Stacking-Order-Dependent Excitonic Properties Reveal Interlayer Interactions in Bulk ReS2.

Rhenium disulfide, a member of the transition metal dichalcogenide family of semiconducting materials, is unique among 2D van der Waals materials due to its anisotropy and, albeit weak, interlayer interactions, confining excitons within single atomic layers and leading to monolayer-like excitonic properties even in bulk crystals. While recent work has established the existence of two stacking modes in bulk, AA and AB, the influence of the different interlayer coupling on the excitonic properties has been poorly explored. Here, we use polarization-dependent optical measurements to elucidate the nature of excitons in AA and AB-stacked rhenium disulfide to obtain insight into the effect of interlayer interactions. We combine polarization-dependent Raman with low-temperature photoluminescence and reflection spectroscopy to show that, while the similar polarization dependence of both stacking orders indicates similar excitonic alignments within the crystal planes, differences in peak width, position, and degree of anisotropy reveal a different degree of interlayer coupling. DFT calculations confirm the very similar band structure of the two stacking orders while revealing a change of the spin-split states at the top of the valence band to possibly underlie their different exciton binding energies. These results suggest that the excitonic properties are largely determined by in-plane interactions, however, strongly modified by the interlayer coupling. These modifications are stronger than those in other 2D semiconductors, making ReS2 an excellent platform for investigating stacking as a tuning parameter for 2D materials. Furthermore, the optical anisotropy makes this material an interesting candidate for polarization-sensitive applications such as photodetectors and polarimetry.

Intermixing-Driven Surface and Bulk Ferromagnetism in the Quantum Anomalous Hall Candidate MnBi6 Te10.

The recent realizations of the quantum anomalous Hall effect (QAHE) in MnBi2 Te4 and MnBi4 Te7 benchmark the (MnBi2 Te4 )(Bi2 Te3 )n family as a promising hotbed for further QAHE improvements. The family owes its potential to its ferromagnetically (FM) ordered MnBi2 Te4 septuple layers (SLs). However, the QAHE realization is complicated in MnBi2 Te4 and MnBi4 Te7 due to the substantial antiferromagnetic (AFM) coupling between the SLs. An FM state, advantageous for the QAHE, can be stabilized by interlacing the SLs with an increasing number n of Bi2 Te3 quintuple layers (QLs). However, the mechanisms driving the FM state and the number of necessary QLs are not understood, and the surface magnetism remains obscure. Here, robust FM properties in MnBi6 Te10 (n = 2) with Tc ≈ 12 K are demonstrated and their origin is established in the Mn/Bi intermixing phenomenon by a combined experimental and theoretical study. The measurements reveal a magnetically intact surface with a large magnetic moment, and with FM properties similar to the bulk. This investigation thus consolidates the MnBi6 Te10 system as perspective for the QAHE at elevated temperatures.

Metamagnetism of Weakly Coupled Antiferromagnetic Topological Insulators.

The magnetic properties of the van der Waals magnetic topological insulators MnBi_{2}Te_{4} and MnBi_{4}Te_{7} are investigated by magnetotransport measurements. We evidence that the relative strength of the interlayer exchange coupling J to the uniaxial anisotropy K controls a transition from an A-type antiferromagnetic order to a ferromagneticlike metamagnetic state. A bilayer Stoner-Wohlfarth model allows us to describe this evolution, as well as the typical angular dependence of specific signatures, such as the spin-flop transition of the uniaxial antiferromagnet and the switching field of the metamagnet.

Visualizing coexisting surface states in the weak and crystalline topological insulator Bi2TeI.

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.

Crystal Chemistry and Bonding Patterns of Bismuth-Based Topological Insulators.

Bismuth is gaining importance as a key element of functional quantum materials. The effects of spin-orbit coupling (SOC) are at the heart of many exciting proposals for next-generation quantum technologies, including topological materials for efficient information transmission and energy-saving applications. The "heavy" element bismuth and its compounds are predestined for SOC-induced topological properties, but materials design is challenged by a complex link between them and the chemical composition and crystal structure. Nevertheless, a lot can be learned about a certain property by testing its limits with compositional and/or structure modifications. We survey a handful of topological bismuth-based materials that bear structural and chemical semblance to the early topological insulators, antimony-doped elemental bismuth, Bi2Se3 and Bi2Te3. Chemical bonding via p orbitals and modular structure underlie all considered bismuth chalcogenides, subhalides, and chalcogenide halides and allow us to correlate the evolution of chemical bonding and structure with variability of the topological properties, although materials design should not be regarded as a building blocks set. Over the past decade, material discoveries have unearthed a plethora of topological properties, and bismuth is very fertile as a progenitor of a rich palette of exotic quantum materials, ranging from strong and weak 3D and crystalline topological insulators over topological metals and semimetals to magnetic topological insulators, while preserving the general layered structure motif.

