Volume 38 Issue 3

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GENERAL
Two-Dimensional Quantum Walk with Non-Hermitian Skin Effects
Tianyu Li, Yong-Sheng Zhang, and Wei Yi
Chin. Phys. Lett. 2021, 38 (3): 030301 .   DOI: 10.1088/0256-307X/38/3/030301
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We construct a two-dimensional, discrete-time quantum walk, exhibiting non-Hermitian skin effects under open-boundary conditions. As a confirmation of the non-Hermitian bulk-boundary correspondence, we show that the emergence of topological edge states is consistent with the Floquet winding number, calculated using a non-Bloch band theory, invoking time-dependent generalized Brillouin zones. Further, the non-Bloch topological invariants associated with quasienergy bands are captured by a non-Hermitian local Chern marker in real space, defined via the local biorthogonal eigenwave functions of a non-unitary Floquet operator. Our work aims to stimulate further studies of non-Hermitian Floquet topological phases where skin effects play a key role.
Effects of Quantum Noise on Quantum Approximate Optimization Algorithm
Cheng Xue, Zhao-Yun Chen, Yu-Chun Wu, and Guo-Ping Guo
Chin. Phys. Lett. 2021, 38 (3): 030302 .   DOI: 10.1088/0256-307X/38/3/030302
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The quantum-classical hybrid algorithm is a promising algorithm with respect to demonstrating the quantum advantage in noisy-intermediate-scale quantum (NISQ) devices. When running such algorithms, effects due to quantum noise are inevitable. In our work, we consider a well-known hybrid algorithm, the quantum approximate optimization algorithm (QAOA). We study the effects on QAOA from typical quantum noise channels, and produce several numerical results. Our research indicates that the output state fidelity, i.e., the cost function obtained from QAOA, decreases exponentially with respect to the number of gates and noise strength. Moreover, we find that when noise is not serious, the optimized parameters will not deviate from their ideal values. Our result provides evidence for the effectiveness of hybrid algorithms running on NISQ devices.
Experimental Protection of the Spin Coherence of a Molecular Qubit Exceeding a Millisecond
Yingqiu Dai, Yue Fu, Zhifu Shi, Xi Qin, Shiwei Mu, Yang Wu, Ji-Hu Su, Yi-Fei Deng, Lei Qin, Yuan-Qi Zhai, Yan-Zhen Zheng, Xing Rong, and Jiangfeng Du
Chin. Phys. Lett. 2021, 38 (3): 030303 .   DOI: 10.1088/0256-307X/38/3/030303
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Molecular qubits are promising as they can benefit from tailoring and versatile design of chemistry. It is essential to reduce the decoherence of molecular qubits caused by their interactions with the environment. Herein the dynamical decoupling (DD) technique is utilized to combat such decoherence. The coherence time for a transition-metal complex {(PPh$_4$)}$_2$[Cu(mnt)$_2$] is prolonged from 6.8 µs to 1.4 ms. The ratio of the coherence time and the length of $\pi$/2 pulse, defined as the single qubit figure of merit ($Q_{\rm M}$), reaches $1.4 \times 10^5$, which is 40 times greater than what previously reported for this molecule. Our results show that molecular qubits, with milliseconds coherence time, are promising candidates for quantum information processing.
Quantum Algorithm for Approximating Maximum Independent Sets
Hongye Yu, Frank Wilczek, and Biao Wu
Chin. Phys. Lett. 2021, 38 (3): 030304 .   DOI: 10.1088/0256-307X/38/3/030304
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We present a quantum algorithm for approximating maximum independent sets of a graph based on quantum non-Abelian adiabatic mixing in the sub-Hilbert space of degenerate ground states, which generates quantum annealing in a secondary Hamiltonian. For both sparse and dense random graphs $G$, numerical simulation suggests that our algorithm on average finds an independent set of size close to the maximum size $\alpha(G)$ in low polynomial time. The best classical algorithms, by contrast, produce independent sets of size about half of $\alpha(G)$ in polynomial time.
