arXiv subjects

Young-Woo Son

Publications and source records attributed to Young-Woo Son.

At least 55 records · Page 3Linked to original sources

arXiv2017

Strain-shear coupling in bilayer MoS2

Layered materials such as graphite and transition metal dichalcogenides have extremely anisotropic mechanical properties owing to orders of magnitude difference between in-plane and out-of-plane interatomic interaction strengths. Although effects of mechanical perturbations on either intra- or inter-layer interactions have been extensively investigated, mutual correlations between them have rarely been addressed. Here we show that layered materials have an inevitable coupling between in-plane uniaxial strain and interlayer shear. Because of this, the uniaxial in-plane strain induces an anomalous splitting of the degenerate interlayer shear phonon modes such that the split shear mode along the tensile strain is not softened but hardened contrary to the case of intralayer phonon modes. We confirm the effect by measuring Raman shifts of shear modes of bilayer MoS2 under strain. Moreover, by analyzing the splitting, we obtain an unexplored off-diagonal elastic constant, demonstrating that Raman spectroscopy can determine almost all mechanical constants of layered materials.

cond-mat.mtrl-sci↗

arXiv2017

Quasiparticle energy bands and Fermi surfaces of monolayer NbSe$_2$

A quasiparticle band structure of a single layer 2H-NbSe$_2$ is reported by using first-principles $GW$ calculation. We show that a self-energy correction increases the width of a partially occupied band and alters its Fermi surface shape when comparing those using conventional mean-field calculation methods. Owing to a broken inversion symmetry in the trigonal prismatic single layer structure, the spin-orbit interaction is included and its impact on the Fermi surface and quasiparticle energy bands are discussed. We also calculate the doping dependent static susceptibilities from the band structures obtained by the mean-field calculation as well as $GW$ calculation with and without spin-orbit interactions. A complete tight-binding model is constructed within the three-band third nearest neighbour hoppings and is shown to reproduce our $GW$ quasiparticle energy bands and Fermi surface very well. Considering variations of the Fermi surface shapes depending on self-energy corrections and spin-orbit interactions, we discuss the formations of charge density wave (CDW) with different dielectric environments and their implications on recent controversial experimental results on CDW transition temperatures.

cond-mat.mes-hall↗

arXiv2017

Atomically flat two-dimensional silicon crystals with versatile electronic properties

Silicon (Si) is one of the most extensively studied materials owing to its significance to semiconductor science and technology. While efforts to find a new three-dimensional (3D) Si crystal with unusual properties have made some progress, its two-dimensional (2D) phases have not yet been explored as much. Here, based on a newly developed systematic $ab$ $initio$ materials searching strategy, we report a series of novel 2D Si crystals with unprecedented structural and electronic properties. The new structures exhibit perfectly planar outermost surface layers of a distorted hexagonal network with their thicknesses varying with the atomic arrangement inside. Dramatic changes in electronic properties ranging from semimetal to semiconducting with indirect energy gaps and even to one with direct energy gaps are realized by varying thickness as well as by surface oxidation. Our predicted 2D Si crystals with flat surfaces and tunable electronic properties will shed light on the development of silicon-based 2D electronics technology.

cond-mat.mtrl-sci↗

arXiv2017

Origins of the structural phase transitions in MoTe$_2$ and WTe$_2$

Layered transition metal dichalcogenides MoTe$_2$ and WTe$_2$ share almost similar lattice constants as well as topological electronic properties except their structural phase transitions. While the former shows a first-order phase transition between monoclinic and orthorhombic structures, the latter does not. Using a recently proposed van der Waals density functional method, we investigate structural stability of the two materials and uncover that the disparate phase transitions originate from delicate differences between their interlayer bonding states near the Fermi energy. By exploiting the relation between the structural phase transitions and the low energy electronic properties, we show that a charge doping can control the transition substantially, thereby suggesting a way to stabilize or to eliminate their topological electronic energy bands.

cond-mat.mes-hall↗

arXiv2016

Inhibiting Klein tunneling in graphene p-n junction without an external magnetic field

