鐵基超導體

鐵基超導體是指化合物中含有鐵，在低溫時具有超導現象，且鐵扮演形成超導的主體的材料。2006年日本東京工業大學細野秀雄教授的團隊發現第一個以鐵為超導主體的化合物LaFeOP^[1]，打破以往普遍認定鐵元素不利形成超導迷思。

LnFePnO的晶體結構，它是ferropnictide化合物之一。

根據BCS理論，產生超導性的必要條件是材料中的電子必須配對，這樣配對的電子稱為庫柏對。庫柏對中的兩個電子自旋相反，所以總自旋為零，因而科學家認為超導性與鐵磁性可能無法共存，材料中如果加入磁性元素（如鐵、鎳）會大大降低超導性。鐵基超導體雖然含有鐵元素且是產生超導的主體，但是鐵和其他元素（如砷、硒）形成鐵基平面後，已不再具有鐵磁性。

2008年1月9日，細野秀雄教授的團隊再度發現鐵基層狀材料La[O_1-xF_x]FeAs（x = 0.05 – 0.12）在絕對溫度26K時存在超導性^[2]。2008年2月26日，細野團隊又發現其在絕對溫度43 K的超導性^[3]。2008年3月28日，中國科學院物理研究所趙忠賢領導的科研小組報告，氟摻雜鐠氧鐵砷化合物的高溫超導臨界溫度可達52 K（-221.15 ℃）。4月13日該科研小組又有新發現：氟摻雜釤氧鐵砷化合物假如在壓力環境下產生作用，其超導臨界溫度可進一步提升至55 K（-218.15 ℃）。此外，中科院物理所聞海虎領導的科研小組還報告，鍶摻雜鑭氧鐵砷化合物的超導臨界溫度為25 K（-248.15 ℃）^[4]，從此研究鐵基超導體便在世界上形成一股熱潮。引起許多科學家的興趣的重要原因之一在於鐵基超導體的結構與高溫超導的銅氧平面類似，超導性發生在鐵基平面上，屬於二維的超導材料。因此儘管鐵基超導體的臨界溫度只有數十開爾文，研究鐵基超導體可能有助於了解高溫超導的機制。

晶格結構

現有的鐵基超導體從結構上可分為四類：（1111）、（122）、（111） 和 (11)。

氮磷族氧化物 (oxypnictide)	臨界溫度 (K)
LaO0.89F0.11FeAs	26[5]
LaO0.9F0.2FeAs	28.5[6]
CeFeAsO0.84F0.16	41[5]
SmFeAsO0.9F0.1	43[5]
La0.5Y0.5FeAsO0.6	43.1[7]
NdFeAsO0.89F0.11	52[5]
PrFeAsO0.89F0.11	52[8]
ErFeAsO1-y	45[9]
Al-32522 (CaAlOFeAs)	30(As), 16.6 (P)[10]
Al-42622 (CaAlOFeAs)	28.3(As), 17.2 (P)[11]
GdFeAsO0.85	53.5[12]
BaFe1.8Co0.2As2	25.3[13]
SmFeAsO~0.85	55[14]

非氮磷族氧化物 (non-oxypnictide)	臨界溫度 (K)
Ba0.6K0.4Fe2As2	38[15]
Ca0.6Na0.4Fe2As2	26[16]
CaFe0.9Co0.1AsF	22[17]
Sr0.5Sm0.5FeAsF	56[18]
LiFeAs	<18 [19] [20][21]
NaFeAs	9–25[22][23]
FeSe	<27[24][25]

參見

• 安德烈夫反射(Andreev reflection)
• 高溫超導
• 近藤效應
• 磁化帆
• 氮磷族氧化物(oxypnictide)

參考文獻

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