單晶四鉛化鉑超導特性研究

Abstract

我們以自我助熔法成長四鉛化鉑(PtPb4)單晶，在經由X光繞射量測確認PtPb4晶體後，進行了電性及磁性量測，可發現其Tc約為2.77 K。由磁場下電阻-溫度的行為，我們獲得了上臨界磁場Hc2、相干長度 ξ 與溫度之關係及釘扎能U和磁場之關係；由磁化強度M-H之關係，我們獲得下臨界磁場Hc1、穿透深度λ、與溫度，及釘扎能與外加磁場之關係。此外，由霍爾電性量測亦發現PtPb4主要傳輸載子在31 K以下為電子，31 K以上則轉變為電洞。在正常態，我們發現PtPb4的磁阻(magnetoresistance, MR)在高磁場下和外加磁場呈線性關係，在磁場6 T溫度5 K下，其磁阻可達300%，而推測其線性磁阻之現象可利用Parish and Littlewood model(PL model)來解釋，其低溫5 K之磁阻亦顯示PtPb4具有拓樸表面態之弱反局域(weak-anti-localization)電導特性。由M-H量測可顯示PtPb4之二類超導磁場穿透性質。透過磁滯曲線，我們可以利用Bean model得到臨界電流密度，進而得到釘扎力隨磁場之變化關係，最後利用Dew-Hughes model來擬合結果，發現PtPb4在低溫2.0 K為一維磁通釘扎，溫度往Tc升高時開始偏向二維磁通釘扎。

PtPb4 single crystals have been grown with self-flux method. After we determine the crystals of PtPb4 with X-ray diffraction, we measure its electric as well as magnetic properties, realizing that the critical temperature Tc is about 2.77 K. According to the characteristic of resistance-temperature behavior under magnetic fields applied, we have obtained the temperature dependances of the upper critical field Hc2(T), coherence length ξ(T), as well as field dependance of pinning energy U(H). From the diagram of field-dependent magnetization M(H) curves, we have obtained the temperature dependances of lower critical field Hc1(T), and penetration depth λ(T). Besides, from the results of Hall measurement, we have noticed that the main carriers below 31 K are electrons, but they transform into holes at temperatures higher than 31 K. At normal state, it is worth noticed that the magnetoresistance (MR) of PtPb4 exhibits a linear correlation with the applied magnetic field at high fields, and reaches to 300 % under 6 T and 5 K. It is predicted that this phenomenon can be described by the Parish and Littlewood (PL) model. The MR behavior of PtPb4 at 5 K indicate that there is a topological surface state existing on PtPb4, accompanying a weak-anti-localization (WAL) conductivity observed. Results of M-H measurements clearly reveal the type-II superconductivity in PtPb4. From magnetic hysteresis loop, we have obtained the critical current density Jc base on the Bean model, and further derived the variation of pinning force Fp at different applied fields. Finally, we use the Dew-Hughes model to fit the result Fp(H) data, and find that PtPb4 exhibits one-dimensional flux-pinning at lower temperature of 2.0 K, then the flux pinning transforms to be two-dimensional as the temperature gets higher and near Tc.