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 Select Radiation force and torque on a two-dimensional circular cross-section of a non-viscous eccentric layered compressible cylinder in acoustical standing waves F G Mitri Chin. Phys. B, 2021, 30 (2): 024302.   DOI: 10.1088/1674-1056/abbbd9 Abstract （69）   HTML （0）    PDF （10104KB）（1067）       The purpose of this study is to develop an analytical formalism and derive series expansions for the time-averaged force and torque exerted on a compound coated compressible liquid-like cylinder, insonified by acoustic standing waves having an arbitrary angle of incidence in the polar (transverse) plane. The host medium of wave propagation and the eccentric liquid-like cylinder are non-viscous. Numerical computations illustrate the theoretical analysis with particular emphases on the eccentricity of the cylinder, the angle of incidence and the dimensionless size parameters of the inner and coating cylindrical fluid materials. The method to derive the acoustical scattering, and radiation force and torque components conjointly uses modal matching with the addition theorem, which adequately account for the multiple wave interaction effects between the layer and core fluid materials. The results demonstrate that longitudinal and lateral radiation force components arise. Moreover, an axial radiation torque component is quantified and computed for the non-absorptive compound cylinder, arising from geometrical asymmetry considerations as the eccentricity increases. The computational results reveal the emergence of neutral, positive, and negative radiation force and torque depending on the size parameter of the cylinder, the eccentricity, and the angle of incidence of the insonifying field. Moreover, based on the law of energy conservation applied to scattering, numerical verification is accomplished by computing the extinction/scattering energy efficiency. The results may find some related applications in fluid dynamics, particle trapping, mixing and manipulation using acoustical standing waves.
 Select High-resolution bone microstructure imaging based on ultrasonic frequency-domain full-waveform inversion Yifang Li(李义方), Qinzhen Shi(石勤振), Ying Li(李颖), Xiaojun Song(宋小军), Chengcheng Liu(刘成成), Dean Ta(他得安), and Weiqi Wang(王威琪) Chin. Phys. B, 2021, 30 (1): 014302.   DOI: 10.1088/1674-1056/abc7aa Abstract （416）   HTML （1）    PDF （9210KB）（798）       The main challenge in bone ultrasound imaging is the large acoustic impedance contrast and sound velocity differences between the bone and surrounding soft tissue. It is difficult for conventional pulse-echo modalities to give accurate ultrasound images for irregular bone boundaries and microstructures using uniform sound velocity assumption rather than getting a prior knowledge of sound speed. To overcome these limitations, this paper proposed a frequency-domain full-waveform inversion (FDFWI) algorithm for bone quantitative imaging utilizing ultrasonic computed tomography (USCT). The forward model was calculated in the frequency domain by solving the full-wave equation. The inverse problem was solved iteratively from low to high discrete frequency components via minimizing a cost function between the modeled and measured data. A quasi-Newton method called the limited-memory Broyden-Fletcher-Goldfarb-Shanno algorithm (L-BFGS) was utilized in the optimization process. Then, bone images were obtained based on the estimation of the velocity and density. The performance of the proposed method was verified by numerical examples, from tubular bone phantom to single distal fibula model, and finally with a distal tibia-fibula pair model. Compared with the high-resolution peripheral quantitative computed tomography (HR-pQCT), the proposed FDFWI can also clearly and accurately presented the wavelength scaled pores and trabeculae in bone images. The results proved that the FDFWI is capable of reconstructing high-resolution ultrasound bone images with sub-millimeter resolution. The parametric bone images may have the potential for the diagnosis of bone disease.
