Jun-hyung Cho - Academia.edu (original) (raw)
Papers by Jun-hyung Cho
Physical Review B, 2022
Based on first-principles calculations, we discover a two-dimensional layered antiferromagnetic (... more Based on first-principles calculations, we discover a two-dimensional layered antiferromagnetic (AFM) electride Gd 2 O, where anionic excess electrons exist in the interstitial spaces between positively charged cationic layers. It is revealed that each cationic layer composed of three-atom-thick Gd−O−Gd stacks has inplane ferromagnetic and out-of-plane AFM superexchange interactions between the localized Gd 4f spins through O 2p orbitals. Furthermore, the interlayer superexchange mediated by the hybridized Gd-5d and interstitial-s-like states involves intimate couplings between the spin, lattice, and charge degrees of freedom, thereby inducing simultaneous magnetic, structural, and electronic phase transitions. The resulting ground state with the simple hexagonal lattice hosts massless Dirac fermions protected by nonsymmorphic magnetic symmetry, as well as massive Dirac fermions. We thus demonstrate that the anionic excess electrons in Gd 2 O play a crucial role in the emergence of magnetic Dirac semimetal states, therefore offering an intriguing interplay between 2D magnetic electrides and topological physics.
Physical Review B, 2017
Low-dimensional electron systems often show a delicate interplay between electron-phonon and elec... more Low-dimensional electron systems often show a delicate interplay between electron-phonon and electronelectron 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 exchangecorrelation 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(1010) and ZnO(1010) 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(1010) and ZnO(1010) 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(1010) prefer the AFM order, while less localized Zn DB electrons in ZnO(1010) 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.
Physical Review Materials, 2021
Lanthanum hydride LaH 10 with a sodalitelike clathrate structure was experimentally realized to e... more Lanthanum hydride LaH 10 with a sodalitelike clathrate structure was experimentally realized to exhibit a room-temperature superconductivity under megabar pressures. Based on first-principles calculations, we reveal that the metal framework of La atoms has the excess electrons at interstitial regions. Such anionic electrons are easily captured to form a stable clathrate structure of H cages. We thus propose that the charge transfer from La to H atoms is mostly driven by the electride property of the La framework. Further, the interaction between La atoms and H cages induces a delocalization of La-5p semicore states to hybridize with H-1s state. Consequently, the bonding nature between La atoms and H cages is characterized as a mixture of ionic and covalent. Our findings demonstrate that anionic and semicore electrons play important roles in stabilizing clathrate H cages in LaH 10 , which can be broadly applicable to other high-pressure rare-earth hydrides with clathrate structures.
Physical Review B, 2020
Recently, the discovery of room-temperature superconductivity (SC) was experimentally realized in... more Recently, the discovery of room-temperature superconductivity (SC) was experimentally realized in the fcc phase of LaH10 under megabar pressure. Specifically, the isotope effect of Tc was measured by the replacement of hydrogen (H) with deuterium (D), demonstrating a driving role of phonons in the observed room-temperature SC. Herein, based on the first-principles calculations within the harmonic approximation, we reveal that (i) the identical electron-phonon coupling constants of fcc LaH10 and LaD10 decrease monotonously with increasing pressure and (ii) the isotope effect of Tc is nearly proportional to M −α (M : ionic mass) with α ≈ 0.465, irrespective of pressure. The predicted value of α agrees well with the experimental one (α = 0.46) measured at around 150 GPa. Thus, our findings provide a theoretical confirmation of the conventional electron-phonon coupling mechanism in a newly discovered room-temperature superconductor of compressed LaH10.
