The discovery of mono-ion intercalated FeSe superconductors [1,2], namely, AxFe2-ySe2 (A = K, Rb, Cs, Tl) with Tc ≈ 30 K, has stimulated studies of unconventional superconductivity in iron-based superconductors. Many studies have shown that the phase separation is inevitably in bulk AxFe2-ySe2 single crystals , which several phases with different stoichiometry and physical properties separate spatially, e.g., an insulating antiferromagnetic phase K2Fe4Se5 with Fe vacancies, √5×√5 ordering (major phase), and a metallic superconducting phase KxFe2Se2 with complete FeSe layers (minor phase).
Chemical doping has been proven to be an effective way to study the interplay of structural parameters and superconductivity in the parent compounds. In particular, isovalent substitution can induce chemical pressure without additional charge carriers into a target system. The substitution of Te for Se in AxFe2-ySe2 induces negative chemical pressure due to larger ionic radius of Te2– dopant, while a positive chemical pressure can be induced by doping S2– with smaller ionic radius. Previous results have shown that both of chemical pressures can result in the suppression of Tc in AxFe2-ySe2, differing from the suppression of Tc by high hydrostatic-pressures in the optimal doped AxFe2-ySe2 (Tc ≈ 32.7 K) crystals. Yet, there is no detailed study on the Te-doped superconducting KxFe2-ySe2 single crystals.
Here we present the results of the Te doping and its influences on the structure and superconductivity in K0.8Fe2Se2-zTez (z = 0–0.6) single crystals grown by optical floating-zone technique , and also show the results of critical current density for the single crystals with z = 0, 0.09 and 0.16. As-grown single crystals were characterized by x-ray diffraction (XRD) with Cu Kα radiation at room temperature. Magnetic susceptibility measurements were performed using a superconducting quantum interference device (SQUID) magnetometer. In-plane resistivity measurements by four probes were carried out on a Physical Property Measurement System (PPMS, Quantum Design).
Figure 1(a) illustrates the schematic crystal graph of K0.8Fe2Se2-zTez with type ThCr2Si2-like structure, containing ordered Fe vacancies (phase of K0.8Fe1.6Se2) with space group I4/m or ordered Fe atoms (phase of KxFe2Se2, x = 0.38–0.58) with I4/mmm. The typical as-grown single crystal ingot and as-cleaved single crystal with z = 0.09 are shown in Fig. 1(b). The end part of the ingot exhibits a shiny surface and the other part is decomposed due to large thermal strains during floating zone growth procedure . The (00ℓ) XRD patterns of K0.8Fe2Se2-zTez single crystal with different Te doping levels are shown in Fig. 1(c). The strong and weak diffraction peaks correspond to the insulating/semiconducting and metallic superconducting phases, respectively. With increasing Te doping content, the (00ℓ) peaks gradually shift to the lower 2θ angle region. It is indicated that the c-axis lattice parameter increases with increasing Te doping in both insulating/semiconducting and metallic superconducting phases, as shown in Fig. 1(d). This can be attributed to the larger ionic Te2– (2.21 Å) substituted for Se2– (1.98 Å).
For K0.8Fe2Se2-zTez single crystals, the temperature dependence of magnetic susceptibility was measured by SQUID, as shown in Fig. 2(a). The Tcmag is suppressed by the Te doping and the shielding volume fraction decreases with increasing Te doping level, indicating that the Te dopant can reduce the amount of precipitation of superconducting phase. The Tc is completely suppressed at z = 0.51 in our samples. The temperature dependence of resistance was performed for z = 0, 0.09, 0.16, and the results are shown Fig. 2(b). The inset of Fig. 2(b) shows the data for z = 0.34 and 0.51 samples. The suppression behavior of Tcres by Te substitution is consistent to magnetization measurements. For AxFe2-ySe2 superconductors, a resistance hump always exists in normal state , characterized as a crossover from semiconducting to metallic behavior at Th (hump temperature). It has been demonstrated that the hump is not correlated to superconductivity and structural phase transition, but arising from the percolating behavior of the metallic and semiconducting phases.
Field dependence of magnetic hysteresis loops (MHLs) for K0.8Fe2Se2-zTez with z = 0, 0.09 and 0.16 was measured under the field H||c, as shown in Figs. 3 (a), (b) and (c), respectively. The features of MHLs observed in our samples are similar to those of post-thermal quenched KxFe2-ySe2 single crystals. At low temperatures, the MHLs curves show symmetric shapes, indicating the dominant pinning centers are from bulk for z = 0, 0.09 and 0.16 samples. As the temperature increases up to 22 K, the opening area of MHLs shrinks with increasing Te dopant, showing a reduction of capability of carrying superconducting current towards high magnetic field. To estimate the critical current density from MHLs curves, we apply extended Bean critical state model, Jc = 20ΔM/[w(1–w/3ℓ)] where w and ℓ are the width and length of the sample, ΔM is the difference between the magnetization values for increasing and decreasing field as measured in emu/cm3 unit. The calculated curves of Jc versus μ0H are shown in Figs. 3 (d), (e) and (f), respectively. The Jc values at low field limit for all three samples are in the range of ≈103–104 A/cm2. These values are comparable to the post-thermal quenched and S-doped KxFe2-ySe2 single crystals. However, the Jc values at low field limit are much smaller than "122" iron-arsenic electron or hole-doped BaFe2As2 superconductors.
In summary, we have investigated the structure, transport and magnetic properties of K0.8Fe2Se2-zTez single crystals. The c-axis lattice parameter is expanded with increasing Te doping level. The superconducting temperature Tc is suppressed by Te dopant and completely vanished at z = 0.51. The Jc values were estimated to be 103–104 A/cm2 for z = 0, 0.09 and 0.16 samples. As the temperature increases, the capability of carrying superconducting current is reduced towards high magnetic field with increase of Te doping content.