B. G. Kim

Tamkang University, Taipei, Taipei, Taiwan

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Publications (13)43.82 Total impact

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    ABSTRACT: We report a sequent phase transition using high-resolution synchrotron x-ray scattering in a single crystal Nd <sub>1/2</sub> Sr <sub>1/2</sub> MnO <sub>3</sub> . By measuring the peak profile of Bragg reflections, upon cooling, we observed an increase in the width of the Bragg reflections around the Curie temperature (252 K) corresponding to the transition from a paramagnetic to a ferromagnetic state. Below approximately 200 K, dramatic changes in the width and integrated intensity were observed. Changes continued until the formation of charge ordering with q=( 1 2 ,0,0) at T<sub> CO </sub>=152 K . This charge ordering was observed to be the first order transition with a large hysteresis width of 10 K. This sequent transition is understood by the formation of different magnetic domains at different temperature ranges as that observed by neutron powder diffraction.
    Journal of Applied Physics 08/2008; · 2.21 Impact Factor
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    ABSTRACT: We report the observation of superstructures associated with the oxygen 2p states in two prototypical manganites using x-ray diffraction at the oxygen K edge. In the stripe order system Bi0.31Ca0.69MnO3, hole-doped O states are orbitally ordered, at the same propagation vector as the Mn orbital ordering, but no oxygen charge stripes are found at this periodicity. In La7/8Sr1/8MnO3, we observe a 2p charge ordering described by alternating hole-poor and hole-rich MnO planes that is consistent with some of the recent predictions.
    Physical Review Letters 12/2007; 99(20):206403. · 7.73 Impact Factor
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    ABSTRACT: We report on the experimental characterization of orbital ordering and the associated lattice distortions in highly doped Bi1−xCaxMnO3. Resonant x-ray diffraction was used at the Mn L-edge for the direct observation of the ordered localized states, and at the Mn K-edge for the sensitivity to the distortions of the manganese-oxygen octahedra. The orbital ordering on Mn atoms was directly observed at x=0.69; the analysis and the numerical simulations of the K-edge spectra allow us to characterize the pattern of the distorted octahedra at x=4/5. These observations support the Wigner-crystal-type model at both dopings; the bi-stripe model is ruled out at x=0.69.
    Phys. Rev. B. 01/2007; 75(8).
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    ABSTRACT: We report an x-ray diffraction study of the collapse of charge-ordered (CO) phase in Bi_0.2Ca_0.8MnO3 (BCMO) and Bi_0.36Sr_0.64MnO3 (BSMO) single crystals. BCMO shows a first-order CO transition and temperature-independent CO structure. In contrast, the structure of the CO phase of BSMO changes as temperature increases and the CO phase melts. In particular, the lattice modulation period and the ratio of the CO peak intensity to its 2nd harmonic both grow with increasing temperature. These results are discussed in relation with different existing models of the CO state in manganites.
    03/2004;
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    ABSTRACT: Our results from various transport experiments on Mg1-xB2 indicate a surprising effect associated with the presence of a Mg deficiency in MgB2: the phase separation between Mg-vacancy rich and Mg-vacancy poor phases. The Mg-vacancy poor phase is superconducting, but the insulating nature of the Mg-vacancy rich phase probably originates from the Anderson (disorder-induced) localization of itinerant carriers. Furthermore, electron diffraction measurements indicate that within vacancy-rich regions these defects tend to order with intriguing patterns. This electronic phase separation in Mg1-xB2 shows similar, but also distinct characteristics compared with that observed in La(2)CuO(4+delta).
    Physical Review Letters 11/2002; 89(16):167003. · 7.73 Impact Factor
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    ABSTRACT: We have studied the magnetic relaxation properties of the phase-separated manganite compound La0.250Pr0.375Ca0.375MnO3. A series of polycrystalline samples was prepared with different sintering temperatures, resulting in a continuous variation of phase fraction between metallic (ferromagnetic) and charge-ordered phases at low temperatures. Measurements of the magnetic viscosity show a temperature and field dependence which can be correlated to the static properties. Common to all the samples, there appears to be two types of relaxation processes: at low fields associated with the reorientation of ferromagnetic domains and at higher fields associated with the transformation between ferromagnetic and nonferromagnetic phases.
    Physical Review B 05/2002; 65(17):174426. · 3.66 Impact Factor
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    ABSTRACT: We investigated temperature (T)- and magnetic field-dependent optical conductivity spectra σ(ω) of a La5/8-yPryCa3/8MnO3(y≈0.35) single crystal, showing intriguing phase coexistence at low T. At TC<T<TCO, a dominant charge-ordered phase produces a large optical gap energy of ∼0.4 eV. At T<TC, at least two absorption bands newly emerge below 0.4 eV. Analyses of σ(ω) indicate that the bands should be attributed to a ferromagnetic metallic phase and a charge-disordered phase that coexist with the charge-ordered phase. This optical study clearly shows that La5/8-yPryCa3/8MnO3(y≈0.35) is composed of multiphases that might have different lattice strains.