Physical Review B, 2020
Recently, the discovery of room-temperature superconductivity (SC) was experimentally realized in... more Recently, the discovery of room-temperature superconductivity (SC) was experimentally realized in the fcc phase of LaH 10 under megabar pressures. This SC of compressed LaH 10 has been explained in terms of strong electron-phonon coupling (EPC), but the mechanism of how the large EPC constant and high superconducting transition temperature T c are attained has not yet been clearly identified. Based on the density-functional theory and the Migdal-Eliashberg formalism, we reveal the presence of two nodeless, anisotropic superconducting gaps on the Fermi surface (FS). Here, the small gap is mostly associated with the hybridized states of H s and La f orbitals on the three outer FS sheets, while the large gap arises mainly from the hybridized state of neighboring H s or p orbitals on the one inner FS sheet. Further, we find that the EPC constant of compressed YH 10 with the same sodalite-like clathrate structure is enhanced due to the two additional FS sheets, leading to a higher T c than LaH 10. It is thus demonstrated that the multiband pairing of hybridized electronic states is responsible for the large EPC constant and room-temperature SC in compressed hydrides LaH 10 and YH 10 .
Scientific reports, Jul 28, 2016
Semiconductor surfaces with narrow surface bands provide unique playgrounds to search for Mott-in... more Semiconductor surfaces with narrow surface bands provide unique playgrounds to search for Mott-insulating state. Recently, a combined experimental and theoretical study of the two-dimensional (2D) Sn atom lattice on a wide-gap SiC(0001) substrate proposed a Mott-type insulator driven by strong on-site Coulomb repulsion U within a single-band Hubbard model. However, our systematic density-functional theory (DFT) study with local, semilocal, and hybrid exchange-correlation functionals shows that the Sn dangling-bond state largely hybridizes with the substrate Si 3p and C 2p states to split into three surface bands due to the crystal field. Such a hybridization gives rise to the stabilization of the antiferromagnetic order via superexchange interactions. The band gap and the density of states predicted by the hybrid DFT calculation agree well with photoemission data. Our findings not only suggest that the Sn/SiC(0001) system can be represented as a Slater-type insulator driven by long-...
Scientific Reports, 2016
Design and synthesis of three-dimensional metallic carbons are currently one of the hot issues in... more Design and synthesis of three-dimensional metallic carbons are currently one of the hot issues in contemporary condensed matter physics because of their fascinating properties. Here, based on first-principles calculations, we discover a novel stable metallic carbon allotrope (termed H18 carbon) in "Equation missing" ("Equation missing") symmetry with a mixed sp2-sp3 hybridized bonding network. The dynamical stability of H18 carbon is verified by phonon mode analysis and molecular dynamics simulations and its mechanical stability is analyzed by elastic constants, bulk modulus and shear modulus. By simulating the x-ray diffraction patterns, we propose that H18 carbon would be one of the unidentified carbon phases observed in recent detonation experiments. The analysis of the band structure and density of states reveal that this new carbon phase has a metallic feature mainly due to the C atoms with sp2 hybridization. This novel 3D metallic carbon phase is anticipate...
![Research paper thumbnail of Erratum: Magnetism of the V(001) surface: Contradictory results from pseudopotential and linearized augmented plane-wave calculations [Phys. Rev. B 63 , 172420 (2001)]](https://attachments.academia-assets.com/103822309/thumbnails/1.jpg)
](Physical Review B, 2001
In this paper two references to the work by Perdew cited as Ref. 2 were unclear. In the fourth li... more In this paper two references to the work by Perdew cited as Ref. 2 were unclear. In the fourth line of the first paragraph, and the last line of the second column on page 2, ''Perdew's'' should be replaced by ''the PW91.''
