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Path integral Monte Carlo simulations of helium: From superfluid droplets to quantum crystals.

機譯：氦氣的路徑積分Monte Carlo模擬：從超流體液滴到量子晶體。

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摘要

Below Tlambda = 2.17 K, bulk 4He is a superfluid and has a non-zero Bose-Einstein condensate fraction. This work will focus primarily on how phenomena such as superfluidity, Bose condensation and superfluid vortices are manifested in microscopic, inhomogeneous helium systems. Path Integral Monte Carlo is a powerful method for calculating the equilibrium properties of quantum systems at finite temperature. We have achieved linear scaling of computer time with number of particles through the use of neighbor lists, allowing us to simulate systems of several thousand atoms.; We have derived a local superfluid estimator and used it to examine the microscopic superfluid response around a molecule rotating in a helium nanodroplet. We found that the first solvation layer is well-described by a two dimensional superfluid, with thermal excitations occuring at a lower temperature than in bulk helium. The effective moment of inertia of a linear impurity in a helium droplet is calculated, and compared with experimental scattering results. In addition, we calculated the vortex formation energy for both pure droplets and droplets doped with linear impurities, and found that the linear impurities had a negligible impact on the formation energy. A possible spectroscopic signature of vortices in doped helium droplets was suggested.; After deriving a local estimator, we calculated the condensate fraction throughout the free helium surface of a semi-infinite slab. These results, along with density-density correlation functions, were used to characterize the surface excitations and calculate the extent to which ripplons are present. In addition, the ripplon dispersion relation was calculated using imaginary-time correlation functions, and found to be in good agreement with experimental results.; Finally, we have calculated the Deybe-Waller factor in solid helium for a range of temperatures and densities, and compared the scaling behavior with the predictions of harmonic theory. The first non-Gaussian contribution to the density distribution was calculated.
機譯：低于Tlambda = 2.17 K時，體4He是超流體，并且玻色-愛因斯坦凝聚分數(shù)不為零。這項工作將主要集中在微觀，不均勻氦系統(tǒng)中如何表現(xiàn)出超流，玻色凝結和超流渦旋等現(xiàn)象。路徑積分蒙特卡洛是一種在有限溫度下計算量子系統(tǒng)平衡特性的強大方法。通過使用鄰居列表，我們實現(xiàn)了具有粒子數(shù)量的計算機時間的線性縮放，從而使我們可以模擬數(shù)千個原子的系統(tǒng)。我們已經(jīng)導出了一個局部超流體估計器，并使用它來檢查圍繞氦納米液滴中旋轉的分子的微觀超流體響應。我們發(fā)現(xiàn)第一溶劑化層由二維超流體很好地描述，熱激發(fā)發(fā)生在比體氦中更低的溫度下。計算了氦滴中線性雜質(zhì)的有效慣性矩，并將其與實驗散射結果進行了比較。此外，我們計算了純液滴和摻雜線性雜質(zhì)的液滴的渦旋形成能，發(fā)現(xiàn)線性雜質(zhì)對形成能的影響可忽略不計。有人提出了摻雜氦滴中渦旋的可能的光譜特征。推導出局部估計量后，我們計算了整個半無限平板的自由氦表面上的冷凝水分數(shù)。這些結果與密度-密度相關函數(shù)一起用于表征表面激發(fā)并計算出ripplons存在的程度。另外，利用虛時相關函數(shù)計算出波紋的色散關系，與實驗結果吻合良好。最后，我們針對一定溫度和密度計算了固態(tài)氦中的Deybe-Waller因子，并將其定標行為與諧波理論的預測進行了比較。計算了密度分布的第一個非高斯貢獻。

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  • 作者

    Draeger, Erik Walter.;

  • 作者單位

    University of Illinois at Urbana-Champaign.;

  • 授予單位 University of Illinois at Urbana-Champaign.;
  • 學科 Physics Condensed Matter.
  • 學位 Ph.D.
  • 年度 2001
  • 頁碼 126 p.
  • 總頁數(shù) 126
  • 原文格式 PDF
  • 正文語種 eng
  • 中圖分類 O49;
  • 關鍵詞

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