We study by first-principles calculations a densely packed island of organic molecules (F4TCNQ) adsorbed on graphene. We find that with electron doping the island naturally forms a p-n junction in the graphene sheet. For example, a doping level of ~ 3 * 10^{13} electrons per cm^2 results in a p-n junction with 800 meV electrostatic potential barrier. Unlike in a conventional p-n junction in graphene, in the case of the junction formed by an adsorbed organic molecular island we expect that the Klein tunneling is inhibited, even without an applied external magnetic field. Here Klein tunneling is inhibited by the ferromagnetic order that spontaneously occurs in the molecular island upon doping. We estimate that the magnetic barrier in the graphene sheet is around 10 mT.

cond-mat.mes-hall↗

arXiv2015

Photocurrent generation at ABA/ABC lateral junction in tri-layer graphene photodetector

Metal-graphene-metal photodetectors utilize photocurrent generated near the graphene/metal junctions and have many advantages including high speed and broad-band operation. Here, we report on photocurrent generation at ABA/ABC stacking domain junctions in tri-layer graphene with a responsivity of 0.18 A/W. Unlike usual metal-graphene-metal devices, the photocurrent is generated in the middle of the graphene channel, not confined to the vicinity of the metal electrodes. The magnitude and the direction of the photocurrent depend on the back-gate bias. Theoretical calculations show that there is a built-in band offset between the two stacking domains, and the dominant mechanism of the photocurrent is the photo-thermoelectric effect due to the Seebeck coefficient difference.

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arXiv2015

Poisson's Ratio of Layered Two-dimensional Crystals

We present first-principles calculations of elastic properties of multilayered two-dimensional crystals such as graphene, h-BN and 2H-MoS2 which shows that their Poisson's ratios along out-of-plane direction are negative, near zero and positive, respectively, spanning all possibilities for sign of the ratios. While the in-plane Poisson's ratios are all positive regardless of their disparate electronic and structural properties, the characteristic interlayer interactions as well as layer stacking structures are shown to determine the sign of their out-of-plane ratios. Thorough investigation of elastic properties as a function of the number of layers for each system is also provided, highlighting their intertwined nature between elastic and electronic properties.

cond-mat.mtrl-sci↗

arXiv2015

A metallic mosaic phase and the origin of Mott insulating state in 1T-TaS2

Electron-electron and electron-phonon interactions are two major driving forces that stabilize various charge-ordered phases of matter. The intricate interplay between the two give rises to a peculiar charge density wave (CDW) state, which is also known as a Mott insulator, as the ground state of layered compound 1T-TaS2. The delicate balance also makes it possible to use external perturbations to create and manipulate novel phases in this material. Here, we study a mosaic CDW phase induced by voltage pulses from the tip of a scanning tunneling microscope (STM), and find that the new phase exhibit electronic structures that are entirely different from the Mott ground state of 1T-TaS2 at low temperatures. The mosaic phase consists of nanometer-sized domains characterized by well-defined phase shifts of the CDW order parameter in the topmost layer, and by altered stacking relative to the layer underneath. We discover that the nature of the new phases is dictated by the stacking order, and our results shed fresh light on the origin of the Mott phase in this layered compound.

cond-mat.mtrl-sci↗

arXiv2015

Anomalous excitonic resonance Raman effects in few-layer MoS2

The resonance effects on the Raman spectra from 5 to 900 cm-1 of few-layer MoS2 thin films up to 14-layers were investigated by using six excitation energies. For the main first-order Raman peaks, the intensity maximum occurs at ~2.8 eV for single layer and at ~2.5 eV for few-layer MoS2, which correspond to the band-gap energy. At the excitation energy of 1.96 eV, several anomalous behaviors are observed. Many second-order peaks are anomalously enhanced even though the main first-order peaks are not enhanced. In the low-frequency region (<100 cm-1), a broad peak centered at ~38 cm-1 and its second order peak at 76 cm-1 appear for the excitation energy of 1.96 eV. These anomalous resonance effects are interpreted as being due to strong resonance with excitons or exciton-polaritons.

