臺灣博碩士論文加值系統

我們對鐵基超導體11 系&;#63900;-FeSe 單晶在磁場平&;#64008;c 軸及平&;#64008;ab
軸進&;#64008;比熱&;#63870;測。從比熱結果可以得知其超導溫&;#64001;約為8 K，&;#63965;用&;#63847;
同的有序&;#63851;&;#63849;去擬合超導態電子比熱的結果，進一步獲悉&;#63930;能隙上的
超導對對稱態。在費米面上同時存在&;#63930;同向性(isotropic)s-wave 及&;#63842;
向性(anisotropic)extended s-wave，而此結果與實驗&;#63849;據γ(H)&;#64008;為相
符。
除此之外，我們還對一系&;#63900;的Ba(Fe1-xCox)2As2 單晶進&;#64008;比熱&;#63870;
測，成份為x=0, 0.08, 0.2。在成份為x=0.08 時，從電子比熱結果顯示
出超導溫&;#64001;出現在Tc=21 K，而超導溫&;#64001;隨著磁場的增加而&;#64009;低。在
極低溫時，比熱明顯出現T2 項，提供&;#63930;能隙上有節點的證據。因此，
我們可以知道Ba(Fe0.92Co0.08)2As2 為一能隙有節點之超導體。

We have measured the low-temperature specific heat of FeSe single
crystals with in-plane and out-of-plane magnetic fields. The symmetry of
the order parameter and the mixed state will be discussed for compounds.
Coexistence of isotropic s-wave and anisotropic extended s-wave is found
in FeSe. The mixed state γ(H) is also in accord with above findings.
Also, we have measured the low-temperature specific heat of
Ba(Fe1-xCox)2As2 (x=0,0.08,0.2) single crystals. The electronic specific
heat of Ba(Fe0.92Co0.08)2As2 in the superconductiong state with Tc=21 K is
revealed. A T2 term was observed at low temperatures, providing the
evidence of nodes in the gap. Furthermore, the data suggest a nodal feature for Ba(Fe0.92Co0.08)2As2.

摘要 …………………………………………………………………i
Abstract ………………………………………………………………...ii
致謝 ………………………………………………………………..iv
目錄 ………………………………………………………………...v
圖次 ……………………………………………………………….vii
第一章 緒論 ........................................................................................... 1
第二章 理論與材料 ............................................................................... 4
2.1. 超導體的基本特性及現象 ........................................................... 4
2.1.1. 零電阻現象 ........................................................................ 4
2.1.2. 臨界溫度Tc ........................................................................ 4
2.1.3. 臨界磁場 Hc ...................................................................... 4
2.1.4. 邁斯納(Meissner)效應 ....................................................... 4
2.2. 超導體的電動力學 ....................................................................... 5
2.3. 超導體微觀理論：BCS(Bardeen-Cooper-Schrieffer)理論 ........ 5
2.4. 基本比熱理論 ............................................................................... 6
2.5. 理論模型 ....................................................................................... 9
2.5.1. d-wave 模型預測： ............................................................ 9
vi
2.5.2. BCS 模型理論?v T ?w
es C ~ e ?{ ? / ： ........................................... 11
2.5.3. 線結(line-node)模型?vC ~ T 2 ?w es ： ...................................... 11
2.5.4. Two-gap 模型理論： ....................................................... 12
2.6. 鐵基超導體(Iron-based superconductor) ................................... 12
第三章 樣品製備與實驗方法 ............................................................. 19
3.1. 樣品製備 ..................................................................................... 19
3.2. 比熱量測 ..................................................................................... 20
3.2.1. PPMS 基本構造 ............................................................... 21
3.2.2. 3He 降溫系統 ................................................................... 23
3.2.3. 比熱測量原理 .................................................................. 25
3.2.4. 比熱實驗操作流程 .......................................................... 28
第四章 結果與討論 ............................................................................. 30
4.1. FeSe-1 的比熱數據量測 ............................................................ 30
4.2. FeSe-2 的比熱數據量測 ............................................................ 38
4.3. Ba(Fe1-xCox)2As2 的比熱數據量測 ............................................. 44
第五章 總結 ......................................................................................... 52
參考文獻 ……………………………………………………………….53

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