Advection and Thermal Diode
Ying Li and Jiaxin Li
Chin. Phys. Lett. 2021, 38 (3): 030501 .   DOI: 10.1088/0256-307X/38/3/030501
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We prove that under the condition of closed boundary to mass flux, pure advection is not a valid mechanism to make a practical thermal diode. Among the various designs of thermal diodes, many of them involve circulating fluid flow, such as in thermosyphons. However, those designs often employ natural convection, which is basically a nonlinear process. It thus remains unclear how the pure advection of temperature field induced by a decoupled velocity field influences the symmetry of heat transfer. Here we study three typical models with pure advection: one with open boundary, one with closed boundary at unsteady state, and one with closed boundary at steady state. It is shown that only the last model is practical, while it cannot become a thermal diode. Finally, a general proof is given for our claim by analyzing the diffusive reciprocity.
Ehrenfest Time at the Transition from Integrable Motion to Chaotic Motion
Chuan Zhao and Biao Wu
Chin. Phys. Lett. 2021, 38 (3): 030502 .   DOI: 10.1088/0256-307X/38/3/030502
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Ehrenfest time depends differently on the Planck constant in integrable motion and chaotic motion. We study how its dependence on the Planck constant changes when there is a continuous transition from regular motion to chaotic motion. We find that the dependence is a weighted compromise between its two distinct dependences in regular and chaotic motions. The study is carried out with the system of periodically driven anharmonic oscillator. As the system is quite typical, the result may apply generally.
NUCLEAR PHYSICS
Quantum Anti-Zeno Effect in Nuclear $\beta$ Decay
Ming Ji  and Chang Xu
Chin. Phys. Lett. 2021, 38 (3): 032301 .   DOI: 10.1088/0256-307X/38/3/032301
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The acceleration of decay induced by frequency measurements, namely the quantum anti-Zeno effect (AZE), was first predicted by Kofman and Kurizki [Nature 405 (2000) 546 ]. The effect of the frequency measurements on nuclear $\beta$ decay rate is analyzed based on the time-dependent perturbation theory. We present a detailed calculation of the decay rates of $^{3}$H, $^{60}$Co ($\beta^{-}$ type), $^{22}$Na, $^{106}$Ag ($\beta^{+}$ type) and $^{18}$F, $^{57}$Co and $^{111}$Sn (EC type) under frequency measurements. It is found that the effects of frequency measurements on the decay rates of $\beta^{+}$ and $\beta^{-}$ cases are different from the case of EC, and the smaller the $\beta$ decay energy is, the more favorable it is to observe the AZE in experiment. Based on our analysis, it is suggested that possible experimental candidates should have a small decay energy and a reasonable half life (such as $^{3}$H) for observing the AZE in $\beta$ decay.
FUNDAMENTAL AREAS OF PHENOMENOLOGY(INCLUDING APPLICATIONS)
Effective Interaction Force between an Electric Charge and a Magnetic Dipole and Locality (or Nonlocality) in Quantum Effects of the Aharonov–Bohm Type
Gianfranco Spavieri, George T. Gillies, Miguel Rodriguez, and Maribel Perez
Chin. Phys. Lett. 2021, 38 (3): 034101 .   DOI: 10.1088/0256-307X/38/3/034101
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Classical electrodynamics foresees that the effective interaction force between a moving charge and a magnetic dipole is modified by the time-varying total momentum of the interaction fields. We derive the equations of motion of the particles from the total stress-energy tensor, assuming the validity of Maxwell's equations and the total momentum conservation law. Applications to the effects of Aharonov–Bohm type show that the observed phase shift may be due to the relative lag between interfering particles caused by the effective local force.
