arXiv subjects

Choongyu Hwang

Publications and source records attributed to Choongyu Hwang.

At least 37 records · Page 2Linked to original sources

arXiv2018

Gapped Nearly Free-Standing Graphene on an SiC(0001) Substrate Induced by Manganese Atoms

The electron band structure of manganese-adsorbed graphene on an SiC(0001) substrate has been studied using angle-resolved photoemission spectroscopy. Upon introducing manganese atoms, the conduction band of graphene completely disappears and the valence band maximum is observed at 0.4 eV below Fermi energy. At the same time, the slope of the valence band decreases, approaching the electron band structure calculated using the local density approximation method. While the former provides experimental evidence of the formation of nearly free-standing graphene on an SiC substrate, concomitant with a metal-to-insulator transition, the latter suggests that its electronic correlations can be modified by foreign atoms. These results pave the way for promising device applications using graphene that is semiconducting and charge neutral.

cond-mat.mtrl-sci↗

arXiv2018

Emergence of Kondo resonance in graphene intercalated with cerium

The interaction between a magnetic impurity, such as cerium (Ce) atom, and surrounding electrons has been one of the core problems in understanding many-body interaction in solid and its relation to magnetism. Kondo effect, the formation of a new resonant ground state with quenched magnetic moment, provides a general framework to describe many-body interaction in the presence of magnetic impurity. In this Letter, a combined study of angle-resolved photoemission (ARPES) and dynamic mean-field theory (DMFT) on Ce-intercalated graphene shows that Ce-induced localized states near Fermi energy, $E_{\rm F}$, hybridized with the graphene $π$ band, exhibit gradual increase in spectral weight upon decreasing temperature. The observed temperature dependence follows the expectations from the Kondo picture in the weak coupling limit. Our results provide a novel insight how Kondo physics emerges in the sea of two-dimensional Dirac electrons.

cond-mat.str-el↗

arXiv2017

Temperature-Dependent Electron-Electron Interaction in Graphene on SrTiO3

The electron band structure of graphene on SrTiO3 substrate has been investigated as a function of temperature. The high-resolution angle-resolved photoemission study reveals that the spectral width at Fermi energy and the Fermi velocity of graphene on SrTiO3 are comparable to those of graphene on a BN substrate. Near the charge neutrality, the energy-momentum dispersion of graphene exhibits a strong deviation from the well-known linearity, which is magnified as temperature decreases. Such modification resembles the characteristics of enhanced electron-electron interaction. Our results not only suggest that SrTiO3 can be a plausible candidate as a substrate material for applications in graphene-based electronics, but also provide a possible route towards the realization of a new type of strongly correlated electron phases in the prototypical two-dimensional system via the manipulation of temperature and a proper choice of dielectric substrates.

cond-mat.mtrl-sci↗

arXiv2017

Hole doping, hybridization gap, and electronic correlation in graphene on a platinum substrate

The interaction between graphene and substrates provides a viable routes to enhance functionality of both materials. Depending on the nature of electronic interaction at the interface, the electron band structure of graphene is strongly influenced, allowing us to make use the intrinsic properties of graphene or to design additional functionality in graphene. Here, we present an angle-resolved photoemission study on the interaction between graphene and a platinum substrate. The formation of an interface between graphene and platinum leads to a strong deviation in the electronic structure of graphene not only from its freestanding form but also from the behavior observed on typical metals. The combined study on the experimental and theoretical electron band structure unveils the unique electronic properties of graphene on a platinum substrate, which singles out graphene/platinum as a model system investigating graphene on a metallic substrate with strong interaction.

cond-mat.mtrl-sci↗

arXiv2017

Realization of Quantum Spin Hall State in Monolayer 1T'-WTe2

A quantum spin Hall (QSH) insulator is a novel two-dimensional quantum state of matter that features quantized Hall conductance in the absence of magnetic field, resulting from topologically protected dissipationless edge states that bridge the energy gap opened by band inversion and strong spin-orbit coupling. By investigating electronic structure of epitaxially grown monolayer 1T'-WTe2 using angle-resolved photoemission (ARPES) and first principle calculations, we observe clear signatures of the topological band inversion and the band gap opening, which are the hallmarks of a QSH state. Scanning tunneling microscopy measurements further confirm the correct crystal structure and the existence of a bulk band gap, and provide evidence for a modified electronic structure near the edge that is consistent with the expectations for a QSH insulator. Our results establish monolayer 1T'-WTe2 as a new class of QSH insulator with large band gap in a robust two-dimensional materials family of transition metal dichalcogenides (TMDCs).

cond-mat.mtrl-sci↗

arXiv2016

ARPES study of the epitaxially grown topological crystalline insulator SnTe(111)

