Hu-Jong Lee

Inelastic scattering in a monolayer graphene sheet: A weak-localization study

Charge carriers in a graphene sheet, a single layer of graphite, exhibit distinct characteristics from those in other two-dimensional electronic systems because of their chiral nature. In this paper, we focus on the observation of weak localization in a graphene sheet exfoliated from a piece of natural graphite and nanopatterned into a Hall-bar geometry. Much stronger chiral-symmetry-breaking elastic intervalley scattering in our graphene sheet restores the conventional weak localization. The resulting carrier density and temperature dependence of the phase coherence length reveal that the electron-electron interaction including a direct Coulomb interaction is the main inelastic-scattering factor while electron-hole puddles enhance the inelastic scattering near the Dirac point.

Fabrication of metal nanowire using carbon nanotube as a mask

We report on the fabrication of metal nanowires on an insulating substrate using carbon nanotubes as a new kind of mask material. By irradiating Ar+ ions of 300 eV energy on a nanotube-coated Au/Ti thin layer on a SiO2 substrate, Au/Ti nanowires were successfully formed just underneath the nanotube, indicating that the carbon nanotubes had acted as a good mask against the argon ion bombardment. The Au/Ti wire of a few nanometers in width was frequently observed among the wires of various widths. After the formation of the Au/Ti nanowires, the carbon nanotube on the metal nanowire could be removed by atomic force microscope.

Josephson-vortex-flow terahertz emission in layered high-Tcsuperconducting single crystals

We report on the successful terahertz emission (0.6-1 THz) that is continuous and tunable in its frequency and power, by driving Josephson vortices in resonance with the collective standing Josephson plasma modes excited in stacked Bi2Sr2CaCu2O8+x intrinsic Josephson junctions. Shapiro-step detection was employed to confirm the terahertz-wave emission. Our results provide a strong feasibility of developing long-sought solid-state terahertz-wave emission devices.

Observation of negative refraction of Dirac fermions in graphene

Negative refraction has now been observed for Dirac fermions in graphene, and is used to create an electronic Veselago lens. Half a century ago, Veselago1 proposed ‘left-handed’ materials with negative permittivity and permeability, in which waves propagate with phase and group velocities in opposite directions. Significant work has been undertaken to attain this left-handed response, such as establishing a negative refractive index in so-called metamaterials, which consist of periodic sub-wavelength structures2,3,4. However, an electronic counterpart has not been demonstrated owing to difficulties in creating repeated structures smaller than the electronic Fermi wavelength of the order ∼10 nm. Here, without needing to engineer sub-wavelength structures, we demonstrate negative refractive behaviour of Dirac fermions in graphene, exploiting its unique relativistic band structure5. Analysis of both electron focusing through an n–p–n flat lens and negative refraction across n–p junctions confirms left-handed behaviour in the electronic system. This approach to electronic optics is of particular relevance to the on-going efforts to develop novel quantum devices with emerging6 layered materials.

Observation of supercurrent in PbIn-graphene-PbIn Josephson junction

Superconductor-graphene-superconductor (SGS) junction provides a unique platform to study relativistic electrodynamics of Dirac fermions combined with proximity-induced superconductivity. We report observation of the Josephson effect in proximity-coupled superconducting junctions of graphene in contact with Pb1-xInx (x=0.07) electrodes for temperatures as high as T = 4.8K, with a large IcRn (~ 255 microV). This demonstrates that Pb1-xInx SGS junction would facilitate the development of the superconducting quantum information devices and superconductor-enhanced phase-coherent transport of graphene.

Effects of two gaps and paramagnetic pair breaking on the upper critical field of SmFeAsO 0.85 and SmFeAsO 0.8 F 0.2 single crystals

We investigated the temperature dependence of the upper critical field

Gate-tuned differentiation of surface-conducting states in Bi1.5Sb0.5Te1.7Se1.3topological-insulator thin crystals

[{H}_{c2}(T)]

Breakdown of the interlayer coherence in twisted bilayer graphene.

of fluorine-free

Complete gate control of supercurrent in graphene p–n junctions

{\text{SmFeAsO}}_{0.85}

Electrically Tunable Macroscopic Quantum Tunneling in a Graphene-Based Josephson Junction

and fluorine-doped