Heng Gao

Probing the electronic states and impurity effects in black phosphorus vertical heterostructures

Atomically thin black phosphorus (BP) is a promising two-dimensional material for fabricating electronic and optoelectronic nano-devices with high mobility and tunable bandgap structures. However, the charge-carrier mobility in few-layer phosphorene (monolayer BP) is mainly limited by the presence of impurity and disorders. In this study, we demonstrate that vertical BP heterostructure devices offer great advantages in probing the electron states of monolayer and few-layer phosphorene at temperatures down to 2 K through capacitance spectroscopy. Electronic states in the conduction and valence bands of phosphorene are accessible over a wide range of temperature and frequency. Exponential band tails have been determined to be related to disorders. Unusual phenomena such as the large temperature-dependence of the electron state population in few-layer phosphorene have been observed and systematically studied. By combining the first-principles calculation, we identified that the thermal excitation of charge trap states and oxidation-induced defect states were the main reasons for this large temperature dependence of the electron state population and degradation of the on-off ratio in phosphorene field-effect transistors.

Electric field effect of GaAs monolayer from first principles

Using first-principle calculations, we investigate two-dimensional (2D) honeycomb monolayer structures composed of group III-V binary elements. It is found that such compound like GaAs should have a buckled structure which is more stable than graphene-like flat structure. This results a polar system with out-of-plane dipoles arising from the non-planar structure. Here, we optimized GaAs monolayer structure, then calculated the electronic band structure and the change of buckling height under external electric field within density functional theory using generalized gradient approximation method. We found that the band gap would change proportionally with the electric field magnitude. When the spin-orbit coupling (SOC) is considered, we revealed fine spin-splitting at different points in the reciprocal space. Furthermore, the valence and conduction bands spin-splitting energies due to SOC at the K point of buckled GaAs monolayers are found to be weakly dependent on the electric field strength. Finally elec...

Global structure search and physical properties of Os2C.

The crystal structures of Os2C were extensively investigated using the structure search method from the first-principles calculations. In contrast to the P6 3 /mmc phase previously proposed as the ground state at ambient pressure, an energetically favorable structure with space group P-6m2 was found more stable at ambient condition. The structural stabilities of the new phase are confirmed by the phonon dispersion and elastic constants. Further calculations indicate that the newly predicted P-6m2 phase is ultra-incompressible with a high bulk modulus of 387 GPa and has a larger ideal shear strength than the P6 3 /mmc phase.

Spin valley and giant quantum spin Hall gap of hydrofluorinated bismuth nanosheet

Spin-valley and electronic band topological properties have been extensively explored in quantum material science, yet their coexistence has rarely been realized in stoichiometric two-dimensional (2D) materials. We theoretically predict the quantum spin Hall effect (QSHE) in the hydrofluorinated bismuth (Bi2HF) nanosheet where the hydrogen (H) and fluorine (F) atoms are functionalized on opposite sides of bismuth (Bi) atomic monolayer. Such Bi2HF nanosheet is found to be a 2D topological insulator with a giant band gap of 0.97 eV which might host room temperature QSHE. The atomistic structure of Bi2HF nanosheet is noncentrosymmetric and the spontaneous polarization arises from the hydrofluorinated morphology. The phonon spectrum and ab initio molecular dynamic (AIMD) calculations reveal that the proposed Bi2HF nanosheet is dynamically and thermally stable. The inversion symmetry breaking together with spin-orbit coupling (SOC) leads to the coupling between spin and valley in Bi2HF nanosheet. The emerging valley-dependent properties and the interplay between intrinsic dipole and SOC are investigated using first-principles calculations combined with an effective Hamiltonian model. The topological invariant of the Bi2HF nanosheet is confirmed by using Wilson loop method and the calculated helical metallic edge states are shown to host QSHE. The Bi2HF nanosheet is therefore a promising platform to realize room temperature QSHE and valley spintronics.

Extreme magnetoresistance and SdH oscillation in compensated semimetals of NbSb2 single crystals

Topological semimetals represent one of the most interesting classes of materials that continue to attract worldwide interest. Here, we report magnetotransport properties of MPn2-type (M = Nb, Ta; Pn = P, As, Sb) NbSb2 single-crystal semimetals with a centrosymmetric C12/m1 space group, paramagnetic ground state, and non-saturation parabolic-like magnetoresistance. The NbSb2 crystals show metallic conductivity down to 2 K and undergo a metal-to-insulator-like transition under a magnetic field B (B ≥ 4 T) and exhibit a resistivity plateau in the low-temperature region (T ≤ 10 K), where the value of resistivity strongly depends on the magnitude and direction of the magnetic field. Upon sweeping the magnetic field from 0 to 14.5 T in the transverse configuration at T = 1.5 K, the NbSb2 crystal shows a large positive magnetoresistance (4.2 × 103% at B = 14.5 T) with Shubnikov–de Haas (SdH) oscillation. Hall measurements reveal that both the carrier compensation between electrons and holes and the high mobilit...