arXiv subjects

Han Woong Yeom

Publications and source records attributed to Han Woong Yeom.

49 records · Page 3Linked to original sources

arXiv2015

Nanoscale manipulation of the Mott insulating state coupled to charge order in 1T-TaS2

Quantum states of strongly correlated electrons are of prime importance to understand exotic properties of condensed matter systems and the controllability over those states promises unique electronic devices such as a Mott memory. As a recent example, a ultrafast switching device was demonstrated using the transition between the correlated Mott insulating state and a hidden-order metallic state of a layered transition metal dichalcogenides 1T-TaS2. However, the origin of the hidden metallic state was not clear and only the macroscopic switching by laser pulse and carrier injection was reported. Here, we demonstrate the nanoscale manipulation of the Mott insulating state of 1T-TaS2. The electron pulse from a scanning tunneling microscope switches the insulating phase locally into a metallic phase which is textured with irregular domain walls in the charge density wave (CDW) order inherent to this Mott state. The metallic state is a novel correlated phase near the Mott criticality with a coherent feature at the Fermi energy, which is induced by the moderate reduction of electron correlation due to the decoherence in CDW. This work paves the avenue toward novel nanoscale electronic devices based on correlated electrons.

cond-mat.mtrl-sci↗

arXiv2015

Interplay of electron-electron and electron-phonon interactions in the low temperature phase of 1T-TaS2

We investigate the interplay of the electron-electron and electron-phonon interactions in the electronic structure of an exotic insulating state in the layered dichalcogenide 1T-TaS2, where the charge-density-wave (CDW) order coexists with a Mott correlation gap. Scanning tunneling microscopy and spectroscopy measurements with high spatial and energy resolution determine unambiguously the CDW and the Mott gap as 0.20-0.24 eV and 0.32 eV, respectively, through the real space electron phases measured across the multiply formed energy gaps. An unusual local reduction of the Mott gap is observed on the defect site, which indicates the renormalization of the on-site Coulomb interaction by the electron-phonon coupling as predicted by the Hubbard-Holstein model. The Mott-gap renormalization provides new insight into the disorder-induced quasi-metallic phases of 1T-TaS2.

cond-mat.mtrl-sci↗

arXiv2015

Electron Quantization in Broken Atomic Wires

We demonstrate using scanning tunneling microscopy and spectroscopy the electron quantization within metallic Au atomic wires self-assembled on a Si(111) surface and segmented by adatom impurities. The local electronic states of wire segments with a length up to 10 nm are investigated as terminated by two neighboring Si adatoms. One dimensional (1D) quantum well states are well resolved by their spatial distributions and the inverse-length-square dependence in their energies. The quantization also results in the quantum oscillation of the conductance at the Fermi level. These results deny the dopant role of the adatoms assumed for a long time but indicate their strong scattering nature. The present approach provides a new and convenient platform to investigate 1D quantum phenomena with atomic precision.

cond-mat.mes-hall↗

arXiv2014

Transforming a Surface State of Topological Insulator by a Bi Capping Layer

We introduce a dinstint approach to engineer a topologically protected surface state of a topological insulator. By covering the surface of a topological insulator, Bi2Te2Se, with a Bi monolayer film, the original surface state is completely removed and three new spin helical surface states, originating from the Bi film, emerge with different dispersion and spin polarization, through a strong electron hybridization. These new states play the role of topological surface states keeping the bulk topological nature intact. This mechanism provides a way to create various different types of topologically protected electron channels on top of a single topological insulator, possibly with tailored properties for various applications.

cond-mat.mes-hall↗

arXiv2014

Impurity-mediated early condensation of an atomic layer electronic crystal

While impurity has been known widely to affect phase transitions, the atomistic mechanisms have rarely been disclosed. We directly show in atomic scale how impurity atoms induces the condensation of a representative electronic phase, charge density wave (CDW), with scanning tunneling microscopy. Oxygen impurity atoms on the self-assembled metallic atomic wire array on a silicon crystal condense CDW locally even above the transition temperature, More interestingly, the CDW along the wires is induced not by a single atomic impurity but by the cooperation of multiple impurities. First principles calculations disclose the mechanism of the cooperation as the coherent superposition of the local lattice strain induced by impurities, stressing the coupled electronic and lattice degrees of freedom for CDW. This newly discovered mechanism can widely be applied to various important electronic orders coupled to lattice, opening the possibility of the atomic scale strain engineering.

cond-mat.mes-hall↗

arXiv2014

Structural versus electronic distortions of symmetry-broken IrTe$_2$

We investigate atomic and electronic structures of the intriguing low temperature phase of IrTe2 using high-resolution scanning tunneling microscopy and spectroscopy. We confirm various stripe superstructures such as $\times$3, $\times$5, and $\times$8. The strong vertical and lateral distortions of the lattice for the stripe structures are observed in agreement with recent calculations. The spatial modulations of electronic density of states are clearly identified as separated from the structural distortions. These structural and spectroscopic characteristics are not consistent with the charge-density wave and soliton lattice model proposed recently. Instead, we show that the Ir (Te) dimerization together with the Ir 5d charge ordering can explain these superstructures, supporting the Ir dimerization mechanism of the phase transition.

cond-mat.str-el↗

arXiv2014

Atomic scale variation of electron tunneling into a Luttinger liquid ? : High resolution scanning tunneling spectroscopy study on Au/Ge(001)

