite and post-perovskite is an important parameter for the thermal dynamics in the Earth. Here, we develop a deep potential of density functional theory quality under entire thermodynamic conditions in the lower mantle, and calculate the $\kappa_{\rm lat}$ by the Green–Kubo relation. Deep potential molecular dynamics captures full-order anharmonicity and considers ill-defined phonons in low-$\kappa_{\rm lat}$ materials ignored in the phonon gas model. The $\kappa_{\rm lat}$ shows negative temperature dependence and positive linear pressure dependence. Interestingly, the $\kappa_{\rm lat}$ undergos an increase at the phase boundary from perovskite to post-perovskite. We demonstrate that, along the geotherm, the $\kappa_{\rm lat}$ increases by 18.2% at the phase boundary. Our results would be helpful for evaluating Earth's thermal dynamics and improving the Earth model.
 Select Dehydrogenation Induced Formation of Chiral Core-Shell Arrays of Melamine on Ag(111) Hexu Zhang, Yuanhao Lyu, Wenqi Hu, Lan Chen, Yi-Qi Zhang, and Kehui Wu Chin. Phys. Lett. 2022, 39 (11): 116401 .   DOI: 10.1088/0256-307X/39/11/116401 Abstract   PDF(pc) (27534KB) ( 22 ) PDF(mobile)(28854KB) ( 2 ) HTML ( 14 )  The structural evolution of supramolecular phases of melamine on Ag(111) surface as a function of annealing temperature is investigated by employing low-temperature scanning tunneling microscopy/spectroscopy (LT-STM/STS). It is found that partial deprotonation of the melamine molecules leads to formation of distinct types of ordered supramolecular arrangements. Apart from two previously reported phases ($\alpha$ and $\beta$), a new phase comprising arrays of close-packed hexagonal core-shell-type clusters is identified for the first time. Based on high-resolution STM images as well as structural modeling, we show that the new phase presents a two-level hierarchical order and chirality is expressed at both levels. Using STS characterization, we further reveal that the chiral arrangement of the clusters confines surface electrons into a honeycomb pathway with handedness, which could give rise to novel interfacial electronic properties such as Dirac fermions as well as flat band.
CONDENSED MATTER: ELECTRONIC STRUCTURE, ELECTRICAL, MAGNETIC, AND OPTICAL PROPERTIES
 Select Evolution of Topological End States in the One-Dimensional Kondo–Heisenberg Model with Site Modulation Neng Xie, Danqing Hu, Shu Chen, and Yi-feng Yang Chin. Phys. Lett. 2022, 39 (11): 117101 .   DOI: 10.1088/0256-307X/39/11/117101 Abstract   PDF(pc) (5854KB) ( 47 ) PDF(mobile)(5858KB) ( 4 ) HTML ( 16 )  We investigate interplay of topological and Kondo effects in a one-dimensional Kondo–Heisenberg model with nontrivial conduction band using the density matrix renormalization group method. By analyzing the density profile, the local hybridization, and the spin/charge gap, we find that the Kondo effect can be destructed at edges of the chain by the topological end state below a finite critical Kondo coupling $J_{\scriptscriptstyle{\rm K}}^{\rm c}$. We construct a phase diagram characterizing the transition of the end states.
 Select Magnetic Interactions and Band Gaps of the (CrO$_2$)$_2$/(MgH$_2$)$_n$ Superlattices Wanfei Shan, Jiangtao Du, and Weidong Luo Chin. Phys. Lett. 2022, 39 (11): 117501 .   DOI: 10.1088/0256-307X/39/11/117501 Abstract   PDF(pc) (6554KB) ( 24 ) PDF(mobile)(6955KB) ( 3 ) HTML ( 12 )  Lattice superlattices constructed with different materials such as ferromagnets and insulators at atomic scale provide an ideal platform for exploring many emergent physical phenomena. In the present work, a new type of superlattices composed of ferromagnetic half-metal CrO$_2$, with a thickness of two atomic layers, together with insulating MgH$_2$ are constructed. Systematic theoretical studies on the (CrO$_2$)$_2$/(MgH$_2$)$_n$ ($n = 2, 3, 4, 5, 6$) superlattices are carried out based on first-principles density-functional theory calculations. These superlattices are ferromagnetic semiconductors with similar intra-layer magnetic exchange couplings between Cr ions. As the thickness of the MgH$_2$ layer increases, the magnetic exchange interaction between inter-layer Cr ions shows oscillating decaying behavior, while the energy band gaps show a small increase. The understanding of magnetic couplings in these superlattices provides a pathway for constructing new ferromagnetic semiconductors.
