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Yong-Jihn Kim Theme Song (Composer: Michael Bonet)
2006 IOP Reception(IPod Nano winner!), Batimore, MD
Hello!
I have been working on superconductivity more than 15 years! During the time, I found some serious fundamental problems in our understanding of superconductivity and tried to resolve those problems. This website summarizes the current status of this effort. I wish it could be helpful for many people.
Yong-Jihn Kim, Associate Professor of Physics, UPR-Mayaguez
I have started a superconducting wire company, called Cutting Edge Superconductors, Inc.
- Recent work 1: Revised Theory of the Josephson Effects: Just published in Journal of Applied Physics, Vol. 103, 103902, (2008)
Highlighted in
Virtual Journal of Applications of Superconductivity, Vol. 14, issue 11, June 1, 2008
In 1962 B. Josephson predicted the Josephson effects. He won the Nobel Prize in Physics in 1973
for the contribution. Nevertheless, there are still some unsolved fundamental problems, such as the sign problem
in the AC Josephson effect, the threshold behavior in the DC Josephson effect, and etc.
This paper introduces a Cooper pair wavefunction approach to the Josephson effects. This method is more powerful and error-free, compared to the previous techniques. We have found that SIS junctions have a material-dependent threshold resistance, shown in the figure below,which explains why MgB2 SIS junctions don't show the supercurrent for the big gap! In addition we have found a more accurate formula for the temperature dependence of the supercurrent.
This understanding may lead to the discovery of better materials for SIS junctions other than Nb and the optimum miniaturization of the SIS junctions for the petaflops superconducting supercomputers.
The preliminary result has been published in the LT24 Proceedings: Threshold resistance in the DC Josephson effect.
Copyright (2006) American Institute of Physics.
In 1998 I predicted this behavior in my booklet "Reinvestigation of Inhomogeneous and high
Tc Superconductors".
cf. 5.5 Josephson effect
The (insulator) thickness dependence of the supercurrents is not well understood!
-
Recent work 2: Rediscovery of Testardi's universal correlation of Tc and resistance ratio [ 1, 2 ]
Around 1975 Testardi and coworkers found this correlation [3,4]. However, P. W. Anderson's theorem [5] does not allow this kind of correlation. Therefore, the experimetnal discovery was forgotten. Recently, we showed that Anderson's theorem breaks down when electrons are (weakly) localized [6], which led to the explanation of the correlation [1]. Furthermore, we noticed that this correlation can probe the phonon mechanism in superconductors [2]. In particular, the Testardi correlation provided definite proof of the phonon mechanism in the recently discovered 39K superconductor, MgB2 [2,7].
From Buzea and Yamashita [7]
Advantage: applicable to any superconductor, including d- and f- electron
superconductors
Limitation: limited applicability in systems, such as
i) highly anisotropic systems where impurity scattering can reduce the Tc to
zero even before electrons are weakly localized
ii) high Tc cuprates where the Coulomb interactions are comparable to the
electron-phonon interaction even at room temperature
=> Nevertheless weak localization effect shows that the electron-phonon interaction is also crucial even in high Tc cuprates : Using weak localization to probe the phonon-mechanism in high Tc superconductors.
- Recent work 3: New Interpretation of Fux Quantization [8]
The Byers and C. N. Yang's theory [9] of flux quantization is not consistent with the
observation of persistent current in normal metal rings [10]. Byers and Yang (BY) assumed
that the normal state free energy Fn does not depend on the flux Φ, i.e., Fn(Φ) = constant.
However, the persistent current is given by I= -c ∂Fn(Φ)/ ∂Φ. Accordingly, the
BY theory leads to the following:
 -2× 
=> 
No flux quantization!
New Interpretation [8] : flux dependence of the pairing interaction leads to flux quantization.
Comment by an expert:
I do not think your finding is actually at odds with the way in which flux quantization
is understood by experts on superconductivity - it is at odds with the way it is often
discussed in text books however.
References
[1] M A Park and Yong-Jihn Kim, Phys. Rev. B 61, 14733 (2000).
[2] M A Park, K. Savran, and Yong-Jihn Kim, Supercond. Sci. Technol.14, L31-L35,
(Rapid Communication), (2001).
[3] L. R. Testardi, R. L. Meek, J. M. Poate, W. A. Royer, A. R. Strom, and
J. H. Wernik, Phys. Rev. B 11, 4304 (1975).
[4] J. M. Poate, L. R. Testardi, A. R. Strom, and W. M. Augustyniak, Phys. Rev. Lett.
35, 1290 (1975)
[5] P. W. Anderson, J. Phys. Chem. Solids 11, 26 (1959).
[6] Yong-Jihn Kim and A. W. Overhauser, Phys. Rev. B 47, 8025 (1993).
[7] C. Buzea and T. Yamashita, Supercond. Sci. Technol.14, R115-R146, (2001).
[8] M A Park and Yong-Jihn Kim, Supercond. Sci. Technol. 17, L10-L16,(Rapid Communication), (2004).
[9] N. Byers and C. N. Yang, Phys Rev. Lett. 7, 46 (1961).
[10] M. Buttiker, Y. Imry, and R. Landauer, Phys. Lett. A 96, 365 (1983).
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