Ta-Kun Chen

Superconductivity in the PbO-type structure α-FeSe

The recent discovery of superconductivity with relatively high transition temperature (Tc) in the layered iron-based quaternary oxypnictides La[O1−xFx] FeAs by Kamihara et al. [Kamihara Y, Watanabe T, Hirano M, Hosono H (2008) Iron-based layered superconductor La[O1-xFx] FeAs (x = 0.05–0.12) with Tc = 26 K. J Am Chem Soc 130:3296–3297.] was a real surprise and has generated tremendous interest. Although superconductivity exists in alloy that contains the element Fe, LaOMPn (with M = Fe, Ni; and Pn = P and As) is the first system where Fe plays the key role to the occurrence of superconductivity. LaOMPn has a layered crystal structure with an Fe-based plane. It is quite natural to search whether there exists other Fe based planar compounds that exhibit superconductivity. Here, we report the observation of superconductivity with zero-resistance transition temperature at 8 K in the PbO-type α-FeSe compound. A key observation is that the clean superconducting phase exists only in those samples prepared with intentional Se deficiency. FeSe, compared with LaOFeAs, is less toxic and much easier to handle. What is truly striking is that this compound has the same, perhaps simpler, planar crystal sublattice as the layered oxypnictides. Therefore, this result provides an opportunity to better understand the underlying mechanism of superconductivity in this class of unconventional superconductors.

Tellurium substitution effect on superconductivity of the α-phase iron selenide

We have carried out a systematic study of the PbO-type compound FeSe1−xTex (x=0–1), where the Te substitution effect on superconductivity is investigated. It is found that the superconducting transition temperature reaches a maximum of Tc=15.2 K at about 50% Te substitution. The pressure-enhanced Tc of FeSe0.5Te0.5 is more than 10 times larger than that of FeSe. Interestingly, FeTe is no longer superconducting. A low-temperature structural distortion changes FeTe from triclinic symmetry to orthorhombic symmetry. We believe that this structural change breaks the magnetic symmetry and suppresses superconductivity in FeTe.

Crystal orientation and thickness dependence of the superconducting transition temperature of tetragonal FeSe1-x thin films.

Superconductivity was recently found in the tetragonal phase FeSe. A structural transformation from tetragonal to orthorhombic (or monoclinic, depending on point of view) was observed at low temperature, but was not accompanied by a magnetic ordering as commonly occurs in the parent compounds of FeAs-based superconductors. Here, we report the correlation between structural distortion and superconductivity in FeSe(1-x) thin films with different preferred growth orientations. The films with preferred growth along the c axis show a strong thickness dependent suppression of superconductivity and low temperature structural distortion. In contrast, both properties are less affected in the films with (101) preferred orientation. These results suggest that the low temperature structural distortion is closely associated with the superconductivity of this material.

Heterojunction of Fe(Se1−xTex) superconductor on Nb-doped SrTiO3

We report the fabrication of heterojunctions formed by the FeSe0.5Te0.5 (FeSeTe) superconductor and Nb-doped SrTiO3 semiconducting substrate and their properties. At high temperature when FeSeTe is in its normal state, the forward bias I−V curves behave like a metal-semiconductor junction with a low Schottky barrier. Direct tunneling through the thin depletion layer of the junction dominates the reverse bias I−V curves. When FeSeTe film becomes superconducting at low temperature, we observed that the Schottky barrier height of the junction increased but was suppressed by an external magnetic field. This deviation provides an estimate of the superconducting energy gap of the FeSeTe film.