Au-induced atomic wires on the Ge(001) surface were recently claimed to be an ideal 1D metal and their tunneling spectra were analyzed as the manifestation of a Tomonaga-Luttinger liquid (TLL) state. We reinvestigate this system for atomically well-ordered areas of the surface with high resolution scanning tunneling microscopy and spectroscopy (STS). The local density-of-states maps do not provide any evidence of a metallic 1D electron channel along the wires. Moreover, the atomically resolved tunneling spectra near the Fermi energy are dominated by local density-of-states features, deviating qualitatively from the power-law behavior. On the other hand, the defects strongly affect the tunneling spectra near the Fermi level. These results do not support the possibility of a TLL state for this system. An 1D metallic system with well-defined 1D bands and without defects are required for the STS study of a TLL state.

cond-mat.str-el↗

arXiv2014

Edge and Interfacial States in a 2D Topological Insulator:Bi(111) Bilayer on Bi$_{2}$Te$_{2}$Se

The electronic states of a single Bi(111) bilayer and its edges, suggested as a two dimensional topological insulator, are investigated by scanning tunneling spectroscopy (STS) and first-principles calculations. Well-ordered bilayer films and islands with zigzag edges are grown epitaxially on a cleaved Bi$_{2}$Te$_{2}$Se crystal. The calculation shows that the band gap of the Bi bilayer closes with a formation of a new but small hybridization gap due to the strong interaction between Bi and Bi$_{2}$Te$_{2}$Se. Nevertheless, the topological nature of the Bi bilayer and the topological edge state are preserved only with an energy shift. The edge-enhanced local density of states are identified and visualized clearly by STS in good agreement with the calculation. This can be the sign of the topological edge state, which corresponds to the quantum spin Hall state. The interfacial state between Bi and Bi$_{2}$Te$_{2}$Se is also identified inside the band gap region. This state also exhibits the edge modulation, which was previously interpreted as the evidence of the topological edge state [F. Yang et al., Phys. Rev. Lett. 109, 016801 (2012)].

cond-mat.mes-hall↗

arXiv2013

Self-Assembled Nanowires with Giant Rashba-Type Band Splitting

We investigated Pt-induced nanowires on the Si(110) surface using scanning tunneling microscopy (STM) and angle-resolved photoemission (ARP). High resolution STM images show a well-ordered nanowire array of 1.6 nm width and 2.7 nm separation. ARP reveals fully occupied one dimensional (1D) bands with a Rashba-type split dispersion. Local dI/dV spectra further indicate well confined 1D electron channels on the nanowires, whose density of states characteristics are consistent with the Rashba-type band splitting. The observed energy and momentum splitting of the bands are among the largest ever reported for Rashba systems, suggesting the Pt-Si nanowire as a unique 1D giant Rashba system. This self-assembled nanowire can be exploited for silicon-based spintronics devices as well as the quest for Majorana Fermions.

cond-mat.mes-hall↗

arXiv2013

Visualizing Atomic-Scale Negative Differential Resistance in Bilayer Graphene

We investigate the atomic-scale tunneling characteristics of bilayer graphene on silicon carbide using the scanning tunneling microscopy. The high-resolution tunneling spectroscopy reveals an unexpected negative differential resistance (NDR) at the Dirac energy, which spatially varies within the single unit cell of bilayer graphene. The origin of NDR is explained by two near-gap van Hove singularities emerging in the electronic spectrum of bilayer graphene under a transverse electric field, which are strongly localized on two sublattices in different layers. Furthermore, defects near the tunneling contact are found to strongly impact on NDR through the electron interference. Our result provides an atomic-level understanding of quantum tunneling in bilayer graphene, and constitutes a useful step towards graphene-based tunneling devices.

cond-mat.mtrl-sci↗

arXiv2012

Topological Solitons versus Non-Solitonic Phase Defects in Quasi-One-Dimensional Charge Density Wave

We investigated phase defects in a quasi-one-dimensional commensurate charge density wave (CDW) system, an In atomic wire array on Si(111), using low temperature scanning tunneling microscopy. The unique four-fold degeneracy of the CDW state leads to various phase defects, among which intrinsic solitons are clearly distinguished. The solitons exhibit a characteristic variation of the CDW amplitude with a coherence length of about 4 nm, as expected from the electronic structure, and a localized electronic state within the CDW gap. While most of the observed solitons are trapped by extrinsic defects, moving solitons are also identified and their novel interaction with extrinsic defects is disclosed.

cond-mat.mes-hall↗

arXiv2011

Radial Band Structure of Electrons in Liquid Metals

The electronic band structure of a liquid metal was investigated by measuring precisely the evolution of angle-resolved photoelectron spectra during the melting of a Pb monolayer on a Si(111) surface. We found that the liquid monolayer exhibits a free-electron-like band and it undergoes a coherent radial scattering, imposed by the radial correlation of constituent atoms, to form a characteristic secondary hole band. This unique double radial bands and their gradual evolution during melting can be quantitatively reproduced, including detailed spectral intensity profiles, with our radial scattering model based on a theoretical prediction of 1962. Our result establishes the radial band structure as a key concept for describing the nature of electrons in strongly disordered states of matter.

cond-mat.mtrl-sci↗

arXiv2010

Nearly Massless Electrons in the Silicon Interface with a Metal Film

We demonstrate the realization of nearly massless electrons in the most widely used device material, silicon, at the interface with a metal film. Using angle-resolved photoemission, we found that the surface band of a monolayer lead film drives a hole band of the Si inversion layer formed at the interface with the film to have nearly linear dispersion with an effective mass about 20 times lighter than bulk Si and comparable to graphene. The reduction of mass can be accounted for by repulsive interaction between neighboring bands of the metal film and Si substrate. Our result suggests a promising way to take advantage of massless carriers in silicon-based thin-film devices, which can also be applied for various other semiconductor devices.

cond-mat.mtrl-sci↗