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研究生:呂正中
研究生(外文):Jenq-Jong Lu
論文名稱:稀土族介金屬化合物RTX2類型之物理性質研究
論文名稱(外文):Investigations on the physical properties of RTX2 type rare earth based intermetallic compounds
指導教授:田聰
指導教授(外文):Cheng Tien
學位類別:博士
校院名稱:國立成功大學
系所名稱:物理學系碩博士班
學門:自然科學學門
學類:物理學類
論文種類:學術論文
論文出版年:2002
畢業學年度:90
語文別:中文
論文頁數:211
中文關鍵詞:自旋玻璃混合價鍵康多效應重費米子
外文關鍵詞:mixed valenceheavy fermionKondo effectspin glass
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稀土族介金屬化合物有關混和價鍵,重費米子,以及近來引起注意的自旋玻璃的現象,長久以來便為凝態物理上所持續研究及感興趣之課題。一般認為,4f電子與傳導電子之隅合以及其間之混域(hybridization)作用,應是稀土族合金及介金屬化合物特殊物理性質形成的主因。
一種新類型的介金屬化合物-RTX2 (其中R為稀土族元素,T為過渡金屬元素,X為Si,Ge,或Sn),於近十年前開始吸引人們注意。這篇論文的內容主要乃針對此一類型的介金屬化合物,對其物理性質做探討。論文主要的內容主要分為底下三部份:
在論文的第一部份,我們所研究的系統為CePdSi2。我們對此樣本做了X-光粉末繞射、直流電阻率、交直流磁化率、Ce之LIII邊緣X-光吸收譜線,以及比熱測定。由其電阻率的測定中,顯現出康多效應以及晶格場的作用。而磁化率的量測上,顯示出樣本在2.7K處存在一反鐵磁相變;除此之外,在T ~ 5 K處,我們觀察到有類似自旋玻璃的現象。比熱的量測上則觀察到存在有兩個峰值,一約在2.7K,另一則在6.8 K附近。前者應是對應於反鐵磁相變,而後者可能是晶格場所造成之肖特基異常(Schotty anomaly)。另外CePdSi2其配適所得之線性比熱係數γ為0.348 Jmole-1K-2,其值較一般金屬來得大得多。造成如此大的γ值可能來自康多效應、晶格場作用以及自旋玻璃相。而由各項之量測顯示:CePdSi2除了可以視為是反鐵磁性康多晶格(Anti-ferromagnetic Kondo Lattice)之外,在磁性上或許也可將之歸納為”重返型自旋玻璃”(Re-entrant spin glass)。
在論文的第二部分,我們則將致力於在CePt1-xIrxSi2(x = 0, 0.2, 0.4, 0.6, 0.8, 1.0)系列中,由重費米子性演進至混和價鍵行為之探討。我們主要進行了X-光粉末繞射、直流電阻率、Ce之LIII邊緣X-光吸收譜線,以及系列樣本之比熱測定。實驗結果顯示:隨著Ir逐漸取代Pt,單位原胞之體積逐漸減小,因而使得Ce之4f電子與鄰近過渡金屬之傳導電子波函數之混域作用強度增大,因而使得4f電子巡游性增加,而呈現混和價鍵的行為,其磁性也因而減弱。除此之外,我們還發現,x ~ 0.6處,應為重費米子性與混和價鍵這兩種行為之交界。
論文的最後一部份,我們所研究的乃於CeNi(Si1-xGex)2 (x = 0, 0.2, 0.4, 0.5, 0.6, 0.8, 1.0)系列樣本中,由混合價鍵行為至反鐵磁性康多晶格之演變。實驗結果顯示:CeNiSi2乃一典型之混和價鍵化合物,CeNiGe2則顯現出反鐵磁性康多晶格的行為。而當逐漸以Ge取代Si時,晶格結構並未改變,然而原胞體積卻逐漸增大,使得Ce之4f電子與鄰近過渡金屬之傳導電子波函數之混域作用強度減小,4f電子之局域性增加,而形成好的磁矩。因而RKKY作用逐漸取得主導的地位,而終使樣本形成長程的磁序-反鐵磁相。因此我們認為,此一由混和價鍵至反鐵磁性之演變,乃歸因於4f電子與傳導電子間混域作用減弱所致。另外,由原胞體積V,比熱線性係數γ.,以及由X-光吸收光譜配適所得價數v之結果均顯示:x = 0 ~ 0.2,樣本呈現混和價鍵化合物之行為;而當x = 0.8 ~ 1.0時,系列樣本則呈現康多晶格之行為;至於當x = 0.4 ~ 0.6時,樣本性質則呈現出”單雜質康多效應( single impurity Kondo Effect)”之特徵。
The mixed-valence (MV) and heavy-fermion (HF) phenomena in cerium based intermetallic compounds are the subjects of continuous interest for experimental study of their physical behaviors for decades. It is believed that the hybridization of highly correlated 4f electrons with the itinerant conducting electrons should be responsible for both HF and MV behaviors.
