Shu-feng Zhang

Prediction of tunable quantum spin Hall effect in methyl-functionalized tin film

The quantum spin Hall (QSH) effect may promote revolutionary device development due to dissipationless propagation of spin currents. The bottleneck preventing applications from the QSH effect, however, is a lack of large nontrivial bulk gap and highly stable two-dimensional (2D) films. In this work, we design a novel 2D honeycomb lattice, namely a SnCH3 monolayer, using comprehensive density-functional theory (DFT) computations. The structural stability is confirmed using a phonon spectrum and molecular dynamics simulations. Interestingly, its nontrivial bulk gap can reach up to 0.34 eV, which is further tunable via external strain. The nontrivial topology stems mainly from band inversion between the s–px,y orbitals, demonstrated by the nonzero topological invariant Z2 and a single pair of gapless helical edge states located in the bulk gap. The effects of a growth substrate on the QSH effect are also checked by hydrogen bonding on a single side in stanene, showing the robustness of the observed QSH phase. Considering its compatibility with the current electronics industry, these findings present an efficient platform to enrich topological phenomena and expand potential applications of 2D stanene at high temperature.

A planar C3Ca2 film: a novel 2p Dirac half metal

The exploration of Dirac materials is a great challenge in condensed matter physics and material chemistry. In this paper we present a novel 2D magnetic graphene-like Dirac material with a Honeycomb–Kagome (HK) lattice, named as C3Ca2. The ground state of C3Ca2 is a half-metal with a 100% spin polarized Dirac cone locating exactly at the Fermi level in the metallic spin channel, and has a large band gap in the insulating spin channel. In particular, C3Ca2 has 2p magnetism with the Dirac cones mainly contributed by pxy orbitals of C atoms, instead of 3d magnetism or pz dominated Dirac cones in other HK or Kagome materials, and it is robust against spin–orbit coupling and biaxial strains. The mechanism of magnetism could be understood by double exchange between carbon anions, using Ca2+ cations as bridges. These outstanding properties of C3Ca2 indicate it to be a promising 2D material for applications in spintronics.

High-temperature Dirac half-metal PdCl3: a promising candidate for realizing quantum anomalous Hall effect

The prospect of a Dirac half-metal (DHM) and the realization of the quantum anomalous Hall effect (QAHE) on a honeycomb lattice without external fields are a great challenge in experiments due to the structural complexities of two-dimensional (2D) crystals. Here, based on density-functional theory calculations, we propose an ideal candidate material for realizing these exotic quantum states in a 2D honeycomb metal–halogen lattice, single-layer PdCl3. We find that the ground state of PdCl3 is a 100% spin-polarized DHM with a ferromagnetic Curie temperature TC = 528 K predicted from Monte Carlo simulations. Upon including spin–orbit coupling (SOC), PdCl3 reveals the QAHE due to the splitting of the manifold of Pd |dxz〉 and |dyz〉 bands near the Fermi level, which is characterized by the nontrivial Chern number (C = −1) and chiral edge states. In particular, the origin of the topological properties of the PdCl3 honeycomb lattice is explained by the tight-binding model. The sensitivity of nontrivial topology to the cooperative effect of the electron correlation of Pd-4d electrons and SOC is demonstrated: when increasing the on-site Coulomb repulsion U, a sizable nontrivial band gap Eg = 68.6 meV is obtained. Additionally, we explore the mechanical and dynamical stability, as well as strain response of PdCl3 for possible epitaxial growth conditions in experiments. The coexistence of a high temperature DHM and the QAHE in PdCl3 presents a promising platform for the emerging area of spintronics devices with dissipationless edge states.

Low-energy electronic properties of a Weyl semimetal quantum dot

It is necessary to study the properties of Weyl semimetal nanostructures for potential applications in nanoelectronics. Here we study the Weyl semimetal quantum dot with a most simple model Hamiltonian with only two Weyl points. We focus on the low-energy electronic structure and show the correspondence to that of three-dimensional Weyl semimetal, such as Weyl point and Fermi arc. We find that there exist both surface and bulk states near Fermi level. The direct gap of bulk states reaches the minimum with the location determined by Weyl point. There exists a quantum number with only several values supporting surface states, which is the projection of Fermi arc. The property of surface state is studied in detail, including circular persistent current, orbital magnetic moment, and chiral spin polarization. Surface states will be broken by a strong magnetic field and evolve into Landau levels gradually. Simple expressions are derived to describe the energy spectra and electronic properties of surface states both in the presence and absence of magnetic field. In addition, this study may help design a method to verify Weyl semimetal by separating out the signal of surface states since quantum dot has the largest surface-to-volume ratio.

Discovery of a novel spin-polarized Nodal ring in two-dimensional HK lattice

Nodal-ring materials with a spin-polarized feature have attracted intensive interest recently due to their exotic properties and potential applications in spintronics. However, such a type of two-dimensional (2D) lattice is rather rare and difficult to realize experimentally. Here, we identify the first 2D Honeycomb-Kagome (HK) lattice, Mn-Cyanogen, as a new single-spin nodal-ring material by using first-principles calculations. Mn-Cyanogen shows gapless and semiconducting properties in spin-up and spin-down orientations, respectively, indicating a spin-gapless semiconductor nature. Remarkably, a spin-polarized nodal ring induced by px,y/pz band inversion is captured from the 3D band structure, which is irrelevant to spin-orbit coupling. The origin of the single-spin nodal-ring can be further clarified by the effective tight-binding (TB) model. These results open a new avenue to achieving spin-polarized nodal-ring materials with promising applications in spintronic devices.