 Select Acoustic radiation force and torque on a lossless eccentric layered fluid cylinder F G Mitri Chin. Phys. B, 2020, 29 (11): 114302.   DOI: 10.1088/1674-1056/aba27a Abstract （140）   HTML    PDF （3021KB）（722）       Exact analytical equations and computations for the longitudinal and transverse acoustic radiation force and axial torque components for a lossless eccentric liquid cylinder submerged in a nonviscous fluid and insonified by plane waves progressive waves (of arbitrary incidence in the polar plane) are established and computed numerically. The modal matching method and the translational addition theorem in cylindrical coordinates are used to derive exact mathematical expressions applicable to any inner and outer cylinder sizes without any approximations, and taking into account the interaction effects between the waves propagating in the layer and those scattered from the cylindrical core. The results show that longitudinal and transverse radiation force components arise, in addition to the emergence of an axial radiation torque component acting on the non-absorptive compound cylinder due to geometrical asymmetry as the eccentricity increases. The computations demonstrate that the axial torque component, which arises due to a geometrical asymmetry, can be positive (causing counter-clockwise rotation in the polar plane), negative (clockwise rotation) or neutral (rotation cancellation) depending on the size parameter of the cylinder and the amount of eccentricity. Furthermore, verification and validation of the results have been accomplished from the standpoint of energy conservation law applied to scattering, and based on the reciprocity theorem.
 Select Quantization of the band at the surface of charge density wave material 2H-TaSe2 Man Li(李满), Nan Xu(徐楠), Jianfeng Zhang(张建丰), Rui Lou(娄睿), Ming Shi(史明), Lijun Li(黎丽君), Hechang Lei(雷和畅), Cedomir Petrovic, Zhonghao Liu(刘中灏), Kai Liu(刘凯), Yaobo Huang(黄耀波), and Shancai Wang(王善才) Chin. Phys. B, 2021, 30 (4): 047305.   DOI: 10.1088/1674-1056/abe9a8 Abstract （543）      PDF （1794KB）（647）       By using angle-resolved photoemission spectroscopy (ARPES) combined with the first-principles electronic structure calculations, we report the quantized states at the surface of a single crystal 2H-TaSe2. We have observed sub-bands of quantized states at the three-dimensional Brillouin zone center due to a highly dispersive band with light effective mass along kz direction. The quantized sub-bands shift upward towards EF while the bulk band at $\varGamma$ shifts downward with the decrease of temperature across charge density wave (CDW) formation. The band shifts could be intimately related to the CDW. While neither the two-dimensional Fermi-surface nesting nor purely strong electron-phonon coupling can explain the mechanism of CDW in 2H-TaSe2, our experiment may ignite the interest in understanding the CDW mechanism in this family.
 Select Numerical analysis of motional mode coupling of sympathetically cooled two-ion crystals Li-Jun Du(杜丽军), Yan-Song Meng(蒙艳松), Yu-Ling He(贺玉玲), and Jun Xie(谢军) Chin. Phys. B, 2021, 30 (7): 073702.   DOI: 10.1088/1674-1056/abfc3e Abstract （49）      PDF （1029KB）（610）       A two-ion pair in a linear Paul trap is extensively used in the research of the simplest quantum-logic system; however, there are few quantitative and comprehensive studies on the motional mode coupling of two-ion systems yet. This study proposes a method to investigate the motional mode coupling of sympathetically cooled two-ion crystals by quantifying three-dimensional (3D) secular spectra of trapped ions using molecular dynamics simulations. The 3D resonance peaks of the 40Ca+-27Al+ pair obtained by using this method were in good agreement with the 3D in- and out-of-phase modes predicted by the mode coupling theory for two ions in equilibrium and the frequency matching errors were lower than 2%. The obtained and predicted amplitudes of these modes were also qualitatively similar. It was observed that the strength of the sympathetic interaction of the 40Ca+-27Al+ pair was primarily determined by its axial in-phase coupling. In addition, the frequencies and amplitudes of the ion pair's resonance modes (in all dimensions) were sensitive to the relative masses of the ion pair, and a decrease in the mass mismatch enhanced the sympathetic cooling rates. The sympathetic interactions of the 40Ca+-27Al+ pair were slightly weaker than those of the 24Mg+-27Al+ pair, but significantly stronger than those of 9Be+-27Al+. However, the Doppler cooling limit temperature of 40Ca+ is comparable to that of 9Be+ but lower than approximately half of that of 24Mg+. Furthermore, laser cooling systems for 40Ca+ are more reliable than those for 24Mg+ and 9Be+. Therefore, 40Ca+ is probably the best laser-cooled ion for sympathetic cooling and quantum-logic operations of 27Al+ and has particularly more notable comprehensive advantages in the development of high reliability, compact, and transportable 27Al+ optical clocks. This methodology may be extended to multi-ion systems, and it will greatly aid efforts to control the dynamic behaviors of sympathetic cooling as well as the development of low-heating-rate quantum logic clocks.
