arXiv subjects

Hyeong-Jin Kim

Publications and source records attributed to Hyeong-Jin Kim.

27 records · Page 2Linked to original sources

arXiv2001

Energy gap, penetration depth and surface resistance of MgB2 thin films determined by microwave resonator mesurements

We have measured the temperature dependence of the microwave surface impedance Zs = Rs + iΩμ_0 \Lamda of two c-axis oriented MgB2 films at a frequency Ω/(2π) of 17.9 GHz employing a dielectric (sapphire) resonator technique. The temperature dependence of the magnetic field penetration depth \Lamda can be well fitted from 5 K close to Tc by the standard BCS integral expression assuming the reduced energy gap Delta(0)/kTc to be as low as 1.13 and 1.03 for the two samples. For the penetration depth at zero temperatures, values of 110 nm and 115 nm were determined from the fit. Our results clearly indicates the s-wave character of the order parameter. The temperature dependence of the surface resistance Rs below Tc/2 is consistent with the low value of the energy gap. The surface resistance below 8 K was found to be below the resolution limit of 100 microOhm of our measurement technique.

cond-mat.supr-con↗

arXiv2001

Superconducting gap and electron-phonon interaction in MgB$_2$ thin film studied by point contacts

The superconducting order parameter and electron-phonon interaction (EPI) in a MgB$_{2}$ c-axis oriented thin film are investigated by point contacts (PCs). Two superconducting gaps $Δ_{Small}\approx 2.6$ meV and $Δ_{Large}\approx 7.4$ meV have been established, which fairly well correspond to the theoretical gap ratio 1:3. The reconstructed PC EPI function along c-axis has above 30 meV similarities with the phonon DOS, while a pronounced maximum around 20 meV is resolved pointing out that the low frequency phonon mode can be of importance for pairing. We have estimated the Fermi velocity in MgB$_2$ about 5$\times 10^7$ cm/s.

cond-mat.supr-con↗

arXiv2001

High current-carrying capability in c-axis-oriented superconducting MgB2 thin films

In high-quality c-axis-oriented MgB2 thin films, we observed high critical current densities (Jc) of 16 MA/cm^2 at 15 K under self fields comparable to those of cuprate high-temperature superconductors. The extrapolated value of Jc at 5 K was estimated to be 40 MA/cm^2. For a magnetic field of 5 T, a Jc of 0.1 MA/cm^2 was detected at 15 K, suggesting that this compound would be a very promising candidate for practical applications at high temperature and lower power consumption. The vortex-glass phase is considered to be a possible explanation for the observed high current-carrying capability.

cond-mat.supr-con↗

arXiv2001

MgB2 superconducting thin films with a transition temperature of 39 Kelvin

We report the growth of high-quality c-axis-oriented epitaxial MgB2 thin films by using a pulsed laser deposition technique. The thin films grown on (1`1 0 2) Al2O3 substrates show a Tc of 39 K. The critical current density in zero field is ~ 6 x 10^6 A/cm2 at 5 K and ~ 3 x 10^5 A/cm^2 at 35 K, suggesting that this compound has great potential for electronic device applications, such as microwave devices and superconducting quantum interference devices. For the films deposited on Al2O3, X-ray diffraction patterns indicate a highly c-axis-oriented crystal structure perpendicular to the substrate surface.

cond-mat.supr-con↗

arXiv2001

Far-infrared transmission studies of c-axis oriented superconducting MgB2 thin film

We reported far-infrared transmission measurements on a c-axis oriented superconducting MgB$_{2}$ thin film in the frequency range of 30 $\sim$ 250 cm$^{-1}$. We found that these measurements were sensitive to values of scattering rate $1/τ$ and superconducting gap $2Δ$. By fitting the experimental transmission spectra at 40 K and below, we obtained $1/τ=$ (700 $\sim$ 1000) cm$^{-1}$ and $2Δ(0)\cong$ 42 cm$^{-1}$. These two quantities suggested that MgB$_{2}$ belong to the dirty limit.

cond-mat.supr-con↗

arXiv2001

Epitaxial MgB2 superconducting thin films with a transition temperature of 39 Kelvin

We report the growth of high-quality epitaxial MgB2 thin films on (1102) Al2O3 and (001) SrTiO3 substrates by using a pulsed laser deposition technique. The thin films grown on Al2O3 substrates show a Tc of 39 K with a sharp transition width of ~ 0.7 K. The critical current density in zero field is ~ 8 x 10^6 A/cm^2 at 5 K, suggesting that this compound has great potential for industrial applications. Also, Al2O3 is a very promising substrate in terms of superconducting device applications because of its high thermal conductivity and small dielectric constant. For the films deposited on Al2O3, X-ray diffraction patterns indicate a c-axis-oriented crystal structure perpendicular to the substrate surface whereas the films deposited on SrTiO3 show {101} plane as the preferred orientation.

cond-mat.supr-con↗

arXiv2001

Origin of the high DC transport critical current density for the MgB2 superconductor

If the critical current density Jc is very high for a superconductor, then estimating its value from transport measurements is not very easy. In such cases, the value of Jc, called Jcm for magnetic Jc, is usually obtained from the measured magnetic hysteresis loop measurements by using a proper critical state model such as Bean's model (ref. 1). However, for bulk polycrystalline high temperature superconductors, the values of Jcm are much higher than the values, Jct, obtained from the transport measurements. This is due to the fact that the Jct is interrupted by weakly linked grain boundaries. However, for the recently discovered superconductor MgB2 (ref. 2), the grain boundary effect is negligible and these two values seem to coincide. Moreover, Jc increases drastically with decreasing the temperature. Consequently, the critical current densities for bulk wires can be very high, suggesting that numerous applications for the power transport. In this letter, we report the origin of the large current carrying capability of MgB2 based on direct measurements of the current-voltage relation in high magnetic fields. A strong coupling between the grains may be one reason for the absence of the weak link effect. Another reason may be the fact that, instead of a weak pinning mechanism such as thermally activated flux hopping, strong pinning due to a vortex glass phase is found in this material. The vortex phase diagram obtained from transport measurements shows that, in H-T space, a wide region below the Hc2 line is covered by a vortex glass phase.

cond-mat.supr-con↗

arXiv2001

Fluctuation Magnetoconductance in MgB2

We report fluctuation magnetoconductance in a MgB2 superconductor prepared under a high pressure of 3 GPa. This excess magnetoconductance, $Δσ(H)$, follows the three-dimensional scaling function for the critical fluctuations proposed by Ullah and Dorsey. This feature is inferred to originate from the isotropic nature of the MgB2 compound.

cond-mat.supr-con↗

arXiv2001

Hole carrier in MgB_{2} characterized by Hall Measurements

The longitudinal resistivity rho_{xx} and Hall coefficient R_H were measured for MgB2 sintered under high pressure. We found that R_H is positive like cuprate high-Tc superconductors, and decreases as the temperature increases for 40 K < T < 300 K. The cotangent of the Hall angle was found to follow a+bT^2 behavior from Tc to 300 K. At T = 100 K, R_H = 4.1 x 10^{-11} m^3/C from which hole carrier density was determined to be 1.5 x 10^{23}/cm^3. This carrier density is 2 - 3 orders of magnitude larger than that of Nb3Sn superconductor and that of optimally doped YBa_2Cu_3O_y.

cond-mat.supr-con↗