Ultra-Broadband Infrared Metamaterial Absorber for Passive Radiative Cooling
Yan-Ning Liu, Xiao-Long Weng, Peng Zhang, Wen-Xin Li, Yu Gong, Li Zhang, Tian-Cheng Han, Pei-Heng Zhou, and Long-Jiang Deng
Chin. Phys. Lett. 2021, 38 (3): 034201 .   DOI: 10.1088/0256-307X/38/3/034201
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Infrared metamaterial absorber (MMA) based on metal-insulator-metal (MIM) configuration with flexible design, perfect and selective absorption, has attracted much attention recently for passive radiative cooling applications. To cool objects passively, broadband infrared absorption (i.e. 8–14 µm) is desirable to emit thermal energy through atmosphere window. We present a novel MMA composed of multilayer MIM resonators periodically arranged on a PbTe/MgF$_{2}$ bilayer substrate. Verified by the rigorous coupled-wave analysis method, the proposed MMA shows a relative bandwidth of about 45% (from 8.3 to 13.1 µm with the absorption intensity over 0.8). The broadband absorption performs stably over a wide incident angle range (below 50$^{\circ}$) and predicts 12 K cooling below ambient temperature at nighttime. Compared with the previous passive radiative coolers, our design gets rid of the continuous metal substrate and provides an almost ideal transparency window (close to 100%) for millimeter waves over 1 mm. The structure is expected to have potential applications in thermal control of integrated devices, where millimeter wave signal compatibility is also required.
PHYSICS OF GASES, PLASMAS, AND ELECTRIC DISCHARGES
Nonlinear Coupling of Reversed Shear Alfvén Eigenmode and Toroidal Alfvén Eigenmode during Current Ramp
Shizhao Wei, Yahui Wang, Peiwan Shi, Wei Chen, Ningfei Chen, and Zhiyong Qiu
Chin. Phys. Lett. 2021, 38 (3): 035201 .   DOI: 10.1088/0256-307X/38/3/035201
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Two novel nonlinear mode coupling processes for reversed shear Alfvén eigenmode (RSAE) nonlinear saturation are proposed and investigated. In the first process, the RSAE nonlinearly couples to a co-propagating toroidal Alfvén eigenmode (TAE) with the same toroidal and poloidal mode numbers, and generates a geodesic acoustic mode. In the second process, the RSAE couples to a counter-propagating TAE and generates an ion acoustic wave quasi-mode. The condition for the two processes to occur is favored during current ramp. Both the processes contribute to effectively saturate the Alfvénic instabilities, as well as nonlinearly transfer of energy from energetic fusion alpha particles to fuel ions in burning plasmas.
Energetic Particle Physics on the HL-2A Tokamak: A Review
Pei-Wan Shi, Wei Chen, and Xu-Ru Duan
Chin. Phys. Lett. 2021, 38 (3): 035202 .   DOI: 10.1088/0256-307X/38/3/035202
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Interaction between shear Alfvén wave (SAW) and energetic particles (EPs) is one of major concerns in magnetically confined plasmas since it may lead to excitation of toroidal symmetry breaking collective instabilities, thus enhances loss of EPs and degrades plasma confinement. In the last few years, Alfvénic zoology has been constructed on HL-2A tokamak and series of EPs driven instabilities, such as toroidal Alfvén eigenmodes (TAEs), revered shear Alfvén eigenmodes (RSAEs), beta induced Alfvén eigenmodes (BAEs), Alfvénic ion temperature gradient (AITG) modes and fishbone modes, have been observed and investigated. Those Alfvénic fluctuations show frequency chirping behaviors through nonlinear wave-particle route, and contribute to generation of axisymmetric modes by nonlinear wave-wave resonance in the presence of strong tearing modes. It is proved that the plasma confinement is affected by Alfvénic activities from multiple aspects. The RSAEs resonate with thermal ions, and this results in an energy diffusive transport process while the nonlinear mode coupling between core-localized TAEs and tearing modes trigger avalanche electron heat transport events. Effective measures have been taken to control SAW fluctuations and the fishbone activities are suppressed by electron cyclotron resonance heating. Those experimental results will not only contribute to better understandings of energetic particles physics, but also provide technology bases for active control of Alfvénic modes on International Thermonuclear Experimental Reactor (ITER) and Chinese Fusion Engineering Testing Reactor (CFETR).