 Select Search for topological defect of axionlike model with cesium atomic comagnetometer Yucheng Yang(杨雨成), Teng Wu(吴腾), Jianwei Zhang(张建玮), and Hong Guo(郭弘) Chin. Phys. B, 2021, 30 (5): 050704.   DOI: 10.1088/1674-1056/abf132 Abstract （413）      PDF （916KB）（594）       Many terrestrial experiments have been designed to detect domain walls composed of axions or axionlike particles (ALPs), which are promising candidates of dark matter. When the domain wall crosses over the Earth, the pseudoscalar field of ALPs could couple to the atomic spins. Such exotic spin-dependent couplings can be searched for by monitoring the transient-in-time change of the atomic spin precession frequency in the presence of a magnetic field. We propose here a single-species cesium atomic comagnetometer, which measures the spin precession frequencies of atoms in different ground-state hyperfine levels, to eliminate the common-mode magnetic-field variations and search for the exotic non-magnetic couplings solely between protons and ALPs. With the single-species atomic comagnetometer, we experimentally rule out the possibility that the decay constant of the linear pseudoscalar couplings of ALPs to protons is $f_{\rm p}\lesssim 3.71\times 10^{7}~\rm{GeV}$. The advanced system has the potential to constrain the constant to be $f_{\rm p}\lesssim 10.7\times 10^{9}~\rm{GeV}$, promising to improve astrophysical constraint level by at least one order of magnitude. Our system could provide a sensitive detection method for the global network of optical magnetometers to search for exotic physics.
 Select Transport property of inhomogeneous strained graphene Bing-Lan Wu(吴冰兰), Qiang Wei(魏强), Zhi-Qiang Zhang(张智强), and Hua Jiang(江华) Chin. Phys. B, 2021, 30 (3): 030504.   DOI: 10.1088/1674-1056/abe3e3 Abstract （473）   HTML （8）    PDF （1974KB）（576）       In analogy to real magnetic field, the pseudo-magnetic field (PMF) induced by inhomogeneous strain can also form the Landau levels and edge states. In this paper, the transport properties of graphene under inhomogeneous strain are studied. We find that the Landau levels have non-zero group velocity, and construct one-dimensional conducting channels. In addition, the edge states and the Landau level states in PMF are both fragile under disorder. We also confirm that the backscattering of these states could be suppressed by applying a real magnetic filed (MF). Therefore, the transmission coefficient for each conducting channel can be manipulated by adjusting the MF strength, which indicates the application of switching devices.
 Select Structural and electrical transport properties of Cu-doped Fe1 -xCuxSe single crystals He Li(李贺), Ming-Wei Ma(马明伟), Shao-Bo Liu(刘少博), Fang Zhou(周放), and Xiao-Li Dong(董晓莉) Chin. Phys. B, 2020, 29 (12): 127404.   DOI: 10.1088/1674-1056/abc3af Abstract （429）   HTML    PDF （859KB）（538）       We report the structural and electrical transport properties of Fe1 -xCuxSe (x = 0, 0.02, 0.05, 0.10) single crystals grown by a chemical vapor transport method. Substituting Cu for Fe suppresses both the nematicity and superconductivity of FeSe single crystal, and provokes a metal-insulator transition. Our Hall measurements show that the Cu substitution also changes an electron dominance at low temperature of un-doped FeSe to a hole dominance of Cu-doped Fe1 -xCuxSe at x = 0.02 and 0.1, and reduces the sign-change temperature (TR) of the Hall coefficient (R H).
