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Yong-Jihn Kim

Publications and source records attributed to Yong-Jihn Kim.

At least 19 recordsLinked to original sources
arXiv2006

Weak Localization Effects On The Electron-Phonon Interaction In Disordered Metals

Weak localization has a strong influence on both the normal and superconducting properties of metals. In particular, since weak localization leads to the decoupling of electrons and phonons, the temperature dependence of resistance decreases with increasing disorder, as manifested by Mooij's empirical rule. In addition, Testardi's universal correlation of Tc and the resistance ratio follows. We show that weak localization leads to the similar correction to other physical quantities, such as electronic thermal resistivity, electronic heat capacity, and ultrasonic attenuation, which are controlled by the electron-phonon interaction.

cond-mat.dis-nn↗
arXiv2006

Threshold Resistance in the DC Josephson Effect

We show that SIS Josephson junctions have a threshold resistance, above which the Josephson coupling and the supercurrents become extremely small, due to the shrinking of the Cooper pair size during the Josephson tunneling. Accordingly, the threshold resistance is smaller for higher Tc superconductors with small Cooper pair size and for the insulating barrier with higher resistance. This understanding agrees with the observations in SIS junctions of low Tc superconductors, such as Sn, Pb, and Nb. For MgB2 it explains why the big gap does not show the supercurrents, unlike the small gap. Furthermore, it is consistent with the fact that high Tc cuprates show the Josephson effects only for SNS type junctions, including the intrinsic Josephson effects.

cond-mat.supr-con↗
arXiv2004

A New Approach to the Josephson Effect

We introduce a new approach to the Josephson effect in SIS tunnel junctions. The Josephson coupling energy is calculated from the overlap of real space Cooper pair wavefunctions in two superconductors through an insulating barrier. It is shown that the Josephson tunneling is limited by the size of the Cooper pair and its shrinking during the tunneling. Therefore, the Josephson coupling energy and the critical current become extremely small in high $T_{c}$ superconductors, including $MgB_{2}$. This shrinking also causes the observed DC supercurrent in low $T_{c}$ superconductors, such as Pb and Sn, to fall off much faster than $1/R_{n}$ for tunneling resistance $R_{n}$ above several ohms. Consequently there is a material-dependent threshold resistance, above which the supercurrent decreases much faster with increasing resistance. The impurity-induced shrinking is also shown to limit the critical current. Furthermore, the (weak) temperature dependence of the Cooper pair size is found to contribute to the temperature dependence of the DC supercurrent.

cond-mat.supr-con↗
arXiv2002

A New Interpretation of Flux Quantization

We study the effect of Aharonov-Bohm flux on the superconducting state in metallic cylinders. Although Byers and Yang attributed flux quantization to the flux-dependent minimum of kinetic energies of the Cooper pairs, it is shown that kinetic energies do not produce any discernible oscillations in the free energy of the superconducting state (relative to that of normal state) as a function of the flux. This result is indeed anticipated by the observation of persistent current in normal metal rings at low temperature. Instead, we have found that pairing interaction depends on the flux, leading to flux quantization. When the flux $(Φ$) is given by $Φ=n\times hc/2e$ (with integer n), the pairing interaction and the free energy become unchanged (even n) or almost unchanged (odd n), due to degenerate-state pairing resulting from the energy level crossing. As a result, flux quantization and Little-Parks oscillations follow.

cond-mat.supr-con↗
arXiv2002

Different Behavior of Magnetic Impurities in Crystalline and Ammorphous States of Superconductors

It has been observed that the effect of magnetic impurities in a superconductor is drastically different depending on whether the host superconductor is in a crystalline or an amorphous state. Based on the recent theory of Kim and Overhauser (KO), it is shown that as the system is getting disordered, the initial slope of the $T_{c}$ depression is decreasing by a factor $\sqrt{\ell/ξ_{0}}$, when the mean free path $\ell$ becomes smaller than the BCS coherence length $ξ_{0}$, which is in agreement with experimental findings. In addition, for a superconductor in a crystalline state in the presence of magnetic impurities the superconducting transition temperature $T_{c}$ drops sharply from about 50% of $T_{c0}$ (for a pure system) to zero near the critical impurity concentration. This {\sl pure limit behavior} was indeed found by Roden and Zimmermeyer in crystalline Cd. Recently, Porto and Parpia have also found the same {\sl pure limit behavior} in superfluid He-3 in aerogel, which may be understood within the framework of the KO theory.

cond-mat.mtrl-sci↗
arXiv2001

Weak localization and the Mooij rule in disordered metals

Weak localization leads to the same correction to both the conductivity and the McMillan's electron-phonon coupling constant $λ$ (and $λ_{tr}$). Consequently the temperature dependence of the thermal electrical resistivity is decreasing as the conductivity is decreasing due to weak localization, which results in the decrease of the temperature coefficient of resistivity (TCR) with increasing the residual resistivity. When $λ$ and $λ_{tr}$ are approaching zero, only residual resistivity part remains and gives rise to the negative TCR. Accordingly, the Mooij rule is a manifestation of weak localization correction to the conductivity and the electron-phonon interaction. This study may provide a new means of probing the phonon-mechanism in exotic superconductors.