CONDENSED MATTER: STRUCTURE, MECHANICAL AND THERMAL PROPERTIES
Novel Superconducting Electrides in Ca–S System under High Pressures
Yun-Xian Liu , Chao Wang, Shuai Han , Xin Chen , Hai-Rui Sun , and Xiao-Bing Liu
Chin. Phys. Lett. 2021, 38 (3): 036201 .   DOI: 10.1088/0256-307X/38/3/036201
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Due to their unique structure properties, most of the electrides that possess extra electrons locating in interstitial regions as anions are insulators. Metallic and superconducting electrides are very rare under ambient conditions. We systematically search possible compounds in Ca–S systems stabilized under various pressures up to 200 GPa, and investigate their crystal structures and properties using first-principles calculations. We predict a series of novel stoichiometries in Ca–S systems as potential superconductors, including $P2_{1}/m$ Ca$_{3}$S, $P$4mbm Ca$_{3}$S, Pnma Ca$_{2}$S, Cmcm Ca$_{2}$S, Fddd CaS$_{2}$, Immm CaS$_{3}$ and $C2/c$ CaS$_{4}$. The $P4mbm$ Ca$_{3}$S phase exhibits a maximum $T_{\rm c}$ value of $\sim $20 K. It is interesting to notice that the $P2_{1}/m$ Ca$_{3}$S and Pnma Ca$_{2}$S stabilized at 60 and 50 GPa behave as superconducting electrides with critical temperatures $T_{\rm c}$ of 7.04 K and 0.26 K, respectively. More importantly, our results demonstrate that $P2_{1}/m$ Ca$_{3}$S and Pnma Ca$_{2}$S are dynamically stable at 5 GPa and 0 GPa, respectively, indicating a high possibility to be quenched to ambient condition or synthesized using the large volume press.
Quantum Transport across Amorphous-Crystalline Interfaces in Tunnel Oxide Passivated Contact Solar Cells: Direct versus Defect-Assisted Tunneling
Feng Li, Weiyuan Duan, Manuel Pomaska, Malte Köhler, Kaining Ding, Yong Pu, Urs Aeberhard, and Uwe Rau
Chin. Phys. Lett. 2021, 38 (3): 036301 .   DOI: 10.1088/0256-307X/38/3/036301
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Tunnel oxide passivated contact solar cells have evolved into one of the most promising silicon solar cell concepts of the past decade, achieving a record efficiency of 25%. We study the transport mechanisms of realistic tunnel oxide structures, as encountered in tunnel oxide passivating contact (TOPCon) solar cells. Tunneling transport is affected by various factors, including oxide layer thickness, hydrogen passivation, and oxygen vacancies. When the thickness of the tunnel oxide layer increases, a faster decline of conductivity is obtained computationally than that observed experimentally. Direct tunneling seems not to explain the transport characteristics of tunnel oxide contacts. Indeed, it can be shown that recombination of multiple oxygen defects in $a$-SiO$_{x}$ can generate atomic silicon nanowires in the tunnel layer. Accordingly, new and energetically favorable transmission channels are generated, which dramatically increase the total current, and could provide an explanation for our experimental results. Our work proves that hydrogenated silicon oxide (SiO$_{x}$:H) facilitates high-quality passivation, and features good electrical conductivity, making it a promising hydrogenation material for TOPCon solar cells. By carefully selecting the experimental conditions for tuning the SiO$_{x}$:H layer, we anticipate the simultaneous achievement of high open-circuit voltage and low contact resistance.
