Please use this identifier to cite or link to this item: http://hdl.handle.net/11455/16972
標題: Thermal Properties of Superconductor MgB2-XCX (X=0、0.02、0.06、0.08)
超導體MgB2-XCX (X=0、0.02、0.06、0.08)之熱傳導物理性質相關研究
作者: 張洵愷
Chang, Hsun-Kai
關鍵字: MgB2
二硼化鎂
thermal conductivity
seebeck
carbon doping
熱傳導係數
熱電效應
碳摻雜
出版社: 物理學系所
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摘要: MgB2的超導臨界溫度為39K,是目前金屬類超導體中臨界溫度最高的,本論文主要在研究摻雜碳後二硼化鎂(MgB2-XCX,X=0、0.02、0.06和0.08)的相關熱傳導物理性質。 實驗中使用美國QUANTUM DESIGN公司出產的PPMS-9 Physical Properties Measurement System中的TTO系統(Thermal Transport Option),分別對Nb、YNi2B2C和MgB2-XCX(X=0、0.02、0.04、0.08)等塊材超導體進行Thermal Conductivity、Seebeck Coefficient及Eletrial Resistivity於8K至300K下的數值量測,並經由Wiedemann- Franzlaw的比值結果中瞭解聲子和電子在熱傳導機制上的貢獻,超導態下的MgB2-XCX則藉由multiband 理論,來觀察分析其TC以下,不同能帶上電子的熱傳導貢獻。Seebeck Coefficient數據的正負值,則可判別樣品於研究溫區中,其電傳導率的主控方式。 實驗結果發現MgB2於超導態時, band上的準粒子是主要的電子熱傳導貢獻者,當C的取代摻雜後, band 和 band上的電子熱傳導貢獻下降,因而使熱傳導係數降低。由Seebeck Coefficient數據中,得到MgB2-XCX因C的取代摻雜,使得 能帶的費米能下降,電子狀態密度減少,並且MgB2-XCX的超導轉變溫度也降低,不過其上臨界磁場卻是升高的。
The transition temperature of MgB2 superconductor is 39 K, which is currently the highest among the metallic superconductors. The purpose of this study is to investigate thermal properties of MgB2-xCx (x = 0, 0.02, 0.06, 0.08) superconductors. The thermal conductivity, Seebeck coefficient and electrical resistivity of the MgB2-xCx (x = 0, 0.02, 0.06, 0.08) samples were measured by using Thermal Transport Option of Physical Properties Measurement System (PPMS) under a wide temperature range between 8 K and 300 K. The effective Lorenz number in the Wiedemann- Franz law was used to estimate the contributions of phonon and electron to the thermal conductivity. A more quantitative analysis of thermal conductivity suggests that the phonon contribution is relatively small in comparison with the field-induced electronic contribution. The contribution of electrons to the thermal conductivity in the superconducting state was further analyzed by the two-band theory. Our result showed that electrons in the π-band play a significant role in the heat transport. The substitution of carbon for boron leads to a considerable reduction of the electronic heat transport in bothπ-band and σ-band. The polarity of the free carriers in this work was mainly determined by the Seebeck coefficient. In the carbon-doped MgB2 compounds the Seebeck coefficient data demonstrates that the Fermi energy and the density of states decrease with the increasing x. Also, the reductions result in a lower transition temperature. But the upper critical fields are surprisingly enhanced by the carbon doping. We believe that the enhancement is mostly attributed to the flux pinning of the defects which act as pinning centers.
URI: http://hdl.handle.net/11455/16972
其他識別: U0005-2507200623301800
文章連結: http://www.airitilibrary.com/Publication/alDetailedMesh1?DocID=U0005-2507200623301800
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