Soon-Gil Jung

Potassium-doped BaFe2As2 superconducting thin films with a transition temperature of 40 K

We report the growth of potassium-doped BaFe2As2 thin films, where the major charge carriers are holes, on Al2O3 (0001) and LaAlO3 (001) substrates by using an ex situ pulsed laser deposition technique. The measured superconducting transition temperatures are 40 K and 39 K for the films grown on Al2O3 and LaAlO3, respectively, and diamagnetism indicates that the films have good bulk superconducting properties below 36 K and 30 K, respectively. The x-ray diffraction patterns for both films indicated a preferred c-axis orientation, regardless of the substrate structures of LaAlO3 and Al2O3. The upper critical field at zero temperature was estimated to be about 155 T.

In situ fabrication of cobalt-doped SrFe2As2 thin films by using pulsed laser deposition with excimer laser

The remarkably high superconducting transition temperature and upper critical field of iron(Fe)-based layered superconductors, despite ferromagnetic material base, open the prospect for superconducting electronics. However, success in superconducting electronics has been limited because of difficulties in fabricating high-quality thin films. We report the growth of high-quality c-axis-oriented cobalt(Co)-doped SrFe2As2 thin films with bulk superconductivity by using an in-situ pulsed laser deposition technique with a 248-nm-wavelength KrF excimer laser and an arsenic(As)-rich phase target. The temperature and field dependences of the magnetization showing strong diamagnetism and transport critical current density with superior Jc-H performance are reported. These results provide necessary information for practical applications of Fe-based superconductors.

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.

Enhanced critical current density in the pressure-induced magnetic state of the high-temperature superconductor FeSe

We investigate the relation of the critical current density (Jc) and the remarkably increased superconducting transition temperature (Tc) for the FeSe single crystals under pressures up to 2.43 GPa, where the Tc is increased by ~8 K/GPa. The critical current density corresponding to the free flux flow is monotonically enhanced by pressure which is due to the increase in Tc, whereas the depinning critical current density at which the vortex starts to move is more influenced by the pressure-induced magnetic state compared to the increase of Tc. Unlike other high-Tc superconductors, FeSe is not magnetic, but superconducting at ambient pressure. Above a critical pressure where magnetic state is induced and coexists with superconductivity, the depinning Jc abruptly increases even though the increase of the zero-resistivity Tc is negligible, directly indicating that the flux pinning property compared to the Tc enhancement is a more crucial factor for an achievement of a large Jc. In addition, the sharp increase in Jc in the coexisting superconducting phase of FeSe demonstrates that vortices can be effectively trapped by the competing antiferromagnetic order, even though its antagonistic nature against superconductivity is well documented. These results provide new guidance toward technological applications of high-temperature superconductors.

Superconductivity at 7.4 K in Few Layer Graphene by Li-intercalation

Superconductivity in graphene has been highly sought after for its promise in various device applications and for general scientific interest. Ironically, the simple electronic structure of graphene, which is responsible for novel quantum phenomena, hinders the emergence of superconductivity. Theory predicts that doping the surface of the graphene effectively alters the electronic structure, thus promoting propensity towards Cooper pair instability (Profeta et al (2012) Nat. Phys. 8 131-4; Nandkishore et al (2012) Nat. Phys. 8 158-63) [1, 2]. Here we report the emergence of superconductivity at 7.4 K in Li-intercalated few-layer-graphene (FLG). The absence of superconductivity in 3D Li-doped graphite underlines that superconductivity in Li-FLG arises from the novel electronic properties of the 2D graphene layer. These results are expected to guide future research on graphene-based superconductivity, both in theory and experiments. In addition, easy control of the Li-doping process holds promise for various device applications.

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.

Hard magnetic properties in nanoflake van der Waals Fe 3 GeTe 2

Two-dimensional van der Waals materials have demonstrated fascinating optical and electrical characteristics. However, reports on magnetic properties and spintronic applications of van der Waals materials are scarce by comparison. Here, we report anomalous Hall effect measurements on single crystalline metallic Fe3GeTe2 nanoflakes with different thicknesses. These nanoflakes exhibit a single hard magnetic phase with a near square-shaped magnetic loop, large coercivity (up to 550 mT at 2 K), a Curie temperature near 200 K and strong perpendicular magnetic anisotropy. Using criticality analysis, the coupling length between van der Waals atomic layers in Fe3GeTe2 is estimated to be ~5 van der Waals layers. Furthermore, the hard magnetic behaviour of Fe3GeTe2 can be well described by a proposed model. The magnetic properties of Fe3GeTe2 highlight its potential for integration into van der Waals magnetic heterostructures, paving the way for spintronic research and applications based on these devices.Exploring the magnetism in the van der Waals materials facilitates two dimensional spintronic devices. Here the authors demonstrate the evolution of magnetic behavior, strong perpendicular magnetic anisotropy and existence of magnetic coupling between atomic layers in Fe3GeTe2 nanoflakes by varying the layer thickness.

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.