Yumi Ko

Yonsei University, Sŏul, Seoul, South Korea

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Publications (10)25.65 Total impact

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    ABSTRACT: Speckle tracking is widely used for elastography in ultrasound imaging. Traditional speckle tracking algorithms accurately detect axial motion, but have difficulty detecting lateral motion, even with a significant computational burden. A modified cross correlation algorithm with a synthetic lateral phase has been developed to reduce the computation time, allowing for real-time implementation. The modified algorithm yields results comparable to those of existing lateral displacement tracking methods. With the modified method, lateral movement in the micrometer range can be successfully tracked while reducing computational time by more than a factor of two. This algorithm will serve as a useful tool to further advance ultrasound elastography.
    Journal- Korean Physical Society 01/2012; 60(1). · 0.43 Impact Factor
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    ABSTRACT: We study the holographic QCD in the hadronic medium by using the soft wall model. We discuss the Hawking-Page transition between Reissner-Nordstr\"{o}m AdS black hole and thermal charged AdS of which the geometries correspond to deconfinement and confinement phases respectively. We also present the numerical result of the vector and axial vector meson spectra depending on the quark density.
    Physical review D: Particles and fields 04/2011; 84.
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    ABSTRACT: It is beyond the present techniques based on perturbation theory to reveal the nature of phase transitions in strongly interacting field theories. Recently, the holographic approach has provided us with an effective dual description, mapping strongly coupled conformal field theories to classical gravity theories. Resorting to the holographic superconductor model, we propose a general criterion for the nature of the superconducting phase transition based on effective interactions between vortices. We find "tricritical" points in terms of the chemical potential for U(1) charges and an effective Ginzburg-Landau parameter, where vortices do not interact to separate the second order (repulsive) from the first order (attractive) transitions. We interpret the first order transition as the Coleman-Weinberg mechanism, arguing that it is relevant to superconducting instabilities around quantum criticality.
    Physical Review B 04/2011; 84(18). · 3.66 Impact Factor
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    ABSTRACT: Based on Sen's entropy function formalism, we consider the Bekenstein–Hawking entropy of the extremal Kerr-(anti-)de Sitter black holes in 4-dimensions. Unlike the extremal Kerr black hole case with flat asymptotic geometry, where the Bekenstein–Hawking entropy S is proportional to the angular momentum J, we get a quartic algebraic relation between S and J by using the known solution to the Einstein equation. We recover the same relation in the entropy function formalism. Instead of full geometry, we write down an ansatz for the near horizon geometry only. The exact form of the unknown functions and parameters in the ansatz are obtained by solving the differential equations which extremize the entropy function. The results agree with the nontrivial relation between S and J.We also study the Gauss–Bonnet correction to the entropy exploiting the entropy function formalism. We show that the term, though being topological thus does not affect the solution, contributes a constant addition to the entropy because the term shifts the Hamiltonian by that amount.
    Annals of Physics 08/2010; · 3.07 Impact Factor
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    ABSTRACT: We study a self-bound dense object in the hard wall model. We consider a spherically symmetric dense object which is characterized by its radial density distribution and non-uniform but spherically symmetric chiral condensate. For this we analytically solve the partial differential equations in the hard wall model and read off the radial coordinate dependence of the density and chiral condensate according to the AdS/CFT correspondence. We then attempt to describe nucleon density profiles of a few nuclei within our framework and observe that the confinement scale changes from a free nucleon to a nucleus. We briefly discuss how to include the effect of higher dimensional operator into our study. We finally comment on possible extensions of our work. Comment: 17 pages, 5 figures, figures replaced, minor revision, to appear in JHEP
    Journal of High Energy Physics 07/2010; · 6.22 Impact Factor