Physical Review B, 1999
The atomic structure of the As-covered GaAs͑110͒ surface in one-monolayer coverage is studied usi... more The atomic structure of the As-covered GaAs͑110͒ surface in one-monolayer coverage is studied using the pseudopotential density-functional theory. We compare the adsorption geometry of As/GaAs͑110͒ with that of the well-studied isoelectronic Sb/GaAs͑110͒ system and discuss the large difference in the desorption temperature found in both systems, based on the calculated adsorption energies. In addition, we calculate the As 3d and Ga 3d core-level shifts at As/GaAs͑110͒ using initial-state theory. Our calculations not only produce well the surface components resolved in a recent photoemission experiment, but predict an additional surface core level for the substrate As atom bonded to the adsorbed As atom. ͓S0163-1829͑99͒02219-5͔
Physical review letters, Jan 28, 2015
The insulating ground state of the 5d transition metal oxide CaIrO_{3} has been classified as a M... more The insulating ground state of the 5d transition metal oxide CaIrO_{3} 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 t_{2g} 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 Ir^{4+} spins. Furthermore, the spin-orbit coupling (SOC) hybridizes the t_{2g} states to open an insulating gap. These results indicate that CaIrO_{3} 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 AF...
Physical Review B, 2003
The structural and bonding properties of adsorbed trimethylamine and dimethylamine on the Si͑001͒... more The structural and bonding properties of adsorbed trimethylamine and dimethylamine on the Si͑001͒ surface are investigated by first-principles density-functional calculations. We find that trimethylamine dissociation is not kinetically feasible because of the existence of a high-energy barrier for the NuCH 3 bond cleavage, but dimethylamine dissociation accompanying the NuH bond cleavage takes place at room temperature. Our initial-state theory calculation for the N ͑C͒ 1s core level of adsorbed trimethylamine obtains 2.77 ͑0.67͒ eV greater binding energy than that of dissociated dimethylamine. The final-state effect due to screening of the N ͑C͒ core hole yields 2.90 ͑0.78͒ eV. These N and C 1s core-level shifts of adsorbed trimethylamine are attributed to rehybridization between the lone-pair state of the N atom and the empty dangling-bond state of the down atom of the Si dimer. In addition, unlike dimethylamine adsorption which attains full coverage with one molecule per surface dimer, we find significant repulsive interactions between adsorbed trimethylamine molecules, disfavoring adsorption at every Si dimer. This result accounts for a relatively lower saturation coverage in trimethylamine adsorption, which was observed from multiple internal reflection Fourier transform infrared spectroscopy and x-ray photoelectron spectroscopy.
The origin of the insulating ground state in 5d transition metal oxide CaIrO3 has been believed t... more The origin of the insulating ground state in 5d transition metal oxide CaIrO3 has been believed to be related to electron correlations, therefore being represented 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 correcting the self-interaction error inherent to local and semilocal DFT calculations 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. It is thus demonstrated for the first time that CaIrO3 can be represented as a spin-orbit Slater insulator, driven by the interplay between an itinerant AFM order and the SOC.
Physical Review B, 2001
The energetics and electronic structures of flat Ag films on the MgO͑001͒ substrate are studied b... more The energetics and electronic structures of flat Ag films on the MgO͑001͒ substrate are studied by firstprinciples density-functional calculations. An oscillatory variation of the film energetics showing the existence of multiple magic thicknesses for smooth growth is found. This oscillatory behavior correlates well with the quantum-well states, which themselves vary with the film thickness. The results demonstrate the importance of the confined motion of the conduction electrons in stabilizing epitaxial metal films, as emphasized in a recent ''electronic growth'' model.
Physical Review B, 2014
The Sn overlayer on the Si(111) surface has been considered as a prototypical system for explorin... more The Sn overlayer on the Si(111) surface has been considered as a prototypical system for exploring twodimensional (2D) correlated physics on the triangular lattice. Most of the previous theoretical studies were based on the presumption that the surface state dominantly originates from Sn dangling-bond (DB) electrons, leading to a strongly correlated 2D electronic system. By contrast, our density-functional theory calculations show that the Sn DB state significantly hybridizes with Si substrate states to form a resonant state. The strong resonance between the Sn 5p z and Si 3p z orbitals facilitates the recently observed antiferromagnetic order through superexchange interactions, giving rise to a band-gap opening. It is thus demonstrated that the insulating ground state of Sn/Si(111) can be characterized as a Slater-type insulator via band magnetism.