 Select Evolution of domain structure in Fe3GeTe2 Siqi Yin(尹思琪), Le Zhao(赵乐), Cheng Song(宋成), Yuan Huang(黄元), Youdi Gu(顾有地), Ruyi Chen(陈如意), Wenxuan Zhu(朱文轩), Yiming Sun(孙一鸣), Wanjun Jiang(江万军), Xiaozhong Zhang(章晓中), and Feng Pan(潘峰) Chin. Phys. B, 2021, 30 (2): 027505.   DOI: 10.1088/1674-1056/abd693 Abstract （494）   HTML （4）    PDF （1057KB）（533）       Two-dimensional (2D) magnets provide an ideal platform to explore new physical phenomena in fundamental magnetism and to realize the miniaturization of magnetic devices. The study on its domain structure evolution with thickness is of great significance for better understanding the 2D magnetism. Here, we investigate the magnetization reversal and domain structure evolution in 2D ferromagnet Fe3GeTe2 (FGT) with a thickness range of 11.2-112 nm. Three types of domain structures and their corresponding hysteresis loops can be obtained. The magnetic domain varies from a circular domain via a dendritic domain to a labyrinthian domain with increasing FGT thickness, which is accompanied by a transition from squared to slanted hysteresis loops with reduced coercive fields. These features can be ascribed to the total energy changes from exchange interaction-dominated to dipolar interaction-dominated with increasing FGT thickness. Our finding not only enriches the fundamental magnetism, but also paves a way towards spintronics based on 2D magnet.
 Select Collective modes of Weyl fermions with repulsive S-wave interaction Xun-Gao Wang(王勋高), Huan-Yu Wang(王寰宇), Jiang-Min Zhang(张江敏), and Wu-Ming Liu(刘伍明) Chin. Phys. B, 2020, 29 (11): 117201.   DOI: 10.1088/1674-1056/abbbdb Abstract （357）   HTML    PDF （731KB）（413）       We calculate the spin and density susceptibility of Weyl fermions with repulsive S-wave interaction in ultracold gases. Weyl fermions have a linear dispersion, which is qualitatively different from the parabolic dispersion of conventional materials. We find that there are different collective modes for the different strengths of repulsive interaction by solving the poles equations of the susceptibility in the random-phase approximation. In the long-wavelength limit, the sound velocity and the energy gaps vary with the different strengths of the interaction in the zero sound mode and the gapped modes, respectively. The particle–hole continuum is obtained as well, where the imaginary part of the susceptibility is nonzero.
 Select Peierls-phase-induced topological semimetals in an optical lattice: Moving of Dirac points, anisotropy of Dirac cones, and hidden symmetry protection Jing-Min Hou(侯净敏) Chin. Phys. B, 2020, 29 (12): 120305.   DOI: 10.1088/1674-1056/abc0de Abstract （256）   HTML    PDF （1439KB）（395）       We propose a square optical lattice in which some of neighbor hoppings have a Peierls phase. The Peierls phase makes the lattice have a special band structure and induces the existence of Dirac points in the Brillouin zone, which means that topological semimetals exist in the system. The Dirac points move with the change of the Peierls phase and the Dirac cones are anisotropic for some vales of the Peierls phase. The lattice has a novel hidden symmetry, which is a composite antiunitary symmetry composed of a translation operation, a sublattice exchange, a complex conjugation, and a local U(1) gauge transformation. We prove that the Dirac points are protected by the hidden symmetry and perfectly explain the moving of Dirac points with the change of the Peierls phase based on the hidden symmetry protection.
 Select Superconductivity in an intermetallic oxide Hf3Pt4Ge2O Chengchao Xu(徐程超), Hong Wang(王鸿), Huanfang Tian(田焕芳), Youguo Shi(石友国), Zi-An Li(李子安), Ruijuan Xiao(肖睿娟), Honglong Shi(施洪龙), Huaixin Yang(杨槐馨), and Jianqi Li(李建奇) Chin. Phys. B, 2021, 30 (7): 077403.   DOI: 10.1088/1674-1056/abfb53 Abstract （298）      PDF （3238KB）（384）       Discovery of a new superconductor with distinct crystal structure and chemistry often provides great opportunity for further expanding superconductor material base, and also leads to better understanding of superconductivity mechanisms. Here, we report the discovery of superconductivity in a new intermetallic oxide Hf3Pt4Ge2O synthesized through a solid-state reaction. The Hf3Pt4Ge2O crystallizes in a cubic structure (space group Fm-3m) with a lattice constant of a = 1.241 nm, whose stoichiometry and atomic structure are determined by electron microscopy and x-ray diffraction techniques. The superconductivity at 4.1 K and type-Ⅱ superconducting nature are evidenced by the electrical resistivity, magnetic susceptibility, and specific heat measurements. The intermetallic oxide Hf3Pt4Ge2O system demonstrates an intriguing structural feature that foreign oxygen atoms can be accommodated in the interstitial sites of the ternary intermetallic framework. We also successfully synthesized a series of Hf3Pt4Ge2O1+δ (-0.25 ≤ δ ≤ 0.5), and found the δ-dependent superconducting transition temperature Tc. The atomic structure and the electronic structure are also substantiated by first-principles calculations. Our results present an entirely new family of superconductors with distinct structural and chemical characteristics, and could attract research interest in further finding new superconductors and exploring novel physics pertaining to the 5d-electron in these intermetallic compound systems.