CONDENSED MATTER: ELECTRONIC STRUCTURE, ELECTRICAL, MAGNETIC, AND OPTICAL PROPERTIES
Superconductivity and Charge Density Wave in Iodine-Doped CuIr$_{2}$Te$_{4}$
Mebrouka Boubeche, Jia Yu, Li Chushan, Wang Huichao, Lingyong Zeng, Yiyi He, Xiaopeng Wang, Wanzhen Su, Meng Wang, Dao-Xin Yao, Zhijun Wang, and Huixia Luo
Chin. Phys. Lett. 2021, 38 (3): 037401 .   DOI: 10.1088/0256-307X/38/3/037401
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We report a systematic investigation on the evolution of the structural and physical properties, including the charge density wave (CDW) and superconductivity of the polycrystalline CuIr$_{2}$Te$_{4- x}$I$_{x}$ for $0.0 \le x \le 1.0$. X-ray diffraction results indicate that both of $a$ and $c$ lattice parameters increase linearly when $0.0 \le x \le 1.0$. The resistivity measurements indicate that the CDW is destabilized with slight $x$ but reappears at $x \ge 0.9$ with very high $T_{\rm CDW}$. Meanwhile, the superconducting transition temperature $T_{\rm c}$ enhances as $x$ increases and reaches a maximum value of around 2.95 K for the optimal composition CuIr$_{2}$Te$_{1.9}$I$_{0.1}$ followed by a slight decrease with higher iodine doping content. The specific heat jump ($\Delta C/\gamma T_{\rm c}$) for the optimal composition CuIr$_{2}$Te$_{3.9}$I$_{0.1}$ is approximately 1.46, which is close to the Bardeen–Cooper–Schrieffer value of 1.43, indicating that it is a bulk superconductor. The results of thermodynamic heat capacity measurements under different magnetic fields [$C_{\rm p}(T, H)$], magnetization $M(T, H)$ and magneto-transport $\rho (T, H)$ measurements further suggest that CuIr$_{2}$Te$_{4- x}$I$_{x}$ bulks are type-II superconductors. Finally, an electronic phase diagram for this CuIr$_{2}$Te$_{4- x}$I$_{x}$ system has been constructed. The present study provides a suitable material platform for further investigation of the interplay of the CDW and superconductivity.
Gate Tunable Supercurrent in Josephson Junctions Based on Bi$_{2}$Te$_{3}$ Topological Insulator Thin Films
Wei-Xiong Wu, Yang Feng, Yun-He Bai, Yu-Ying Jiang, Zong-Wei Gao, Yuan-Zhao Li, Jian-Li Luan, Heng-An Zhou, Wan-Jun Jiang, Xiao Feng, Jin-Song Zhang, Hao Zhang, Ke He, Xu-Cun Ma, Qi-Kun Xue, and Ya-Yu Wang
Chin. Phys. Lett. 2021, 38 (3): 037402 .   DOI: 10.1088/0256-307X/38/3/037402
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We report transport measurements on Josephson junctions consisting of Bi$_{2}$Te$_{3}$ topological insulator (TI) thin films contacted by superconducting Nb electrodes. For a device with junction length $L = 134$ nm, the critical supercurrent $I_{\rm c}$ can be modulated by an electrical gate which tunes the carrier type and density of the TI film. $I_{\rm c}$ can reach a minimum when the TI is near the charge neutrality regime with the Fermi energy lying close to the Dirac point of the surface state. In the p-type regime the Josephson current can be well described by a short ballistic junction model. In the n-type regime the junction is ballistic at 0.7 K $ < T < 3.8$ K while for $T < 0.7$ K the diffusive bulk modes emerge and contribute a larger $I_{\rm c}$ than the ballistic model. We attribute the lack of diffusive bulk modes in the p-type regime to the formation of p–n junctions. Our work provides new clues for search of Majorana zero mode in TI-based superconducting devices.
Superconductivity and Normal-State Properties of Kagome Metal RbV$_{3}$Sb$_{5}$ Single Crystals
Qiangwei Yin, Zhijun Tu, Chunsheng Gong, Yang Fu , Shaohua Yan , and Hechang Lei
Chin. Phys. Lett. 2021, 38 (3): 037403 .   DOI: 10.1088/0256-307X/38/3/037403
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We report the discovery of superconductivity and detailed normal-state physical properties of RbV$_{3}$Sb$_{5}$ single crystals with V kagome lattice. RbV$_{3}$Sb$_{5}$ single crystals show a superconducting transition at $T_{\rm c}\sim 0.92$ K. Meanwhile, resistivity, magnetization and heat capacity measurements indicate that it exhibits anomalies of properties at $T^{*}\sim 102$–103 K, possibly related to the formation of charge ordering state. When $T$ is lower than $T^{*}$, the Hall coefficient $R_{\rm H}$ undergoes a drastic change and sign reversal from negative to positive, which can be partially explained by the enhanced mobility of hole-type carriers. In addition, the results of quantum oscillations show that there are some very small Fermi surfaces with low effective mass, consistent with the existence of multiple highly dispersive Dirac band near the Fermi energy level.
