In collaboration with the Institute of Physics, the latest recognition of spinel oxide superconductors

LiTi2O4 (LTO) is the only oxide superconductor with spinel structure discovered so far. Its superconductivity is dominated by the 3d electrons of Ti atoms. There is currently no high-quality LTO single crystal, the specific heat data obtained on polycrystalline samples and the Andreev reflection spectrum show the experimental characteristics of traditional BCS electro-acoustic interaction superconductors, but soft X-ray scattering and nuclear magnetic resonance measurements were found in the system. There is a strong electronic-electronic relationship. So is it similar to other high-temperature superconductors, spin/orbit fluctuations also play an important role in the LTO superconducting mechanism?

Another typical feature of LTO is that its basic unit of Ti atoms has a tetrahedral configuration, which is not conducive to the appearance of long-range antiferromagnetic sequences. Copper-based and iron-based superconducting materials, which are familiar to everyone, usually have a high superconducting transition temperature after breaking the long process. Therefore, an in-depth study of LTO is an effective complement to high-temperature superconductors such as copper-based and iron-based ones, which is conducive to a comprehensive understanding of the role of 3D electrons in superconductivity.

Recently, the Institute of Physics, Chinese Academy of Sciences/Beijing National Laboratory for Condensed Matter State Superconducting State Key Laboratory of Gold Condensation and the Professor Takeuchi Group of the University of Maryland team, using pulsed laser coating technology in the (001)-Mg Al2O4-based A high-quality LTO single crystal thin film was prepared on-chip, and the electrical transport properties under magnetic field and point contact tunnel spectra were comprehensively studied.

In the normal state, it was found for the first time that under 50 K LTO showed a significant anisotropic positive magnetic reluctance, while above 50 K it showed an isotropic negative reluctance. If the magnetic field is rotated along the plane, it is found that its in-plane magnetoresistance is doubled below 100 K, and the intensity of this duality has a jump around 50 K. Abnormal magnetoresistance behavior must be accompanied by competition between different factors. In the LTO system there is a competition between orbital order and spin orbital fluctuations. The magnetic susceptibility results further support that spin fluctuations dominate below 100 K, magnetic fields suppress fluctuations and cause negative magnetoresistance; orbital correlations below 50 K The sequence dominates, increasing the magnetic field results in a positive magnetoresistance. In the superconducting state, the tunneling spectrum study for the first time shows that the LTO superconducting energy gap decreases linearly with the square of the magnetic field. The previous theoretical model did not predict this rule. However, if there is a symmetry breaking of the electronic state in the LTO, the dependence of the energy gap and the square of the magnetic field can be obtained based on the GL theory. This further supports the existence of orbital correlations below 50 K (as shown in the figure).

At present, the mainstream view is that copper oxide high-temperature superconductors originate from spin fluctuations, whereas spin-fluctuations in LTO systems are far away from the superconductor region, which may be the reason why high-temperature superconductivity is not observed in LTO. . Also involved in this work were the superconductor room researcher Dan Lei, a professor at Loughborough University, a theorist Kusmartsev, and the University of Maryland professor Greene. Part of the results of this work was published in Nature Communications 6, 7183 (2015) and supported by the National Natural Science Foundation of China and the Chinese Academy of Sciences Class B pilot project.

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