Detuning the Honeycomb of the α-RuCl3 Kitaev Lattice: A Case of Cr3+ Dopant.

Fine-tuning chemistry by doping with transition metals enables new perspectives for exploring Kitaev physics on a two-dimensional (2D) honeycomb lattice of α-RuCl3, which is promising in the field of quantum information protection and quantum computation. The key parameters to vary by doping are both Heisenberg and Kitaev components of the nearest-neighbor exchange interaction between the Jeff = 1/2 Ru3+ spins, depending strongly on the peculiarities of the crystal structure. Here, we present crystal growth by chemical vapor transport and structure elucidation of a solid solution series Ru1- xCr xCl3 (0 ≤ x ≤ 1), with Cr3+ ions coupled to the Ru3+ Kitaev host. The Cr3+ substitution preserves the honeycomb type lattice of α-RuCl3 and creates mixed occupancy of Ru/Cr sites without cationic order within the layers as confirmed by single-crystal X-ray diffraction and transmission electron microscopy investigations. In contrast to high-quality single crystals of α-RuCl3 with ABAB-stacked layers, the ternary compounds demonstrate a significant stacking disorder along the c-axis direction as evidenced by X-ray diffraction and high resolution scanning transmission electron microscopy (HR-STEM). Raman spectra of substituted samples are in line with the symmetry conservation of the parent lattice upon chromium doping. At the same time, our magnetic susceptibility data indicate that the Kitaev physics of α-RuCl3 is increasingly suppressed by the dominant spin-only driven magnetism of Cr3+ ( S = 3/2) in Ru1- xCr xCl3.

A bismuth triiodide monosheet on Bi2Se3(0001).

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.

Unusual Phonon Heat Transport in α-RuCl_{3}: Strong Spin-Phonon Scattering and Field-Induced Spin Gap.

The honeycomb Kitaev-Heisenberg model is a source of a quantum spin liquid with Majorana fermions and gauge flux excitations as fractional quasiparticles. Here we unveil the highly unusual low-temperature heat conductivity κ of α-RuCl_{3}, a prime candidate for realizing such physics: beyond a magnetic field of B_{c}≈7.5 T, κ increases by about one order of magnitude, both for in-plane as well as out-of-plane transport. This clarifies the unusual magnetic field dependence unambiguously to be the result of severe scattering of phonons off putative Kitaev-Heisenberg excitations in combination with a drastic field-induced change of the magnetic excitation spectrum. In particular, an unexpected, large energy gap arises, which increases linearly with the magnetic field, reaching remarkable ℏω_{0}/k_{B}≈50 K at 18 T.

Synthesis and Characterization of the New Manganese Hydroxide Chloride γ-Mn(OH)Cl.

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.

The Intermetalloid Cluster Cation (CuBi8 )3.

The reaction of Bi, BiCl3 , and CuCl in the ionic liquid [BMIm]Cl⋅4 AlCl3 (BMIm=1-n-butyl-3-methylimidazolium) at 180 °C yielded air-sensitive shiny black crystals of (CuBi8 )[AlCl4 ]2 [Al2 Cl7 ] and (CuBi8 )[AlCl4 ]3 . For both compounds X-ray diffraction on single crystals revealed monoclinic structures that contain the intermetalloid cluster (CuBi8 )3+ . It is the first pure bismuth cluster with a 3d metal and the first with a metal that does not form binary intermetallics with bismuth under ambient pressure. The cluster can be interpreted either as a copper(I) cation, η4 -coordinated by a square-antiprismatic Bi82+ polycation (Bi-Cu 267 pm), or as a nine-atomic intermetalloid nido-cluster with 22 skeletal electrons and the C4v symmetry. One of the chloride ions of a tetrahedral [AlCl4 ]- group coordinates the copper atom (Cu-Cl 228 pm) and thereby completes its 18 electron count. DFT-based calculations, followed by real-space bonding analysis, revealed a multicenter bonding situation between copper and bismuth atoms with about seven shared electrons.

Synthesis of a Cu-Filled Rh17S15 Framework: Microwave Polyol Process Versus High-Temperature Route.