cond-mat.mtrl-sci↗

arXiv2015

Incommensurate double-walled carbon nanotubes as one-dimensional moiré crystals

Cylindrical multishell structure is one of the prevalent atomic arrangements in nanowires. Being multishell, the well-defined atomic periodicity is hardly realized in it because the periodic units of individual shells therein generally do not match except for very few cases, posing a challenge to understand its physical properties. Here we show that moiré patterns generated by superimposing atomic lattices of individual shells are decisive in determining its electronic structures. Double- walled carbon nanotubes, as an example, are shown to have spectacular variations in their electronic properties from metallic to semiconducting and further to insulating states depending on their moiré patterns, even when they are composed of only semiconducting nanotubes with almost similar energy gaps and diameters. Thus, aperiodic multishell nanowires can be classified into new one-dimensional moiré crystals with distinct electronic structures.

cond-mat.mes-hall↗

arXiv2014

Manifestation of axion electrodynamics through magnetic ordering on edges of topological insulator

Based on a first-principles approach, we show that in a single crystal of a prototypical topological insulator such as Bi$_2$Se$_3$ the difference in the work function between adjacent surfaces with different crystal-face orientations generates a built-in electric field around facet edges. Owing to the topological magnetoelectric coupling for a given broken time-reversal symmetry in the crystal, the electric field, in turn, forces effective magnetic dipoles to accumulate along the edges, realizing the facet-edge magnetic ordering. We demonstrate that the predicted magnetic ordering which depends only on the work function difference between facets, is in fact a manifestation of the axion electrodynamics in real solids.

cond-mat.mes-hall↗

arXiv2014

Optical absorption of twisted bilayer graphene with interlayer potential asymmetry

We investigate the band structure and the optical absorption spectrum of twisted bilayer graphenes with changing interlayer bias and Fermi energy simultaneously. We show that the interlayer bias lifts the degeneracy of the superlattice Dirac point, while the amount of the Dirac point shift is significantly suppressed in small rotation angles, and even becomes opposite to the applied bias. We calculate the optical absorption spectrum in various asymmetric potentials and Fermi energies, and associate the characteristic spectral features with the band structure. The spectroscopic features are highly sensitive to the interlayer bias and the Fermi energy, and widely tunable by the external field effect.

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arXiv2014

Low-bias Negative Differential Resistance effect in armchair graphene nanoribbon junctions

Graphene nanoribbons with armchair edges (AGNRs) have bandgaps that can be flexibly tuned via the ribbon width. A junction made of a narrower AGNR sandwiched between two wider AGNR leads was recently reported to possess two perfect transmission channels close to the Fermi level. Here, we report that by using a bias voltage to drive these transmission channels into the gap of the wider AGNR lead, we can obtain a negative differential resistance (NDR) effect. Owing to the intrinsic properties of the AGNR junctions, the on-set bias reaches as low as ~ 0.2 V and the valley current almost vanishes. We further show that such NDR effect is robust against details of the atomic structure of the junction, substrate and whether the junction is made by etching or by hydrogenation.

cond-mat.mes-hall↗

arXiv2014

Large magnetoresistance from long-range interface coupling in armchair graphene nanoribbon junctions

In recent years, bottom-up synthesis procedures have achieved significant advancements in atomically-controlled growth of several-nanometer-long graphene nanoribbons with armchair-shaped edges (AGNRs). This greatly encourages us to explore the potential of such well-defined AGNRs in electronics and spintronics. Here, we propose an AGNR based spin valve architecture that induces a large magnetoresistance up to 900%. We find that, when an AGNR is connected perpendicularly to zigzag-shaped edges, the AGNR allows for long-range extension of the otherwise localized edge state. The huge magnetoresistance is a direct consequence of the coupling of two such extended states from both ends of the AGNR, which forms a perfect transmission channel. By tuning the coupling between these two spin-polarized states with a magnetic field, the channel can be destroyed, leading to an abrupt drop in electron transmission.

