Lorentz transmission electron microscopy for magnetic skyrmions imaging

Magnetic skyrmions have interesting properties,including their small size,topological stability,and extremely low threshold current for current-driven motion.Therefore,they are regarded as promising candidates for next-generation magnetic memory devices.Lorentz transmission electron microscopy(TEM)has an ultrahigh magnetic domain resolution(~2 nm),it is thus an ideal method for direct real-space imaging of fine magnetic configurations of ultra-small skyrmions.In this paper,we describe the basic principles of Lorentz-TEM and off-axis electron holography and review recent experimental developments in magnetic skyrmion imaging using these two methods.

Contactless Microwave Detection of Shubnikov–De Haas Oscillations in Three-Dimensional Dirac Semimetal ZrTe_5

We report Shubnikov–de Haas(SdH)oscillations of a three-dimensional(3D) Dirac semimetal candidate of layered material ZrTe_5 single crystals through contactless electron spin resonance(ESR)measurements with the magnetic field up to 1.4 T.The ESR signals manifest remarkably anisotropic characteristics with respect to the direction of the magnetic field,indicating an anisotropic Fermi surface in ZrTe_5.Further experiments demonstrate that the ZrTe_5 single crystals have the signature of massless Dirac fermions with nontrivialBerry phase,key evidence for 3D Dirac/Weyl fermions.Moreover,the onset of quantum oscillation of our ZrTe_5 crystals revealed by the ESR can be derived down to 0.2 T,much smaller than the onset of SdH oscillation determined by conventional magnetoresistance measurements.Therefore,ESR measurement is a powerful tool to study the topologically nontrivial electronic structure in Dirac/Weyl semimetals and other topological materials with low bulk carrier density.

Non-Monotonic Evolution of Carrier Density and Mobility under Thermal Cycling Treatments in Dirac Semimetal Cd_(3)As_(2) Microbelts

Tunable carrier density plays a key role in the investigation of novel transport properties in three-dimensional topological semimetals.We demonstrate that the carrier density,as well as the mobility,of Dirac semimetal Cd_(3)As_(2) nanoplates can be effectively tuned via in situ thermal treatment at 350 K for one hour,resulting in non-monotonic evolution by virtue of the thermal cycling treatments.The upward shift of Fermi level relative to the Dirac nodes blurs the surface Fermi-arc states,accompanied by an anomalous phase shift in the oscillations of bulk states,due to a change in the topology of the electrons.Meanwhile,the oscillation peaks of bulk longitudinal magnetoresistivity shift at high fields,due to their coupling to the oscillations of the surface Fermi-arc states.Our work provides a thermal control mechanism for the manipulation of quantum states in Dirac semimetal Cd_(3)As_(2) at high temperatures,via their carrier density.

Crystalline-amorphization-recrystallization structural transition and emergent superconductivity in van der Waals semiconductor SiP under compression

van der Waals(vdW)semiconductors have gained significant attention due to their unique physical properties and promising applications,which are embedded within distinct crystallographic symmetries.Here,we report a pressure-induced crystallineamorphization-recrystallization transition under compression in binary vdW semiconductor SiP.Upon compression to 52 GPa,bulk SiP undergoes a consecutive phase transition from pristine crystalline to amorphous phase,ultimately to recrystallized phase.By employing synchrotron X-ray diffraction experiments in conjunction with high-pressure crystal structure searching techniques,we reveal that the recrystallized Si P hosts a tetragonal structure(space group I4mm)and further transforms partially into a cubic phase(space group Fm3m).Consistently,electrical transport and alternating-current magnetic susceptibility measurements indicate the presence of three superconducting phases,which are embedded in separate crystallographic symmetries—the amorphous,tetragonal,and cubic structures.Furthermore,a high superconducting transition temperature of 12.3 K is observed in its recovered tetragonal phase during decompression.Our findings uncover a novel phase evolution path and elucidate a pressure-engineered structure-property relationship in vdW semiconductor SiP.These results not only offer a new platform to explore the transformation between different structures and functionalities,but also provide new opportunities for the design and exploration of advanced devices based on vdW materials.

