Shyam Mohan

Effects of particle irradiations on vortex states in iron-based superconductors

Various kinds of energetic particles are irradiated into iron-based superconductors, and their effects on the critical current density (Jc) and vortex dynamics have been systematically studied. It is found that Jc is enhanced and vortex dynamics is strongly suppressed by energetic particles having a sufficient energy deposition rate, similar to the case of high temperature cuprate superconductors. The enhancement of Jc, in general, persists up to much higher irradiation doses than in cuprates. However, details of the effect of irradiation depend on the kind of ion species and their energies. Even with the same ions and energies, the effect is not universal for different kinds of iron-based superconductors. The correlated nature of defects created by heavy-ion irradiation is confirmed by the angular dependence of irreversible magnetization.

Low-temperature synthesis of FeTe0.5Se0.5 polycrystals with a high transport critical current density

We have prepared high-quality polycrystalline FeTe0.5Se0.5 at temperature as low as 550 °C. The transport critical current density evaluated from the current–voltage characteristics is over 700 A cm − 2 at 4.2 K under zero field, which is several times larger than that for FeTe0.5Se0.5 superconducting wires. The critical current density estimated from magneto-optical images of flux penetration is also similar to this value. The upper critical field of the polycrystalline FeTe0.5Se0.5 at T = 0 K estimated using Werthamer–Helfand–Hohenberg theory is 585 kOe, which is comparable to that of single crystals. This study gives some insight into how to improve the performance of FeTe0.5Se0.5 superconducting wires.

Magneto-optical imaging and transport properties of FeSe superconducting tapes prepared by the diffusion method

FeSe tapes have been prepared by using a diffusion method. By adjusting the reaction temperature and the thickness systematically, a transport critical current density as high as 600?A?cm?2 at 4.2?K under self-field is achieved. Magneto-optical images indicate considerable distribution of Tc and weak-link features in the FeSe tape. This study gives some insight into how to further improve the quality and performance of FeSe tapes and other superconducting tapes prepared through the diffusion method.

Growth and characterization of n-type electron-induced ferromagnetic semiconductor (In,Fe)As

We show that by introducing isoelectronic iron (Fe) magnetic impurities and Beryllium (Be) double-donor atoms into InAs, it is possible to grow a n-type ferromagnetic semiconductor (FMS) with the ability to control ferromagnetism by both Fe and independent carrier doping by low-temperature molecular-beam epitaxy. We demonstrate that (In,Fe)As doped with electrons behaves as an n-type electron-induced FMS. This achievement opens the way to realize novel spin-devices such as spin light-emitting diodes or spin field-effect transistors, as well as helps understand the mechanism of carrier-mediated ferromagnetism in FMSs.

Transport and magnetic properties of Co-doped BaFe2As2 epitaxial thin films grown on MgO substrate

We report resistivity, Hall coefficient, current-voltage characteristics, and magneto-optical imaging measurements of epitaxial Co-doped BaFe_{2}As_{2} thin films deposited on MgO(001) substrate. The Hall resistivity of the films has a substantial contribution arising from anomalous Hall effect of ferromagnetic components. The critical current density (J_{c}) of the films is ~2 MA/cm^{2} at low temperatures. Differential magneto-optical images of the remanent state give similar J_{c} values and also exhibit presence of extended defects in the film.

Magneto-optical imaging of polycrystalline FeTe1−xSex prepared at various conditions

FeTe0.5Se0.5 superconducting wires have been fabricated using ex-situ PIT method with an Fe sheath. Among the other FeAs-based superconductor, FeTe0.5Se0.5 has great advantage for application due to binary composition and less toxic. Surprisingly, superconducting current was observed in the as-fabricated wire without any heat treatments. Zero resistivity and transport critical current density (Jc) were 3.2 K and 2.8 A/cm2, respectively. In addition, by annealing at 200 degC for 2 h, critical temperature Tczero and Jc were enhanced up to 9.1 K, and 64.1 A/cm2, respectively. This suggests that the grain connectivity was improved by heat treatment, and superconducting property of FeTe0.5Se0.5 wire was enhanced.

Experimental demonstration of shrinkage of magnetic domains in a superconductor/ferromagnet bilayer

The shrinkage of magnetic domains in a superconductor/ferromagnet bilayer is studied experimentally using Pb as a superconductor and a garnet film with maze domains as a ferromagnet. The effects of trapped flux in a superconductor and the pinning of magnetic domain boundaries are carefully minimized by using a high-quality Pb single crystal and an AC demagnetization technique. A temperature independent shrinkage of magnetic domains by a factor ~ 0.89 is obtained below the superconducting transition of Pb. The slightly suppressed shrinkage ratio as compared with the theoretical prediction can be explained by a finite gap between the Pb crystal and the garnet film.

Magneto-Optical Characterization of Iron-Based Superconducting Wires and Tapes

The critical current density, <i>J</i>_c, in high-quality single crystals of iron-based superconductors (IBS) has reached larger than 10⁵ A/cm² at 4.2 K. This large value of <i>J</i>_c urged the development of potential superconducting wires or tapes based on IBS. We have fabricated superconducting wires and tapes of IBS using several techniques and made an extensive characterization by structural, transport, and magnetic measurements. The powder-in-tube method is used for (Ba, K)Fe₂As₂ and Ba(Fe, Co)₂As₂ wires and the diffusion method is used for FeSe tape. Intra- and inter-granular <i>J</i>_cs have been evaluated by direct transport and indirect magnetization measurements as well as using magneto-optical (MO) imaging. The intergranular <i>J</i>_cs at 4.2 K under self-field for (Ba, K)Fe₂As₂ and Ba(Fe, Co)₂As₂ wires and FeSe tapes are 1.3 × 10⁴ A/cm², 3.8 × 10³ A/cm², and 600 A/cm², respectively. On the other hand, MO images have demonstrated that the intragranular <i>J</i>_c is at least one order of magnitude larger than the intergranular <i>J</i>_c, which is consistent with the magnetic evaluation.

FeSe superconducting tapes with a high critical current density fabricated by diffusion method

We have prepared high-quality FeSe tape through diffusion method. The transport critical current density evaluated by the current-voltage characteristics is 600 A/cm2 at 4.2 K under zero field. The upper critical field of the FeSe tape at T = 0 K estimated by Werthamer-Helfand-Hohenberg theory is 243 kOe, which is similar to that of single crystals.

Instability of vortex-antivortex interface in optimally doped Ba(Fe1−xCox)2As2

We explore the flux front patterns during virgin penetration and after remagnetization in the iron-based superconductor Ba(Fe_{1-x}Co_{x})_{2}As_{2}. After remagnetization we observe an instability characterized by turbulent dynamics at the vortex-antivortex boundary. Associated with the turbulent flux boundary a band of excess current density is observed. The turbulence is contrasted with the instability observed in the cuprate superconductors. Our results suggest turbulent instability at the vortex-antivortex interface may be a common feature in all remagnetized type-II superconductors.