Prof Yulin Chen

Abstract:

The superconducting diode effect (SDE) is a fascinating nonreciprocal phenomenon where the critical current is different for opposite current directions. It is widely believed that realizing SDE requires breaking both inversion symmetry and time-reversal symmetry (TRS), which are usually achieved via heterostructure engineering and applying external magnetic fields. Here, we report a pressure-induced magnetic-field-free SDE in NbSe2 flakes without any heterostructures. We show that pressure alone breaks the inversion symmetry, as confirmed by the second harmonic generation. Crucially, upon applying an out-of-plane magnetic field (B), the SDE exhibits even-in-B behavior, implying the absence of explicit TRS breaking. This finding challenges the prevailing theoretical paradigm and demonstrates that a magnetic-field-free SDE can emerge without explicitly breaking TRS. Thereby, our Letter establishes pressure engineering as a powerful tool for inducing nonreciprocal superconductivity and designing versatile, magnetic-field-free superconducting devices.

Abstract:

The interplay among orbital-selective electron correlation, Hund's coupling, tunable structural motifs, and Kondo-like scattering establishes a compelling paradigm for understanding and engineering correlated multiorbital systems, as vividly exemplified by nickelate superconductors. Here, using high-resolution angle-resolved photoemission spectroscopy combined with theoretical calculations, we systematically investigate the electronic properties of trilayer nickelates. In La4Ni3O10, we observe pronounced interorbital hybridization, whereas in Pr4Ni3O10, the flat dz2 band becomes markedly incoherent and diminishes in spectral weight. By contrast, the dispersive dx2−y2 bands retain coherence in both compounds. This striking incoherence/coherence dichotomy identifies an orbital differentiation modulated by the interlayer Ni-O-Ni bonding angle. The depletion of the dz2 orbitals further suppresses the interorbital hybridization and influences the density-wave transition in Pr4Ni3O10. Moreover, the density-wave gap is substantially reduced in Pr4Ni3O10, likely due to extra scattering channels provided by the local moments of Pr3+ cations. Our findings reveal a structural control parameter for the multiorbital correlated state in trilayer nickelates, providing important insights into the emergence of superconductivity under high pressure.

Abstract:

The recent discovery of high-temperature superconductivity in pressurized Ruddlesden-Popper nickelates has prompted intensive research into their correlated electron physics. Establishing the diversity of ground states across different Ruddlesden-Popper phases is crucial for elucidating the electron-pairing mechanism in these nickelates. In this work, we synthesized and investigated the long-range ordered 1313-type La3Ni2O7 single crystal. Unlike the bilayer nickelate, the 1313-type La3Ni2O7 exhibits semiconducting behavior at ambient pressure, characterized by a distinct anomaly at 170 K. This behavior is consistently corroborated by magnetic susceptibility and specific heat measurements. 139La nuclear magnetic resonance spectroscopy unambiguously reveals a spin-density-wave transition occurring at 170 K. High-pressure electrical transport measurements reveal pressure-induced metallization but no discernible signs of superconductivity up to 65 GPa. Our findings establish the 1313-type La3Ni2O7 as a new platform for investigating the interplay among crystal structure, density-wave orders, and electron pairing in hybrid nickelates.

Abstract:

The intrinsic electronic structures of narrow bandgap thermoelectric (TE) materials serve as a platform for the investigation of coupling effects of quasi-particles under high pressure, enabling the exploration of emerging electronic and phonon transport, superconductivity, and topological transitions. Here, we report the discovery of pressure-induced superconductivity in the TE semiconductor Mg₃Sb₂. Upon increased pressure, metallization occurs at ∼8.7 GPa, followed by a superconducting transition concomitant with a carrier-type crossover from p- to n-type. This phenomenon arises from a pressure-induced structural phase transition from the semiconducting P3̅m1 to the metallic C2/m-I phase. The superconducting critical temperature (T_c) exhibits a dome-shaped pressure dependence, peaking at 3.3 K at 12.6 GPa. Combined theoretical calculations, high-pressure Raman spectroscopy, and X-ray diffraction (XRD) measurements reveal an additional structural transition above ∼20 GPa, yielding a distinct C2/m-II phase. Our findings establish the high-pressure phase diagram of Mg₃Sb₂, elucidate its pressure-dependent electronic properties, and provide valuable insights for future investigations of TE materials under high pressure.

Abstract:

4Hb-TaS2, a van der Waals superlattice of alternating Ising-superconducting 1H-TaS2 and cluster-Mott-insulating 1T-TaS2 layers, exhibits emergent phenomena, including time-reversal symmetry-breaking superconductivity and spontaneous vortex phases, driven by interlayer interactions. Using area-selective angle-resolved photoemission spectroscopy, we directly probe the electronic structures of 1T- and 1H-terminated surfaces. Metallic states of subsurface 1H layers are folded to the Brillouin zone center by the 13 × 13 modulation of the surface 1T layer, forming chiral “windmill” Fermi surfaces via Umklapp scattering. These states hybridize with the incipient flat band of the surface 1T layer, producing a Kondo-like peak at the Fermi level. Interlayer charge transfer induces distinct 3×3 and 2×2 charge orders on surface and subsurface 1H layers, segmenting Fermi surfaces and shifting van Hove singularities. Our results reconcile competing Kondo and Mott models and highlight the interplay of flat bands, van Hove singularities, charge orders, and unconventional superconductivity in correlated superlattices.