Ultra-broadband,fast,and polarization-sensitive photoresponse of low-symmetry 2D NdSb_(2)

Broadband photodetectors with polarization-sensitive ability have received extraordinary attention for modern optoelectronic devices.Ideal photodetectors should possess high responsivity,fast response,and good stability,which are rare to meet at the same time in one low-symmetric two-dimentional(2D)material.In this work,neodymium diantimonides(RSb_(2)),a member of light rare-earth diantimonides RSb_(2)(R=La–Nd,Sm)with low-symmetry structure,is introduced as a fascinating highly anisotropic 2D material for broadband detection(532 nm to 4μm).The photodetector exhibits a responsivity of 0.49 mA·W^(−1)with 15μs response time at 532 nm and highly stable performance under ambient conditions over 8 months.Furthermore,we identify the polarization-sensitive photoresponse of the detector and demonstrate a high anisotropic factor~1.6.In addition,strong inplane anisotropy is revealed by anisotropic phonon response and the photodetection mechanism is investigated by scanning photocurrent microscopy measurements.This pioneer work on RSb_(2)paves the way for further exploration of 2D RSb_(2)for high performance polarized photodetectors with fast photothermoelectric response.

Current-driven transformations of a skyrmion tube and a bobber in stepped nanostructures of chiral magnets

Magnetic skyrmion tubes and bobbers are two types of different nanoscale spin configurations that can coexist in nanostructures of chiral magnets.They are then proposed to be utilized as binary bits to build racetrack memory devices.The ability to manipulate the two magnetic objects controllably by current in nanostructures is the prerequisite to realize the device.Here,we demonstrate by numerical simulations that a magnetic bobber and a skyrmion tube can be transformed to each other using spinpolarized current in nanostripes with stepped shape.We also show such stepped nanostructures can be readily applied as the write head for the skyrmion-bobber-based racetrack memory.

将磁斯格明子和反斯格明子通过磁布洛赫点四极子相连

类似于粒子-反粒子,斯格明子与反斯格明子具有相反的拓扑序参量.斯格明子-反斯格明子耦合态具有特殊的拓扑磁电特性,例如霍尔天平动力学,可以用来开发赛道存储等新型拓扑自旋电子学器件.本文结合理论计算与实空间观测手段,在一类焦点材料S_(4)对称FeNiPdP中报道了一种新型三维斯格明子-反斯格明子耦合态.通过磁布洛赫点四极子,斯格明子与反斯格明子能在厚度方向上缝合并堆垛在一起形成稳定的三维管状结构.作者提出了一种高效分辨斯格明子-反斯格明子管和斯格明子管的判据,澄清了潜在的磁结构分辨疑云,重新定义了该系列材料的应用前景.研究结果建立了S_(4)对称斯格明子材料的完整三维磁结构模型,提供了一种斯格明子-反斯格明子耦合新方案,将有力推动拓扑磁性理论的发展和拓扑自旋电子学器件的构筑.

利用轨道转移力矩实现室温范德华磁存储

具有非易失性的磁阻式随机存取存储器(MRAM)促进了诸如存内计算、神经形态计算和随机计算等在内的新兴应用.二维范德华异质结具有原子级平滑的界面和高度可调的物理性质,为MRAM的发展提供了新的技术路线.本文报道了基于WTe_2/Fe_(3)GaTe_(2)/BN/Fe_(3)GaTe_(2)异质结构的全二维范德华磁存储器件,实现了室温下数据读取和写入的全电学调控.其数据读取基于由Fe_(3)GaTe_(2)/BN/Fe_(3)GaTe_(2)构筑的磁隧道结,而数据写入是通过WTe_(2)中电流诱导的轨道磁矩极化来实现的.该磁矩对近邻的磁性层Fe_(3)GaTe_(2)施加力矩作用,称为轨道转移力矩(OTT)效应.与传统的自旋转移力矩和自旋轨道力矩相比,OTT效应充分利用了WTe_2中面外取向的轨道磁矩,通过施加界面电流实现无外磁场辅助的垂直磁化翻转.该研究结果表明,基于轨道转移力矩的OTT-MRAM极有希望作为低功耗、高性能的存储器件应用.