 Select Modulation of the second-harmonic generation in MoS2 by graphene covering Chunchun Wu(吴春春), Nianze Shang(尚念泽), Zixun Zhao(赵子荀), Zhihong Zhang(张智宏), Jing Liang(梁晶), Chang Liu(刘畅), Yonggang Zuo(左勇刚), Mingchao Ding(丁铭超), Jinhuan Wang(王金焕), Hao Hong(洪浩), Jie Xiong(熊杰), and Kaihui Liu(刘开辉) Chin. Phys. B, 2021, 30 (2): 027803.   DOI: 10.1088/1674-1056/abd77f Abstract （286）   HTML （6）    PDF （864KB）（375）       Nonlinear optical frequency mixing, which describes new frequencies generation by exciting nonlinear materials with intense light field, has drawn vast interests in the field of photonic devices, material characterization, and optical imaging. Investigating and manipulating the nonlinear optical response of target materials lead us to reveal hidden physics and develop applications in optical devices. Here, we report the realization of facile manipulation of nonlinear optical responses in the example system of MoS2 monolayer by van der Waals interfacial engineering. We found that, the interfacing of monolayer graphene will weaken the exciton oscillator strength in MoS2 monolayer and correspondingly suppress the second harmonic generation (SHG) intensity to 30% under band-gap resonance excitation. While with off-resonance excitation, the SHG intensity would enhance up to 130%, which is conjectured to be induced by the interlayer excitation between MoS2 and graphene. Our investigation provides an effective method for controlling nonlinear optical properties of two-dimensional materials and therefore facilitates their future applications in optoelectronic and photonic devices.
 Select Density functional theory investigation on lattice dynamics, elastic properties and origin of vanished magnetism in Heusler compounds CoMnVZ (Z= Al, Ga) Guijiang Li(李贵江), Enke Liu(刘恩克), Guodong Liu(刘国栋), Wenhong Wang(王文洪), and Guangheng Wu(吴光恒) Chin. Phys. B, 2021, 30 (8): 083103.   DOI: 10.1088/1674-1056/ac0a6a Abstract （366）      PDF （1511KB）（360）       The lattice dynamics, elastic properties and the origin of vanished magnetism in equiatomic quaternary Heusler compounds CoMnVZ (Z=Al, Ga) are investigated by first principle calculations in this work. Due to the similar constituent atoms in CoMnVAl and CoMnVGa compounds, they are both stable in LiMgPdSn-type structure with comparable lattice size, phonon dispersions and electronic structures. Comparatively, we find that CoMnVAl is more structurally stable than CoMnVGa. Meanwhile, the increased covalent bonding component in CoMnVAl enhances its mechanical strength and Vickers hardness, which leads to better comprehensive mechanical properties than those of CoMnVGa. Practically and importantly, structural and chemical compatibilities at the interface make non-magnetic semiconductor CoMnVAl and magnetic topological semimetals Co2MnAl/Ga more suitable to be grown in heterostructures. Owing to atomic preferential occupation in CoMnVAl/Ga, the localized atoms Mn occupy C (0.5, 0.5, 0.5) Wyckoff site rather than B (0.25, 0.25, 0.25) and D (0.75, 0.75, 0.75) Wyckoff sites in LiMgPdSn-type structure, which results in symmetric band filling and consequently drives them to be non-magnetic. Correspondingly, by tuning localized atoms Mn to occupy B (0.25, 0.25, 0.25) or/and D (0.75, 0.75, 0.75) Wyckoff sites in off-stoichiometric Co-Mn-V-Al/Ga compounds and keeping the total valence electrons as 24, newly compensated ferrimagnetic compounds are theoretically achieved. We hope that our work will provide more choices for spintronic applications.
