Jae-Hyun Choi

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.

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 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.

Complete gate control of supercurrent in graphene p–n junctions

In a conventional Josephson junction of graphene, the supercurrent is not turned off even at the charge neutrality point, impeding further development of superconducting quantum information devices based on graphene. Here we fabricate bipolar Josephson junctions of graphene, in which a p-n potential barrier is formed in graphene with two closely spaced superconducting contacts, and realize supercurrent ON/OFF states using electrostatic gating only. The bipolar Josephson junctions of graphene also show fully gate-driven macroscopic quantum tunnelling behaviour of Josephson phase particles in a potential well, where the confinement energy is gate tuneable. We suggest that the supercurrent OFF state is mainly caused by a supercurrent dephasing mechanism due to a random pseudomagnetic field generated by ripples in graphene, in sharp contrast to other nanohybrid Josephson junctions. Our study may pave the way for the development of new gate-tuneable superconducting quantum information devices.

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

Stochastic switching-current distribution in a graphene-based Josephson junction exhibits a crossover from the classical to quantum regime, revealing the macroscopic quantum tunneling of a Josephson phase particle at low temperatures. Microwave spectroscopy measurements indicate a multiphoton absorption process occurring via discrete energy levels in washboard potential well. The crossover temperature for macroscopic quantum tunneling and the quantized level spacing are controlled with the gate voltage, implying its potential application to gate-tunable superconducting quantum bits.

Pseudogap behavior revealed in interlayer tunneling in overdoped Bi 2 Sr 2 CaCu 2 O 8 + x

We report on heating-compensated interlayer tunneling spectroscopy (ITS) performed on stacks of overdoped

Heating-compensated constant-temperature tunneling measurements on stacks of Bi2Sr2CaCu2O8+x intrinsic junctions

{text{Bi}}_{2}{text{Sr}}_{2}{text{CaCu}}_{2}{text{O}}_{8+x}

Collective Josephson vortex dynamics in a finite number of stacked intrinsic Josephson junctions

intrinsic junctions, where most of the bias-induced heating in the ITS was eliminated. The onset temperature of a pseudogap (PG), which was revealed in the hump structure of the electronic excitation spectra, nearly reached room temperature for our overdoped intrinsic junctions, which represented the genuine PG onset. At a temperature below but close to

Heating‐free Interlayer Tunneling Spectroscopy in Bi2Sr2CaCu2O8+x Intrinsic Junctions

{T}_{c}

Scanning Photoemission Microscopy of Graphene Sheets on SiO2

, both the superconducting coherence peak and the pseudogap hump coexisted, implying that the two gaps are of separate origins. The hump voltage increased below