Application of high-resolution dilatometry to the study of critical phenomena in antiferromagnetic systems.

機譯：高分辨率膨脹計在反鐵磁系統中關鍵現象研究中的應用。

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Thermal expansion is an under-utilized physical property with enormous potential in its application to the study of classical critical phenomena. The Pippard relation scales the coefficient of volume thermal expansion multiplied by temperature with heat capacity in the vicinity of a continuous phase transition. This justifies the study of critical behavior, characterized by critical-exponent alpha, with the coefficient of volume thermal expansion instead of heat capacity. We evaluate potential advantages and disadvantages and develop strategies uniquely suited to the analysis of critical behavior exhibited by the coefficient of thermal expansion. The most notable disadvantages arise as a result of numerically differentiating thermal-expansion data to obtain its coefficient. In the course of assessing the detrimental effects of this procedure, we developed a critical expression for thermal expansion, with which we quantitatively evaluate the effect numerical differentiation has on the study of critical phenomena. Antiferromagnets are less susceptible than ferromagnets to long-range dipole contributions, which adjust critical behavior away from theoretical predictions. Therefore, three antiferromagnetic materials were selected to test the suitability of studying critical behavior with thermal expansion. Single crystals of CaMn2O4 and Bi2CuO 4 were grown and characterized by methods described in careful detail. The coefficient of thermal expansion is studied along the principal crystallographic axes of each material demonstrating that the critical behavior exhibited along each axis is the same as that exhibited by the volume. Both transitions belong to the three-dimensional Ising universality class which settles a long-standing question by suggesting that Bi2CuO4 exhibits easy-axis anisotropy rather than easy-plane anisotropy. The thermal expansion of each material (as opposed to the coefficient of thermal expansion) is studied with our critical expression in order to increase the critical temperature range close to the Neel temperature. The limitations of studying the critical behavior of a polycrystalline sample are demonstrated when we investigate the antiferromagnetic transition of alpha-Mn. This element has a surprisingly complicated magnetic structure and our results constrain its transition to be in either the three-dimensional Heisenberg or n = 4 universality class.

機譯：熱膨脹是一種未被充分利用的物理性質，在將其應用于經典臨界現象的研究中具有巨大的潛力。皮帕德關系在連續相變附近縮放體積熱膨脹系數乘以溫度與熱容的關系。這證明了對以臨界指數α為特征的臨界行為進行研究的合理性，該行為具有體積熱膨脹系數而不是熱容。我們評估潛在的優勢和劣勢，并開發出獨特的策略來分析熱膨脹系數所顯示的關鍵行為。最顯著的缺點是由于對熱膨脹數據進行數值微分以獲得其系數而導致的。在評估此過程的有害影響的過程中，我們開發了一個熱膨脹的臨界表達式，通過它我們可以定量評估數值微分對臨界現象研究的影響。與鐵磁體相比，反鐵磁體不易受到遠程偶極子貢獻的影響，遠距離偶極子貢獻使臨界行為偏離了理論預測。因此，選擇了三種反鐵磁材料來測試研究熱膨脹臨界行為的適用性。生長CaMn2O4和Bi2CuO 4的單晶并通過詳細描述的方法進行表征。沿每種材料的主結晶軸研究熱膨脹系數，表明沿每個軸顯示的臨界行為與由體積顯示的臨界行為相同。兩種轉變都屬于三維Ising通用性類別，該類別通過提出Bi2CuO4表現出易軸各向異性而不是易平面各向異性來解決一個長期存在的問題。用我們的臨界表達式研究每種材料的熱膨脹（與熱膨脹系數相反），以增加接近Neel溫度的臨界溫度范圍。當我們研究α-Mn的反鐵磁躍遷時，證明了研究多晶樣品臨界行為的局限性。此元素具有令人驚訝的復雜磁性結構，我們的結果將其轉換限制為三維Heisenberg或n = 4通用類。

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作者
White, Benjamin Dakotah.;
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Montana State University.;

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授予單位 Montana State University.;
學科 Physics Condensed Matter.
學位 Ph.D.
年度 2010
頁碼 341 p.
總頁數 341
原文格式 PDF
正文語種 eng
中圖分類
關鍵詞
入庫時間 2022-08-17 11:36:57

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Application of high-resolution dilatometry to the study of critical phenomena in antiferromagnetic systems.

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