Native point defects and their implications for the Dirac point gap at MnBi2Te4(0001)

We study the surface crystalline and electronic structures of the antiferromagnetic topological insulator MnBi2Te4 using scanning tunneling microscopy/spectroscopy (STM/S), micro(μ)-laser angle-resolved photoemission spectroscopy (ARPES), and density functional theory calculations. Our STM images reveal native point defects at the surface that we identify as BiTe antisites and MnBi substitutions. Bulk X-ray diffraction further evidences the presence of the Mn-Bi intermixing. Overall, our characterizations suggest that the defects concentration is nonuniform within crystals and differs from sample to sample. Consistently, the ARPES and STS experiments reveal that the Dirac point gap of the topological surface state is different for different samples and sample cleavages, respectively. Our calculations show that the antiparallel alignment of the MnBi moments with respect to those of the Mn layer can indeed cause a strong reduction of the Dirac point gap size. The present study provides important insights into a highly debated issue of the MnBi2Te4 Dirac point gap.

The Research on the Complexity of 1T-TaS2 at Ultra-low Temperatures

Graphene, as a successfully industrialized two-dimensional material, has greatly promoted the development of other two-dimensional materials, such as transition metal dichalcogenide (TMDs). 1T-TaS2 is a classical TMDs material, which presents metallicity at high temperature. It undergoes a variety of charge density wave (CDW) phase transitions during the temperature declining process, and presents insulating properties at low temperature. During the temperature rise period, 1T-TaS2 goes through a phase transition, from an energy band insulator to Mott insulator, followed by an insulation-metal phase transition. The complexity of 1T-TaS2 phase diagram encourages researchers to conduct extensive research on it. This paper, via means of resistance, magnetic susceptibility and other technical methods, finds out that the ultra-low temperature of 1T-TaS2 suggests additional complexity. In addition, with the angle resolved photoemission spectroscopy (ARPES) technique of in-situ alkali metal evaporation, this paper proposes that the 1T-TaS2 ultra-low temperature ground state may exist a combination of state and surface state. Our findings provide more experimental evidence for the physical mechanism of this system.

Spectroscopic Evidence for a Three-Dimensional Charge Density Wave in Kagome Superconductor CsV3Sb5

The recently discovered AV3Sb5 (A=K, Rb, Cs) family, possessing V kagome nets, has received considerable attention due to the topological electronic structure and intriguing correlated phenomena, including an exotic charge density wave (CDW) and superconductivity. Detailed electronic structure studies are essential to unravel the characteristics and origin of the CDW as well as its interplay with superconductivity. Here, we present angle-resolved photoemission spectroscopy (ARPES) measurements for CsV3Sb5 at multiple temperatures and photon energies to reveal the nature of the CDW from an electronic structure perspective. We present evidence for a three-dimensional (3D) CDW order. In the process we also pinpoint a surface state attributed to a Cs terminated surface. This state was previously attributed to band folding band due to a CDW along the c direction or a quantum well state from quantum confinement. The CDW expected 2-fold lattice reconstruction along c axis is observed to be a quadrupling of the unit cell, thus for the first time directly demonstrating the 3D nature of the CDW from the electronic structure perspective. Moreover, this 3D CDW configuration originates from two distinct types of distortions in adjacent kagome layers. These present results not only provide key insights into the nature of the unconventional CDW in CsV3Sb5 but also provides an important reference for further studies on the relationship between the CDW and superconductivity.

Discovery of Two Families of Vsb-Based Compounds with V-Kagome Lattice

We report the structure and physical properties of two newly discovered compounds AV8Sb12 and AV6Sb6 (A = Cs, Rb), which have C 2 (space group: Cmmm) and C 3 (space group: R 3 ¯ m ) symmetry, respectively. The basic V-kagome unit appears in both compounds, but stacking differently. A V2Sb2 layer is sandwiched between two V3Sb5 layers in AV8Sb12, altering the V-kagome lattice and lowering the symmetry of kagome layer from hexagonal to orthorhombic. In AV6Sb6, the building block is a more complex slab made up of two half-V3Sb5 layers that are intercalated by Cs cations along the c-axis. Transport property measurements demonstrate that both compounds are nonmagnetic metals, with carrier concentrations at around 1021 cm−3. No superconductivity has been observed in CsV8Sb12 above 0.3 K under in situ pressure up to 46 GPa. Compared to CsV3Sb5, theoretical calculations and angle-resolved photoemission spectroscopy reveal a quasi-two-dimensional electronic structure in CsV8Sb12 with C 2 symmetry and no van Hove singularities near the Fermi level. Our findings will stimulate more research into V-based kagome quantum materials.

Observation of a flat band and bandgap in millimeter-scale twisted bilayer graphene

Magic-angle twisted bilayer graphene, consisting of two graphene layers stacked at a special angle, exhibits superconductivity due to the maximized density of states at the energy of the flat band. Generally, experiments on twisted bilayer graphene have been performed using micrometer-scale samples. Here we report the fabrication of twisted bilayer graphene with an area exceeding 3 × 5 mm2 by transferring epitaxial graphene onto another epitaxial graphene, and observation of a flat band and large bandgap using angle-resolved photoemission spectroscopy. Our results suggest that the substrate potential induces both the asymmetrical doping in large angle twisted bilayer graphene and the electron doped nature of the flat band in magic-angle twisted bilayer graphene.