cond-mat.mtrl-sci↗

arXiv2014

Gate-tunable Phase Transitions in 1T-TaS$_2$

The ability to tune material properties using gate electric field is at the heart of modern electronic technology. It is also a driving force behind recent advances in two-dimensional systems, such as gate-electric-field induced superconductivity and metal-insulator transition. Here we describe an ionic field-effect transistor (termed "iFET"), which uses gate-controlled lithium ion intercalation to modulate the material property of layered atomic crystal 1T-TaS$_2$. The extreme charge doping induced by the tunable ion intercalation alters the energetics of various charge-ordered states in 1T-TaS$_2$, and produces a series of phase transitions in thin-flake samples with reduced dimensionality. We find that the charge-density-wave states in 1T-TaS$_2$ are three-dimensional in nature, and completely collapse in the two-dimensional limit defined by their critical thicknesses. Meanwhile the ionic gating induces multiple phase transitions from Mott-insulator to metal in 1T-TaS$_2$ thin flakes at low temperatures, with 5 orders of magnitude modulation in their resistance. Superconductivity emerges in a textured charge-density-wave state induced by ionic gating. Our method of gate-controlled intercalation of 2D atomic crystals in the bulk limit opens up new possibilities in searching for novel states of matter in the extreme charge-carrier-concentration limit.

cond-mat.mtrl-sci↗

arXiv2014

Polarization dependence of double resonant Raman scattering band in bilayer graphene

The polarization dependence of the double resonant Raman scattering (2D) band in bilayer graphene (BLG) is studied as a function of the excitation laser energy. It has been known that the complex shape of the 2D band of BLG can be decomposed into four Lorentzian peaks with different Raman frequency shifts attributable to four individual scattering paths in the energy-momentum space. From our polarization dependence study, however, we reveal that each of the four different peaks is actually doubly degenerate in its scattering channels, i.e., two different scattering paths with similar Raman frequency shifts for each peak. We find theoretically that one of these two paths, ignored for a long time, has a small contribution to their scattering intensities but are critical in understanding their polarization dependences. Because of this, the maximum-to-minimum intensity ratios of the four peaks show a strong dependence on the excitation energy, unlike the case of single-layer graphene (SLG). Our findings thus reveal another interesting aspect of electron-phonon interactions in graphitic systems.

cond-mat.mes-hall↗

arXiv2013

Anomalous Optical Phonon Splittings in Sliding Bilayer Graphene

We study the variations of electron-phonon coupling and their spectroscopic consequences in response to sliding of two layers in bilayer graphene using first-principles calculations and a model Hamiltonian. Our study shows that the long wave-length optical phonon modes change in a sensitive and unusual way depending on the symmetry as well as the parity of sliding atomic structures and that, accordingly, Raman- and infrared-active optical phonon modes behave differently upon the direction and size of the sliding. The renormalization of phonon modes by the interlayer electronic coupling is shown to be crucial to explain their anomalous behavior upon the sliding. Also, we show that the crystal symmetry change due to the sliding affects the polarized Stokes Raman-scattering intensity, which can be utilized to detect tiny misalignment of graphene layers using spectroscopic tools.

cond-mat.mes-hall↗

arXiv2013

Origin of Anomalous Water Permeation through Graphene Oxide Membrane

Water inside the low dimensional carbon structures has been considered seriously owing to fundamental interest in its flow and structures as well as its practical impact. Recently, the anomalous perfect penetration of water through graphene oxide membrane was demonstrated although the membrane was impenetrable for other liquids and even gases. The unusual auxetic behavior of graphene oxide in the presence of water was also reported. Here, based on first-principles calculations, we establish atomistic models for hybrid systems composed of water and graphene oxides revealing the anomalous water behavior inside the stacked graphene oxides. We show that formation of hexagonal ice bilayer in between the flakes as well as melting transition of ice at the edges of flakes are crucial to realize the perfect water permeation across the whole stacked structures. The distance between adjacent layers that can be controlled either by oxygen reduction process or pressure is shown to determine the water flow thus highlighting a unique water dynamics in randomly connected two-dimensional spaces.

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