arXiv subjects

Sun-Woo Kim

Publications and source records attributed to Sun-Woo Kim.

26 records · Page 2Linked to original sources

arXiv2017

Competing charge density wave and antiferromagnetism of metallic atom wires in GaN(10${\overline{1}}$0) and ZnO(10${\overline{1}}$0)

Low-dimensional electron systems often show a delicate interplay between electron-phonon and electron-electron interactions, giving rise to interesting quantum phases such as the charge density wave (CDW) and magnetism. Using the density-functional theory (DFT) calculations with the semilocal and hybrid exchange-correlation functionals as well as the exact-exchange plus correlation in the random-phase approximation (EX + cRPA), we systematically investigate the ground state of the metallic atom wires containing dangling-bond (DB) electrons, fabricated by partially hydrogenating the GaN(10${\overline{1}}$0) and ZnO(10${\overline{1}}$0) surfaces. We find that the CDW or antiferromagnetic (AFM) order has an electronic energy gain due to a band-gap opening, thereby being more stabilized compared to the metallic state. Our semilocal DFT calculation predicts that both DB wires in GaN(10${\overline{1}}$0) and ZnO(10${\overline{1}}$0) have the same CDW ground state, whereas the hybrid DFT and EX+cRPA calculations predict the AFM ground state for the former DB wire and the CDW ground state for the latter one. It is revealed that more localized Ga DB electrons in GaN(10${\overline{1}}$0) prefer the AFM order, while less localized Zn DB electrons in ZnO(10${\overline{1}}$0) the CDW formation. Our findings demonstrate that the drastically different ground states are competing in the DB wires created on the two representative compound semiconductor surfaces.

cond-mat.mtrl-sci↗

arXiv2017

Microscopic mechanism of tunable band gap in potassium doped few-layer black phosphorus

Tuning band gaps in two-dimensional (2D) materials is of great interest in the fundamental and practical aspects of contemporary material sciences. Recently, black phosphorus (BP) consisting of stacked layers of phosphorene was experimentally observed to show a widely tunable band gap by means of the deposition of potassium (K) atoms on the surface, thereby allowing great flexibility in design and optimization of electronic and optoelectronic devices. Here, based on the density-functional theory calculations, we demonstrates that the donated electrons from K dopants are mostly localized at the topmost BP layer and such a surface charging efficiently screens the K ion potential. It is found that, as the K doping increases, the extreme surface charging and its screening of K atoms shift the conduction bands down in energy, i.e., towards higher binding energy, because they have more charge near the surface, while it has little influence on the valence bands having more charge in the deeper layers. This result provides a different explanation for the observed tunable band gap compared to the previously proposed giant Stark effect where a vertical electric field from the positively ionized K overlayer to the negatively charged BP layers shifts the conduction band minimum $Γ_{\rm 1c}$ (valence band minimum $Γ_{\rm 8v}$) downwards (upwards). The present prediction of $Γ_{\rm 1c}$ and $Γ_{\rm 8v}$ as a function of the K doping reproduces well the widely tunable band gap, anisotropic Dirac semimetal state, and band-inverted semimetal state, as observed by angle-resolved photoemission spectroscopy experiment. Our findings shed new light on a route for tunable band gap engineering of 2D materials through the surface doping of alkali metals.

cond-mat.mtrl-sci↗

arXiv2017

Comment on "Quasi-One-Dimensional Metal-Insulator Transitions in Compound Semiconductor Surfaces"

In a recent Letter, Zhao et al. [1] reported the origin of quasi-one-dimensional metal-insulator (MI) transitions in compound semiconductor surfaces. Based on a density-functional theory (DFT) calculation within the generalized gradient approximation (GGA), they claimed that one-atom-wide metallic structures formed by a selective bonding of H or Li atoms to GaN(10-10) and ZnO(10-10) undergo the Peierls-type MI transitions, leading to a charge-density-wave (CDW) formation with periodic lattice distortion. However, we here demonstrate that such a CDW phase is due to the artifact of the GGA, while the antiferromagnetic (AFM) ground state is predicted by the hybrid DFT calculation and the exact-exchange plus correlation in the random-phase approximation (EX + cRPA). [1] J. Z. Zhao, W. Fan, M. J. Verstraete, Z. Zanolli, J. Fan, X. B. Yang, H. Xu, and S. Y. Tong, Phys. Rev. Lett. 117, 116101 (2016).

cond-mat.mes-hall↗

arXiv2016

Origin of the Metal-Insulator Transition of Indium Atom Wires on Si(111)

