Prof Yulin Chen

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.

Abstract:

Superconducting radio frequency (SRF) cavities constitute the cornerstone of high-efficiency particle accelerators. While traditional bulk niobium cavities have dominated the field, copper substrates with niobium films deposited inside the cavity represent a transformative approach for cost reduction and thermal management. However, achieving conformal superconducting films on complex cavity geometries remains a fundamental challenge, especially on the adhesive behavior of the film. Here, we present a breakthrough high-power impulse magnetron re-sputtering/sputtering (HiPIMRS) system engineered for uniform Nb film depositions on 1.3 GHz copper cavity interiors. Through a re-sputtering process on the copper substrates prior to deposition, we achieve atomic-scale interfacial integrity and eliminate interfacial oxides or degradation. Energy-dispersive x-ray spectroscopy confirms an oxide-free Nb/Cu interface, and atomic force microscopy reveals ultra-smooth surfaces (R_a < 20 nm for 3 μm films). Crucially, electrical transport measurements show that the niobium film has a critical temperature of 8.5 K throughout the cavity interior. XRD demonstrates a (110)-oriented crystalline structure. This work establishes HiPIMRS as a viable pathway for next-generation SRF cavity production, with interfacial engineering protocols offering generational advancements in film conformity and superconducting performance.

Abstract:

Shandite kagome materials have attracted great research attention due to their intriguing properties, such as the magnetic Weyl semimetal phase, endless nodal lines, and pressure-induced superconductivity. In this work, by combining angle-resolved photoemission spectroscopy and ab initio calculation, we systematically investigate the electronic band structure of the shandite kagome compound Ni₃In₂Se₂. The measured band structure is in good agreement with ab initio calculations including the spin-orbit coupling effect. The experimental spectra are predominantly characterized by surface resonance states exhibiting highly anisotropic band dispersions near the Fermi level. These features dominate the electronic states near the Fermi level, which are likely associated with the anisotropic transport properties observed in Ni₃In₂Se₂. Notably, the large spin-orbit coupling in this material leads to the formation of a massive Dirac-like band dispersion in the surface resonance states, contrasting with the gapless Dirac dispersion found in the surface states of its sister compound Ni₃In₂Se₂. Our work will help understand the influence of the spin-orbit coupling effect on both the surface and bulk electronic states of shandite compounds. Furthermore, it establishes a foundation for exploring the potential applications of surface resonance states in surface science.