Physical Review B, 2001
We study the adsorption structure of 1,5-cyclooctadiene on the Si͑001͒ surface using density-func... more We study the adsorption structure of 1,5-cyclooctadiene on the Si͑001͒ surface using density-functional theory. The ''bridge'' structure where the two CϭC double bonds react with different Si dimers is found to be significantly more stable than the ''upright'' structure where only one of the two CϭC bonds reacts with a Si dimer. These two structures exhibit drastically different features in the simulated scanning tunneling microscopy ͑STM͒ image for adsorbed molecules: for the upright structure a single bright-spot image due to an unreacted bond is predicted, whereas the bridge structure produces two bright-spot images. The result of the bridge structure is in good agreement with STM data, and therefore both the energetics and STM image support the bridge structure rather than the upright one.
The adsorption of water on the Si͑001͒ surface is studied by using density-functional total-energ... more The adsorption of water on the Si͑001͒ surface is studied by using density-functional total-energy calculations within the generalized gradient approximation. We find that water can adsorb molecularly on the down atom of the Si dimer, but a dissociative adsorption wherein OH ͑H͒ forms a bond to the down ͑up͒ atom of the Si dimer is more favored over the molecular adsorption ͑by 1.8 eV͒. The decay of the molecular state to the dissociative state occurs via a transition state with the energy barrier of only 0.15 eV. While the interaction between water molecules is repulsive, that between dissociated OH species is attractive by hydrogen bonding.
Physical Review Letters, 2013
The precise driving force of the phase transition in indium nanowires on Si(111) has been controv... more The precise driving force of the phase transition in indium nanowires on Si(111) has been controversial whether it is driven by a Peierls instability or by a simple energy lowering due to a periodic lattice distortion. The present van der Waals (vdW) corrected hybrid density functional calculation predicts that the lowtemperature 8×2 structure whose building blocks are indium hexagons is energetically favored over the room-temperature 4×1 structure. We show that the correction of self-interaction error and the inclusion of vdW interactions play crucial roles in describing the covalent bonding, band-gap opening, and energetics of hexagon structures. The results manifest that the formation of hexagons occurs by a simple energy lowering due to the lattice distortion, not by a charge density wave formation arising from Fermi surface nesting.
Physical Review Letters, 2013
We have performed the semilocal and hybrid density-functional theory (DFT) studies of the Sn/Ge(1... more We have performed the semilocal and hybrid density-functional theory (DFT) studies of the Sn/Ge(111) surface to identify the origin of the observed insulating √ 3× √ 3 phase below ∼30 K. Contrasting with the semilocal DFT calculation predicting a metallic 3×3 ground state, the hybrid DFT calculation including van der Waals interactions shows that the insulating ferrimagnetic structure with √ 3× √ 3 structural symmetry is energetically favored over the metallic 3×3 structure. It is revealed that the correction of self-interaction error with a hybrid exchange-correlation functional gives rise to a band-gap opening induced by a ferrimagnetic order. The results manifest that the observed insulating phase is attributed to the Slater mechanism via itinerant magnetic order rather than the hitherto accepted Mott-Hubbard mechanism via electron correlations.
Physical Review B, 2012
A recent density-functional calculation for fcc C 60 H n (n = odd) [K. W. Lee and C. E. Lee, Phys... more A recent density-functional calculation for fcc C 60 H n (n = odd) [K. W. Lee and C. E. Lee, Phys. Rev. Lett. 106, 166402 (2011)] proposed the existence of Stoner ferromagnetism based on an itinerant band model. However, our density-functional calculation shows that the antiferromagnetic (AFM) configuration is slightly more stable than the ferromagnetic (FM) one. This preference for antiferromagnetism over ferromagnetism is analogous to the case of a dimer (C 60 H) 2 , where each C 60 H is spin polarized by an intramolecular exchange and the two magnetic moments are antiferromagnetically coupled with each other. The results demonstrate that the underlying mechanism of the magnetic order in fcc C 60 H n is associated with the AFM superexchange between the magnetic moments created by H dopants.
