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Register a new userEpitaxial growth of FeSe$_{0.5}$Te$_{0.5}$ thin films on CaF$_2$ substrates with high critical current density
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In-situ epitaxial growth of FeSe$_{0.5}$Te$_{0.5}$ thin films is demonstrated on a non-oxide substrate CaF$_2$. Structural analysis reveals that compressive stress is moderately added to 36-nm thick FeSe$_{0.5}$Te$_{0.5}$, which pushes up the critical temperature above 15 K, showing higher values than that of bulk crystals. Critical current density at $T$ = 4.5 K reaches 5.9 x 10$^4$ Acm$^{-2}$ at $mu_0H$ = 10 T, and 4.2 x 10$^4$ Acm$^{-2}$ at $mu_0H$ = 14 T. These results indicate that fluoride substrates have high potential for the growth of iron-based superconductors in comparison with popular oxide substrates.
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We have investigated the crystal structures and superconducting properties of thin films of FeSe$_{0.5}$Te$_{0.5}$ grown on eight different substrates. Superconductivity is not correlated with the lattice mismatch; rather it is correlated with the degree of in-plane orientation and with the lattice parameter ratio $c/a$. The best superconducting properties were observed in films on MgO and LaAlO$_3$ ($T_mathrm{c}^mathrm{zero}$ of 9.5 K). TEM observation showed that the presence or absence of an amorphous-like layer at the substrate surface plays a key role in determining the structural and superconducting properties of the grown films.
Using a field-effect transistor (FET) configuration with solid Li-ion conductor (SIC) as gate dielectric, we have successfully tuned carrier density in FeSe$_{0.5}$Te$_{0.5}$ thin flakes, and the electronic phase diagram has been mapped out. It is found that electron doping controlled by SIC-FET leads to a suppression of the superconducting phase, and eventually gives rise to an insulating state in FeSe$_{0.5}$Te$_{0.5}$. During the gating process, the (001) peak in XRD patterns stays at the same position and no new diffraction peak emerges, indicating no evident Li$^+$ ions intercalation into the FeSe$_{0.5}$Te$_{0.5}$. It indicates that a systematic change of electronic properties in FeSe$_{0.5}$Te$_{0.5}$ arises from the electrostatic doping induced by the accumulation of Li$^+$ ions at the interface between FeSe$_{0.5}$Te$_{0.5}$ and solid ion conductor in the devices. It is striking that these findings are drastically different from the observation in FeSe thin flakes using the same SIC-FET, in which $T_c$ is enhanced from 8 K to larger than 40 K, then the system goes into an insulating phase accompanied by structural transitions.
The high upper critical field characteristic of the recently discovered iron-based superconducting chalcogenides opens the possibility of developing a new type of non-oxide high-field superconducting wires. In this work, we utilize a buffered metal template on which we grow a textured FeSe$_{0.5}$Te$_{0.5}$ layer, an approach developed originally for high temperature superconducting coated conductors. These tapes carry high critical current densities (>1$times10^{4}$A/cm$^{2}$) at about 4.2K under magnetic field as high as 25 T, which are nearly isotropic to the field direction. This demonstrates a very promising future for iron chalcogenides for high field applications at liquid helium temperatures. Flux pinning force analysis indicates a point defect pinning mechanism, creating prospects for a straightforward approach to conductor optimization.
We present direct measurements of the superconducting order parameter in nearly optimal FeSe$_{0.5}$Te$_{0.5}$ single crystals with critical temperature $T_C approx 14$ K. Using intrinsic multiple Andreev reflection effect (IMARE) spectroscopy and measurements of lower critical field, we directly determined two superconducting gaps, $Delta_L approx 3.3 - 3.4$ meV and $Delta_S approx 1$ meV, and their temperature dependences. We show that a two-band model fits well the experimental data. The estimated electron-boson coupling constants indicate a strong intraband and a moderate interband interaction.
Polycrystalline samples of FeSe$_{0.5}$Te$_{0.5}$ were synthesized using a conventional solid-state reaction method. The onset of bulk superconductivity transition was confirmed by SQUID magnetometry at 12.5~K. $^{57}$Fe Mossbauer spectra in transmission geometry were recorded at temperatures between 6.0 and 320 K. Both the isomer shift and the total absorption started to drop about $T_c$, indicating a softening of the lattice. The drop is estimated to correspond to at least 60~K from the original Debye temperature $theta_{rm D}approx 460$~K. Seebeck measurements indicate that the samples are $n$-type conductors at low temperatures with a cross-over to $p$-type conductivity around 135 K. The zero Seebeck coefficient is seen below $10.6$~K.
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