Exotic Dielectric Behaviors Induced by Pseudo-Spin Texture in Magnetic Twisted Bilayer
Yu-Hao Shen, Wen-Yi Tong, He Hu, Jun-Ding Zheng, and Chun-Gang Duan
Chin. Phys. Lett. 2021, 38 (3): 037501 .   DOI: 10.1088/0256-307X/38/3/037501
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Twisted van der Waals bilayers provide an ideal platform to study the electron correlation in solids. Of particular interest is the 30$^{\circ}$ twisted bilayer honeycomb lattice system, which possesses an incommensurate moiré pattern, and uncommon electronic behaviors may appear due to the absence of phase coherence. Such a system is extremely sensitive to further twist and many intriguing phenomena will occur. Based on first-principles calculations we show that, for further twist near 30$^{\circ}$, there could induce dramatically different dielectric behaviors of electron between left and right-twisted cases. Specifically, it is found that the left and right twists show suppressed and amplified dielectric response under vertical electric field, respectively. Further analysis demonstrate that such an exotic dielectric property can be attributed to the stacking dependent charge redistribution due to twist, which forms twist-dependent pseudospin textures. We will show that such pseudospin textures are robust under small electric field. As a result, for the right-twisted case, there is almost no electric dipole formation exceeding the monolayer thickness when the electric field is applied. Whereas for the left case, the system could even demonstrate negative susceptibility, i.e., the induced polarization is opposite to the applied field, which is very rare in the nature. Such findings not only enrich our understanding on moiré systems but also open an appealing route toward functional 2D materials design for electronic, optical and even energy storage devices.
Observation of Ferroelastic and Ferroelectric Domains in AgNbO$_{3}$ Single Crystal
Wei Zhao, Zhengqian Fu, Jianming Deng, Song Li, Yifeng Han, Man-Rong Li, Xueyun Wang, and Jiawang Hong
Chin. Phys. Lett. 2021, 38 (3): 037701 .   DOI: 10.1088/0256-307X/38/3/037701
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Compared to AgNbO$_{3}$ based ceramics, the experimental investigations on the single crystalline AgNbO$_{3}$, especially the ground state and ferroic domain structures, are not on the same level. Here, based on successfully synthesized AgNbO$_{3}$ single crystal using a flux method, we observed the coexistence of ferroelastic and ferroelectric domain structures by a combination study of polarized light microscopy and piezoresponse force microscopy. This finding may provide a new aspect for studying AgNbO$_{3}$. The result also suggests a weak electromechanical response from the ferroelectric phase of AgNbO$_{3}$, which is also supported by the transmission electron microscope characterization. Our results reveal that the AgNbO$_{3}$ single crystal is in a polar ferroelectric phase at room temperature, clarifying its ground state which is controversial from the AgNbO$_{3}$ ceramic materials.
CROSS-DISCIPLINARY PHYSICS AND RELATED AREAS OF SCIENCE AND TECHNOLOGY
Phase Stability and Hydroxyl Vibration of Brucite Mg(OH)$_{2}$ at High Pressure and High Temperature
Wei-Bin Gui , Chao-Shuai Zhao , and Jin Liu
Chin. Phys. Lett. 2021, 38 (3): 038101 .   DOI: 10.1088/0256-307X/38/3/038101
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Brucite Mg(OH)$_{2}$ is an archetypal hydrous mineral and it has attracted a great deal of attention. However, little is known about the evolution of hydroxyl groups in brucite with respect to subduction fluids. We carried out Raman measurements up to 15.4 GPa and 874 K via an externally heated diamond anvil cell, investigating the stability of brucite under the conditions relevant to subducting slabs. The hydroxyl vibration mode $A_{\rm{1_g}}$(I) of brucite is weakened under simultaneous high pressure-temperature conditions. Meanwhile, the presence of carbonated solution can destabilize the hydroxyl groups of brucite at low pressure. Our results suggest that brucite releases water when reacting with hydrogen carbonate ion to form magnesite MgCO$_{3}$ in subduction zones. This implies that the global water cycle is largely coupled with the deep carbon cycle in Earth's interior.
