Fengfeng Zhu

Epitaxial growth of two-dimensional stanene

Following the first experimental realization of graphene, other ultrathin materials with unprecedented electronic properties have been explored, with particular attention given to the heavy group-IV elements Si, Ge and Sn. Two-dimensional buckled Si-based silicene has been recently realized by molecular beam epitaxy growth, whereas Ge-based germanene was obtained by molecular beam epitaxy and mechanical exfoliation. However, the synthesis of Sn-based stanene has proved challenging so far. Here, we report the successful fabrication of 2D stanene by molecular beam epitaxy, confirmed by atomic and electronic characterization using scanning tunnelling microscopy and angle-resolved photoemission spectroscopy, in combination with first-principles calculations. The synthesis of stanene and its derivatives will stimulate further experimental investigation of their theoretically predicted properties, such as a 2D topological insulating behaviour with a very large bandgap, and the capability to support enhanced thermoelectric performance, topological superconductivity and the near-room-temperature quantum anomalous Hall effect.

Spatial and Energy Distribution of Topological Edge States in Single Bi(111) Bilayer

Recent studies show that two low-energy van Hove singularities (VHSs) seen as two pronounced peaks in the density of states could be induced in a twisted graphene bilayer. Here, we report angle-dependent VHSs of a slightly twisted graphene bilayer studied by scanning tunneling microscopy and spectroscopy. We show that energy difference of the two VHSs follows ΔE(vhs)∼ℏν(F)ΔK between 1.0° and 3.0° [here ν(F)∼1.1 × 10(6) m/s is the Fermi velocity of monolayer graphene, and ΔK = 2Ksin(θ/2) is the shift between the corresponding Dirac points of the twisted graphene bilayer]. This result indicates that the rotation angle between graphene sheets does not result in a significant reduction of the Fermi velocity, which quite differs from that predicted by band structure calculations. However, around a twisted angle θ∼1.3°, the observed ΔE(vhs)∼0.11 eV is much smaller than the expected value ℏν(F)ΔK∼0.28 eV at 1.3°. The origin of the reduction of ΔE(vhs) at 1.3° is discussed.By combining scanning tunneling microscopy and spectroscopy, angle-resolved photoemission spectroscopy, and density functional theory band calculations, we directly observe and resolve the one-dimensional edge states of single bilayer (BL) Bi(111) islands on clean Bi(2)Te(3) and Bi(111)-covered Bi(2)Te(3) substrates. The edge states are localized in the vicinity of step edges having an ∼2  nm wide spatial distribution in real space and reside in the energy gap of the Bi(111) BL. Our results demonstrate the existence of nontrivial topological edge states of single Bi(111) bilayer as a two-dimensional topological insulator.

Quasiparticle dynamics in reshaped helical Dirac cone of topological insulators

Topological insulators and graphene present two unique classes of materials, which are characterized by spin-polarized (helical) and nonpolarized Dirac cone band structures, respectively. The importance of many-body interactions that renormalize the linear bands near Dirac point in graphene has been well recognized and attracted much recent attention. However, renormalization of the helical Dirac point has not been observed in topological insulators. Here, we report the experimental observation of the renormalized quasiparticle spectrum with a skewed Dirac cone in a single Bi bilayer grown on Bi2Te3 substrate from angle-resolved photoemission spectroscopy. First-principles band calculations indicate that the quasiparticle spectra are likely associated with the hybridization between the extrinsic substrate-induced Dirac states of Bi bilayer and the intrinsic surface Dirac states of Bi2Te3 film at close energy proximity. Without such hybridization, only single-particle Dirac spectra are observed in a single Bi bilayer grown on Bi2Se3, where the extrinsic Dirac states Bi bilayer and the intrinsic Dirac states of Bi2Se3 are well separated in energy. The possible origins of many-body interactions are discussed. Our findings provide a means to manipulate topological surface states.