cond-mat.dis-nn↗
arXiv2001

A New Method of Probing the Phonon Mechanism in Superconductors including MgB$_{2}$

Weak localization has a strong influence on both the normal and superconducting properties of metals. In particular, since weak localization leads to the decoupling of electrons and phonons, the temperature dependence of resistance (i.e., $λ_{tr}$) is decreasing with increasing disorder, as manifested by Mooij's empirical rule. In addition, Testardi's universal correlation of $T_{c}$ (i.e., $λ$) and the resistance ratio (i.e., $λ_{tr}$) follows. This understanding provides a new means to probe the phonon mechanism in superconductors including MgB$_{2}$. The merits of this method are its applicability to any superconductors and its reliability because the McMillan's electron-phonon coupling constant $λ$ and $λ_{tr}$ change in a broad range, from finite values to zero, due to weak localization. Karkin et al's preliminary data of irradiated MgB$_{2}$ show the Testardi correlation, indicating that the dominant pairing mechanism in MgB$_{2}$ is the phonon-mediated interaction.

cond-mat.supr-con↗
arXiv2000

Effect of Magnetic Impurities in Crystalline and Amorphous States of Superconductors

It has been observed that the effect of magnetic impurities in a superconductor is drastically different depending on whether the host superconductor is in a crystalline or an amorphous state. Based on the recent theory of Kim and Overhauser, it is shown that as the system is getting disordered, the initial slope of the $T_{c}$ depression is decreasing by a factor $\sqrt{\ell/ξ_{0}}$, when the mean free path $\ell$ becomes smaller than the BCS coherence length $ξ_{0}$, which is in agreement with experimental findings. In addition, for a superconductor in a pure crystalline state the superconducting transition temperature $T_{c}$ drops sharply from about 50% of $T_{c0}$ (for a pure system) to zero near the critical impurity concentration. This {\sl pure limit behavior} was indeed found by Roden and Zimmermeyer in crystalline Cd.

cond-mat.supr-con↗
arXiv2000

On the Mooij Rule

Weak localization leads to the same correction to both the conductivity and the electron-phonon coupling constant $λ$ (and $λ_{tr}$). Consequently the temperature dependence of the (thermal) electrical resistivity is decreasing as the conductivity is decreasing due to weak localization, which results in the decrease of the temperature coefficient of resistivity (TCR) with increasing the residual resistivity. When $λ$ is approaching zero, only residual resistivity part remains and gives rise to the negative TCR. Accordingly, the Mooij rule is a manifestation of weak localization correction to the conductivity and the electron-phonon interaction. This study may provide a new means of probing the phonon-mechanism in exotic superconductors.

cond-mat.dis-nn↗
arXiv1999

Absence of the Vortex Solution in Gor'kov's Formalism

It is shown that the Abrikosov's vortex solution or its corresponding two-particle pair potential is not the solution of the self-consistency equation in Gor'kov's formalism. Since the self-consistency equation leads to a superposition of different types of off-diagonal long-range order (ODLRO) instead of one type of ODLRO only, it may not handle the vortex problem appropriately. A possible resolution is suggested.

cond-mat.supr-con↗
arXiv1999

Weak Localization Effect in Superconductors by Radiation Damage

Large reductions of the superconducting transition temperature $T_{c}$ and the accompanying loss of the thermal electrical resistivity (electron-phonon interaction) due to radiation damage have been observed for several A15 compounds, Chevrel phase and Ternary superconductors, and $\rm{NbSe_{2}}$ in the high fluence regime. We examine these behaviors based on the recent theory of weak localization effect in superconductors. We find a good fitting to the experimental data. In particular, weak localization correction to the phonon-mediated interaction is derived from the density correlation function. It is shown that weak localization has a strong influence on both the phonon-mediated interaction and the electron-phonon interaction, which leads to the universal correlation of $T_{c}$ and resistance ratio.

cond-mat.supr-con↗
arXiv1998

P-wave Pairing and Colossal Magnetoresistance in Manganese Oxides

We point out that the existing experimental data of most manganese oxides show the {\sl frustrated} p-wave superconducting condensation in the ferromagnetic phase in the sense that the superconducting coherence is not long enough to cover the whole system. The superconducting state is similar to the $A_{1}$ state in superfluid He-3. The sharp drop of resistivity, the steep jump of specific heat, and the gap opening in tunneling are well understood in terms of the p-wave pairing. In addition, colossal magnetoresistance (CMR) is naturally explained by the superconducting fluctuations with increasing magnetic fields. The finite resistivity may be due to some magnetic inhomogeneities. This study leads to the possibility of room temperature superconductivity.

