Eun-Mi Choi

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.

Fabrication of FeSe1−x superconducting films with bulk properties

Abstract We have fabricated high-quality FeSe 1− x superconducting films with a bulk T c of 11–12xa0K on different substrates, Al 2 O 3 (0xa00xa00xa01), SrTiO 3 (1xa00xa00), MgO(1xa00xa00), and LaAlO 3 (1xa00xa00), by using a pulsed laser deposition technique. All the films were grown at a high substrate temperature of 610xa0°C, and were preferentially oriented along the (1xa00xa01) direction, the latter being to be a key to fabricating of FeSe 1− x superconducting thin films with high T c . According to the energy dispersive spectroscopy data, the Fe:Se composition ratio was 1:0.90xa0±xa00.02. The FeSe 1− x film grown on a SrTiO 3 substrate showed the best quality with a high upper critical magnetic field [ H c 2 (0)] of 56xa0T.

Observation of trapped-modes excited in double-layered symmetric electric ring resonators

We report on experimental observations of trapped-mode resonances in double-layered symmetric electric ring resonators separated by dielectric inserts. The resulting metamaterial introduces trapped-mode resonances that were thought to be produced only by asymmetric metamaterial unit cells. Experimental verification of the newly observed trapped modes, along with the analysis of the stacked metamaterial geometry reported in this paper, opens an alternative way of forming sharp resonances in a symmetric metamaterial structure extended in all three dimensions.

Suppression of dendritic flux jumps in MgB2 films coated with a gold rim

An effective method for suppressing dendritic flux avalanches in MgB2 films is demonstrated by coating the films with a metal rim along the edges, which is where the avalanches nucleate. The effect of the partial coating has been investigated by means of magneto-optical imaging on a series of samples with golden rims of different width. Measurements of the onset field of instability reveal that such rims can substantially improve the thermo-magnetic stability of the superconducting films.

Growth of MgB2 thin films by using a novel laser-assisted chemical vapor deposition technique

We have successfully fabricated c-axis-oriented MgB2 thin films by using a laser-assisted chemical vapor deposition (LACVD) technique. Mg and B sources were supplied by pulsed laser ablation and a mixture of B2H6 gas in H2, respectively. The MgB2 thin films grown on Al2O3(0001) substrates at 400u2009°C showed a superconducting transition temperature of 40.2xa0K with a sharp transition width of 0.2xa0K and a high critical current density of about 107xa0Axa0cm−2 at 20xa0K and zero field, suggesting that this low-temperature process would be a promising technique for electronic devices operating at high temperatures above 20xa0K.

Magnetic instability of MgB2 thin film triggered by the various sweeping rates of an applied magnetic field

Up to now, the vortex avalanche is known to depend critically on the changing of some external parameters, such as the applied magnetic field, the temperature, and the demagnetization factor. In this study, we found the sweeping rate (SR) of the applied magnetic field to be another parameter that contributed to the appearance of the vortex avalanche. For example, a fast SR 500 Oe/s enhanced the vortex avalanche quietly compared to the case of a slow SR of 3 Oe/s. The dependence of the appearance of the vortex avalanche on the SR was quite strong, especially at low temperatures. A H-T phase diagram that distinguishes the regions of magnetic stability and instability is drawn for each SR.

Saw-tooth pattern from flux jumps observed by high resolution M-H curves in MgB2 thin films

While flux jumps have been observed in the magnetic hysteresis loops of superconductors, a saw-tooth pattern of the flux jump is known to appear only in a bulk superconductor. But in this study, we were able to observe the saw-tooth pattern in MgB2 thin film with the careful data acquisition method enhancing the data taking capability and report the details of the distribution of the field interval between jumps Bfj, and the size of the flux jump, Mfj. The theory based on Bean’s model in the adiabatic approach was adapted and it was compared with experimental results. In addition, we observe the cross-over between the saw-tooth pattern and a rounded saw-tooth pattern, as a byproduct. A patterns diagram of the vortex jump was drawn on the H-T plane.

A comparative study of the dendritic avalanche in MgB2 thin films synthesized by pulsed laser deposition and hybrid physical chemical vapor deposition methods

It is known that MgB2 thin films synthesized by using hybrid physical chemical vapor deposition (HPCVD) do not show dendritic avalanche, which is in contrast to those prepared by using pulsed laser deposition (PLD). To find the cause that makes the difference between the two cases, we studied the microscopic film structure by the scanning electron microscopy and the magnetic hysteresis by using the superconducting quantum interference device magnetometry. The critical current density (Jc), estimated from the magnetic hysteresis based on the Bean’s critical-state model, shows a much higher Jc in the PLD film than in a HPCVD film. This indicates higher vortex pinning in the PLD film. We surmise that high local joule heating beyond the high Jc in the PLD film, as a vortex penetrates into the superconducting thin film, gives a path for the next vortex and induces a positive feedback effect that is absent in the HPCVD film.

Geometric tuning of charge and spin correlations in manganite superlattices

We report a modulation of the in-plane magnetotransport in artificial manganite superlattice [(NdMnO3)n/(SrMnO3)n/(LaMnO3)n]m by varying the layer thickness n while keeping the total thickness of the structure constant. Charge transport in these heterostructures is confined to the interfaces and occurs via variable range hopping. Upon increasing n, the interfacial separation rises, leading to a suppression of the electrostatic screening between carriers of neighboring interfaces and the opening of a Coulomb gap at the Fermi level (EF). The high-field magnetoresistance (MR) is universally negative due to progressive spin alignment. However, at a critical thickness of nu2009=u20095 unit cells (u.c.), an exchange field coupling between ferromagnetically ordered interfaces results in positive MR at low magnetic field (H). Our results demonstrate the ability to geometrically tune the electrical transport between regimes dominated by either charge or spin correlations.