Physical Review B, 2022
Based on first-principles calculations, we discover a two-dimensional layered antiferromagnetic (... more Based on first-principles calculations, we discover a two-dimensional layered antiferromagnetic (AFM) electride Gd 2 O, where anionic excess electrons exist in the interstitial spaces between positively charged cationic layers. It is revealed that each cationic layer composed of three-atom-thick Gd−O−Gd stacks has inplane ferromagnetic and out-of-plane AFM superexchange interactions between the localized Gd 4f spins through O 2p orbitals. Furthermore, the interlayer superexchange mediated by the hybridized Gd-5d and interstitial-s-like states involves intimate couplings between the spin, lattice, and charge degrees of freedom, thereby inducing simultaneous magnetic, structural, and electronic phase transitions. The resulting ground state with the simple hexagonal lattice hosts massless Dirac fermions protected by nonsymmorphic magnetic symmetry, as well as massive Dirac fermions. We thus demonstrate that the anionic excess electrons in Gd 2 O play a crucial role in the emergence of magnetic Dirac semimetal states, therefore offering an intriguing interplay between 2D magnetic electrides and topological physics.
Physical Review B, 2017
Low-dimensional electron systems often show a delicate interplay between electron-phonon and elec... more Low-dimensional electron systems often show a delicate interplay between electron-phonon and electronelectron 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 exchangecorrelation 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(1010) and ZnO(1010) 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(1010) and ZnO(1010) 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(1010) prefer the AFM order, while less localized Zn DB electrons in ZnO(1010) 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.
Physical Review Materials, 2021
Lanthanum hydride LaH 10 with a sodalitelike clathrate structure was experimentally realized to e... more Lanthanum hydride LaH 10 with a sodalitelike clathrate structure was experimentally realized to exhibit a room-temperature superconductivity under megabar pressures. Based on first-principles calculations, we reveal that the metal framework of La atoms has the excess electrons at interstitial regions. Such anionic electrons are easily captured to form a stable clathrate structure of H cages. We thus propose that the charge transfer from La to H atoms is mostly driven by the electride property of the La framework. Further, the interaction between La atoms and H cages induces a delocalization of La-5p semicore states to hybridize with H-1s state. Consequently, the bonding nature between La atoms and H cages is characterized as a mixture of ionic and covalent. Our findings demonstrate that anionic and semicore electrons play important roles in stabilizing clathrate H cages in LaH 10 , which can be broadly applicable to other high-pressure rare-earth hydrides with clathrate structures.
Physical Review B, 2020
Recently, the discovery of room-temperature superconductivity (SC) was experimentally realized in... more Recently, the discovery of room-temperature superconductivity (SC) was experimentally realized in the fcc phase of LaH10 under megabar pressure. Specifically, the isotope effect of Tc was measured by the replacement of hydrogen (H) with deuterium (D), demonstrating a driving role of phonons in the observed room-temperature SC. Herein, based on the first-principles calculations within the harmonic approximation, we reveal that (i) the identical electron-phonon coupling constants of fcc LaH10 and LaD10 decrease monotonously with increasing pressure and (ii) the isotope effect of Tc is nearly proportional to M −α (M : ionic mass) with α ≈ 0.465, irrespective of pressure. The predicted value of α agrees well with the experimental one (α = 0.46) measured at around 150 GPa. Thus, our findings provide a theoretical confirmation of the conventional electron-phonon coupling mechanism in a newly discovered room-temperature superconductor of compressed LaH10.
