Particulate Generation on Surface of Iron Selenide Films by Air Exposure

Nanometer-sized particular structures are generated on the surfaces of FeSe epitaxial films directly after exposure to air; this phenomenon was studied in the current work because these structures are an obstacle to field-induced superconductivity in electric double-layer transistors using FeSe channel layers. Chemical analyses using field-effect scanning Auger electron spectroscopy revealed no clear difference in the chemical composition between the particular structures and the other flat surface region. This observation limits the possible origins of the particulate formation to light elements in air such as O, C, H, and N.Comment: Accepted for publication in J. Supercond. Nov. Mag

Highly hydrogen-sensitive thermal desorption spectroscopy system for quantitative analysis of low hydrogen concentration (~1 x 10^16 atoms/cm3) in thin-film samples

We developed a highly hydrogen-sensitive thermal desorption spectroscopy (HHS-TDS) system to detect and quantitatively analyze low hydrogen concentrations in thin films. The system was connected to an in situ sample-transfer chamber system, manipulators, and an rf magnetron sputtering thin-film deposition chamber under an ultra-high-vacuum (UHV) atmosphere of ~10^-8 Pa. The following key requirements were proposed in developing the HHS-TDS: (i) a low hydrogen residual partial pressure, (ii) a low hydrogen exhaust velocity, and (iii) minimization of hydrogen thermal desorption except from the bulk region of the thin films. To satisfy these requirements, appropriate materials and components were selected, and the system was constructed to extract the maximum performance from each component. Consequently, ~2000 times higher sensitivity to hydrogen than that of a commercially available UHV-TDS system was achieved using H+-implanted Si samples. Quantitative analysis of an amorphous oxide semiconductor InGaZnO4 thin film (1 cm x 1 cm x 1um thickness, hydrogen concentration of 4.5 x 10^17 atoms/cm3) was demonstrated using the HHS-TDS system. This concentration level cannot be detected using UHV-TDS or secondary ion mass spectroscopy (SIMS) systems. The hydrogen detection limit of the HHS-TDS system was estimated to be ~1 x 10^16 atoms/cm3, which implies ~2 orders of magnitude higher sensitivity than that of SIMS and resonance nuclear reaction systems (~10^18 atoms/cm3)

Superconducting Properties and Phase Diagram of Indirectly Electron-Doped (Sr1-xLax)Fe2As2 Epitaxial Films Grown by Pulsed Laser Deposition

A non-equilibrium phase (Sr1-xLax)Fe2As2 was formed by epitaxial film-growth. The resulting films emerged superconductivity along with suppression of the resistivity anomaly that is associated with magnetic and structural phase transitions. The maximum critical temperature was 20.8 K, which is almost the same as that of directly electron-doped Sr(Fe1-xCox)2As2. Its electronic phase diagram is much similar to that of Sr(Fe1-xCox)2As2, indicating that the difference in the electron doping sites does not influence the superconducting properties of 122-type SrFe2As2

Recent advances in iron-based superconductors toward applications

Iron with a large magnetic moment was widely believed to be harmful to the emergence of superconductivity because of the competition between the static ordering of electron spins and the dynamic formation of electron pairs (Cooper pairs). Thus, the discovery of a high critical temperature (Tc) iron-based superconductor (IBSC) in 2008 was accepted with surprise in the condensed matter community and rekindled extensive study globally. IBSCs have since grown to become a new class of high-Tc superconductors next to the high-Tc cuprates discovered in 1986. The rapid research progress in the science and technology of IBSCs over the past decade has resulted in the accumulation of a vast amount of knowledge on IBSC materials, mechanisms, properties, and applications with the publication of more than several tens of thousands of papers. This article reviews recent progress in the technical applications (bulk magnets, thin films, and wires) of IBSCs in addition to their fundamental material characteristics. Highlights of their applications include high-field bulk magnets workable at 15-25 K, thin films with high critical current density (Jc) > 1 MA/cm2 at ~10 T and 4 K, and an average Jc of 1.3*104 A/cm2 at 10 T and 4 K achieved for a 100-m-class-length wire. These achievements are based on the intrinsically advantageous properties of IBSCs such as the higher crystallographic symmetry of the superconducting phase, higher critical magnetic field, and larger critical grain boundary angle to maintain high Jc. These properties also make IBSCs promising for applications using high magnetic fields.Comment: Published online in Materials Today. Open Acces

High critical-current density with less anisotropy in BaFe2(As,P)2 epitaxial thin films: Effect of intentionally grown c-axis vortex-pinning centers

We report herein a high and isotropic critical-current density Jc for BaFe2(As,P)2 epitaxial films. The isotropy of Jc with respect to the magnetic-field direction was improved significantly by decreasing the film growth rate to 2.2 {\AA}/s. The low growth rate served to preferentially align dislocations along the c-axis, which work well as c-axis vortex-pinning centers. Because of the intentional introduction of effective pinning, the absolute Jc at 9 T was larger than that obtained for other iron-based superconductors and conventional alloy superconducting wires