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    ABSTRACT: In this paper we propose a way of determining the subleading corrections to the Bekenstein-Hawking black hole entropy by considering a modified generalized uncertainty principle with two parameters. In the context of modified generalized uncertainty principle, coefficients of the correction terms of black hole entropy are written in terms of combination of the parameters. We also obtained the corrections to the Stefan-Boltzman law and the black hole evaporation in terms of the parameters. By estimating those parameters, say by experiment, one can test results from other context of quantum gravity theories such as black hole entropy. KeywordsGeneralized uncertainty principle-Black hole thermodynamics
    International Journal of Theoretical Physics 06/2010; 49(6):1384-1395. · 1.19 Impact Factor
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    Youngman Kim, Yumi Ko
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    ABSTRACT: We study the density driven symmetry breaking in holographic superconductors by considering the positive mass squared case. We show that even for the positive m 2 , a scalar condensation still forms, provided the chemical potential is high enough. As m 2 increases, the phase space folds due to the non-linearity of the equations of motion, and two nearby points in the phase space can represent symmetry breaking and preserving configurations respectively. The phase space defined by the set of initial conditions of field variables at the horizon undergoes a non-linear radial evolution to result in the phase space folding, a characteristic phenomenon in a non-linear system. We then calculate the specific heat, which characterizes superconductors and has been measured in experiments. We observe a discontinuity in the specific heat at the transition point and compare our results with experimentally observed numbers. The electrical conductivity for various m 2 is also calculated.
    Physical review D: Particles and fields 01/2010; 20.
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    ABSTRACT: We study the spectra of light mesons as well as the dissociation of a quarkonium and monopole-anti monopole bound state in a gluon condensate background. In order to describe the confining phase we introduce an IR cutoff in two ways, namely the hard wall and the braneless approaches. We find that the meson spectra strongly depend on the gluon condensate in the braneless approach, while they do not in the hard wall model. The latter result appears to be a more probable feature of QCD. Comment: 16 pages, 5 figures, figures changed, references added, arguments improved, published version
    Journal of High Energy Physics 12/2009; 2010(4). · 6.22 Impact Factor
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    ABSTRACT: We study the density driven symmetry breaking in holographic superconductors by considering the positive mass squared case. We show that even for the positive $m^2$, a scalar condensation still forms, provided the chemical potential is high enough. As $m^2$ increases, the phase space folds due to the non-linearity of the equations of motion, and two nearby points in the phase space can represent symmetry breaking and preserving configurations respectively. The phase space defined by the set of initial conditions of field variables at the horizon undergoes a non-linear radial evolution to result in the phase space folding, a characteristic phenomenon in a non-linear system. We then calculate the specific heat, which characterizes superconductors and has been measured in experiments.We observe a discontinuity in the specific heat at the transition point and compare our results with experimentally observed numbers. The electrical conductivity for various $m^2$ is also calculated. Comment: 16 pages, v2: title changed, abstract and discussion updated, typos corrected, references added, to appear in PRD
    Physical Review D 04/2009; · 4.86 Impact Factor
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    ABSTRACT: In this paper we propose a way of determining the subleading corrections to the Bekenstein-Hawking black hole entropy by considering a modified generalized uncertainty principle with two parameters. In the context of modified generalized uncertainty principle, coefficients of the correction terms of black hole entropy are written in terms of combination of the parameters. We also calculate corrections to the Stefan-Boltzman law of Hawking radiation corresponding to modified generalized uncertainty principle. By comparing the entropy with one from black holes in string theory compactified on a Calabi-Yau manifold, we point out that the topological information of the compactified space can not easily be related to the parameters in modified generalized uncertainty principle.
    09/2006;

Publication Stats

50 Citations
25.65 Total Impact Points

Institutions

  • 2012
    • Yonsei University
      • Division of Biomedical Engineering
      Sŏul, Seoul, South Korea
  • 2010–2011
    • Asia Pacific Center for Theoretical Physics
      Urusan, Ulsan, South Korea
    • Kyung Hee University
      • Research Institute for Basic Science
      Sŏul, Seoul, South Korea
  • 2009–2010
    • Sogang University
      • Department of Physics
      Sŏul, Seoul, South Korea