FeSe與Fe1.02Se0.33Te0.67磁阻在脈衝磁場之非均向性研究與脈衝式磁場系統設計

Abstract

自從2008年鐵基超導體被發現，鐵基超導體一直是熱門的研究議題，而FeSe在鐵基超導體裡是最簡單的結構，FeSe層狀的結構推測與高溫銅氧化合物超導體類似。到目前為止，FeSe的實驗結果還是具有爭議的，例如有數據顯示FeSe單晶的第二臨界磁場的性質為均向性，但是我們實驗室藉由比熱量測的結果顯示FeSe為非均向性，因此我們將專注在FeSe與Fe1.02Se0.33Te0.67的第二臨界磁場。 我們對鐵基超導體11 Type FeSe與Fe1.02Se0.33Te0.67單晶對磁場平行c軸與磁場垂直c軸進行磁阻量測。由磁阻結果可以得到不同軸向的臨界磁場，我們發現FeSe在低溫為非均向性，Fe1.02Se0.33Te0.67在低溫為均向性。我們利用WHH Theory計算的臨界磁場與實驗值比較發現，Fe1.02Se0.33Te0.67在垂直c軸的臨界磁場與理論值計算不符，證實Fe1.02Se0.33Te0.67垂直c軸的臨界磁場與Pauli limit有關，由霍爾效應實驗也發現FeSe在120 K以下有載子mobility改變的情況，比電子結構與晶格結構改變的90 K還早發生變化，在90 K以下霍爾係數與磁場的大小有關，而且由ρ_xy(H)發現在50 K以下可以發現FeSe內部有多種以上的載子，而這可能與費米面的重建和nematic order有關。 除此之外，我們在台灣建造第一個脈衝磁場系統，脈衝磁場系統建造完成之後，我們量測線圈產生的磁場值，Coil 1在2 mF充電1700 V可產生約31 T；Coil 2在2 mF充電1700 V可產生約13 T。利用脈衝磁場系統，我們設計了磁阻量測的系統，藉由La0.33Sr0.67MnO3薄膜的磁阻量測來測試系統的可靠度。此系統具有高磁場低成本的優點。

Iron-based superconductor have become highly relevant since the discovery of iron pnictides in 2008. The relatively high superconducting transition temperature and quasi-two-dimensional sheet structure of the iron-based superconductors initially suggested analogy with cuprates. Thus far, the experimental results of FeSe are still controversial. Very recently, Vedeneev et al. reported the isotropic Hc2 of single-crystal FeSe. However, the upper critical field of FeSe determined by specific heat measurements suggested a large anisotropic at low-temperature. In this study, to elucidate the above issues, we focused on the upper critical field of FeSe, and compared the results with that of the Fe1.02Se0.33Te0.67. Longitudinal resistivity of FeSe and Fe1.02Se0.33Te0.67 single crystal were measured using a typical four-contact method in pulsed magnetic fields up to 55 T. FeSe shows anisotropy in Hc2 but Fe1.02Se0.33Te0.67 shows isotropic Hc2 at lowest temperatures. The results of the measured zero-temperature upper critical field and WHH modeling showed that the WHH model can not explain our results very well. The anisotropic (isotropic) behavior of FeSe (Fe1.02Se0.33Te0.67) is suggested to be originated from the competition between the Pauli limit and the orbital limit. Doping Te ions into FeSe leads to a crossover from the Pauli limit dominating behavior to the orbital limit dominating one. The Hall experiments show that FeSe has a big jump in carrier mobility under 120 K. The field-dependent Hall coefficient changes abruptly, suggesting a drastic reconstruction of the Fermi surface at the transition of 90 K. We found that FeSe have two kinds of carrier when T < 50 K. When T < 50 K, and the sign change of Hall coefficient suggested the reconstruction of the Fermi surface and possible existence of nematic order. We have constructed the first pulsed-magnetic-field system in Taiwan. The magnetic field can be measured by inductance method. The system can generate 31 T with charging 1700 V to condenser bank. We integrated a magnetoresistance measurement system in the pulsed-field system. The magnetoresistance measurement of La0.33Sr0.67MnO3 thin film shows that the pulsed magnetic field system is reliable.