硫氫化物與硒化鐵中晶體振動與超導性質之第一原理理論研究

Abstract

自從實驗物理學家發現釔鋇銅氧化物的高溫超導性質之後，原本的Bardeen-Cooper-Schrieffer (BCS)理論無法完整解釋其現象，而理論物理學家也不斷提出相關的高溫超導理論來解釋這個新現象。近年來，有理論物理學家透過BCS理論結合第一原理計算，發現在某些高氫化材料中也存在高溫超導，相關實驗也驗證了此一現象。因此，物理學家又重新檢視BCS理論是否可以解釋某些超導物質的超導現象。此論文透過第一原理密度泛函理論計算與BCS理論結合，研究硫氫化物(H3S)與硒化鐵(FeSe)的電子與聲子結構，並探討其超導特性。對於H3S，實驗的數據顯示它的超導溫度在約150 GPa達到最高，並且隨著壓力升高有逐漸下降的趨勢。然而，在我們的理論研究中並沒有發現此結果。另外，藉由參考一些文獻，我們發現在計算中加入非諧效應可以使計算結果與實驗更加相符。對於FeSe，因為其為層狀結構，我們在計算中加入凡得瓦爾效應，但是所計算出的超導溫度仍遠小於實驗的量測(8.5 K)。對於此一結果，藉由參考相關文獻，我們認為在FeSe的計算中引入瓦尼爾(Wannier)函數將可能提升計算之超導溫度並解釋其超導現象。而透過計算FeSe的電子比熱與實驗的比熱做比較，並經由公式估算出電聲子耦合強度，我們成功估算出與實驗接近的超導溫度(8.4 K)。

Ever since the discovery of superconductivity in Yttrium Barium Copper Oxide (YBa2Cu3O7), it became clear that Bardeen-Cooper-Schrieffer (BCS) theory was insuf-ficient in explaining its superconductivity, leading many theoretical physicists to pro-pose their own high-temperature superconductivity theories. In recent years, calcula-tions involving a combination of BCS theory and first-principle calculations led to the discovery of superconductivity in some superhydride materials, which was later veri-fied by experiments. This led to many physicists reconsidering whether BCS theory could explain superconductivity in some superconducting materials or not. In this thesis, we compute the electron and phonon properties of H3S and FeSe using a combination of the first principle density functional theory and BCS theory and dis-cuss their superconducting properties. Experimental H3S analysis shows that its su-perconducting temperature peaks at around 150 GPa and decreases as pressure in-creases further; however, our theoretical calculations do not find this phenomenon. Through investigating references, we discover that incorporating the anharmonic effect into calculations causes results to match up more with the experiments. In our FeSe computations, we include the Van der Waals effect due to the material’s layered struc-ture, but the resulting superconducting temperatures remain much lower than the ex-perimental value (8.5 K). Regarding this result, we concluded from references that the inclusion of the Wannier functions (GW method) may increase the calculated Tc as well as explain its superconductivity. Through computing the specific heat and com-paring it with the experiment, we evaluate the electron-phonon coupling strength of FeSe by empirical formulae and successfully evaluate a superconducting temperature 8.4 K that closely matches the experiment.