Jiaqiang Yan

Two-dimensional superconductivity is now well established in graphene-based systems, with many such realizations showing evidence for unconventional pairing^1,2,3,4,5,6. Yet, in several of the gate-tuned phases that otherwise exhibit clear signatures of superconductivity, the resistance does not vanish as temperature is lowered, instead saturating at a finite value^{2,6,7,8,9,10,11,12,13,14,15}. Here we report a systematic study of this behaviour in rhombohedral graphene on a WSe₂ substrate, finding regions of gate space with zero-resistance superconductivity alongside others with finite saturation resistance. At zero magnetic field, these regions appear as isolated pockets in gate space that otherwise exhibit very similar phenomenology, including abrupt transitions to the normal state as temperature, perpendicular magnetic field and current are raised above critical values. A small in-plane field expands and merges these pockets without qualitatively altering their behaviour, producing a sharp boundary at millikelvin base temperature between states of zero or finite resistance. The finite-resistance state reproduces key phenomenology associated with the anomalous metal, a state that has been observed in thin-film superconductors for decades but lacks an accepted theoretical explanation^16,17. The tunability and reproducibility of ultra-clean rhombohedral graphene place strong constraints on extrinsic explanations and provide a new platform for understanding this behaviour.

Recent experimental and theoretical studies have suggested a possible Dirac magnon gap in the two-dimensional ferromagnetic semiconductor CrSiTe 3 . Detailed neutron scattering measurements were performed to shed light on the existence of the magnon gap, and suggest that the gap is very small or nonexistent, with previous measurements being complicated by experimental factors. During these measurements, it was found that the out-of-plane couplings could explain the usual property of the increase in the magnetic transition temperature when CrSiTe 3 is exfoliated to monolayers. Furthermore, the material was shown to have anisotropic magnons along the out-of-plane direction, through the proposed Dirac point. We speculate that this is due to an exchange anisotropy, though Kitaev-like interactions alone cannot explain the spectra.

Here, we present our synthesis and characterization of the LnTi₃(Sb,Sn)₄ (Ln: Ce, Pr, Nd, Sm, Gd) family of cleavable kagome metals. While these materials are isostructural to the LnTi₃Bi₄ family, they only form as (Sb,Sn) solid-solutions with no corresponding LnTi₃Sb₄ or LnTi₃Sn₄ phases. We use a combination of first-principles density functional theory (DFT) and crystal orbital Hamilton population (COHP) calculations to show that (Sb,Sn) alloying has a stabilizing effect on the structure by adjusting the Fermi level, filling bonding states, depopulating antibonding states, and adjusting the density of states (DOS) toward local minima, an effect we call "synergistic doping." Through a detailed characterization of the SmTi₃(Sb,Sn)₄ series, we further demonstrate that the tunable Fermi level has a profound effect on the physical properties. We observe multiple magnetic phases that stem from a competition between antiferromagnetic (AFM) and ferromagnetic-like (FM) ground states. Furthermore, the (Sb,Sn) ratio allows us to tune from direct AFM-FM competition toward a complex admixture with properties reminiscent of the recently discovered TbTi₃Bi₄. Ultimately, our work demonstrates how the concept of synergistic doping provides a means to stabilize new structures while developing systems with intrinsically tunable chemical, magnetic, and electronic properties.

The topological magnet MnBi 2 Te 4 (MBT), with gapped topological surface state, is an attractive platform for realizing quantum anomalous Hall and axion insulator states. However, the experimentally observed surface state gaps fail to meet theoretical predictions, although the exact mechanism behind the gap suppression has been debated. Recent theoretical studies suggest that intrinsic antisite defects push the topological surface state away from the MBT surface, closing its gap and making it less accessible to scanning probe experiments. Here, we report on the local effect of defects on the MBT surface states and demonstrate that high defect concentrations lead to a displacement of the surface states well into the MBT crystal, validating the theorized mechanism. The local and global influence of antisite defects on the topological surface states are studied with samples of varying defect densities by combining scanning tunneling microscopy, angle-resolved photoemission spectroscopy, and density functional theory. Our findings identify a combination of increased defect density and reduced defect spacing as the primary factors underlying the displacement of the surface states and suppression of surface gap, guiding further development of topological quantum materials.

Van der Waals twisted bilayers of transition-metal dichalcogenides exhibit diverse excitonic phenomena arising from moiré superlattices and lattice reconstruction. While interlayer strain and relaxation reshape their electronic bands, their nanoscale influence on the excitonic fine structure remains poorly understood. Here, we study the interplay between spin-orbit coupling (SOC) and the moiré lattice by probing excitons in H-type twisted (∼8-layer × 8-layer) MoTe₂ using atomic-resolution scanning transmission electron microscopy (STEM) combined with monochromated electron energy loss spectroscopy (EELS) at cryogenic temperature. Spatially resolved spectra reveal moiré-site-dependent variations in excitonic absorption, including modulation of the X_B exciton across the moiré unit cell. We find that SOC in MoTe₂ increases with the twist angle within the studied range (2-4.5°), with the largest value observed at 4°. These findings establish the twist angle as a tunable parameter for SOC via moiré engineering and link local structural relaxation to excitonic fine structure in twisted MoTe₂.

The experimental observation of half-integer-quantized thermal Hall conductivity in the Kitaev candidate material α-RuCl_{3} has served as a signature of non-Abelian anyons through an associated chiral Majorana edge mode. However, both the reproducibility of the quantized thermal Hall conductivity and the fundamental nature of the associated heat carriers, whether bosonic or fermionic, are subjects of ongoing and vigorous debate. In a recent theoretical work, it was proposed that varying the sample geometry through creating constrictions can distinguish between different origins of the thermal Hall effect in magnetic insulators. Here, we provide experimental evidence of chiral fermion edge modes by comparing the thermal Hall effect of a geometrically constricted α-RuCl_{3} sample with that of an unconstricted sample. In contrast to the unconstricted crystals where the thermal Hall signal fades below 5 K, the constricted crystals display a significant thermal Hall signal that remains measurable even at 2 K. This sharp difference agrees well with the theoretical prediction and provides compelling evidence for the contribution of chiral fermion edge modes to the thermal Hall effect in α-RuCl_{3}. More broadly, this work confirms that the geometry dependence of the thermal Hall effect can help identify chiral spin liquids in candidate materials like α-RuCl_{3} and paves the way for the experimental realization of thermal anyon interferometry.