    Physical Review B 03/2002; 65(11). · 3.66 Impact Factor
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    ABSTRACT: We report near-field and far-field optical microscopic studies of photoinduced effects in charge-ordered Bi(0.3)Ca(0.7)MnO(3). Unlike previously reported transient photoinduced effects in manganites, we have observed permanent reflectivity changes following local sample illumination with 488 nm light. High-resolution images of exposed regions reveal optical contrast on a submicrometer scale. This observation indicates that photonic band-gap structures may be created using holographic recording in manganites. We also present images of photoinduced charge-order domain switching in Bi(0.3)Ca(0.7)MnO(3).
    Physical Review Letters 10/2001; 87(12):127204. · 7.73 Impact Factor
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    ABSTRACT: In this paper, we report polarized optical microscopy and electrical transport studies of manganese oxides that reveal that the charge ordering transition in these compounds exhibits typical signatures of a martensitic transformation. We demonstrate that specific electronic properties of charge-ordered manganites stem from a combination of martensitic accommodation strain and effects of strong electron correlations. This intrinsic strain is strongly affected by the grain boundaries in ceramic samples. Consistently, our studies show a remarkable enhancement of low field magnetoresistance and the grain size effect on the resistivity in polycrystalline samples and suggest that the transport properties of this class of manganites are governed by the charge-disordered insulating phase stabilized at low temperature by virtue of martensitic accommodation strain. High sensitivity of this phase to strains and magnetic field leads to a variety of striking phenomena, such as unusually high magnetoresistance (10^10 %) in low magnetic fields. Comment: Short paper, 4 figures, to appear in Rapid Communication
    Physical Review B 07/2001; · 3.66 Impact Factor
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    ABSTRACT: We report x-ray scattering studies of short-range structural correlations and diffuse scattering in Nd0.5Sr0.5MnO3. On cooling, this material undergoes a series of transitions, first from a paramagnetic insulating (PI) to a ferromagnetic metallic (FM) phase, and then to a charge-ordered (CO) insulating state. Highly anisotropic structural correlations were found in both the PI and FM states of this compound. The correlations increase with decreasing temperature, reaching a maximum at the CO transition temperature. Below this temperature, they abruptly collapsed. Single-polaron diffuse scattering was also observed in both the PI and FM states suggesting that substantial local lattice distortions are present in these phases. We argue that our measurements indicate that nanoscale regions exhibiting layered orbital order exist in the paramagnetic and ferromagnetic phases of Nd0.5Sr0.5MnO3.
    Physical review. B, Condensed matter 03/2001; 63(14). · 3.77 Impact Factor
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    ABSTRACT: In doped Mott insulators, charge carriers tend to self-organize in such a way as to form stripe patterns at low temperatures. In the case of doped manganites, orbital degrees of freedom associated with Mn^3+ can be involved in the striped ordering. This striped charge/orbital ordering at low temperatures in manganites is associated with various length scales. Particularly, recent results indicate the presence of very-short-range or dynamic correlation of charge/orbital ordering at high temperatures, far above any phase transition temperatures. This significant fluctuation is directly associated with the origin of the colossal magnetoresistance effect.
    03/2001;
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    ABSTRACT: The insulator-metal transition in single crystal La5/8-yPryCa3/8MnO3 with y≈0.35 was studied using synchrotron x-ray diffraction, electric resistivity, magnetic susceptibility, and specific heat measurements. Despite the dramatic drop in the resistivity at the insulator-metal transition temperature TMI, the charge-ordering (CO) peaks exhibit no anomaly at this temperature and continue to grow below TMI. Our data suggest then, that in addition to the CO phase, another insulating phase is present below TCO. In this picture, the insulator-metal transition is due to the changes that occur within this latter phase. The CO phase does not appear to play a major role in this transition. We propose that a percolationlike insulator-metal transition occurs via the growth of ferromagnetic metallic domains within the parts of the sample that do not exhibit charge ordering. Finally, we find that the low-temperature phase-separated state is unstable against x-ray irradiation, which destroys the CO phase at low temperatures.
    Physical Review B 12/2000; 63(2). · 3.66 Impact Factor
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    ABSTRACT: We have found an unexpected effect associated with the presence of an Mg deficiency in MgB_2: the phase separation between Mg--vacancy rich and Mg--vacancy poor phases. The Mg--vacancy poor phase is superconducting, but the insulating nature of the Mg--vacancy rich phase probably originates from the Anderson (disorder--induced) localization of itinerant carriers. Furthermore, within vacancy--rich regions, these defects tend to order with intriguing patterns. This electronic phase separation in Mg_1-xB2 shows similar, but also distinct characteristics compared with that observed in La_2CuO_4+delta. This work was supported by NSF-DMR-0103858.