arXiv subjects

Kyung-Hwan Jin

Publications and source records attributed to Kyung-Hwan Jin.

27 records · Page 2Linked to original sources

arXiv2017

Creation of Half-metallic f -orbital Dirac Fermion with Superlight Elements in Orbital-Designed Molecular Lattice

Magnetism in solids generally originates from the localized $d$- or $f$-orbitals that are hosted by heavy transition-metal elements. Here, we demonstrate a novel mechanism for designing half-metallic $f$-orbital Dirac fermion from superlight $sp$-elements. Combining first-principles and model calculations, we show that bare and flat-band-sandwiched (FBS) Dirac bands can be created when C$_{20}$ molecules are deposited into a 2D hexagonal lattice, which are composed of $f$-molecular orbitals (MOs) derived from $sp$-atomic orbitals (AOs). Furthermore, charge doping of the FBS Dirac bands induces spontaneous spin-polarization, converting the system into a half-metallic Dirac state. Based on this discovery, a model of spin field effect transistor is proposed to generate and transport 100\% spin-polarized carriers. Our finding illustrates a novel concept to realize exotic quantum states by manipulating MOs, instead of AOs, in orbital-designed molecular crystal lattices.

cond-mat.mes-hall↗

arXiv2016

Bending Strain Engineering of Spin Transport in Quantum Spin Hall Systems: Topological Nano-mechanospintronics

Quantum spin Hall (QSH) system can exhibit exotic spin transport phenomena, mediated by its topological edge states. Here a novel concept of bending strain engineering to tune the spin transport properties of a QSH system is demonstrated by both model and first-principles calculations. Interestingly, we discover that bending strain can be used to mitigate the spin conservation of a QSH system to generate a non-zero spin current (SC), meanwhile the preservation of time reversal symmetry renders its edge states topologically protected to transport robust SC without back scattering. This novel physics mechanism can be applied to effectively tune the SC and spin Hall current in a QSH system by control of its bending curvature. Furthermore, the realization of QSH systems with controllable curvature can be achieved by the concept of "topological nanomechnical architecture". Taking Bi/Cl/Si(111) as a material example, we demonstrate that the relative spin orientations between two edge states of a Bi/Cl/Si(111) film can indeed be tuned dramatically by its self-bending behaviors induced by the pre-designed inherent strain. Therefore, this concept of "bending strain engineering of spins" via topological nanomechanical architecture affords a promising route towards the realization of topological nano-mechanospintronics.

cond-mat.mes-hall↗

arXiv2016

Topological phase transition and quantum spin Hall edge states of antimony few layers

While two-dimensional topological insulators (2D TI) initiated the field of topological materials, only very few materials were discovered to date and the direct access to their quantum spin Hall edge states has been challenging due to material issues. Here, we introduce a new 2D TI material, Sb few layer films. Electronic structures of ultrathin Sb islands grown on Bi2Te2Se are investigated by scanning tunneling microscopy. The maps of local density of states clearly identify robust edge electronic states over the thickness of three bilayers in clear contrast to thinner islands. This indicates the topological edge states emerged through a 2D topological phase transition predicted between three and four bilayer films in recent theory. The non-trivial phase transition and edge states are confirmed for epitaxial films by extensive density-functional-theory calculations. This work provides an important material platform to exploit miscroscopic aspects of the quantum spin Hall phase and its quantum phase transition.

cond-mat.mes-hall↗

arXiv2015

Proximity Effect Induced Electronic Properties of Epitaxial Graphene on Bi2Te2Se

We report that the π-electrons of graphene can be spin-polarized to create a phase with a significant spin-orbit gap at the Dirac point (DP) using a graphene-interfaced topological insulator hybrid material. We have grown epitaxial Bi2Te2Se (BTS) films on a chemical vapor deposition (CVD) graphene. We observe two linear surface bands both from the CVD graphene notably flattened and BTS coexisting with their DPs separated by 0.53 eV in the photoemission data measured with synchrotron photons. We further demonstrate that the separation between the two DPs, ΔD-D, can be artificially fine-tuned by adjusting the amount of Cs atoms adsorbed on the graphene to a value as small as ΔD-D = 0.12 eV to find any proximity effect induced by the DPs. Our density functional theory calculation shows a spin-orbit gap of ~20 meV in the π-band enhanced by three orders of magnitude from that of a pristine graphene, and a concomitant phase transition from a semi-metallic to a quantum spin Hall phase when ΔD-D $\leq$ 0.20 eV. We thus present a practical means of spin-polarizing the π-band of graphene, which can be pivotal to advance the graphene-based spintronics.

cond-mat.mtrl-sci↗

arXiv2015

Topological fate of edge states of Bi single bilayer on Bi(111)

We address the topological nature of electronic states of step edges of Bi(111) films by first principles band structure calculations. We confirm that the dispersion of step edge states reflects the topological nature of underlying films. This result unambiguously denies recent claims that the step edge state on the surface of a bulk Bi(111) crystal or a sufficiently thick Bi(111) films represents non-trivial edge states of the two dimensional topologcial insulator phase expected for a very thin Bi(111) film. The trivial step edge states have a gigantic spin splitting of one dimensional Rashba bands and the substantial intermixing with electronic states of the bulk, which might be exploited further.

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

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

Spin-Chiral Bulk Fermi Surfaces of BiTeI Proven by Quantum Oscillations

We present the Fermi-surface map of the spin-chiral bulk states for the non-centrosymmetric semiconductor BiTeI using de Haas-van Alphen and Shubnikov-de Haas oscillations. We identify two distinct Fermi surfaces with a unique spindle-torus-type topology and the non-trivial Berry phases, confirming the spin chirality with oppositely circulating spin-texture. Near the quantum limit at high magnetic fields, we find a substantial Zeeman effect with an effective g-factor of ~ 60 for the Rashba-split Fermi surfaces. These findings provide clear evidence of strong Rashba and Zeeman coupling in the bulk states of BiTeI, suggesting that BiTeI is a good platform hosting the spin-polarized chiral states.

cond-mat.mtrl-sci↗

arXiv2012

Proximity-induced giant spin-orbit interaction in epitaxial graphene on topological insulator

Heterostructures of Dirac materials such as graphene and topological insulators provide interesting platforms to explore exotic quantum states of electrons in solids. Here we study the electronic structure of graphene-Sb2Te3 heterostructure using density functional theory and tight-binding methods. We show that the epitaxial graphene on Sb2Te3 turns into quantum spin-Hall phase due to its proximity to the topological insulating Sb2Te3. It is found that the epitaxial graphene develops a giant spin-orbit gap of about ~20 meV, which is about three orders of magnitude larger than that of pristine graphene. We discuss the origin of such enhancement of the spin-orbit interaction and possible outcomes of the spin-Hall phase in graphene.

cond-mat.mes-hall↗