- 學位論文
銅氧化物高溫超導體中的膺能隙，電子密度波，時間反演對稱破缺，費米環，以及超導電性的理論
Theory on the Pseudogap, Charge Density Wave, Time Reversal Symmetry Breaking Fluctuation, Fermi Arc Formation, and Superconductivity in High Temperature Cuprate Superconductors
摘要
自從1986年發現以來，銅氧化物高溫超導體帶來數不盡的驚奇與問號，因無法用一般的藍道費米液體來解釋而困擾了物理學界將近40年。時至今日，許多重要的現象，如膺位能相變、電子密度波、微弱的順磁性、d-波超導電性，以及費米環的形成的機制都仍然沒有一個普遍的共識。在這篇論文中，我們使用了一個非典型的方法來處理強關聯電子的問題：推測出由系統中強關聯產生的自由度與電子之間的交互作用，利用這樣的模型，我們可以解釋上述的每個重要現象。這些重要的自由度，包含銅氧化物超導體未加離子植入時的反磁性所產生的自旋關聯，以及電子自旋的Berry phase。由我們的場論模型，可以推得系統的等效晶格模型在均勻情況為一般的Hubbard model，而在非均勻系統，強關聯效應使其轉變為negative-U Hubbaed model. 我們的模型預測的銅氧化合物超導體的相圖，也吻合實驗所觀測到的。
並列摘要
Since their discovery in 1986, high transition temperature cuprate superconductors have raised tons of surprises and questions beyond the standard Fermi liquid theory and prompted intensive studies in nearly 40 years. However, until now, many of their properties remain elusive and under debate, including the pseudogap transition, the inhomogeneous charge density modulation order, the ferromagnetic time-reversal-symmetry-breaking fluctuation, the d-wave superconductivity and the Fermi arc formation under doping, which are attributed to the effects of strong correlations between the electrons. We provide an unconventional method of solving the problem of electron correlations by extracting the relevant degrees of freedom of the correlation itself, whose dynamics conspires with that of the electrons to explain all the above phenomena consistently. The relevant correlation degrees of freedom in cuprates include the electron’s antiferromagnetic spin fluctuation, and the spin Berry’s connection from the electrons’ spin wavefunction overlap. The resulting electron pairing below the superconducting temperature is a real-space pairing, instead of the momentum-space Cooper pairing in conventional superconductors. The homogeneous effective quantum lattice model is the positive-U Hubbard model, while under the effect of quantum fluctuations it becomes the negative-U extended Hubbard model. The phase diagram of our model is also proposed to reproduce the experimentally observed one in cuprates.