Choong H. Kim

Seoul National University, Sŏul, Seoul, South Korea

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Publications (19)67.87 Total impact

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    ABSTRACT: The temperature ($T$) dependence of the optical conductivity spectra $\sigma(\omega)$ of a single crystal SrRuO$_3$ thin film is studied over a $T$ range from 5 to 450 K. We observed significant $T$ dependence of the spectral weights of the charge transfer and interband $d$-$d$ transitions across the ferromagnetic Curie temperature ($T_c$ ~ 150 K). Such $T$ dependence was attributed to the increase in the Ru spin moment, which is consistent with the results of density functional theory calculations. $T$ scans of $\sigma(\Omega, T)$ at fixed frequencies $\Omega$ reveal a clear $T^2$ dependence below $T_c$, demonstrating that the Stoner mechanism is involved in the evolution of the electronic structure. In addition, $\sigma(\Omega, T)$ continues to evolve at temperatures above $T_c$, indicating that the local spin moment persists in the paramagnetic state. This suggests that SrRuO$_3$ is an intriguing oxide system with itinerant ferromagnetism.
    Physical Review Letters 09/2013; 110(24). · 7.73 Impact Factor
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    ABSTRACT: We report on a fundamental thickness limit of the itinerant ferromagnetic oxide SrRuO3 that might arise from the orbital-selective quantum confinement effects. Experimentally, SrRuO3 films remain metallic even for a thickness of 2 unit cells (uc), but the Curie temperature TC starts to decrease at 4 uc and becomes zero at 2 uc. Using the Stoner model, we attributed the TC decrease to a decrease in the density of states (No). Namely, in the thin film geometry, the hybridized Ru dyz,zx orbitals are terminated by top and bottom interfaces, resulting in quantum confinement and reduction of No.
    · 7.73 Impact Factor
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    ABSTRACT: We present an effective tight-binding Hamiltonian for Li2IrO3 based on maximally localized Wannier functions for states near the Fermi level as obtained from first-principles electronic structure calculations. The majority of the Wannier orbitals are positioned on the center site with dominant jeff=1/2 character, while relatively small jeff=3/2 tails lie on the three nearest-neighbor sites. Interestingly, the spin quantization axis of the jeff=1/2 components deviates from the local octahedral axis and points toward the nearest-neighbor Ir direction. In our tight-binding model, there are relatively strong next-nearest- and the third-nearest-neighbor hopping terms within the two-dimensional Ir honeycomb lattice in addition to the relatively small but significant interlayer hopping terms. The ratio between the nearest-neighbor and the third-nearest-neighbor hoppings, which can be controlled by the lattice strain, plays a critical role in determinating the Z2-invariant character of Li2IrO3. From our tight-binding model, we also derive an effective Hamiltonian and its parameters for the magnetic exchange interactions. Due to the complex spin-dependent next-nearest-neighbor hopping terms, our pseudospin Hamiltonian includes significant next-nearest-neighbor antiferromagnetic Kitaev terms as well as Dzyaloshinskii-Moriya and Heisenberg interactions. From our model Hamiltonian we estimate classical energies of collinear magnetic configurations as functions of the Hund's coupling of the Ir atom, from which zigzag-type magnetic order gives the lowest energy.
    Physical review. B, Condensed matter 04/2013; 87(16). · 3.77 Impact Factor
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    ABSTRACT: We performed angle-resolved photoemission studies on Cu(111) and Au(111) surface states with circularly polarized light to investigate local orbital angular momentum (OAM) structures. Existence of OAM is confirmed, as predicted, to exist in systems with an inversion symmetry breaking. Cu(111) surface state bands are found to have chiral OAM in spite of very small spin-orbit coupling, consistent with the theoretical prediction. As for Au(111), we observe split bands for which OAM for the inner and outer bands are parallel, unlike the Bi2Se3 case. We also performed first-principles calculations and the results are found to be consistent with experimental results. Moreover, the majority of OAM is found to have dorbital origin while a small contribution comes from porbitals. An effective Hamiltonian that incorporates the role of OAM is derived and is used to extract the spin and OAM structures. We discuss the evolution of angular momentum structures from a pure OAM system to a strongly spin-orbit-entangled state.