Physical Review B, 2020
Recently, the discovery of room-temperature superconductivity (SC) was experimentally realized in... more Recently, the discovery of room-temperature superconductivity (SC) was experimentally realized in the fcc phase of LaH 10 under megabar pressures. This SC of compressed LaH 10 has been explained in terms of strong electron-phonon coupling (EPC), but the mechanism of how the large EPC constant and high superconducting transition temperature T c are attained has not yet been clearly identified. Based on the density-functional theory and the Migdal-Eliashberg formalism, we reveal the presence of two nodeless, anisotropic superconducting gaps on the Fermi surface (FS). Here, the small gap is mostly associated with the hybridized states of H s and La f orbitals on the three outer FS sheets, while the large gap arises mainly from the hybridized state of neighboring H s or p orbitals on the one inner FS sheet. Further, we find that the EPC constant of compressed YH 10 with the same sodalite-like clathrate structure is enhanced due to the two additional FS sheets, leading to a higher T c than LaH 10. It is thus demonstrated that the multiband pairing of hybridized electronic states is responsible for the large EPC constant and room-temperature SC in compressed hydrides LaH 10 and YH 10 .
Scientific reports, Jul 28, 2016
Semiconductor surfaces with narrow surface bands provide unique playgrounds to search for Mott-in... more Semiconductor surfaces with narrow surface bands provide unique playgrounds to search for Mott-insulating state. Recently, a combined experimental and theoretical study of the two-dimensional (2D) Sn atom lattice on a wide-gap SiC(0001) substrate proposed a Mott-type insulator driven by strong on-site Coulomb repulsion U within a single-band Hubbard model. However, our systematic density-functional theory (DFT) study with local, semilocal, and hybrid exchange-correlation functionals shows that the Sn dangling-bond state largely hybridizes with the substrate Si 3p and C 2p states to split into three surface bands due to the crystal field. Such a hybridization gives rise to the stabilization of the antiferromagnetic order via superexchange interactions. The band gap and the density of states predicted by the hybrid DFT calculation agree well with photoemission data. Our findings not only suggest that the Sn/SiC(0001) system can be represented as a Slater-type insulator driven by long-...
Scientific Reports, 2016
Design and synthesis of three-dimensional metallic carbons are currently one of the hot issues in... more Design and synthesis of three-dimensional metallic carbons are currently one of the hot issues in contemporary condensed matter physics because of their fascinating properties. Here, based on first-principles calculations, we discover a novel stable metallic carbon allotrope (termed H18 carbon) in "Equation missing" ("Equation missing") symmetry with a mixed sp2-sp3 hybridized bonding network. The dynamical stability of H18 carbon is verified by phonon mode analysis and molecular dynamics simulations and its mechanical stability is analyzed by elastic constants, bulk modulus and shear modulus. By simulating the x-ray diffraction patterns, we propose that H18 carbon would be one of the unidentified carbon phases observed in recent detonation experiments. The analysis of the band structure and density of states reveal that this new carbon phase has a metallic feature mainly due to the C atoms with sp2 hybridization. This novel 3D metallic carbon phase is anticipate...
Physical Review B, 2001
In this paper two references to the work by Perdew cited as Ref. 2 were unclear. In the fourth li... more In this paper two references to the work by Perdew cited as Ref. 2 were unclear. In the fourth line of the first paragraph, and the last line of the second column on page 2, ''Perdew's'' should be replaced by ''the PW91.''
Physical Review B, 1999
The atomic structure of the As-covered GaAs͑110͒ surface in one-monolayer coverage is studied usi... more The atomic structure of the As-covered GaAs͑110͒ surface in one-monolayer coverage is studied using the pseudopotential density-functional theory. We compare the adsorption geometry of As/GaAs͑110͒ with that of the well-studied isoelectronic Sb/GaAs͑110͒ system and discuss the large difference in the desorption temperature found in both systems, based on the calculated adsorption energies. In addition, we calculate the As 3d and Ga 3d core-level shifts at As/GaAs͑110͒ using initial-state theory. Our calculations not only produce well the surface components resolved in a recent photoemission experiment, but predict an additional surface core level for the substrate As atom bonded to the adsorbed As atom. ͓S0163-1829͑99͒02219-5͔
Physical review letters, Jan 28, 2015
The insulating ground state of the 5d transition metal oxide CaIrO_{3} has been classified as a M... more The insulating ground state of the 5d transition metal oxide CaIrO_{3} 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 t_{2g} 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 Ir^{4+} spins. Furthermore, the spin-orbit coupling (SOC) hybridizes the t_{2g} states to open an insulating gap. These results indicate that CaIrO_{3} 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 AF...