 Select Plasmonic properties of graphene on uniaxially anisotropic substrates Shengchuan Wang(汪圣川), Bin You(游斌), Rui Zhang(张锐), Kui Han(韩奎), Xiaopeng Shen(沈晓鹏, and Weihua Wang(王伟华) Chin. Phys. B, 2021, 30 (3): 037801.   DOI: 10.1088/1674-1056/abd168 Abstract （185）   HTML （2）    PDF （17038KB）（356）       Most of the current graphene plasmonic researches are based on the substrates with isotropic dielectric constant such as silicon. In this work, we investigate optical properties of graphene nanoribbon arrays placed on a uniaxially anisotropic substrate, where the anisotropy provides an additional freedom to tune the behaviors of graphene plasmons, and its effect can be described by a simple effective formula. In practice, the substrates of semi-infinite and finite thickness are discussed by using both the formula and full wave simulations. Particularly, the dielectric constants $\varepsilon_ \parallel$ and $\varepsilon_ \bot$ approaching zero are intensively studied, which show different impacts on the transverse magnetic (TM) surface modes. In reality, the hexagonal boron nitride (hBN) can be chosen as the anisotropic substrate, which is also a hyperbolic material in nature.
 Select Reversible waveform conversion between microwave and optical fields in a hybrid opto-electromechanical system Li-Guo Qin(秦立国), Zhong-Yang Wang(王中阳), Jie-Hui Huang(黄接辉), Li-Jun Tian(田立君), and Shang-Qing Gong(龚尚庆) Chin. Phys. B, 2021, 30 (6): 068502.   DOI: 10.1088/1674-1056/abea8f Abstract （285）      PDF （1493KB）（350）       We present a scheme of reversible waveform conversion between microwave and optical fields in the hybrid opto-electromechanical system. As an intermediate interface, nanomechanical resonator optomechanically couples both optomechanical cavities in the optical and microwave frequency domains. We find the double-optomechanically induced transparency and achieve coherent signal waveform bi-directional transfer between microwave and optical fields based on quantum interference. In addition, we give an analytical expression of one-to-one correspondence between the microwave field and the optical output field, which intuitively shows the reversible waveform conversion relationship. In particular, by numerical simulations and approximate expression, we demonstrate the conversion effects of the three waveforms and discuss the bi-directional conversion efficiency and the bandwidth. such a hybrid opto- and electro-mechanical device has significant potential functions for electro-optic modulation and waveform conversion of quantum microwave-optical field in optical communications and further quantum networks.
 Select A concise review of Rydberg atom based quantum computation and quantum simulation Xiaoling Wu(吴晓凌), Xinhui Liang(梁昕晖), Yaoqi Tian(田曜齐), Fan Yang(杨帆), Cheng Chen(陈丞), Yong-Chun Liu(刘永椿), Meng Khoon Tey(郑盟锟), and Li You(尤力) Chin. Phys. B, 2021, 30 (2): 020305.   DOI: 10.1088/1674-1056/abd76f Abstract （280）   HTML （12）    PDF （3178KB）（328）       Quantum information processing based on Rydberg atoms emerged as a promising direction two decades ago. Recent experimental and theoretical progresses have shined exciting light on this avenue. In this concise review, we will briefly introduce the basics of Rydberg atoms and their recent applications in associated areas of neutral atom quantum computation and simulation. We shall also include related discussions on quantum optics with Rydberg atomic ensembles, which are increasingly used to explore quantum computation and quantum simulation with photons.
