Jung-Dae Kim

Pohang University of Science and Technology, Geijitsu, Gyeongsangbuk-do, South Korea

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Publications (9)16.42 Total impact

  • Xin Lu · W.K. Park · Jung-Dae Kim · Sunmog Yeo · Sung-Ik Lee · L.H. Greene
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    ABSTRACT: We employ the point-contact Andreev reflection spectroscopy technique in LuNi2B2C (Tc∼16.0K) single crystals along three different crystallographic orientations as a function of magnetic field and temperature. Our data taken on the [001] and [110] faces show gap values at ∼2K of ∼2.4meV, and ∼2.6meV, respectively, as analyzed by the one-gap Blonder–Tinkham–Klapwijk model. Both the energy gap and the Dynes broadening parameter exhibit similar dependences on magnetic field along the two directions. Data taken as a function of temperature follow the BCS-like temperature dependence yielding 2Δ0/kBTc∼3.4 and ∼3.6, respectively. The measured gap values in [100] direction vary from 1.6 to 2.5meV at low temperatures and disappear at the same bulk Tc.
    Physica B Condensed Matter 04/2008; 403(5):1098-1100. DOI:10.1016/j.physb.2007.10.282 · 1.32 Impact Factor
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    ABSTRACT: The equilibrium magnetization of grain-aligned HgBa2Ca4Cu5O12+y (Hg1245) with Tc~=108K was measured. The Ginzburg-Landau parameter (kappa) , obtained by using the Hao-Clem model, increased unphysically with increasing temperature for T/Tc=>0.7 , which indicates that the fluctuation is greatly enhanced. The two-dimensional scaling behavior of the magnetization and the temperature variation of the irreversibility field Hirr(T) from a power-law behavior at low fields to an exponential behavior at high fields indicate that Hg1245 is highly anisotropic, which seems to be affected by the appearance of antiferromagnetism at the inner CuO2 planes while blocking the coupling of the superconductivity between the outer CuO2 layers.
    Physical review. B, Condensed matter 01/2008; 77(2). DOI:10.1103/PhysRevB.77.024519 · 3.66 Impact Factor
  • Kyung-Hee Kim · H.-G. Lee · Soo-Hee Oh · Jung-Dae Kim · Sung-Ik Lee
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    ABSTRACT: We synthesized Ca1−xNaxCuO2Cl2 single crystals for 0.06 ⩽ x ⩽ 0.18 in the several GPa pressure range. The Na content, x, critically depended on the applied pressure, not the initial stoichiometric composition. A superconducting transition temperature of 27 K was obtained for x ∼ 0.18, which is the highest Tc reported so far for this compound.
    Physica C Superconductivity 09/2007; s 460–462:78–79. DOI:10.1016/j.physc.2007.03.070 · 0.94 Impact Factor
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    ABSTRACT: We have investigated the two-gap nature of bulk (Mg1-xAlx)B-2 for 0 <= x <= 0.07 by studying the upper critical field H-c2(T). The H-c2(T) is analyzed based on the dirty-limit two-gap model. While the effective diffusivities of the pi-band (D-sigma(e) (f) (f)) decreased gradually, those of the sigma-band (D-pi(e f f)) were relatively unchanged, which indicated that unlike the 2 dimensional a-band, the isotropic pi-band is seriously affected by the Al doping. The upward curvature of H-c2(T) near T-c is also explained by the dirty-limit two-gap model.
    International Journal of Modern Physics B 07/2007; 21:3314-3316. DOI:10.1142/S0217979207044445 · 0.94 Impact Factor
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    ABSTRACT: To understand the effect of carbon doping on the superconductivity in MgB(2), we obtained the angle- and temperature-dependent upper critical fields [H(c2)(θ) and H(c2)(T)] for Mg(B(1-x)C(x))(2) single crystals (x = 0.06 and 0.1) from resistivity measurements while varying the temperature, the field, and the direction of the field. The detailed values of the diffusivity for two different directions for each σ-band and π-band were obtained to explain both the temperature- and the angle-dependent H(c2) by using the dirty-limit two-gap model. The induced impurity scattering of the σ-band and the π-band for both the ab-plane and the c-direction is studied.
