ABSTRACT
Dual topological materials are unique topological phases that host coexisting surface states of different topological nature on the same or on different material facets. Here, we show that Bi2TeI is a dual topological insulator. It exhibits band inversions at two time reversal symmetry points of the bulk band, which classify it as a weak topological insulator with metallic states on its 'side' surfaces. The mirror symmetry of the crystal structure concurrently classifies it as a topological crystalline insulator. We investigated Bi2TeI spectroscopically to show the existence of both two-dimensional Dirac surface states, which are susceptible to mirror symmetry breaking, and one-dimensional channels that reside along the step edges. Their mutual coexistence on the step edge, where both facets join, is facilitated by momentum and energy segregation. Our observation of a dual topological insulator should stimulate investigations of other dual topology classes with distinct surface manifestations coexisting at their boundaries.
ABSTRACT
A stable BiI3 monosheet has been grown for the first time on the (0001) surface of the topological insulator Bi2Se3 as confirmed by scanning tunnelling microscopy, surface X-ray diffraction, and X-ray photoemision spectroscopy. BiI3 is deposited by molecular beam epitaxy from the crystalline BiTeI precursor that undergoes decomposition sublimation. The key fragment of the bulk BiI3 structure, [Formula: see text][I-Bi-I] layer of edge-sharing BiI6 octahedra, is preserved in the ultra-thin film limit, but exhibits large atomic relaxations. The stacking sequence of the trilayers and alternations of the Bi-I distances in the monosheet are the same as in the bulk BiI3 structure. Momentum resolved photoemission spectroscopy indicates a direct band gap of 1.2 eV. The Dirac surface state is completely destroyed and a new flat band appears in the band gap of the BiI3 film that could be interpreted as an interface state.
ABSTRACT
A new modification of Mn(OH)Cl was obtained under high-pressure/high-temperature conditions in a Walker-type multianvil device. The pale pink, hygroscopic compound crystallizes in the orthorhombic space group Pnma (no. 62) with a = 602.90(4), b = 350.98(2), c = 1077.69(7) pm, and V = 228 × 106 pm3. The layered centrosymmetric structure consists of edge-sharing Mn(OH)3Cl3 octahedra arranged in sheets parallel to the (001) plane. The comparatively long H···Cl distance of 275 pm suggests only weak hydrogen bonds between neighboring layers. Spin-polarized scalar-relativistic DFT+U calculations predict a non-conducting magnetically ordered ground state with a band gap of at least 3.2 eV and an effective magnetic moment of 4.65 µB/f. u. The experimentally determined magnetic response of Mn(OH)Cl is paramagnetic in the range of 10-300 K. The estimated moment of 5.6 µB/f. u. indicates the high-spin d 5 configuration of manganese(II). We find hints for a long-range magnetic ordering below 10 K.