Metal-rich, mixed copper-rhodium sulfide Cu3-δRh34S30 that represents a new Cu-filled variant of the Rh17S15 structure has been synthesized and structurally characterized. Copper content in the [CuRh8] cubic cluster was found to vary notably dependent on the chosen synthetic route. Full site occupancy was achieved only in nanoscaled Cu3Rh34S30 obtained by a rapid, microwave-assisted reaction of CuCl, Rh2(CH3CO2)4 and thiosemicarbazide at 300 °C in just 30 min; whereas merely Cu-deficient Cu3-δRh34S30 (2.0 ≥ δ ≥ 0.9) compositions were realized via conventional high-temperature ceramic synthesis from the elements at 950 °C. Although Cu3-δRh34S30 is metallic just like Rh17S15, the slightly enhanced metal content has a dramatic effect on the electronic properties. Whereas the Rh17S15 host undergoes a superconducting transition at 5.4 K, no signs of the latter were found for the Cu-derivatives at least down to 1.8 K. This finding is corroborated by the strongly reduced density of states at the Fermi level of the ternary sulfide and the disruption of long-range Rh-Rh interactions in favor of Cu-Rh interactions as revealed by quantum-chemical calculations.

The Triply Deprotonated Acetonitrile Anion CCN3- Stabilized in a Solid.

The unprecedented, fully deprotonated form of acetonitrile, the acetonitriletriide anion CCN3- , is experimentally realized for the first time in the stabilizing bulk host framework of the Ba5 [TaN4 ][C2 N] nitridometalate via a one-pot synthesis from the elements under moderate conditions (920 K). The molecular structure of this long-sought acetonitrile derivative is confirmed by X-ray diffraction, as well as NMR, IR, and Raman spectroscopy. The anion is isoelectronic to the CO2 molecule, and, in contrast to acetonitrile (H3 C-C≡N), the electron pairs are shifted towards two double bonds, that is, [C=C=N]3- .

Correlation between topological band character and chemical bonding in a Bi14Rh3I9-based family of insulators.

Recently the presence of topologically protected edge-states in Bi14Rh3I9 was confirmed by scanning tunnelling microscopy consolidating this compound as a weak 3D topological insulator (TI). Here, we present a density-functional-theory-based study on a family of TIs derived from the Bi14Rh3I9 parent structure via substitution of Ru, Pd, Os, Ir and Pt for Rh. Comparative analysis of the band-structures throughout the entire series is done by means of a unified minimalistic tight-binding model that evinces strong similarity between the quantum-spin-Hall (QSH) layer in Bi14Rh3I9 and graphene in terms of Pz-molecular orbitals. Topologically non-trivial energy gaps are found for the Ir-, Rh-, Pt- and Pd-based systems, whereas the Os- and Ru-systems remain trivial. Furthermore, the energy position of the metal d-band centre is identified as the parameter which governs the evolution of the topological character of the band structure through the whole family of TIs. The d-band position is shown to correlate with the chemical bonding within the QSH layers, thus revealing how the chemical nature of the constituents affects the topological band character.

A novel quasi-one-dimensional topological insulator in bismuth iodide ß-Bi4I4.

Recent progress in the field of topological states of matter has largely been initiated by the discovery of bismuth and antimony chalcogenide bulk topological insulators (TIs; refs ,,,), followed by closely related ternary compounds and predictions of several weak TIs (refs ,,). However, both the conceptual richness of Z2 classification of TIs as well as their structural and compositional diversity are far from being fully exploited. Here, a new Z2 topological insulator is theoretically predicted and experimentally confirmed in the ß-phase of quasi-one-dimensional bismuth iodide Bi4I4. The electronic structure of ß-Bi4I4, characterized by Z2 invariants (1;110), is in proximity of both the weak TI phase (0;001) and the trivial insulator phase (0;000). Our angle-resolved photoemission spectroscopy measurements performed on the (001) surface reveal a highly anisotropic band-crossing feature located at the  point of the surface Brillouin zone and showing no dispersion with the photon energy, thus being fully consistent with the theoretical prediction.

Neuroprotective activity of creatylglycine ethyl ester fumarate.

BACKGROUND: We have recently shown neuroprotective activity of the creatine amides in the focal cerebral ischemia in rats on the 280 mg/kg administration. In the present study, neuroprotective properties of creatylglycine ethyl ester fumarate (CrGEt) in rats with focal cerebral ischemia were explored in a wide dosage range (30-280 mg/kg, intravenous and intragastric). METHODS: Focal cerebral ischemia was induced by the middle cerebral artery occlusion (MCAO). RESULTS: The CrGEt administration 30 minutes before and at the last 5 minutes of MCAO dose dependently attenuated cerebral ischemic damage on 35%-65%, reduced neurobehavioral deficits, led to high neuronal survival in ischemic rat brains. The neuroprotective activity of CrGEt was mediated by its following abilities: (1) normalize the energy metabolism in the ischemic brains, maintaining adenosine triphosphate levels, and reducing lactate concentration; (2) inhibit the ischemia-reperfusion-related oxidative stress as evidenced by the increased activity of superoxide dismutase and the reduced levels of malondialdehyde. CrGEt served as a substrate for creatine kinase and a partial agonist of N-methyl-D-aspartate receptors; this partly explains mechanism of its neuroprotective action. CONCLUSIONS: In view of the previously mentioned results, CrGEt holds a promise as a compound for treatment of ischemic brain disorders.