An overview of the Fe-chalcogenide superconductors

This review intends to summarize recent advancements in FeSe and related systems. The FeSe and related superconductors are currently receiving considerable attention for the high critical temperature (T C) observed and for many similar features to the high T C cuprate superconductors. These similarities suggest that understanding the FeSe-based compounds could potentially help our understanding of the cuprates. We begin the review by presenting common features observed in the FeSe- and FeAs-based systems. Then we discuss the importance of careful control of the material preparation allowing for a systematic structure characterization. With this control, numerous rich phases have been observed. Importantly, we suggest that the Fe-vacancy ordered phases found in the FeSe-based compounds, which are non-superconducting magnetic Mott insulators, are the parent compounds of the superconductors. Superconductivity can emerge from the parent phases by disordering the Fe vacancy order, often by a simple annealing treatment. Then we review physical properties of the Fe chalcogenides, specifically the optical properties and angle-resolved photoemission spectroscopy (ARPES) results. From the literature, strong evidence points to the existence of orbital modification accompanied by a gap-opening, prior to the structural phase transition, which is closely related to the occurrence of superconductivity. Furthermore, strong lattice to spin coupling are important for the occurrence of superconductivity in FeSe. Therefore, it is believed that the iron selenides and related compounds will provide essential information to understand the origin of superconductivity in the iron-based superconductors, and possibly the superconducting cuprates.

Disordered Fe vacancies and superconductivity in potassium-intercalated iron selenide (K2−xFe4+ySe5)

In the high-T c potassium-intercalated FeSe, there has been significant debate regarding what the exact parent compound is. Here we show that the Fe-vacancy ordered K2Fe4Se5 is the magnetic, Mott insulating parent compound of the superconducting state. Non-superconducting K2Fe4Se5 becomes a superconductor after high-temperature annealing, and the overall picture indicates that superconductivity in K2−x Fe4+y Se5 originates from the Fe-vacancy order-to-disorder transition. Thus, the long-pending question as to whether magnetic and superconducting state are competing or cooperating for cuprate superconductors may also apply to the Fe-chalcogenide superconductors. It is believed that the iron selenides and related compounds will provide essential information to understand the origin of superconductivity in the iron-based superconductors, and possibly to the superconducting cuprates.

Convective solution transport - An improved technique for the growth of big crystals of the superconducting α-FeSe using KCl as solvent

An improved technique of convective solution transport using KCl as solvent at 840–790 °C (where mass transport takes place across a vertical temperature gradient) is described for the growth of crystals of the recently discovered superconductor α-FeSex (x = 1−0.8). The crystals were annealed in situ at 400–350 °C for 20–30 h to improve the superconducting properties. Hexagonal plate like crystals measuring 5–6 mm across and 0.25–0.5 mm thick were obtained. High resolution transmission electron microscopy (HRTEM) measurements show good crystallinity and the energy dispersive x-ray analysis (EDX) gives a composition very close to the starting powders. The zero resistance temperature of the crystals was found to increase from 6.5 to 9 K as the composition is decreased from x = 0.95 to 0.9 and decrease thereafter. Similar behavior was also observed in the powder x-ray diffraction (XRD) patterns and Raman spectra with the main peak shifting to higher value until 0.9 and decrease thereafter. In addition the XR...

Structural characteristics and phase separation of superconducting Fe1+ySe1−xTex nanowires

Superconducting Fe1+y Se1?x Te x nanowires with considerable differences in the diameter and aspect ratio were synthesized via annealing thin films with particular composition prepared by pulsed laser deposition. Using transmission electron microscopy and related techniques, we found they were all crystallized in a PbO-type tetragonal structure with a growth direction of [010] (equivalent to [100]). All the nanowires were covered by a thin amorphous oxide layer (?5 nm) determined by electron energy loss spectroscopy. Chemical analysis and the calculated lattice constants reveal the fact that these nanowires are different from chemical compositions. High-resolution electron microscopy images show that the two thick nanowires (with diameters ?80 nm) are composed of multiple phases, while the thin nanowires (with diameters of 14 and 22 nm) are single phase. Digital moir? analysis reveals that the thin nanowires contain surface defects, as well as dislocations, inside the nanowires.

Publisher’s Note: “Convective solution transport—An improved technique for the growth of big crystals of the superconducting α-FeSe using KCl as solvent” [J. Appl. Phys. 110, 113919 (2011)]

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