    03/2013;
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    ABSTRACT: The phenomenon of Rashba spin-orbit coupling is examined theoretically for an ultrathin magnetic layer in contact with a nonmagnetic heavy-metal layer. From a first-principles calculation, a large Rashba parameter of order 1 eV Å is obtained, which makes the Rashba interaction a powerful tool for enhancing the spin-transfer torque. Interestingly, the magnitude and sign of the parameter vary from energy band to energy band, which we attribute to band-specific chiral ordering of the orbital angular momentum. We argue that the band dependence of the Rashba parameter provides a natural explanation for a host of recent experiments and may enhance the potential for device application of the Rashba-active magnetic layer.
    Physical review. B, Condensed matter 01/2013; 87(4). · 3.77 Impact Factor
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    ABSTRACT: Theory of Rashba spin-orbit coupling in magnetic metals is worked out from microscopic Hamiltonian describing d-orbitals. When structural inversion symmetry is broken, electron hopping between $d$-orbitals generates chiral ordering of orbital angular momentum, which combines with atomic spin-orbit coupling to result in the Rashba interaction. Rashba parameter characterizing the interaction is band-specific, even reversing its sign from band to band. Large enhancement of the Rashba parameter found in recent experiments is attributed to the orbital mixing of 3d magnetic atoms with non-magnetic heavy elements as we demonstrate by first-principles and tight-binding calculations.
    06/2012;
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    ABSTRACT: We performed angle-resolved photoemission studies on Cu(111) and Au(111) surface states with circularly polarized light to investigate local orbital angular momentum (OAM) structures. Existence of OAM is confirmed, as predicted, to exist in systems with an inversion symmetry breaking. Cu(111) surface state bands are found to have chiral OAM in spite of very small spin-orbit coupling, consistent with the theoretical prediction. As for Au(111), we observe split bands for which OAM for the inner and outer bands are parallel, unlike the Bi2Se3 case. We also performed first-principles calculations and the results are found to be consistent with experimental results. Moreover, the majority of OAM is found to have d-orbital origin while a small contribution comes from p orbitals. An effective Hamiltonian that incorporates the role of OAM is derived and is used to extract the spin and OAM structures. We discuss the evolution of angular momentum structures from a pure OAM system to a strongly spin-orbit-entangled state. We predict that the transition occurs through a reversal of the OAM direction at a k point in the inner band if the system has a proper spin-orbit coupling strength.
    Physical Review B 05/2012; 85(19). · 3.66 Impact Factor
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    ABSTRACT: We show, by way of tight-binding and first-principles calculations, that a one-to-one correspondence between an electron's crystal momentum k and nonzero orbital angular momentum (OAM) is a generic feature of surface bands. The OAM forms a chiral structure in momentum space much as its spin counterpart in Rashba model does, as a consequence of the inherent inversion symmetry breaking at the surface but not of spin-orbit interaction. This is the orbital counterpart of conventional Rashba effect and may be called the “orbital Rashba effect.” The circular dichroism (CD) angle-resolved photoemission (ARPES) method is an efficient way to detect this new order, and we derive formulas explicitly relating the CD-ARPES signal to the existence of OAM in the band structure. The cases of degenerate p- and d-orbital bands are considered.
    Physical review. B, Condensed matter 05/2012; 85(19). · 3.77 Impact Factor
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    ABSTRACT: We predict a quantum phase transition from normal to topological insulators in the 5d transition metal oxide Na2IrO3, where the transition can be driven by the change of the long-range hopping and trigonal crystal field terms. From the first-principles-derived tight-binding Hamiltonian, we determine the phase boundary through the parity analysis. In addition, our first-principles calculations for Na2IrO3 model structures show that the interlayer distance can be an important parameter for the existence of a three-dimensional strong topological insulator phase. Na2IrO3 is suggested to be a candidate material which can have both a nontrivial topology of bands and strong electron correlations.
