Emergent phenomena driven by electronic reconstructions in oxide heterostructures have been intensively discussed. However, the role of these phenomena in shaping the electronic properties in van der Waals heterointerfaces has hitherto not been established. By reducing the material thickness and forming a heterointerface, we find two types of charge-ordering transitions in monolayer VSe on graphene substrates. Angle-resolved photoemission spectroscopy (ARPES) uncovers that Fermi-surface nesting becomes perfect in ML VSe. Renormalization-group analysis confirms that imperfect nesting in three dimensions universally flows into perfect nesting in two dimensions. As a result, the charge-density wave-transition temperature is dramatically enhanced to a value of 350 K compared to the 105 K in bulk VSe. More interestingly, ARPES and scanning tunneling microscopy measurements confirm an unexpected metal-insulator transition at 135 K that is driven by lattice distortions. The heterointerface plays an important role in driving this novel metal-insulator transition in the family of monolayer transition-metal dichalcogenides.

Recently SnSe, a layered chalcogenide material, has attracted a great deal of attention for its excellent p-type thermoelectric property showing a remarkable ZT value of 2.6 at 923 K. For thermoelectric device applications, it is necessary to have n-type materials with comparable ZT value. Here, we report that n-type SnSe single crystals were successfully synthesized by substituting Bi at Sn sites. In addition, it was found that the carrier concentration increases with Bi content, which has a great influence on the thermoelectric properties of n-type SnSe single crystals. Indeed, we achieved the maximum ZT value of 2.2 along b axis at 733 K in the most highly doped n-type SnSe with a carrier density of −2.1 × 1019 cm−3 at 773 K.

SnSe single crystals have recently been found to exhibit excellent thermoelectric performance with an extremely high figure of merit (ZT) value of 2.6. Although this high ZT value has attracted considerable attention, the microscopic origin of the p-type characteristics of SnSe is not yet clearly understood. Here, we directly observed and identified intrinsic point defects existing on SnSe via scanning tunneling microscopy (STM) and investigated the effect of defects on the electronic properties using density functional theory (DFT) calculations. Our results demonstrate that the most dominant Sn vacancies move the Fermi energy inside the dispersive valence band and produce extra holes throughout the system. On the other hand, Se vacancies create a nondispersive donor level and generate immobile electrons localized near the vacancy site. Our combined STM/DFT studies show that the p-type characteristics of SnSe originate from extra holes in the dispersive Bloch-like band created by Sn vacancies. We expect that our results provide important information for the development of highly efficient SnSe-based thermoelectric devices.

Microscopic structures and magnetic properties are investigated for Fe5−xGeTe2 single crystal, recently discovered as a promising van der Waals (vdW) ferromagnet. An Fe atom (Fe(1)) located in the outermost Fe5Ge sublayer has two possible split‐sites which are either above or below the Ge atom. Scanning tunneling microscopy shows √3 × √3 superstructures which are attributed to the ordering of Fe(1) layer. The √3 × √3 superstructures have two different phases due to the symmetry of Fe(1) ordering. Intriguingly, the observed √3 × √3 ordering breaks the inversion symmetry of crystal, resulting in substantial antisymmetric exchange interaction. The temperature dependence of magnetization reveals a sharp magnetic anomaly suggesting helical magnetism of the Fe5−xGeTe2 due to its non‐centrosymmetricity. Analytical study also supports that the observed ordering can give rise to the helimagnetism. The work will provide essential information to understand the complex magnetic properties and the origin of the new vdW ferromagnet, Fe5−xGeTe2 for future topology‐based spin devices.

Reducing thickness of three dimensional samples on appropriate substrates is a promising way to control electron-electron interactions, responsible for so called electronic reconstruction phenomena. Although the electronic reconstruction has been investigated both extensively and intensively in oxide heterostructure interfaces, this paradigm is not well established in the van der Waals heterointerface system, regarded to be important for device applications. In the present study we examine nature of a charge ordering transition in monolayer vanadium diselenide (V Se2), which would be distinguished from that of V Se2 bulk samples, driven by more enhanced electron-electron correlations. We recall that V Se2 bulk samples show a charge density wave (CDW) transition around TCDW ∼ 105 K, expected to result from Fermi surface nesting properties, where the low temperature CDW state coexists with itinerant electrons of residual Fermi surfaces. Recently, angle resolved photoemission spectroscopy measurements [Nano Lett. 18, 5432 (2018)] uncovered that the Fermi surface nesting becomes perfect, where the dynamics of hot electrons is dispersionless along the orthogonal direction of the nesting wave-vector. In addition, scanning tunneling microscopy measurements [Nano Lett. 18, 5432 (2018)] confirmed that the resulting CDW state shows essentially the same modulation pattern as the three dimensional system of V Se2. Here, we perform the renormalization group analysis based on an effective field theory in terms of critical CDW fluctuations and hot electrons of imperfect Fermi-surface nesting. As a result, we reveal that the imperfect nesting universally flows into perfect nesting in two dimensions, where the Fermi velocity along the orthogonal direction of the nesting vector vanishes generically. We argue that this electronic reconstruction is responsible for the observation [Nano Lett. 18, 5432 (2018)] that the CDW transition temperature is much more enhanced to be around TCDW ∼ 350 K than that of the bulk sample. arXiv:1804.03766v3 [cond-mat.str-el]