K-Y Choi

Chung-Ang University, Sŏul, Seoul, South Korea

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Publications (12)48.26 Total impact

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    ABSTRACT: We report electron spin resonance (ESR), Raman scattering, and interband absorption measurements of the multiferroic FeTe2O5Br with two successive magnetic transitions at TN1 = 11.0 K and TN2 = 10.5 K. ESR measurements show all characteristics of a low-dimensional frustrated magnet: (i) the appearance of an antiferromagnetic resonance (AFMR) mode at 40 K, a much higher temperature than TN1, and (ii) a weaker temperature dependence of the AFMR linewidth than in classical magnets, ΔHpp(T) ∝ T(n) with n = 2.2-2.3. Raman spectra at ambient pressure show a large variation of phonon intensities with temperature while there are no appreciable changes in phonon numbers and frequencies. This demonstrates the significant role of the polarizable Te(4+) lone pairs in inducing multiferroicity. Under pressure at P = 2.12-3.04 GPa Raman spectra undergo drastic changes and absorption spectra exhibit an abrupt drop of a band gap. This evidences a pressure-induced structural transition related to changes of the electronic states at high pressures.
    Journal of Physics Condensed Matter 02/2014; 26(8):086001. · 2.36 Impact Factor
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    ABSTRACT: We report on coherent manipulation of electron spins in an antiferromagnetically coupled spin triangle {Cu3-X} (X=As, Sb) impregnated in freestanding nanoporous silicon (NS) by using 240 GHz microwave pulses. Rabi oscillations are observed and the spin coherence time is found to be T(2)=1066 ns at 1.5 K. This demonstrates that the {Cu3-X}:NS hybrid material provides a promising scheme for implementing spin-based quantum gates. By measuring the spin relaxation times of samples with different symmetries and environments we give evidence that a spin chirality is the main decoherence source of spin triangle molecules.
    Physical Review Letters 02/2012; 108(6):067206. · 7.94 Impact Factor
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    ABSTRACT: Doping and temperature dependent studies of optical phonon modes in Fe-122 pnictides are performed using Raman scattering experiments and compared with model calculations to elucidate the role of electron-phonon and spin-phonon interaction in this family of compounds. The frequency and linewidth of the B(1g) mode at around 210 cm(-1) is highlighted as appreciable anomalies at the superconducting and spin density wave transitions are observed that strongly depend on composition. We give estimates of the electron-phonon coupling related to this renormalization and calculate the phonon self-energy on the basis of a four-band model comparing different symmetries of the order parameters. In addition, we observe a pronounced quasi-elastic Raman response for the undoped compound, suggesting persisting magnetic fluctuations in the spin density wave state.
    Journal of Physics Condensed Matter 03/2010; 22(11):115802. · 2.36 Impact Factor
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    ABSTRACT: We report 51V zero-field NMR of the manganese vanadate spinel compound, MnV2O4, together with both ac and dc magnetization measurements. The field and temperature dependences of ac susceptibilities show a re-entrant-spin-glass-like behavior below the ferrimagnetic (FEM) ordering temperature. The zero-field NMR spectrum consists of multiple lines ranging from 240 to 320 MHz. Its temperature and field dependences are discussed in terms of the persistence of a small fraction of the cubic phase within the FEM ordered ground state. Due to strong spin–orbit couplings the disordered phase induces an anomalous structural and electronic state in an external field. This suggests a close correlation between magnetism and structure.
    Journal of Physics Condensed Matter 03/2008; 20(13):135218. · 2.36 Impact Factor
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    ABSTRACT: We used a continuously rotating torsion balance instrument to measure the acceleration difference of beryllium and titanium test bodies towards sources at a variety of distances. Our result Deltaa(N),(Be-Ti)=(0.6+/-3.1)x10(-15) m/s2 improves limits on equivalence-principle violations with ranges from 1 m to infinity by an order of magnitude. The Eötvös parameter is eta(Earth,Be-Ti)=(0.3+/-1.8)x10(-13). By analyzing our data for accelerations towards the center of the Milky Way we find equal attractions of Be and Ti towards galactic dark matter, yielding eta(DM,Be-Ti)=(-4+/-7)x10(-5). Space-fixed differential accelerations in any direction are limited to less than 8.8x10(-15) m/s2 with 95% confidence.
    Physical Review Letters 03/2008; 100(4):041101. · 7.94 Impact Factor
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    ABSTRACT: We report coherent spin manipulation on Cr(5+) (S = 1/2, I = 0) doped K(3)NbO(8), which constitutes a dilute two-level model relevant for use as a spin qubit. Rabi oscillations are observed for the first time in a spin system based on transition metal oxides up to room temperature. At liquid helium temperature the phase coherence relaxation time T2 reaches approximately 10 micros and, with a Rabi frequency of 20 MHz, yields a single-qubit figure of merit Q(M) of about 500. This shows that a diluted ensemble of Cr(5+) (S = 1/2) doped K(3)NbO(8) is a potential candidate for solid-state quantum information processing.