BaFe2(As1-xPx)2 (x = 0.22-0.42) thin films grown on practical metal-tape substrates and their critical current densities

We optimized the substrate temperature (Ts) and phosphorus concentration (x) of BaFe2(As1-xPx)2 films on practical metal-tape substrates for pulsed laser deposition from the viewpoints of crystallinity, superconductor critical temperature (Tc), and critical current density (Jc). It was found that the optimum Ts and x values are 1050 degree C and x = 0.28, respectively. The optimized film exhibits Tc_onset = 26.6 and Tc_zero = 22.4 K along with a high self-field Jc at 4 K (~1 MA/cm2) and relatively isotropic Jc under magnetic fields up to 9 T. Unexpectedly, we found that lower crystallinity samples, which were grown at a higher Ts of 1250 degree C than the optimized Ts = 1050 degree C, exhibit higher Jc along the ab plane under high magnetic fields than the optimized samples. The presence of horizontal defects that act as strong vortex pinning centers, such as stacking faults, are a possible origin of the high Jc values in the poor crystallinity samples

Stabilization and heteroepitaxial growth of metastable tetragonal FeS thin films by pulsed laser deposition

Pulsed laser deposition, a non-equilibrium thin-film growth technique, was used to stabilize metastable tetragonal iron sulfide (FeS), the bulk state of which is known as a superconductor with a critical temperature of 4 K. Comprehensive experiments revealed four important factors to stabilize tetragonal FeS epitaxial thin films: (i) an optimum growth temperature of 300 {\deg}C followed by thermal quenching, (ii) an optimum growth rate of ~7 nm/min, (iii) use of a high-purity bulk target, and (iv) use of a single-crystal substrate with small in-plane lattice mismatch (CaF2). Electrical resistivity measurements indicated that none of all the films exhibited superconductivity. Although an electric double-layer transistor structure was fabricated using the tetragonal FeS epitaxial film as a channel layer to achieve high-density carrier doping, no phase transition was observed. Possible reasons for the lack of superconductivity include lattice strain, off-stoichiometry of the film, electrochemical etching by the ionic liquid under gate bias, and surface degradation during device fabrication

Water-induced superconductivity in SrFe2As2

It has been considered that FeAs-based high transition temperature (high-Tc) superconductors need electron or hole doping by aliovalent ion substitution or large off-stoichiometry in order to induce superconductivity. We report that exposure of undoped SrFe2As2 epitaxial thin films to water vapor induces a superconducting transition. These films exhibit a higher onset-Tc (25 K) and larger magnetic field anisotropy than those of cobalt-doped SrFe2As2 epitaxial films, suggesting that the mechanism for the observed superconducting transition differs from that of the aliovalent-ion doped SrFe2As2. The present finding provides a new approach to induce superconductivity with a higher Tc in FeAs-based superconductors

Electric field-induced superconducting transition of insulating FeSe thin film at 35 K

It is thought that strong electron correlation in an insulating parent phase would enhance a critical temperature (Tc) of superconductivity in a doped phase via enhancement of the binding energy of a Cooper pair as known in high-Tc cuprates. To induce a superconductor transition in an insulating phase, injection of a high density of carriers is needed (e.g., by impurity doping). An electric double-layer transistor (EDLT) with an ionic liquid gate insulator enables such a field-induced transition to be investigated and is expected to result in a high Tc because it is free from deterioration in structure and carrier transport that are in general caused by conventional carrier doping (e.g., chemical substitution). Here, for insulating epitaxial thin films (~10 nm thick) of FeSe, we report a high Tc of 35 K, which is four times higher than that of bulk FeSe, using an EDLT under application of a gate bias of +5.5 V. Hall effect measurements under the gate bias suggest that highly accumulated electron carrier in the channel, whose area density is estimated to be 1.4x10^15 cm-2 (the average volume density of 1.7x10^21 cm-3), is the origin of the high-Tc superconductivity. This result demonstrates that EDLTs are useful tools to explore the ultimate Tc for insulating parent materials.Comment: PNAS Early Editio

Superconductivity in Epitaxial Thin Films of Co-Doped SrFe2As2 with Bilayered FeAs Structures and their Magnetic Anisotropy

Superconducting epitaxial films of Fe-based layered arsenide, Co-doped SrFe2As2, were grown at 700oC on mixed perovskite (La, Sr)(Al, Ta)O3 (001) single-crystal substrates by pulsed-laser deposition. Both the epitaxial film and an (001)-oriented film grown at 600oC exhibited superconducting transitions at ~ 20 K. The zero-resistance states of the epitaxial film were sustained under a magnetic field (H) of 9 T at 9 K when H was parallel to the c-axis, while they were sustained at higher temperatures up to 10 K for H parallel to the a-axis. This is the first demonstration of superconducting thin films of FeAs-based new superconductors.Comment: Revised manuscript: Accepted for publication in Appl. Phys. Expres