SnTe is a prototypical topological crystalline insulator, in which the gapless surface state is protected by a crystal symmetry. The hallmark of the topological properties in SnTe is the Dirac cones projected to the surfaces with mirror symmetry, stemming from the band inversion near the L points of its bulk Brillouin zone, which can be measured by angle-resolved photoemission. We have obtained the (111) surface of SnTe film by molecular beam epitaxy on BaF2(111) substrate. Photon-energy-dependence of in situ angle-resolved photoemission, covering multiple Brillouin zones in the direction perpendicular tothe (111) surface, demonstrate the projected Dirac cones at the Gamma_bar and M_bar points of the surface Brillouinzone. In addition, we observe a Dirac-cone-like band structure at the Gamma point of the bulk Brillouin zone,whose Dirac energy is largely different from those at the Gamma_bar and M_bar points.

cond-mat.mtrl-sci↗

arXiv2016

Study on the high spectral intensity at the Dirac energy of single-layer graphene on an SiC substrate

We have investigated electron band structure of epitaxially grown graphene on an SiC(0001) substrate using angle-resolved photoemission spectroscopy. In single-layer graphene, abnormal high spectral intensity is observed at the Dirac energy whose origin has been questioned between in-gap states induced by the buffer layer and plasmaron bands induced by electron-plasmon interactions. With the formation of double-layer graphene, the Dirac energy does not show the high spectral intensity any longer different from the single-layer case. The inconsistency between the two systems suggests that the main ingredient of the high spectral intensity at the Dirac energy of single-layer graphene is the electronic states originating from the coupling of the graphene $π$ bands to the localized $π$ states of the buffer layer, consistent with the theoretical prediction on the presence of in-gap states.

cond-mat.mtrl-sci↗

arXiv2016

Magnetic effects in sulfur-decorated graphene

The interaction between two different materials can present novel phenomena that are quite different from the physical properties observed when each material stands alone. Strong electronic correlations, such as magnetism and superconductivity, can be produced as the result of enhanced Coulomb interactions between electrons. Two-dimensional materials are powerful candidates to search for the novel phenomena because of the easiness of arranging them and modifying their properties accordingly. In this work, we report magnetic effects of graphene, a prototypical non-magnetic two-dimensional semi-metal, in the proximity with sulfur, a diamagnetic insulator. In contrast to the well-defined metallic behaviour of clean graphene, an energy gap develops at the Fermi energy for the graphene/sulfur compound with decreasing temperature. This is accompanied by a steep increase of the resistance, a sign change of the slope in the magneto-resistance between high and low fields, and magnetic hysteresis. A possible origin of the observed electronic and magnetic responses is discussed in terms of the onset of low-temperature magnetic ordering. These results provide intriguing insights on the search for novel quantum phases in graphene-based compounds.

cond-mat.mtrl-sci↗

arXiv2016

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 (MBE). The combination of angle-resolved photoemission (ARPES), scanning tunneling microscopy/spectroscopy (STM/STS), 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 3D semiconductors, yet small as compared to other 2D 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.

cond-mat.mtrl-sci↗

arXiv2015

Tight-binding approach to understand photoelectron intensity from graphene for circularly polarized light

We have investigated the effect of imperfect circular polarization on the angle-resolved photoemission spectroscopy signal, using graphene as a prototypical system that can be understood within tight-binding formalism. We found that perfect left- and right-circularly polarized lights give the same photoelectron intensity distribution around a constant energy contour of the graphene $π$ band. On the other hand, upon breaking the purity of the polarization, photoelectron intensity starts to show circular dichroism, which is enhanced with further increasing the imperfection. Our results predict the existence of an additional factor for the circular dichroism observed in the photoemission signal from graphene and hence suggest the importance of experimental conditions to understand circular dichroism observed via photoemission spectroscopy.

cond-mat.mtrl-sci↗

arXiv2014

Ytterbium-driven strong enhancement of electron-phonon coupling in graphene

We present high-resolution angle-resolved photoemission spectroscopy study in conjunction with first principles calculations to investigate how the interaction of electrons with phonons in graphene is modified by the presence of Yb. We find that the transferred charges from Yb to the graphene layer hybridize with the graphene $π$ bands, leading to a strong enhancement of the electron-phonon interaction. Specifically, the electron-phonon coupling constant is increased by as much as a factor of 10 upon the introduction of Yb with respect to as grown graphene ($\leq$0.05). The observed coupling constant constitutes the highest value ever measured for graphene and suggests that the hybridization between graphene and the adatoms might be a critical parameter in realizing superconducting graphene.

