Won Kyung Seong

Josephson effects in weakly coupled MgB2 intergrain nanobridges prepared by focused ion beam

We have fabricated weakly coupled intergrain nanobridges from MgB2 films by a focused ion beam (FIB) patterning technique and studied their transition properties. The bridges were nominally 300 nm wide and 100 nm long, and crossed a single grain boundary perpendicularly. The temperature-dependent resistance data showed a two-step transition after FIB pattern with more-than-two-decade increase in the resistivity. Current-voltage curves showed the characteristics of an ideal Josephson junction. The measured data were perfectly matched with the theory of the resistively shunted junction model with thermal fluctuations at all measured temperatures. At 4.2 K, the measured data showed the effect of hysteresis in agreement with the estimated McCumber parameter βc>0.3. The hysteresis effect disappeared above 6 K. The critical currents obtained from fitting to the RSJ model were linearly dependent on temperature, implying that the grain boundary played an insulating barrier.

Effect of columnar grain boundaries on flux pinning in MgB2 films

Columnar grain boundaries are widely known to be a very effective source for flux pinning in MgB2 films. In this study, we have investigated the pinning effect of a columnar grain boundary at various temperatures of 5, 10, 20, 30, and 35 K in columnar structured MgB2 films with an average grain size of ∼300 nm. The average vortex-vortex spacing (a0) is estimated at a specific magnetic field, Bpeak, where the Bpeak is the magnetic field when the flux pinning force density (Fp) reaches a maximum. The values of a0/2, which largely affect the vortex-vortex interaction, are much closer to the coherence length of MgB2, than to the penetration depth, which indicates that the vortices can be strongly pinned to the columnar grain boundaries. Furthermore, we found that the columnar grain boundaries acted as strong pinning sources over a wide temperature region, although their effectiveness began to lessen slowly at temperatures above ∼20 K, which was determined on the basis of the flux-line lattice-shearing mechanism.

A simple method for the enhancement of Jc in MgB2 thick films with an amorphous SiC impurity layer

We investigated the effect of SiC doping on the critical current density (Jc) in MgB2 thick films using amorphous SiC impurity layers of various thicknesses: 7, 14, 35, and 70?nm. SiC impurity layers were first deposited on the Al2O3(0001) substrates at room temperature by using a pulsed laser deposition system, after which MgB2 films were grown on the SiC deposited precursor substrates by using a hybrid physical?chemical vapor deposition technique at a low growth temperature of 480??C. All samples showed a high transition temperature of ~40?K irrespective of the thickness of the impurity layer. The grain sizes of the MgB2 films slightly increased from 400 to 488?nm with increasing thickness of the impurity layer. The MgB2 thick film with a 35?nm thick SiC impurity layer exhibited the highest Jc, while all SiC doped samples showed a higher Jc than a pure MgB2 thick film throughout the whole magnetic field region. These results suggest that the SiC particles of the impurity layer diffused into the MgB2 films during film growth, and the SiC particles, along with the columnar grain boundaries in the MgB2 thick films, act as strong pinning centers.

Observation of strong intrinsic pinning in MgB2 films

By using three types of MgB2 superconductors, such as c-axis-oriented single-crystal films, c-axis-oriented columnar-structure films and films without c-axis orientation perpendicular to the substrate surface, we have investigated the intrinsic pinning effect in MgB2 superconductors. The strong field performance of Jc was observed by turning the orientation of grains from the c axis to the a axis. No c-axis-oriented MgB2 films showed a noticeable increase of Jc at high fields compared with c-axis-oriented films, whether they had columnar structures or not. Our results clearly show that MgB2 has strong intrinsic pinning caused by the large anisotropy of the superconducting energy gap in the boron layers like high-Tc cuprate superconductors with a layered structure.

All focused ion beam fabricated MgB2 inter-grain nanobridge dc SQUIDs

We have fabricated MgB2 dc SQUIDs (superconducting quantum interference devices) containing inter-grain nanobridges as Josephson elements by a focused ion beam (FIB) etching method and measured their transport properties. The entire structure including the SQUID loop was patterned only using a FIB. The beam energy was 30 kV and the current was 0.9 nA for larger structures and 34 and 1.5 pA for the nanobridge pattern. Each bridge with a nominal width of 100 nm crossed a single grain boundary in the normal direction. The SQUID loop had a 3.1 µm × 3.1 µm hole with a 2 µm average linewidth, corresponding to an inductance of 5.1 pH. The nanobridges had a two-step transition with an increase in the resistivity of more than a decade and a substantial decrease in the critical current density. Current–voltage characteristics showed a resistively shunted junction behavior at all temperatures below Tc, which implies that the current in the inter-grain nanobridges was determined mainly by the Josephson coupling. These results are believed to be due to discernible damage on the grain boundary caused by FIB irradiation, resulting in the formation of a tunneling barrier in the boundary. The SQUID voltage showed well-behaved modulations in response to the external flux with maximum modulation depths of 80 µV at 15.2 K and 130 µV at 5.9 K.