 Select Optically Detected Magnetic Resonance of Diamond Nitrogen-Vacancy Centers under Megabar Pressures Jian-Hong Dai, Yan-Xing Shang, Yong-Hong Yu, Yue Xu, Hui Yu, Fang Hong, Xiao-Hui Yu, Xin-Yu Pan, and Gang-Qin Liu Chin. Phys. Lett. 2022, 39 (11): 117601 .   DOI: 10.1088/0256-307X/39/11/117601 Abstract   PDF(pc) (9646KB) ( 319 ) PDF(mobile)(12133KB) ( 46 ) HTML ( 110 )  Megabar pressures are of crucial importance for cutting-edge studies of condensed matter physics and geophysics. With the development of diamond anvil cell (DAC), laboratory studies of high pressure have entered the megabar era for decades. However, it is still challenging to implement in situ magnetic sensing under ultrahigh pressures. In this work, we demonstrate optically detected magnetic resonance and coherent quantum control of diamond nitrogen-vacancy (NV) center, a promising quantum sensor inside the DAC, up to 1.4 Mbar. The pressure dependence of optical and spin properties of NV centers in diamond are quantified, and the evolution of an external magnetic field has been successfully tracked at about 80 GPa. These results shed new light on our understanding of diamond NV centers and pave the way for quantum sensing under extreme conditions.
CROSS-DISCIPLINARY PHYSICS AND RELATED AREAS OF SCIENCE AND TECHNOLOGY
 Select High-Performance Indium-Gallium-Zinc-Oxide Thin-Film Transistors with Stacked Al$_{2}$O$_{3}$/HfO$_{2}$ Dielectrics Yue Li, Li Zhu, Chunsheng Chen, Ying Zhu, Changjin Wan, and Qing Wan Chin. Phys. Lett. 2022, 39 (11): 118501 .   DOI: 10.1088/0256-307X/39/11/118501 Abstract   PDF(pc) (2244KB) ( 46 ) PDF(mobile)(2242KB) ( 5 ) HTML ( 13 )  High-performance amorphous indium-gallium-zinc-oxide thin-film transistors (a-IGZO TFTs) gated by Al$_{2}$O$_{3}$/ HfO$_{2}$ stacked dielectric films are investigated. The optimized TFTs with Al$_{2}$O$_{3}$ (2.0 nm)/HfO$_{2}$ (13 nm) stacked gate dielectrics demonstrate the best performance, including low total trap density $N_{\rm t}$, low subthreshold swing voltage, large switching ratio $I_{\rm ON/OFF}$, high mobility $\mu_{_{\scriptstyle \rm FE}}$, and low operating voltage, equal to $1.35 \times 10^{12}$ cm$^{-2}$, 88 mV/dec, $5.24 \times 10^{8}$, 14.2 cm$^{2}$/V$\cdot$s, and 2.0 V, respectively. Furthermore, a low-voltage-operated resistor-loaded inverter has been fabricated based on such an a-IGZO TFT, showing ideal full swing characteristics and high gain of $\sim$27 at 3.0 V. These results indicate a-IGZO TFTs gated by optimized Al$_{2}$O$_{3}$/HfO$_{2}$ stacked dielectrics are of great interests for low-power, high performance, and large-area display and emerging electronics.
GEOPHYSICS, ASTRONOMY, AND ASTROPHYSICS
 Select Strongly Lensed Transient Sources: A Review Kai Liao, Marek Biesiada, and Zong-Hong Zhu Chin. Phys. Lett. 2022, 39 (11): 119801 .   DOI: 10.1088/0256-307X/39/11/119801 Abstract   PDF(pc) (17430KB) ( 148 ) PDF(mobile)(17449KB) ( 23 ) HTML ( 41 )  The past decades have witnessed a lot of progress in gravitational lensing with two main targets: stars and galaxies (with active galactic nuclei). The success is partially attributed to the continuous luminescence of these sources making the detection and monitoring relatively easy. With the running of ongoing and upcoming large facilities/surveys in various electromagnetic and gravitational-wave bands, the era of time-domain surveys would guarantee constant detection of strongly lensed explosive transient events, for example, supernovae in all types, gamma ray bursts with afterglows in all bands, fast radio bursts, and even gravitational waves. Lensed transients have many advantages over the traditional targets in studying the Universe, and magnification effect helps to understand the transients themselves at high redshifts. In this review article, on base of the recent achievements in literature, we summarize the methods of searching for different kinds of lensed transient signals, the latest results on detection and their applications in fundamental physics, astrophysics, and cosmology. At the same time, we give supplementary comments as well as prospects of this emerging research direction that may help readers who are interested in entering this field.