Au Films Composed of Nanoparticles Fabricated on Liquid Surfaces for SERS
Xunheng Ye , Jiawei Shen , Xiangming Tao , Gaoxiang Ye , and Bo Yang
Chin. Phys. Lett. 2021, 38 (3): 038102 .   DOI: 10.1088/0256-307X/38/3/038102
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A series of Au films with different nominal deposition thickness $d$ were fabricated on ionic liquid surfaces by thermal evaporation at room temperature, taken as surface-enhanced Raman scattering (SERS) substrates. Au atoms deposited on the liquid surfaces can diffuse and aggregate randomly and eventually form films with ramified structure, which consist of nanoparticles (NPs). There are amounts of ultrasmall ($\sim $1 nm or smaller) nanogaps among the Au NPs, which can dramatically enhance Raman signal. Raman spectra of R6G were investigated with the assistance of the Au films. The results indicate that the Au films with higher thickness possess better SERS performance when $5.0 \le d \le 30.0$ nm. A random distribution model of Au NPs was used in the finite-difference time-domain method and the simulation results are in good agreement with the experimental findings.
A Linear Frequency Principle Model to Understand the Absence of Overfitting in Neural Networks
Yaoyu Zhang, Tao Luo, Zheng Ma, and Zhi-Qin John Xu
Chin. Phys. Lett. 2021, 38 (3): 038701 .   DOI: 10.1088/0256-307X/38/3/038701
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Why heavily parameterized neural networks (NNs) do not overfit the data is an important long standing open question. We propose a phenomenological model of the NN training to explain this non-overfitting puzzle. Our linear frequency principle (LFP) model accounts for a key dynamical feature of NNs: they learn low frequencies first, irrespective of microscopic details. Theory based on our LFP model shows that low frequency dominance of target functions is the key condition for the non-overfitting of NNs and is verified by experiments. Furthermore, through an ideal two-layer NN, we unravel how detailed microscopic NN training dynamics statistically gives rise to an LFP model with quantitative prediction power.
GEOPHYSICS, ASTRONOMY, AND ASTROPHYSICS
Klein–Nishina Effect and the Cosmic Ray Electron Spectrum
Kun Fang, Xiao-Jun Bi, Su-Jie Lin, and Qiang Yuan
Chin. Phys. Lett. 2021, 38 (3): 039801 .   DOI: 10.1088/0256-307X/38/3/039801
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Radiative energy losses are very important in regulating the cosmic ray electron and/or positron (CRE) spectrum during their propagation in the Milky Way. Particularly, the Klein–Nishina (KN) effect of the inverse Compton scattering (ICS) results in less efficient energy losses of high-energy electrons, which is expected to leave imprints on the propagated electron spectrum. It has been proposed that the hardening of CRE spectra around 50 GeV observed by Fermi-LAT, AMS-02, and DAMPE could be due to the KN effect. We show in this work that the transition from the Thomson regime to the KN regime of the ICS is actually quite smooth compared with the approximate treatment adopted in some previous works. As a result, the observed spectral hardening of CREs cannot be explained by the KN effect. It means that an additional hardening of the primary electrons spectrum is needed. We also provide a parameterized form for the accurate calculation of the ICS energy-loss rate in a wide energy range.
COMMENTS AND ERRATA
Erratum: A Ubiquitous Thermal Conductivity Formula for Liquids, Polymer Glass, and Amorphous Solids [Chin. Phys. Lett. 37 (2020) 104401]
Qing Xi, Jinxin Zhong, Jixiong He, Xiangfan Xu, Tsuneyoshi Nakayama, Yuanyuan Wang, Jun Liu, Jun Zhou, and Baowen Li
Chin. Phys. Lett. 2021, 38 (3): 039901 .   DOI: 10.1088/0256-307X/38/3/039901
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23 articles