Enhancement in High-Field J c Properties and the Flux Pinning Mechanism of ZnO-Buffered MgB2 Films.

We investigated the flux pinning properties in terms of the critical current density (J c) and pinning force density (F p) of MgB2 films with ZnO buffer layers of various thicknesses. At higher thicknesses of the buffer layer, significantly larger J c values are observed in the high-field region, whereas J c values in the low- and intermediate-field regions remain largely unaffected. A secondary point-pinning mechanism other than primary grain boundary pinning is observed in the F p analysis, which depends on the thickness of the ZnO buffer layer. Moreover, a close relationship between the Mg and B bond ordering and the fitting parameter of secondary pinning is obtained, indicating that the local structural distortion of MgB2 induced by ZnO buffer layers with different thicknesses may contribute to flux-pinning enhancement in the high-field region. Discovering further advantages of ZnO as a buffer layer other than the delamination resistance it provides will help to develop a MgB2 superconducting cable with a high J c for power applications.

High critical current density and high-tolerance superconductivity in high-entropy alloy thin films.

High-entropy alloy (HEA) superconductors-a new class of functional materials-can be utilized stably under extreme conditions, such as in space environments, owing to their high mechanical hardness and excellent irradiation tolerance. However, the feasibility of practical applications of HEA superconductors has not yet been demonstrated because the critical current density (Jc) for HEA superconductors has not yet been adequately characterized. Here, we report the fabrication of high-quality superconducting (SC) thin films of Ta-Nb-Hf-Zr-Ti HEAs via a pulsed laser deposition. The thin films exhibit a large Jc of >1 MA cm-2 at 4.2 K and are therefore favorable for SC devices as well as large-scale applications. In addition, they show extremely robust superconductivity to irradiation-induced disorder controlled by the dose of Kr-ion irradiation. The superconductivity of the HEA films is more than 1000 times more resistant to displacement damage than that of other promising superconductors with technological applications, such as MgB2, Nb3Sn, Fe-based superconductors, and high-Tc cuprate superconductors. These results demonstrate that HEA superconductors have considerable potential for use under extreme conditions, such as in aerospace applications, nuclear fusion reactors, and high-field SC magnets.

Giant proximity effect in single-crystalline MgB2 bilayers.

Although giant proximity effect (GPE) can shed important information on understanding superconducting pairing mechanisms and superconducting electronics, reports on the GPE are few because the fabrication of the junctions with GPE is technologically difficult. Here, we report a GPE in the single-crystalline MgB2 bilayers (S'/S), where the S' is the damaged MgB2 layer by cobalt (Co)-ion irradiation and the S is the undamaged MgB2 layer. Superconducting properties of the S' is remarkably degraded by the irradiation, whereas those of the S is uninfluenced by the irradiation. The degraded superconductivity in the S' is fully recovered by increasing the thickness of undamaged MgB2 layer S despite almost ten times larger thickness ~ 95 nm of S' than the superconducting coherence length ξab(0) ~ 8.5 nm of the S, indicating a presence of GPE in the S'/S MgB2 bilayers. A diffusion of electrons in the S' into the S can reduce a pair breaking scattering in the S', and the similar electronic structures of S' and S layers and a finite attractive electron-electron interaction in the S' are thought to be origins of unpredicted GPE between the same superconducting materials. Both upper critical field (µ0Hc2) and in-field critical current density (Jc) of S'/S bilayers show a significant enhancement, representing a strong correlation between S' and S. These discoveries provide the blue print to the design of the superconducting multilayers for fundamental researches on the mechanism of the GPE as well as their technological applications.

Enhanced magnetic and thermoelectric properties in epitaxial polycrystalline SrRuO3 thin films.

Transition metal oxide thin films show versatile electric, magnetic, and thermal properties which can be tailored by deliberately introducing macroscopic grain boundaries via polycrystalline solids. In this study, we focus on the modification of magnetic and thermal transport properties by fabricating single- and polycrystalline epitaxial SrRuO3 thin films using pulsed laser epitaxy. Using the epitaxial stabilization technique with an atomically flat polycrystalline SrTiO3 substrate, an epitaxial polycrystalline SrRuO3 thin film with the crystalline quality of each grain comparable to that of its single-crystalline counterpart is realized. In particular, alleviated compressive strain near the grain boundaries due to coalescence is evidenced structurally, which induced the enhancement of ferromagnetic ordering of the polycrystalline epitaxial thin film. The structural variations associated with the grain boundaries further reduce the thermal conductivity without deteriorating the electronic transport, and lead to an enhanced thermoelectric efficiency in the epitaxial polycrystalline thin films, compared with their single-crystalline counterpart.

