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  • 學位論文

硫族化合物硒化錳硫元素摻雜的磁性研究

The study of magnetic behavior in chalcogenide MnSe1-xSx system

指導教授 : 吳茂昆
共同指導教授 : 張嘉升
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摘要


近年來,過渡金屬硫化物在光學、傳輸性質和應用引起廣泛的討論,尤其是在硒化鐵系統中發現超導特性(常壓Tc= 8 K)。這個結果引起我們尋找其他過渡金屬硫化物是否有超導特性存在的興趣。目前發現在高壓條件下磷化錳(8 GPa下Tc= 1 K)和砷化鉻(8 kbar下超導Tc= 2 K)具有超導特性。此外,在砷化鉻系統也發現鉀元素的參雜也發現超導(K2Cr3As3常壓下Tc= 6.1 K)。以上結果,讓我們引起對尋找常壓下錳基超導體興趣。我們選擇了磁性行為接近的硒化錳化合物利用硫元素取代硒元素,尋找是否有超導產生。本論文,我們將呈現硒化錳硫取代一系列樣品的成長方法以及原子結構、磁性、電性、熱性、磁結構分析。同時搭配密度泛函理論計算硒化錳的能帶結構,以及使用高壓鑽石鉆研究硒化錳在高壓下的結構相變。 多晶硒化錳粉末樣品可藉由固態燒結成相,成長出來的樣品是氯化鈉型式立方晶系。同步輻射變溫X-ray粉末繞射指出,在常壓下,硒化錳樣品在降溫過程中150 K會產生部分立方晶系結構相轉變到六角立方晶系,有趣的是,系統回溫至150 K時此結構相轉變並未消失,一直延續到270 K才消失。在不使用高壓條件下,我們利用硫元素取代硒元素讓整個系統產生等效晶格應力。儘管在硒(硫)化錳系統沒有發現超導訊號,但是發現硒化錳的複雜的反鐵磁性以及結構相變可以藉著硫元素參雜而抑制。此外,低溫中子繞射數據指出硒化錳在120 K到200 K的複雜磁性來自於立方晶系及六角晶系所對應的反鐵磁結構所產生的耦合現象。因此,硒化錳硫取代所造成的晶格應力不足以誘發超導。我們亦根據同步輻射X-ray粉末繞射得到的結果,進行密度泛函理論(CASTEP)模擬計算。發現六角晶系的能隙(0.28 eV)比立方晶系的硒化錳能隙(0.67eV)來的小。推論六角晶系的硒化錳可能有機會誘發超導。然而,在近期文獻指出,硒化錳在機械高壓30 GPa下會產生立方晶系結構相變至正交晶系。因此,我們利用高壓鑽石鉆,量測硒化錳在高壓條件下的晶體結構以及電性變化。發現在26 GPa條件下疑似有超導訊號產生。

並列摘要


Recently, transition metal chalcogenides have drawn great attention because of its optical and transport property for potential practical applications. The discovery of superconductivity in FeSe system (ambient condition transition temperature 8 K) has extended further the interests. This result inspired us to search for other superconductor in the transition-metal chalcogenide family. Now, MnP (under 8 GPa condition, Tc = 1 K) and CrAs (under 8 kbar condition, Tc = 2 K) were both found to be superconducting under external pressure. More recently, superconductivity in CrAs was induced by potassium doped at ambient condition (K2Cr3As3 with Tc = 6.1 K). The above results trigger our attention to investigate whether there exists other Mn-based superconductor at ambient condition. We select MnSe as the candidate, because its magnetic property was very similar to that of MnP. We first decide to study whether superconductivity could be induced in MnSe system by substituting Selenium with Sulfur. In this thesis, we will present the results of detailed studies of atomic and magnetic structural variation, and the physics properties including magnetic susceptibility, electrical property, and heat capacity of MnSe1-xSx samples. Additionally, based on the results of synchrotron X-ray powder diffraction, we calculate the electron band structure and energy gap of MnSe. We also used diamond anvil cell to study the structural variation under high pressure in MnSe system. Polycrystalline MnSe1-xSx can be synthesized by solid state reaction, and its structural type and space group are NaCl-type cubic and Fm3 ̅m, respectively. The investigation of structural variation of MnSe is performed by temperature dependent synchrotron X-ray, and the results indicate MnSe undergoes partial structural transformation from cubic to hexagonal at 150 K in cooling process. More interestingly, as this measurement is done in warming cycle, the hexagonal to cubic structural transformation occurs at 270 K instead of 150 K. In order to cause lattice stress into MnSe without applied mechanical pressure, partial Selenium element in MnSe is substituted by Sulfur. Although no superconductivity is observed in MnSe1-xSx system, we find thE complicated anti-ferromagnetism and cubic-hexagonal structural transformation in MnSe can be gradually suppressed by S substitution. Additionally, the results of temperature dependent neutron diffraction patterns show that both cubic and hexagonal phase in MnSe are anti-ferromagnetic. The result explains the huge magnetic anomaly between 100 K to 200 K in MnSe, which is due to the coupling between the cubic and hexagonal phase. The energy band structure and energy gap of cubic and hexagonal phase in MnSe are simulated by density functional theory (CASTEP package software), and the results indicate the energy gap of cubic and hexagonal phase of MnSe are 0.67 and 0.28 eV, respectively. Hence, we propose hexagonal type MnSe might be more favorable for superconductivity. Recently Wang et al. reported the atomic structure of MnSe transformed from cubic to orthorhombic at 30 GPa. We have therefore carrier out the structural variation and temperature dependent electrical property of MnSe under applied pressure. Preliminary results indicate superconductivity could be induced by pressure in MnSe.

參考文獻


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