四丁基銨電化學插層鉀銠氧單晶之電磁傳輸研究

Magnetotransport Properties of KxRhO2 Single Crystals with TBA Intercalation by Electrochemical method

黃律翔(Lu-Hsiang Huang)

指導教授 ： 王立民

國立臺灣大學/理學院/物理學研究所/碩士(2021年)

https://doi.org/10.6342/NTU202100359

摘要

本研究以助熔劑成長法合成鉀銠氧單晶，使用能量色散光譜儀進行元素分析，成功合成出高品質樣品K0.58RhO2並研究其電磁傳輸特性。另一方面，透過X射線繞射儀量測其c軸晶格長度為12.25 Å。經過磁性的量測後，於低溫時觀察到隨磁場變大而飽和的磁矩，且重複施加正反方象磁場對樣品量測，並無發現磁滯現象。於是我們推論K0.58RhO2可能於低溫時生成反鐵磁對，對整體磁化強度作出貢獻，並引入超交換作用模型與Goodenough-Kanehori rules解釋其機制。分析磁化率的倒數與溫度的關係我們得到其居里-外斯(Curie-Weiss)溫度θ為2.68 K。
此外我們使用電化學方法將K0.58RhO2樣品進行插層(intercalation)反應，透過氧化還原反應將四丁基銨(TBA)分子還原並插入層銠氧層間，最後再透過XRD以及EDS確認其成分比例為K0.58(TBA)0.9RhO2。
本研究將樣品進行一系列的霍爾及電性量測，由電阻率與溫度的次方關係，發現鉀銠氧於低溫時展現費米液體的特性，並於轉換溫度(T*)以上轉換為1.5次方關係，也就是所謂的Fermi Liquid state與vibronic state混和態。我們將過去實驗室曾經合成過的鉀銠氧單晶經過比較後，發現隨著鉀含量的增加或是TBA插層之後，其c軸晶格常數越小，T*也越高，這部分的結論與莫特相變的理論原理互相吻合。
RhO2層的結構上看似三角平面，其中卻隱藏著Kagome晶格，由理論計算的結果可知道霍爾係數將於高溫時與溫度成正比的關係，此線性關係與本次實驗的樣品於高溫時(> 220 K)的結果互相吻合，同時也由霍爾係數得知此材料之主要傳輸載子為電洞。另一方面，計算磁阻後觀察到隨著鉀的參雜越多，材料開始出現零磁阻的特徵。最後，我們也透過拓樸霍爾效應的計算求得樣品的拓樸霍爾電阻率並觀察到其隨磁場的變化趨勢。

關鍵字

助熔劑成長法 ； 鉀銠氧 ； 反鐵磁 ； 超交換作用 ； 電化學插層 ； 莫特相變 ； 拓樸霍爾效應

並列摘要

We grow the single crystal of KxRhO2 with the flux growth method and analyzed the composition of the elements with the Energy-dispersive X-ray spectroscopy (EDS). We successfully synthesized high quality K0.58RhO2 single crystals and investigated the magnetotransport properties of them. The c-axis of the sample was characterized to be 12.25 Å. In the magnetic measurement, we found that the magnetization would saturate with the increasing magnetic field at the low temperature, while the hysteresis curve was not observed in this case. It is inferred that the saturation magnetization is caused by the existence of antiferromagnetic pairs, and the mechanism is related to superexchange and Goodenough-Kanehori rules. Through the relation between the reciprocal of susceptibility and the Curie-Weiss temperatureθof 2.68 K for K0.58RhO2 has been obtained.
After that, the tetra-butyl-ammonium (TBA) molecules were intercalated via the electrochemical method into the sample. By the redox reactions, reduced TBA molecules were placed it between the RhO2 layers, and it was confirmed that its composition was K0.58(TBA)0.9RhO2 by using XRD and EDS.
In Hall effect and electrical measurement, we have found that the KxRhO2 has properties similar to Fermi liquid (FL), and the resistivity of KxRhO2 will transform the T2 dependent behavior into T1.5 dependence, so-called the mixed state of FL state and vibronic state, when the temperature is higher than the transition temperature T*. Compare the case of KxRhO2 samples formerly synthesized, we find that the c-axis will decrease, being accompanied with an increased T* for samples with higher potassium proportion as well as TBA intercalation. The result is explainable with the Mott-Hubbard transition.
It’s known that there is a triangular structure existing in the RhO2 plane, and which will reveal the so-called Kagome lattices. According to the theoretical calculation, thus the Hall coefficient RH of KxRhO2 is predicted to be proportional to temperature at high temperatures. This kind of linear relation of RH(T) was observed on K0.58RhO2 single crystals at T > 220 K. The obtained the positive Hall coefficient, indicated that the dominant carriers are the hole-type in K0.58RhO2. On the other hand, when the proportion of potassium x in samples is enhanced, the magnetoresistance tends to disappear. Finally, we have calculated the topological hall resistivity and obtain the trends in the variant magnetic fields.

並列關鍵字

flux growth method ； KxRhO2 ； antiferromagnetic ； superexchange ； electro-chemical intercalation ； mott transition ； topological hall effect

參考文獻

[1] Takada, K., Sakurai, H., Takayama-Muromachi, E., Izumi, F., Dilanian, R. A., Sasaki, T, Nature, 422(6927), 53 (2003)

[2] Shibasaki, S., Nakano, T., Terasaki, I., Yubuta, K., Kajitani, T., Journal of Physics: Condensed Matter, 22(11), 115603 (2010)

[3] Jia-Wei Huang. Anomalous Magnetotransport Characteristics of Single Crystal KxRhO2 (2018)

[4] Zhou, J., Liang, Q. F., Weng, H., Chen, Y. B., Yao, S. H., Chen, Y. F., ... Guo, G. Y., Physical review letters, 116(25), 256601 (2016)

[5] S. H. Yao, B. B. Zhang, J. Zhou, Y. B. Chen, S. T. Zhang, Z. B. Gu, S. T. Dong, and Y. F. Chen, AIP Adv. 2, 042140 (2012)