Tuning electromagnetic properties of SrRuO3 epitaxial thin films via atomic control of cation vacancies.

Elemental defect in transition metal oxides is an important and intriguing subject that result in modifications in variety of physical properties including atomic and electronic structure, optical and magnetic properties. Understanding the formation of elemental vacancies and their influence on different physical properties is essential in studying the complex oxide thin films. In this study, we investigated the physical properties of epitaxial SrRuO3 thin films by systematically manipulating cation and/or oxygen vacancies, via changing the oxygen partial pressure (P(O2)) during the pulsed laser epitaxy (PLE) growth. Ru vacancies in the low-P(O2)-grown SrRuO3 thin films induce lattice expansion with the suppression of the ferromagnetic T C down to ~120 K. Sr vacancies also disturb the ferromagnetic ordering, even though Sr is not a magnetic element. Our results indicate that both A and B cation vacancies in an ABO3 perovskite can be systematically engineered via PLE, and the structural, electrical, and magnetic properties can be tailored accordingly.

Revisiting optical properties of MgB2 with a high-quality sample prepared by a HPCVD method.

We investigated a high-quality MgB2 thin film with a thickness of ~1000 nm on an Al2O3 substrate using optical spectroscopy. We measured the reflectance spectra of the film at various temperatures both below, and above, the superconducting transition temperature, T c [Formula: see text] 40 K. An earlier study showed that when the sample surface is exposed to air the optical properties of the surface change immediately, however, the saturated change is negligibly small in the far-infrared region. The optical conductivity spectrum in the normal state shows two (narrow and broad) Drude modes, with the narrow Drude mode being dominant in the low frequency region below 1000 cm-1. Our study, which uses a good-quality sample, provides more reliable data on the optical properties of MgB2, in a similar spectral range. The optical data is analyzed further using an extended Drude model, and the electron-phonon spectral density function, α 2 F(ω), is extracted. The spectral density function α 2 F(ω) features two peaks: a small one near 114 cm-1, and a strong peak around the 550 cm-1 where the B-B bond stretching phonon exists. Our data in the superconducting state does not show the expected energy shift of the onset of scattering associated with the α 2 F(ω) peaks.

Magnetic characteristics of copper ion-modified DNA thin films.

We developed a new method of fabricating a divalent copper ion (Cu(2+)) modified DNA thin film on a glass substrate and studied its magnetic properties. We evaluated the coercive field (Hc), remanent magnetization (Mr), susceptibility (χ), and thermal variation of magnetization with varying Cu(2+) concentrations [Cu(2+)] resulting in DNA thin films. Although thickness of the two dimensional DNA thin film with Cu(2+) in dry state was extremely thin (0.6 nm), significant ferromagnetic signals were observed at room temperature. The DNA thin films with a [Cu(2+)] near 5 mM showed the distinct S-shape hysteresis with appreciable high Hc, Mr and χ at low field (≤600 Oe). These were primarily caused by the presence of small magnetic dipoles of Cu(2+) coordination on the DNA molecule, through unpaired d electrons interacting with their nearest neighbors and the inter-exchange energy in the magnetic dipoles making other neighboring dipoles oriented in the same direction.

Doping effect of CNT and nano-carbon in magnesium diboride bulk.

We fabricated carbon nanotube (CNT)- and nano-carbon (NC)-doped MgB2 using an in-situ process in order to improve the critical current density (J(c)) at high magnetic field. We then evaluated the effects of the doped carbon content on phase formation, microstructure, and critical properties. CNT had a diameter and length of 5-10 nm and 0.5-1 microm, respectively, and NC was a sphere with a diameter of 5-30 nm. The bulk MgB(2-x)C(x) samples with x = 0, 0.05, and 0.1 for NC and CNT were fabricated by pressing into pellets and then sintered at 900 degrees C for 30 min. NC was more effective than CNT for carbon doping at the B site in MgB2 and, therefore, the NC-doped MgB2 samples had a lower critical temperature (T(c)) of 35.0-34.7 K than that of the CNT-doped samples (36.4-36.1 K). In addition, the J(c)(B) behavior was improved when NC and CNT were doped due to doping effect. Microstructural observation suggested that the nano-sized and unreacted NC particles and the nanodomain MgB2 acted as effective flux pinning centers for the NC- and CNT-doped MgB2, respectively.