Magnetoresistance Oscillations of Ultrathin Pb Bridges

Pb nanobridges with a thickness of less than 10 nm and a width of several hundred nm have been fabricated from single-crystalline Pb fi lms using low-temperature molecular beam epitaxy and focus ion beam microfabrication techniques.We observed novel magnetoresistance oscillations below the superconducting transition temperature(TC)of the bridges.The oscillations which were not seen in the crystalline Pb fi lmsmay originate from the inhomogeneity of superconductivity induced by the applied magnetic fi elds on approaching the normal state,or the degradation of fi lm quality by thermal evolution.

De Haas–van Alphen study on three-dimensional topological semimetal pyrite PtBi2

We present the systematic de Haas–van Alphen(d Hv A) quantum oscillations studies on the recently discovered topological Dirac semimetal pyrite PtBi2 single crystals. Remarkable d Hv A oscillations are emerged at a low field about 1.5 T. From the analyses of the d Hv A oscillations, we extract the high quantum mobilities, light effective masses and phase shift factors for the Dirac fermions in pyrite PtBi2. From the angular dependence of the d Hv A oscillations, we map out the topology of the Fermi surface.Furthermore, we identify two additional oscillation frequencies that are not probed by the Sd H oscillations, which provides us with opportunities to further understand its Fermi surface topology.

Photophysical and electrical properties of organic waveguide nanorods of perylene-3,4,9,10-tetracarboxylic dianhydride

The single crystalline nanostructure of organic semiconductors provides a very promising class of materials for applications in modern optoelectronic devices. However, morphology control and optoelectronic property modulation of high quality single crystalline samples remain a challenge. Here, we report the morphology-controlled growth of single crystalline nanorod arrays of perylene- 3,4,9,10-tetracarboxylic dianhydride （IrFCDA）. We demonstrate that, unlike FTCDA film, PTCDA nanorods exhibits optical waveguide features, enhanced absorption, and Frenkel excitation emission in the visible region. Additionally, we measured the electrical properties of PTCDA nanorods, including the conductivity along the growth direction of the nanorod, which is roughly 0.61 S-m i （much higher than that of pure crystalline PTCDA films）.

Sign-reversed anomalous Nernst effect in the ferromagnetic Weyl-semimetal Fe_(3-δ)GeTe_(2): the role of Fe vacancies

Anomalous Nernst effect, as a thermal partner of anomalous Hall effect, is particularly sensitive to the Berry curvature anomaly near the Fermi level, and has been used to probe the topological nature of quantum materials. In this work, we report the observation of both effects in the ferromagnetic Weyl-semimetal Fe_(3-δ)GeTe_(2) with tunable Fe vacancies. With decreasing Fe vacancies,the anomalous Hall conductivity evolves as a function of the longitudinal conductivity from the hopping region to the region where the intrinsic Berry curvature contribution dominates. Concomitant evolutions in the anomalous Nernst signal and the anomalous off-diagonal thermoelectric coefficient are observed below the Curie temperature, displaying a unique sign change caused by the Fe vacancies. Combining these results with first-principles calculations, we argue that the Fe-vacancy concentration plays a unique role in simultaneously tuning the chemical potential and ferromagnetism, which in turn controls the Berry curvature contribution in this family of ferromagnetic topological semimetals.

Transport signatures of the topological surface state induced by the size effect in superconductor β-PdBi_(2)

The superconductivity and nontrivial topological electronic state are key hallmarks of topological superconductors.Here,we focus on the transport signals of possible topological surface state in the topological superconductor candidate β-PdBi_(2) nanoflake with a thickness of 21 nm.The resistance demonstrates a semiconductor-metal transition followed by an upturned behavior as the temperature decreases.A large and unsaturated longitudinal magnetoresistance(MR),accompanied by distinct Shubnikov-de Hass oscillation in Hall resistance,is observed.An analysis of Hall resistance reveals that the carriers present the characteristics of relativistic particles with small effective mass and extremely high mobility.The angle-dependent quantum oscillations demonstrate a two-dimensional Fermi surface topology.A giant anisotropic MR as large as 98%is detected when rotating the magnetic field.These results provide the possible transport signals of a nontrivial topological electronic state,establishing a further understanding of the topological properties of the low-dimensional topological superconductor candidate β-PdBi_(2).