Yijun Yu

All Publications

• Interlayer engineering of Fe3GeTe2: From 3D superlattice to 2D monolayer. Proceedings of the National Academy of Sciences of the United States of America Wu, Y., Wang, B. Y., Yu, Y., Li, Y., Ribeiro, H. B., Wang, J., Xu, R., Liu, Y., Ye, Y., Zhou, J., Ke, F., Harbola, V., Heinz, T. F., Hwang, H. Y., Cui, Y. 2024; 121 (4): e2314454121

  Abstract

  The discoveries of ferromagnetism down to the atomically thin limit in van der Waals (vdW) crystals by mechanical exfoliation have enriched the family of magnetic thin films [C. Gong et al., Nature 546, 265-269 (2017) and B. Huang et al., Nature 546, 270-273 (2017)]. However, compared to the study of traditional magnetic thin films by physical deposition methods, the toolbox of the vdW crystals based on mechanical exfoliation and transfer suffers from low yield and ambient corrosion problem and now is facing new challenges to study magnetism. For example, the formation of magnetic superlattice is difficult in vdW crystals, which limits the study of the interlayer interaction in vdW crystals [M. Gibertini, M. Koperski, A. F. Morpurgo, K. S. Novoselov, Nat. Nanotechnol. 14, 408-419 (2019)]. Here, we report a strategy of interlayer engineering of the magnetic vdW crystal Fe3GeTe2 (FGT) by intercalating quaternary ammonium cations into the vdW spacing. Both three-dimensional (3D) vdW superlattice and two-dimensional (2D) vdW monolayer can be formed by using this method based on the amount of intercalant. On the one hand, the FGT superlattice shows a strong 3D critical behavior with a decreased coercivity and increased domain wall size, attributed to the co-engineering of the anisotropy, exchange interaction, and electron doping by intercalation. On the other hand, the 2D vdW few layers obtained by over-intercalation are capped with organic molecules from the bulk crystal, which not only enhances the ferromagnetic transition temperature (TC), but also substantially protects the thin samples from degradation, thus allowing the preparation of large-scale FGT ink in ambient environment.

  View details for DOI 10.1073/pnas.2314454121

  View details for PubMedID 38232283

• Linear-in-temperature resistivity for optimally superconducting (Nd,Sr)NiO2. Nature Lee, K., Wang, B. Y., Osada, M., Goodge, B. H., Wang, T. C., Lee, Y., Harvey, S., Kim, W. J., Yu, Y., Murthy, C., Raghu, S., Kourkoutis, L. F., Hwang, H. Y. 2023; 619 (7969): 288-292

  Abstract

  The occurrence of superconductivity in proximity to various strongly correlated phases of matter has drawn extensive focus on their normal state properties, to develop an understanding of the state from which superconductivity emerges1-4. The recent finding of superconductivity in layered nickelates raises similar interests5-8. However, transport measurements of doped infinite-layer nickelate thin films have been hampered by materials limitations of these metastable compounds: in particular, a high density of extended defects9-11. Here, by moving to a substrate (LaAlO3)0.3(Sr2TaAlO6)0.7 that better stabilizes the growth and reduction conditions, we can synthesize the doping series of Nd1-xSrxNiO2 essentially free from extended defects. In their absence, the normal state resistivity shows a low-temperature upturn in the underdoped regime, linear behaviour near optimal doping and quadratic temperature dependence for overdoping. This is phenomenologically similar to the copper oxides2,12 despite key distinctions-namely, the absence of an insulating parent compound5,6,9,10, multiband electronic structure13,14 and a Mott-Hubbard orbital alignment rather than the charge-transfer insulator of the copper oxides15,16. We further observe an enhancement of superconductivity, both in terms of transition temperature and range of doping. These results indicate a convergence in the electronic properties of both superconducting families as the scale of disorder in the nickelates is reduced.

  View details for DOI 10.1038/s41586-023-06129-x

  View details for PubMedID 37438595

  View details for PubMedCentralID 7812792