鈦酸鋰多晶摻雜氟離子的超導電性和反常磁阻

Abstract

本研究採用固相合成法合成尖晶石型鈦酸鋰，合成原料是碳酸鋰、二氧化鈦和鈦。透過在上述原料中加入不同計量的氟化鋰來合成摻雜的樣品。合成後用X射線繞射儀測量繞射峰。本研究中未摻雜和摻雜的樣品吸收峰位置都與理論位置大致相同，晶格常數有在摻雜後有略為變大。在確認結構後分別用磁化率和電阻隨溫度的變化曲線測定了超導臨界溫度，從電阻的量測顯示超導臨界溫度在摻雜後略微上升，從未摻雜時的11.1K上升至最高13.3K，在摻雜過量後大幅降低到11.0K。之後還量測了超導臨界磁場，結果顯示在摻雜後超導臨界磁場略為下降。接著量測了Hall電阻並計算出Hall係數和載子濃度，結果顯示較少量摻雜的LiTi_2 O_4和LiTi_2 O_4 F_0.05是空穴型載子，隨著溫度上升空穴減少，LiTi_2 O_4 F_0.075和LiTi_2 O_4 F_0.1是電子型載子，隨著溫度上升電子增加。我們還量測了樣品在不同溫度的磁阻，並發現對於空穴型載子的LiTi_2 O_4和LiTi_2 O_4 F_0.05，空穴越少負磁阻越顯著。對於電子型載子的LiTi_2 O_4 F_0.075電子越少負磁阻越顯著。從這裡可以推論鈦酸鋰中負磁阻來源於晶體中混價的磁性Ti³⁺離子對載子的磁性相關散射。

In this study, spinel-type lithium titanate was synthesized using the solid-state reaction method, with lithium carbonate, titanium dioxide, and titanium metal as starting materials. Doped samples were prepared by adding different amounts of lithium fluoride to the raw materials. After synthesis, X-ray diffraction (XRD) was used to measure the diffraction peaks. The positions of the absorption peaks for both undoped and doped samples were generally consistent with the theoretical values, and the lattice constants showed a slight increase after doping. After confirming the crystal structure, the superconducting transition temperature was determined using measurements of magnetic susceptibility and electrical resistance as a function of temperature. Resistance measurements showed that the superconducting critical temperature slightly increased after doping, rising from 11.1 K in the undoped sample to a maximum of 13.3 K, and then significantly dropping to 11.0 K with excessive doping. The upper critical field was also measured, showing a slight decrease after doping. Hall resistance was measured to calculate the Hall coefficient and carrier concentration. The results showed that lightly doped samples (LiTi_2 O_4and LiTi_2 O_4 F_0.05) had hole-type carriers, with hole concentration decreasing as temperature increased. In contrast, LiTi_2 O_4 F_0.075and LiTi_2 O_4 F_0.1had electron-type carriers, with electron concentration increasing with temperature. Magnetoresistance measurements at different temperatures revealed that for hole-type samples (LiTi_2 O_4and LiTi_2 O_4 F_0.05, the negative magnetoresistance became more pronounced as hole concentration decreased. For the electron-type sample LiTi_2 O_4 F_0.075, the negative magnetoresistance became more significant as electron concentration decreased. These observations suggest that the negative magnetoresistance in lithium titanate originates from spin-dependent scattering of carriers by mixed-valence magnetic Ti³⁺ ions in the crystal.