Epitaxial Growth of Ultraflat Bismuthene with Large Topological Band Inversion Enabled by Substrate-Orbital-Filtering Effect.

Quantum spin Hall (QSH) systems hold promises of low-power-consuming spintronic devices, yet their practical applications are extremely impeded by the small energy gaps. Fabricating QSH materials with large gaps, especially under the guidance of design principles, is essential for both scientific research and practical applications. Here, we demonstrate that large on-site atomic spin-orbit coupling can be directly exploited via the intriguing substrate-orbital-filtering effect to generate large-gap QSH systems and experimentally realized on the epitaxially synthesized ultraflat bismuthene on Ag(111). Theoretical calculations reveal that the underlying substrate selectively filters Bi pz orbitals away from the Fermi level, leading pxy orbitals with nonzero magnetic quantum numbers, resulting in large topological gap of ∼1 eV at the K point. The corresponding topological edge states are identified through scanning tunneling spectroscopy combined with density functional theory calculations. Our findings provide general strategies to design large-gap QSH systems and further explore their topology-related physics.

Ag2S monolayer: an ultrasoft inorganic Lieb lattice.

Lieb lattice, a two-dimensional edge-centered square lattice, has attracted considerable interest due to its exotic electronic and topological properties. Although various optical and photonic Lieb lattices have been experimentally demonstrated, it remains challenging for an electronic Lieb lattice to be realized in real material systems. Here, based on first-principles calculations and tight-binding modeling, a silver sulfide (Ag2S) monolayer is reported as a long-sought-after inorganic electronic Lieb lattice. This Lieb-lattice Ag2S is further found to be ultrasoft, which enables its electronic properties and topological states near the Fermi level to be finely tuned, as evidenced by the strain-induced topologically non-trivial edge states near the valence band edge. These results not only provide an ideal platform to further explore and harvest interesting quantum properties but also pave a way to pursue other inorganic electronic Lieb lattices in a broader material domain.

Realization of a Buckled Antimonene Monolayer on Ag(111) via Surface Engineering.

The degree of buckling of two-dimensional (2D) materials can have a dramatic impact on their corresponding electronic structures. Antimonene (ß-phase), a new 2D material with air stability and promising electronic properties, has been engineered to adopt flat or two-heights-buckling geometries by employing different supporting substrates for epitaxial growth. However, studies of the antimonene monolayer with a more buckled configuration are still lacking. Here, we report the synthesis of an antimonene monolayer with a three-heights-buckling configuration overlaid on SbAg2 surface alloy-covered Ag(111) by molecular beam epitaxy, in which the underlying surface alloy provides interfacial interactions to modulate the structure of the antimonene monolayer. The atomic structure of the synthesized antimonene has been precisely identified through a combination of low-temperature scanning tunneling microscopy and density functional theory calculations. The successful fabrication of a buckled antimonene monolayer could provide a promising way to modulate the structures of 2D materials for future electronic and optoelectronic applications.

Designing Kagome Lattice from Potassium Atoms on Phosphorus-Gold Surface Alloy.

Materials with flat bands are considered as ideal platforms to explore strongly correlated physics such as the fractional quantum hall effect, high-temperature superconductivity, and more. In theory, a Kagome lattice with only nearest-neighbor hopping can give rise to a flat band. However, the successful fabrication of Kagome lattices is still very limited. Here, we provide a new design principle to construct the Kagome lattice by trapping atoms into Kagome arrays of potential valleys, which can be realized on a potassium-decorated phosphorus-gold surface alloy. Theoretical calculations show that the flat band is less correlated with the neighboring trivial electronic bands, which can be further isolated and dominate around the Fermi energy with increased Kagome lattice parameters of potassium atoms. Our results provide a new strategy for constructing Kagome lattices, which serve as an ideal platform to study topological and more general flat band phenomena.

Tailoring sample-wide pseudo-magnetic fields on a graphene-black phosphorus heterostructure.