Flux Pinning Mechanism in Single-Crystalline MgB2 Thin Films

We investigate the flux pinning mechanism for single-crystalline MgB2 thin films showing a peculiar flux pinning force behavior. The vortex–vortex distance is considered in this study through an analysis of the critical current density (\(J_{\text{c}}\)) and the flux pinning force density (\(F_{\text{p}}\)) over a wide range of temperatures from 5 to 35 K. Two unusual kinds of peaks, a near zero-field peak like a hump and another peak with a broad shape, are observed in the field dependences of the \(F_{\text{p}}\). The second peak with a broad shape begins to be depressed at temperatures above 25 K while the first sharp peak is only detected at temperatures above 30 K. In the single-crystalline MgB2 thin films weak collective pinning is dominant at all temperatures, and the pinning mechanism shows a crossover from δTc-pinning to δl-pinning with increasing magnetic field. We describe these unique features by considering the relation between the inter-vortex distances and the vortex–vortex interaction energy.

Flux pinning in columnar-structured and single crystalline MgB2 films

We have found that single-crystal films can be grown on (0001) Al2O3 substrates through the golden relation of a perfect lattice-matching ratio between the a-axis lattice constants of MgB2 and Al2O3. Selected area electron diffraction patterns evidently indicate hexagonal MgB2 film with a 30 degrees in-plane rotation with respect to the Al2O3 substrate. The films grown on Al2O3 show a zero-resistance transition temperature of 39.6 K with a transition width of 0.1 K. The critical current density at zero field is comparable to the depairing critical current density and rapidly decreases with increasing applied field due to the lack of pinning sites, as observed for high-quality MgB2 single crystals.

Influence of Mg deficiency on superconductivity in MgB2 thin films grown by HPCVD

The effects of Mg deficiency in MgB2 films grown by hybrid physical–chemical vapor deposition were investigated after vacuum annealing at various temperatures. High-quality MgB2 films grown on c-cut Al2O3 substrates with different superconducting transition temperatures (Tcs) of 40.2 and 41 K were used in this study. As the annealing temperature was increased from 200 to 800 °C, the Mg content in the MgB2 films systemically decreased, but the Tcs did not change, within ± 0.12 K, until the annealing temperature reached 700 °C. For MgB2 films annealed at 800 °C for 30 min, however, no superconductivity was observed, and the temperature dependence of the resistivity showed a semiconducting behavior. We also found that the residual resistivity ratio decreased with increasing annealing temperature.

Superconducting MgB2 flowers: growth mechanism and their superconducting properties

We report for the first time the growth and the systematic study of the growth mechanism for flower-like MgB2 structures fabricated on the substrates for solid-state electronics by the hybrid physical-chemical vapor deposition (HPCVD) technique. The MgB2 flower has a width of 30 μm and a height of 10 μm. The superconductivity of MgB2 flowers was confirmed by a magnetization measurement, and the transition temperature is 39 K, which is comparable with high-quality bulk samples. The excellent current-carrying capability was demonstrated by MgB2 flowers. To understand the nucleation and growth mechanism of MgB2 flowers a very systematic study was performed by a high-resolution transmission electron microscope (HRTEM) and atom probe (AP) microscopy. The HRTEM revealed that the seed grain of a MgB2 flower has a [100] direction, and the flower is composed of micro-columnar MgB2 grains having pyramidal tips and which are grown along the (0001) plane. A clear understanding of the growth mechanism for MgB2 flowers could lead to the growth of other low-dimensional MgB2 structures for superconducting electronic devices.

Hall conductivity and the vortex phase in MgB2 thin films

In a MgB2 thin film superconductor, we have found that Hall conductivity (?xy) is described by the sum of two terms, ?xy = C1/H+C3H, where C1 and C3 are independent of the magnetic fields and have positive values. C1 is observed to be proportional to (1?t)n with n = 4.2, where t is the reduced temperature (T/Tc), and C3 is weakly dependent on the temperature. These results are consistent with those of the overdoped La2?xSrxCuO4 superconductors. Based on Hall angle data, we obtained a vortex phase diagram with three regions, vortex-solid, crossover, and vortex-liquid?regions in the H?T plane.