    Physical Review Letters 03/2012; 108(10):106401. · 7.73 Impact Factor
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    ABSTRACT: We demonstrate that the chiral orbital angular momentum (OAM) structure can emerge as a result of broken inversion symmetry especially at the metal surfaces. The surface-normal electric field is responsible for chiral OAM states even if spin-orbit interaction is negligible. Such chiral OAM structure can be measured by a circular dichroism (CD) in angle-resolved photoemission spectroscopy (ARPES). To confirm the existence of OAM and its detection by CD-ARPES, we perform simulation of CD-ARPES for Cu surface states by first-principles calculation and the results agree well with our CD-ARPES experiment. Addition of the spin-orbit interaction to the chiral OAM structure produces a chiral spin angular momentum (SAM) pattern and the corresponding Rashba-type band splitting. We assert that OAM polarization should be a more widespread feature than the chiral spin structure which requires strong spin-orbit coupling.
    02/2012;
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    ABSTRACT: Using first-principles calculations, we show that topological quantum phase transitions are driven by external electric fields in thin films of Sb(2)Te(3). The film, as the applied electric field normal to its surface increases, is transformed from a normal insulator to a topological insulator or vice versa depending on the film thickness. We identify the band topology by directly calculating the Z(2) invariant from electronic wave functions. The dispersion of edge states is also found to be consistent with the bulk band topology in view of the bulk-boundary correspondence. We present possible applications of the topological phase transition as an on/off switch of the topologically protected edge states in nano-scale devices.
    Proceedings of the National Academy of Sciences 12/2011; 109(3):671-4. · 9.81 Impact Factor
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    ABSTRACT: We show, by way of tight-binding and first-principles calculations, that a one-to-one correspondence between electron's crystal momentum k and non-zero orbital angular momentum (OAM) is a generic feature of surface bands. The OAM forms a chiral structure in momentum space much as its spin counterpart in Rashba model does, as a consequence of the inherent inversion symmetry breaking at the surface but not of spin-orbit interaction. Circular dichroism (CD) angle-resolved photoemission (ARPES) experiment is an efficient way to detect this new order, and we derive formulas explicitly relating the CD-ARPES signal to the existence of OAM in the band structure. The cases of degenerate p- and d-orbital bands are considered.
    12/2011;
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    ABSTRACT: We propose that the existence of local orbital angular momentum (OAM) on the surfaces of high-Z materials plays a crucial role in the formation of Rashba-type surface band splitting. Local OAM state in a Bloch wave function produces an asymmetric charge distribution (electric dipole). The surface-normal electric field then aligns the electric dipole and results in chiral OAM states and the relevant Rashba-type splitting. Therefore, the band splitting originates from electric dipole interaction, not from the relativistic Zeeman splitting as proposed in the original Rashba picture. The characteristic spin chiral structure of Rashba states is formed through the spin-orbit coupling and thus is a secondary effect to the chiral OAM. Results from first-principles calculations on a single Bi layer under an external electric field verify the key predictions of the new model.
    Physical Review Letters 10/2011; 107(15):156803. · 7.73 Impact Factor
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    ABSTRACT: Rashba-split surface band is characterized by a one-to-one correspondence between the electron's momentum k and its spin orientation. Here we show that a similar correspondence between momentum and orbital angular momentum (OAM) must exist on surface bands once the inversion symmetry is broken. The correspondence is valid even when there is no spin-orbit interaction. Tight-binding and first-principles calculations are presented to support our claim. As a method to detect such OAM-momentum correspondence, we propose the circular dichroism (CD) experiment using the angle-resolved photoemission (ARPES) setup. CD-ARPES experiment performed on Cu surface confirms the existence of chiral OAM. A new concept of "orbital Galvanic effect" is proposed.