Physical Review B, 2003
The structural and bonding properties of adsorbed trimethylamine and dimethylamine on the Si͑001͒... more The structural and bonding properties of adsorbed trimethylamine and dimethylamine on the Si͑001͒ surface are investigated by first-principles density-functional calculations. We find that trimethylamine dissociation is not kinetically feasible because of the existence of a high-energy barrier for the NuCH 3 bond cleavage, but dimethylamine dissociation accompanying the NuH bond cleavage takes place at room temperature. Our initial-state theory calculation for the N ͑C͒ 1s core level of adsorbed trimethylamine obtains 2.77 ͑0.67͒ eV greater binding energy than that of dissociated dimethylamine. The final-state effect due to screening of the N ͑C͒ core hole yields 2.90 ͑0.78͒ eV. These N and C 1s core-level shifts of adsorbed trimethylamine are attributed to rehybridization between the lone-pair state of the N atom and the empty dangling-bond state of the down atom of the Si dimer. In addition, unlike dimethylamine adsorption which attains full coverage with one molecule per surface dimer, we find significant repulsive interactions between adsorbed trimethylamine molecules, disfavoring adsorption at every Si dimer. This result accounts for a relatively lower saturation coverage in trimethylamine adsorption, which was observed from multiple internal reflection Fourier transform infrared spectroscopy and x-ray photoelectron spectroscopy.
The origin of the insulating ground state in 5d transition metal oxide CaIrO3 has been believed t... more The origin of the insulating ground state in 5d transition metal oxide CaIrO3 has been believed to be related to electron correlations, therefore being represented 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 correcting the self-interaction error inherent to local and semilocal DFT calculations 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. It is thus demonstrated for the first time that CaIrO3 can be represented as a spin-orbit Slater insulator, driven by the interplay between an itinerant AFM order and the SOC.
Physical Review B, 2001
The energetics and electronic structures of flat Ag films on the MgO͑001͒ substrate are studied b... more The energetics and electronic structures of flat Ag films on the MgO͑001͒ substrate are studied by firstprinciples density-functional calculations. An oscillatory variation of the film energetics showing the existence of multiple magic thicknesses for smooth growth is found. This oscillatory behavior correlates well with the quantum-well states, which themselves vary with the film thickness. The results demonstrate the importance of the confined motion of the conduction electrons in stabilizing epitaxial metal films, as emphasized in a recent ''electronic growth'' model.
Physical Review B, 2014
The Sn overlayer on the Si(111) surface has been considered as a prototypical system for explorin... more The Sn overlayer on the Si(111) surface has been considered as a prototypical system for exploring twodimensional (2D) correlated physics on the triangular lattice. Most of the previous theoretical studies were based on the presumption that the surface state dominantly originates from Sn dangling-bond (DB) electrons, leading to a strongly correlated 2D electronic system. By contrast, our density-functional theory calculations show that the Sn DB state significantly hybridizes with Si substrate states to form a resonant state. The strong resonance between the Sn 5p z and Si 3p z orbitals facilitates the recently observed antiferromagnetic order through superexchange interactions, giving rise to a band-gap opening. It is thus demonstrated that the insulating ground state of Sn/Si(111) can be characterized as a Slater-type insulator via band magnetism.