 Select Twistronics in graphene-based van der Waals structures Ya-Ning Ren(任雅宁), Yu Zhang(张钰), Yi-Wen Liu(刘亦文), and Lin He(何林) Chin. Phys. B, 2020, 29 (11): 117303.   DOI: 10.1088/1674-1056/abbbe2 Abstract （332）   HTML    PDF （2345KB）（322）       The electronic properties of van der Waals (vdW) structures can be substantially modified by the moiré superlattice potential, which strongly depends on the twist angle among the compounds. In twisted bilayer graphene (TBG), two low-energy Van Hove singularities (VHSs) move closer with decreasing twist angles and finally become highly non-dispersive flat bands at the magic angle (∼ 1.1°). When the Fermi level lies within the flat bands of the TBG near the magic angle, Coulomb interaction is supposed to exceed the kinetic energy of the electrons, which can drive the system into various strongly correlated phases. Moreover, the strongly correlated states of flat bands are also realized in other graphene-based vdW structures with an interlayer twist. In this article, we mainly review the recent scanning tunneling microscopy (STM) advances on the strongly correlated physics of the magic-angle TBG (MATBG) and the small-angle twisted multilayer graphene. Lastly we will give out a perspective of this field.
 Select Correlated insulating phases in the twisted bilayer graphene Yuan-Da Liao(廖元达), Xiao-Yan Xu(许霄琰), Zi-Yang Meng(孟子杨), and Jian Kang(康健) Chin. Phys. B, 2021, 30 (1): 017305.   DOI: 10.1088/1674-1056/abcfa3 Abstract （305）   HTML （6）    PDF （1150KB）（318）       We review analytical and numerical studies of correlated insulating states in twisted bilayer graphene, focusing on real-space lattice models constructions and their unbiased quantum many-body solutions. We show that by constructing localized Wannier states for the narrow bands, the projected Coulomb interactions can be approximated by interactions of cluster charges with assisted nearest neighbor hopping terms. With the interaction part only, the Hamiltonian is SU(4) symmetric considering both spin and valley degrees of freedom. In the strong coupling limit where the kinetic terms are neglected, the ground states are found to be in the SU(4) manifold with degeneracy. The kinetic terms, treated as perturbation, break this large SU(4) symmetry and propel the appearance of intervalley coherent state, quantum topological insulators, and other symmetry-breaking insulating states. We first present the theoretical analysis of moir\'e lattice model construction and then show how to solve the model with large-scale quantum Monte Carlo simulations in an unbiased manner. We further provide potential directions such that from the real-space model construction and its quantum many-body solutions how the perplexing yet exciting experimental discoveries in the correlation physics of twisted bilayer graphene can be gradually understood. This review will be helpful for the readers to grasp the fast growing field of the model study of twisted bilayer graphene.
 Select Nodal superconducting gap in LiFeP revealed by NMR: Contrast with LiFeAs A F Fang(房爱芳), R Zhou(周睿), H Tukada, J Yang(杨杰), Z Deng(邓正), X C Wang(望贤成) , C Q Jin(靳常青), and Guo-Qing Zheng(郑国庆) Chin. Phys. B, 2021, 30 (4): 047403.   DOI: 10.1088/1674-1056/abec37 Abstract （214）      PDF （1513KB）（310）       Identifying the uniqueness of FeP-based superconductors may shed new lights on the mechanism of superconductivity in iron-pnictides. Here, we report nuclear magnetic resonance (NMR) studies on LiFeP and LiFeAs which have the same crystal structure but different pnictogen atoms. The NMR spectrum is sensitive to inhomogeneous magnetic fields in the vortex state and can provide the information on the superconducting pairing symmetry through the temperature dependence of London penetration depth ΛL. We find that Λ L saturates below T ∼ 0.2 T c in LiFeAs, where T c is the superconducting transition temperature, indicating nodeless superconducting gaps. Furthermore, by using a two-gaps model, we simulate the temperature dependence of ΛL and obtain the superconducting gaps of LiFeAs, as $\varDelta_1 = 1.2$ kB Tc and $\varDelta_2 = 2.8$ kB T c, in agreement with previous result from spin-lattice relaxation. For LiFeP, in contrast, Λ L does not show any saturation down to T ∼ 0.03 T c, indicating nodes in the superconducting gap function. Finally, we demonstrate that strong spin fluctuations with diffusive characteristics exist in LiFeP, as in some cuprate high temperature superconductors.
ISSN 1674-1056   CN 11-5639/O4