As a prototypical one-dimensional electron system, self-assembled indium (In) nanowires on the Si(111) surface have been believed to drive a metal-insulator transition by a charge-density-wave (CDW) formation due to electron-phonon coupling. Here, our first-principles calculations demonstrate that the structural phase transition from the high-temperature 4x1 phase to the low-temperature 8x2 phase occurs through an exothermic reaction with the consecutive bond-breaking and bond-making processes, giving rise to an energy barrier between the two phases as well as a gap opening. This atomistic picture for the phase transition not only identifies its first-order nature but also solves a long-standing puzzle of the origin of the metal-insulator transition in terms of the x2 periodic lattice reconstruction of In hexagons via bond breakage and new bond formation, not by the Peierls instability-driven CDW formation.

cond-mat.mes-hall↗

arXiv2015

Nature of the Insulating Ground State of the 5d PostPerovskite CaIrO3

The insulating ground state of the 5d transition metal oxide CaIrO3 has been classified as a Mott-type insulator. Based on a systematic density functional theory (DFT) study with local, semilocal, and hybrid exchange-correlation functionals, we reveal that the Ir t2g states exhibit large splittings and one-dimensional electronic states along the c axis due to a tetragonal crystal field. Our hybrid DFT calculation adequately describes the antiferromagnetic (AFM) order along the c direction via a superexchange interaction between Ir4+ spins. Furthermore, the spin-orbit coupling (SOC) hybridizes the t2g states to open an insulating gap. These results indicate that CaIrO3 can be represented as a spin-orbit Slater insulator, driven by the interplay between a long-range AFM order and the SOC. Such a Slater mechanism for the gap formation is also demonstrated by the DFT + dynamical mean field theory calculation, where the metal-insulator transition and the paramagnetic to AFM phase transition are concomitant with each other.

cond-mat.str-el↗

arXiv2015

Equivalence of Electronic and Mechanical Stresses in Structural Phase Stabilization: A Case Study of Indium Wires on Si(111)

It was recently proposed that the stress state of a material can also be altered via electron or hole doping, a concept termed electronic stress (ES), which is different from the traditional mechanical stress (MS) due to lattice contraction or expansion. Here we demonstrate the equivalence of ES and MS in structural stabilization, using In wires on Si(111) as a prototypical example. Our systematic density-functional theory calculations reveal that, first, for the same degrees of carrier doping into the In wires, the ES of the high-temperature metallic 4x1 structure is only slightly compressive, while that of the low-temperature insulating 8x2 structure is much larger and highly anisotropic. As a consequence, the intrinsic energy difference between the two phases is significantly reduced towards electronically phase-separated ground states. Our calculations further demonstrate quantitatively that such intriguing phase tunabilities can be achieved equivalently via lattice-contraction induced MS in the absence of charge doping. We also validate the equivalence through our detailed scanning tunneling microscopy experiments. The present findings have important implications in understanding the underlying driving forces involved in various phase transitions of simple and complex systems alike.

cond-mat.mes-hall↗

arXiv2015

Contrasting interedge superexchange interactions of graphene nanoribbons embedded in h-BN and graphane

Based on first-principles density-functional theory calculations, we present a comparative study of the elec- tronic structures of ultranarrow zigzag graphene nanoribbons (ZGNRs) embedded in hexagonal boron nitride (BN) sheet and fully hydrogenated graphene (graphane) as a function of their width N (the number of zigzag C chains composing the ZGNRs). We find that ZGNRs/BN have the nonmagnetic ground state except at N = 5 and 6 with a weakly stabilized half-semimetallic state, whereas ZGNRs/graphane with N >= 2 exhibit a strong antiferromagnetic coupling between ferromagnetically ordered edge states on each edge. It is revealed that the disparate magnetic properties of the two classes of ZGNRs are attributed to the contrasting interedge superexchange interactions arising from different interface structures: i.e., the asymmetric interface structure of ZGNRs/BN gives a relatively short-range and weak superexchange interaction between the two inequivalent edge states, while the symmetric interface structure of ZGNRs/graphane gives a long-range, strong interedge superexchange interaction.

cond-mat.mes-hall↗

arXiv2013

Antiferromagnetic ordering of dangling-bond electrons at the stepped Si(001) surface

Using first-principles density-functional calculations, we explore the possibility of magnetic order at the rebonded $D_B$ step of the Si(001) surface. The rebonded $D_B$ step containing threefold coordinated Si atoms can be treated as a one-dimensional dangling-bond (DB) wire along the step edge. We find that Si atoms composing the step edge are displaced up and down alternatively due to Jahn-Teller-like distortion, but, if Si dimers on the terrace are passivated by H atoms, the antiferromagnetic (AFM) order can be stabilized at the step edge with a suppression of Jahn-Teller-like distortion. We also find that the energy preference of AFM order over Jahn-Teller-like distortion is enhanced in an oscillatory way as the length of DB wires decreases, showing the so-called quantum size effects.

cond-mat.mes-hall↗