超導體Dy0.18 Ca0.82 (Fe0.92 Co0.08 As)2 的 交流磁化率研究

在本實驗中，我們將Dy、Ca、CoAs、FeAs以適當的莫爾數比例，混和製作單晶Dy0.18 Ca0.82 (Fe0.92 Co0.08 As)2樣品。我們使用 PPMS 的 AC Measurement System(ACMS)套件和電阻套件對我們的單晶樣品進行量測。 由電阻率和交流磁化率的量測，我們得知樣品的超導臨界溫度Tc,onset 為 17 K。從不同交流磁場的交流磁化率量測中 我們發現Tc,onset 會隨交流磁場變大而下降；在相同溫度下的交流磁化率虛部隨著頻率增加而增加。我們也量測不同外加直流磁場下的磁化強度，並經由 Bean's model 計算得到不同磁場下的臨界電流密度。 本實驗確認單晶Dy0.18 Ca0.82 (Fe0.92 Co0.08 As)2樣品的下臨界磁場(Hc1 )很小 即，使在地磁的環境下，將樣品溫度經'零磁場冷卻(ZFC)'到臨界溫度之下時，樣品已經處於超導混和態(mixed state)，有磁束被釘札在樣品內部。In this research, the Dy0.18 Ca0.82 (Fe0.92 Co0.08 As)2 single crystal with non-stoichiometric compositions of Dy、Ca、CoAs and FeAs was prepared by the self-flux method. We used the AC Measurement System (ACMS) option and resistivity option of PPMS to measure our single crystal samples. The superconducting transition temperature Tc,onset is obtained 17 K by the resistivity and ac susceptibility measurements. The results of ac susceptibility measurement show that Tc,onset decreases with increasing ac magnetic field. In addition, the imaginary part susceptibility enhances with increasing frequency is observed. The field dependence of the inductive critical current density is calculated by the Bean's model according to the hysteresis loop of the dc magnetization measurement. This research confirms that the superconducting lower critical field (Hc1 ) of the Dy0.18 Ca0.82 (Fe0.92 Co0.08 As)2 single crystal is extremely small. The specimen is in the superconducting mixed state as cooling down to T < Tc in the earth-field environment. The magnetic flux is pinned in the sample even under the 'Zero-Field-Cooled (ZFC)' process.摘要 ..................................................................................................i Abstract ..................................................................................................ii 致謝 .................................................................................................. iii 目錄 ................................................................................................. iv 圖次 .................................................................................................. vi 第一章前言 .............................................................................................................. 1 第二章簡介 .............................................................................................................. 2 2.1超導體簡介 ................................................................................................................. 2 2.2超導體的特性 ............................................................................................................. 2 2.3超導體的分類 ............................................................................................................. 4 2.4London 穿透深度 ........................................................................................................ 6 2.5超導體的理論 ............................................................................................................. 6 2.4鐵基超導 .......................................................................................... 8 2.5交流磁化率 ............................................................................................................... 12 第三章儀器介紹 .................................................................................................... 17 2.3物理特性量測 (Physical Property Measurement System，PPMS) ......................... 17 2.4交流量測系統 ( AC measurement system，ACMS ).............................................. 20 2.5電阻率量測系統 ....................................................................................................... 22 第四章實驗流程與量測方法 ................................................................................ 23 4.1樣品製備 ................................................................................................................... 23 4.2磁化強度量測 ........................................................................................................... 23 4.3電阻率量測 ............................................................................................................... 24 第五章數據探討 .................................................................................................... 25 第六章結論 ............................................................................................................ 33 參考文獻 .................................................................................................. 3