arXiv subjects

W. N. Kang

Publications and source records attributed to W. N. Kang.

At least 37 records · Page 2Linked to original sources

arXiv2002

Local threshold field for dendritic instability in superconducting MgB2 films

Using magneto-optical imaging the phenomenon of dendritic flux penetration in superconducting films was studied. Flux dendrites were abruptly formed in a 300 nm thick film of MgB2 by applying a perpendicular magnetic field. Detailed measurements of flux density distributions show that there exists a local threshold field controlling the nucleation and termination of the dendritic growth. At 4 K the local threshold field is close to 12 mT in this sample, where the critical current density is 10^7 A/cm^2. The dendritic instability in thin films is believed to be of thermo-magnetic origin, but the existence of a local threshold field, and its small value are features that distinctly contrast the thermo-magnetic instability (flux jumps) in bulk superconductors.

cond-mat.supr-con↗

arXiv2002

Reflection of two-gap nature in penetration depth measurements of MgB$_2$ film

The magnetic penetration depth, $λ^{-2}(T)$, in the basal plane of a magnesium diboride (MgB$_2$) film was measured using a two-coil mutual inductance technique at 50 kHz. This film has $T_c\simeq 38$ K, $ΔT_c \leq 1$ K, and $λ(0)\sim 1500$ Å. At low temperatures, $λ^{-2}(T)$ shows a clear exponential temperature dependence, indicating s-wave superconducting order parameter symmetry. However, the data are not quantitatively well described by theory assuming a single gap. From the data fit by the full BCS calculation assuming a double gap, the values of the two distinct gaps were obtained: $Δ_S(0)=2.61\pm0.41$ meV and $Δ_L(0)=6.50\pm0.33$ meV. The contributions of the small and the large gaps to the total superfluid density at T=0 were estimated to be 21% and 79%, respectively. Finally, we consider the effect of gap anisotropy on the penetration depth measurements, and find that the anisotropy does not play a significant role in determining the temperature dependence of the penetration depth.

cond-mat.supr-con↗

arXiv2002

Current-induced dendritic magnetic instability in superconducting MgB2 films

Magneto-optical imaging reveals that in superconducting films of MgB2 a transport current creates avalanche-like flux dynamics where highly branching dendritic penetration patterns are formed. The instability is triggered when the current exceeds a threshold value, and the superconductor, shaped as a long strip, is initially in the critical state. The instability exists up to 19 K, which is a much wider temperature range than in previous experiments, where dendrites were formed by applying a magnetic field. The instability is believed to be of thermo-magnetic origin indicating that thermal stabilization may become crucial in applications of MgB2.

cond-mat.supr-con↗

arXiv2002

Complicated nature of the gap in MgB2: magnetic field-dependent optical studies

We investigated the magnetic field (H)-dependent optical conductivity spectra of MgB2 thin film in the far-infrared region. The H-dependences can be explained by the increase of normal metallic regions embedded in the superconducting background. The area fraction of the normal metallic region increases rather quickly at low field, but slowly at high field. It follows neither H- nor H^{1/2}-dependences. The results suggest the complicated nature of the superconducting gap in MgB2.

cond-mat.str-el↗

arXiv2002

Superconducting energy gap distribution in c-axis oriented MgB$_2$ thin film from point contact study

We have analyzed about 100 voltage-dependent differential resistance dV/dI(V) curves of metallic point contacts between c-axis oriented MgB$_2$ thin film and Ag, which exhibit clear Andreev reflection features connected with the superconducting gap. About one half of the curves show the presence of a second larger gap. The histogram of the double-gap distribution reveals distinct maxima at 2.4 and 7 meV, while curves with a single-gap features result in more broad maximum at 3.5 meV. The double-gap distribution is in qualitative agreement with the distribution of gap values over the Fermi surface calculated by H. J. Choi et al. (cond-mat/0111183). The data unequivocally show the presence of two gaps $Δ_{S}=2.45\pm 0.15$ meV and $Δ_{L}=7.0\pm 0.45$ meV in MgB$_2$ with gap ratio $2.85\pm 0.15$. Our observations prove further a widely discussed multi-gap scenario for MgB$_2$, where two distinct gaps are seen in the clean limit, while a single averaged gap is present in the dirty one.

cond-mat.supr-con↗

arXiv2002

Universal scaling of the Hall resistivity in MgB2 superconductors

The mixed-state Hall resistivity and the longitudinal resistivity in superconducting MgB2 thin films have been investigated as a function of the magnetic field over a wide range of current densities from 100 to 10000 A/cm^2. We observe a universal Hall scaling behavior with a constant exponent of 2.0, which is independent of the magnetic field, the temperature, and the current density. This result can be interpreted well within the context of recent theories.

cond-mat.supr-con↗

arXiv2002

In-plane Hall effect in c-axis-oriented MgB2 thin films

We have measured the longitudinal resistivity and the Hall resistivity in the ab-plane of highly c-axis-oriented MgB2 thin films. In the normal state, the Hall coefficient (R_H) behaves as R_H ~ T with increasing temperature (T) up to 130 K and then deviates from that linear T-dependence at higher temperatures. The T^2 dependence of the cotangent of the Hall angle is only observed above 130 K. The mixed-state Hall effect reveals no sign anomaly over a wide range of current densities from 10^2 to 10^4 A/cm^2 and for magnetic fields up to 5 T.