New ternary intermetallic compounds of type CeTX2 (where T = transition metals and X= Si, Ge) remain a considerable focus. In this work, we studied on this new type of Ce-based system. The thesis mainly consists of three parts.
In the first part, we studied on the physical properties of CePdSi2. We examined x-ray diffraction, dc electrical resistivity, dc and ac magnetic susceptibilities, specific heat and the Ce LIII-edge x-ray absorption spectrum of this cerium based ternary compound. Electrical resistivity ρ(T) indicates the presence of Kondo and crystal-field effects. The results of magnetic susceptibility measurements for CePdSi2 exhibit a spin-glass behavior at Tf ~ 5 K and the antiferromagnetism at TN = 2.7 K. The M(t) measurement and magnetic entropy calculation also indicate the presence of spin-glass phase. The specific-heat measurement shows two peaks in C(T), one is at around 2.7 K, another is at ~ 6.8 K. The former is likely due to an antiferromagnetic transition, the latter might be due to the Schottky anomaly with spin-glass contribution. The linear specific-heat coefficient γ of CePdSi2 is 0.348 Jmole-1K-2, which is much larger than those of normal metals. This large γ value might result from Kondo effect, crystal effect and spin-glass magnetism. From these measurements, in additional to an antiferromagnetic Kondo Lattice, this compound might be magnetically classified as a re-entrant spin glass.
The second part of this thesis, we worked on the evolution from heavy-fermion to mixed valence behavior in the series CePt1-xIrxSi2. To study the evolution from heavy-fermion to mixed-valence behavior upon substituting Pt for Ir in the ternary intermetallic compound CePtSi2, we carried out the x-ray diffraction, dc electric resistivity, Ce LIII-edge x-ray absorption spectra (XAS), dc susceptibilities, and specific heat measurements in the series CePt1-xIrxSi2 (x = 0, 0.2, 0.4, 0.6, 0.8. 1.0). The results obtained allowed us to suggest that the evolution from heavy-fermion to mixed-valence behavior is likely due to the enhancement of hybridization between the Ce 4f-electron wave function and sp wave function of the adjacent transition metal ion. Besides, it was found that x ~ 0.6 is the borderline region between heavy-fermion and mixed-valence regimes.
The last part of this thesis, we studied on the evolution from mixed-valence to anti-ferromagnetic Kondo-Lattice behavior in the series CeNi(Si1-xGex)2, we carried out the x-ray diffraction, dc electric resistivity, Ce LIII-edge x-ray absorption spectra (XAS), dc and ac susceptibilities, and specific heat measurements in the series CeNi(Si1-xGex)2 (x = 0, 0.2, 0.4, 0.5, 0.6, 0.8. 1.0). The results show that CeNiSi2 is a mixed-valence compound. On the other hand, CeNiGe2 is a Kondo-Lattice with an anti-ferromagnetic phase transition at ~ 5.8 K. When we substituted Ge for Si, with an increase in x, there is a discernible tendency towards a increasing in unit-cell volume. The increasing in unit cell volume will decrease the hybridization of Ce 4f electron wave function with the sp wave function of the neighboring transition metal atoms and increase the localization of 4f electrons and finally drive the system from MV behavior to anti-ferromagnetism. The results allowed us to suggest that the evolution from mixed-valence to Kondo-lattice behavior is likely due to the dehybridization between the Ce 4f-electron wave function and sp wave function of the adjacent transition metal ion. Besides, from these measurements, it was found that x = 0 ~ 0.2, the series show basically mixed-valence behavior. When x = 0.8 ~ 1.0, the series show Kondo-Lattice behavior. Between x = 0.4 ~ 0.6, the behaviors of the samples can be characterized as single impurity Kondo effect.
