Magneto Transport Properties of Superconducting LiTi_1-xSn_xO₄ Poly-crystals

ZHU JIA LEI LEI^1*, Li-min Wang¹, I-Nan Chen¹

¹物理系, 國立臺灣大學, 臺北市, Taiwan

* Presenter:ZHU JIA LEI LEI, email:2561895243@qq.com

In this study, a high temperature solid phase method was used to synthesize lithium tetraoxodititanate (LiTi₂O₄) , and Sn⁴⁺ was doped by adding different proportions of SnO₂ to the solid phase to replace part of TiO₂ to produce tin-doped lithium tetraoxodititanate (LiTi_2-xSn_xO₄). X-ray diffraction was used to obtain the X-ray diffraction (XRD) pattern to confirm the product, It was found that polycrystalline LiTi₂O₄, LiTi_1.98Sn_0.02O₄, LiTi_1.95Sn_0.05O₄ and LiTi_1.85Sn_0.15O₄ samples were achieved for study. The lattice constants of samples are all close to the standard values of LiTi₂O₄. By measuring the change of magnetization M with temperature for the four samples, one can see that with the increase of Sn⁴⁺ doping, Tc does not change much, where T_c^onset is about 12 K, but the proportion of superconductor content decreases, according to the magnetic susceptibility at 2 K. By measuring the magnetization M(H) for the four samples at different temperatures, the lower critical magnetic field H_c1(T) and the upper critical magnetic field H_c2(T) can be obtained. The M(H) can confirm that LiTi_2-xSn_xO₄ is the type-Ⅱ superconductor with κ ≈ 6. The coherence length ξ can be calculated through the upper critical magnetic field H_c2; The coherence length ξ(0) of the sample LiTi₂O₄ is 5.971 nm, while the coherence length ξ(0) of the sample LiTi_1.95Sn_0.05O₄ is 8.549 nm, and one can see that ξ(0) will increase after doping. The London penetration depth λ is calculated by the lower critical magnetic field H_c1, the λ(0) of the sample LiTi₂O₄ is 40.59 nm, while the λ(0) of the sample LiTi_1.95Sn_0.05O₄ is 42.03 nm, and one can see that λ(0) is not much different with the Sn⁴⁺ doping. Combined with the λ(T) formula, it has a relationship with the temperature T in (Δλ(T))/(λ(0))≈√((πΔ₀)/2T)exp(-Δ₀/T), which conforms to the characteristics of s-wave superconductors.Moreover, using the Bean model to analyze the hysteresis curve, the temperature dependence of critical current J_c(T) is obtained. It is found that the maximum J_c is not at H = 0, but at H* = 400-1200 Oe , showing a trend of increasing first and then decreasing. The calculated pinning force F_p has similar properties, for samples with different Sn⁴⁺ doping. Using the Dew-Hughes model to the F_p(H) fit results, we infer that LiTi₂O₄ belongs to two-dimensional magnetic flux pinning. The H-T phase diagram of flux pinning in LiTi_2-xSn_xO₄ is obtained also.

Keywords: LiTi₂O₄, LiTi_2-xSn_xO₄, Doping, Type II Superconductivity, flux pinning