Prof Yulin Chen

Abstract:

The discovery of superconductivity in pressurized Ruddlesden-Popper (RP) nickelates has provided new perspectives on the mechanism of high-temperature superconductivity. Up to now, most experiments concentrated on the lanthanum-related RP phase, so the discovery of new superconducting RP nickelates is highly desirable to reveal their generality. Here we report the observation of superconductivity in Pr4Ni3O10 single crystals above 10 GPa, achieving a maximum Tc of 39 K without saturation, significantly exceeding the value of 25–30 K of La4Ni3O10. Ultrasensitive magnetic susceptibility measurements under high pressure indicate bulk superconductivity with appreciable superconducting volume fractions. Unlike La4Ni3O10, the electronic structure of the high-pressure phase of Pr4Ni3O10 exhibits a dramatic metallization of the σ-bonding band consisting of three dz2$$d_{z^{2}}$$ orbitals and van Hove singularity of coupled bands of dx2−y2$$d_{x^{2}-y^{2}}$$ orbitals near the Fermi level, similar to La3Ni2O7. These findings reveal some generic features of both crystal and electronic structures for high-temperature superconductivity in nickelates and multi-layer cuprates.

Abstract:

Graphene‐based systems have emerged as a rich platform for exploring emergent quantum phenomena—including superconductivity, magnetism, and correlated insulating behavior—arising from flat electronic bands that enhance many‐body interactions. Realizing such flat bands has thus far relied primarily on moiré graphene superlattices or rhombohedral stacking graphene systems, both of which face challenges in reproducibility and tunability. Here, we introduce an artificial Kagome superlattice in bilayer graphene, engineered via nanopatterning of the dielectric substrate to create a precisely defined and electrostatically tunable periodic potential. Magnetotransport measurements reveal the emergence of a stack of correlated insulating states at moderate superlattice potentials, characteristic of strong electron–electron interactions within Kagome‐induced flat bands. As temperature increases, these correlated gaps collapse, signaling the thermal suppression of interaction‐driven states. Continuum‐model calculations confirm the formation of multiple flat minibands and reproduce the observed evolution of band reconstruction. Our results establish dielectric‐patterned graphene superlattices as a robust and controllable architecture for realizing flat‐band–induced correlated phenomena beyond moiré systems. We create a reproducible, gate‐tunable Kagome superlattice in bilayer graphene by nanopatterning the dielectric substrate. Magnetotransport reveals a stack of correlated insulating states that emerges at moderate superlattice potential and collapses with increasing temperature, consistent with interaction‐driven gaps in Kagome‐induced flat minibands. Continuum calculations reproduce the miniband flattening and band reconstruction trends.

Abstract:

The pursuit of emergent quantum phenomena lies at the forefront of modern condensed matter physics. A recent breakthrough in this arena is the discovery of the fractional quantum anomalous Hall effect (FQAHE) in twisted bilayer MoTe₂ (tbMoTe₂), marking a paradigm shift and establishing a versatile platform for exploring the intricate interplay among topology, magnetism, and electron correlations. While significant progress has been made through both optical and electrical transport measurements, direct experimental insights into the electronic structure – crucial for understanding and modeling this system – have remained elusive. Here, using spatially and angle-resolved photoemission spectroscopy (μ-ARPES), we directly map the electronic band structure of tbMoTe₂. We identify the valence band maximum, whose partial filling underlies the FQAHE, at the K points, situated approximately 150 meV above the Γ valley. By fine-tuning the doping level via in-situ alkali metal deposition, we also resolve the conduction band minimum at the K point, providing direct evidence that tbMoTe₂ exhibits a direct band gap – distinct from all previously known moiré bilayer transition metal dichalcogenide systems. These results offer critical insights for theoretical modeling and advance our understanding of fractionalized excitations and correlated topological phases in this emergent quantum material.

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.