cond-mat.supr-con↗

arXiv2002

Effect of High Energy Heavy Ion Irradiation on c-axis Oriented MgB2 Films

We report on the transport, magnetization, and scanning tunneling spectroscopy measurements on c-axis oriented thin films of MgB2 irradiated with high energy heavy ions of uranium and gold. We find a slight shift in the irreversibility and upper critical field lines to higher temperatures after irradiation. In addition, we observe an increase in the critical current at high temperatures near Tc2 and only a small change at low temperatures. Furthermore, we find no evidence for the existence of anisotropic pinning induced by heavy ion irradiation in this material. Tunneling spectra in an irradiated sample show a double gap structure with a flat background and very low zero-bias conductance, behaving in much the same way as the pristine unirradiated sample.

cond-mat.supr-con↗

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

Suppression of Superconducting Critical Current Density by Small Flux Jumps in $MgB_2$ Thin Films

By doing magnetization measurements during magnetic field sweeps on thin films of the new superconductor $MgB_2$, it is found that in a low temperature and low field region small flux jumps are taking place. This effect strongly suppresses the central magnetization peak leading to reduced nominal superconducting critical current density at low temperatures. A borderline for this effect to occur is determined on the field-temperature (H-T) phase diagram. It is suggested that the small size of the flux jumps in films is due to the higher density of small defects and the relatively easy thermal diffusion in thin films in comparison with bulk samples.

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

Magnetic relaxation and critical current density of $MgB_2$ thin films

Magnetic relaxation and critical current density have been measured on a $MgB_2$ thin film in a wide region of temperature with the magnetic field up to 8 T. The irreversibility line has also been determined. It is found that the relaxation rate has a very weak temperature dependence below $1/2 T_c$ showing a clear residual relaxation rate at zero K, which cannot be easily explained as due to thermally activated flux creep. Furthermore the relaxation rate has a strong field dependence. The flux dynamics of thin films are very similar to that of high pressure synthesized bulks although the relaxation rate in thin film is systematically higher than that of a bulk sample. All the results here together with those from the bulk samples suggest that the flux dynamics may be dominated by the quantum effects, such as quantum fluctuation and tunneling.

cond-mat.supr-con↗

arXiv2001

Spectroscopic Evidence for Anisotropic S-Wave Pairing Symmetry in MgB2

Scanning tunneling spectroscopy of superconducting MgB$_2$ ($T_c = 39$ K) were studied on high-density pellets and c-axis oriented films. The sample surfaces were chemically etched to remove surface carbonates and hydroxides, and the data were compared with calculated spectra for all symmetry-allowed pairing channels. The pairing potential ($Δ_k$) is best described by an anisotropic s-wave pairing model, with $Δ_k = Δ_{xy} \sin ^2 θ_k + Δ_z \cos ^2 θ_k$, where $θ_k$ is the angle relative to the crystalline c-axis, $Δ_z \sim 8.0$ meV, and $Δ_{xy} \sim 5.0$ meV.

cond-mat.supr-con↗

arXiv2001

Dendritic flux patterns in MgB2 films

Magneto-opitcal studies of a c-oriented epitaxial MgB2 film with critical current density 10^7 A/cm^2 demonstrate a breakdown of the critical state at temperatures below 10 K [cond-mat/0104113]. Instead of conventional uniform and gradual flux penetration in an applied magnetic field, we observe an abrupt invasion of complex dendritic structures. When the applied field subsequently decreases, similar dendritic structures of the return flux penetrate the film. The static and dynamic properties of the dendrites are discussed.

cond-mat.supr-con↗

arXiv2001

Dendritic magnetic instability in superconducting MgB2 films

Magneto-opitcal studies of a c-oriented epitaxial MgB2 film show that below 10 K the global penetration of vortices is dominated by complex dendritic structures abruptly entering the film. We suggest that the observed behavior is due to a thermo-magnetic instability which is supported by vortex dynamics simulations. The instability is also responsible for large fluctuations in the magnetization curves in MgB2 at low temperatures.

cond-mat.supr-con↗

arXiv2001

Optical studies of charge dynamics in c-axis oriented superconducting MgB2 films

Temperature dependent optical conductivities and DC resistivity of c-axis oriented superconducting (Tc = 39.6 K) MgB2 films (~ 450 nm) have been measured. The normal state ab-plane optical conductivities can be described by the Drude model with a temperature independent Drude plasma frequency of ω_{p,D}=13,600 +/- 100 cm-1 or 1.68 +/- 0.01 eV. The normal state resistivity is fitted by the Bloch-Gruneisen formula with an electron-phonon coupling constant λ_{tr} = 0.13 +/- 0.02. The optical conductivity spectra below T_c of these films suggest that MgB2 is a multi-gap superconductor.

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

Superconducting properties of well-shaped MgB2 single crystal

We report measurements of the transport and the magnetic properties of high-quality, sub-millimeter-sized MgB2 single crystals with clear hexagonal-plate shapes. The low-field magnetization and the magnetic hysteresis curves show the vortex pinning of these crystals to be very weak. The Debye temperature of $Θ_{D}\sim 1100$ K, obtained from the zero-field resistance curve, suggests that the normal-state transport properties are dominated by electron-phonon interactions. The resistivity ratio between 40 K and 300 K was about 5, and the upper critical field anisotropy ratio was 3 $\pm$ 0.2 at temperatures around 32 K.

cond-mat.supr-con↗