2023year2moon22On the same day, the team of Academician Zhu Jing from Tsinghua University, together with Professor Che Renchao from Fudan University and Associate Professor Li Yuan from Peking University, published an article titled "Nature" in the journal” Topological spin texture in the pseudogap phase of a high-Tc superconductor”^[1] The article. This research was conducted using a Thermo Fisher transmission electron microscope（TEM）For the first time in the pseudo bandgap stateYBa₂Cu₃O_6.5The existence of topological magnetic vortex structures has been discovered in the material. The discovery of this topological magnetic vortex structure provides direct image evidence for the temporal inversion symmetry breaking under pseudo gap states at the microscale in real space. It was also found that the topological magnetic vortex structure was destroyed during charge density wave states and reappeared when entering superconducting states. This discovery is of great significance for revealing the microscopic mechanism of high-temperature superconductivity, and advanced transmission electron microscopy plays an indispensable role in this discovery. Academician Zhu Jing, Professor Che Renchao, and others have been deeply involved in the research of superconducting materials, Lorentz low-temperature in-situ transmission electron microscopy, and electron microscopy for many years, and have achieved a series of important research results. In this study, the research team utilized the newly installed electron microscope laboratory at Fudan UniversitySpectra 300The transmission electron microscope conducted low-temperature Lorentz sample testing and made this significant discovery.

2021In the year, Thermo Fisher Shanghai Nanoport（Shanghai NanoPort, Thermo Fisher Scientific）I am fortunate to have participated in some of the experimental work and actively cooperated and supported in creating a frozen experimental environment and collecting in-situ data. This article will mainly introduce two electron microscopy techniques——Lorentz transmission electron microscope（LTEM）Integral differential phase contrast（iDPC）The key role played in this job.

Lorentz transmission electron microscope（LTEM）

normalTEMUnder the optical path, the objective lens is in the open state, and the sample is in the middle of the upper and lower polar shoes of the objective lens~2TIn a strong magnetic field, the intrinsic magnetic structure of the sample will be disrupted by the strong magnetic field of the objective lens. In order to observe the intrinsic magnetic structure of the sample in a non-magnetic environment, Thermo Fisher Field Emission Transmission Electron Microscopy was usedTalosCorrection of spherical aberration using transmission electron microscopySpectraThe sample can be placed in a zero magnetic field environment by turning off the objective lens current, and then the microscopic intrinsic magnetic structure of the sample can be observed by the Lorentz magnetic lens located inside the pole shoe under the objective lens.LTEMThere are two main imaging modes:FresnelImaging mode andFoucaultImaging mode.FresnelThe imaging mode is achieved by changing the defocus amount of the image to observe magnetic domains or domain walls. The main characteristic of its image is that the contrast of magnetic domain walls is opposite under under and over focus conditions, while the positive focus image has no magnetic contrast.FoucaultImaging is achieved by masking or preserving diffraction signals related to magnetic domains on the back focal plane (similar to dark field imaging),Suitable for observing magnetic domains with different magnetization orientations. image1a-cPlease refer to the pseudo bandgap states mentioned in this articleYBa₂Cu₃O_6.5Under the conditions of positive focus, over focus, and under focus of the sampleFresnelImage, defocus amount is±1.08 mmThe contrast characteristics of its inversion effectively prove the existence of topological domain structures in the sample. In addition, the Lorentz function on the Thermo Fisher transmission electron microscope can not only achieve a non-magnetic environment, but also easily change the magnetic field by changing the objective current, which is used for in-situ study of the variation of magnetic structure with magnetic field strength. In this study, the author applied an external magnetic field to the sample by changing the objective current, and the topological features disappeared, further proving that this effect is caused by magnetic properties. The author uses the strength transfer equation（Transport of Intensity Equation, TIE）Phase reconstruction technology^[2]Yes, that's rightLTEMThe image is processed to obtain the magnetic field direction and relative intensity distribution of the topological magnetic vortex structure (Figure)1d-e, i-l）. image1m-nis caused byLTEMTwo possible schematic diagrams of magnetic vortex structures inferred from the results. In this articleLTEMThe experiments were conducted at Thermo Fisher ScientificSpectra300，ThemisandTitanThe machine underwent repeated verification and observed topological magnetic vortex structures.