cond-mat.supr-con↗

arXiv2013

Photoelectron spin-flipping and texture manipulation in a topological insulator

Recently discovered materials called three-dimensional topological insulators constitute examples of symmetry protected topological states in the absence of applied magnetic fields and cryogenic temperatures. A hallmark characteristic of these non-magnetic bulk insulators is the protected metallic electronic states confined to the material's surfaces. Electrons in these surface states are spin polarized with their spins governed by their direction of travel (linear momentum), resulting in a helical spin texture in momentum space. Spin- and angle-resolved photoemission spectroscopy (spin-ARPES) has been the only tool capable of directly observing this central feature with simultaneous energy, momentum, and spin sensitivity. By using an innovative photoelectron spectrometer with a high-flux laser-based light source, we discovered another surprising property of these surface electrons which behave like Dirac fermions. We found that the spin polarization of the resulting photoelectrons can be fully manipulated in all three dimensions through selection of the light polarization. These surprising effects are due to the spin-dependent interaction of the helical Dirac fermions with light, which originates from the strong spin-orbit coupling in the material. Our results illustrate unusual scenarios in which the spin polarization of photoelectrons is completely different from the spin state of electrons in the originating initial states. The results also provide the basis for a novel source of highly spin-polarized electrons with tunable polarization in three dimensions.

cond-mat.mtrl-sci↗

arXiv2012

Fermi velocity engineering in graphene by substrate modification

The Fermi velocity 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 Fermi velocity. Indeed, several efforts have succeeded in modifying Fermi velocity by varying charge carrier concentration. Here we present a powerful but simple new way to engineer Fermi velocity while holding the charge carrier concentration constant. We find that when the environment embedding graphene is modified, the Fermi velocity of graphene is (i) inversely proportional to its dielectric constant, reaching ~2.5$\times10^6$ 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.

cond-mat.mes-hall↗

arXiv2011

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 Berry's 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 π/2, we found that graphene with a Berry's phase of nπ(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 π/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.

cond-mat.mtrl-sci↗

arXiv2011

Electron-Phonon Coupling in Highly-Screened Graphene

Photoemission studies of graphene have resulted in a long-standing controversy concerning the strength of the experimental electron-phonon interaction in comparison with theoretical calculations. Using high-resolution angle-resolved photoemission spectroscopy we study graphene grown on a copper substrate, where the metallic screening of the substrate substantially reduces the electron-electron interaction, simplifying the comparison of the electron-phonon interaction between theory and experiment. By taking the nonlinear bare bandstructure into account, we are able to show that the strength of the electron-phonon interaction does indeed agree with theoretical calculations. In addition, we observe a significant bandgap at the Dirac point of graphene.

cond-mat.mtrl-sci↗

arXiv2011

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 like 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.

cond-mat.mtrl-sci↗

arXiv2011

Quasi-Freestanding Multilayer Graphene Films on the Carbon Face of SiC

The electronic band structure of as-grown and doped graphene grown on the carbon face of SiC is studied by high-resolution angle-resolved photoemission spectroscopy, where we observe both rotations between adjacent layers and AB-stacking. The band structure of quasi-freestanding AB- bilayers is directly compared with bilayer graphene grown on the Si-face of SiC to study the impact of the substrate on the electronic properties of epitaxial graphene. Our results show that the C-face films are nearly freestanding from an electronic point of view, due to the rotations between graphene layers.

cond-mat.mtrl-sci↗

arXiv2011

Self-trapping nature of Tl nanoclusters on Si(111)-7$\times$7 surface

We have investigated electronic and structural properties of thallium (Tl) nanoclusters formed on the Si(111)-7$\times$7 surface at room temperature (RT) by utilizing photoemission spectroscopy (PES) and high-resolution electron-energy-loss spectroscopy (HREELS) combined with first principles calculations. Our PES data show that the state S2 stemming from Si restatoms remains quite inert with Tl coverage $θ$ while S1 from Si adatoms gradually changes, in sharp contrast with the rapidly decaying states of Na or Li nanoclusters. No Tl-induced surface state is observed until $θ$=0.21 ML where Tl nanoclusters completely cover the faulted half unit cells (FHUCs) of the surface. These spectral behaviors of surface states and a unique loss peak L$_2$ associated with Tl in HREELS spectra indicate no strong Si-Tl bonding and are well understood in terms of gradual filling of Si dangling bonds with increasing $θ$. Our calculational results further reveal that there are several metastable atomic structures for Tl nanoclusters at RT transforming from each other faster than 10$^{10}$ flippings per second. We thus conclude that the highly mobile Tl atoms form self-trapped nanoclusters within FHUC at RT with several metastable phases. The mobile and multi-phased nature of Tl nanoclusters not only account for all the existing experimental observations including the fuzzy scanning tunneling microscope images and a dynamical model proposed by recent x-ray study but also provides an example of self-trapping of atoms in a nanometer-scale region.

cond-mat.mes-hall↗