    Physical Review Letters 10/2007; 99(13):137601. · 7.94 Impact Factor
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    ABSTRACT: We have tested the proportionality of force and acceleration in Newton's second law, F=ma, in the limit of small forces and accelerations. Our tests reach well below the acceleration scales relevant to understanding several current astrophysical puzzles such as the flatness of galactic rotation curves, the Pioneer anomaly, and the Hubble acceleration. We find good agreement with Newton's second law at accelerations as small as 5 x 10(-14) m/s(2).
    Physical Review Letters 05/2007; 98(15):150801. · 7.94 Impact Factor
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    ABSTRACT: We report inelastic light scattering experiments on FeCr2S4, which shows colossal magnetoresistance (CMR) effects around TC≈170 K. Several distinct features have been observed: (i) anomalies in phonon frequencies and linewidths around TC, (ii) the vanishing of one-magnon excitation close to 0.83 TC, and (iii) the presence of an electronic background at low temperature. This highlights the significance of an intricate interplay among electron, spin, orbital, and lattice degrees of freedom in inducing CMR effects.
    Journal of Physics Condensed Matter 03/2007; 19(14):145260. · 2.36 Impact Factor
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    ABSTRACT: We report on pulsed field magnetization and ESR measurements of the family of copper(II)-substituted polyoxotungstates [Cu3(H2O)3(α-XW9O33)2]12−. (X = As, Sb) where the three Cu2+(S = 1/2) ions form an antiferromagnetically-coupled triangle. The distinct features are the observation of half step magnetization and hysteresis loops as well as asymmetric/symmetric magnetization between a positive and negative field, depending on the diamagnetic heteroatom X. This is attributed to the interplay between an adiabatic magnetization and Dzyaloshinskii-Moriya interactions. Moreover, the more symmetric magnetization curve of X = Sb is discussed in terms of dynamical mixing of a S = 1/2 state via a phonon mode.
    Journal of Physics Conference Series 12/2006; 51(1):95.
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    ABSTRACT: We report Raman scattering measurements on the spin ladder compound (C5H12N)2CuBr4. Pronounced multiphonon scattering is observed up to eighth-order, arising from the Franck–Condon process. Furthermore, a two-magnon continuum is seen with frequencies around 18 cm−1 at low temperatures. Its symmetric line shape suggests a substantial hopping of triplets along the leg of the ladder. Thus, the studied system is considered to represent a two-leg ladder with coupling constants in the intermediate regime between the strong and the isotropic limit.
    Journal of Physics Condensed Matter 06/2005; 17(27):4237. · 2.36 Impact Factor
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    ABSTRACT: We report on polarized Raman scattering of single crystals of Nd1−xSrxMnO3 (x = 0.3, 0.5). Raman spectra of Nd0.7Sr0.3MnO3 show a significant change through the metal–insulator transition. In the ferromagnetic metallic phase phonon modes grow in intensity and number while the electronic continuum becomes more pronounced. We suggest that these effects are due to the strong competition between the localization and the delocalization of carriers which is the origin of the largest colossal magnetoresistance effect ever reported for the manganites. Raman spectra of Nd0.5Sr0.5MnO3, upon cooling through the charge-ordering temperature TCO = 148 K, exhibit several new lines which undergo a substantial hardening. This hardening is interpreted as a freezing of the Jahn–Teller distortions with a gradual decrease of a fraction of the ferromagnetic phase in the CE-type charge/orbital ordered state.
    Journal of Physics Condensed Matter 05/2003; 15(19):3333. · 2.36 Impact Factor
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    ABSTRACT: We report on polarized Raman scattering of single crystals of Nd1-xSrxMnO3(x 0.3, 0.5). Raman spectra of Nd0.7Sr0.3MnO3show a significant change through the metal-insulator transition. In the ferromagnetic metallic phase phonon modes grow in intensity and number while the electronic continuum becomes more pronounced. We suggest that these effects are due to the strong competition between the localization and the delocalization of carriers which is the origin of the largest colossal magnetoresistance effect ever reported for the manganites. Raman spectra of Nd0.5Sr0.5MnO3, upon cooling through the charge-ordering temperature TCO 148 K, exhibit several new lines which undergo a substantial hardening. This hardening is interpreted as a freezing of the Jahn-Teller distortions with a gradual decrease of a fraction of the ferromagnetic phase in the CE-type charge/orbital ordered state.
    Journal of Physics Condensed Matter 01/2003; 15(19). · 2.36 Impact Factor

Publication Stats

197 Citations
48.26 Total Impact Points

Institutions

  • 2012–2014
    • Chung-Ang University
      • Department of Physics
      Sŏul, Seoul, South Korea
  • 2010
    • Technische Universität Braunschweig
      • Institut für Physik der kondensierten Materie
      Braunschweig, Lower Saxony, Germany
  • 2006–2008
    • National High Magnetic Field Laboratory
      Tallahassee, Florida, United States
  • 2005
    • Tohoku University
      • Institute for Materials Research
      Sendai-shi, Miyagi-ken, Japan