Choongyu Hwang

Fermi velocity engineering in graphene by substrate modification

The Fermi velocity, vF, is one of the key concepts in the study of a material, as it bears information on a variety of fundamental properties. Upon increasing demand on the device applications, graphene is viewed as a prototypical system for engineering vF. Indeed, several efforts have succeeded in modifying vF by varying charge carrier concentration, n. Here we present a powerful but simple new way to engineer vF while holding n constant. We find that when the environment embedding graphene is modified, the vF of graphene is (i) inversely proportional to its dielectric constant, reaching vF ~ 2.5×106 m/s, the highest value for graphene on any substrate studied so far and (ii) clearly distinguished from an ordinary Fermi liquid. The method demonstrated here provides a new route toward Fermi velocity engineering in a variety of two-dimensional electron systems including topological insulators.

Many-body interactions in quasi-freestanding graphene

The Landau–Fermi liquid picture for quasiparticles assumes that charge carriers are dressed by many-body interactions, forming one of the fundamental theories of solids. Whether this picture still holds for a semimetal such as graphene at the neutrality point, i.e., when the chemical potential coincides with the Dirac point energy, is one of the long-standing puzzles in this field. Here we present such a study in quasi-freestanding graphene by using high-resolution angle-resolved photoemission spectroscopy. We see the electron–electron and electron–phonon interactions go through substantial changes when the semimetallic regime is approached, including renormalizations due to strong electron–electron interactions with similarities to marginal Fermi liquid behavior. These findings set a new benchmark in our understanding of many-body physics in graphene and a variety of novel materials with Dirac fermions.

Photoelectron spin-flipping and texture manipulation in a topological insulator

In a topological insulator, the surface-state electron spins are ‘locked’ to their direction of travel. But when an electron is kicked out by a photon through the photoelectric effect, the spin polarization is not necessarily conserved. In fact, the ejected spins can be completely manipulated in three dimensions by the incident photons.

Electronic Structure, Surface Doping, and Optical Response in Epitaxial WSe2 Thin Films

High quality WSe2 films have been grown on bilayer graphene (BLG) with layer-by-layer control of thickness using molecular beam epitaxy. The combination of angle-resolved photoemission, scanning tunneling microscopy/spectroscopy, and optical absorption measurements reveal the atomic and electronic structures evolution and optical response of WSe2/BLG. We observe that a bilayer of WSe2 is a direct bandgap semiconductor, when integrated in a BLG-based heterostructure, thus shifting the direct-indirect band gap crossover to trilayer WSe2. In the monolayer limit, WSe2 shows a spin-splitting of 475 meV in the valence band at the K point, the largest value observed among all the MX2 (M = Mo, W; X = S, Se) materials. The exciton binding energy of monolayer-WSe2/BLG is found to be 0.21 eV, a value that is orders of magnitude larger than that of conventional three-dimensional semiconductors, yet small as compared to other two-dimensional transition metal dichalcogennides (TMDCs) semiconductors. Finally, our finding regarding the overall modification of the electronic structure by an alkali metal surface electron doping opens a route to further control the electronic properties of TMDCs.

Direct measurement of quantum phases in graphene via photoemission spectroscopy

Quantum phases provide us with important information for understanding the fundamental properties of a system. However, the observation of quantum phases, such as Berrys phase and the sign of the matrix element of the Hamiltonian between two non-equivalent localized orbitals in a tight-binding formalism, has been challenged by the presence of other factors, e.g., dynamic phases and spin/valley degeneracy, and the absence of methodology. Here, we report a new way to directly access these quantum phases, through polarization-dependent angle-resolved photoemission spectroscopy (ARPES), using graphene as a prototypical two-dimensional material. We show that the momentum- and polarization-dependent spectral intensity provides direct measurements of (i) the phase of the band wavefunction and (ii) the sign of matrix elements for non-equivalent orbitals. Upon rotating light polarization by \pi/2, we found that graphene with a Berrys phase of n\pi (n=1 for single- and n=2 for double-layer graphene for Bloch wavefunction in the commonly used form) exhibits the rotation of ARPES intensity by \pi/n, and that ARPES signals reveal the signs of the matrix elements in both single- and double-layer graphene. The method provides a new technique to directly extract fundamental quantum electronic information on a variety of materials.

Observation of the intrinsic bandgap behaviour in as-grown epitaxial twisted graphene.

Twisted graphene is of particular interest due to several intriguing characteristics, such as its the Fermi velocity, van Hove singularities and electronic localization. Theoretical studies recently suggested the possible bandgap opening and tuning. Here, we report a novel approach to producing epitaxial twisted graphene on SiC (0001) and the observation of its intrinsic bandgap behaviour. The direct deposition of C60 on pre-grown graphene layers results in few-layer twisted graphene confirmed by angular resolved photoemission spectroscopy and Raman analysis. The strong enhanced G band in Raman and sp(3) bonding characteristic in X-ray photoemission spectroscopy suggests the existence of interlayer interaction between adjacent graphene layers. The interlayer spacing between graphene layers measured by transmission electron microscopy is 0.352 ± 0.012 nm. Thermal activation behaviour and nonlinear current-voltage characteristics conclude that an intrinsic bandgap is opened in twisted graphene. Low sheet resistance (~ 160 Ω □(-1) at 10 K) and high mobility (~2,000 cm(2) V(-1) s(-1) at 10 K) are observed.

Stability of graphene band structures against an external periodic perturbation: Na on graphene

The electronic structure of Na-adsorbed graphenes formed on the 6H-SiC(0001) substrate was studied using angle-resolved photoemission spectroscopy with synchrotron photons and ab initio pseudopotential calculations. It was found that the band of the graphenes sensitively changes upon Na adsorption especially at low temperature. With increasing Na dose, the

Electron–phonon coupling and intrinsic bandgap in highly-screened graphene

\ensuremath{\pi}

Ultrafast quenching of electron–boson interaction and superconducting gap in a cuprate superconductor

band appears to be quickly diffused into the background at 85 K whereas it becomes significantly enhanced with its spectral intensity at room temperature (RT). A new parabolic band centered at

Quasi-Freestanding multilayer graphene films on the carbon face of SiC

k\ensuremath{\sim}1.15\text{ }{\text{\AA{}}}^{\ensuremath{-}1}