Creation of helical Dirac fermions by interfacing two gapped systems of ordinary fermions

Topological insulators are a unique class of materials characterized by a Dirac cone state of helical Dirac fermions in the middle of a bulk gap. When the thickness of a three-dimensional topological insulator is reduced, however, the interaction between opposing surface states opens a gap that removes the helical Dirac cone, converting the material back to a normal system of ordinary fermions. Here we demonstrate, using density function theory calculations and experiments, that it is possible to create helical Dirac fermion state by interfacing two gapped films-a single bilayer Bi grown on a single quintuple layer Bi(2)Se(3) or Bi(2)Te(3). These extrinsic helical Dirac fermions emerge in predominantly Bi bilayer states, which are created by a giant Rashba effect with a coupling constant of ~4 eV·Å due to interfacial charge transfer. Our results suggest that this approach is a promising means to engineer topological insulator states on non-metallic surfaces.

Electronic structure of black phosphorus studied by angle-resolved photoemission spectroscopy

Electronic structures of single crystalline black phosphorus were studied by state-of-art angleresolved photoemission spectroscopy. Through high resolution photon energy dependence measurements, the band dispersions along out-of-plane and in-plane directions are experimentally determined. The electrons were found to be more localized in the ab-plane than that is predicted in calculations. Beside the kz-dispersive bulk bands, resonant surface state is also observed in the momentum space. Our finds strongly suggest that more details need to be considered to fully understand the electronic properties of black phosphorus theoretically.

Large magnetic moment of gadolinium substituted topological insulator: Bi1.98Gd0.02Se3

The crystal structure, electronic, and magnetic properties of Gadolinium (Gd) substituted Bi2Se3—represented by Bi1.98Gd0.02Se3—were investigated systematically by scanning tunneling microscopy, angle-resolved photoemission spectroscopy, and superconducting quantum interference device. Gd dopants with valence of 3+ were mainly found to substitute Bi atoms. Each Gd3+ ion has a magnetic moment as large as ∼6.9μB in the bulk paramagnetic state.

Electronic structure of a superconducting topological insulator Sr-doped Bi2Se3

Using high-resolution angle-resolved photoemission spectroscopy and scanning tunneling microscopy/spectroscopy, the atomic and low energy electronic structure of the Sr-doped superconducting topological insulators (SrxBi2Se3) was studied. Scanning tunneling microscopy shows that most of the Sr atoms are not in the van der Waals gap. After Sr doping, the Fermi level was found to move further upwards when compared with the parent compound Bi2Se3, which is consistent with the low carrier density in this system. The topological surface state was clearly observed, and the position of the Dirac point was determined in all doped samples. The surface state is well separated from the bulk conduction bands in the momentum space. The persistence of separated topological surface state combined with small Fermi energy makes this superconducting material a very promising candidate for the time reversal invariant topological superconductor.

Carriers dependence of the magnetic properties in magnetic topological insulator Sb1.95−xBixCr0.05Te3

Bulk carrier density and magnetic properties of Sb1.95−xBixCr0.05Te3 single crystals were studied. We found that the carrier density can be tuned by Bi substitution. Both the Curie temperature and magnetic moments are strongly related to the carrier density, which suggests the existence of intrinsic ferromagnetism in Cr-doped Sb2Te3 magnetic topological insulator that is very important for spin-related applications.

Carrier density dependence of the magnetic properties in iron-doped Bi2Se3 topological insulator

The electronic and magnetic properties of iron-doped topological insulator Bi1.84−xFe0.16CaxSe3 single crystals were studied. By co-doping Fe and Ca atoms, ferromagnetic bulk states with different carrier density (from n-type to p-type) were obtained. Effective magnetic moments for each Fe atom was estimated as small as about 0.07μB. Magnetic and non-magnetic phases separation was observed in all samples. Our results suggest that the bulk ferromagnetism in Fe-doped Bi2Se3 is not intrinsic and regardless of carrier density.

Fully gappeds-wave-like superconducting state and electronic structure in Ir0.95Pd0.05Te2single crystals with strong spin-orbital coupling

Due to the large spin-orbital coupling in the layered 5d-transition metal chalcogenides compound, the occurrence of superconductivity in Ir2-xPdxTe2 offers a good chance to search for possible topological superconducting states in this system. We did comprehensive studies on the superconducting properties and electronic structures of single crystalline Ir0.95Pd0.05Te2 samples. The superconducting gap size, critical fields and coherence length along different directions were experimentally determined. Macroscopic bulk measurements and microscopic low temperature scanning tunneling spectroscopy results suggest that Ir0.95Pd0.05Te2 possesses a BCS-like s-wave state. No sign of zero bias conductance peak were found in the vortex core at 0.4K.