中文摘要 ………………………………………………………...I
英文摘要 ……………………………………………..………..IV
誌謝 ………………………………………………………….VII
目錄 ……………………………………………………..….VIII
表目錄 ………………………………………….…………….XII
圖目錄 ……………………………………………………….XIII
第一章 緒論 …………………………………….…...……..1-1
第二章 理論 …………………………………………….….2-1
第一節 原子和離子固有磁矩的由來 ………..……..….2-1
第二節 稀土族離子之磁矩 …………………..….…...2-2
第三節 磁相 ……………………………….….…..…..2-4
第四節 磁交互作用 .. ………………………….………2-7
(一)直接交換作用 ….…………………….…….2-7
(二)RKKY交換作用 ……………………..…….2-8
(三)磁偶極作用 …………………………….…..2-9
(四)超交換作用 ………………………………..2-10
(五)s-d 交換作用 ………………….……..…….2-11
第五節 安德生模型及混合價鍵 ……….…….……….2-12
第六節 康多效應及重費米子性 ……………………...2-16
(一)康多效應 …………………………..……...2-16
(二)重費米子性 …………………………..…...2-21
第七節 晶格場作用 …………..……………………..…2-23
(一)克拉瑪斯簡併 ………………………….....2-24
(二)肖特基異常 …………………………….…2-24
(三)Cornut-Coqblin模型 ……..…………..…...2-25
第八節 自旋玻璃 …………………………….….….….2-29
(一) 自旋玻璃的現象 ………………..…...…….2-29
(二) 自旋玻璃的種類 ……………………..……2-30
(三) 自旋玻璃的特徵 ………………………..…2-31
(四) 自旋玻璃的理論模型 …………….…….…2-33
第三章 實驗細節及數據分析 ………………………….…..3-1
第一節 樣本的製作及處理 ……………………..……....3-1
第二節 X-光粉末繞射 ……………………….….…..…..3-4
第三節 直流電阻率的量測 ……………..….…….…....3-10
第四節 比熱的量測 …………………………….…..…3-12
第五節 磁性的量測 …………………………...….….3-16
第六節 X-光吸收光譜實驗 …………………...…..…3-20
(一) X-光吸收光譜簡介 ………….……..……..3-20
(二) Wiggler光束線之實驗操作 ………..….…...3-21
(三) 實驗數據之處理與分析 ……………..……..3-25
第四章 結果與討論 …………….………………..….….….4-1
第一節 CePdSi2之物理性質研究 ……………………..4-1
(一)實驗結果 ……………………….………..…..4-1
(二)討論 …………………………….……….…...4-9
(三)結論 ………………….………………...……4-23
第二節 CePt1-xIrxSi2系列由重費米子性至混和價鍵行為之演進 ………………………….………..…...4-24
(一)實驗結果 ………………….………….…..4-24
(二)討論 ……………………………..…...…...4-38
(三)結論 ……………….…..……………...…..4-41
第三節 CeNi(Si1-xGex)2系列由混和價鍵至反鐵磁康多晶格行為之演進…………………….…..…...4-43
(一)實驗結果 ……..…………….…..…….…..4-43
(二)討論 ……………………...…………..…...4-61
(三)結論 …….……………………..………….4-62
第四節 總結與未來展望………………………………….4-67
參考文獻 …………………………………..……….….…….R-1
附錄 …………………………………………..………….…..A-1
A. 直接交換作用能量 ………………….…..…..A-1
B. RKKY間接交換作用 ……………….……....A-4
C. 安德生模型 ………………………….………A-9
D. 克拉瑪斯簡併 …………………………..… A-14
E. 居理-外斯定律 …………….……..…..…….A-16
(一) 居理定律 ……………………..….……A-16
(二) 外斯定律 …………………..….………A-18
(三) 尼爾的反鐵磁模型 …………..………A-21
F. Cornut-Coqblin 模型 …………..…….…….A-24
G. 康多效應 ………………………….………..A-25
H. 傳導電子之順磁性 …………………..……..A-29
I. SK模型之磁序參數q及磁化強度m的
自洽方程式 …………….………….………....A-31
著作 ………………………………………………………W-1
作者簡歷 …………………………………………………W-6
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