Structure and bonding of Bi4Ir: a difficult-to-access bismuth iridide with a unique framework structure.

Crystals of Bi(4)Ir, a new intermetallic compound, were obtained by the reaction of an iridium-containing intermetallic precursor with liquid bismuth. X-ray diffraction on a single crystal revealed a rhombohedral structure [R3̅m, a = 2656.7(2) pm, and c = 701.6(4) pm]. Bi(4)Ir is not isostructural to Bi(4)Rh but combines motifs of the metastable superconductor Bi1(4)Rh(3) with those found in the weak topological insulator Bi1(4)Rh(3)I(9). The two crystallographically independent iridium sites in Bi(4)Ir have square-prismatic and skewed-square-antiprismatic bismuth coordination with Bi-Ir distances of 283-287 pm. By sharing common edges, the two types of [IrBi(8)] units constitute a complex three-dimensional network of rings and helices. The bonding in the heterometallic framework is dominated by pairwise Bi-Ir interactions. In addition, three-center bonds are found in the bismuth triangles formed by adjacent [IrBi(8)] polyhedra. Density functional theory based band-structure calculations suggest metallic properties.

Low-temperature topochemical transformation of Bi13Pt3I7 into the new layered honeycomb metal Bi12Pt3I5.

Ordered single-crystals of the metallic subiodide Bi13 Pt3 I7 were grown and treated with n-butyllithium. At 45 °C, complete pseudomorphosis to Bi12 Pt3 I5 was achieved within two days. The new compound is air-stable and contains the same ${{{\hfill 2\atop \hfill \infty }}}$[(PtBi8/2 )3 I](n+) honeycomb nets and iodide layers as the starting material Bi13 Pt3 I7 , but does not include ${{{\hfill 1\atop \hfill \infty }}}$[BiI2 I4/2 ](-) iodidobismuthate strands. Electron microscopy and X-ray diffraction studies of solid intermediates visualize the process of the topochemical crystal-to-crystal transformation. In the electronic band structures of Bi13 Pt3 I7 and Bi12 Pt3 I5 , the vicinities of the Fermi levels are dominated by the intermetallic fragments. Upon the transformation of Bi13 Pt3 I7 into Bi12 Pt3 I5 , the intermetallic part is oxidized and the Fermi level is lowered by 0.16 eV. Whereas in Bi13 Pt3 I7 the intermetallic layers do not interact across the iodidobismuthate spacers (two-dimensional metal), they couple in Bi12 Pt3 I5 and form a three-dimensional metal.

A metallic room-temperature oxide ion conductor.

Nanoparticles of Bi3 Ir, obtained from a microwave-assisted polyol process, activate molecular oxygen from air at room temperature and reversibly intercalate it as oxide ions. The closely related structures of Bi3 Ir and Bi3 IrOx (x≤2) were investigated by X-ray diffraction, electron microscopy, and quantum-chemical modeling. In the topochemically formed metallic suboxide, the intermetallic building units are fully preserved. Time- and temperature-dependent monitoring of the oxygen uptake in an oxygen-filled chamber shows that the activation energy for oxide diffusion (84 meV) is one order of magnitude smaller than that in any known material. Bi3 IrOx is the first metallic oxide ion conductor and also the first that operates at room temperature.

Stacked topological insulator built from bismuth-based graphene sheet analogues.

Commonly, materials are classified as either electrical conductors or insulators. The theoretical discovery of topological insulators has fundamentally challenged this dichotomy. In a topological insulator, the spin-orbit interaction generates a non-trivial topology of the electronic band structure dictating that its bulk is perfectly insulating, whereas its surface is fully conducting. The first topological insulator candidate material put forward--graphene--is of limited practical use because its weak spin-orbit interactions produce a bandgap of ~0.01 K. Recent reexaminations of Bi2Se3 and Bi2Te3, however, have firmly categorized these materials as strong three-dimensional topological insulators. We have synthesized the first bulk material belonging to an entirely different, weak, topological class, built from stacks of two-dimensional topological insulators: Bi14Rh3I9. Its Bi-Rh sheets are graphene analogues, but with a honeycomb net composed of RhBi8 cubes rather than carbon atoms. The strong bismuth-related spin-orbit interaction renders each graphene-like layer a topological insulator with a 2,400 K bandgap.