Spatially tailored pseudo-magnetic fields (PMFs) can give rise to pseudo-Landau levels and the valley Hall effect in graphene. At an experimental level, it is highly challenging to create the specific strain texture that can generate PMFs over large areas. Here, we report that superposing graphene on multilayer black phosphorus creates shear-strained superlattices that generate a PMF over an entire graphene-black phosphorus heterostructure with edge size of tens of micrometres. The PMF is intertwined with the spatial period of the moiré pattern, and its spatial distribution and intensity can be modified by changing the relative orientation of the two materials. We show that the emerging pseudo-Landau levels influence the transport properties of graphene-black phosphorus field-effect transistor devices with Hall bar geometry. The application of an external magnetic field allows us to enhance or reduce the effective field depending on the valley polarization with the prospect of developing a valley filter.

Phonon-Mediated Colossal Magnetoresistance in Graphene/Black Phosphorus Heterostructures.

There is a huge demand for magnetoresistance (MR) sensors with high sensitivity, low energy consumption, and room temperature operation. It is well-known that spatial charge inhomogeneity due to impurities or defects introduces mobility fluctuations in monolayer graphene and gives rise to MR in the presence of an externally applied magnetic field. However, to realize a MR sensor based on this effect is hampered by the difficulty in controlling the spatial distribution of impurities and the weak magnetoresistance effect at the monolayer regime. Here, we fabricate a highly stable monolayer graphene-on-black phosphorus (G/BP) heterostructure device that exhibits a giant MR of 775% at 9 T magnetic field and 300 K, exceeding by far the MR effects from devices made from either monolayer graphene or few-layer BP alone. The positive MR of the G/BP device decreases when the temperature is lowered, indicating a phonon-mediated process in addition to scattering by charge impurities. Moreover, a nonlocal MR of >10 000% is achieved for the G/BP device at room temperature due to an enhanced flavor Hall effect induced by the BP channel. Our results show that electron-phonon coupling between 2D material and a suitable substrate can be exploited to create giant MR effects in Dirac semimetals.

Temperature- and Phase-Dependent Phonon Renormalization in 1T'-MoS2.

Polymorph engineering of 2H-MoS2, which can be achieved by alkali metal intercalation to obtain either the mixed 2H/1T' phases or a homogeneous 1T' phase, has received wide interest recently, since this serves as an effective route to tune the electrical and catalytic properties of MoS2. As opposed to an idealized single crystal-to-single crystal phase conversion, the 2H to 1T' phase conversion results in crystal domain size reduction as well as strained lattices, although how these develop with composition is not well understood. Herein, the evolution of the phonon modes in Li-intercalated 1T'-MoS2 (Li xMoS2) are investigated as a function of different 1T'-2H compositions. We observed that the strain evolution in the mixed phases is revealed by the softening of four Raman modes, Bg ( J1), Ag ( J3), E12g, and A1g, with increasing 1T' phase composition. Additionally, the first-order temperature coefficients of the 1T' phonon mode vary linearly with increasing 1T' composition, which is explained by increased electron-phonon and strain-phonon coupling.

Pressure induced topological phase transition in layered Bi2S3.

A large bulk band gap and tunable Dirac carriers are desired for practical device applications of topological insulators. However, most known topological insulators are narrow gap materials and the manipulation of their Dirac surface states is limited by residual bulk charge carriers originating from intrinsic defects. In this study, via density functional theory based first-principles calculations, we predict that a layered hexagonal structure of Bi2S3 is stable, and it becomes a topological insulator under a moderate compressive pressure of about 5.3 GPa. Interestingly, we find that the strength of the spin-orbit interaction in Bi2S3 can be effectively enhanced by the applied pressure. This leads to an increased inverted band gap with pressure, which can reach 0.4 eV with a pressure of 13.7 GPa. Compared to Bi2Se3, intrinsic defects are suppressed in Bi2S3 under both cation- and anion-poor growth conditions. Our calculations predict a new Bi-based topological insulator, and also shed light on control over spin-orbit interactions in Bi2S3 and tuning of its topological properties.