Superconductivity in Iron Telluride Thin Films under Tensile Stress

By realizing in thin films a tensile stress state, superconductivity of 13 K was introduced into FeTe, a nonsuperconducting parent compound of the iron pnictides and chalcogenides, with a transition temperature higher than that of its superconducting isostructural counterpart FeSe. For these tensile stressed films, superconductivity is accompanied by a softening of the first-order magnetic and structural phase transition, and also, the in-plane extension and out-of-plane contraction are universal in all FeTe films independent of the sign of the lattice mismatch, either positive or negative. Moreover, the correlations were found to exist between the transition temperatures and the tetrahedra bond angles in these thin films.

Figure 1

X-ray diffraction spectra measured at room temperature and related analysis. (a)–(d) XRD spectra of films on MgO, ${\mathrm{SrTiO}}_3$, LSAT, and ${\mathrm{LaAlO}}_3$, respectively, with $F$ denoting film. (e) Summary of $c$-axes lattice constants for 32 FeTe films on 4 different substrates. Arrows indicate 4 samples the high-resolution XRD of which were performed to deduce the Fe-Te-Fe angles. (f) Fe-Te-Fe bond angle dependencies of zero resistance transition temperatures $T_{c,0}$ for 4 FeTe films indicated by arrows in (e). Inset shows schematically the definition of the angle 1 and angle 2, which are Fe4-Te-Fe1 and Fe4-Te-Fe2, respectively.

Figure 2

Scanning electron microscopy and energy dispersive analysis of x-ray. (a) SEM photograph of a FeTe film on ${\mathrm{SrTiO}}_3$. (b) EDAX of the surface area within the frame shown in (a), giving a $\mathrm{Fe}:\mathrm{Te}$ ratio of $1.08:1$.

Figure 3

Resistivity and magnetization versus temperature measurements for a FeTe film on MgO substrate measured from 300 K down to 2 K. (a) Four probe resistivity measurement result for a film and a FeTe bulk crystal given as a reference. Inset gives the enlargement of part of that of the film. The first-order phase transition at 70 K can be seen clearly in the FeTe bulk crystal. (b) Subtraction result of the dc magnetization of the FeTe film, with magnetic field perpendicular to the film surface plane. Inset shows the original dc magnetization measurement results for the FeTe film on MgO, and for the MgO substrate. Measurements were performed under 500 Oe and with the field perpendicular to the film surface. (c) Enlargement of part of the subtraction result in (b). (d) The ac susceptibility measurement result.

Figure 4

Transport properties of our FeTe thin films. (a) Temperature dependencies of the Hall coefficients of 90 nm thick FeTe films on MgO and on ${\mathrm{SrTiO}}_3$ measured at fixed magnetic field 5 T by scanning the temperature. Inset gives the Hall resistivity as a function of applied magnetic field measured at fixed temperatures for the film on MgO. Results from temperature scan and field scan match well to each other. A bar of $300\times 900\mu {\mathrm{m}}^2$ was etched. (b) Influence of magnetic field on superconductivity for a 90 nm FeTe film on ${\mathrm{SrTiO}}_3$. Inset shows upper critical fields deduced from the midpoint transition temperatures with $H_{c2}(0)$ extrapolated to be 123.0 T. A microbridge of $10\times 100\mu {\mathrm{m}}^2$ was etched. (c) Current versus voltage measurements for the microbridge given in (b) performed at 2 K under magnetic field up to 7 T. Critical current densities at 2 K under 0 and 7 T are $6.7\times {10}^4\mathrm{A}{\mathrm{cm}}^{-2}$ and $3.0\times {10}^4\mathrm{A}{\mathrm{cm}}^{-2}$, respectively. Critical currents were read at $1\times {10}^{-5}\mathrm{V}$ criteria.