Physical Review B, 2001
We study the adsorption structure of 1,5-cyclooctadiene on the Si͑001͒ surface using density-func... more We study the adsorption structure of 1,5-cyclooctadiene on the Si͑001͒ surface using density-functional theory. The ''bridge'' structure where the two CϭC double bonds react with different Si dimers is found to be significantly more stable than the ''upright'' structure where only one of the two CϭC bonds reacts with a Si dimer. These two structures exhibit drastically different features in the simulated scanning tunneling microscopy ͑STM͒ image for adsorbed molecules: for the upright structure a single bright-spot image due to an unreacted bond is predicted, whereas the bridge structure produces two bright-spot images. The result of the bridge structure is in good agreement with STM data, and therefore both the energetics and STM image support the bridge structure rather than the upright one.
The adsorption of water on the Si͑001͒ surface is studied by using density-functional total-energ... more The adsorption of water on the Si͑001͒ surface is studied by using density-functional total-energy calculations within the generalized gradient approximation. We find that water can adsorb molecularly on the down atom of the Si dimer, but a dissociative adsorption wherein OH ͑H͒ forms a bond to the down ͑up͒ atom of the Si dimer is more favored over the molecular adsorption ͑by 1.8 eV͒. The decay of the molecular state to the dissociative state occurs via a transition state with the energy barrier of only 0.15 eV. While the interaction between water molecules is repulsive, that between dissociated OH species is attractive by hydrogen bonding.
Physical Review Letters, 2013
The precise driving force of the phase transition in indium nanowires on Si(111) has been controv... more The precise driving force of the phase transition in indium nanowires on Si(111) has been controversial whether it is driven by a Peierls instability or by a simple energy lowering due to a periodic lattice distortion. The present van der Waals (vdW) corrected hybrid density functional calculation predicts that the lowtemperature 8×2 structure whose building blocks are indium hexagons is energetically favored over the room-temperature 4×1 structure. We show that the correction of self-interaction error and the inclusion of vdW interactions play crucial roles in describing the covalent bonding, band-gap opening, and energetics of hexagon structures. The results manifest that the formation of hexagons occurs by a simple energy lowering due to the lattice distortion, not by a charge density wave formation arising from Fermi surface nesting.
Physical Review Letters, 2013
We have performed the semilocal and hybrid density-functional theory (DFT) studies of the Sn/Ge(1... more We have performed the semilocal and hybrid density-functional theory (DFT) studies of the Sn/Ge(111) surface to identify the origin of the observed insulating √ 3× √ 3 phase below ∼30 K. Contrasting with the semilocal DFT calculation predicting a metallic 3×3 ground state, the hybrid DFT calculation including van der Waals interactions shows that the insulating ferrimagnetic structure with √ 3× √ 3 structural symmetry is energetically favored over the metallic 3×3 structure. It is revealed that the correction of self-interaction error with a hybrid exchange-correlation functional gives rise to a band-gap opening induced by a ferrimagnetic order. The results manifest that the observed insulating phase is attributed to the Slater mechanism via itinerant magnetic order rather than the hitherto accepted Mott-Hubbard mechanism via electron correlations.
Physical Review B, 2012
A recent density-functional calculation for fcc C 60 H n (n = odd) [K. W. Lee and C. E. Lee, Phys... more A recent density-functional calculation for fcc C 60 H n (n = odd) [K. W. Lee and C. E. Lee, Phys. Rev. Lett. 106, 166402 (2011)] proposed the existence of Stoner ferromagnetism based on an itinerant band model. However, our density-functional calculation shows that the antiferromagnetic (AFM) configuration is slightly more stable than the ferromagnetic (FM) one. This preference for antiferromagnetism over ferromagnetism is analogous to the case of a dimer (C 60 H) 2 , where each C 60 H is spin polarized by an intramolecular exchange and the two magnetic moments are antiferromagnetically coupled with each other. The results demonstrate that the underlying mechanism of the magnetic order in fcc C 60 H n is associated with the AFM superexchange between the magnetic moments created by H dopants.