Minjun Lee

Energy Bandgap and Edge States in an Epitaxially Grown Graphene/h-BN Heterostructure.

Securing a semiconducting bandgap is essential for applying graphene layers in switching devices. Theoretical studies have suggested a created bulk bandgap in a graphene layer by introducing an asymmetry between the A and B sub-lattice sites. A recent transport measurement demonstrated the presence of a bandgap in a graphene layer where the asymmetry was introduced by placing a graphene layer on a hexagonal boron nitride (h-BN) substrate. Similar bandgap has been observed in graphene layers on metal substrates by local probe measurements; however, this phenomenon has not been observed in graphene layers on a near-insulating substrate. Here, we present bulk bandgap-like features in a graphene layer epitaxially grown on an h-BN substrate using scanning tunneling spectroscopy. We observed edge states at zigzag edges, edge resonances at armchair edges, and bandgap-like features in the bulk.

Ab initio pseudopotential study of the structural and electronic properties of ZnTe under high pressure

We have performed ab initio pseudopotential calculations within the local density approximation to investigate the structural phase transition of ZnTe under pressure. Zn 3d and Te 4d orbitals are treated as part of the valence states in order to describe the structural properties of ZnTe accurately. By calculating the total energy, atomic force, and stress tensors, we theoretically determine the structural phase transition of ZnTe from the zinc-blende (ZB) to the cinnabar to the distorted NaCl structure under increasing pressure. Calculational results are compared in detail with the recent experimental data obtained using angle-dispersive techniques and an image-plate detector. We also demonstrate that rotation of bonds toward lower-symmetry positions occurs at the critical pressure to relieve excessive strain. Detailed electronic structures of each phase are also presented.

Electronic properties of the generalized Fibonacci lattices: energy spectrum and wavefunction

The authors study the electronic properties of a one-dimensional diagonal tight-binding model with potentials (Vn) arranged in generalized Fibonacci (GF) sequences. Using the negative-eigenvalue theorem, they calculate the density of states (DOS). The DOS and the V dependence of energy spectra for silver-mean (SM) and copper-mean (CM) series clearly show distinctive features. The relation of the energy spectral feature to the geometry of the underlying lattices is emphasized. Various states of the CM lattice are examined in detail by means of wavefunctions, resistances and a multifractal analysis. Critical states are characterized both by scaling transformations and by multifractal behaviours. They find that states with a strongly localized wavefunction under a given system size exhibit additional wavepackets with increasing system size. This shows that the localized behaviour of allowed states is a feature of the finite size of the system, and implies the absence of strongly localized states in a system of infinite size.

Morphological effect of light emission from gold nanoparticles on Si(111)

The detection efficiency of a newly built photon detection system was measured using photons emitted from a scanning tunneling microscope junction. The efficiency was estimated from the instrumental yield and the geometry of the system on a clean Ag(111) surface using a measured photon map and a simultaneously measured topography image. The photon generation rates of gold three-dimensional (3D) islands and two-dimensional flat layers grown on a Si(111) surface were compared, and an enhanced photon generation rate was observed for the 3D island nanostructures.

Surface reconstruction and charge modulation in BaFe2As2 superconducting film

Whether or not epitaxially grown superconducting films have the same bulk-like superconducting properties is an important concern. We report the structure and the electronic properties of epitaxially grown Ba(Fe1-x Co x )2As2 films using scanning tunneling microscopy and scanning tunneling spectroscopy (STS). This film showed a different surface structure, [Formula: see text]R45° reconstruction, from those of as-cleaved surfaces from bulk crystals. The electronic structure of the grown film is different from that in bulk, and it is notable that the film exhibits the same superconducting transport properties. We found that the superconducting gap at the surface is screened at the Ba layer surface in STS measurements, and the charge density wave was observed at the surface in sample in the superconducting state.

Note: Development of a wideband amplifier for cryogenic scanning tunneling microscopy

A wideband cryogenic amplifier has been developed for low temperature scanning tunneling microscopy. The amplifier consisting of a wideband complementary metal oxide semiconductor field effect transistors operational amplifier together with a feedback resistor of 100 kΩ and a capacitor is mounted within a 4 K Dewar. This amplifier has a wide bandwidth and is successfully applied to scanning tunneling microscopy applications at low temperatures down to ∼7 K. The quality of the designed amplifier is validated by high resolution imaging. More importantly, the amplifier has also proved to be capable of performing scanning tunneling spectroscopy measurements, showing the detection of the Shockley surface state of the Au(111) surface and the superconducting gap of Nb(110).

Localization and multifractal wave functions in a one-dimensional incommensurate system under an electric field

The authors study the Kronig-Penney model with incommensurate potentials in the presence of an electric field. Both localization and delocalization by electric fields are observed, which are results intermediate between those obtained in periodic and disordered systems under electric fields. By performing a multifractal analysis, they show that the wave functions for large fields are multifractal.