cond-mat.mtrl-sci↗
arXiv1998

Reinvestigation of Inhomogeneous Superconductors and High $T_{c}$ Superconductors

We review the recent progress in the theory of inhomogeneous superconductors. It was shown that Gor'kov's self-consistency equation needs a pairing constraint derived from the Anomalous Green's function. The Bogoliubov-de Gennes equations also need a pairing constraint in order to obtain a correct vacuum state by the corresponding unitary transformation. This new study opens up a reinvestigation of inhomogeneous superconductors. We discuss (i) problems of the conventional Green's function theory, (ii) reinvestigation of impure superconductors, and (iii) impurity doping effect in high $T_{c}$ superconductors. It is also pointed out that a new formalism is required to tackle the macroscopically or mesoscopically inhomogeneous systems such as the junction and the vortex problems.

cond-mat.supr-con↗
arXiv1998

Impurity Doping Effect in High $T_{c}$ Superconductors

It has been observed that impurity doping and/or ion-beam-induced damage in high $T_{c}$ superconductors cause a metal-insulator transition and thereby suppress the critical temperature. Based on our recent theory of the weak localization effect on superconductivity, we examine the variation of $T_{c}$ with increasing of impurity concentration $\rm (x)$ in $\rm{La_{1.85}Sr_{0.15}Cu_{1-x}A_{x}O_{4}}$ systems, where A $=$ Fe, Co, Ni, Zn, or Ga. We find that the doping impurity decreases the scattering matrix elements for electron-electron attractions, such as $V_{nn'}=-V[1-{2\over πk_{F}\ell} ln(L/\ell)]$, where $L$ and $\ell$ are the inelastic and elastic mean free paths, respectively. Using the mean free path $\ell$ determined from resistivity data, we find good agreements between our calculated values for $T_{c}$ and experimental data except for Ni-doped samples, where Ni impurities may enhance the pairing interaction.

cond-mat.supr-con↗
arXiv1997

Strong-Coupling Theory of Impure Superconductors : Correspondence with Weak-Coupling Theory

We reconsider the ordinary impurity effect on the transition temperature $T_{c}$ of superconductors using the Eliashberg formalism. It is shown that the correspondence principle, which relates strong-coupling and weak-coupling theories, works only when Anderson's pairing condition between the time-reversed scattered-states is used. For an Einstein phonon model, the change of the electron density of states caused by the impurity scattering leads to a $T_{c}$ decrease proportional to ${1/ E_{F}τ}$ in the dirty limit. It is pointed out that the phonon-mediated interaction decreases by the same weak localization correction term as that of the conductivity. Accordingly, for strongly localized states the phonon-mediated interaction is exponentially small. We also discuss the case of Debye phonon model.

cond-mat.supr-con↗
arXiv1997

Compensation of Magnetic Impurity Effect in Superconductors by Radiation Damage

Compensation of the reduction of $T_{c}$ caused by magnetic impurities has been observed as a consequence of radiation damage. Using the recent theory by Kim and Overhauser (KO) we consider the effect of radiation damage on the $T_{c}$ of superconductors having magnetic impurities. We find a good fitting to the experimental data. It is also pointed out that Gor'kov's formalism with the pairing constraint derived from the Anomalous Green's function leads to KO theory.

cond-mat.supr-con↗
arXiv1997

Impurity scattering in a d-wave superconductor

The influence of (non-magnetic and magnetic) impurities on the transition temperature of a d-wave superconductor is studied anew within the framework of BCS theory. Pairing interaction decreases linearly with the impurity concentration. Accordingly $T_{c}$ suppression is proportional to the (potential or exchange) scattering rate, $1/τ$, due to impurities. The initial slope versus $1/τ$ is found to depend on the superconductor contrary to Abrikosov-Gor'kov type theory. Near the critical impurity concentration $T_{c}$ drops abruptly to zero. Because the potential scattering rate is generally much larger than the exchange scattering rate, magnetic impurities will also act as non-magnetic impurities as far as the $T_{c}$ decrease is concerned. The implication for the impurity doping effect in high $T_{c}$ superconductors is also discussed.

cond-mat.supr-con↗
arXiv1997

Weak Localization Effect in Superconductors

We study the effect of weak localization on the transition temperatures of superconductors using time-reversed scattered state pairs, and find that the weak localization effect weakens electron-phonon interactions. With solving the BCS $T_{c}$ equation, the calculated values for $T_c$ are in good agreement with experimental data for various two- and three-dimensional disordered superconductors. We also find that the critical sheet resistance for the suppression of superconductivity in thin films does not satisfy the universal behavior but depends on sample, in good agreement with experiments. but depends on sample, in good agreement with experiments.

cond-mat.supr-con↗