Nanjing University has made important progress in the research of high-temperature superconducting mechanism
Recently, the team of Professor Wen Haihu and Professor Yang Huan and their collaborators directly observed the newly emerging incommensurate antiferromagnetic phase in a copper oxide high-temperature superconductor doped with iron impurities. The research results were published online on December 21, 2021 in the international core journal Proceedings of the National Academy of Sciences [PNAS 118, No. 51 e2115317118, with the title "Direct visualization of a static incommensurate antiferromagnetic order in Fe-doped Bi2Sr2CaCu2O8+δ" (2021)] on.
The work was completed by the team of Professor Wen Haihu and Professor Yang Huan in cooperation with the theoretical collaborator, Professor Eremin's team from Ruhr University in Bochum, Germany. Professor Gu Gen from the Brookhaven National Laboratory in the United States provided high-quality samples. Nanjing University It is the unit of the first author and the first corresponding author. The co-first authors of the article are Wan Siyuan, Li Huazhou, Peayush Choubey and Gu Qiangqiang; the corresponding authors are Professor Yang Huan, Professor Eremin and Professor Wen Haihu. This work was supported by the Natural Science Foundation of China, the "Quantum Control Program" of the Ministry of Science and Technology, and the "Collaborative Innovation Center for Artificial Microstructure Science and Technology" in the 2011 program. I would like to express my gratitude.
The mechanism of high-temperature superconductivity is one of the 125 major scientific issues currently facing mankind selected by Science magazine. The understanding of it will fundamentally break through Bardeen-Cooper- which was founded in 1957 and has dominated the field of superconductivity for more than 60 years. The scope of Schrieffer (BCS) theory will bring a profound revolution in basic science and the development of applications.
Since the discovery of copper oxide high-temperature superconductivity in 1986, its mechanism has always been an important research content in condensed matter physics. Due to the electronic relevance, the parent body of the copper oxide superconductor exhibits insulation, and its ground state is an antiferromagnetic Mott insulator, which is no longer described by the well-known solid-state physical energy band theory. With the doping of hole carriers, the strongly correlated electrons are loosened, so the long-range antiferromagnetic sequence is destroyed and only short-range antiferromagnetic fluctuations are left, and the antiferromagnetic fluctuations are thought to be high-temperature hyperthermia. The driving force for pairing of conductive electrons. In order to further study the internal relationship between high-temperature superconductivity and antiferromagnetism, the team of Professor Wen Haihu and Yang Huan of the School of Physics used the spin-polarized scanning tunnel developed by themselves on the iron-doped copper oxide high-temperature superconductor Bi2Sr2CaCu2O8+δ. With microscopy, an incommensurate antiferromagnetic sequence was directly observed near the iron impurity. This work has fully revealed the close relationship between superconductivity and antiferromagnetism in copper oxides, and imposed strong restrictions on various models of unconventional high-temperature superconducting mechanisms.
The results are described in detail: Through tunnel spectrum measurement, they found that iron impurities in the Bi-2212 superconductor destroyed the superconducting coherence near it, but did not have much influence on the characteristics of the pairing, which itself gave important enlightenment. Furthermore, they used their own spin-polarized scanning tunneling spectroscopy technology to carry out a series of quasi-particle coherent scattering measurements on the samples. Figure 1A and B show the measurement of quasi-particle coherent scattering in the same area with a positive and negative magnetic field polarized tip. The bright spot is the doped iron atoms. There is basically no difference between the two figures, indicating that the non-spin-related charge signal is dominant. The Fourier transform results (Figure 1C, D) are similar to those measured by ordinary needle tips. Above the superconducting energy gap, Q" scattering patterns in the charge modulation can be seen. If you subtract the data in Figure 1A and B, you can get The related information of spin resolution, as shown in Figure 1E, around the iron atom, a similar antiferromagnetic signal can be found between the positive and negative phases. Figure 1F is the result of the Fourier transform corresponding to Figure 1E, which is approximately equal to (0.85, 0.85)π At the position of the /a0 wave vector, four new static antiferromagnetic patterns appear, because they do not match the lattice parameters, indicating that this is an incommensurate antiferromagnetic sequence.
Figure 1 (A, B) The coherent scattering spectrum of quasi-particles obtained by polarizing the tip with positive and negative magnetic fields, and their Fourier transform intensity diagrams are shown in (C, D). (E) The spin-resolved quasi-particle coherent scattering image obtained by subtracting the two images A and B, there are alternating positive and negative antiferromagnetic patterns near the position of the Fe atom (green five-pointed star). (F) The intensity map after Fourier transform of E, the scattering speckle Q" of charge modulation in C and D disappears, and the antiferromagnetic scattering speckle Q0 newly appears.
After that, they continued to perform high-pixel measurement and magnification analysis of this incommensurate antiferromagnetic sequence (Figure 2A-C). In fact, the spatial period is slightly larger than the period of the lattice lattice (the black dots in Figure 2B). For the analysis of the spin-resolved quasi-particle coherent scattering signal after Fourier transform of different energy measurements, it can be found that the position of this antiferromagnetic sequence in the momentum space with a wide energy range of 25 to 150 meV hardly moves, that is, It is said that there is basically no energy dispersion. The theoretical collaborator, Professor Eremin of Ruhr University, Germany, through the assumption of a single-band Hubbard model, plus d-wave superconductivity and antiferromagnetic fluctuations, and finally found that weak magnetic impurities will stabilize the scattering between diagonal nodes The antiferromagnetic sequence of the wave vector (shown by the black arrow in Figure 2F), so the theoretical calculation and our experimental data are basically consistent.
Figure 2 The spatial spin-resolved quasi-particle coherent scattering signal (B) measured in a small area (A), you can see the obvious spatial oscillation of the antiferromagnetic sequence (C), and the result of the Fourier transform (D) can be seen To obvious speckles, this antiferromagnetic sequence appears in a large range above the energy gap, and there is no dispersion (E). The theoretical calculation results (G-J) are basically consistent with the experimental results.
This work opened up a successful example of spin-resolved scanning tunneling microscopy in the study of copper oxide superconductors, and successfully visualized a new incommensurate antiferromagnetic sequence, which was highly praised by the reviewers as copper oxide An important milestone in physical research (milestone). In the nearly optimally doped copper oxide superconductor, the iron impurity not only suppresses the superconducting coherence, but also produces a new incommensurable antiferromagnetic sequence, revealing the close relationship between high temperature superconducting and antiferromagnetic The internal relationship strongly restricts many models of the high-temperature superconducting mechanism currently appearing. This result provides important experimental evidence and theoretical support for the in-depth understanding of the antiferromagnetic correlation in the unconventional superconducting mechanism.
Paper link:https://doi.org/10.1073/pnas.2115317118