    07/2011;
  • Jiyeon Kim, Choong H. Kim, Rokyeon Kim, Jaejun Yu
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    ABSTRACT: Recently defect induced ferroelectricity in SrTiO3 has been reported at room temperature. Strontium-oxygen vacancies were suggested as a possible source of electric polarization regarding to the existence of mid-gap states. To understand the detailed electronic structures induced by defects and their formation energies, we carried out density-functional-theory calculations for various defects such as Sr, Ti, O, Sr-O, Sr-O- O vacancies. We employed the LDA+U method as implemented in the VASP code to describe the d-orbital occupation at the Ti- site due to the presence of oxygen vacancy. A complex of Sr-O-O vacancies is found to contribute to the localized electronic states in the band gap and its formation energy is small enough to form easily under the poor oxygen limit. We conclude that the vacancy-complex defects play a crucial role in determination of the physical properties of SrTiO3 thin films.
    03/2011;
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    ABSTRACT: We have investigated the electronic structure of double perovskites, Ba(2)FeReO(6) (metallic) and Ca(2)FeReO(6) (insulating) using optical and x-ray absorption spectroscopy. By comparing the experimental results with the density functional theory calculations, we found that the electronic structure of Ba(2)FeReO(6) could be determined from the interaction of the electron correlation and spin-orbit coupling. On the other hand, for Ca(2)FeReO(6), the lattice distortion and electron correlation are important in determining the electronic structure. Additionally, the insulating gap in Ca(2)FeReO(6) is realized by the spin-orbit coupling. Our work shows that the subtle interplay of the spin-orbit interaction, electron correlation, and lattice distortion should be taken into account to understand the electronic structure of the 5d transition metal oxides.
    Journal of Physics Condensed Matter 09/2010; 22(34):345602. · 2.22 Impact Factor
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    ABSTRACT: We report on a fundamental thickness limit of the itinerant ferromagnetic oxide SrRuO$_3$ that might arise from the orbital-selective quantum confinement effects. Experimentally, SrRuO$_3$ films remain metallic even for a thickness of 2 unit cells (uc), but the Curie temperature, T$_C$, starts to decrease at 4 uc and becomes zero at 2 uc. Using the Stoner model, we attributed the T$_C$ decrease to a decrease in the density of states (N$_o$). Namely, in the thin film geometry, the hybridized Ru-d$_yz,zx$ orbitals are terminated by top and bottom interfaces, resulting in quantum confinement and reduction of N$_o$.
    Physical Review Letters 07/2009; 103:057201. · 7.73 Impact Factor
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    ABSTRACT: We present an anti-ferromagnetically ordered ground state of Na$_{2}$IrO$_{3}$ based on density-functional-theory calculations including both spin-orbit coupling and on-site Coulomb interaction $U$. We show that the splitting of $e_{g}'$ doublet states by the strong spin-orbit coupling is mainly responsible for the intriguing nature of its insulating gap and magnetic ground state. Due to its proximity to the spin-orbit insulator phase, the magnetic ordering as obtained with finite $U$ is found to exhibit a strong in-plane anisotropy. The phase diagram of Na$_{2}$IrO$_{3}$ suggests a possible interplay between spin-orbit insulator and Mott anti-ferromagnetic insulator phases. Comment: 5 pages, 4 figures
    07/2009;
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    Choong H. Kim, Jaejun Yu
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    ABSTRACT: We present an extension of the local moment approach to the Anderson impurity model with spin-dependent hybridization. By employing the two-self-energy description, as originally proposed by Logan and co-workers, we applied the symmetry restoration condition for the case with spin-dependent hybridization. Self-consistent ground states were determined through variational minimization of the ground state energy. The results obtained with our spin-dependent local moment approach applied to a quantum dot system coupled to ferromagnetic leads are in good agreement with those obtained from previous work using numerical renormalization group calculations.
    Journal of Physics Condensed Matter 09/2007; 19(45). · 2.22 Impact Factor