Twist-Angle Tuning of Electronic Structure in Two-Dimensional Dirac Nodal Line Semimetal Au2Ge on Au(111).

Topological semimetals have emerged as quantum materials including Dirac, Weyl, and nodal line semimetals, and so on. Dirac nodal line (DNL) semimetals possess topologically nontrivial bands crossing along a line or a loop and are considered precursor states for other types of semimetals. Here, we combine scanning tunneling microscopy/spectroscopy (STM/S) measurements and density functional theory (DFT) calculations to investigate a twist angle tuning of electronic structure in two-dimensional DNL semimetal Au2Ge. Theoretical calculations show that two bands of Au2Ge touch each other in Γ-M and Γ-K paths, forming a DNL. A significant transition of electronic structure occurs by tuning the twist angle from 30° to 24° between monolayer Au2Ge and Au(111), as confirmed by STS measurements and DFT calculations. The disappearing of DNL state is a direct consequence of symmetry breaking.

Two-Dimensional Artificial Ge Superlattice Confining in Electronic Kagome Lattice Potential Valleys.

Constructing two-dimensional (2D) artificial superlattices based on single-atom and few-atom nanoclusters is of great interest for exploring exotic physics. Here we report the realization of two types of artificial germanium (Ge) superlattice self-confined by a 37×37 R25.3° superstructure of bismuth (Bi) induced electronic kagome lattice potential valleys. Scanning tunneling microscopy measurements demonstrate that Ge atoms prefer to be confined in the center of the Bi electronic kagome lattice, forming a single-atom superlattice at 120 K. In contrast, room temperature grown Ge atoms and clusters are confined in the sharing triangle corner and the center, respectively, of the kagome lattice potential valleys, forming an artificial honeycomb superlattice. First-principle calculations and Mulliken population analysis corroborate that our reported atomically thin Bi superstructure on Au(111) has a kagome surface potential valley with the center of the inner Bi hexagon and the space between the outer Bi hexagons being energetically favorable for trapping Ge atoms.

Thermoelectric properties of hydrogenated Sn2Bi monolayer under mechanical strain: a DFT approach.

Bismuth based structures are among the most promising candidates for thermoelectric applications. Recently, a semiconducting binary compound with stoichiometry of Sn2Bi has been synthesized, showing a strong spin-orbit coupling effect and high electron-hole asymmetry. Motivated by the experiment, we performed a density functional theory calculation combined with the semiclassical Boltzmann transport equation to investigate the thermoelectric properties of the stabilized Sn2Bi monolayer. It is demonstrated that the mobility is strongly dependent on the strain. It is 2389 (186) cm2 V-1 s-1 for hole (electron) in relaxed monolayer, but it becomes 1758 (1758) cm2 V-1 s-1 by applying a 2.5% tensile strain. Spin-orbit coupling (SOC) induces a huge spin splitting in the conduction and valence bands as high as 350 and 270 meV, respectively, coming from p orbitals of bismuth atoms. Also, the thermoelectric efficiency of the monolayer could be directly controlled by doping and strain where the maximum room temperature figure of merit of 1.01 is obtained under the strain of 3% for n-type doping with inclusion of SOC, making it a promising candidate for thermoelectric applications.

Edge-Dependent Electronic and Magnetic Characteristics of Freestanding ß 12-Borophene Nanoribbons.

This work presents an investigation of nanoribbons cut from ß 12-borophene sheets by applying the density functional theory. In particular, the electronic and magnetic properties of borophene nanoribbons (BNR) are studied. It is found that all the ribbons considered in this work behave as metals, which is in good agreement with the recent experimental results. ß 12-BNR has significant diversity due to the existence of five boron atoms in a unit cell of the sheet. The magnetic properties of the ribbons are strongly dependent on the cutting direction and edge profile. It is interesting that a ribbon with a specific width can behave as a normal or a ferromagnetic metal with magnetization at just one edge or two edges. Spin anisotropy is observed in some ribbons, and the magnetic moment is not found to be the same in both edges in an antiferromagnetic configuration. This effect stems from the edge asymmetry of the ribbons and results in the breaking of spin degeneracy in the band structure. Our findings show that ß 12 BNRs are potential candidates for next-generation spintronic devices.

Freestanding χ3-borophene nanoribbons: a density functional theory investigation.

Experimental observation of borophene nanoribbons (BNRs) motivated us to carry out a comprehensive investigation on BNRs, decomposed from a χ3 sheet, using density functional theory. Our results show that the stability and also the electrical and magnetic properties of the ribbons are strongly dependent on the edge configurations. We have studied two categories of ribbon: XBNRs and YBNRs. The first one is a nonmagnetic metal with armchair shaped edges, while YBNRs can be magnetic or nonmagnetic depending on the edge shape. YBNRs have four different edge types and we show that two of them are magnetic (a- and b-type edges) while the other two are nonmagnetic (c- and d-type edges). There are 10 distinct configurations possible by arranging the different edges of YBNRs. 10 percent of YBNRs are polarized asymmetrically at the edges, leading to the loss of degeneracy of the spin-up and spin-down bands in the antiferromagnetic configuration. 40 percent of YBNRs have one magnetic edge and can be promising candidates for spintronic applications due to the separation of the spin in the real space in addition to the energy space. Electronic transmission properties of the ribbons were also studied and we found that transmission channels are suppressed at the edges of XBNRs due to electron localization.

Current-voltage characteristics of borophene and borophane sheets.

Motivated by recent experimental and theoretical research on a monolayer of boron atoms, borophene, the current-voltage characteristics of three different borophene sheets, 2Pmmn, 8Pmmn, and 8Pmmm, are calculated using density functional theory combined with the nonequilibrium Green's function formalism. Borophene sheets with two and eight atoms in a unit cell are considered. Their band structure, electron density, and structural anisotropy are analyzed in detail. The results show that the 8Pmmn and 8Pmmm structures that have eight atoms in the unit cell have less anisotropy than 2Pmmn. In addition, although 8Pmmn shows a Dirac cone in the band structure, its current is lower than that of the other two. We also consider a fully hydrogenated borophene, borophane, and find that the hydrogenation process reduces the structural anisotropy and the current significantly. Our findings reveal that the current-voltage characteristics of the borophene sheets can be used to detect the type and the growth direction of the sample because it is strongly dependent on the direction of the electron transport, anisotropy, and details of the unit cell of the borophene.