    Journal of Physics Condensed Matter 06/2007; 19(24):242201. DOI:10.1088/0953-8984/19/24/242201 · 2.35 Impact Factor
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    ABSTRACT: The superconducting gap anisotropy in non-magnetic members of the intermetallic borocarbide family still remains controversial. Several scenarios have been proposed including the s+g pairing symmetry and multi-band/multi-gap superconductivity. In order to address this issue, especially the puzzling existence of point nodes along a- and b-axis, we apply the point-contact spectroscopy technique to investigate the superconducting gap structure of single crystals LuNi2B2C (Tc˜16.5 K) along three different crystallographic orientations. ab-plane surfaces are prepared by embedding and polishing crystals and their orientations are confirmed by X-ray diffraction. Our preliminary conductance data, analyzed by the one-band Blonder-Tinkham-Klapwijk model, show anisotropic gap values, ˜1.6 meV and ˜2.6 meV, along (001) and (110) directions, respectively. We will discuss the possible origin for the gap anisotropy.
  • Jung-Dae Kim · Heon-Jung Kim · Hyeonjin Doh · Sung-Ik Lee
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    ABSTRACT: The coexistence of superconductivity and magnetism in Ho1-xDyxNi2B2C is studied by using a phenomenological model that is based on the Ginzburg-Landau theory. This model includes two magnetic and two superconducting order parameters accounting for the multiband structure of Ho1-xDyxNi2B2C. The doping dependence of T-c and the effects of antiferromagnetic and spiral order in this material are analyzed.
    Journal of the Physical Society of Japan 01/2007; 76(Suppl.A):144-145. DOI:10.1143/JPSJS.76SA.144 · 1.59 Impact Factor
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    ABSTRACT: We report the reversible magnetization of the Ca2−xNaxCuO2Cl2 (x=0.18) single crystal for H‖c up to 1.2 T to study the effect of oxygen replacement by chlorine at the apical site. The superconducting transition temperature Tc (∼27 K) of this single crystal is the highest reported in this family. Various thermodynamic parameters, such as the penetration depth λab(0), the coherence length ξab(0), the critical field Hc(0), the upper critical field Hc2(0) and the Ginzburg-Landau parameter κ, are also reported. Compared to the iso-structural compound La1.82Sr0.18CuO4 with apical oxygen, Ca1.82Na0.18CuO2Cl2 has two significant differences: (1) λab(0) is larger, so the Cooper pair density is quite reduced, and (2) the anisotropic nature is more enhanced. From these observations, chlorine at the apical site is thought to be less effective than oxygen in supplying charge carriers to the CuO2 planes; as a result, the interlayer coupling between the CuO2 planes is weakened.
    Physical Review B 12/2005; 72(22). DOI:10.1103/PhysRevB.72.224510 · 3.74 Impact Factor
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    ABSTRACT: We report magnetization measurements of grain-aligned Ba2Ca2Cu3O6(O,F)2 with Tc≃108 K. The interlayer distance of the material is the shortest among known tri-layer superconductors. Unexpectedly, the magnetization data show that the coupling strength between CuO2 layers is rather weak. A direct reflection of the weak coupling is highly suppressed irreversibility line, i.e. a broad reversible region in H–T plane. The decoupling field obtained from the irreversibility line is less than 0.1 T, which is comparable with that of quasi two-dimensional superconductor Bi2Sr2CaCu2O8+δ. Comparison of data with the Hao–Clem model gives characteristic parameters [ξab(0) and λab(0)] and the critical fields [Hc(0) and Hc2c(0)]. A large value of penetration depth, λab(0)=240 nm reflects a small carrier concentration in CuO2 planes, and explains the reason of the weak interlayer coupling.
    Solid State Communications 02/2005; 133(7):459–463. DOI:10.1016/j.ssc.2004.11.044 · 1.90 Impact Factor