Prof Yulin Chen

Abstract:

In the two-dimensional limit, the interplay between Néel order and band topology in van der Waals topological antiferromagnets can give rise to novel quantum phenomena in the quantum anomalous Hall state. However, because of the absence of net magnetization in antiferromagnets, probing the energetically degenerate Néel orders has long remained a significant challenge. In this Letter, we demonstrate deterministic control over the Néel orders in MnBi2Te4 thin flakes through surface anisotropy engineering enabled by the AlOx capping layer. By tuning the surface anisotropy, we uncover parity-dependent symmetry breaking states that manifest as distinct odd-even boundary architectures, including 180° domain walls or continuous spin structures. Comparative studies between AlOx-capped and pristine odd-layer MnBi2Te4 flakes using domain-resolved magnetic force microscopy reveal pronounced differences in coercivity and magnetization-reversal dynamics. Notably, an unconventional giant exchange bias, which arises from perpendicular magnetic anisotropy, has been discovered. Our findings establish a pathway for manipulating Néel order through surface modification in topological antiferromagnets.

Abstract:

Since the discovery of high-temperature superconductivity, studying the upper critical field and its anisotropy has been crucial for understanding the superconducting mechanism and guiding applications. Here, we perform in situ high-pressure angular-dependent electrical transport measurements on Pr₄Ni₃O₁₀ single crystals using a custom diamond anvil cell (DAC) rotator, confirming its anisotropic superconductivity. The anisotropy parameter γ, derived from the upper critical fields (μ₀H_c2) for H⊥ab and H//ab, is approximately 1.6, decreasing with increasing temperature and approaching 1 near T_c. Comparing effective mass anisotropy and interblock distance in cuprates and iron-based superconductors (FeSCs) reveals that Pr₄Ni₃O₁₀ single-crystal superconductors are consistent with a two-band model, where intralayer quantum confinement within the unit cell induces interlayer coherence, thereby leading to three-dimensional (3D) superconductivity. This study not only establishes the existence of weakly anisotropic superconductivity in bulk Ruddlesden-Popper nickelates but also provides critical insight into the role of dimensionality in high-temperature superconductivity.

Abstract:

Magnetic kagome materials provide a platform for exploring magneto-transport phenomena, symmetry breaking and charge ordering driven by the intricate interplay among electronic structure, topology and magnetism. Yet geometric frustration in conventional kagome magnets limits their tunability. Here we propose a design strategy for interweaving quasi-one-dimensional magnetic Tb zigzag chains with non-magnetic Ti-based kagome bilayers in TbTi3Bi4. Comprehensive spectroscopic analyses reveal coexisting elliptical-spiral magnetic and spin-density-wave orders accompanied by a large ~90 meV band-folding gap. The combined magnetic and electronic state leads to a giant anomalous Hall conductivity of 105 Ω−1 cm−1, which exceeds that observed in frustrated kagome analogues. These results establish TbTi3Bi4 as a model system of magnetic kagome metals with strong electron–magnetism interactions and underscore the necessity of interweaving designed magnetic and charge layers separately to achieve tunable transport properties. This design strategy will enable the discovery of emergent quantum states and next-generation electronic materials.

Abstract:

Based on high-throughput screening and experimental data, we find that CeCo2P2 is unique in heavy-fermion materials: It has a Kondo effect at a high temperature which is nonetheless below a Co antiferromagnetic ordering temperature. This begs the question: How is the Kondo singlet formed? All other magnetic Kondo materials do not first form magnetism on the atoms whose electrons are supposed to screen the local moments. We theoretically explain these observations and show the multifaceted uniqueness of CeCo2P2: a playground for Kondo, magnetism, flat band, and topological physics. At high temperatures, the itinerant Co c electrons of the system form nonatomic bands with a narrow bandwidth, leading to a high antiferromagnetic transition temperature. We show that the quantum geometry of the bands promotes in-plane ferromagnetism, while the weak dispersion along the z direction facilitates out-of-plane antiferromagnetism. At low temperatures, we uncover a phase that manifests the coexistence of Co antiferromagnetism and the Kondo effect, linked to the PT-protected Kramers' doublets and the filling-enforced metallic nature of c electrons in the antiferromagnetic phase. Subsequently, the emergence of the Kondo effect, in cooperation with glide-mirror-z symmetry, creates nodal-line excitation near the Fermi energy. Our results emphasize the importance of lattice symmetry and quantum geometry, Kondo physics, and magnetism in the understanding of the correlation physics of this unique compound. We also test our theory on the structurally similar compound LaCo2P2 and show how we are able to understand its vastly different phase diagram.