image1 （a-c）LTEM FresnelPseudo bandgap state in modeYBa₂Cu₃O_6.5The positive focus, over focus, and under focus images of the sample (with a defocus amount of)±1.08 mm）The sample is in300 KIn a zero magnetic field environment, the ruler is500 nm；（d-e）To pass throughTIEThe magnetic field and magnetic field intensity images obtained by the algorithm; （f-j）Crop and enlarge the image corresponding to the red box; （k-l）A magnetic field and magnetic field intensity map for a single magnetic vortex structure; （m-n）Schematic diagram of two possible topological magnetic vortex structures^[1]

Besides the regular onesLTEMIn addition to imaging, Thermo Fisher aberration correction transmission electron microscopySpectraThe series can be corrected by the objective lens aberration correctorLTEMPerform aberration correction on the optical axis. Aberration correction in Lorentz mode can achieve better results than1nmThe information resolution helps researchers observe smaller magnetic structures.

Integral differential phase contrast（iDPC）

The ultra-high spatial resolution of aberration corrected transmission electron microscopy provides more information about the relationship between the appearance of topological spin structures and local crystal structures. The doping or absence of oxygen atoms in copper based superconducting materials has a significant impact on material properties, and directly observing the occupation of oxygen atoms is of great significance for revealing the relationship between material microstructure and properties. However, the widely used scanning transmission electron microscope（STEM）The high angle circular dark field（HAADF）The image, due to its main reception of high angle Rutherford scattering signals, cannot simultaneously image light and heavy elements,CTheNTheOLight atoms cannot be observed.STEMCircular bright field（ABF）Although light elements can be observedABFThe image cannot be directly interpreted, and there are issues such as high requirements for sample thickness and poor signal-to-noise ratio in the image. In order to solve the above problems, Thermo Fisher Scientific proposed and developed integral differential phase contrast（iDPC）Technology.iDPCThis brand newSTEMThe emergence of imaging modes greatly improves the ability of transmission electron microscopy to capture atoms.iDPCThe technology has the advantages of achieving simultaneous imaging of light and heavy atoms, low electron dose, high resolution and high signal-to-noise ratio imaging, and easy interpretation of image contrast^[3]Currently,iDPCTechnology has become a hot topic in the field of material characterization, with important applications in characterizing light element occupancy, two-dimensional materials, electron beam sensitive materials, superconductors, and other fields.iDPCImaging technology has now been integrated into the Thermo Fisher aberration corrected electron microscopeSpectraField emission electron microscopyTalosUp, can be achievediDPCOnline collection and display of images.

image2 (a) YBa₂Cu₃O_6.0，(b) YBa₂Cu₃O_6.5and(c) YBa₂Cu₃O_6.9Atomic resolutioniDPCimage^[1]

image2forYBa₂Cu₃O_6.0TheYBa₂Cu₃O_6.5andYBa₂Cu₃O_6.9High resolutioniDPCThe image clearly shows the position of oxygen atoms, and with different oxygen doping contents,Cu-OThe oxygen occupancy on the chain gradually increases. It is worth noting that the pseudo bandgap stateYBa₂Cu₃O_6.5ofCu-OThere is an ordered arrangement of oxygen enrichment and oxygen deficiency on the chain. The author believes that this ordered arrangement of oxygen is beneficial for the topological magnetic vortex structure alongcThe free arrangement of axes is the best area for observing magnetic vortex structures. The author believes that at present, the possibility of oxygen filled chains exciting magnetism cannot be ruled out.

Thermo Fisher Scientific will be committed to the research and application of related electron microscopy technologies, providing stronger assistance for the study of the electrical and magnetic properties of materials.

Author: Liu Jian

References

[1] Zechao Wang, Ke Pei, Liting Yang, Chendi Yang, Guanyu Chen, Xuebing Zhao, Chao Wang, Zhengwang Liu, Yuan Li, Renchao Che & Jing Zhu. Topological spin texture in the pseudogap phase of a high-Tc superconductor. Nature (2023). https://doi.org/10.1038/s41586-023-05731-3

[2] M. Beleggia, M.A. Schofield, V.V. Volkov, Y. Zhu. On the transport of intensity technique for phase retrieval. Ultramicroscopy 102 (2004) 37–49.

[3] Ivan Lazi? , Eric G.T. Bosch and Sorin Lazar. Phase contrast STEM for thin samples: Integrated differential phase contrast. Ultramicroscopy 160, 265-280 (2016).