Ya li you dao fei dian shi chao dao ti shen hua ge ji xiang guan lin zu hua he wu lin hua ge de ci shu yun xing zhi yan jiu

Ph.D.CrAs belongs to 3d transition metal pnictides which have attracted considerable attention for their rich variety of magnetic properties. As the representative of double helical magnetic compounds, CrAs has been studied for its magnetic structure and properties over 50 years. Until recently, high pressure study revealed a superconducting phase where the helimagnetic orders are suppressed above a critical pressure. The temperature pressure phase diagram is reminiscent of many unconventional superconducting systems and typical behaviours associated with antiferromagnetic quantum criticality were observed near the critical pressure, indicating that the observed superconductivity may be related to the critical spin fluctuations. This discovery provides another platform to investigate the interplay of superconductivity and magnetism.With the aim of probing electronic structure at the paramagnetic phase, we carried out magnetotransport study of CrAs under pressure. The magnetoresistance shows non-saturating quasilinear dependence with magnetic field at a wide range of pressure and temperature. Band structure calculation suggests that the linear magnetoresistance is attributed to a small Fermi pocket at Y point. Here all the carriers reach extreme quantum limit under magnetic field and dominate the magnetoresistance. The relatively low crossover field leads to the absence of Shubnikov-de Hass oscillations. Besides, a strong violation of Kohler’s rule was observed for CrAs under pressure while a new H–T scaling is applicable.Comparison with CrP, the isostructural high pressure analogue, provides additional support for our explanation, including quadratic magnetoresistance, obedience to Kohler’s rule and clear observation of Shubnikov-de Hass oscillation for CrP. Moreover, CrP exhibits other interesting features such as the nonsaturating large magnetoresistance as well as the ‘turn-on’ temperature behaviour similar to extremely large magnetoresistance observed in semi-metals. Our detail magnetotransport study attributes these features to the carriers compensation in CrP, indicating that the extremely large magnetoresistance is not restricted to topological semimetals. The Shubnikov-de Hass oscillation and the angular dependence study shows multiple frequencies which agree with band structure calculation. A small frequency near 200 T is supposed to come from the Fermi pocket at Y point and a magnetic breakdown orbit arises at 3.44 kT due to the nonsymmorphic symmetry protected degeneracy. Thus, CrP also provides another platform to examine the nonsymmorphic symmetry protection of the space group.3d轉變金屬磷族化合物因為其豐富多樣的磁學性質吸引了研究者大量的關注。其中砷化鉻作為雙螺旋磁性結構的代表被廣泛研究長達五十多年。最近，相關高壓實驗結果顯示在螺旋磁序被高壓抑制以後會出現超導現象。其溫度壓力相圖和其他許多非常規超導體體系的相圖十分相像，而且在臨界壓力附近我們也觀察到了反鐵磁量子臨界點相關的典型行為，預示著本體系的超導現象應該和臨界自旋漲落有關。這個發現為研究超導和磁性之間的相互作用提供了又一個很好的平台。本論文旨在通過測量砷化鉻在壓力下的磁性輸運性質得到其在順磁相的電子結構。我們發現，在很廣的溫度和壓力範圍內，砷化鉻的磁阻和磁場表現出非飽和準線性的依賴關係。能帶計算顯示在布里淵區的Y點存在一個很小的費米包，其中的載流子在磁場的作用下可以達到量子極限，從而主宰了整個體系的磁阻性質，導致線性磁阻的出現。同時相對較低的轉變磁場也導致了無法觀測到舒布尼科夫-德哈斯量子震蕩現象。除此之外，我們還觀測到砷化鉻的磁阻不服從科勒定則，而是遵守一種新的溫度磁場標度。為了檢驗我們對砷化鉻線性磁阻的解釋，我們還測量了磷化鉻的磁性輸運性質，並與砷化鉻進行比較。磷化鉻具有和砷化鉻相同的結構，其差別在於磷原子的半徑比砷原子小，因此其晶格常數也相對變小，可以將其類比作在高壓極限下的砷化鉻。測量結果顯示出與我們的理論預期相符的行為，包括磁阻對磁場的平方依賴關係、服從科勒定則以及觀測到了舒布尼科夫-德哈斯量子震蕩現象。除此之外，磷化鉻還表現出其他有趣的性質，比如和拓撲半金屬的極大磁阻特性類似的非飽和大磁阻以及開啟溫度現象。我們詳盡的磁性輸運研究結果將這些性質歸因于磷化鉻的載流子補償效應，並顯示極大磁阻效應并不僅僅局限于拓撲半金屬材料。舒布尼科夫-德哈斯量子震蕩以及其角依賴性測量結果顯示出與能帶計算結果大致相符的多個頻率。其中，一個大約200特斯拉的頻率應該來自於布里淵區Y點的小費米包。而另外一個大約3440特斯拉的頻率則對應於其他兩個頻率所對應的準粒子軌道塌縮造成的新軌道，這個軌道塌縮則是由空間群的非點式對稱性引起的簡并導致的，因此磷化鉻還可以作為一個研究非點式對稱性表現的平台。Niu, Qun = 壓力誘導非點式超導體砷化鉻及相關磷族化合物磷化鉻的磁輸運性質研究./ 牛群.Thesis Ph.D. Chinese University of Hong Kong 2018.Includes bibliographical references (leaves 97-108).Abstracts also in Chinese.Title from PDF title page (viewed on …).Niu, Qun = Ya li you dao fei dian shi chao dao ti shen hua ge ji xiang guan lin zu hua he wu lin hua ge de ci shu yun xing zhi yan jiu / Niu Qun